Lode Gold Deposits

Both gold-rich sulphides and ultra-high grade native gold oreshoots are common but poorly understood phenomenon in orogenic-type mineral systems, partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations. The world-class Obuasi gold deposit, Ghana, has gold-rich arsenopyrite spatially associated with quartz veins, which have extremely high, localised concentrations of native gold, contained in micro-crack networks within the quartz veins where they are folded. Here, we examine selected samples from Obuasi using a novel combination of quantitative electron backscatter diffraction analysis, ion microprobe imaging, synchrotron XFM mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations. The auriferous arsenopyrites are shown to have undergone partial replacement (>15%) by Au-poor, nickeliferous arsenopyrite, during localised crystal-plastic deformation, intragranular microfracture and metamorphism (340–460 °C, 2 kbars). Our results show the dominant replacement mechanism was pseudomorphic dissolution-reprecipitation, driven by small volumes of an infiltrating fluid that had relatively low f S 2 and carried aqueous NiCl 2. We find that arsenopyrite replacement produced strong chemical gradients at crystal-fluid interfaces due to an increase in f S2 during reaction, which enabled efficient removal of gold to the fluid phase and development of anomalously gold-rich fluid (potentially 10 ppm or more depending on sulphur concentration). This process was facilitated by precipitation of ankerite, which removed CO2 from the fluid, increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation. Gold re-precipitation occurred over distances of 10 lm to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides. We suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic/ultramafic rocks, which act as a source of Ni and Co scavenged by chloride-bearing fluids. Both the preference of the

The Jílové deposit in the central part of the Bohemian Massif represents a vein to stockwork type of orogenic-type gold deposit. It is hosted by Neoproterozoic rocks of the Jílové Belt and by various magmatic dikes related to the ~355 to ~335 Ma Central Bohemian Plutonic Complex. The deposit is situated along the terrane boundary of the Teplá Barrandian and Moldanubian units. The deposit offered an exceptional opportunity to trace O, C, S and Sr stable isotope evolution of parent fluids based on combined mineralogical and geochemical study of carbonate, quartz, scheelite, and sulfide minerals, which represent six stages of mineralization, including the gold-bearing event. Stable isotope data and mineral and isotope thermometry indicate gangue and ore mineral formation between ~350 °C and b100 °C, which can be divided into 6 stages. Scheelite-bearing assemblages (stages 2–3) precipitated at 292 ± 8 °C from a fluid with calculated values: δ 18 O SMOW = +4.2 ± 0.5‰ and δ 13 C PDB = −11 ± 1‰. Gold precipitation (stage 5) probably started at about 300 °C, but the major event probably occurred at 230 ± 30 °C from a fluid with more variable isotope values (δ 18 O SMOW = +2.5 to +5‰ and δ 13 C PDB = −9 to −13.5‰). The carbon speciation was characterized by predomination of dissolved CO 2 (H 2 CO 3ap.) in the fluids. Some gold, however, undoubtedly precipitated from bicarbonate dominated fluids even at b120 °C. Extreme variations in the δ 18 O values of carbonate minerals, obtained from sampling profiles across individual veins with macroscopic gold, revealed severe thermal gradients during vein formation (~50 to ~ 100 °C difference of crystallization temperatures between the vein margin and core). The sulfur stable isotope composition of sulfide minerals indicates the dominant role of sulfur remobilization from Neoproterozoic rocks and stratiform mineralizations of the Jílové Belt by Variscan hydrothermal fluids. Similarly, the Sr-isotope composition of carbonates indicates both relatively primitive (87 Sr/ 86 Sr = 0.7055) and more evolved (87 Sr/ 86 Sr ~ 0.7090) fluid compositions, probably indicating fluid exchange with the Jílové Belt and the Central Bohemian Plutonic Complex rocks, respectively. Age determination of hydrothermal muscovite (related to stage 2) via 40 Ar/ 39 Ar indicated an age of 339.0 ± 1.5 Ma for the quartz veins. The mineralization is essentially coeval with the late intrusive phases of the Central Bohemian Plutonic Complex (i.e. the ultrapotassic suite) and with late-orogenic large-scale tec-tonic movements at the boundary between the two crustal terranes (Teplá-Barrandian and Moldanubian). Based on evaluation of the available age data on the hydrothermal and magmatic activity within the broader area of the Central Bohemian Plutonic Complex, we suggest two intervals of gold mineralization: 347 to 341 Ma and 340 to 337 Ma. The former interval overlaps with the intrusive activity of the Blatná high-K suite (granodiorite). The associated gold deposits (Mokrsko and Petráčkova hora) exhibit strong affiliation to the intrusion-related-gold-type deposit. The later interval overlaps with the ultrapotassic magmatism and is associated with more or less " classical " orogenic-gold-type deposits (Jílové, Bělčice, Libčice deposits).

Ore mineralogy of the Kedrovskoe-Irokinda ore field (northern Transbaikalia) has been studied. The ore field comprises ca. 200 quartz veins. Vein 3 and the Kvartsevaya and Serebryakovskaya veins of the Irokinda deposit and the Shamanovskaya, Pineginskaya, Osinovaya, and Barguzinskaya veins of the Kedrovskoe deposit have been described. Quartz-pyrite assemblage (quartz-1, pyrite, pyrrho-tite, and marcasite) and quartz-gold-sulfide assemblage (quartz-2, galena, chalcopyrite, sphalerite, electrum, fahlore, Ag tellurides, and sulfosalts of Ag, Cu, Sb, Pb, and Sn) have been revealed. Major ore minerals were investigated by EPMA and LA-ICP-MS. An increase in Ag content in electrum (from 5.5 to 72.4 wt.%) and fahlores (from 5 to 35 wt.%) and in the abundance of Ag minerals during the ore formation has been established. Galena contains impurities of Sb and Ag (thousands of ppm), Se, Cd, Te, and Bi (hundreds of ppm), Cu, Zn, As, and Sn (tens of ppm). It is shown that the Kedrovskoe-Irokinda ore field is a rare type of orogenic deposits with considerable variations in the composition of major ore minerals (electrum, sphalerite, and fahlores), which is explained by the diversity of the host rocks.

The Roudný gold deposit, hosted by a large-scale NNE–SSW trending fault zone (the Blanice Graben), represents a type locality of low-fineness gold (Au–Ag) mineralization in the Bohemian Massif. In order to decipher its structural evolution, we performed a detailed analysis of brittle to brittle–ductile structures on outcrops and of structures described in unpublished historical materials. Three stress phases were distinguished: compressional, strike-slip, and extensional. Most hydrothermal veins originated during the strike-slip and extensional phases. Based on a comparison with the nearby Ratibořské Hory–Stará Vožice deposit, a representative of Ag–Pb–Zn vein type mineralization in the Blanice Graben, we conclude that the structural pattern of both ore deposits/mineralizations (Ag–Pb–Zn and Au–Ag) is compatible with the overall sinistral strike-slip evolution of the Blanice Graben (c. 280–270 Ma). Whereas most of the Ag–Pb–Zn veins represent typical extension veins/fractures, early...

Tectonic processes and the resulting deformation of Earth's crust are known to control and influence the location as well as shape of ore bodies. Furthermore, ore body geometries and associated spatial relationships provide fundamental clues to the genesis of ore deposits. Therefore, it is important to establish consistent 3D geological models to visualise and analyse ore delineation in order to understand the controls on mineralisation processes. Implicit modelling generates objective 3D geological models directly from drill-hole data. Instead of manual linkage of hand-digitised 2D cross-sections, mathematical interpolation functions are used to generate 3D isosurfaces. These surfaces can represent ore grade shells, lithological boundaries or structural trends. The 3D implicit model can be used to define ore body geometries, grade continuity direction(s) and spatial relationships between mineralisation, lithological boundaries and major structures, helping to explain the struct...

The chemical and physical properties of carbonaceous matter (CM) were studied in Paleoproterozoic metasediment-hosted, orogenic-type gold deposits in Burkina Faso (the Inata deposit), in Mali (the Syama deposit), and in Ghana (the Obuashi and Bogoso deposits). Two types of CM occur in all the studied deposits: metamorphosed and hydrothermal. Metamorphosed CM prevails in all the deposits. Hydrothermal CM occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub-parallel to C-type cleavage within the shear zones. The origin of hydrothermal CM, which occurs in paragenesis with Au-bearing arsenopyrite or pyrite, seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels. The isotopic composition of carbon in bulk CM (−33.1 to −26.2‰, VPDB) indicates its biogenic origin. The isotopic composition of carbon in hydrothermal carbonates ranges from −14.5 to −4.4‰ (VPDB), which suggest mixing of carbon derived from a deep-seated source with carbon derived from an organic source. The interaction of hydrothermal fluids with metamorphosed CM could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization. The optical properties and Raman spectra of the metamorphosed CM particles in the individual studied mineral deposits differ considerably. The temperatures calculated on the basis of the Raman spectra of metamorphosed CM vary between 280 and 440 • C, depending on the thermometer used, and correspond to temperatures of meta-morphism of upper sub-grenschist and greenschist facies. The temperatures calculated for hydrothermal CM at the individual deposits, are only slightly lower compared to the metamorphosed CM at the same deposits, which indicates approximately the same temperature of the metamorphic and hydrothermal processes.