Low temperature recrystallisation of alluvial gold in paleoplacer deposits

Detrital gold particles in paleoplacer deposits develop recrystallised rims, with associated expulsion of Ag, leading to the formation of Ag-poor rims which have been recognised in most placer gold particles around the world. Recrystallisation is facilitated by accumulation of strain energy as the gold particles are deformed, particularly on particle margins, during transportation in a fluvial system. The recrystallisation process ensues after sedimentary deposition and can occur at low temperatures (<40 °C) over long geological time scales (millions of years). In the Otago placer goldfield of southern New Zealand, paleoplacers of varying ages contain gold with varying transport distances and these display differing degrees of rim formation. Narrow (1–10 µm) recrystallised rims with 0–3 wt% Ag formed on gold particles that had been transported <10 km from their source and preserved in Eocene sediments. Relict, coarse grained (∼100 µm) gold particle cores have 3–10 wt% Ag, which is representative of the source gold in nearby basement rocks. Gold in the Miocene paleoplacers was recycled from the Eocene deposits and transported >20 km from their source. The gold particles now have wider recrystallised rims (up to 100 µm), so that some particles have essentially no relict cores preserved. Gold in Cretaceous paleoplacers have wide (∼100 µm) recrystallised low-Ag rims, even in locally-derived particles, partly as a result of diagenetic effects not seen in the younger placers. Gold particles in all the paleoplacers have delicate gold overgrowths that are readily removed during recycling, but are replaced by groundwater dissolution and reprecipitation on a time scale of <1 Ma. The recrystallisation that leads to Ag-poor rim formation is primarily related to the amount of deformation imposed on particles during sedimentary transport, and is therefore broadly linked to transport distance, but is also partly controlled by the age of the paleoplacer on time scales of tens of millions of years. Gold particles that have been derived directly from basement sources can retain their original composition for long distances (tens to hundreds of kilometres) in a river system, with only minor recrystallised rim development. Gold particles that have been recycled through paleoplacer deposits can lose this link to source composition after relatively short transport distances because of extensive recrystallisation.     

The Niassa Gold Belt,northern Mozambique – A segment of a continental-scale Pan-African gold-bearing structure?

The Niassa Gold Belt, in northernmost Mozambique, is hosted in the Txitonga Group, a Neoproterozoic rift sequence overlying Paleoproterozoic crust of the Congo–Tanzania Craton and deformed during the Pan-African Orogeny. The Txitonga Group is made up of greenschist-facies greywacke and schist and is characterized by bimodal, mainly mafic, magmatism. A zircon U–Pb age for a felsic volcanite dates deposition of the sequence at 714 ± 17 Ma. Gold is mined artisanally from alluvial deposits and primary chalcopyrite-pyrite-bearing quartz veins containing up to 19 ppm Au have been analyzed. In the Cagurué and M’Papa gold fields, dominantly N–S trending quartz veins, hosted in metagabbro and schist, are regarded as tension gashes related to regional strike-slip NE–SW-trending Pan-African shear zones. These gold deposits have been classified as mesozonal and metamorphic in origin. Re–Os isotopic data on sulfides suggest two periods of gold deposition for the Cagurué Gold Field. A coarse-crystalline pyrite–chalcopyrite assemblage yields an imprecise Pan-African age of 483 ± 72 Ma, dating deposition of the quartz veins. Remobilization of early-formed sulfides, particularly chalcopyrite, took place at 112 ± 14 Ma, during Lower Cretaceous Gondwana dispersal. The ～483 Ma assemblage yields a chondritic initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.123 ± 0.058. This implies a juvenile source for the ore fluids, possibly involving the hosting Neoproterozoic metagabbro. The Niassa Gold Belt is situated at the eastern end of a SW–NE trending continental-scale lineament defined by the Mwembeshi Shear Zone and the southern end of a NW–SE trending lineament defined by the Rukwa Shear Zone. We offer a review of gold deposits in Zambia and Tanzania associated with these polyphase lineaments and speculate on their interrelation.     

Texture and chemistry of pyrite at Chah Zard epithermal gold–silver deposit,Iran

Gold mineralization at Chah Zard, Iran, is mostly concentrated in breccia and veins, and is closely associated with pyrite. Optical and scanning electron microscopy-backscattered electron observations indicate four different pyrite types, each characterized by different textures: porous and fractured py1, simple-zoned, oscillatory-rimmed, framboidal and fibrous py2, colloform py3, and inclusion-rich py4. Laser ablation ICP–MS analysis and elemental mapping reveal the presence of invisible gold in all pyrite types. The highest concentrations (161–166 ppm Au) are found in py2 and py4, which correlate with the highest As concentrations (73,000–76,000 ppm). In As-poor grains, Au concentrations decrease by about two orders of magnitude. Copper, Pb, Zn, Te, Sb, and Ag occur with invisible gold, suggesting that at least part of the gold occurs in nanoparticles of sulfosalts of these metals and metalloids. Gold distribution patterns suggest that only negligible Au was originally trapped in py1 from the initial ore fluids. However, most, if not all, Au was transported and deposited during subsequent overprinting hydrothermal fluid flow in overgrowth rims around the margins of the py2 and within microfractures of py4 grains. Oscillatory zonation patterns for Co, Ni, Sb, Cu, Pb, and Ag in pyrite reflect fluctuations in the hydrothermal fluid chemistry. The LA-ICP–MS data reveal that Cu, Pb and Ag show systematic variations between different pyrite types. Thus, Cu/Pb and Pb/Ag ratios in pyrite may provide a potentially powerful exploration vector to epithermal gold mineralization at Chah Zard district and elsewhere.     

The world-class Wona-Kona gold deposit,Burkina Faso

The world-class > 4 Moz Wona-Kona gold deposit is hosted within the Paleoproterozoic Birimian Houndé greenstone belt which is the most important gold mineralized belt in the western part of Burkina Faso, with a cumulative reserve of ~ 11 Moz. The mineralization consists of a pervasive silicification with disseminated pyrite–arsenopyrite crosscut by quartz–carbonate veinlets (1 to 10 cm wide) forming a vertical, thick (up to 40 m) and laterally extensive (5 km) northeast trending orebody hosted within a large (200 m wide) shear zone of regional extent. Gold occurs in association with 3 generations of pyrite and 2 generations of arsenopyrite. Free gold, interpreted as the last mineralizing event, occurs as late fracture filling in the pervasive silicification zone.     

Trace element and sulfur isotopic evidence for redox changes during formation of the Wallaby Gold Deposit,Western Australia

Wallaby is a major gold deposit of the St Ives Gold field of Western Australia, with an estimated resource of 8 million ounces of gold. It has a well-established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time; from hematite to magnetite. Micro-analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions. The sulfur isotope composition has a δ³⁴S range of −7.7 to +9.8‰ using 3 μm spots on the Sensitive High Resolution Iron Micro Probe-Stable Isotope (SHRIMP-SI). Negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite (which coexists with hematite) that also contains high concentrations of As, Ni, Zn, Ag, Sb, Cu and Pb. Conversely, positive values representative of reduced sulfur are found in later generations of pyrite, with lower concentrations of As, Ni, Sb, Cu, Zn and Pb. These later pyrite crystals display higher As/Ni, As/Sb, and As/Bi, and lower Cu/Te. These geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure. Previous studies suggested a single orogenic event formed the Wallaby Deposit. This is not supported by the present study. Trace element ratios such as As/Ni clearly delineate the high Au generations and could be used as an exploration tool. It is suggested that pyrite from the Wallaby Gold Deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated. Utilizing pyrite to link the entire fluid history of Wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold-bearing fluid systems.     

Geology,geochronology and fluid characteristics of the Pingqiu gold deposit,Southeastern Guizhou Province,China

The junction of the southeastern Guizhou, the southwestern Hunan, and the northern Guangxi regions is located within the southwestern Jiangnan orogen and forms a NE-trending ∼250 km gold belt containing more than 100 gold deposits and occurrences. The Pingqiu gold deposit is one of the numerous lode gold deposits in the southeastern Guizhou district. Gold mineralization is hosted in Neoproterozoic lower greenschist facies metamorphic rocks and controlled by fold-related structures. Vein types present at Pingqiu include bedding-parallel and discordant types, with saddle-reefs and their down limb extensions dominating but with lesser discordant types. The major sulfide minerals are arsenopyrite and pyrite, with minor sphalerite, galena, chalcopyrite, and rare pyrrhotite, marcasite, and tetrahedrite. Much of the gold is μm- to mm-sized grains, and occurs as fracture-controlled isolated grains or filaments in quartz, galena, sphalerite, pyrite, and wallrock.Three types of fluid inclusions are distinguished in hydrothermal minerals. Type 1 aqueous inclusions have homogenization temperatures of 171–396 °C and salinities of 1.4–9.8 wt% NaCl equiv. Type 2 aqueous-carbonic inclusions yield final homogenization temperatures of 187–350 °C, with salinities of 0.2–7.7 wt% NaCl equiv. Type 3 inclusions are carbonic inclusions with variable relative content of CO₂ and CH₄, and minor amounts of N₂ and H₂O. The close association of CO₂-rich inclusions and H₂O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. The calculated δ¹⁸O_H2O values range from 4.3‰ to 8.3‰ and δD_H2O values of fluid inclusions vary from −55.8‰ to −46.9‰. A metamorphic origin is preferred on the basis of geological background and analogies with other similar deposit types.Two ore-related sericite samples yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 425.7 ± 1.7 Ma and 425.2 ± 1.3 Ma, respectively. These data overlap the duration of the Caledonian gold mineralization along the Jiangnan orogen, and suggest that gold mineralization was post-peak regional metamorphism and occurred during the later stages of the Caledonian orogeny.Overall, the Pingqiu gold deposit displays many of the principal characteristics of the Bendigo gold mines in the western Lachlan Orogen (SE Australia) and the Dufferin gold deposit in the Meguma Terrane (Nova Scotia, Canada) but also some important differences, which may lead to the disparity in gold endowment. However, the structural make-up at deposit scale, and the shallow mining depth at present indicate that the Pingqiu gold deposit may have considerable gold potential at depth.     

Gold losses by cementation and thermal reduction in the gold recovery circuits

In this study, gold losses in a carbon-in-pulp (CIP) cyanidation gold recovery process and potential sources of these losses were investigated. Gold was found in samples from different streams through the CIP-cyanidation process, pointing to incidental losses. Mineralogical studies showed that gold losses occurred in two main forms, either as attached to larger entities or in the form of dendritic precipitates. SEM and EDS studies revealed that iron bearing minerals acted as the major media in cases when gold associations were observed as losses. The highly alkaline pH (≈ 13), elevated process temperature (≈ 145 °C), and high cyanide concentration (≥ 250 ppm) in the elution column along with a fine iron bearing material implied that gold attachment occurred through an electrochemical cementation mechanism. It was anticipated that the presence of iron in the process, which facilitated gold cementation, relied on the oxidative breakdown of the iron bearing minerals in the ore and/or due to the formation of porous iron oxides due to the roasting of iron sulfides in the regeneration kiln. In the elution column some part of the auro–cyanide complexes would remain non-eluted and be discharged into the carbon generation kiln and the carbon generation kiln was another section promoting gold losses. The high temperature condition in the carbon regeneration kiln (> 500 °C) caused thermal reduction of the non-eluted auro–cyanide complexes to metallic gold, leading to the formation of dendritic gold precipitates and their eventual loss.     

Flotation kinetics of a pyritic gold ore

Kinetics of semi-batch flotation behavior of a gold ore from North-Western Quebec, Canada was investigated with respect to fineness of grind (P₈₀). The gold was mostly associated with pyrite in native form as inclusions and was also in contact with pyrite along the boundaries of non-sulphide gangue. Gold recoveries were found to increase from 91.8% to 95.8% as the particles size decreased from an F₈₀ of 205 μm to 53 μm. Data treatment according to the graphical method of kinetic analysis indicated a much better fit to second order kinetics for both gold and pyrite with correlation coefficients higher than 0.998 compared to first order kinetics with correlation coefficients of less than 0.95. Variation and implication of flotation rate constants are discussed with respect to chemical conditions and particle size.     

Genesis of Neoproterozoic Au-bearing volcanogenic sulfides and quartz veins in the Ar Rjum goldfield,Saudi Arabia

The Ar Rjum goldfield is an example of late Neoproterozoic Au mineralization that is hosted by submarine arc assemblage and syn-anorogenic intrusive rocks. Apart from ancient workings, recent exploration in the goldfield defined three main targets along 3 km N–S corridor (Um Na'am, Ghazal and Wasema), and indicated that Wasema alone hosts 11.8 Mt @ 2.5 g/t Au. The majority of gold and sulfide mineralization is confined to diorite, where gold content increases with shearing, pyrite–sericite–carbonate alteration and development stockworks of quartz–carbonate–pyrite veins and stringers. Generally, the concentration of gold increases in the diorite samples that experienced variable degrees of hydrothermal alterations near local shear zones. Anomalous gold content (up to 11.76 g/t) in some metachert is the result of the remobilization of volcanogenic lattice-bound (refractory) Au into free Au due to post-metamorphic hydrothermal alterations. The chemistry of pyrite from the mineralized veins and stringers indicates considerable amounts of gold that reaches ~ 0.3 wt.%.Chlorite that co-exists with pyrite in the hydrothermally altered metavolcanics is mostly sheridanite with up to ~ 25 wt.% FeO^t and minor amounts of ripidolite. Chlorite geothermometry suggests that two temperature ranges affecting the area. The first temperature range (290–334 °C) is consistent with regional greenschist facies metamorphism, and the second (306–355 °C) is interpreted to be related to recrystallization-submarine hydrothermal alteration related to the gold mineralization. Stable isotope (δ³⁴S, δ¹⁸O and δ¹³C) data suggest an original volcanogenic arc signature that has been slightly modified by low-grade metamorphism, and finally by the late interaction of hydrothermal fluids. Ore evolution model for the Ar Rjum goldfield includes seafloor sulfide alteration, several deformation episodes and intrusive effects, and in this context the ore resulted from the reduction of seawater sulfates. The gold-rich veins interpreted as orogenic lode deposits are confined to localized shear zones in a syn-orogenic diorite.     

The effect of the crystal size in sample on the trace-element concentration

The dependence of trace-element concentration on the size of crystal in sample is experimentally studied by the example of gold distribution among single crystals of different sizes of hydrothermally grown pyrite, As-pyrite, and magnetite. The effect is modeled on the assumption that the Au uptake is due to a nonautonomous phase (NAP) at crystal surface. The structurally bound gold admixture is estimated from the dependence of the average content of evenly distributed gold on the specific surface of average crystal (1.5, 0.5, and 0.7 ppm for pyrite, As-pyrite with 0.02–0.08 wt.% As, and magnetite, respectively). The gold concentrations in hypothetical “pure” NAPs have been estimated by the extrapolation of the concentration dependence to the characteristic size of an NAP. The coefficients of fractionation of Au into an NAP relative to the bulk phase are 1.1 × 10³, 3.5 × 10³, and 2.4 × 10³ for pyrite, As-pyrite, and magnetite, respectively. Thus, the above effect is comparable in magnitude with the known effect of trace-element trapping by defects of crystal structure. Arsenic admixture favors the fractionation of gold into an NAP. We also considered other manifestations of this effect and its significance for solving problems of experimental geochemistry and analytical chemistry of trace elements and mineral processing. The data obtained substantiate the new mechanism of uptake of incompatible elements (including noble metals) during endogenic ore formation as more common and more effective than classical adsorption, including reducing adsorption of mercury and noble metals on mineral phases.     

Re–Os isochron ages for arsenopyrite from Carlin-like gold deposits in the Yunnan–Guizhou–Guangxi “golden triangle”, southwestern China

The Yunnan–Guizhou–Guangxi “golden triangle” is considered to be one of the regions hosting Carlin-like gold deposits in China. Gold deposits in this region can be grouped into lode type that are controlled by faults and layer-like type controlled by stratigraphy. Arsenopyrite is one of the major gold-bearing minerals in these deposits. Rhenium–Os isotopic dating of arsenopyrite from the lode type Lannigou and Jinya and the layer-like type Shuiyindong gold deposits yields isochron ages of 204 ± 19 Ma, 206 ± 22 Ma, and 235 ± 33 Ma, respectively. The data suggest that the Carlin-like gold deposits formed in Late Triassic to Early Jurassic, which is clearly earlier than the ca. 100–80 Ma acid to ultra-basic magmatism in this part of southwestern China. The ages are consistent with ore formation during a period of post-collisional lateral transpression, which is similar to that of the Carlin-like gold deposits in western Qinling of China, but quite different from Carlin-type gold deposits in Nevada, U.S.A.     

Ore-forming process of the Huijiabao gold district,southwestern Guizhou Province,China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO₂ inclusions, CO₂–H₂O inclusions, hydrocarbon inclusions, and hydrocarbon–H₂O inclusions. The ore-forming fluids are characterized by a H₂O + CO₂ + CH₄ ± N₂ system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H₂O + NaCl → H₂O + NaCl + CO₂ + CH₄ ± N₂ → H₂O + NaCl ± CH₄ ± CO₂ with a slight decrease in homogenization temperature and salinity. The δ¹⁸O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δD_H2O and calculated δ¹⁸O_H2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ¹³C_PDB and δ¹⁸O_SMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H₂O–NaCl–CO₂ fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.     

The Variscan Tamlalt–Menhouhou gold deposit,Eastern High-Atlas,Morocco

The Tamlalt–Menhouhou gold deposit belongs to the Neoproterozoic–Palaeozoic Tamlalt inlier located in the Eastern High-Atlas (Morocco). It occurs in altered Upper Neoproterozoic bimodal volcanic and volcano-sedimentary units outcropping in the Tamlalt–Menhouhou area. Gold mineralization has been identified in quartz veins related to shear-zones associated with a strong quartz-phyllic-argillic alteration. Visible free gold is related to goethite–malachite–barite boxworks in quartz veins. The other alteration minerals accompanying gold mineralization are mainly carbonates, chlorite, hematite, albite and pyrite whose relative proportion defines three alteration types. ⁴⁰Ar/³⁹Ar geochronology performed on phengite grains from phyllic alteration and the auriferous quartz veins, yields plateau ages ranging from 300 ± 5 Ma to 284 ± 12 Ma with a weighted mean age of 293 ± 7 Ma. This identifies a Late Variscan age for the Tamlalt–Menhouhou “shear zones-related” gold deposit and emphasizes the consequences of the Variscan orogeny for gold mineralization in the High-Atlas and Anti-Atlas Neoproterozoic inliers.     

The Youga gold deposit,Burkina Faso

The Youga gold deposits are located in southern Burkina Faso, close to the border with Ghana and classified as epigenetic mesothermal orogenic type gold deposits. They are hosted within or adjacent to Tarkwaian-type metasediments of the Youga Basin, composed of a succession of arkosic sandstones, conglomerates and subordinate chlorite schists. The Youga deposits are characterized by two distinct styles of mineralization; the moderately to weakly silicified host rock with quartz stockwork veining and pyrite as the predominant sulphide which generally grades between 0.5 and 2 g/t and the intensely silicified arkose with abundant quartz veins and more diverse sulphides (pyrite, arsenopyrite, chalcopyrite, pyrrhotite and galena) which generally grades > 3 g/t. The alteration paragenesis associated with the mineralized vein stockwork is characterized by quartz, ankerite, albite, chlorite and pyrite. The first mineralization episode occurred under brittle-ductile conditions during the D1_Y deformation event characterized by E–W trending penetrative to discrete structures. Gold is concentrated in zones affected by irregular fracturing, quartz veining and occasional brecciation. Reworking of these structures during D2_Y, by N–S to NE-trending sub-vertical shear zones, lead to further economic concentration of gold found in eight individual deposits, all localized in or immediately adjacent to Tarkwaian-type sediments (Main, East, West Zone 1, 2, and 3, A2NE, NTV and Zergoré). Absolute age of mineralization is unknown as well as that of the host sediments; however stratigraphic and structural craton-wide correlations suggest that the mineralization occurred after 2110 Ma if not much later. Commercial production was achieved at the Youga Gold Mine in 2008 and as of December 31st, 2014 has produced 537,621 oz of gold.     

Aresenopyrite-pyrite paragenesis in gold deposits (thermodynamic modeling)

The compositions of parageneses including arsenopyrite (Asp), pyrite (Py), and As-pyrite (As-Py) have been calculated by the solution of primal physicochemical modeling problems. The numerical models for the interaction of Py matrix with hydrothermal solution saturated with Asp are considered for three variants of the solution penetration into the Py matrix: percolation, spreading, and tightening at 100-300 °C and 300 bars. It is shown that Asp forms in the zone of an ore column where fluid (solution) is predominant, with the Py matrix being replaced independently of the type of the solution transfer. Prevailing As and Fe complexes are considered. The calculated models for the three types of interaction show that the redox potential in the solution varies from -0.055 V at the ends of the Asp-containing ore column to -0.55 V in its central zone. This difference makes an electrochemical geochemical barrier at the interface, where metallic gold is deposited with Asp-Py ores.     

Genetic investigation and comparison of Kartaldağ and Madendağ epithermal gold deposits in Çanakkale,NW Turkey

Two epithermal gold deposits (Kartaldağ and Madendağ) located in NW Turkey have been characterized through the detailed examinations involving geologic, mineralogical, fluid inclusion, stable isotope, whole-rock geochemistry, and geochronology data.The Kartaldağ deposit (0.01–17.65 ppm Au), hosted by Eocene dacite porphyry, is associated with four main alteration types with characteristic assemblage of: i) chlorite/smectite–illite ± kaolinite, ii) quartz–kaolinite, iii) quartz–alunite–pyrophyllite, iv) quartz–pyrite, the last being characterized by three distinct quartz generations comprising massive/vuggy (early), fine–medium grained, vug-lining (early), and banded, colloform, comb (late) textures. Observed sulfide minerals are pyrite, covellite, and sphalerite. Oxygen and sulfur isotope analyses, performed on quartz (δ¹⁸O_(quartz): 7.93 to 8.95‰ and calculated δ¹⁸O_(H2O): − 7.95 to 1.49‰) and pyrite (δ³⁴S_(pyrite): − 4.8‰ and calculated δ³⁴S_(H2S): − 6.08 to − 7.20‰) separates, suggest a meteoric water source for water in the hydrothermal fluid, and an igneous source for the sulfur dissolved in ore-related fluids. Microthermometric analyses of primary fluid inclusion assemblages performed on quartz (late quartz generation) yield temperatures (Th) dominantly in the range of 245–285 °C, and generally low salinity values at 0 to 1.7 wt.% NaCl eq. Based on the quartz textures and the associated base metal concentrations, along with fluid inclusion petrography, the early vug-lining quartz is considered to have been associated with the mineralization possibly through a boiling and a late mixing process at > 285 °C.The Madendağ deposit (0.27–20.60 ppm Au), hosted by Paleozoic mica schists, is associated with two main alteration types: sericite–illite–kaolinite, and quartz–pyrite dominated by two distinct quartz generations i) early colloform, comb and banded quartz and ii) late quartz, forming the cement in hydrothermal breccia. Whereas oxygen isotope analyses of quartz (δ¹⁸O_(quartz): 9.55 to 18.19‰ and calculated δ¹⁸O_(H2O): − 2.97 to 5.54‰) suggest varying proportions of meteoric and magmatic sources for the ore bearing fluid, sulfur isotope ratios (δ³⁴S_(pyrite): − 2.2‰ and calculated δ³⁴S_(H2S): (− 3.63) to (− 3.75) ‰) point to an essentially magmatic source for sulfur with or without contribution from sedimentary sources. Microthermometric analysis carried out on primary fluid inclusion populations of a brecciated sample (early quartz), give a temperature (Th) range of 235–255 °C and 0.0 to 0.7 wt.% NaCl eq. salinity. Based on the textural relationship, base metal and high gold contents, the ore precipitation stage is associated with late stage quartz formation via a possible boiling process.The presence of alunite, pyrophyllite and kaolinite, vuggy quartz and covellite suggest a high-sulfidation type of epithermal deposit for Kartaldağ. On the other hand, Madendağ is identified as an adularia-sericite type owing to the presence of significant sericite, neutral pH clays (mostly illite, chlorite/smectite, and kaolinite), low temperature quartz textures (e.g., colloform, comb, and banded quartz), and limited sulfide minerals.Given the geographical proximity of Kartaldağ and Madendağ deposits, the similar temperature and salinity ranges obtained from their fluid inclusions, and the similar ages of igneous rocks in both deposits (Kartaldağ: 40.80 ± 0.36 to 42.19 ± 0.45 Ma, Madendağ: 43.34 ± 0.85 Ma) the mineralizing systems in both deposits are considered to be genetically related.     

Oxidative pre-treatment in thiosulphate leaching of sulphide gold ores

A diagnostic leaching showed that partial oxidation of the sulphide minerals in a gold ore was beneficial for thiosulphate leaching of gold. A pre-treatment process with oxidative ammoniacal solution enhanced the thiosulphate leaching of the sulphide ore, while the thiosulphate consumption was substantially reduced. The sulphide minerals partially decomposed in the pre-treatment process, exposing gold to the leach solution. Oxygen input by air bubbling and a longer contact time enhanced the oxidative ammonia pre-treatment process and hence accelerated subsequent thiosulphate leaching of the sulphide ore. Gold extraction in 0.8 M ammonia and 0.1 M thiosulphate solution after 24 h increased from 69% without pre-treatment to 81%, 84%, 90% and 94% respectively after 1, 3, 7 and 22 h pre-treatment. The consumption of sodium thiosulphate was 2.37 kg/t after 24 h leaching without pre-treatment, but was negligible after over 1 h oxidative ammonia pre-treatment. A counter-current leaching process was conducted in the leaching of the sulphide ore. The fresh leachant still gave higher leaching rates in contact with the pre-leached ore, while the pre-used leachant had significantly lower leaching kinetics and overall gold extraction in contact with the fresh ore. This 2-step counter-current leaching process proved that the leachant, other than the passivation, was the determinant factor causing the gold leaching rates to decrease after a certain time of leaching. The findings enable the thiosulphate leaching of high sulphide containing gold ores to be more efficient at lower thiosulphate consumption following the oxidative ammoniacal pre-treatment.     

Geology and geochemistry of the sediment-hosted Cheshmeh-Konan redbed-type copper deposit,NW Iran

The several-hundred-m-thick Miocene Upper Red Formation in northwestern Iran hosts stratiform and fault-controlled copper mineralization. Copper enrichment in the percent range occurs in dm-thick carbonaceous sandstone and shale units within the clastic redbed sequence and consists of fine-grained disseminated copper sulfides (chalcopyrite, bornite, chalcocite) and supergene alteration minerals (covellite, malachite and azurite). The copper mineralization formed after calcite cementation of the primary rock permeability. Copper sulfides occur mainly as replacement of diagenetic pyrite, which, in turn, replaced organic matter. Electron microprobe analysis on bornite, chalcocite and covellite identifies elevated silver contents in these minerals (up to 0.12, 0.72 and 1.21 wt%, respectively), whereas chalcopyrite and pyrite have only trace amounts of silver (<0.26 and 0.06 wt%, respectively). Microthermometric data on fluid inclusions in authigenic quartz and calcite indicate that the Cu mineralization is related to a diagenetic fluid of moderate-to low temperature (Th = 96–160 °C) but high salinity (25–38 wt% CaCl₂ equiv.). The range of δ³⁴S in pyrite is −41.9 to −16.4‰ (average −31.4‰), where framboidal pyrite shows the most negative values between −41.9 and −31.8‰, and fine-grained pyrite has relatively heavier δ³⁴S values (−29.2 to −16.4‰), consistent with a bacteriogenic derivation of the sulfur. The Cu-sulfides (chalcopyrite, bornite and chalcocite) show slightly heavier values from −14.6 to −9.0‰, and their sulfur sources may be both the precursor pyrite-S and the bacterial reduction of sulfate-bearing basinal brines. Carbonates related to the ore stage show isotopically light values of δ¹³C_V-PDB from −8.2 to −5.1‰ and δ¹⁸O_V-PDB from −10.3 to −7.2‰, indicating a mixed source of oxidation of organic carbon (ca. −20‰) and HCO₃⁻ from seawater/porewater (ca. 0‰). The copper mineralization is mainly controlled by organic matter content and paleopermeability (intragranular space to large fracture patterns), enhanced by feldspar and calcite dissolution. The Cheshmeh-Konan deposit can be classified as a redbed-type sediment-hosted stratiform copper (SSC) deposit.     

Electron microprobe analyses of ore minerals and H–O,S isotope geochemistry of the Yuerya gold deposit,eastern Hebei,China: Implications for ore genesis and mineralization

The Yuerya gold deposit in eastern Hebei Province, China, is located on the eastern margin of the North China Craton and is hosted by Mesozoic Yanshanian granitoid rocks and adjacent Mesoproterozoic Gaoyuzhuang Formation carbonates. The auriferous quartz veins in this deposit are dominated by pyrite, with subordinate sphalerite, chalcopyrite, and galena in a quartz-dominated gangue that also contains calcite, dolomite, barite, apatite, and fluorite. Gold is present as native gold and electrum, which are generally present as micron-size infillings in microfissures within pyrite and less commonly as tiny inclusions within pyrite, quartz, and tellurobismuthite. The pyrite in this deposit has high Co/Ni ratios and contains elevated concentrations of both of these elements, suggesting that the Yuerya gold deposit has a magmato-hydrothermal origin and that the ore-forming fluids that formed the deposit leached trace elements such as Co, Ni, As, and Au during passage through Archean metamorphic rocks, Mesoproterozoic carbonates, and the Yanshanian Yuerya granitoid. Pyrite in the study area has S/Se ratios and S isotopic compositions that suggest that the sulfur (and by inference the gold) within the deposit was sourced from magmato-hydrothermal fluids that were probably originally derived from Archean metamorphic rocks and Yanshanian granitoids. Tellurobismuthite in the study area is closely intergrown with gold and was the single telluride phase identified during this study. The fineness of gold associated with tellurobismuthite is greater than the fineness of gold associated with pyrite, although the fine particle size of the gold surrounded by tellurobismuthite means that the recovery of this gold is difficult, in turn meaning that the tellurobismuthite has little significance to the economics of the Yuerya gold deposit. Only trace amounts of sulfides are associated with the tellurobismuthite within the Yuerya gold deposit, suggesting that this mineral was deposited under conditions of low fS₂ and/or high fTe₂. In addition, the presence of tellurides within the Yuerya gold deposit reflects a genetic relationship between the deposit and magmatism. Quartz from mineralized veins in the study area has δ¹⁸O values of 11.2‰–12.9‰ and the fluids that formed these veins have δD values of − 78.3‰ to − 72.1‰. The δ³⁴S values of pyrite within the deposit are rather restricted (2.3‰–3.5‰). These data, combined with the trace element geochemistry of sulfides within the deposit, suggest that the formation of the Yuerya gold deposit was closely related to both Archean metamorphic rocks and the Yanshanian Yuerya granitoid.     

Uraniferous paleoplacers of the Mesoarchean Mahagiri Quartzite,Singhbhum craton,India: Depositional controls,nature and source of > 3.0 Ga detrital uraninites

The quartz-pebble conglomerate (QPC)-hosted detrital uranium mineralization is unique in character in terms of their restricted distribution before 2.2 Ga atmosphere during pre-Great Oxidation Event (pre-GOE). Such QPC paleoplacer deposits over the world are good targets for moderate to high tonnage and low grade uranium deposits and more importantly for their gold content. The Mahagiri Quartzite, dated c. 3.02 Ga for their youngest detrital zircon population, is developed unconformably over the Mesoarchean Singhbhum Granite (3.44 Ga to 3.1 Ga). The Mahagiri Quartzite includes a conglomerate-pebbly sandstone dominated subaerial alluvial fan to coastal braided plain sequence in the lower parts and shallow marine mature quartz arenite in the upper parts. The alluvial fan-braided plain deposits in the lower parts host a number of pyritiferous and uraniferous conglomerate and pebbly sandstone beds. The uraninite grains are rounded to subrounded in outline suggesting mechanical transport and detrital origin. Together with detrital pyrite and uraninite constitute the example of > 3.0 Ga paleoplacer closely comparable to the Witwatersrand Au–U deposits. EPMA and SEM-EDS studies suggest that the uraninite grains are rich in Th (> 4 wt.%), S and REE-Y. Chemical formula calculations from EPMA analyses suggest uraninite grains belong to two populations with different oxidation states as revealed from Y/REE and cation U^4 +: U^6 + [apfu] ratios. The U contents of the detrital uraninite grains from Mahagiri are significantly lower than that of the ideal stoichiometric composition of UO₂. This is mainly due to higher amount of heterovalent cationic substitution by Th, REE, Y, Pb, and Ca in Mahagiri QPC uraninite structures, and partial alteration and metamictization of uraninites. Alteration due to metamictization resulted in elevated concentration of Si, Al, P, and Ca in more altered and metamict uraninite grains. The REE pattern is typically flat with comparable LREE–HREE concentration. The high Th content flat REE-pattern suggests that the uraninitere presents high temperature phases (> 350 °C) and are magmatic in origin. The Mahagiri detrital uraninite grains suggest existence of highly felsic and K-rich (richer than TTG) granodiorite–granite–monzogranite suites (GGM) of rocks older than 3.1 Ga in the Singhbhum craton.