No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Structural Constraints and Localization of Gold Mineralization in Leather Jacket Lodes, Ballarat, Victoria, Australia
Abstract
Gold mineralization at the Ballarat East deposit, central Victoria, Australia, is hosted in lodes that are historically known as "leather jackets." These are quartz-dominant vein arrays related to low-displacement, W-dipping faults (≤45°) that transect the core and/or eastern limb of tight, asymmetric N-S-trending anticlines. The leather jacket lodes typically have dip extents from 5 to 65 m, widths of ≤20 m, and strike lengths up to hundreds of meters, but their along-strike continuity is disrupted by oblique, low-displacement listric faults known as "cross courses." The gold lodes are characterized by distinct phases of sulfide paragenesis with minor gold + arsenopyrite + pyrite defining the early sulfide stage. Late-stage coarse gold was precipitated with galena + sphalerite ± pyrrhotite ± chalcopyrite (late pyrite also occurs). The gold mineralization events are linked to low-strain mineralized fracture networks, which are closely related to the final deformation stages and the amplification of the major folds enclosing the lodes. This amplification produced domal fold culminations, with plunges ≤30°, and localized minor parasitic folds with shallower plunges (≤10°). A network of dilation sites, on the W-dipping faults, preferentially developed in the cores of anticlines, particularly in zones where there are changes in strike of bedding or fault bifurcation and refraction through contrasting sandstone and interbedded packages of sandstone and shale. Numerical three-dimensional simulations were undertaken to test our geologic observations and replicate conditions controlling the emplacement of the leather jacket lodes. Two different scenarios were investigated: first, to determine how changes in the local stress field orientation influences dilation and fluid infiltration; secondly, to test variations in fault geometry during the last stages of deformation-that is, within the final 2% of shortening, when most of the mineralized sites were created. Results show that strain and fluid flow localized along refracted sections of faults and around changes in dip, specifically on the shallower dipping sections within subvertical sandstone units. This is consistent with the observation that high-grade gold-bearing quartz is associated with localized changes in fault dip in thicker sandstone and sandstone-shale packages. There was also a component of strike-slip motion and near-field NW-to-SE or N-to-S stress fields, which can be attributed to the development of a component of out-of-plane motion during the development of fold culminations. The preferred model for the distribution of the high-grade auriferous vein arrays defining the leather jacket lodes is one of fold amplification and extension parallel to the fold axes, which produced an increasing out-of-plane relaxation. The main fluid conduits responsible for the leather jacket style of mineralization involve infiltration along steep bedding-parallel faults and veins that link up with the arrays of low-displacement W-dipping faults.
... The deformational history at Hill End indicates that goldbearing bedding-parallel veins formed repeatedly as a consequence of the development of bedding-parallel movement zones concentrated in shale beds during east-west shortening and flexural-slip folding of a metaturbiditic sequence (Tanner, 1989;Fowler, 1996;Horne and Culshaw, 2001;Fagereng, 2015;Wilson et al., 2016). The upright, close to tight, chevronstyle F 0 2 folds plunge gently north-northeast with a subvertical axial planar S2 slaty cleavage striking at 005°. ...
... Axial-planar cleavage development and boudinage of both bedding-parallel veins and lithological BPVs strongly modified by late reverse faults layering during flattening of the folds effectively terminated bedding-parallel slip. Late, initially bedding-parallel, reverse faults with dip angles up to 70° do not cross the bedding (cf. in the goldfields of Victoria; Willman, 2010;Wilson and Leader, 2014;Wilson et al., 2016) and are only rarely encountered in preexisting weak zones on the limbs of anticlinal hinges containing saddle reefs (Fig. 6D). Therefore, shortening did not continue to a stage where bedding discordant thrusts became the preferred deformation mechanism during fold tightening. ...
... Where fold-related, high-angle, bedding-parallel reverse faults link on adjacent W-dipping fold limbs, the associated dilational jogs contain fault-fill lodes and extension vein arrays (Wattle Gully; Cox et al., 1995;Willman, 2007). At Ballarat East (the Leather Jacket Lodes; Wilson et al., 2016) beddingparallel veins, faults, and related extension veins initially formed during flexural-slip folding along shale and siltstone beds. Subsequent strong fold tightening (tight to isoclinal) caused a failure in the hinges in more competent sandstoneshale units. ...
... The deformational history at Hill End indicates that goldbearing bedding-parallel veins formed repeatedly as a consequence of the development of bedding-parallel movement zones concentrated in shale beds during east-west shortening and flexural-slip folding of a metaturbiditic sequence (Tanner, 1989;Fowler, 1996;Horne and Culshaw, 2001;Fagereng, 2015;Wilson et al., 2016). The upright, close to tight, chevronstyle F 0 2 folds plunge gently north-northeast with a subvertical axial planar S2 slaty cleavage striking at 005°. ...
... Axial-planar cleavage development and boudinage of both bedding-parallel veins and lithological BPVs strongly modified by late reverse faults layering during flattening of the folds effectively terminated bedding-parallel slip. Late, initially bedding-parallel, reverse faults with dip angles up to 70° do not cross the bedding (cf. in the goldfields of Victoria; Willman, 2010;Wilson and Leader, 2014;Wilson et al., 2016) and are only rarely encountered in preexisting weak zones on the limbs of anticlinal hinges containing saddle reefs (Fig. 6D). Therefore, shortening did not continue to a stage where bedding discordant thrusts became the preferred deformation mechanism during fold tightening. ...
... Where fold-related, high-angle, bedding-parallel reverse faults link on adjacent W-dipping fold limbs, the associated dilational jogs contain fault-fill lodes and extension vein arrays (Wattle Gully; Cox et al., 1995;Willman, 2007). At Ballarat East (the Leather Jacket Lodes; Wilson et al., 2016) beddingparallel veins, faults, and related extension veins initially formed during flexural-slip folding along shale and siltstone beds. Subsequent strong fold tightening (tight to isoclinal) caused a failure in the hinges in more competent sandstoneshale units. ...
The Reward mine at Hill End hosts structurally controlled orogenic gold mineralization in moderately S plunging, high-grade gold shoots located at the intersection between a late, steeply W dipping reverse fault zone and E-dipping, bedding-parallel, laminated quartz veins (the Paxton’s vein system). The mineralized bedding-parallel veins are contained within the middle Silurian to Middle Devonian age, turbidite-dominated Hill End trough forming part of the Lachlan orogen in New South Wales. The Hill End trough was deformed in the Middle Devonian (Tabberabberan orogeny), forming tight, N-S–trending, macroscopic D2 folds (Hill End anticline) with S2 slaty cleavage and associated bedding-parallel veins.
Structural analysis indicates that the D2 flexural-slip folding mechanism formed bedding-parallel movement zones that contained flexural-slip duplexes, bedding-parallel veins, and saddle reefs in the fold hinges. Bedding-parallel veins are concentrated in weak, narrow shale beds between competent sandstones with dip angles up to 70° indicating that the flexural slip along bedding occurred on unfavorably oriented planes until fold lockup. Gold was precipitated during folding, with fluid-flow concentrated along bedding, as fold limbs rotated, and hosted by bedding-parallel veins and associated structures. However, the gold is sporadically developed, often with subeconomic grades, and is associated with quartz, muscovite, chlorite, carbonates, pyrrhotite, and pyrite.
East-west shortening of the Hill End trough resumed during the Late Devonian to early Carboniferous (Kanimblan orogeny), producing a series of steeply W dipping reverse faults that crosscut the eastern limb of the Hill End anticline. Where W-dipping reverse faults intersected major E-dipping bedding-parallel veins, gold (now associated with galena and sphalerite) was precipitated in a network of brittle fractures contained within the veins, forming moderately S plunging, high-grade gold shoots. Only where major bedding-parallel veins were intersected, displaced, and fractured by late W-dipping reverse faults is there a potential for localization of high-grade gold shoots (>10 g/t). A revised structural history for the Hill End area not only explains the location of gold shoots in the Reward mine but allows previous geochemical, dating, and isotope studies to be better understood, with the discordant W-dipping reverse faults likely acting as feeder structures introducing gold-bearing fluids sourced within deeply buried Ordovician volcanic units below the Hill End trough.
A comparison is made between gold mineralization, structural style, and timing at Hill End in the eastern Lachlan orogen with the gold deposits of Victoria, in the western Lachlan orogen. Structural styles are similar where gold mineralization is formed during folding and reverse faulting during periods of regional east-west shortening. However, at Hill End, flexural-slip folding-related weakly mineralized bedding-parallel veins are reactivated to a lesser degree once folds lock up (cf. the Bendigo zone deposits in Victoria) due to the earlier effects of fold-related flattening and boudinage. The second stage of gold mineralization was formed by an array of crosscutting, steeply W dipping reverse faults fracturing preexisting bedding-parallel veins that developed high-grade gold shoots. Deformation and gold mineralization in the western Lachlan orogen started in the Late Ordovician to middle Silurian Benambran orogeny and continued with more deposits forming in the Bindian (Early Devonian) and Tabberabberan (late Early-Middle Devonian) orogenies. This differs from the Hill End trough in the eastern Lachlan orogen, where deformation and mineralization started in the Tabberabberan orogeny and culminated with the formation of high-grade gold shoots at Hill End during renewed compression in the early Carboniferous Kanimblan orogeny.
... In 2D sections showing the `saddle reefs', the common elements include faults propagating from or associated with bedding-parallel veins and neck and leg reefs with spur or extensional veins on fold limbs (Fig. 10). However, the 3D geometries suggest much of the gold mineralization in the Bendigo Zone post-dates the Benambran (455-440 Ma) E-W-compression event (Pabst, 1919;Chance, 1949;Forde and Bell, 1994;Leader et al., 2013;Wilson et al., 2013Wilson et al., , 2016. In particular Pabst (1919) was the first to identify a later phase of mineralization and stated that there was "a very close association between the major strike-faulting and the occurrences of economic gold shoots". ...
... 14D, 13F). These V 3 -V 4 veins and associated gold lodes formed as low-strain fracture networks closely related to the final fault movements as the major folds were modified as the regional stress field changed (Wilson et al., 2016).) ...
... Cross-course faults range from centimeter-to meter-scale thickness and have variable orientations ( Fig. 14D) with some containing V 3 -V 4 auriferous quartz-carbonate veins. The V 2 -V 4 vein arrays are also off-set by small-displacement cross-courses ( Fig. 7 in Wilson et al., 2016). In contrast, the larger cross-course faults have no new quartz and are dominated by gouge, but localize the mineralization into 'compartments' (Fig. 14). ...
... The 3D visualization of ore-forming space and the quantification of the correlations between the mineralization and extracted ore-controlling indicators make it possible to obtain more useful spatial information, such as distance (Carranza, 2009;Mao et al., 2011a;Chen et al., 2012;Liu et al., 2013), fluid pathways (Almasi et al., 2017), geochemical anomalies (Zuo, 2014;Gao et al., 2016;Zuo et al., 2016a), and shape undulation (Mao et al., 2012(Mao et al., , 2016Yuan et al., 2014;Hu et al., 2018). While the strength of spatial analysis is in discovering geological features and their quantitative correlation to ores, current spatial analysis methods are still limited in analysis of the controls of complex shape patterns in mineralization, such as stepped ore patterns typical of structurally-controlled hydrothermal deposits (e.g., Bons et al., 2012;Henry et al., 2014;Song et al., 2015;Wilson et al., 2016). The ability of 3D geologic models to provide reliable spatial data is completely dependent on the quality of datasets, modeling techniques, expert knowledge, and the complexity of the geological setting (Houlding, 1994;Fallara et al., 2006;Lindsay et al., 2012;Jessell et al., 2014;Liu et al., 2016). ...
... 5,8). Fault development was accompanied by extensional shear opening at relatively gentle segments while the steep parts occurred during compressional stress (e.g., Bons et al., 2012;Henry et al., 2014;Wilson et al., 2016). Consequently, the ore fluids preferentially gathered into these dilation zones with low dip angles. ...
... (4) It must be noted that the Zhaoping fault has many local undulations (Fig. 8) where the most gold enrichment occurs ( Fig. 16d and e). These areas, with consistently different dip angles, (Fig. 15b) indicate the different stress state distributions (Henry et al., 2014;Wilson et al., 2016;Li et al., 2018b;Yang et al., 2018). The stress changes in these local bends were likely the major triggers for the decoupling of gold complexes during fluid flow up along the fault conduits. ...
The Jiaojia-type gold deposits, hosting >80% of gold resource in the Jiaodong Peninsula, Eastern China, are characterized by veinlet-and disseminated-style mineralization associated with the Mesozoic detachment faults. In this study, we performed multi-constraint geological modeling and spatial analysis involving 3D buffer analysis , shape analysis, and field analysis for the Dayingezhuang gold deposit to quantitatively assess the gold distribution and its association with geological features. The obtained spatial data were further integrated into three dimensional (3D) prospectivity modeling by fuzzy weights-of-evidence (WofE) and continuous WofE methods to evaluate mineral potential. Our results determine a quantitative correlation between phyllic alteration thickness and tectono-geochemical anomaly to construct the geometric models of alteration zone. The hydrothermal intensity extracted from the models shows a bimodal distribution and it is significantly high in the center of No. 2 orebodies, indicating an overprinting gold mineralization. The spatial analysis on the Zhaoping fault reveals that the most probable locations for gold deposition were determined to be in segments of the Zhaoping fault with a slope of 20° to 40°, dip angle changes of −5°, and undulation of near 0 m. All of the features likely result from structural controls on fluid flow and infiltration, as well as variations in the stability of Au-bearing complexes related to fault morphology. 3D prospectivity models generated by continuous transformed spatial evidence values with lower bias and uncertainty yielded a higher predictive efficiency than classified evidential layers. Our study not only highlights that gold enrichment of the Jiaojia-type deposit is essentially controlled by shape features of detachment faults, but also emphasizes the applicability of 3D prospectivity modeling in identifying potential gold mineralization at depth in the Jiaodong Peninsula.
... gold mineralization period. The apparent undulations may imply localized stress inhomogeneities during the structural activities, which contributed to the enhancement in permeability or fluid fluxes in local host rocks, thereby facilitating gold precipitation (Henry et al., 2014;Wilson et al., 2016). The ore-forming simulation results encompass a series of rock and fluid conditions, such as volumetric strain, shear strain, temperature, and fluid flux or divergence (Zhao et al., 2009;Itasca, 2012), all of which contribute to fluid flow and interaction with rocks and gold precipita-tion. ...
... Substantial dilation (positive volumetric strain) and extensive shear are commonly observed in the gentle areas ( Figure 10A). This implies that the localization of volume strain and shear strain that further causes rock fracture and deformation is closely attributed to the fault geometry (Wilson et al., 2016). The hydrothermal fluid flows upward along with the Jiaojia fault (Figure 10), supporting a source of gold and fluid derived from deep (Deng et al., 2020a;Liu et al., 2021b). ...
Gold mineralization in the Jiaojia gold belt was formed in a structurally-dominant hydrothermal mineral system showing a close spatial association with the Jiaojia detachment fault. This study delves into the Jiaojia gold belt from the perspective of coupled spatial association and ore-forming processes by employing spatial analysis of three-dimensional (3D) models, 3D ore-forming numerical modeling, and 3D prospectivity modeling using machine learning techniques (random forest (RF) and multilayer perceptron (MLP)). The overarching goal is to gain insight into the structural-hydrothermal gold system and pinpoint potential areas of deep-seated gold deposits for future exploration endeavors. The spatial analysis of ore-controlling faults uncovers a close correlation between gold enrichment and specific fault geometrical attributes, including a dip angle ranging from 20° to 40°, minimal variations in dip angle (less than 5°), and convex topographical features. These attributes likely stem from the influence of fault morphology on the flow and pooling of fluids. In conjunction with this, 3D ore-forming numerical modeling of structural deformation and fluid flow reveals that gold mineralization is intertwined with moderate volumetric strain and shear strain of rock and fluid divergence. This interaction seems particularly pronounced in areas characterized by channel-like or gentle features. Consequently, it is plausible that gold distribution in the Jiaojia region is the outcome of a comprehensive coupling process involving strain localization, rock deformation, fluid flow, heat transfer and/or interaction. The deep gold prospectivity models of RF and MLP for the Jiaojia district jointly using the predictive variables of fault geometry features and ore-forming simulation data (volume strain, shear strain, temperature variation, and fluid flux) exhibit higher AUC (area under the curve) values compared to models employing individual predictor variable datasets. This improvement underscores their enhanced predictive capability. The prospectivity results thus were used for identifying gold potential within the Jiaojia region, where five promising gold targets at depth were ultimately determined.
... Rock deformation provides channels and facilitates ore-forming fluid flows to form various hydrothermal deposits [71][72][73]. In hydrothermal systems, the sites of shear strain and the related local dilation tend to increase the fluid flux, often resulting in favorable locations for mineral precipitation [18,24,27,29,74]. Based on the Mohr-Coulomb plasticity criterion, shear induces dilatancy, which increases the pore volume of the rock [18,75], and thus, we carried out several experiments to determine the deformation with tectonic stress in various directions (Figures 7-10) to better understand the relationship of fluid flow and shear strain, as well as related dilation. ...
... Tectonic stress is a crucial factor in the fluid migration mechanism, promoting the rapid fluid migration from deep to shallow levels [1,4]. Therefore, when the fluid rises to the dilation sites of the fault, which are the pooling centers for fluids [74,80], the flow of the fluid slows down significantly, and gold may be precipitated during this process ( Figure 13). Structural controls in the Dayingezhuang deposit are represented by strain localization and the formation of shear zones and fractures, which eventually influence fluid activity and mineral precipitation [30]. ...
The Dayingezhuang orogenic gold deposit, located in the northwestern Jiaodong Peninsula, is hosted by the Zhaoping detachment fault, but the paleostress regime during the mineralization period remains poorly understood. In this study, a series of numerical modeling experiments with variable stress conditions were carried out using FLAC3D software to determine the orientation of paleostress and the fluid migration processes during the ore-forming period. The results show that the simple compression or tension stress model led to fluid downward or upward flow along the fault, respectively, accompanying the expansion deformation near the hanging wall or footwall of the Zhaoping fault, which is inconsistent with the known NE oblique mineralization distribution at Dayingezhuang. The reverse and strike-slip model shows that the shear stress was distributed in the gentle dip sites of the fault, and the expansion space occurred in the geometric depression sites of the fault, which is also inconsistent with the known mineralization distribution. The normal and strike-slip model shows that shear stress was distributed in the sites where the fault geometry transforms from steep to gentle. In addition, the expansion deformation zones appeared at sites with dip angles of 35~60° in the footwall and extended along with the NE-trending distribution from shallow to deep levels. The numerical results are quite consistent with the known mineralization, suggesting that the fault movement during the mineralization stage is a combination of the local strike-slip and the NW–SE extension in the Dayingezhuang deposit. Under this stress regime (σ1 NE–SW, σ2 vertical, σ3 NW–SE), the NE dilation zones associated with fault deformation served as channels for the ore-forming fluid migration. Based on the numerical modeling results, the deeper NE levels of the No. 2 orebody in the Dayingezhuang deposit have good prospecting potential. Thus, our study not only highlights that gold mineralization at Dayingezhuang is essentially controlled by the detachment fault geometry associated with certain stress directions but also demonstrates that numerical modeling is a robust tool for identifying potential mineralization.
... The resulting reefs are generally 100s of meters long but have up and/or downdip extents <20 m. In the Ballarat-East district arrays of mineralized quartz veins are locally developed along small displacement faults, which cut tight, gently plunging, asymmetric folds (Wilson et al., 2016). More extensive vein arrays are confined to fault segments whose orientations are close to orthogonal to bedding, within zones of particularly tight, short wavelength, doubly plunging parasitic folds, where they cut thicker sandstone units. ...
... The greater thickness of sandstone and shale beds at Bendigo and Ballarat-East, compared to the Greenland Group, may account for the lack of auriferous quartz veins along anticline hinges in the Reefton goldfield. Rocks that surround the Birthday reef are lithologically homogeneous at scales <10 to 20 m (Cox, 2000), whereas major lodes at Ballarat-East coincide with the intersection of mineralized faults and 10 to 20+-m-thick sandstone units flanked by shales whose rheological contrast was a key factor that affected both the trajectory and dilatant behavior of the faults (Wilson et al., 2016). ...
This paper describes the structural setting of the Globe-Progress and Blackwater orogenic gold deposits, the only two +1-million ounce (Moz) deposits discovered to date within the Reefton goldfield of central New Zealand. Most of the Globe-Progress deposit is located along the Globe-Progress shear zone, a curviplanar, brittle-ductile, structure that juxtaposes tightly folded rocks in its footwall against largely unfolded rocks in its hanging wall. At its eastern end the Globe-Progress shear zone merges with the fold-parallel Oriental shear zone, with the Globe-Progress shear zone acting as a tear fault that transferred contractional deformation on the Oriental shear zone across the structural grain of the adjacent folded rocks, during or soon after the end of greenschist facies metamorphism. An early phase of mineralization at ca. 438 Ma (K-Ar, sericite) accompanied this contractional deformation, whereas a second phase of mineralization occurred during later brittle, oblique-normal, reactivation of the two shear zones at ca. 386 Ma (K-Ar, sericite). The resulting ore shoots plunge moderately towards the southwest, along intersections between (1) the two shear zones, (2) the Globe-Progress shear zone and minor normal faults in its hanging wall, and (3) the Globe-Progress shear zone and W-dipping bedding in its hanging wall. The kilometer-long Birthday reef in the Blackwater mine is located on a narrow, mineralized, brittle-ductile fault that can be traced for 3+ km through the adjacent Greenland Group sandstones and argillites. The reef occupies that part of the fault that is close to parallel with bedding on a relatively gently (~60°) W-dipping segment of the western limb of the Waiuta anticline. The reef pinches out where the dip of the host fault steepens and its strike swings from north-northeast toward north. This change in orientation occurs where the fault refracts onto the near vertical axial plane of the Waiuta anticline above/north of the reef, and follows an open flexure in the orientation of bedding on the western limb of the Waiuta anticline below/south of the reef. Both ends of the reef, and its more intensely mineralized internal ore shoots, plunge 35° toward the north-northeast, parallel to the intersection of the host fault and the axial planar cleavage associated with the adjacent Waiuta anticline. © 2018 Society of Economic Geologists, Inc.All Rights Reserved.
... Leader et al. (2013) suggested that a numerical model of compressional reactivation of reef margins by oblique slip best simulated the observed distribution of strain and location of quartz reefs hosting gold mineralization in the Bendigo goldfield of Australia. Wilson et al. (2016) demonstrated that the local geometries of faults and stress fields controlled the localization of dilation and shear strain and thus the gold mineralization in the Ballarat gold deposit of Australia. ...
... The Mohr-Coulomb isotropic elastic-plastic constitutive model in FLAC3D, which has been previously used in many studies related to tectonics and ore deposits (e.g., Strayer et al., 2001;Oliver et al., 2006;Potma et al., 2008;Zhang et al., 2009Zhang et al., , 2011aLeader et al., 2013;Wilson and Leader, 2014;Liu et al., 2016;Wilson et al., 2016), was selected to simulate the interactions between deformation and fluid flow in porous media. ...
Unconformity-related uranium deposits in the Athabasca Basin (Canada) are spatially associated with reactivated basement faults that cut the unconformity surface between Archean to Paleoproterozoic basement and Paleoproterozoic to Mesoproterozoic sedimentary rocks of the Athabasca Group. The Sue deposits (Sue A, B, C, D, and E) are located in the eastern Athabasca Basin along a 2-km-long north-northeast-trending structural corridor and have both sandstone-hosted (Sue A and Sue B; to the north) and basement-hosted (Sue C, Sue D, and Sue E; to the south) orebodies. All the deposits are structurally controlled by north-northeast-trending basement faults that demonstrate different degrees of reverse displacements of the unconformity surface. However, it is not clear why the mineralization is distributed in such a pattern along the corridor. In this study, both 2-dimensional (2D) and 3-dimensional (3D) numerical modeling of deformation were conducted to examine the relationship between the distribution of strain and uranium mineralization. The modeling shows that dilation (positive volumetric strain), thought to correlate with the development of extensional fracture systems and dilational jogs that represent potential mineralization sites, is primarily controlled by the rheological contrasts in basement rocks and the degree of deformation. The presence of faulted graphitic pelitic gneiss associated with the sandstone-hosted Sue A and Sue B deposits favours dilation localized within the sandstone at low degrees of deformation mainly due to the overlying competent hangingwall granitic gneiss. In contrast, the basement-hosted Sue C, Sue D and Sue E deposits are associated with dilation zones localized along the contact between the faulted graphitic pelitic gneiss and competent footwall silicified gneiss at higher degrees of deformation. The modeling results highlight the importance of both graphitic basement structures and strong rheological contrasts between basement rocks in uranium exploration in the Athabasca Basin.
... The local region is determined by the voxel's viewing direction and projection distance, and the region size was negatively correlated with projection distance. This is consistent with the local fault controls on orogenic Au mineralization at the orebody scale associated with local fluids entrapment (Henry et al., 2014;Wilson et al., 2016;Mao et al., 2019;Liu et al., 2021a). The projection distance is added to the multichannel image for prospectivity modeling to clarify the fault buffer's effect on ores. ...
... For example, Liu et al. [13], Hu et al. [48], and Shan et al. [49] used the dilation space, mineralization rate, and/or geochemical interaction as ore proxies to find the favorable metal deposition location at depth. Ford et al. [50] and Wilson et al. [51] discussed the fault geometry controls on the rock deformation and fluid flow regular pattern and determined the mineralization exploration strategies in strike-slip fault fluid systems. Thus, ore-forming numerical modeling has the powerful ability to provide ore-predictive information in terms of the metallogenic geodynamic system. ...
Finding new, effective predictive variables for 3D mineral prospectivity modeling is both important and challenging. The 3D ore-forming numerical modeling quantitively characterizes the complex coupling-mineralization process of the structure, fluid, heat, and wall rock, which may be potential indicators for mineral exploration. We here conducted 3D mineral prospectivity modeling with the integration of ore-forming computational simulation information in the Xiadian orogenic gold deposit, China, to examine whether the simulation data input can improve the reliability of prospectivity modeling. First, we constructed the 3D models of the orebody and fault to extract the fault geometric features using spatial analysis, as they are always considered to be the crucial controls of gold distribution. Second, we performed 3D numerical modeling of the deformation–fluid–heat-coupling process of the structurally controlled hydrothermal Au system using the FLAC3D platform. Finally, the fault-geometry features (buffer, dip, dip variation, and undulation) and the ore-formation-simulation indices (volume strain, shear strain, temperature variation, and fluid flux) were integrated using Bayesian decomposition modeling, which has a promising nonlinear model ability and a flexible variable-integration ability. The prospectivity modeling results demonstrated that the model generated by combining geometry and simulation variables achieved significantly higher AUC, precision, accuracy, Kappa, and F1 scores compared to other models using a single-predictor-variable dataset. This suggests that the joint use of geometry and simulation variables construct a comprehensive association between gold and its ore-controlling factors, thereby resulting in a highly reliable prospectivity model. Thus, the approach of 3D mineral prospectivity modeling aided by ore-forming numerical simulation proves to be more useful in guiding mineral exploration, especially in the condition of fewer variables. Based on the prospectivity modeling outcomes, we identified four gold targets at depth in the Xiadian district that warrant focused exploration efforts.
... The lens-like structure hosting the Zun-Kholba deposit probably was formed as a large transpressional zone. Transpressional shear zones often control the location of orogenic gold deposits [61,62]. The rocks of the Tuva-Mongol microcontinent basement and cover, as well as blocks of ophiolites and fragments of granitoids, were sheared together along this lens-like structural zone. ...
In this paper, we present a new point of view on the deposit geology and genesis of the largest gold deposit in Eastern Sayan, Zun-Kholba. Quartz–sulfide replacement ores and shallow quartz veins are of great economic importance. At the deposit, quartz–pyrite ore bodies are dominant, whereas paragenetically late base metal-bearing quartz veins occur only at shallow levels. The study of the fluid inclusions, stable isotopes (C–O–S), and mineral geothermometry allowed us to determine the P–T conditions of ore deposition. It is established that the temperature of ore formation changed from 380 to 433 °C at the deepest levels of the deposit to 316 °C at the shallowest levels. The pressure estimates for gold deposition at 570–950 bar were calculated for the 1490-m level in the center of the deposit. The determined P–T conditions can help estimate the average depth of deposit formation at about 3.6 km. Ore mineral assemblages were formed from homogeneous fluids of low to moderate salinity (2.4–7.9 wt.% eq. NaCl). The sulfur isotope composition of sulfide minerals (δ34S = 0–4.6‰), as well as pyrite geochemistry, corresponds to that of older volcanogenic massive sulfide (VMS) occurrences, which are widespread within the Eastern Sayan ophiolitic belts. Lens-like fragments of metamorphosed VMS-type sulfide ores are also present in the Zun-Kholba deposit. The oxygen isotope data indicate a metamorphic origin for the ore-forming fluids. Migrating metamorphic fluids may have become enriched in gold, sulfur, and other metals during the flow through the complex ore-hosting lithologies, including ophiolitic bodies with sulfide-rich VMS fragments that are characterized by a relatively high content of gold and related ore elements. The obtained data allow us to suggest a metamorphic devolatilization model for the genesis of the Zun-Kholba gold deposit.
... The main (2) 'saddle reef' type veins that are concentrated in the hinges of upright anticlines and commonly occur as a series of stacked veins that can each be up to 10 m thick; (3) other vein types commonly associated with folded strata (a-c and radial veins), and (4) late-tectonic veins, commonly with abundant wing veins, associated with post-folding high-angle reverse faults that typically cut across the main anticlines (e.g. Leader et al. 2010Leader et al. , 2012Wilson et al. 2016). All of these vein types typically have low sulfide contents, including pyrite, arsenopyrite, and locally trace amounts of base metals (galena, sphalerite + stibnite) and pyrrhotite. ...
Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: 1) crustal scale fault associated (CSF) subtype, 2) sediment-hosted orogenic gold (SHOG) subtype, 3) forearc (FA) subtype, and 4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.
... Mineralization has been discovered to be closely correlated to the shape of the geological boundaries such as contacts and faults (Carranza & Hale, 2002;Groves et al., 2018;Robb, 2020;Song et al., 2015;Wilson et al., 2016). The 3D geometry of geological boundaries controls not only the transportation and trapping of ore-forming fluids but also the deposition and preservation of ores (Squire et al., 2008;Leader et al., 2013;Jacques et al., 2018;Yang et al., 2018;Hronsky et al., 2020). ...
The shapes of geological boundaries such as contacts and faults play a crucial role in the transportation, deposition and preservation of metals in magmatic and hydrothermal systems. Analyzing the shapes of geological boundaries, in particular those associated with mineralization, is an important step in 3D mineral prospectivity modeling. However, existing methods of shape analysis are limited in the adaptation of various shapes, scales and topologies of geological boundaries. This paper presents a general method of shape analysis based on mathematical morphology (MM), which is a generalization of the original MM method for shape analysis. The generalization extends the applicability of the original MM method from closed surfaces to general surfaces, while inheriting the real 3D and multi-scale analysis capabilities of the original method. This is achieved by regarding MM operations on 3D sphere structural elements as their equivalent operations, and redefining the operations to general surfaces. The generalized MM method enables us to handle complex 3D shapes such as overturned and/or recumbent geological boundaries as well as incomplete shapes due to weathering processes and data unavailability. The proposed method was applied to analyze the shape of an intrusive contact in the Fenghuangshan Cu ore field, Eastern China, whose shape was in the form of a non-closed surface. This analysis revealed a stronger spatial association between the large concave parts of the contact zone and the mineralization. Due to its enhanced adaptability to different shapes, the generalized MM method, compared with the original MM method, allows us to capture shape features that are more plausible for the geological setting.
... The lens-like structure hosting the Zun-Kholba deposit probably was formed as a large transpressional zone. Transpressional shear zones often control the location of orogenic gold deposits [61,62]. The rocks of the Tuva-Mongol microcontinent basement and cover, as well as blocks of ophiolites and fragments of granitoids, were sheared together along this lens-like structural zone. ...
... In contrast to the 2D MPM, one of the most important data sources for 3D MPM is the 3D geological models. The models of the geological boundaries potentially indicate the areas with high permeability (Snow, 1969;Lisle, 1994;Liu et al., 2012), the locations of fluids conduits (Kyne et al., 2019;Wilson et al., 2016), and the transition zone of physical and chemical properties (Ord et al., 2002;Hu et al., 2020;Cao et al., 2020), where are typically in favor of metal deposition. The 3D geological models have been thought to be the key data source used for deriving predictor variables in 3D MPM Nielsen et al., 2015;Xiao et al., 2015). ...
The three-dimensional (3D) geological models are the typical and key data source in the 3D mineral prospecitivity modeling. Identifying prospectivity-informative predictor variables from the 3D geological models is a challenging and tedious task. Motivated by the ability of convolutional neural networks (CNNs) to learn the intrinsic features, in this paper, we present a novel method that leverages CNNs to learn 3D mineral prospectivity from the 3D geological models. By exploiting the learning ability of CNNs, the presented method allows for disentangling complex correlation to the mineralization and thus opens a door to circumvent the tedious work for designing the predictor variables. Specifically, to explore the unstructured 3D geological models with the CNNs whose input should be structured, we develop a 2D CNN framework in which the geometry of geological boundary is compiled and reorganized into multi-channel images and fed into the CNN. This ensures an effective and efficient training of CNNs while allowing the prospective model to approximate the ore-forming process. The presented method is applied to a typical structure-controlled hydrothermal deposit, the Dayingezhuang gold deposit, eastern China, in which the presented method was compared with the prospectivity modeling methods using hand-designed predictor variables. The results demonstrate the presented method capacitates a performance boost of the 3D prospectivity modeling and empowers us to decrease work-load and prospecting risk in prediction of deep-seated orebodies.
... 8. Histogram of the residuals (a) and plot of the probability density estimated via KDE (b) for the Bayesian model. Weatherley and Henley, 2013), and local transient changes in stress state during deformation (Deng et al., 2011;Henry et al., 2014;Wilson et al., 2016). During the Early Cretaceous, the Zhaoping detachment fault developed normal sinistral strike-slip movements with local compression or tensile torsions induced by an overall NW-SE extension setting (Lin et al., 2000;Charles et al., 2011;Deng et al., 2015). ...
The inference of deep alterations plays an important role in the analysis of deep geological processes and the targeting of buried mineral resources. However, the construction of three-dimensional (3D) models of alteration zones based on geochemical data is challenging due to the diverse geological processes and scarcity of information related to deep alterations. In this paper, we propose an interpretable machine-learning method to infer 3D geological models of alteration zones based on geochemical data from the Dayingezhuang area, China. To expose the information hidden in geochemical data that is indirectly and weakly related to alteration zones, a generalized additive model (GAM) is used. It learns a nonlinear, intelligible model associating geochemical anomalies (extracted by PCA) with alteration features. The model learned via GAM and is then combined with a Bayesian framework to infer the thickness of alterations in a no-drilling area, thus enabling 3D modelling of deep alteration zones. With such an intelligible model, geologists are capable of inspecting the reliability of the model and better understanding the association between the distribution of geochemical elements and deep alterations. The inference results show that the relationship between geochemical anomalies and alteration depth is significant , while the relationship with alteration thickness is complex. The results of this study provide the underlying insights needed to guide the analysis of deep geological mineralization processes.
... As a significant gold-producing district in the Jiangnan Orogenic Belt, the Changsha-Pingjiang (Chang-Ping) Fault Zone in the central part of the Orogenic Belt, hosts Liling, Huangjindong and Wangu goldfields with totally Au reserves of~215 t (Fig. 1b;Xu et al., 2017). In common with quartz vein-type gold deposit in the world (Deng et al., 2003;Yang et al., 2014Yang et al., , 2016aGoldfarb and Groves, 2015;Wilson et al., 2016), gold deposits along the Chang-Ping Fault are dominated by structures, generating ore-bearing quartz vein, and mineralized host rocks during the fluid-rock reacting Sun et al., 2018Sun et al., , 2019. The ore grade varies from 1 g/t to hundreds of grams per ton (Liling Zhengchong Mining Co. Ltd, pers. ...
The Zhengchong gold deposit, with a gold reserve of 19 t, is located in the Liling Goldfield, central Jiangnan Orogen, Southern China. The orebodies are structurally bound by NW- and NNE-NE-trending faults, hosted in the Neoproterozoic slates. The ore styles primarily comprise auriferous pyrite-arsenopyrite-quartz vein, disseminated pyrite-arsenopyrite-sericite-quartz mineralized slate, and intense mineralized slate breccia. The gold-bearing sulfide-quartz veins have average Au grades up to 13.5 g/t, generally higher than that of mineralized rock in all orebodies of 0.6 - 2.0 g/t and intense mineralized breccia within quartz veins of 2.5 g/t - 4.0 g/t. Three primary and successive paragenetic sequences are identified: (i) Barren quartz - muscovite; (ii) Quartz - polymetallic sulfides - native gold - muscovite - minor chlorite; (iii) Barren quartz - calcite veins. Native gold has only been observed in the fractures of early barren quartz, pyrite and arsenopyrite in NW-trending auriferous veins. According to textural differences, five types of pyrite and four types of arsenopyrite are recognized from NW- and NNE-NE-trending orebodies.
Based on LA-ICPMS spot and imaging analyses of different types of pyrites and arsenopyrites, the refractory, invisible gold in pyrites and arsenopyrites range from <1 to >500 ppm and was incorporated in these sulfides through substitution of As¹⁻ for S²⁻ as the Au and As data plotting below the both gold solubility lines for Carlin and orogenic gold deposits. The invisible gold as the form of Au⁺ concentrates in sulfides. Besides, minor micro-scale gold with Cu-, Pb-, Sb-, Ag-, and Bi-minerals occur in the pores of pyrite and arsenopyrite. The initial ore-bearing fluid with abundant Si, S, Fe, As and minor Au, Co, Ni, Cu, Zn, Sb, Te, Pb, Bi reacting with Neoproterozoic slates to generate low-grade pyrite (Py-I and Py-III) in both-trending orebodies. For NNE-trending sulfide-quartz veins, cyclic fluid pressure dropping resulted fluid phase separation and destabilization of bisulphide-gold. Due to the small fluid flux in narrow NNE-trending faults, Au⁺ exhausted after the precipitation of high-grade Py-II, Apy-II and Au-rich rim of Py-I with invisible gold. In NW-trending auriferous quartz veins, hydrofracturing under high fluid pressure in thrust shear-faults, generated massive slate breccias. The sharply pressure decrease in dilations along the ore-controlling faults resulted in significant auriferous fluid phase separation and the solubility reducing of trace elements and gold, which in turn led in the deposition of native gold with chalcopyrite, sphalerite and tetrahedrite in fractures of Py-IV and Apy-III.
... The Benambran deformation is dominant in the Bendigo Zone and was accompanied by east-west oriented shortening, producing north-northwest-trending upright folds and steepdipping (> 60°) limb thrust faults which show evidence of multiple slip events (Willman, 2007;Wilson and Leader, 2014). These structures are intimately associated with auriferous quartz veins and can be observed in a number of orogenic gold deposits (Wilson et al., , 2016 , both the Avoca and Heathcote fault zones have been interpreted to reach potential gold source regions at mid crustal levels. It is thought that fluids passing through or released from the potential gold source regions were repeatedly channelized and ascended along these deep-seated faults. ...
At the Fosterville gold deposit, high concentrations of visible and refractory gold are hosted in arrays of quartz and quartz-carbonate veins. They are distributed in elongate zones to form south plunging ore shoots within a folded and fractured turbidite sequence. Here we distinguish the influence of faults versus folds at Fosterville, by assessing the deposit-scale structural framework and hydrothermal fluid flow using a combination of 3D implicit modelling, structural analysis and 3D fault slip/dilation tendency analyses. Gold assay and bedding trend data, from a large drillhole dataset, were spatially interpolated and used to analyse and measure the orientation of ore bodies and folded host rocks. Field measurements of fold axis orientations were compared to bedding measurements in oriented drillcore, Bingham axial analysis of bedding trends, and orientation data from a 3D implicit model. Both field and drill core structural data show that ore shoots at Fosterville do not parallel the first order, gently (≤10°) south-plunging folds. Instead the orientation of the ore shoots is coincident with an intersection lineation between steeply west-dipping bedding-parallel faults and moderately dipping linkage faults that obliquely cut across the main structural grain, resulting in ore shoot plunges larger than 20°. Slip and dilation tendencies suggest that linkage faults had a higher likelihood of activity in the inferred far-field stress regimes at the time of mineralisation, relative to other structures at Fosterville. These linkage faults are therefore interpreted to have been preferential fluid flux conduits in the system despite appearing to be subordinate structures at the scale of the deposit. This new structural model for Fosterville highlights that fluid flow and ore shoot formation are strongly controlled by obliquely oriented structures that are unlike the main structural grain of first order folds and faults.
... Furthermore, if the D4 structures predate the stratiform mineralization, the host strata should have been already folded and dipping to the SSW before the D2 stage ESE-directed compression and such a sequence of deformation should Table 1. have resulted in a sub-horizontal ore-bearing vein system crosscutting the bedding or S1 foliation (e.g., Wilson et al., 2016), which is apparently not the case. The timing of this event is uncertain. ...
... The sites of shear strain and related dilation, which tend to localize fluid flux, have been considered to be favourable places for mineral precipitation in hydrothermal systems (Oliver et al. 2006;Ford et al. 2009;Zhang et al. 2013;Wilson et al. 2016). The dilation zones in our modeling correlate well with the known orebodies of the Gryphon and Phoenix deposits. ...
... The sites of shear strain and related dilation, which tend to localize fluid flux, have been considered to be favourable places for mineral precipitation in hydrothermal systems (Oliver et al. 2006;Ford et al. 2009;Zhang et al. 2013;Wilson et al. 2016). The dilation zones in our modeling correlate well with the known orebodies of the Gryphon and Phoenix deposits. ...
Previous studies on unconformity-related uranium deposits in the Athabasca Basin (Canada) suggest that egress flow and ingress flow can develop along single fault systems at different stages of compressional deformation. This research aims to examine whether or not both ingress and egress flow can develop at the same time within an area under a common compressional stress field, as suggested by the reverse displacement of the unconformity surface by the basement faults. The study considers the Phoenix and Gryphon uranium deposits in the Wheeler River area in the southeastern part of the Athabasca Basin. Two-dimensional numerical modeling of fluid flow, coupled with compressional deformation and thermal effects, was carried out to examine the fluid flow pattern. The results show that local variations in the basement geology under a common compressional stress field can result in both egress and ingress flow at the same time. The fault zone at Phoenix underwent a relatively low degree of deformation, as reflected by minor reverse displacement of the unconformity, and egress flow developed, whereas the fault zone at Gryphon experienced a relatively high degree of deformation, as demonstrated by significant reverse displacement of the unconformity, and ingress flow was dominant. The correlation between strain development and location of uranium mineralization, as exemplified by Gryphon and Phoenix uranium deposits, suggests that the localization of dilation predicted by numerical modeling may represent favourable sites for uranium mineralization in the Athabasca Basin.
... Gold mineralisation at Ballarat is hosted by metamorphosed Lower Ordovician turbiditic sandstones and shales of the Castlemaine Group and occurs preferentially in shaledominated sequences (Taylor, Whitehead, Olshina, & Leonard, 1996). The stratigraphy is folded into a series of inclined folds with steeply west-dipping axial surfaces and cut by a series of west-dipping reverse faults known locally as 'leather jackets' (Boucher, Osborne, & D'Auvergne, 2008;Wilson, Osborne, Robinson, & Miller, 2016). Quartz veins and associated tension vein arrays are typically associated with these reverse faults and host coarse free gold. ...
HyLogger hyperspectral data obtained from seven orogenic gold deposits in central Victoria, including Bendigo, Ballarat, Maldon, Fosterville, Costerfield, Castlemaine and Wildwood, are presented. The data demonstrate that fresh diamond drill core displays substantial mineralogical variation that can be attributed to the effects of cryptic hydrothermal alteration that might not otherwise be recognised. The most significant hyperspectral response lies in the white mica compositions, which vary in a systematic manner between high-Al muscovitic zones (Al?OH absorption around 2208 nm) that define a phyllic alteration halo around mineralised structures, and low-Al phengitic?chlorite zones (Al?OH absorption >2014 nm) inferred to represent either more distal alteration or possibly regional metamorphic background. An extensive ferroan dolomite alteration halo overlaps the phyllic and sulfidic alteration zones and extends beyond the sampled core in most instances. This ferroan dolomite halo has previously been defined petrographically, geochemically and using carbonate staining techniques, and is further characterised using thermal infrared hyperspectral data in drill core from the Ballarat goldfield. The mineralogical trends identified by the hyperspectral data are best developed in diamond drill core from the Costerfield, Fosterville and Ballarat goldfields, and are less pronounced at the other deposits. At Bendigo and Castlemaine the reasons for this are not immediately clear, but may be related to the close timing of gold mineralisation relative to peak metamorphism. The Maldon area lies within the contact aureole of the Harcourt Batholith and so has been thermally overprinted leading to the recrystallisation of earlier hydrothermal assemblages. The Wildwood deposit is similar to the Magdala deposit at Stawell and differs from the other goldfields in its geological setting, host rock lithologies and style of hydrothermal alteration, with the development of Fe-rich chlorite closely associated with gold mineralisation. The results demonstrate how hyperspectral data can be used to define large hydrothermal alteration footprints associated with orogenic gold mineralisation in central Victoria that are of direct benefit to mineral explorers, as well as independently characterising lithological variations in drill core.
Three-dimensional (3D) geological models are typical data sources in 3D mineral prospectivity modeling. However, identifying prospectivity-informative predictor variables from 3D geological models is a challenging and work-intensive task. Motivated by the ability of convolutional neural networks (CNNs) to learn intrinsic features, in this paper, we present a novel method that leverages CNNs to learn 3D mineral prospectivity from 3D geological models. By exploiting this learning ability, the proposed method simplifies the complex correlations of mineralization and circumvent the need for designing the predictor variables. Specifically, to analyze unstructured 3D geological models using CNNs—whose inputs should be structured—we develop a 2D CNN framework where the geometry of geological boundary is compiled and reorganized into multi-channel images and fed into the CNN. This ensures the effective and efficient training of the CNN while facilitating the representation of mineralization control. The presented method is applied to a typical structure-controlled hydrothermal deposit, the Dayingezhuang gold deposit in eastern China; the presented method is compared with prospectivity modeling methods using designed predictor variables. The results show that the presented method has a performance boost in terms of the 3D prospectivity modeling and decreases the workload and prospecting risk in the prediction of deep-seated orebodies.
This report provides the numerical modeling results of Thermal-Hydraulic-Mechanical-Chemical (THMC) processes related to the unconformity-related uranium mineralization in the Athabasca Basin. It is supported by NSERC-CRD and Orano Canada Inc.
New ⁴⁰Ar/³⁹Ar geochronological data suggest orogenic gold mineralisation at the Ballarat East deposit, southeast Australia, occurred in three main episodes at ca. 445-435Ma, ca. 420-415Ma and ca. 380-370Ma. The gold mineralisation is localised in muscovite-bearing quartz and quartz-carbonate veins hosted in the steep faults (70-90°), on limbs of tight and isoclinal folds in an Ordovician turbidite sequence, and within west-dipping (≤45°) faults, historically known as leather jacket lodes. Initiation of the ≤45° faults that are confined to fold culminations, begins at ca. 445Ma, with peak metamorphic conditions at 440Ma. The earliest vein sets (V1), were emplaced on limb thrusts at ca. 445-435Ma and are characterised by arsenopyrite-dominated quartz veins. These V1 veins parallel arsenopyrite-rich shale units, historically referred to as 'indicator beds'. Both the steep and ≤45° faults were reactivated during fold amplification with deposition of the V2 auriferous veins at ca. 420-415Ma. A later set of auriferous veins (V3-V4) with ages of 380-370Ma, dominated by pyrite-sphalerite-galena-white-mica quartz-(V3) or carbonate-rich (V4) veins are predominantly associated with reactivation of the ≤45° west-dipping faults. This new geochronological data constrains the local kinematic history of the Ballarat East deposit and has regional implications. The V1-V2 vein development appears to be synchronous across the entire western section of the Lachlan Orogen, where previous studies have suggested that initial gold mineralisation was linked to orogenesis at ∼440Ma, asa result of metamorphic devolatilisation reactions in the lower crust. In contrast, a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the V3-V4 veins. The deformation that accompanies V3-V4 vein development is attributed to small, localised events during east-west shortening, utilising pre-existing fold and fault structures. The origin of the fluids producing the V3-V4 veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central Victoria.
Gold mineralisation in the Lachlan Orogen of western Victoria, is generally hosted in turbidites with very low-grade metamorphic assemblages. Metamorphic data from these turbidites are relatively rare because of the fine-grained nature of the pelitic component and lack of suitable assemblages for thermobarometric estimates. In this study, ‘illite crystallinity’ (Kübler Index) and b-lattice spacing measurements were carried out on white micas in metapelites, collected from near the inferred western margin of the Selwyn Block, as well as three exploration targets, in an attempt to relate thermal and barometric conditions to mineralisation. Higher-grade (epizone) metamorphic conditions are recorded in sequences west of the Whitelaw Fault and lower-grade (anchizone) metamorphic conditions to the east of the fault. The change from epizonal to anchizonal grade is abrupt, resolved to a distance of a few hundred metres. The b-spacing values change adjacent to the Muckleford Fault. This is due to rocks to the east being exhumed as the edge of the Selwyn Block moved westward during the Middle Devonian Tabberabberan Orogeny at ∼380 Ma. We propose that the juxtaposition of rocks with contrasting thermal and barometric histories represents expression of the upper crustal location of the western margin of the Selwyn Block at the time of peak deformation, at about 440 Ma, and this crustal structure controlled the distribution of the major quartz-vein-type gold deposits. The Middle Devonian orogenic activity (∼380 Ma) was accompanied by the formation of disseminated gold deposits such as Fosterville. This represents a mineralising event that overprints the earlier gold deposits in a corridor at least 50 km wide and to the west of the Whitelaw Fault, that parallels the margin of the Selwyn Block. The correlation between gold assays and ‘illite crystallinity’ results, from X-ray diffraction and from short-wave infrared-reflectance field-spectroscopy data, were ambiguous. Kübler Indices are not found to be effective in targeting of mineralisation as the values obtained from the alteration and the host-rock assemblages were similar and reflected the ambient P–T conditions at the time of mineralisation.
A metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many goldfields such as those found in Archean greenstone belts, slate-belts and other gold-only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist–amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H2O and CO2 but also S and, in turn, Au. Elevated gold in solution is achieved by complexing with reduced S, and by H2CO3 weak acid buffering near the optimal fluid pH for gold solubility (the buffering is more important than being at the point of maximum gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre-scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold. Deposition of gold from solution necessitates breakdown of the gold–thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon-bearing host rocks and/or by sulphidation of wallrocks to generate iron-bearing sulphide and precipitated gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite-bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major gold deposits with rock type, even when the gold is primarily in veins, argues for rock-dominated depositional systems, not fluid-dominated ones. As a consequence, a general role in gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at gold-only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher-grade metamorphism and deformation, and/or (palaeo)-weathering may provide many of the most-obvious features of goldfields including their mineralogy, geochemistry, geometry, small-scale timing features, geophysical response and even mesoscopic gold distribution.
Download full paper as PDF for free until April 24, 2015:
http://authors.elsevier.com/a/1QeYTcTGxnimv
We introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3D implicit modelling in data-rich environments with field observations. We apply this approach to the early Palaeozoic Navachab gold deposit which is located in the Damara orogenic belt, Namibia. Compared to traditional modelling methods, 3D implicit modelling reduces user-based modelling bias by generating open or closed surfaces from geochemical, lithological or structural data without manual digitisation and linkage of sections or level plans. Instead, a mathematically defined spatial interpolation is used to generate 3D models that show trends and patterns that are embedded in large drillhole datasets. In our 3D implicit model of the Navachab gold deposit, distinctive high-grade mineralisation trends were identified and directly related to structures observed in the field. The 3D implicit model and field data suggest that auriferous semi-massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome, where shear strain reached peak values during fold amplification. This setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit. Both bedding-parallel and highly discordant sets of auriferous quartz-sulphide veins are interpreted to have formed during the later lock-up stage of the regional scale dome. Additionally, pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set. We propose that kilometre-scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures (traps and pathways for fluids) at different preferred sites and orientations. We also propose that areas that experience high shear strain, located along the inflection lines of folds can act as preferred sites for syn-deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts. Our research also suggests that examination of existing drillhole datasets using 3D implicit modelling is a powerful tool for spatial analysis of mineralisation patterns. When combined with fieldwork, this approach has the potential to improve structural understanding of a variety of ore deposits.
We use numerical simulations to identify sites of dilation and areas of high, shear strain and fluid flow that may be related to gold deposition in faults that transect the western sub-province of the Lachlan Orogen. Our results can explain how a late tectonic history consisting of a switch from east-west compression to north-south transpression contributes to the formation of gold deposits in association with late fault movements. The models simulate incremental east-west shortenings of 4% and a superimposed 1% north-south shortening on major crustal scale (intrazonal) faults and suggest that strain and fluid flow was greatest in the shallow-dipping fault segments (first order faults) that lie within the mafic rocks of the lower crust. The areas above the shallow-dipping segments of intrazonal faults (second order) become sites for the initiation of later (third order) faults and fracture networks, within the higher-level metasedimentary rocks, strain decreases in the steeper segments of the intrazonal faults. These second order faults are inferred to act as highly permeable channel-ways for fluid discharge and it is their geometry and a shift from east-west reverse-dip-slip to north-south reverse-oblique-slip fault kinematics that controls the final distribution of gold mineralization in the folded metasedimentary rocks. Changing the direction of principle compression, has a dramatic effect on the extent and location of volumetric strain (dilation) and fluid flow with localized deformation within bedding-parallel veins dispersing fluid flow. Increased dilation facilitates the influx of gold-bearing fluids that in combination with a fault-fracture network, and geochemical factors, have led to significant amounts of localized gold mineralization.
Many algorithms assume plane strain to construct, model and restore fault-cored folds. Using mechanical models that allow heterogeneous transport in three dimensions, we explore the distribution and magnitude of out-of-plane transport in plunging fault-cored anticlines and provide guidelines of where plane strain should and should not be applied. We developed a new technique of incrementing infinitesimal elastic strains to produce folds with aspect ratios similar to natural folds. Map views of displacement vectors show that in general, out-of-plane displacement is localized near the lateral fold tips. Cross-sections show that out-of-plane transport is depth dependent with out-of-plane displacement increasing toward the surface. Flexural slip surfaces compartmentalize out-of-plane transport within distinct mechanical units, with the maximum out-of-plane displacement near the tops of mechanical units. Two-dimensional models with additional frictionally slipping bed contacts suggest that freely slipping contacts can approximate the deformation of many frictionally slipping contacts. We show that out-of-plane transport is significant in the simplest non-cylindrical folds, and suggest that complex non-cylindrical structures should not be modeled using plane strain exclusively. We also show that flexural slip surfaces exert a significant control on the magnitude and structural position of out-of-plane transport in our models.
In the internationally significant Victorian goldfields a complex system of faults dismembers the 5 million ounce Magdala gold deposit. These faults represent a combination of neoformed faults and inherited faults that reflect deformation associated with stress tensors of variable orientation and stress shape ratio (φ). The fault geometry is strongly controlled by the pre-existing rheology. Faults have propagated around the flanks of an antiformal basalt dome, along earlier ductile cleavages and the margins of porphyry dykes. Many of the faults do not have Andersonian geometries and there is no correlation between the orientation of the faults and the palaeostress directions. Much of the faulting is associated with the emplacement of porphyry dykes, additional gold mineralisation related to plutonism and late-stage deformation post-dating the intrusion of the Stawell pluton. Systematic mapping of extension veins associated with faults, striations and conjugate joint sets allowed the construction of a revised and more robust history of brittle deformation. This successfully predicted the offset direction of the currently mined Magdala ore body beneath the studied system of faults. The use of extension veins was a critical aspect of the analysis. If striations on the fault surfaces had solely been used, the offset direction of the new Golden Gift orebody would not have been correctly ascertained. The palaeostress history was delineated via use of compression and tension dihedra, stress inversion of slip data and calculation of theoretical resolved shear stress for faults with orientations similar to those mapped. The calculation of theoretical resolved shear stress directions highlights the importance that the intermediate stress has on the slip direction for faults whose pole does not lie in the plane containing σ1 and σ3.
Multilayered rock complexes with regular alternations of competent and incompetent layers of thickness t1 and t2, respectively, and with high ductility contrast form folds of the chevron style when subjected to compression along the layering. The geometric forms of progressively developing chevron folds are analyzed using a model whose properties are based on the geometric forms of naturally deformed rock layers. It is found that the chevron fold style is only stable where no strongly marked variations in competent layer thickness t1 exists. The thickness of the incompetent layer exerts no influence on fold model stability. Slight variations of competent layer thickness can be accommodated by local modifications of the fold style, such as limb faults, bulbous hinge zones, or layer boudinage, but if any strongly marked variation exists, the fold limbs become curved. The chevron model involves dilation at the hinge zones, and saddle reef formation, incompetent layer flow into the hinge, or slow hinge collapse generally results from actual or potential dilation. The speed of development of chevron folds is calculated under conditions of constant stress and of constant load. Folding starts slowly but rapidly accelerates; the later stages are characterized by a progressive slowing in shortening rate and fold growth, leading either to a stage of locking up of the fold or to modification of its geometry by limb thinning and hinge thickening; toward a more similar geometric style. The strains in the hinge zone region are related to the rates of shear taking place in the fold limbs; the geometric model is not completely stable throughout the fold development, and complex progressive strain increments can occur in the hinge zones and lead to the development of superposed small-scale structures indicating reversals of principal axes of incremental strain.
Variable distribution of elevated gold grades in the Magdala Central Lode system is controlled by preferential and localised reactivation of pre-existing faults in a progressively rotating stress field. Populations of slickenline lineations on fault surfaces in combination with extension vein arrays associated with the faults indicate Central Lode formed initially during southwest – northeast compression, but was subsequently locally reactivated during east – west compression on lower angle fault segments to produce a series of ore-shoots within the overall lode system. A series of 2D cross-section numerical models have been used to examine how deformation and fluid flow are portioned at different stages of the lode evolution. Models have been constructed in the plane estimated to contain maximum and minimum principal stresses during the events leading up to, and during, mineralisation as indicated by the lineation and extension vein data. Results of the models show that during southwest – northeast compression, a major shear zone forms along the boundary between the Stawell Facies and the overlying Albion Formation consistent with the extent and location of the Central Lode Shear Zone. In a second series of models it is shown that during east – west compression, shear failure is considerably more localised to regions overlying lower angle lithological contacts. This localisation of shear failure in these east – west compression models shows strong correlation with the distribution of gold in grade shell models, confirming the structural and lithological factors identified as controlling the mineralisation.
Several gold deposits occurring in the western Lachlan orogen have geological, geochemical, and geochronological characteristics that distinguish them from typical vein-hosted orogenic gold deposits of the central Victorian gold province. The later are responsible for more than 90 percent, of primary (hard-rock) gold production from this region and are generally considered to represent the only economically significant type of gold deposit in the western Lach an orogen. Atypical gold occurrences at Malmsbury, Myrtle Creek, Mount Piper, and the Wonga deposit in the Stawell goldfield are characterized by a close spatial and temporal association with posttectonic felsic intrusions, disseminated to stockwork-style mineralization, alteration dominated by sericitization, sulfidation, silicification, carbonatization and tourmalinization, and associated complex An ± Mo-W-Bi-Te-Cu. The deposits have a number of features in common with intrusion-related gold deposits elsewhere in Phanerozoic orogenic belts. Although production from this type of gold mineralization in the western Lachlan orogen has been small compared to orogenic gold deposits, the possible existence of intrusion-related gold deposits has potentially important implications for exploration in this region and also provides significant clues to the tectonic framework and Paleozoic metallogeny of eastern Australia.
Age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central Victoria, Australia occurred over a prolonged period of time ranging from prior to the Late Silurian to the Late Devonian. An early phase of gold mineralisation associated with regional deformation pre-dated the emplacement of Late Silurian felsic porphyry dykes at 413±3 Ma in the central lode system of the Magdala deposit, Stawell. Gold mineralisation associated with the final stages of the main regional deformation also pre-dated the intrusion of Early (ca. 400 Ma) to Late Devonian (ca. 370 Ma) plutons at Stawell (Wonga deposit), Mt. Bute, Tarnagulla, Tallangalook, Maldon and Heathcote in both the Stawell, Pyrenees and Ballarat metallogenic provinces, consistent with preliminary dating studies of hydrothermal micas from major gold deposits.
We present coupled 3D deformation – fluid-flow models which place constraints on the importance of basalt dome shape and interpreted synmineralising shortening direction in localising gold mineralisation around basalt domes in the Stawell corridor, western Victoria. Gold mineralisation in the Magdala orebody at the Stawell mine occurs predominantly within a thin metasomatised unit named the Stawell Facies which blankets the basalt domes and also occurs close to parasitic fold-like basalt lobes on the basalt domes. In dome-scale models that do not contain basalt lobes, areas with the maximum fluid-flow rates occur on the tops of the flanks of the domes where there is a dramatic change in dip of the basalt, and a change from contraction to dilation which creates a significant pore-pressure gradient. In models that contain basalt lobes, the location of high fluid-flow rates is strongly controlled by the presence of these lobes. High fluid-pressure gradients are created between the contracting Stawell Facies in the area between the lobe and the main domes and those areas dilating above. Areas of significant dilation occur on the shallow-dipping portion at the top of the dome and cause fluid to flow towards them. Areas that have significant dilation are also areas of tensile failure in some cases and are coincident with areas of known quartz vein-associated mineralisation. In the Magdala Dome models, only the east-northeast – west-southwest- and east – west-shortened models record high fluid-flow rates in areas of known mineralisation, which is consistent with the interpreted synmineralisation-shortening directions. Therefore in this situation, fluid-flow rates during east-northeast – west-southwest- and east – west-shortening can be used to indicate the potential location of gold mineralisation. In numerical models of the Kewell Dome (a prospect to the north), the position of areas of high fluid-flow rate when shortened in the east-northeast – west-southwest and east – west direction, combined with information from limited drilling, indicated the potential for gold mineralisation at the southwest end of the dome. Diamond drillholes in this area yielded significant gold values.
We present coupled mechanical, fluid-flow and chemical numerical models that place constraints on some of the processes that formed gold deposits in the Bendigo–Ballarat Zone of Victoria, Australia, and specifically the Bendigo goldfield. Although many goldfields in this region are located close to major intrazone faults, these major faults are rarely mineralised, and such goldfields are commonly located in the hangingwall of the faults, 5–10 km from the fault plane. Individual gold deposits are hosted within quartz veins associated with reverse faults and chevron folds. Regional-scale models are aimed at determining if the intrazone faults were more or less permeable than the surrounding host rocks, if these faults acted as conduits to supply fluids to goldfields, if the presence of a blind fault below the Bendigo goldfield was necessary to deliver fluids to the area, and if the upper units of the Castlemaine Supergroup (Darriwilian and Yapeenian) acted as a low permeability cap to facilitate gold and quartz precipitation in the lower units (Castlemainian to Lancefieldian). The models indicate that the combination of permeable intrazone faults, and relatively low permeability units at the top of the Castlemaine Supergroup, allows the greatest fluid flux to occur in the area of the Bendigo goldfield. The presence of a blind fault beneath the Bendigo goldfield is not critical to the supply of fluids to this area. Small-scale models of a chevron fold with a permeable fault simulate fluid pumping and suggest that fluids transported along the fault mixed with those in the host rocks. Fluids are expelled from, and drawn back into, the fault in a cyclical manner and are controlled by deformation-induced changes in volume. Intermittent high fluid pressures in the host rocks relative to the fault cause the rocks to yield in tension, simulating hydrofracturing and the formation of quartz veins associated with gold-bearing reverse faults. Models coupling deformation, fluid flow and chemical reactions demonstrate that gold is precipitated within the fault region when CH4 and H2S are sourced at depth and CO2 is transported along the fault. In the models presented, gold precipitation is strongly controlled by H2S, although the effects of fluids interacting with graphite have not been modelled. Rates of gold mineralisation of 0.09–0.023 g/t over 1 million years suggest that the permeability of the fault was at least two orders of magnitude higher than the permeability modelled here.
40Ar/39Ar data for muscovite separates and hydrothermally altered whole-rock samples from the Ballarat West and the Ballarat East goldfields indicate that mesothermal gold mineralisation at Ballarat occurred during several episodic pulses, ranging in age from the Late Ordovician to the Early Devonian. Initial formation of auriferous structures in the Ballarat goldfields coincided with folding and thrusting associated with the development of the western Lachlan Fold Belt between 460 and 440 Ma. Subsequent fault reactivation and magmatism resulted in remobilisation and additional mineralisation between 410 and 380 Ma, and around 370 Ma. The results presented herein are in agreement with findings for other major gold deposits in central Victoria and further constrain the history of deformation, metamorphism and mineralisation in the western subprovince of the Lachlan Fold Belt.
Gold-bearing quartz vein systems in metamorphic terranes are one of the most important types of lode gold resource. Major vein-type Au mineralisation of this style in central Victoria is restricted to narrow, structurally-controlled domains in a low grade metamorphosed quartz-rich turbidite sequence. Vein systems in these domains have developed in fault-related and fold-related dilatant fractures which were generated at supralithostatic fluid pressures during regional deformation and metamorphism.The timing of mineralisation, the nature of associated hydrothermal alteration, and the isotopic and chemical compositions of the fluids point to ore genesis involving large volumes of COH metamorphic fluids whose flow has been channelised along high-permeability fault zones. The development of auriferous vein systems has been controlled by the coincident development of structural and geochemical traps. The major structural traps are dilatant jogs on reverse faults, extension fracture arrays adjacent to faults, and saddle reefs and related structures in fold hinges. Gold precipitation is ascribed largely to structurally controlled mixing of secondary CH4-bearing fluids with more oxidised primary gold-bearing fluids traversing the dilational fracture systems. The CH4-bearing fluids have been produced by interaction of the primary fluid with graphitic slates adjacent to fault zones. Cyclic fluctuations in fluid pressure and shear stress accompanying episodes of fault motion are shown to control repeated episodes of fracture opening, fluid mixing, and mineralisation.
a b s t r a c t Despite how common folds are as structural features, along-strike fold propagation has proven elusive to document. However, if a fold grows laterally along its axis, early-formed fold-related joints may differ significantly in orientation from joints that form later. In this paper, we integrate mechanical stratigraphy with joint pattern analysis to determine relative timing of jointing. Additionally, we demonstrate that joint patterns can be related to stresses on both the top and bottom of the bed during flexure. We present joint data from eight sedimentary beds on the fold terminus at Sheep Mountain Anticline, Wyoming, USA. The joint patterns around the terminus show two distinct patterns: joints in six of the beds show a radial pattern around the terminus whereas joint patterns in the two remaining beds differ from proximal units, despite being in the same structural position. Fracture resistance calculations confirm that the beds with mis-oriented fractures are less resistant to fracturing than other units in the study, and therefore would have fractured earlier in fold growth history. We present a plate bending model that illustrates potential joint patterns around a plunging fold nose from stresses along both the top and bottom of the bed. The joint strike predictions for the area in front of the inflection line on the fold nose match the orientations in our less resistant beds, which are now positioned behind the inflection line, suggesting that the fold grew laterally. The combined analysis of fracture pattern and mechanical stratigraphy provides a new way to investigate fold evolution.
The Bendigo and Castlemaine goldfields are classic examples of structurally controlled orogenic gold deposits in the Bendigo
Zone of central Victoria, SE Australia. Detailed mapping and biostratigraphic interpretation has led to a better understanding
of the regional structural controls of this type of gold-quartz mineralisation. Mineralised quartz veins are hosted by the
Castlemaine Group, an Early-to-Middle Ordovician turbidite succession at least 3,000m thick. Gold deposits are controlled
by low-displacement faults that are clustered into several belts (the goldfields) indicating a regional structural control.
The timing of mineralisation overlapped with that of the major period of deformation including folding, cleavage development
and regional faulting. The Bendigo and Castlemaine goldfields are located in an area termed the Whitelaw thrust sheet bounded
by two unmineralised, high-displacement, regional-scale faults. Mapping has revealed an interrelationship between the regional-scale
faults, regional structural style and goldfield location. The goldfields lie immediately west of the boundary between the
upper and lower portions of the thrust sheet and are characterised by symmetric folds with sub-horizontal to synclinal enveloping
surfaces, relatively low co-axial strains and moderate cleavage development. The non-gold-bearing areas immediately east of
each goldfield correspond with the lower part of the Whitelaw thrust sheet and are characterised by higher non-coaxial strains,
stronger cleavage and folds with wide west-dipping limbs giving rise to easterly vergent sections and steeply west-dipping
enveloping surfaces. That mineralisation was an integral part of the thin-skinned style of deformation in the central Bendigo
Zone is indicated by timing relationships and the interrelationship between local-scale mineralised structures and regional-scale
features such as large-displacement unmineralised faults, regional variations in fold style and overall thrust sheet geometry.
The work supports previous models that suggest mineralised fluids were focussed along a linked system of deep-seated faults.
The primary conduits may have been major regional-scale ‘intrazone’ faults, which are inferred to sole into detachments near
the base of the Castlemaine Group. It is proposed that these structures linked with minor intrazone faults and then with networks
of low-displacement mineralised faults that were strongly controlled by folds. The location of minor intrazone faults was
probably controlled by internal thrust sheet geometry. The distribution of gold deposits and of gold production suggests that
maximum fluid flow was concentrated along the eastern margins of networks of low-displacement faults.
Flexural-slip folding has been established for two anticlines in the Meguma Group, Nova Scotia. Flexural slip postdates the main Acadian episode of flexural-flow fold growth and represents a late, brittle-ductile reactivation of the fold belt. Flexural-slip structures are dominated by bedding-parallel movement horizons, but include a linked system of frontal and lateral ramps and conjugate movement horizons. Slip amount calculated from displacement of discordant quartz veins, combined with vein spacing, indicates an average local shear strain of approximately 0.5. However, because flexural-slip occurred when the fold limbs were steep, the shear strain accounts for a small change in limb dip (4–8°). Variation of slip amount with spacing of movement horizons suggests progressive flexural slip was accommodated by continuous initiation of new movement horizons as well as continuing slip on existing movement horizons.The presence of quartz veins along flexural-slip structures together with evidence for ductile deformation coeval with flexural slip suggests that slip episodes were fluid assisted and intermittent, separated by periods of ductile deformation and build up of fluid pressure. Flexural-slip veins in the Ovens Anticline are auriferous and a flexural slip model may be appropriate for other similar Meguma gold deposits.Thrust sheets, up to 10 m thick, occurring on a fold limb are interpreted to have originated in flat box fold hinges and to have been placed onto the steep limbs of the chevron folds during the flexural slip episode. Thrusting may have assisted the transformation of a box fold into a chevron.