The granite‑hosted Variscan gold deposit from Santo António mine in the Iberian Massif (Penedono, NW Portugal): constraints from mineral chemistry, fuid inclusions, sulfur and noble gases isotopes

The study area is located in the Central Iberian Zone, a major tectonic unit of the Iberian Massif (Variscan belt). In this region the basement is composed of Cambrian-Ordovician sedimentary and minor volcanic rocks that underwent deformation and metamorphism during the Carboniferous. These metamorphic rocks host ca. 331–308 Ma granitic plutons emplaced during the D2 extensional and D3–D4 contractional deformation phases. The gold-bearing quartz veins from the Santo António mine (Penedono region) occur in granite formed at 310.1 ± 1.1 Ma and post-dated the peak of metamorphism. Gold–silver alloy is included in quartz, but mainly occurs in spaces between grains or micro-fractures within arsenopyrite of all three generations and less in pyrite. Late sulphides and sulphosalts were deposited along fractures mainly in arsenopyrite, and locally surrounding the gold–silver alloy grains. Ferberite, scheelite and stolzite replace arsenopyrite. The abundant aqueous carbonic fluids and the occurrence of a low-salinity fluid and their minimum possible entrapment temperature of 360–380 °C suggest that this gold-forming event began during the waning stages of the Variscan orogeny. The mean δ34S values of arsenopyrite and pyrite are − 4.7‰ and − 3.8‰, respectively. He–Ar–Ne isotopic data suggest a crustal origin. The ascent of the granite magma has provided the heat for remobilization of gold, other metals and metalloids from the metamorphic rocks. This gold-arsenopyrite deposit has thus similar characteristics as other selected gold-arsenopyrite deposits from the Iberian Massif, but it contains tungstates.El área de estudio está ubicada en la Zona Centroibérica, una importante unidad tectónica del Macizo Ibérico (cinturón varisco). En esta región el basamento está compuesto por rocas sedimentarias y volcánicas del Cámbrico-Ordovícico tectonizadas y metamorfzadas durante el Carbonífero. Estas rocas metamórfcas sirven como caja de los plutones graníticos datados en torno a 331–308 Ma y que fueron emplazados durante la fase de deformación extensional D2 y las fases de deformación contraccional D3 y D4. Las venas de cuarzo ricas en oro de la mina de Santo António (región de Penedono) que aparecen en un granito datado a los 310.1 ± 1.1 Ma son posteriores al pico metamórfco regional. La aleación de oro y plata se incluye en el cuarzo, pero se produce principalmente en los espacios entre granos o micro-fracturas dentro de arsenopirita de las tres generaciones y menos en pirita. Los sulfuros y sulfuros tardíos se depositaron a lo largo de las fracturas principalmente en arsenopirita, y alrededor de los granos de aleación de oro y plata. Ferberita, scheelita y la estolzita sustituyen a la arsenopirita. Los abundantes líquidos acuosos carbónicos y la presencia de un fuido de baja salinidad y su posible temperatura de atrapamiento mínima en torno de 360-380 ºC sugieren que este evento de formación de oro comenzó durante las etapas fnales de la orogenia varisca. Los valores medios de S de arsenopirita y pirita son − 4.7 ‰ y − 3.8 ‰, respectivamente. Los datos isotópicos de He–Ar–Ne sugieren que en el origen de los fuidos mineralizados participa la corteza continental. El ascenso del magma granítico ha provisto el calor para la movilización del oro, otros metales y metaloides desde las rocas metamórfcas. Este depósito de oroarsenopirita tiene así características similares a otros yaciamientos con arsenopirita y oro del Macizo Ibérico, pero sin embargo contienen tungstates.This research was financially supported by Fundação para a Ciência e Tecnologia through the projects GOLDGranites, Orogenesis, Long-term strain/stress and Deposition of ore metals—PTDC/GEO-GEO/2446/2012: COMPETE: FCOMP-01-0124-FEDER-029192 and UID/GEO/04035/2013

Phase equilibria at sub-solidus conditions in the Fe-Mg-Zn-O system in air

The phase equilibria in the Fe-Mg-Zn-O system in the temperature range 1100-1550degreesC in air have been experimentally studied using equilibration and quenching followed by electron probe X-ray microanalysis. The compositions of condensed phases in equilibrium in the binary MgO-ZnO system and the ternary Fe-Mg-O system have been reported at sub-solidus in air. Pseudo-ternary sections of the quaternary Fe-Mg-Zn-O system at 1100, 1250 and 1400degreesC in air were constructed using the experimental data. The solid solution of iron oxide, MgO and ZnO in the periclase (Mg, Zn, Fe)O, spinel (Mg2+, Fe2+, Zn2+)(x)Fe(2+y)3+O4 and zincite (Zn, Mg, Fe)O phases were found to be extensive under the conditions investigated. A continuous spinel solid solution is formed between the magnesioferrite (Mg2+, Fe2+)(x)Fe(2+y)3+O4 and franklinite (Zn2+, Fe2+)(x)Fe(2+y)3+O4 end-members at 1100 and 1250degreesC, extending to magnetite (Fe2+)(x)Fe(2+y)3+O4 at 1400degreesC in air. The compositions along the spinel boundaries were found to be non-stoichiometric, the magnitude of the non-stoichiometry being a function of composition and temperature in air. It was found that hematite dissolves neither MgO nor ZnO in air

Subsolidus Phase Relations of the CaO-WO3-CuO System at 800 °C in Air

The subsolidus phase relations of the CaO-WO3-CuO system were investigated in air. The samples were equilibrated at 800 degrees C. Under these conditions, five binary oxide phases are stable: Ca2CuO3, Cu3WO6, CuWO4, CaWO4 and Ca3WO6. The pseudo-ternary section is characterised by 6 three-phase regions. No Ca equivalents to the Sr2CuWO6, Ba2CuWO6 or Sr8CuW3O18 phases were formed

Gold scavenged by bismuth melts: An example from Alpine shear-remobilizates in the Highis Massif, Romania

The original publication can be found at www.springerlink.comGold mineralization occurs in the Şoimuş Ilii vein, the main Cu prospect in the Highiş Massif, Western Apuseni Mts., Romania. The Highiş Massif is part of the Highiş Biharia Shear Zone, a 320–300 Ma Variscan greenschist belt, with a 114–100 Ma Alpine overprint. In Highiş, phyllonites enclose an igneous core consisting of an Early Permian basic complex intruded by Middle Permian granitoids. The vein is hosted within basalt hornfels at its contact with the 264 Ma Jernova granite. Gold is not only present as native gold, but also as jonassonite (ideally AuBi5S4). The latter occurs as inclusions 1–30 µm in size in chalcopyrite; microanalysis gives the empirical formulae Au1.02(Pb0.47Bi4.51)4.98S4. The two Au minerals are spatially associated with Bi–(Pb) sulfosalts (oversubstituted bismuthinite, cosalite) and sulfotellurides/selenides (ingodite, ikunolite and laitakarite) in blebs/patches, mainly hosted in chalcopyrite. This Au–Bi–Te association overprints an earlier, chalcopyrite-quartz assemblage, occurring as trails along discrete zones of brecciation that crosscut former mineral boundaries. Curvilinear and cuspate boundary textures within the blebs/patches suggest deposition in a molten form. Mineral associations in combination with phase relations indicate that the Au–Bi–Te association formed as a result of melting of pre-existing native Bi (and possibly sulfosalts) at 400 °C under sulfidation conditions. These melts incorporated Au, Pb, Te and S as they moved in the vein during shearing and were locked within dilational sites. Native Bi occurs as coarse aggregates along vein margins, but in the Au–Bi–Te association, it is present only as small droplets in shear gashes, never together with other Bi- and Au-minerals. The Bi-derived melts are part of an internal remobilizate which also includes chlorite and adularia. Minerals in the system Au–Bi–Te were deposited from a neutral low reducing fluid during Alpine shearing in the Early Cretaceous. The fluid also assisted solid-state mobilisation of chalcopyrite and cobaltite. This study illustrates the significant potential of Bi, a low melting-point chalcophile element (LMCE), to act as Au scavenger at temperatures as low as 400 °C.C. L. Ciobanu, N. J. Cook, F. Damian and G. Damia