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a-Simplified geological map of the Brno Batholith (modified from Leichmann and Höck 2008); the outlined square area is enlarged in b. b-Simplified sketch of the SW part of the Brno Batholith, modified from Mittrenga et al. (1976). According to a new scheme (Leichmann and Höck 2008) the Střelice, Kounice and Tetčice subtypes are now classified as the Tetčice suite, the Krumlovský les, Réna and Leskoun subtypes belong to the Réna suite and the Hlína subtype is now classified as the Hlína suite. Inset shows position of the studied region within the Bohemian Massif.
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Layered, garnet-bearing aplite-pegmatite dykes of the Hlína granitic suite, ˜2 to 50 m thick and up to ˜200 m long with general NW-SE orientation and dip 40-80° to NE or SW, cut granodiorites to granites at the SW part of the Brno Batholith, Brunovistulicum, Czech Republic. Aplite-pegmatite bodies are characterized by alternation of two main textur...
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Citations
... Morfostrukturní prvky, uvedené Hrádkem (1995a, b), se však nepodařilo v terénu potvrdit, a to především dilatační rozsedliny a elevace. Novější výzkumy potvrdily, že elevace jsou tvořeny zvětralými odolnějšími magmatickými horninami -žilnými formami (do 50 m mocnými) felsických aplitů až pegmatitů podtypu Hlína uvnitř podtypu granodioritů Krumlovského lesa (Hönig et al. 2010). ...
New knowledge on the occurrence of Lower and Middle Miocene sediments in the southwestern surroundings of Brno has been acquired through geological mapping and documentation work during the recent years. Specific work was focused on: • preparation of the publication on the local national history of the cadastral territory of the village of Bohutice; • salvage study of the former reserved bentonite deposit Ivančice – Réna, which is being gradually remediated and turned into a recreational area; • the area in the foreground of the New Ivančice viaduct, which is susceptible to long-term slope instability; • the western edge of the village of Němčičky near Židlochovice, where extensive construction of family houses is underway; • the area between the municipalities of Moravany, Nebovidy and Ostopovice, where the excavations for new extra-high voltage pylons were documented. The detailed mapping, petrographic and biostratigraphic studies allowed to refine the distribution, lithological characteristics and age of Miocene sediments; specifically, a more extensive occurrence of Lower Miocene sediments compared to previous findings was confirmed (localities Ostopovice, Moravské Bránice – locality 5). These findings support earlier results from the area N of the City of Brno, where the known extent of Ottnangian sediments was expanded at the expense of Badenian sediments. The documentation and sampling of the new excavations (for family houses and extra-high voltage pylons) and old mining pits enabled the description and further study of the sediments. The acquired litho- and biostratigraphic data were correlated with engineering geological findings. In a construction pit in Bohutice, a completely new occurrence of tuffitic sediments of the Ottnangian age was discovered and geochemically verified. Furthermore, the Ottnangian gravels in Němčičky were newly discovered. These contain a large proportion of granitoid pebbles probably derived from the Brno Massif. It was found that the weakly consolidated lithologically variable Lower Miocene sediments are prone to landslides. In the case of the New Ivančice viaduct, extensive suffusion doline were identified, resulting from the ingress of rainwater from the wider area of the railway embankment foreground. Biostratigraphy of the sediments was based on micropaleontological analysis of foraminiferas. Ottnangian sediments were usually fossil- free and/or they contained reworked Cretaceous foraminiferas. Lower Badenian sediments are characterized by occurrence of abundant and diversified fauna represented by foraminifera species such as Martinotiella karreri (Cush.), Globigerinoides bisphericus Todd, Vaginulina legumen (L.), etc.
... Metamict Nb-Ti-Th-Y-bearing minerals of composition close to euxenite-aeschynite coexist with Tioxides (ilmenite-pyrophanite), which are typical mineral species of NYF pegmatites (Černý 1991b;Ercit et al. 2005). The high Y and REE contents of garnet are similar to garnet from NYF pegmatites in the Sveconorwegian province (Hönig et al. 2010(Hönig et al. , 2014Müller et al. 2012). The high Ca content of garnet is also comparable to that of some NYF pegmatites, mixed LCT + NYF pegmatites, or A-type pegmatites (Dahlquist et al. 2007;Feng et al. 2017;Hönig et al. 2014, Fig. 11a). ...
The Mangodara district (southwestern of Burkina Faso, West African Craton) consists of a regional-scale Eburnean dome cored by granitoid-gneisses comprising rafts of migmatitic paragneisses and amphibolites of the Paleoproterozoic Birimian series. The occurrence of rare metal-bearing pegmatites in diffuse contact with these migmatitic and granitoid gneisses suggests that they originated from the segregation of a residual melt of these partially molten hosts. In this paper, we constrain the petrogenetic link between pegmatites and their hosts, and the mechanisms of rare metal fractionation in Lithium-Cesium-Tantalum (LCT) vs Niobium-Yttrium-Fluorine (NYF) petrogenetic signatures by the geochemistry of micas, apatite, columbite-group minerals, garnet, and zircon. Titanite-allanite pegmatites (containing titanite, allanite, epidote, zircon, and apatite as accessory minerals) and their evolved equivalent, apatite-zircon pegmatites (richer in apatite, lower K/Rb and Fe/Mn ratio in biotite but Li-depleted) are poorly mineralized metaluminous pegmatites. They display a continuous evolution trend in K/Rb and Fe/Mn in biotite and similar REE patterns in apatite, which favor an origin by segregation of residual melt within tonalitic-trondhjemitic gneiss in the core of the Mangodara dome. Garnet-columbite pegmatites containing REE-bearing phosphates and Zr-U-Th-bearing metamict minerals are mixed LCT + NYF pegmatites. Their micas, slightly enriched in Li, LREE-rich apatite, and Nb–Ta-U-rich garnet, are consistent with an origin by partial melting of a metasedimentary source, with dehydration of biotite (reservoir of Li, Rb, Nb) and dissolution of apatite-monazite (reservoir of REE). Apatite crystals in one garnet-columbite pegmatite reveal an inherited REE signature typical of apatite-zircon pegmatite, which suggests mingling of a LCT pegmatite-forming melt with the residual melt derived from crystallization of metaluminous pegmatites. Garnet-REE pegmatites, containing ilmenite-pyrophanite and euxenite-aeschynite), are NYF pegmatites that should originate from melt segregation within granodioritic gneiss associated with breakdown/entrainment of amphibole (reservoir of REE, Y) and LREE segregation by allanite and phosphates in the source. These data show that the LCT vs NYF signature of pegmatites of the Mangodara district results primarily from the chemical composition of the partially-molten source and the minerals involved in the partial melting reactions, which vary as a function of increasing depth (mica, phosphate, amphibole, garnet). The trace-element signature of anatectic peraluminous pegmatite-forming melt might then be affected by mingling with residual Nb-enriched metaluminous melt.
... Metamict Nb-Ti-Th-Y-bearing minerals of composition close to euxenite-195 aeschynite coexist with Ti-oxides (ilmenite-pyrophanite), which are typical mineral species of NYF pegmatites (Černý 1991b;Ercit et al. 2005;. The high Y and REE contents of garnet are similar to existing data of garnet from NYF pegmatites in Sveconorwegian province (Hönig et al. 2010(Hönig et al. , 2014. High Ca content of garnet is also comparable with the garnet Ca content of some NYF pegmatites, mixed LCT+NYF pegmatites or A-type pegmatites ( Figure. ...
Les métaux rares sont essentiels au développement de l'industrie de pointe et à la transition vers une énergie décarbonnée. Les pegmatites granitiques sont des métallotectes de ces métaux, dont l'origine est discutée entre la fusion partielle de métasédiments, ou la différenciation magmatique d'un pluton enrichi. Le secteur de Mangodara (Sud-Ouest du Burkina Faso) expose des pegmatites à métaux rares associées à des roches métamorphiques, migmatitiques et magmatiques, dans un contexte de gneiss-granitoïdes du Craton Ouest-Africain Paléoprotérozoïque. Nous investiguons la pétrogenèse de ces pegmatites à travers une approche multiple pétrographique, géochimique et géochronologique. Le secteur étudié constitue un dôme régional, représentatif de la croûte moyenne à inférieure (4-7 kbar, baromètre Aluminium dans la hornblende), avec au cœur un gneiss tonalitique- trondhjémitique riche en plagioclase, enveloppé par un gneiss granodioritique interprété comme étant une diatexite, contenant des lambeaux d'amphibolites, paragneiss (sillimanite-grenat/staurotide) et migmatites de protolithes birimiens. Des roches plutoniques, formant deux séries géochimiques l'une Na-riche et l'autre K-riche, sont en contact diffus avec ces gneiss ou intrusifs dans ces derniers. Quatre types de pegmatites ont été identifiées dans le secteur. Les pegmatites de type titanite-allanite et apatite-zircon sont métalumineuses, peu évoluées, et issues la ségrégation tardive du gneiss tonalitique-trondhjémitique. Les pegmatites de type grenat-colombite (Li, Nb) sont peralumineuses, de famille Mixte Lithium-Césium-Tantale et Niobium-Yttrium-Fluor (LCT+NYF). Elles sont issues de la fusion partielle de paragneiss avec un contrôle du fractionnement des métaux rares dans la source par les micas, les phosphates et le grenat, et une contamination par les liquides résiduels des pegmatites métalumineuses. Les pegmatites de type grenat-REE (Ti, Y, HREE) sont des pegmatites NYF, issues de la ségrégation tardive du gneiss granodioritique, avec un contrôle du fractionnement des métaux rares par l'allanite, les phosphates et l'amphibole. La cristallisation de la pegmatite de type titanite-allanite est datée à 2094,3 ± 8,8 Ma par la méthode U-Pb sur zircon. Le refroidissement régional à moins de 500 °C est indiqué par des âges U-Pb sur apatite dans le gneiss tonalitique-trondhjémitique (2094 ± 21 Ma), dans le gneiss granodioritique (2041 ± 33 Ma) et dans la pegmatite de type apatite-zircon (2055 ± 20 Ma). La genèse des pegmatites à métaux rares de Mangodara dérive à la fois de processus de ségrégation et de fusion partielle dans la croûte moyenne, mais le contrôle de minéraux porteurs de métaux rares est déterminant sur le fractionnement des métaux rares, et la signature pétrogénétique des pegmatites. Cela ouvre des perspectives de recherche des minéralisations liées aux métaux rares dans les zones de transition migmatitiques-magmatique.
... Cette pegmatite stockscheider achève l'étape magmatique, il est donc cohérent que sa mise en place (309,7 ± 4,5 Ma) soit sub-synchrone de celle du leucogranite. Le stockscheider de Montebras constitue un exemple représentatif de formations magmatiques de contact caractérisées par des textures de cristallisation unidirectionnelles telles qu'on peut les rencontrer dans différentes intrusions composites (Shannon et al., 1982 ;Breiter, 2002 ;Hönig et al., 2010). L'orthose, largement dominante (le stockscheider contient 62 % de SiO 2 ), cristallise de manière unidirectionnelle en grands cristaux décimétriques rougesroses au sommet de la coupole. ...
The Montebras cupola is a small massif of granite with rare metals (Sn, W, Li, Nb-Ta) located north of the French Massif Central which emplaced during the upper Carboniferous in an older host rock, the granite of Chanon (357.2 ± 2.1 Ma). Two episodes, a microgranite (316.1 ± 4.3 Ma), and an albitic leucogranite (309.8 ± 3.9 Ma) are distinguished. The second develops contact formations at its roof, in particular a thick stockscheider pegmatite (309.7 ± 4.5 Ma), passing east to lithium greisens and tin-bearing flat quartz veins, previously exploited (303.8 ± 4.8 Ma).
The cassiterite deposition (associated with manganocolumbite) spreads from the magmatic phase with crystals disseminated in leucogranite, until the end of the pneumatolytic phase marked by veins of stanniferous quartz. This spreading is marked by a progressive drop in the concentrations of trace elements (Nb, Ta, Fe, Mn, Mg, Ti) in cassiterite but without reaching the domain of typically hydrothermal compositions. During the pneumatolytic phase, cassiterite is accompanied by rare scheelite, the rare qitianlingite and a frequent wolframite, the composition of which (hubnerite) indicates a magmatic origin for the metal and the fluids involved in the deposition of tungsten. The deposition of sulfides rich in Cu, As and Sn (lollingite, chalcopyrite, tennantite, stannoidite, mawsonite…) marks the transition to the hydrothermal phase and suggests an origin in host rocks for certain metals. The liasic fluorite-baryte event is manifested by the local appearance of veinlets with violet fluorite, barite and manganapatite. The Montebras dome provides a representative example of the rare metal granites of the Variscan orogen. It is contemporary with the other rare elements granitic magmas of the northern French Massif Central with which it presents points of similarity but also differences. Its emplacement could fall under a mechanism of the cauldron subsidence type.
... Cette pegmatite stockscheider achève l'étape magmatique, il est donc cohérent que sa mise en place (309,7 ± 4,5 Ma) soit sub-synchrone de celle du leucogranite. Le stockscheider de Montebras constitue un exemple représentatif de formations magmatiques de contact caractérisées par des textures de cristallisation unidirectionnelles telles qu'on peut les rencontrer dans différentes intrusions composites (Shannon et al., 1982 ;Breiter, 2002 ;Hönig et al., 2010). L'orthose, largement dominante (le stockscheider contient 62 % de SiO 2 ), cristallise de manière unidirectionnelle en grands cristaux décimétriques rougesroses au sommet de la coupole. ...
La coupole de Montebras est un petit massif de granite à métaux rares (Sn, W, Li, Nb-Ta) situé au nord du Massif Central Français qui se met en place au Carbonifère supérieur dans un encaissant plus ancien, le granite de Chanon (357,2 ± 2,1 Ma). Deux épisodes magmatiques, un microgranite (316,1 ± 4,3 Ma) et un leucogranite albitique (309,8 ± 3,9 Ma), sont distingués. Le second développe à son toit des formations de contact, notamment une puissante pegmatite stockscheider (309,7 ± 4,5 Ma), passant vers l’est à des greisens à lithium et des filons plats de quartz stannifères anciennement exploités (303,8 ± 4,8 Ma). Le dépôt de cassitérite (associée à la manganocolumbite) s’étale depuis la phase magmatique avec des cristaux disséminés dans le leucogranite, jusqu’à la fin de la phase pneumatolytique marquée par des filons de quartz stannifères. Cet étalement se traduit par une baisse progressive des concentrations en éléments-traces (Nb, Ta, Fe, Mn, Mg, Ti) dans la cassitérite mais sans que soit atteint le domaine des compositions typiquement hydrothermales. Lors de la phase pneumatolytique, la cassitérite est accompagnée de rare scheelite, de la rarissime qitianlingite et d’une wolframite fréquente dont la composition (hübnérite) indique une origine magmatique pour le métal et les fluides impliqués dans le dépôt du tungstène. La paragenèse à sulfures riches en Cu, As et Sn (löllingite, chalcopyrite, tennantite, stannoïdite, mawsonite…) marque le passage à la phase hydrothermale et suggère une origine dans les roches encaissantes pour le cuivre et l’arsenic. L’événement fluo-barytique liasique se manifeste par l’apparition locale de fissures à fluorine violette, barytine et manganapatite. La coupole de Montebras fournit un exemple représentatif des granites à métaux rares de la chaîne varisque. Elle est contemporaine des autres magmas granitiques à éléments rares du nord Massif central avec lesquels elle présente des points de similitude mais aussi des différences. Sa mise en place pourrait relever d’un mécanisme de type cauldron subsidence .
... Leukokratní granitové a aplo-pegmatitové žíly suity Hlína (Hönig et al. 2010), prorážející granity a granodiority brněnského masivu, jsou sice silně frakcionované, ale téměř bezslídové, a tedy Li-chudé. ...
... At the contact in the cupola of the pegmatite (i.e. the centre of the upper dome), there is a metre layer of K-feldspar with graphic quartz. This pinches out distally, and is underlain by a fine quartz and feldspar microgranite facies, which grades into a fine quartz, feldspar and biotite intergrowth zone with localised biotite splays (0.3 m) and 0.2 m K-feldspar megacrysts.The wall rocks of pegmatites are known to contain unidirectional solidification textures (USTs) which represent magmatic layering; inwardly extended and flared crystals of feldspar and mica (e.g.London, 2008) and stripes or trains of garnet line rocks (e.g.Hönig, 2010).London (2008) suggests that liquidus undercooling (of 150 -250°C) and nucleation delay, in addition to isothermal subsolidus fractional crystallisation, constitutional zone refinement and far field chemical diffusion, are required for the development of orientated textures and indeed the internal zonation of pegmatites in general (London 2014). Hönig (2010) echoes that undercooling and rapid, multistage non-equilibrium crystallisation from a water saturated melt is necessary for the development of USTs, and qualifies the production of garnet line rocks as a form of magmatic layering which requires the formation of a boundary chemical layer bordering the crystallisation front. ...
... Cooling of the Anenský mlýn quartz diorite was dated at 596.1 ± 2.1 Ma by Ar-Ar method on amphibole (Fritz et al. 1996). Small lenticular bodies of felsic, garnet-bearing granite of A-type affinity intruding Cadomian granites of the WGC were dated (WR-Grt Sm-Nd) at 430.1 ± 6.4 Ma (Hönig et al. 2010;Leichmann et al. 2013). The westerly equivalent of the WGC, the Svratka Massif in paraautochton of Moravicum, was dated at 576 ± 2 Ma by Ar-Ar method on amphibole (Fritz et al. 1996) and at 634 ± 6 Ma by LA-ICP-MS U-Pb method on zircon (Soejono et al. 2017). ...
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The Brno Massif is the largest exposed part of the Brunovistulicum (eastern Bohemian Massif) representing Precambrian basement incorporated into the Central European Variscan Belt. Two well-known Cadomian granodiorite complexes of magmatic-arc origin are separated by N–S-trending belt of mafic rocks previously compared to ophiolite. This so-called Central Basic Belt is formed by a slightly metamorphosed volcanic part (Metabasite Zone) in the east and dominantly plutonic Diorite Zone in the west. Our new geological, geochemical and isotopic data including U–Pb zircon dating reveal two distinct Precambrian magmatic events within the Central Basic Belt preceding the Cadomian arc. The geochemical signatures of the dominant late Tonian (c. 730 Ma) tholeiitic basalts ( ε725Nd = + 7.8 to + 6.7) in the Metabasite Zone suggest a direct derivation from a mantle source in an extensional setting. Also, the associated sporadic rhyolitic lavas and tuffs are primitive, showing a short mean crustal residence ( ε725Nd = + 6.0 and + 5.7; TNdDM .2stg ~ 0.9 Ga). By contrast, the Cryogenian (c. 650 Ma) magmatism of the Diorite Zone clearly demonstrates features of a magmatic-arc origin. Rather primitive whole-rock geochemistry and radiogenic Nd isotopic signature ( ε655Nd values typically falling between + 7 and + 6) show that this arc was either intraoceanic, or developed on recently accreted, immature mafic crust. Based on all the available data, three successive tectono-magmatic stages have been identified in the Brno Massif in the Neoproterozoic times (c. 730–600 Ma), as products of a single long-lived, multi-stage subduction system spanning nearly full Neoproterozoic supercontinent cycle from the break-up of Rodinia to the assembly of Pannotia.
... Commonly, they show quartz and feldspar precipitation, indicating fast diffusive elements such as F, Li and P (Gardien et al., 2016). Minor commonly zoned fluorite, sulfide, and REE oxides are also observed (Hönig et al., 2010). Crystal zoning in plagioclases and alkali-feldspars, as alternate repetition of mineral layers (Hönig et al., 2010) could be explained by oscillatory crystallization close to eutectic composition where element saturation repeatedly occurs (Bogoch et al., 1997). ...
... Minor commonly zoned fluorite, sulfide, and REE oxides are also observed (Hönig et al., 2010). Crystal zoning in plagioclases and alkali-feldspars, as alternate repetition of mineral layers (Hönig et al., 2010) could be explained by oscillatory crystallization close to eutectic composition where element saturation repeatedly occurs (Bogoch et al., 1997). Their composition, rich in Rb, Cs, Sn, Zn, attests the presence of an abundant MVP rich in halogens (Breiter, 2002). ...
Many precious and base metals (Cu, Au, Ag, Mo, W, Sn) in porphyry-type deposits are extracted from a granitic to granodioritic silicate melt and transported by magmatic-hydrothermal fluids to the site of ore deposition. Metals abundance increases from some ppb in the melt to % in ore bodies. Assuming a unique magmatic source for metals, it implies an exceptional combination of enrichment process acting over up to 4 orders of magnitude. Previous models of magma differentiation and hydrothermal circulations omitted to consider the role of metal segregation during magma chamber evolution. Models for the formation and evolution of intrusions have recently shifted from the so-called melting-storage-assimilation-homogenization (MASH) paradigm, basically steady state, to the mantle-melting-segregation-ascent-emplacement (m(M-SAE)) paradigm, dynamic and discontinuous in time. Successive magma injections of variable compositions progressively build the magma chamber. This is supported by field relations (e.g. cross-cutting dykes and stocks), textures (e.g. partially resorbed enclaves, or mineral fabrics), geochemistry (e.g. hybridization signatures), and age dating aswell as
istopic studies. As melt crystallizes and forms a mush (> 50% crystals), volatiles also exsolve forming a magmatic volatile phase (MVP) while melt motion slows down. Metals partition between the three phases: melt; crystals; and MVP. They generally prefer the MVP owing to more favorable partition coefficients. This is indicated by metal content in coeval melt and fluid inclusions with homogenization temperatures above 600 °C, or in volcanic fumaroles. Here, by simulating the physical interactions between the three phases we suggest a model of fluid sparging, during which metals segregate towards the MVP by diffusion and are further transported
by advection as metal complexes. The model also provides estimates of metal enrichment. An undimensional Péclet number rules the competition between diffusion and advection, basically scaling with respect to the inverse of the product of melt viscosity (η) by metal diffusivity (D). The threshold value of the Péclet number between diffusion and advection is roughly 10−9. Fast diffusive metals (Au, Cu, Ag, W) readily diffuse from silicate melt and towards bubbles of the MVP. To escape the mush through tubular structures, the MVP must
overcome a critical gas saturation level (about 20% vol.) usually reached after several magma injections. Advection then takes over and transports the metal-enriched MVP towards the top of the magma chamber. This leads to nearly coeval (1) separation between a high salinity-liquid phase and a low-salinity vapor phase, (2) fluid-rock interactions resulting in potassic, advanced argillic and phyllic alterations and (3) metal deposition. The metal enrichment scales as the ratio between partition coefficient (i.e. related to the gradient in chemical potential), diffusivity (i.e. related to the gradient of concentration), and melt viscosity (i.e. related to the gradient of momentum). The rates at which all such gradients relax determe metals enrichment, inducing chemical and physical instabilities, leading to a cyclic process. The whole cycle also encompasses the case of a partial, noncompleted, full enrichment yielding to barren intrusions. A tentative model generalizes the sparging fluid model
to other metal deposits linked to an intrusion. Such generalization should be interpreted as predicting metal enrichment by 3–4 orders of magnitude, rather than predicting an exact value.
... Within galleries and boreholes in the Passa Três granite, we observed rocks, textures and structures that clearly indicate the presence of magmatic-hydrothermal transitional features, such as aplites, pegmatites, UST (Unidirectional Solidification Texture, Hönig et al., 2010;Shafaroudi et al., 2015;Yang et al., 2016) and quartz veins with a Kfeldspar border (herein denominated transitional veins) (Figs. 4 and 5). All these features are intensively developed in the upper part of the granite body, and are described below. ...
... USTs are constituted by parallel layers of quartz and K-feldspar (Figs. 4I and 5) (Hönig et al., 2010;Shafaroudi et al., 2015;Yang et al., 2016). In detail, this texture consists in alternating crystallization of Kfeldspar and quartz with some evidence of growth direction expressed by the fan shape of comb K-feldspars and quartz (Fig. 5). ...
The Passa Três granite is a NNE-SSW elongated shape intrusion located in the Paraná State (southern Brazil). This intrusion was emplaced within metapelites of the Mesoproterozoic Votuverava Group, which are part of the N040°E trending Lancinha Shear Zone. Gold mineralisation within the Passa Três granite is held by meter-scale quartz veins forming orebodies with variable massive, banded, sheared and/or brecciated internal textures. Structural data indicate the existence of two orthogonal directions of structures, N-S and E-W, dipping 60–75°W and 45–70°S respectively. Mineralised veins contain, in addition to the quartz of the gangue, and in chronological order: fluorite, pyrite, chalcopyrite, gold, aikinite, molybdenite, muscovite, carbonate and baryte. Gold occurs as native grains within fractures that affect pyrite, commonly associated with chalcopyrite and aikinite. Quartz veins are sometimes bordered by aplitic dykes. Additionally, some of the veins exhibit a thin margin of K-feldspar minerals that could represent the early stage of vein formation. These observations, the presence of Unidirectional Solidification Textures (UST), along with abundant expression of pegmatite-rich pockets within granite that crops out close to the surface allow the reconstruction of the granite architecture in terms of late magmatic evolution, magmatic-hydrothermal transition and position of the cupola. N-S and E-W systems are interpreted to be contemporaneous and conjugate. Normal displacements are predominant and main mineralised veins are essentially located within extensional pull-apart structures. The structural model suggests that the normal faults were initiated along former low-angle planes associated with aplite and/or early quartz veins that subsequently controlled the opening of the pull-apart structures that represent the economic orebodies. Zircon from the granite and muscovite grains from mineralised veins were dated by U-Pb and Ar-Ar methods, respectively. Zircons from the main facies (medium grained: GEM and microgranite: GEF) provided undistinguishable ages which were pooled to give an age of 611.9 ± 3.6 Ma. A less abundant, leucocratic facies (“white granite”: GEB) yielded a significantly younger age of 592.8 ± 7.1 Ma. Muscovites from quartz veins gave Ar-Ar ages of 612.9 ± 2 to 608.8 ± 2 Ma (transitional vein with a K-feldspar border), 611.7 ± 2 to 608.8 ± 2 Ma (mineralised veins) and 608.4 ± 2 Ma (barren quartz vein). Thus, the Passa Três granite and its gold mineralisation appear to represent a unique example of an intrusion-related gold system in which the mineralisation concentrates in the core of the magmatic intrusion in the form of meter-scale veins strongly controlled by tectonics.
