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... Some gold mineralization is hosted in veins developed during NNW-SSE-directed shortening coupled with preferential alteration of a primary gabbro by albite-arsenopyrite-associated gold mineralization that postdates peak metamorphic, calc-silicate assemblages. The most significant gold mineralization is, however, in vein quartz developed at intersections with NW-and N-trending structures that concentrated veins during northeast-southwest extension (Witt et al., 2001). ...
The structural geology and tectonic setting of hydrothermal gold deposits are paramount for understanding their genesis and for their exploration. Strong structural control on mineralization is one of the defining features of these deposits and arises because the permeabilities of crustal rocks are too low to allow the formation of hydrothermal deposits on realistic timescales unless rocks are deformed. Deformation zones and networks of deformation zones are the fundamental structures that control mineralization. Systematically analyzing deposit geometry, kinematics, and dynamics leads to the most thorough comprehension of a deposit. Geometric analysis relates orebody shape to controlling structures, and networks of deformation zones can be analyzed using topol-ogy to understand their connectivity and mineralizing potential. Kinematic analysis determines the location of permeability creation and mineralization. New views of shear zone kinematics allow for variable ratios of pure to simple shear, which change likely directions of mineralization. Multiple orientations of mineralized deformation zones may form simultaneously and symmetrically about the principal strain axes. Dynamic analysis is necessary for a mechanical understanding of deformation, fluid flow, and mineralization and can be achieved through numerical modeling. The relationship between deformation (kinematics) and stress (dynamics) constitutes the rheology; rheological contrasts are critical for the localization of many deposits. Numerous gold deposits, especially the largest, have evidence for multiple mineralizing events that may be separated by tens to hundreds of millions of years. In these cases, reactivation of structures is common, and a range of orientations of preexisting structures are predicted to be reactivated, given that they are weaker than intact rock. Physical and chemical processes of mineralization can be integrated using a nonequilibrium thermodynamics approach. Hydrothermal gold deposits form in contractional, strike-slip, and extensional tectonic settings. However, there may be great variation in the spatial scale over which the tectonic setting applies, and tectonic settings may also change on rapid timescales, so that it is inadvisable to infer local tectonics from deposit-scale patterns, and vice versa. It is essential to place mineralizing events within a complete geologic history in order to distinguish pre-and postmineralizing structures from synmineralization deformation features.
... Some gold mineralization is hosted in veins developed during NNW-SSE-directed shortening coupled with preferential alteration of a primary gabbro by albite-arsenopyrite-associated gold mineralization that postdates peak metamorphic, calc-silicate assemblages. The most significant gold mineralization is, however, in vein quartz developed at intersections with NW-and N-trending structures that concentrated veins during northeast-southwest extension (Witt et al., 2001). ...
... Some gold mineralization is hosted in veins developed during NNW-SSE-directed shortening coupled with preferential alteration of a primary gabbro by albite-arsenopyrite-associated gold mineralization that postdates peak metamorphic, calc-silicate assemblages. The most significant gold mineralization is, however, in vein quartz developed at intersections with NW-and N-trending structures that concentrated veins during northeast-southwest extension (Witt et al., 2001). ...
... wt.% NaCl), CO 2 -bearing ore fluids. Furthermore, detailed structural-hydrothermal studies at the Copperhead, Yilgarn Star, and Westonia gold deposits suggest that the calc-silicate alteration assemblages are pre-, syn-, and post-orogenic and are not necessarily contemporaneous with spatially associated felsic intrusions or indicative of a magmatic source for gold Hagemann and Cassidy, 1999;Witt et al., 2001a;Grainger and Hagemann, 2002). In contrast, the Nevoria and Corinthian deposits are classified as syn-magmatic Au-W ± Mo ± Ag metasomatic skarns based on their metal content, gangue mineralogy (Mueller, 1997), and their spatial and temporal association with late (2640-2630 Ma) "post-orogenic", I-type granitic and pegmatite intrusions (Mueller et al., 2004). ...
The Yilgarn Craton hosts three types of Archean granitoid-associated metallogenic systems: orogenic gold, intrusion-related, and porphyry systems. These systems may occur in the same terrane and share many broad characteristics, including spatial coincidence with granitoids, gangue silicate mineralogy, metal associations, and local structural controls. Features of Archean orogenic gold systems spatially associated with granitoids include (i) a broad range of granitoid host compositions; (ii) gold mineralization is mostly late with respect to Yilgarn-wide, granitoid emplacement, peak metamorphism, and deformation; (iii) orebodies are structurally controlled, and (iv) gold deposition occurs over a large range of temperatures, pressures, and crustal depths (2 to 15 km) from a CO2-rich, low to moderate salinity, reduced ore fluid that was derived from a metamorphic and/or distal magmatic fluid source. These systems do not display deposit-scale vertical or lateral zonation of metals away from granitoid contacts. Host granitoid age, composition, or oxidation state are not important in controlling gold mineralization processes, whereas structural setting and fluid flux are paramount. Archean intrusion-related Au–Mo–W and Mo ± Au systems in the Yilgarn Craton are commonly small (< 10 t Au), and are spatially and temporally associated with felsic intrusions emplaced at < 5 to 14 km crustal depths. Orebodies are associated with pervasive alteration in the granitoid-host and proximal supracrustal countryrock. Where available, fluid inclusion and metal association data indicate the involvement of an aqueous, CO2-bearing, moderate to high salinity fluid during ore deposition. Archean porphyry Cu–Mo–Au, Cu–Mo, and Cu–Au systems are spatially and temporally associated with volumetrically small, pervasively altered, felsic plutons and dikes surrounded by altered and mineralized supracrustal countryrock. Lateral zonation of metals away from porphyry centers commonly includes a decrease in Cu:Au and Ag:Au ratios with distance from the source pluton. Microthermometry studies on fluid inclusions in gold-bearing veins suggest that early ore fluids were high-temperature and high-salinity, and that mineralization occurred at < 5 km crustal depths. Exploration for Archean granitoid-associated orogenic gold systems in the Yilgarn Craton lies primarily in identifying mineralized structures in granitoids or adjacent supracrustal rocks and predicting low mean stress areas where ore fluids may concentrate. In contrast, exploration strategies for Archean syn-magmatic systems hinges upon establishing temporal connections between granitoid emplacement and associated mineralization, and testing this relationship in coeval granitoids in the same terrane. Fractionation trends in granitoid composition and ore metal associations, using geochemical indices such as Rb/Sr versus Fe2O3/FeO, are demonstrated in Phanerozoic terranes but are presently ill-defined in the Yilgarn Craton. Crustal depth estimation is important in defining areas likely to have preserve shallow-forming, Archean porphyry systems.
... A these studies and Witt et al (2001), the FCSZ-Taurus Shear Zone system is considered to have developed during D3 regional E-W compression. This followed the NE-SW shortening that generated the upright N-S F2 folds. ...
Golden Pig mine is in the central part of the Southern Cross greenstone belt within the Archaean Yilgarn craton. The mine has produced approximately 2.5 Mt of ore at 5.7 g/t Au to yield approximately 450,000oz. At Golden Pig, the majority of gold mineralisation is hosted within extensively deformed and quartz veined BIF. Within the sheared and isoclinally folded BIF units the primary textures are replaced by texturally destructive auriferous quartz-pyrrhotite-diopside-amphibole dominant calc silicate assemblages or carbonate-quartz-pyrrhotite assemblages.
In addition to the BIF-hosted mineralisation, an increasingly important, part of the resources lie in the Taurus Lode, which is an amphibolite facies shear zone that trends N-NNW. The Taurus Lode shear zone has a moderate to steeply WSW dipping foliation and a mineral lineation of biotite, amphibole and pyrrhotite plunging very consistently 10º south. Drive faces, perpendicular to the foliation and lineation, consistently show evidence (from sigma clasts, dilatational jogs and asymmetric shear band type boudins) for syn-mineralization reverse shear. Exposures parallel to the mineral lineation and perpendicular to the foliation reveal both sinistral and dextral asymmetry’s, sometimes within centimetres of each other and conjugate, syn-deformational veins, oblique to the shear zone, are folded. These characteristics suggest that the vorticity axis of the Taurus shear zone is parallel to the mineral lineation, and that the simple shear sense is reverse. Such geometry can arise when there is a major component of flattening across the shear zone, in which case the vorticity axis can be parallel to the finite stretch and lineation.
The Taurus Lode shear zone is considered to be part of the Fraser’s-Corinthia shear zone, a regional scale structure that apparently links several major deposits in the Southern Cross greenstone belt. It has been suggested that the Fraser’s-Corinthia shear zone has a strike-slip simple shear component that changes from sinistral to the north of Southern Cross to dextral southwards. The shear zone has also been linked with diapiric rise of the Ghooli dome to the East. However, the above observations are not compatible with either strike slip deformation or the normal sense of shear anticipated from the simple application of a diapiric model. Instead, it is possible that the Taurus strains indicate a more complicated geometry due to interference between two adjacent domes.
The BIF-hosted lodes plunge consistently 15o NNW coincident with the intersection of the axial planar cleavage of the regional folds and the Taurus shear. However, where the penetrative shear characteristics control the orientation and geometry of the mineralisation, it plunges parallel to the stretching and mineral lineations defined within the shear. The predetermination of the opposing SSW plunge to strike extensive and laterally confined high-grade, economic ore shoots within the Taurus shear ultimately proves imperative to defining the economic mineralisation within this lode. Identifying the zones of maximum continuity and establishing the optimal position for future development will maximise ore extraction.
The new Taurus mineralisation model is consistent with the kinematic indicators and south plunge of the lineations observed, which is pertinent to the major ore-shoots within the shear, thereby strongly influencing the orientation, continuity and geometry of the Taurus resource.
The Pedra Branca gold deposit is a recent discovery conducted by private exploration companies in the Troia Massif of the northern Borborema Province, Northeast Brazil. It comprises an orogenic gold deposit hosted by the amphibolite-facies Serra das Pipocas greenstone belt. Airborne hyperspectral images (ProSpecTIR-VNIR-SWIR system) covering the Pedra Branca deposit as well as spectroscopic data (ASD-Fieldspec and SisuCHEMA instruments) from representative rock samples in the area were used in this study to investigate the spectro-mineralogical patterns of alteration minerals at both district and deposit scales, respectively. The spectroscopic data were integrated with geological information from the deposit, and also with X-ray powder diffraction and petrographic analyses, in order to reveal alteration footprints that could guide gold exploration targeting in the region. The results indicate the occurrence of a main hydrothermal calc-silicate alteration in ferromagnesian host rocks, consistent with proximal alteration assemblages found in amphibolite-facies gold deposits. White mica compositional trends suggest a later hydrothermal alteration that overprinted the pre-existing mineral assemblages and crystallized Al-rich white mica in distal zones and Al-poor white mica in the ore zone. Reflectance spectroscopy analyses of outcrop samples indicate that well-ordered kaolinite occurs as a weathering product of Al-poor white mica-bearing samples, and therefore could be used as a proxy to mineralization at the deposit scale. Mineral maps produced by processing airborne hyperspectral images, combined with soil geochemical anomalies, further support the notion that well-ordered kaolinite can be used as a proxy to mineralization in weathered ferromagnesian host rocks. Target areas for gold exploration can be defined by airborne hyperspectral data based on the identification of spectral mixtures of nontronite and well-ordered kaolinite. The mineral and physicochemical trends identified in this study set an important baseline for mineral exploration of amphibolite-facies orogenic gold deposits in greenstone belt terranes. The mineral guides defined here can be sensed by VNIR-SWIR hyperspectral sensors at multiple scales using portable, core scanning, wall-imaging, airborne or satellite-borne instruments, respectively.
Part 3 of the Yilgarn Gold Exploration Targeting Atlas presents 12 targeting criteria for deposit- or mine-scale exploration
for gold lodes that are based on empirical, qualitative observations or reports from lode gold deposits in the Yilgarn
Craton. These can be subdivided into structural criteria, which are largely predictive in nature, and geochemical and
mineralogical criteria, which are used to directly detect evidence for the presence of ore shoots. Structural targeting
includes well-established criteria such as fault intersections, fault intersections with specific lithological units, and fault
bends and jogs. All three of these settings potentially provide anomalies in structurally induced permeability resulting
from tensile failure of rock units and consequent focusing of ore fluid flux. Examples of deposits where hydraulic seals
and constriction zones (bottlenecks) may have contributed to localization of ore shoots are also presented. As crustal level
deepens and metamorphic grade increases, folds and boudins become increasingly important structural controls on the
location and orientation of ore shoots. Common elements in all of these structural targeting criteria are rock rheology and
steep rheological gradients. Previous studies of geochemical dispersion of gold and pathfinder elements around gold lodes
indicate that their extent orthogonal to gold lodes is very limited. Similarly limited dispersion has been demonstrated where
alteration indices and stable isotopes have been studied. However, application of these geochemical parameters to detect
conduits between ore shoots holds some promise. The relatively recently developed rare alkali index is the most useful of
these parameters as it remains anomalous for up to tens of metres beyond the visible alteration halo around gold lodes in
many of the deposits where it has been applied, including Red October (a new study completed for this Atlas). Another
recent technological development with potential application to deposit- and mine-scale exploration is the automated
spectral identification of hydrothermal minerals associated with gold mineralization. Spectra generated in the VNIR–SWIR
range have been used to develop vectors to gold ore at the Golden Mile and Kanowna. The very recent introduction of a
thermal infrared (TIR) spectrometer to the automated HyLogger instrument allows identification of anhydrous minerals
such as feldspars and pyroxenes, which has application in higher-temperature gold deposits such as those in the Southern
Cross district. Application of combined VNIR–SWIR and TIR spectral data to six diamond cores from gold deposits at
Southern Cross has allowed distinction of the main lithological groups in the greenstone sequence and identified vectors
to gold lodes, including changes in mineral abundance, the composition of plagioclase, and the presence of K-feldspar,
none of which are easily identified by conventional logging of the core.
The formation of structurally controlled lode-gold deposits is a controversial subject with both magmatic and metamorphic fluids considered a feasible source for gold. Here we present a study from the Southern Cross district (Yilgarn Craton, Western Australia), where structurally controlled Neoarchean gold deposits are hosted in amphibolite facies greenstones. We combine geochronological data with structural analysis to constrain the timing and structural control of ore deposition. We show that gold mineralization took place between c. 2638 and 2630 Ma, contemporaneous with the onset of voluminous intrusions of Low-Ca, I-type granites, which are ubiquitous in Southern Cross and elsewhere in the Yilgarn Craton. We argue that the timing of the intrusions, their spatial association with gold deposits, their chemical composition, and isotopic signature are consistent with Low-Ca granites being a major fluid source for mineralization. We propose that the fluid release by cooling plutons was facilitated by transport along active amphibolite facies shear zones. As gold mineralization was synchronous with Low-Ca magmatism across large areas of the Yilgarn Craton, our model may be applicable to orogenic gold deposits in other parts of the Yilgarn Craton as well as other Archean granite-greenstone terrains with a ‘late granite bloom’.
The grade of ore at Marvel Loch mine is directly related to the presence
or absence of shear zones and quartz-sulfide-rich hornblende (± diopside)
veining within areas of plagioclase alteration. An experienced geologist
can identify these features in hand specimen. Practical rules have been
developed to assist this visual identification. The structural sites and
styles of alteration and veining most conducive to gold mineralisation
have also been determined. The work has enabled a better understanding
of the geological controls on mineralisation at Marvel Loch, and has
clarified geological understanding for optimisation of grade control
modeling, ore block mark-ups, and minimisation of ore loss and dilution
during excavation. In addition to this, it has enabled a detailed
understanding of mine scale structural controls, geometry and relative
grade relationships.
The Southern Cross belt is located in the Youanmi Terrane of the Yilgarn Craton, directly north of the Forrestania belt, which contains economic komatiite-hosted Ni–Cu mineralisation. The mafic and ultramafic greenstone assemblage in the Southern Cross belt, encompasses a continuum from olivine-rich cumulates through to evolved high MgO komatiitic basalts and gabbros. Inspection of geochemical data shows that komatiites can be divided into a high-Al group with Al2O3/TiO2 17–19, slightly lower than typical Munro-type komatiites, and a low-Al group with Al2O3/TiO2 17–19, intermediate between typical Munro-type and Barberton-type komatiites. Parent magmas for the high-Al and low-Al groups had MgO contents ranging between 10 and 15 wt% and between 15 and 25wt%, respectively. Volcanic facies show a transition from distal thin flow facies in the north, changing to lava lakes and sills further south. Olivine-rich channel or conduit facies assemblages are absent from both suites, suggesting low prospectivity for nickel sulfide mineralisation. The high-Al group displays flat to very weakly LREE enriched patterns with flat HREE that are typical of Munro-type komatiites. The low-Al group is weakly LREE enriched, has distinctly negative sloping REE patterns from La to Lu, and minor HREE depletion relative to MREE. Direct comparison with the Forrestania belt shows that the Southern Cross komatiites overlap with the transitional Munro-type komatiites of upper Central and Takashi belts that represent the unmineralised portion of the Forrestania belt. The predominance of Munro-type komatiites in the Southern Cross belt implies that the Southern Cross komatiites were derived from shallower melting than the Barberton-type komatiites that host mineralisation in Forrestania greenstone belt. Furthermore, the variation between Al/Ti and V/Ti implies that the Southern Cross ultramafic suite may represent the transition from Barberton-type to Munro-type komatiites, and formed close to thelower pressure limit of stability of majorite garnet in the mantle residue. It is suggested that the transitional character of the Southern Cross komatiites may be associated with a fundamental difference in the crustal architecture and craton keel thickness underlying the southeastern Youanmi Craton.
