Relative timing of deformation and two-stage gold mineralization at the Hutti Mine, Dharwar Craton, India

Gold mineralization at Hutti is confined to a series of nine parallel, N–S to NNW–SSE trending, steeply dipping shear zones. The host rocks are amphibolites and meta-rhyolites metamorphosed at peak conditions of 660±40°C and 4±1 kbar. They are weakly foliated (S₁) and contain barren quartz extension veins. The auriferous shear zones (reefs) are typically characterized by four alteration assemblages and laminated quartz veins, which, in places, occupy the entire reef width of 2–10 m, and contain the bulk of gold mineralization. A <1.5 m wide distal chlorite-sericite (+biotite, calcite, plagioclase) alteration zone can be distinguished from a 3–5 m wide proximal biotite-plagioclase (+quartz, muscovite, calcite) alteration zone. Gold is both spatially and temporally associated with disseminated arsenopyrite and pyrite mineralization. An inner chlorite-K-feldspar (+quartz, calcite, scheelite, tourmaline, sphene, epidote, sericite) alteration halo, which rims the laminated quartz veins, is characterized by a pyrrhotite, chalcopyrite, sphalerite, ilmenite, rutile, and gold paragenesis. The distal chlorite-sericite and proximal biotite-plagioclase alteration assemblages are developed in microlithons of the S₂–S₃ crenulation cleavage and are replaced along S₃ by the inner chlorite-K-feldspar alteration, indicating a two-stage evolution for gold mineralization. Ductile D₂ shearing, alteration, and gold mineralization formed the reefs during retrograde evolution and fluid infiltration under upper greenschist to lower amphibolite facies conditions (560±60°C, 2±1 kbar). The reefs were reactivated in the D₃ dextral strike-slip to oblique-slip environment by fault-valve behavior at lower greenschist facies conditions (ca. 300–350°C), which formed the auriferous laminated quartz veins. Later D₄ crosscutting veins and D₅ faults overprint the gold mineralization. The alteration mineralogy and the structural control of the deposit clearly points to an orogenic style of gold mineralization, which took place either during isobaric cooling or at different levels of the Archean crust. From overlaps in the tectono-metamorphic history, it is concluded that gold mineralization occurred during two tectonic events, affecting the eastern Dharwar craton in south India between ca. 2550 – 2530 Ma: (1) The assemblage of various terranes of the eastern block, and (2) a tectono-magmatic event, which caused late- to posttectonic plutonism and a thermal perturbation. It differs, however, from the pre-peak metamorphic gold mineralization at Kolar and the single-stage mineralization at Ramagiri. Notably, greenschist facies gold mineralization occurred at Hutti 35–90 million years later than in the western Dharwar craton. Editorial handling: G. Beaudoin     

Genesis of the Bangbu Orogenic Gold Deposit,Tibet: Evidence from Fluid Inclusion,Stable Isotopes,and Ar-Ar Geochronology

The Bangbu gold deposit is a large orogenic gold deposit in Tibet formed during the AlpineHimalayan collision. Ore bodies(auriferous quartz veins) are controlled by the E-W-trending Qusong-Cuogu-Zhemulang brittle-ductile shear zone. Quartz veins at the deposit can be divided into three types: pre-metallogenic hook-like quartz veins, metallogenic auriferous quartz veins, and postmetallogenic N-S quartz veins. Four stages of mineralization in the auriferous quartz veins have been identified:(1) Stage S1 quartz+coarse-grained sulfides,(2) Stage S2 gold+fine-grained sulfides,(3) Stage S3 quartz+carbonates, and(4) Stage S4 quartz+ greigite. Fluid inclusions indicate the oreforming fluid was CO_2-N_2-CH_4 rich with homogenization temperatures of 170–261°C, salinities 4.34–7.45 wt% Na Cl equivalent. δ~(18)Ofluid(3.98‰–7.18‰) and low δDV-SMOW(-90‰ to-44‰) for auriferous quartz veins suggest ore-forming fluids were mainly metamorphic in origin, with some addition of organic matter. Quartz vein pyrite has δ~(34)SV-CDT values of 1.2‰–3.6‰(an average of 2.2‰), whereas pyrite from phyllite has δ~(34)SV-CDT 5.7‰–9.9‰(an average of 7.4‰). Quartz vein pyrites yield 206Pb/204 Pb ratios of 18.662–18.764, 207Pb/204 Pb 15.650–15.683, and ~(208)Pb/204 Pb 38.901–39.079. These isotopic data indicate Bangbu ore-forming materials were probably derived from the Langjiexue accretionary wedge. 40Ar/39 Ar ages for sericite from auriferous sulfide-quartz veins yield a plateau age of 49.52 ± 0.52 Ma, an isochron age of 50.3 ± 0.31 Ma, suggesting that auriferous veins were formed during the main collisional period of the Tibet-Himalayan orogen(~65–41 Ma).     

Structural control of low-sulfidation epithermal gold mineralization in the Rosario–Bunawan district, East Mindanao Ridge, Philippines

The Rosario–Bunawan district is situated about 200 km north of Davao City, the capital of the Mindanao Island, Southern Philippines. Gold is produced from the Co-O mine, containing about 2,034,000 t of ore at 10.9 g/t Au, and in numerous small-scale operations by local miners. Epithermal gold mineralization in the Rosario–Bunawan district and the Co-O mine is confined to narrow (0.2–4 m) low-sulfidation quartz–chalcedony–calcite veins in volcanic and volcaniclastic wall rocks. Three major vein orientations are distinguished: (1) the NNW–SSE-trending set with a sinistral strike-slip sense of deformation (Philippine Fault trend); (2) the ENE–WSW-trending dextral strike-slip set (Palawan trend) and associated veins in the Riedel geometry; and (3) the WNW–ESE-trending conjugate set (Co-O trend). Three structural stages are defined: (1) extensional shear or shear veins formed in the Co-O, the Philippine Fault, and Palawan trends during regional NW–SE compression and near vertical vein opening (D₁); (2) reactivation of veins in the Philippine Fault, veins associated with the Palawan, and, to a lesser extent, the Co-O trends during E–W compression and near horizontal N–S-oriented vein opening (D₂). New D₂ extensional shear or shear veins formed in the Philippine Fault, and structures associated with the Palawan and associated Riedel trends; (3) the D₃-stage block faulting subsequently displaced all of the auriferous veins. The auriferous Rosario–Bunawan district is situated between two splays of the Philippine Fault, which acted as a lateral ramp system during the oblique convergence of the Philippine Sea plate and the Eurasian plate. The oblique convergence resulted in a change from a compressional (D₁) to a transpressional (D₂) regime, which was a prerequisite for the two-stage vein opening and hydrothermal mineralization, leading to an economic gold enrichment. D₁ compressional tectonics may have caused an elevated geothermal gradient in shallow crustal levels, forming the heat source for the fluid plumbing system, which is at variance to typical epithermal deposits formed in extensional zones. D₂ thrusting of a limestone nappe together with syn-tectonic diorite intrusions may have further increased the geothermal gradient, maintaining the fluid plumbing system. The limestone nappe may, at the same time, have represented an aquitard forcing the hydrothermal fluids into the volcanic and volcaniclastic wall rocks, which is regarded as critical for the two-stage gold mineralization in the Rosario–Bunawan district.     

Genesis of the Maanqiao gold deposit in the western Qinling orogen,central China: Constraints from fluid inclusions and in-situ sulfur isotopes

The western Qinling orogen (WQO) is one of the most important prospective gold provinces in China. The Maanqiao gold deposit, located on the southern margin of the Shangdan suture, is a representative gold deposit in the WQO. The Maanqiao deposit is hosted by the metasedimentary rocks of the Upper Devonian Tongyusi Formation. The EW-trending brittle-ductile shear zone controls the orebodies; they occur as disseminated, and auriferous quartz–sulfide vein. The ore-related hydrothermal alteration comprises silicification, sulfidation, sericitization, chloritization, and carbonatization. Native gold is visible and mainly associated with pyrite and pyrrhotite. Mineralization can be classified into the following three stages: bedding-parallel barren quartz–pyrite–(pyrrhotite) (early-stage), auriferous quartz–polymetallic (middle-stage), and carbonate–(quartz)–sulfide (late-stage).Detailed fluid inclusion (FI) studies revealed three types of inclusions in quartz and calcite: aqueous (W-type), CO₂–H₂O (C-type), and pure carbonic (PC-type) FIs. The primary FIs in the early-stage quartz are C- and PC-type, in the middle-stage quartz are mainly W- and C-type, and in the late-stage calcite are only W-type. During gold mineralization, the total FI homogeneous temperatures evolved from 189–375 °C (mostly 260–300 °C) to 132–295 °C (mostly 180–240 °C) to 123–231 °C (mostly 130–150 °C), and the salinities varied among 2.2–9.1 wt.% NaCl equiv. (mostly 5–8 wt.%) to 0.2–9.0 wt.% NaCl equiv. (mostly 3–6 wt.%) to 0.3–3.6 wt.% NaCl equiv. (mostly 2–4 wt.%). The ore-forming fluid was characterized as an H₂O–NaCl−CO₂−CH₄–(N₂) system with medium-low temperature and low salinity. The fluid immiscibility and fluid-rock interaction may be responsible for the precipitation of the sulfides and gold at the Maanqiao gold deposit. Three types of pyrite corresponding to the three mineralization stages, as well as pyrrhotite and arsenopyrite in the middle stage, are micro-analyzed for in-situ sulfur isotopic composition by LA-ICP-MS. Py1 yield near-zero δ³⁴S values of −2.5‰ to 3.0‰, which are somewhat lower than that of the granite hosted pyrites (Py-g, 4.8‰ to 6.6‰). The result suggests a mixed sulfur source from magmatic-hydrothermal fluids and the metamorphism of diagenetic pyrite. Pyrite + pyrrhotite + arsenopyrite assemblages in the middle-stage have relatively higher δ³⁴S values (6.6‰ to 12.3‰) and are mainly developed due to the metamorphism of the ore-host and underlying Devonian sedimentary sequences. The low δ³⁴S values of the late-stage fracture-filled Py3 (−21.9‰ to −17.0‰) resulted from an increasing oxygen fugacity, which was caused by the inflow of oxidized meteoric waters.Based on our studies, the Maanqiao gold deposit is considered to be an orogenic type and closely related to the Indosinian Qinling orogeny.     

Geology and Metallogenesis of the Sawayaerdun Gold Deposit in the Southwestern Tianshan Mountains, Xinjiang, China

The Sawayaerdun gold deposit, located in Wuqia County, Southwest Tianshan, China, occurs in Upper Silurian and Lower Devonian low‐grade metamorphic carbonaceous turbidites. The orebodies are controlled by a series of NE‐NNE‐trending, brittle–ductile shear zones. Twenty‐four gold mineralized zones have been recognized in the Sawayaerdun ore deposit. Among these, the up to 4‐km‐long and 200‐m wide No. IV mineralized zone is economically the most important. The average gold grade is 1–6 g/t. Gold reserves of the Sawayaerdun deposit have been identified at approximately 37 tonnes and an inferred resource of 123 tonnes. Hydrothermal alteration is characterized by silicification, pyritization, arsenopyritization, sericitization, carbonatization and chloritization. On the basis of field evidence and petrographic analysis, five stages of vein emplacement and hydrothermal mineralization can be distinguished: stage 1, early quartz stage, characterized by the occurrence of quartz veins; stage 2, arsenopyrite–pyrite–quartz stage, characterized by the formation of auriferous quartz veinlets and stockworks; stage 3, polymetallic sulfide quartz stage, characterized by the presence of auriferous polymetallic sulfide quartz veinlets and stockworks; stage 4, antimony–quartz stage, characterized by the formation of stibnite–jamesonite quartz veins; and stage 5, quartz–carbonate vein stage. Stages 2 and 3 represent the main gold mineralization, with stage 4 representing a major antimony mineralization episode in the Sawayaerdun deposit. Two types of fluid inclusion, namely H₂O–NaCl and H₂O–CO₂–NaCl types, have been recognized in quartz and calcite. Aqueous inclusions show a wide range of homogenization temperatures from 125 to 340°C, and can be correlated with the mineralization stage during which the inclusions formed. Similarly, salinities and densities of these fluids range for each stage of mineralization from 2.57 to 22 equivalent wt% NaCl and 0.76 to 1.05 g/cm³, respectively. The ore‐forming fluids thus are representative of a medium‐ to low‐temperature, low‐ to medium‐salinity H₂O–NaCl–CO₂–CH₄–N₂ system. The δ³⁴S_CDT values of sulfides associated with mineralization fall into a narrow range of ?3.0 to +2.6‰ with a mean of +0.1‰. The δ¹³C_PDB values of dolomite and siderite from the Sawayaerdun gold deposit range from ?5.4 to ?0.6‰, possibly reflecting derivation of the carbonate carbon from a mixed magmatic/sedimentary source. Changes in physico‐chemical conditions and composition of the hydrothermal fluids, water–rock exchange and immiscibility of hydrothermal fluids are inferred to have played important roles in the ore‐forming process of the Sawayaerdun gold–antimony deposit.     

The origin of fluids and nature of fluid–rock interaction in mid-crustal auriferous mylonites of the Renco mine, southern Zimbabwe

Geothermometric constraints on auriferous shear zones of the Renco mine in the Northern Marginal Zone of the late-Archaean, granulite-facies Limpopo Belt in southern Zimbabwe indicate that deformation and associated mineralization occurred at temperatures of at least 600 °C up to more likely 700 °C. Mid- to upper-amphibolite facies conditions during mineralization correspond to the regional-scale retrogression of granulite facies wall rocks during the late-Archaean thrusting of high-grade metamorphic rocks of the Northern Marginal Zone onto low- to medium-grade granite-greenstone terrains of the Zimbabwe craton. Mineral assemblages indicate that the ore fluid was moderately oxidized with log f_O2 values between 10⁻¹⁷ and 10⁻¹⁸ bars with high H₂S activities of 0.25–0.75. Elements enriched in the shear zones include Au, S, Fe, Cu, Mo, Bi, Te, Ni, Co, and H₂O, Au and Cu being the most enriched. Geochemically, Au correlates with Cu but not with S, which, together with the fact that gold is only rarely intergrown or in direct contact with sulfides, possibly indicates a transport of gold as a chloride complex. The siting of gold along fractures or within implosion breccias suggests that gold was precipitated due to fluid immiscibility induced by catastrophic fluid pressure drops during seismic slip events. Fluid inclusions are predominantly CO₂ (±CH₄ ± N₂)-rich, but petrographic work indicates that fluid inclusions have undergone extensive post-entrapment modifications due to the pervasive recrystallization of mineral textures in the high-temperature shear zones. The mineralized shear zones are enriched in ¹⁸O compared to wall-rock enderbites, which is interpreted to represent an influx of externally derived fluids of probably metamorphic origin. Based on temporal and spatial relationships between mineralization, late-Archaean overthrusting of the Northern Marginal Zone onto the Zimbabwe craton, and coeval amphibolite-facies hydration of granulites, we suggest that the Renco mineralization formed in a mid-crustal environment from metamorphic fluids that were generated from dehydration of subcreted greenstone terrains of the Zimbabwe craton. Received: 27 October 1998 / Accepted: 13 August 1999     

Nature and source of the ore-forming fluids associated with orogenic gold deposits in the Dharwar Craton

Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include(1) the famous Kolar mine and the world class Hutti deposit;(2) small mines at HiraBuddini, Uti, Ajjanahalli, and Guddadarangavanahalli;(3) prospects at Jonnagiri; and(4) old mining camps in the Gadag and Ramagiri-Penakacherla belts. The existing diametric views on the source of ore fluid for formation of these deposits include fluids exsolved from granitic melts and extracted by metamorphic devolatilization of the greenstone sequences. Lode gold mineralization occurs in structurally controlled higher order splays in variety of host rocks such as mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. Estimated metamorphic conditions of the greenstones vary from lower greenschist facies to mid-amphibolite facies and mineralizations in all the camps are associated with distinct hydrothermal alterations. Fluid inclusion microthermometric and Raman spectroscopic studies document low salinity aqueous-gaseous(H_2O + CO_2 ± CH_4 + NaCl) ore fluids,which precipitated gold and altered the host rocks in a narrow P-T window of 0.7-2.5 kbar and 215-320℃. While the calculated fluid O-and C-isotopic values are ambiguous, S-isotopic compositions of pyrite-precipitating fluid show distinct craton-scale uniformity in terms of its reduced nature and a suggested crustal sulfur source.Available ages on greenstone metamorphism, granitoid plutonism and mineralization in the Hutti Belt are tantamount, making a geochronology-based resolution of the existing debate on the metamorphic vs.magmatic fluid source impossible. In contrast, tourmaline geochemistry suggests involvement of single fluid in formation of gold mineralization, primarily derived by metamorphic devolatilization of mafic greenstones and interlayered sedimentary rocks, with minor magmatic contributions. Similarly, compositions of scheelite, pyrite and arsenopyrite point toward operation of fault-valves that caused pressure fluctuation-induced fluid phase separation, which acted as the dominant process of gold precipitation,apart from fluid-rock sulfidation reactions. Therefore, results from geochemistry of hydrothermal minerals and those from fluid inclusion microthermometry corroborate in constraining source of ore fluid,nature of gold transport(by Au-bisulfide complex) and mechanism of gold ore formation in the Dharwar Craton.     

Geological and geochemical characteristics of the Baogudi Carlin-type gold district (Southwest Guizhou,China) and their geological implications

The newly discovered Baogudi gold district is located in the southwestern Guizhou Province, China, where there are numerous Carlin-type gold deposits. To better understand the geological and geochemical characteristics of the Baogudi gold district, we carried out petrographic observations, elemental analyses, and fluid inclusion and isotopic composition studies. We also compared the results with those of typical Carlin-type gold deposits in southwestern Guizhou. Three mineralization stages, namely, the sedimentation diagenesis, hydrothermal (main-ore and late-ore substages), and supergene stages, were identified based on field and petrographic observations. The main-ore and late-ore stages correspond to Au and Sb mineralization, respectively, which are similar to typical Carlin-type mineralization. The mass transfer associated with alteration and mineralization shows that a significant amount of Au, As, Sb, Hg, Tl, Mo, and S were added to mineralized rocks during the main-ore stage. Remarkably, arsenic, Sb, and S were added to the mineralized rocks during the late-ore stage. Element migration indicates that the sulfidation process was responsible for ore formation. Four types of fluid inclusions were identified in ore-related quartz and fluorite. The main-ore stage fluids are characterized by an H₂O–NaCl–CO₂–CH₄ ± N₂ system, with medium to low temperatures (180–260 °C) and low salinity (0–9.08% NaCl equivalent). The late-ore stage fluids featured H₂O–NaCl ± CO₂ ± CH₄, with low temperature (120–200 °C) and low salinity (0–7.48% NaCl equivalent). The temperature, salinity, and CO₂ and CH₄ concentrations of ore-forming fluids decreased from the main-ore stage to the late-ore stage. The calculated δ¹³C, δD, and δ¹⁸O values of the ore-forming fluids range from − 14.3 to − 7.0‰, −76 to −55.7‰, and 4.5–15.0‰, respectively. Late-ore-stage stibnite had δ³⁴S values ranging from − 0.6 to 1.9‰. These stable isotopic compositions indicate that the ore-forming fluids originated mainly from deep magmatic hydrothermal fluids, with minor contributions from strata. Collectively, the Baogudi metallogenic district has geological and geochemical characteristics that are typical of Carlin-type gold deposits in southwest Guizhou. It is likely that the Baogudi gold district, together with other Carlin-type gold deposits in southwestern Guizhou, was formed in response to a single widespread metallogenic event.

     

Multistage gold mineralization at the Lapa mine, Abitibi Subprovince: insights into auriferous hydrothermal and metasomatic processes in the Cadillac–Larder Lake Fault Zone

The Lapa gold deposit contains reserves of 2.4 Mt at 6.5 g/t Au and is one of the few deposits located directly within the Cadillac–Larder Lake Fault Zone (CLLFZ), a first-order crustal-scale fault that separates the Archean Abitibi Subprovince from the Pontiac Subprovince to the south. Gold mineralization is predominantly hosted in highly strained and altered, upper greenschist–lower amphibolite facies mafic to ultramafic rocks of the Piché Group. Auriferous ore zones consist of finely disseminated auriferous arsenopyrite–pyrrhotite?±?pyrite and native gold disseminated in biotite- and carbonate-altered wall rocks. Native gold, which is also present in quartz ± dolomite–calcite veinlets, is locally associated with Sb-bearing minerals, especially at depth ≤1 km from surface where the deposit is characterized by a Au–Sb–As association. At vertical depth greater than 1 km, gold is associated with arsenopyrite and pyrrhotite (Au–As association). The mineralogy and paragenesis of the Lapa deposit metamorphosed ore and alteration assemblages record the superposition of three metamorphic episodes (M1, M2, and M3) and three gold mineralizing events. Spatial association between biotitized wall rocks and auriferous arsenopyrite indicates that arsenopyrite precipitation is concomitant with potassic alteration. The predominant Au–As association recognized across the deposit is related to gold in solid solution in arsenopyrite as part of a pre-M2 low-grade auriferous hydrothermal event. However, the occurrence of hornblende?+?oligoclase porphyroblasts overprinting the biotite alteration, and the presence of porous clusters and porphyroblasts of arsenopyrite with native gold and pyrrhotite indicate an auriferous metasomatic event associated with peak M2 prograde metamorphism. Late retrograde metamorphism (M3) overprints the hornblende–oligoclase M2 assemblage within the host rocks proximal to ore by an actinolite–albite assemblage by precipitation of free gold and Sb–sulfosalts at lower P–T. The complex relationships between ore, structural features, and metamorphic assemblages at Lapa are related to the tectonometamorphic evolution of the Cadillac–Larder Lake Fault Zone at different times and crustal levels, and varying heat and fluid flow regimes. The Lapa deposit demonstrates that early, low-grade gold mineralization within the Cadillac–Larder Lake Fault Zone has benefited from late gold enrichment(s) during prograde and retrograde metamorphism, suggesting that multi-stage processes may be important to form gold-rich orogenic deposits in first order crustal-scale structures.     

The New Consort Gold Mine,Barberton greenstone belt,South Africa: orogenic gold mineralization in a condensed metamorphic profile

The New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa is one of the oldest recognized orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units that occur at, or close to, the contact between the mafic and ultramafic volcanic rocks of the c. 3,280 Ma Onverwacht Group and the mainly metasedimentary rocks of the overlying c. 3,260–3,230 Ma Fig Tree Group. This contact, locally referred to as the Consort Bar, formed during ductile D₁ imbrication of the metavolcanosedimentary sequence and predates the main stage of the gold mineralization. The imbricate stack is situated in the immediate hanging wall of the basal granitoid–greenstone contact along the northern margin of the greenstone belt. It is characterized by a condensed metamorphic profile in which the metamorphic grade increases from upper greenschist facies conditions (510–530°C, 4 kbar) in rocks of the Fig Tree Group to upper amphibolite facies grades (600–700°C, 6–8 kbar) in the basal Onverwacht Group. Detailed structural and petrological investigations indicate that the Consort Bar represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural sequence. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc–silicate alteration assemblage characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht Group. This early mineralization may correlate with the local D₁ deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D₂. The D₂ deformation resulted in the reactivation of earlier structures, and the formation of a NNW trending, steeply dipping shear zone system, the Shires Shear Zone, which separates two regional SE plunging D₁ synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes that have previously been dated at c. 3040 Ma. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of gold mineralization, which affected a crustal profile of ca. 1.5 km, was characterized by increasing temperatures (c. 520 to 600°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade, ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest to arsenopyrite + pyrrhotite + chalcopyrite + loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly syn-peak metamorphic with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally underlying Onverwacht Group. This indicates that the mineralization coincided with the juxtaposition of the two units. As the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence can be ruled out as the source of the mineralizing fluids.     

Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow

The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D₂ shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D₃ shearing and intense quartz veining. The development of a S₂–S₃ crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D₂ shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D₃ is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S₃. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ¹⁸O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D₂ deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D₃ deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization.     

Geological,fluid inclusion,and O–C–S–Pb–He–Ar isotopic constraints on the genesis of the Honghuagou lode gold deposit,northern North China Craton

The Honghuagou Au deposit is located in the Chifeng-Chaoyang region within the northern margin of the North China Craton. The auriferous quartz veins are mainly hosted in the mafic gneiss and migmatite of the Neoarchean Xiaotazigou Formation along NNW- and NE-striking faults, with pyrite as the predominant ore mineral. The gold mineralization process can be divided into two stages, involving stage I quartz-pyrite and stage II quartz-calcite-polymetallic sulfide. Three types of fluid inclusions (FIs) have been identified in the Honghuagou deposit, namely, carbonic inclusions, aqueous‑carbonic inclusions, and aqueous inclusions. Quartz of stage I contains all types of FIs, whereas only aqueous inclusions are evident in stage II veins. The FIs of stages I and II yield homogenization temperatures of 275–340 °C and 240–290 °C with salinities of 3.4–10.7 wt% and 1.4–9.7 wt% NaCl eqv., respectively. The ore-forming fluids are characterized by medium temperature and low salinity, belonging to the H₂O–NaCl–CO₂ system. The δ¹⁸O_H2O values of the ore fluids are between 2.1‰ and 5.9‰, within the range of enriched mantle-derived fluids in the North China Craton. The carbon isotope compositions of calcite (δ¹³C_PDB = −4.4‰ to −4‰) are also similar to mantle carbon. He-Ar isotope data (³He/⁴He = 0.38–0.44 Ra; ⁴⁰Ar/³⁶Ar = 330–477) of fluid inclusions in pyrite indicate a mixed crustal and mantle source for the ore-forming fluids. Whereas, S-Pb isotope compositions of sulfides reveal that ore metals are principally derived from crustal rocks. On the basis of available geological and geochemical evidence, we suggest that the Honghuagou deposit is an orogenic gold deposit.     

Stable isotopes and noble gases in the Xishimen gold deposit,central North China Craton: metallogeny associated with lithospheric thinning and crust–mantle interaction

Quartz-vein type gold mineralization at Xishimen is a recently discovered gold deposit in the central North China Craton. More than 50 auriferous quartz veins occur in this region within a NNW–SSE-trending fault zone 4600 m in length and 3–10 m wide. Wall rocks are mainly Precambrian tonalite–trondhjemite–granodiorite (TTG) gneisses and associated supracrustals, modified by K-feldspathization and pyrite-phyllic hydrothermal alteration. Based on detailed field and petrographic studies, we identify five episodes of mineralization: pyrite-phyllic stage (I), coarse-grained pyrite-milky white quartz stage (II), fine-grained smoky grey quartz-pyrite stage (III), fine-grained smoky grey quartz-polymetallic sulphide stage (IV), and quartz-carbonate stage (V). We present results of δ³⁴S analysis of sulphide minerals from the different stages which show tightly clustered values in the range of –1.0‰ to 2.1‰, close to those of mantle and meteorite sulphur. Lead isotopic ratios of pyrite from the early to main stages also show restricted ranges with ²⁰⁶Pb/²⁰⁴Pb of 16.289–17.286, ²⁰⁷Pb/²⁰⁴Pb of 15.217–15.453, ²⁰⁸Pb/²⁰⁴Pb of 37.012–38.232, implying lower crustal input. ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios of fluid trapped in pyrite are 0.68 Ra to 1.20 Ra (where Ra is the ³He/⁴He ratio of air = 1.4 × 10^?6) and 540.9–1065, respectively. ³He and ⁴Ar concentrations vary from 10.05 to 18.5 (10^?7 cm³STP/g) and 6.15 to 17.4 (10^?7cm³STP/g), respectively, with calculated mantle helium ranging from 8.47% to 14.96% (average 11.01%). δ¹⁸O_Q and δ¹⁸D_Q values of quartz range from 8.0‰ to 13.2‰ and –101.9‰ to –70.5‰, respectively, with calculated δ¹⁸O_W values of the mineralizing fluid ranging from 1.11‰ to 5.72‰, suggesting the mixing of magmatic aqueous fluid with meteoric water during gold precipitation. We correlate the mixed crust–mantle signature of the ore-forming sources to magmatism and metallogeny associated with Mesozoic inhomogeneous lithosphere thinning in the central North China Craton.     

Characteristic Features of REE and Pb–Zn–Ag Mineralizations in the Na Son Deposit,Northeastern Vietnam

The Na Son deposit is a small‐scale Pb–ZnPb–Zn–Ag deposit in northeast Vietnam and consists of biotite–chlorite schist, reddish altered rocks, quartz veins and syenite. The biotite–chlorite schist is intruded by syenite. Reddish altered rocks occur as an alteration halo between the biotite–allanite‐bearing quartz veins and the biotite–chlorite schist. Allanite occurs in the biotite–allanite‐bearing quartz veins and in the proximal reddish altered rocks. Rare earth element (REE) fluorocarbonate minerals occur along fractures or at rim of allanite crystals. The later horizontal aggregates of sulfide veins and veinlets cut the earlier reddish altered rocks. The earlier Pb–Zn veins consist of a large amount of galena and lesser amounts of sphalerite, pyrite and molybdenite. The later Cu veins cutting the Pb–Zn veins include chalcopyrite and lesser amounts of tetrahedrite and pyrite. The occurrences of two‐phase H₂O–CO₂ fluid inclusions in quartz from biotite–allanite‐bearing quartz veins and REE‐bearing fluorocarbonate minerals in allanite suggest the presence of CO₂ and F in the hydrothermal fluid. The oxygen isotopic ratios of the reddish altered rocks, biotite–chlorite schist, and syenite range from +13.9 to +14.9 ‰, +11.5 to +13.3 ‰, and +10.1 to +11.6 ‰, respectively. Assuming an isotopic equilibrium between quartz (+14.6 to +15.8 ‰) and biotite (+8.6 ‰) in the biotite–allanite‐bearing quartz vein, formation temperature was estimated to be 400°C. At 400°C, δ¹⁸O values of the hydrothermal fluid in equilibrium with quartz and biotite range from +10.5 to +11.7 ‰. These δ¹⁸O values are consistent with fluid that is derived from metamorphism. Assuming an isotopic equilibrium between galena (+1.5 to +1.7 ‰) and chalcopyrite (+3.4 ‰), the formation temperature was estimated to be approximately 300°C. The formation temperature of the Na Son deposit decreased with the progress of mineralization. Based on the geological data, occurrence of REE‐bearing minerals and oxygen isotopic ratios, the REE mineralization is thought to result from interaction between biotite–chlorite schist and REE‐, CO₂‐ and F‐bearing metamorphic fluid at 400°C under a rock‐dominant condition.     

Geological and Fluid Inclusion Constraints on Gold Deposition Processes of the Dayingezhuang Gold Deposit,Jiaodong Peninsula,China

The Dayingezhuang gold deposit, hosted mainly by Late Jurassic granitoids on Jiaodong Peninsula in eastern China, contains an estimated 170 t of gold and is one of the largest deposits within the Zhaoping fracture zone. The orebodies consist of auriferous altered pyrite–sericite–quartz granites that show Jiaojia-type (i.e., disseminated and veinlet) mineralization. Mineralization and alteration are structurally controlled by the NE- to NNE-striking Linglong detachment fault. The mineralization can be divided into four stages: (K-feldspar)–pyrite–sericite–quartz, quartz–gold–pyrite, quartz–gold–polymetallic sulfide, and quartz–carbonate, with the majority of the gold being produced in the second and third stages. Based on a combination of petrography, microthermometry, and laser Raman spectroscopy, three types of fluid inclusion were identified in the vein minerals: NaCl–H2O (A-type), CO2–H2O–NaCl (AC-type), and pure CO2 (PC-type). Quartz crystals in veinlets that formed during the first stage contain mainly AC-type fluid inclusions, with rare PC-type inclusions. These fluid inclusions homogenize at temperatures of 251°C–403°C and have low salinities of 2.2–9.4 wt% NaCl equivalent. Quartz crystals that formed in the second and third stages contain all three types of fluid inclusions, with total homogenization temperatures of 216°C–339°C and salinities of 1.8–13.8 wt% NaCl equivalent for the second stage and homogenization temperatures of 195°C–321°C and salinities of 1.4–13.3 wt% NaCl equivalent for the third stage. In contrast, quartz crystals that formed in the fourth stage contains mainly A-type fluid inclusions, with minor occurrences of AC-type inclusions; these inclusions have homogenization temperatures of 106°C–287°C and salinities of 0.5–7.7 wt% NaCl equivalent. Gold in the ore-forming fluids may have changed from Au(HS)0 as the dominant species under acidic conditions and at relatively high temperatures and fO2 in the early stages, to Au(HS)2– under neutral-pH conditions at lower temperatures and fO2 in the later stages. The precipitation of gold and other metals is inferred to be caused by a combination of fluid immiscibility and water–rock interaction.     

A two-stage evolution model for the Amantaytau orogenic-type gold deposit in Uzbekistan

A lithogeochemical, mineral chemical, isotopic, and fluid inclusion study of barren, low-, and high-grade Au-mineralized samples from the shear zone-hosted Amantaytau gold deposit, Uzbekistan, shows that the local host rocks, Late Ordovician–Earlz Silurian carbonacous shales, are likely to have been an important source of Au, As, Ni, and S in the formation of the deposit. Syn-depositional pyrite in these shales contains on average 0.23 ppm Au, 1,083 ppm As, and 861 ppm Ni. The distribution of rare earth elements (REE) indicates a homogeneous source of light REE, whereas the heavy REE distribution reflects most likely primary variations in the sediments. The mineralized zone is marked by a positive Eu anomaly, which supports reducing conditions during the mineralization. A hydrothermal overprint by an aqueous–carbonic fluid is reflected in a high-grade Au-mineralized sample by δ¹³C values of ?13.0?‰ (V-PDB). The δ ³⁴S values in pyrite (?0.13 to +7.30?‰ CDT) from barren and mineralized samples are consistent with marine sulfate being the principal source of the ore sulfur. Assuming a formation temperature of between 300 and 400 °C for the main stage of mineralization, as indicated by the alteration mineral assemblage, the calculated δ ¹⁸O_fluid is between 9.5 and 13.4?‰ V-SMOW, which points at a metamorphic origin of the ore fluid.     

Gold–quartz deposits of the Zhdaninsky ore–placer cluster,eastern Yakutia: Structural control and formation conditions

Gold deposits and occurrences small in reserves and high in Au grade conventionally determine the line of prospecting in terrigenous sequences of the Verkhoyansk–Kolyma region. In this paper, the geological structure of such gold objects is considered with the example of the deposits and prospects making up the Zhdaninsky ore–placer cluster in the Republic of Sakha (Yakuia). From lithological, structural, and mineralogical–geochemical data, the formation conditions of ore-bearing complexes are specified, the geological evolution history of the northern Ol’chan Zone of the Kular–Nera Belt is reconstructed, and the zonal distribution of mineralization within the ore–placer cluster is revealed. The structural–compositional complexes were formed in the following succession: (1) sedimentation at the shelf of the passive margin accompanied by synsedimentation deformations; (2) metagenesis of sediments and the development of bedding-plane intraformational detachments of collision stage D₁ under conditions of tangential compression and accompanied by the formation of carbon dioxide–aqueous metamorphic fluid at a temperature of 300°C and under a pressure of 1.4 kbar; (3) folding and faulting of orogenic stage D₂ with the formation of synkinematic magmatic bodies, metasomatic alteration, and Au-bearig mineral assemblages. Small Au-bearing objects with veined mineralization and high Au grade are localized in structures of stage D₂ transverse to bedding-plane schistosity S₁. They form at the collision stage above intraformational detachment surfaces and are controlled by shear structures of the orogenic stage with misalignment of these deformations. The ore zoning is determined by the distribution of Co and Ni minerals and by variations in the anionic composition of ore (S, As, Sb).     

Lithologic controls on mineralization at the Lagunas Norte high-sulfidation epithermal gold deposit, northern Peru

The 13.1-Moz high-sulfidation epithermal gold deposit of Lagunas Norte, Alto Chicama District, northern Peru, is hosted in weakly metamorphosed quartzites of the Upper Jurassic to Lower Cretaceous Chimú Formation and in overlying Miocene volcanic rocks of dacitic to rhyolitic composition. The Dafne and Josefa diatremes crosscut the quartzites and are interpreted to be sources of the pyroclastic volcanic rocks. Hydrothermal activity was centered on the diatremes and four hydrothermal stages have been defined, three of which introduced Au ± Ag mineralization. The first hydrothermal stage is restricted to the quartzites of the Chimú Formation and is characterized by silice parda, a tan-colored aggregate of quartz-auriferous pyrite–rutile ± digenite infilling fractures and faults, partially replacing silty beds and forming cement of small hydraulic breccia bodies. The δ³⁴S values for pyrite (1.7–2.2?‰) and digenite (2.1?‰) indicate a magmatic source for the sulfur. The second hydrothermal stage resulted in the emplacement of diatremes and the related volcanic rocks. The Dafne diatreme features a relatively impermeable core dominated by milled slate from the Chicama Formation, whereas the Josefa diatreme only contains Chimú Formation quartzite clasts. The third hydrothermal stage introduced the bulk of the mineralization and affected the volcanic rocks, the diatremes, and the Chimú Formation. In the volcanic rocks, classic high-sulfidation epithermal alteration zonation exhibiting vuggy quartz surrounded by a quartz–alunite and a quartz–alunite–kaolinite zone is observed. Company data suggest that gold is present in solid solution or micro inclusions in pyrite. In the quartzite, the alteration is subtle and is manifested by the presence of pyrophyllite or kaolinite in the silty beds, the former resulting from relatively high silica activities in the fluid. In the quartzite, gold mineralization is hosted in a fracture network filled with coarse alunite, auriferous pyrite, and enargite. Alteration and mineralization in the breccias were controlled by permeability, which depends on the type and composition of the matrix, cement, and clast abundance. Coarse alunite from the main mineralization stage in textural equilibrium with pyrite and enargite has δ³⁴S values of 24.8–29.4?‰ and $ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $ values of 6.8–13.9?‰, consistent with H₂S as the dominant sulfur species in the mostly magmatic fluid and constraining the fluid composition to low pH (0–2) and logfO₂ of ?28 to ?30. Alunite–pyrite sulfur isotope thermometry records temperatures of 190–260 °C; the highest temperatures corresponding to samples from near the diatremes. Alunite of the third hydrothermal stage has been dated by ⁴⁰Ar/³⁹Ar at 17.0?±?0.22 Ma. The fourth hydrothermal stage introduced only modest amounts of gold and is characterized by the presence of massive alunite–pyrite in fractures, whereas barite, drusy quartz, and native sulfur were deposited in the volcanic rocks. The $ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $ values of stage IV alunite vary between 11.5 and 11.7?‰ and indicate that the fluid was magmatic, an interpretation also supported by the isotopic composition of barite (δ³⁴S?=?27.1 to 33.8?‰ and $ {\delta^{18 }}{{\mathrm{O}}_{{\mathrm{S}{{\mathrm{O}}_4}}}} $ ?=?8.1 to 12.7?‰). The Δ³⁴S_py–alu isotope thermometry records temperatures of 210 to 280 °C with the highest values concentrated around the Josefa diatreme. The Lagunas Norte deposit was oxidized to a depth of about 80 m below the current surface making exploitation by heap leach methods viable.     

Fluid inclusion geochemistry and Ar–Ar geochronology of the Cenozoic Bangbu orogenic gold deposit,southern Tibet,China

Located along the southern part of the Yarlung Zangbo suture zone in southern Tibet, Bangbu is one of the largest gold deposits in Tibet. Auriferous sulfide-bearing quartz veins are controlled by second- or third-order brittle fractures associated with the regional Qusong–Cuogu–Zhemulang brittle-ductile shear zone. Fluid inclusion studies show that the auriferous quartz contains aqueous inclusions, two-phase and three-phase CO₂-bearing inclusions, and pure gaseous hydrocarbon inclusions. The CO₂-bearing inclusions have salinities of 2.2–9.5% NaCl_eq, and homogenization temperatures (Th) of 167–336 °C. The δD, δ¹⁸O, and δ¹³C compositions of the Bangbu ore-forming fluids are − 105.5 to − 44.4‰, 4.7 to 9.0‰ and − 5.1 to − 2.2‰, respectively, indicating that the ore-forming fluid is mainly of metamorphic origin, with also a mantle-derived contribution. The ³He/⁴He ratio of the ore-forming fluids is 0.174 to 1.010 R_a, and ⁴⁰Ar/³⁶Ar ranges from 311.9 to 1724.9. Calculations indicate that the percentage of mantle-derived He in fluid inclusions from Bangbu is 2.7–16.7%. These geochemical features are similar to those of most orogenic gold deposits. Dating by ⁴⁰Ar/³⁹Ar of hydrothermal sericite collected from auriferous quartz veins at Bangbu yielded a plateau age of 44.8 ± 1.0 Ma, with normal and inverse isochronal ages of 43.6 ± 3.2 Ma and 44 ± 3 Ma, respectively. This indicates that the gold mineralization was contemporaneous with the main collisional stage between India and Eurasia along the Yarlung Zangbo suture, which resulted in the development of near-vertical lithospheric shear zones. A deep metamorphic fluid was channeled upward along the shear zone, mixing with a mantle fluid. The mixed fluids migrated into the brittle structures along the shear zone and precipitated gold, sulfides, and quartz because of declining temperature and pressure or fluid immiscibility. The Bangbu is a large-scale Cenozoic syn-collisional orogenic gold deposit     

P–T conditions of mineralization in the Jonnagiri granitoid-hosted gold deposit,eastern Dharwar Craton,southern India: Constraints from fluid inclusions and chlorite thermometry

Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H₂O–CO₂–CH₄ + salt) fluid that underwent phase separation and gave rise to gaseous (CO₂–CH₄), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O₂), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems.