Geological features and origin of gold deposits occurring in the Baotou–Bayan Obo district, south-central Inner Mongolia, People's Republic of China

Located at western portion of northern margin of North China craton, the Baotou–Bayan Obo district is one of the most important Fe–REE–Nb and Au metallogenic provinces in China. Presently, about 52 gold deposits and prospects have been discovered, explored and mined, among which Shibaqinhao, Laoyanghao, Houshihua, Saiyinwusu, Wulashan and Donghuofang are the most important ones. All these gold occurrences can be subdivided into three groups (or types) according to its host rocks: (1) hosted by Archean high-grade metamorphic rocks; (2) hosted by Proterozoic sedimentary rocks; (3) hosted by or related to Hercynian alkaline intrusive rocks. The first group contains the Shibaqinhao, Laoyanghao and Houshihua gold deposits. Gold mineralization at these three deposits occurs within Archean amphibolite, gneiss and granulite as gold-bearing quartz veins and veinlet groups containing native gold, electrum, pyrite and chalcopyrite. The Saiyinwusu deposit belongs to the second group, and occurs within Proterozoic sandstone, quartzite and carbonaceous slate as quartz veins and replacement bodies along the fracture zones. Pyrite, marcasite, arsenopyrite, native gold and electrum are identified. The third group includes the Wulashan, Donghuofang and Luchang deposits. Gold mineralization at these three deposits occurs predominantly within the Hercynian alkaline syenite or melagabbro stocks and dyke swarms or along their contacts with Archean metamorphic wall rocks as K-feldspar–quartz veins, dissemination and veinlets. Pyrite, galena, chalcopyrite, native gold and calaverite are major metallic minerals.δ³⁴S value of sulfides (pyrite, galena and pyrrhotite) separates from groups 1 and 2 varies from −4.01‰ to −0.10‰ and −3.01‰ to 2.32‰, respectively. δ³⁴S values of Archean and Proterozoic metamorphic wall rocks for groups 1 and 2 deposits range from −20.2‰ to −17.0‰ and −15.8‰ to −16.2‰, respectively. The values are much lower than their hosted gold deposits. All these pyrite separates from Hercynian alkaline intrusions associated with the gold deposits show positive δ³⁴S values of 1.3‰ to 4.8‰, which is higher than those Precambrian metamorphic wall rocks and their hosted gold deposits. δ³⁴S values of the sulfides (pyrite and galena) from the Donghuofang and Wulashan deposits (group 3) increase systematically from veins (−14.8‰ to −2.4‰) to the Hercynian alkaline igneous wall rocks (2.8‰ to 4.8 ‰). All of these deposits in groups 1, 2 and 3 show relatively radiogenic lead isotopic compositions compared to mantle or lower crust curves. Most lead isotope data of sulfides from the gold ores plot between the Hercynian alkaline intrusions and Precambrian metamorphic wall rocks. Data are interpreted as indicative of a mixing of lead from mantle-derived alkaline magma with lead from Precambrian metamorphic wall rocks.Isotopic age data, geological and geochemical evidence suggest that the ore fluids for the groups 1 and 2 deposits were generated during the emplacement of the Hercynian alkaline syenite and mafic intrusions. The Hercynian alkaline magma may provide heat, volatiles and metals for these groups 1 and 2 deposits. Evolved metamorphic fluids produced by the devolatilization, which circulated the wall rocks, were also progressively involved in the alkaline magmatic hydrothermal system, and may have dominate the ore fluids during late stage of ore-forming processes. Most of these gold deposits hosted by Archean high-grade metamorphic rocks occur at or near the intersections of the NE- and E–W-trending fracture systems. The ore fluid of the group 3 deposits may have resulted from the mixing of Hercynian alkaline magmatic fluids and evolved meteoric waters. The deposits are believed to be products of Hercynian alkaline igneous processes along deep-seated fault zones within Archean terrain.     

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have ³He/⁴He ratios of 0.16 to 0.86 R_a, whereas their ⁴⁰Ar/³⁶Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ³⁴S values of pyrite are of 0.7–4.2‰ (n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ¹⁸O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.     

Gold and sulphide minerals in Tertiary quartz pebble conglomerate gold placers, Southland, New Zealand

Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.     

Rb–Sr Dating of Gold‐bearing Pyrites from Wulaga Gold Deposit and its Geological Significance

Wulaga epithermal gold deposit is located in northeast China. Gold mineralization mainly occurs within the crypto‐explosive breccia belt of subvolcanic intrusion. Constraints on the precise timing of mineralization are of fundamental importance for understanding the ore genesis of the Wulaga gold deposit and its mineralization potential. Three hydrothermal stages have been identified: the early veiny quartz–euhedral pyrite stage; the fine pyrite–marcasite–gray or black chalcedony stage; and the late carbonate–pyrite stage. The Rb–Sr dating of gold‐bearing pyrites from the fine pyrite–marcasite–gray or black chalcedony stage is 113.8 ± 4.4 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.706346 ± 0.000019. The age of the gold deposit is consistent with the age of ore‐bearing volcanic (109–113 Ma) and subvolcanic intrusion (103–112 Ma) within the error limits, and the pyrite initial ratio has an identical value of ⁸⁷Sr/⁸⁶Sr to subvolcanic intrusion (0.705547 ± 0.000012). These indicate that crystallization of the wall rock and epithermal gold mineralization was coeval and likely cogenetic. Moreover, a lot of epithermal gold deposits that formed in Early Cretaceous volcanic and subvolcanic intrusions have been discovered in recent years in Heilongjiang province. Combined with the studies of tectonic and magmatic activities, we propose that the formation of the Wulaga gold deposit might be caused by the heated circum‐flow water related to the volcanic–subvolcanic intrusive hydrothermal event triggered by the ancient subduction of the Izanagi plate in the Early Cretaceous.     

Structural controls,temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine,south Eastern Desert,Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (L_CO2 + L_aq ± V_CO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (L_CO2 ± V_CO2) and aqueous H₂O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H₂O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm³, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f _S2 to the range of 10⁻¹⁰ to 10^−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f _O2 through pressure fluctuation and CO₂ effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.     

Gold and copper mineralization at the El Indio deposit, Chile

A Middle Tertiary volcanic belt in the High Andes of north-central Chile hosts numerous precious- and base-metal epithermal deposits over its 150 km north-south trend. The El Indio district, believed to be associated with a hydrothermal system in the late stages of development of a volcanic caldera, consists of a series of separate vein systems located in an area of 30 km² which has undergone intense argillic-sericitic-solfataric alteration. The majority of the known gold-copper-silver mineralization occurs within a structural block only 150 by 500 m in surface area, with a recognized vertical extent exceeding 300 m. This block is bounded by two high-angle northeast-trending faults oriented subparallel to the mineralized veins.Hypogene mineralization at El Indio is grouped into two main ore-forming stages: Copper and Gold. The Copper stage is composed chiefly of enargite and pyrite forming massive veins up to 20 m wide, and is accompanied by alteration of the wall rocks to alunite, kaolinite, sericite, pyrite and quartz. The Gold stage consists of vein-filling quartz, pyrite, native gold, tennantite and subordinate amounts of a wide variety of telluride minerals. Associated with this stage is pervasive alteration of the wall rocks to sericite, kaolinite, quartz and minor pyrophyllite. The transition from copper to gold mineralization is marked by the alteration of enargite to tennantite and by minor deposition of sphalerite, galena, huebnerite, chalcopyrite and gold. Mineral stability relations indicate that there was a general decrease in the activity of S₂ accompanied by variations in the activity of Te₂ during the Gold stage.Fluid-inclusion data show homogenization temperatures ranging from about 220 to 280°C, with salinities on the order of 3–4 eq. wt. % NaCl for the Copper stage. The Gold-stage inclusions indicate a similar range in homogenization temperatures, but significantly lower salinities (0.1–1.4 eq. wt. % NaCl). Fluid inclusions of transition minerals show a weak inverse relationship between homogenization temperatures (190–250°C) and salinities (3.4–1.4 eq. wt. % NaCl), which may represent mixing of hotter Gold-stage fluids with cooler late-Copper-stage fluids. No evidence of boiling was found in fluid inclusions, but CO₂ vapor-rich inclusions were identified in wall-rock quartz phenocrysts which pre-date copper and gold mineralization.Mineral stability calculations indicate that given a fairly restricted range of solution compositions, the Copper-, Transition- and Gold-stage minerals at El Indio could have been deposited from a single solution, with constant total dissolved sulfur which underwent reduction through time. Limited sulfur-isotope data indicates that pyrite from the Copper stage was not in isotopic equilibrium with Copper-stage alunite or Transition-stage sphalerite. The sulfur-isotope and fluid-inclusion data indicate that two fluids with comparable temperatures but different compositions flowed through the El Indio system. The earlier fluid deposited copper attended by sericite-alunite-kaolinite alteration, and later epithermal fluids deposited gold with quartz-sericite-kaolinite-pyrite alteration.     

Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration, fluid inclusions and stable isotope data

The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO₂-rich, H₂O-rich and mixed H₂O–CO₂ fluid inclusions in the gold-bearing quartz veins. The H₂O–CO₂ inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ³⁴S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO₂.     

Fluid evolution,wallrock alteration,and ore mineralization associated with the Rodalquilar epithermal gold-deposit in southeast Spain

At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS) ₂ ^– , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO₂. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.     

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.     

Genetic environment of germanium-bearing gold-silver vein ores from the Wolyu mine,Republic of Korea

The Wolyu mine is one of the largest vein-type gold-silver-bearing epithermal systems in the Youngdong district and is the first gold-silver deposit in Korea found to contain significant germanium, in the form of argyrodite (Ag₈GeS₆). Mineralized veins (78.9 ± 1.2 Ma) crosscutting Late Cretaceous hostrock tuff and quartz porphyry (81.5 ± 1.8 Ma) consist of three stages of quartz and carbonates, the first of which contains pyrite, basemetal sulfides and Au-Ag-minerals. Stage I Au-Ag-Ge-mineralized veins show a systematic variation of mineral assemblage with time: (1) quartz + pyrite; (2) quartz + pyrite + sphalerite + electrum + argentite; (3) carbonate + quartz + sphalerite + electrum + argentite; (4) carbonate + native silver + argentite + Ag-sulfosalts + argyrodite + sphalerite. Calculated values of temperature and sulfur activity are: assemblage (1), 360-280°C and 10^–7-10^–10; (2), 280-210°C and 10^–10-10^–14; (3), 210-180°C and 10^–14-10^–16; (4), 180-155°C and 10^–17-10^–18. These data, the frequent association of gold with sulfides, and the abundance of pyrite in alteration zones indicate that decreasing sulfur activity and cooling were important in triggering gold deposition. Hydrogen and oxygen isotope compositions of ore fluids display a systematic variation with increasing time. Within the main Ag-Au-Ge mineralization, D and ¹⁸O values decrease with the transition from quartz to carbonate deposition (from -78 and –2.8% to –90 and –8.7%., respectively), indicating increasing involvement (mixing) of less evolved meteoric water which resulted in progressive cooling and dilution of ore fluids in the shallow ( 370–600 m) Wolyu epithermal system.     

Listvenite–lode association at the Barramiya gold mine,Eastern Desert,Egypt

Several occurrences of gold-bearing quartz veins are situated along the east–northeast-trending Barramiya–Um Salatit ophiolitic belt in the central Eastern Desert of Egypt. In the Barramiya mine, gold mineralization within carbonaceous, listvenized serpentinite and adjacent to post-tectonic granite stocks points toward a significant role of listvenitization in the ore genesis. The mineralization is related to quartz and quartz–carbonate lodes in silicified/carbonatized wallrocks. Ore minerals, disseminated in the quartz veins and adjacent wallrocks are mainly arsenopyrite, pyrite and trace amounts of chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, galena, gersdorffite and gold. Partial to complete replacement of arsenopyrite by pyrite and/or marcasite is common. Other secondary phases include covellite and goethite. Native gold and gold–silver alloy occur as tiny grains along micro-fractures in the quartz veins. However, the bulk mineralization can be attributed to auriferous arsenopyrite and arsenic-bearing pyrite (with hundreds of ppms of refractory Au), as evident by electron microprobe and LA-ICP-MS analyses.The mineralized quartz veins are characterized by abundant carbonic (CO₂ ± CH₄ ± H₂O) and aqueous-carbonic (H₂O–NaCl–CO₂ ± CH₄) inclusions along intragranular trails, whereas aqueous inclusions (H₂O–NaCl ± CO₂) are common in secondary sites. Based on the fluid inclusions data combined with thermometry of the auriferous arsenopyrite, the pressure–temperature conditions of the Barramiya gold mineralization range from 1.3 to 2.4 kbar at 325–370 °C, consistent with mesothermal conditions. Based on the measured δ³⁴S values of pyrite and arsenopyrite intimately associated with gold, the calculated δ³⁴S_Σs values suggest that circulating magmatic, dilute aqueous-carbonic fluids leached gold and isotopically light sulfur from the ophiolitic sequence. As the ore fluids infiltrated into the sheared listvenite rocks, a sharp decrease in the fluid fO₂ via interaction with the carbonaceous wallrocks triggered gold deposition in structurally favorable sites.     

Gold distribution in As-deficient pyrite and telluride mineralogy of the Yangzhaiyu gold deposit,Xiaoqinling district,southern North China craton

The Mesozoic Yangzhaiyu lode gold deposit is situated in the southern edge of the North China craton. Gold mineralization is hosted in Archean amphibolite facies metamorphic rocks, and consists mainly of auriferous quartz veins. Pyrite is the predominant sulfide mineral, with minor amounts of chalcopyrite, sphalerite, and galena. Based on morphology and paragenesis, there are three generations of pyrite, termed as first generation (G1), second generation (G2), and third generation (G3). They have distinct contents, occurrences, and distribution patterns of gold. The coarse-grained, euhedral G1 pyrite contains negligible to low levels of gold, whereas both invisible and visible gold are present in the fine- to medium-grained G2 pyrite that is characterized by abundance of microfractures and porosities, forming a foam-like texture. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) depth profiles indicate that invisible gold occurs either as solid solution or as nanoparticles of gold-bearing tellurides in the G2 pyrite. Visible gold is widespread and present as irregular grains and stringers of native gold mostly along grain boundaries or filling microfractures of pyrite, likely resulting from remobilization of invisible gold once locked in the G2 pyrite. The G3 pyrite, invariably intergrown with chalcopyrite, sphalerite, and galena, contains the highest levels of invisible gold. There is a positive correlation between Au, Ag, and Te, indicating that gold occurs as submicroscopic Au-bearing telluride inclusions in the host minerals. Whenever gold, either invisible or visible, is present, As is always below or only marginally higher than the detection limit of LA-ICP-MS. This indicates that As played an insignificant role in gold mineralization. Tellurides are widespread in the auriferous quartz veins, consisting mainly of petzite, calaverite, hessite, altaite, and tellurobismuthite. Native gold commonly occurs as intergrowths with tellurides. Textural evidence indicates a precipitation sequence, in a temporal order, of calcaverite, petzite, altaite, tellurobismuthite, and hessite. Little amount of sulfide phases has been found in association with the tellurides, indicating that tellurides were deposited under low S fugacity (fS ₂ ) and/or high Te fugacity (fTe ₂ ) conditions. The textural relationships, when combined with fluid inclusion microthermometric data of auriferous quartz veins and tellurides thermodynamic data, permit estimation for logfTe ₂ during telluride formation, which are −6.8 to −10.8 at 300°C and −9.6 to −17.6 at 250°C. Available geochronological and geochemical data suggest that Te was most likely derived from the late Mesozoic magmatic rocks widespread in the Xiaoqinling district and other parts of the southern North China craton, which were emplaced broadly contemporaneous with gold mineralization at Yangzhaiyu. This study highlights the role of Te and tellurides as important gold scavengers in As-deficient ore fluids.     

新疆南天山阿沙哇义金矿床的成因与找矿启示:来自流体和硫化物成分的限定

阿沙哇义金矿位于中国新疆南天山造山带,属于著名的中亚南天山锑-汞-金成矿带的东延部分。该矿床严格受断裂所控制,以浸染状黄铁矿化、毒砂化为特征。矿化可分为三个阶段:早期无矿或贫矿石英阶段,中期石英多金属硫化物阶段,晚期石英-碳酸盐阶段。其中,中期是主要成矿阶段。成矿流体气相成分以H_2O为主,摩尔含量为75%～93%,其次为CO_2,摩尔含量为6%～25%,其余为CH_4、C_2H_6、H_2S、N_2和Ar;液相成分阳离子以Na~+为主,含少量K~+、Ca~(2+)离子,阴离子以Cl~-为主,SO~(2-)次之;矿石的Au含量与其流体的CO_2含量呈反相关,与K~+含量呈正相关。硫化物成分分析结果表明:(1)围岩地层和矿石中的黄铁矿和毒砂是重要的载金矿物,黄铁矿Au含量为0～0. 09%,平均值0. 03%;毒砂Au含量为0～0. 28%,平均值0. 07%;(2)黄铁矿和毒砂Au含量与其自形程度没有明显的相关性;(3)环带状黄铁矿较均质结构黄铁矿具有更高的Au含量;(4)岩体中的黄铁矿几乎不含Au。在成矿构造环境、成矿流体特征及演化、金矿富集机制、成矿温压条件等方面,该矿床与世界上大多数造山型金矿显示出一致性,成矿类型应属于剥蚀程度较浅的造山型金矿。断层阀作用控制的断层愈合-破裂导致的流体不混溶作用是本区金富集、沉淀的最重要机制,但流体混合机制对金的富集沉淀也发挥了作用。黄铁矿、毒砂发育及较多的含炭物质三者共存是本区寻找富矿的关键标志。     

Origin and evolution of the calcic and magnesian skarns hosting the El Valle-Boinás copper–gold deposit, Asturias (Spain)

The El Valle-Boinás copper–gold deposit is located in the southern part of the Rio Narcea Gold Belt 65 km west of Oviedo (NW Spain), within the Cantabrian Zone (Iberian Hercynian Massif). The deposit is related to the Boinás stock, which ranges from quartz-monzonite to monzogranite and intruded (303 Ma) the carbonated Láncara Formation (early Cambrian) and the siliciclastic Oville Formation (middle-late Cambrian).A copper–gold skarn was developed along the contact between the igneous rock and the carbonated sedimentary rocks. The skarn distribution and mineralogy reflects both structural and lithologic controls. Two types of skarn exists: a calcic skarn mainly developed in the upper calcic member of the Láncara Formation, and a magnesian skarn developed in the lower dolomitic and organic-rich member. The former mainly consists of garnet, pyroxene and wollastonite. Retrograde alteration consists of K-feldspar, epidote, quartz, calcite, magnetite, ferroactinolite, titanite, apatite, chlorite and sulfides. Magnesian skarn mainly consists of diopside with interbedded forsterite zones. Pyroxene skarn is mainly altered to tremolite, with minor phlogopite and serpentine. Olivine skarn is pervasively altered to serpentine and magnetite, and is commonly accompanied by high sulfide and gold concentrations. This altered skarn results in a very dark rock, referred to as “black skarn”, which has great importance in gold reserves. Sulfide mineralization mainly consists of chalcopyrite, bornite, arsenopyrite, pyrrhotite and pyrite, while wittichenite, sphalerite, digenite, bismuthinite, native bismuth and electrum occur as accessory minerals.After extensive erosion, reactivation of the northeast-trending fracture zone provided conduits for the subsequent emplacement of porphyritic dikes (285±4 Ma) and diabasic dikes (255±5 Ma). Alteration, characterized by sericitization, silicification, carbonatization and hypogene oxidation took place, as did sulfide mineralization (pyrite, arsenopyrite, sphalerite, chalcopyrite, galena, bournonite, and Fe–Pb–Sb sulfosalts). Veins with quartz, carbonate, adularia and sulfide minerals crosscut all previous lithologies. Jasper and jasperoid breccias developed at the upper parts of the deposits.The fluid inclusion and stable isotope studies suggest a predominantly magmatic prograde-skarn fluid characterized by high-salinity (26–28 wt.% KCl and 32–36 wt.% NaCl) and high temperature, above 580°C. This fluid evolved into two immiscible fluids: a CO₂- and/or CH₄-rich, high-salinity aqueous fluid. Temperatures for the first retrograde-stage are between 350 and 425°C. A second stage is related to a more diluted aqueous fluid (3–6.2 wt.% NaCl eq.) and temperatures from 280 to 325°C. The fluid inclusion study performed on quartz from low-temperature mineralization indicates a very low salinity (0.2–6.2 wt.% NaCl eq.), low-temperature aqueous fluid (from 150 to 250°C), and trapping pressure conditions less than 0.2 kbar. In addition, the stable isotope study suggests that an influx of metamorphic waters derived from the country rocks produced these lower temperature fluids. The last control for the Au mineralization is the Alpine tectonism, which developed fault breccias (cataclasites to, locally, protomylonites) and gold remobilization from previous mineralization.     

A three stage fluid flow model for Variscan gold metallogenesis in northern Portugal

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits (Grovelas and Penedono), together with a deposit linked to sub-vertical structures in silicified metasediments at Três-Minas, and several intra-metamorphic occurrences at Vila Pouca de Aguiar. They all possess similar mineral assemblages, deformational state, fluid flow characteristics, ore fluid composition and have comparable P–T conditions. Three successive crystallisation stages are recorded during the formation of gold-bearing structures independent of their location or host rocks (granites or metasediments). They are:Stage 1 — the development of milky quartz veins that formed primarily after the emplacement of peraluminous two-mica granites (315–310 Ma) at P–T conditions reflecting high temperature and low pressure. They are similar to those from pluton induced metamorphism (P=300–350 MPa and T=500–550°C). No clear evidence was found for gold deposition during this stage.Stage 2 — during orogenic uplift and repeated tectonic reactivation a clear quartz was deposited in the early milky quartz veins (Stage 1) at P–T conditions between 100 and 300 MPa and 300 and 450°C. Local sulphide deposition (arsenopyrite II and pyrite II) occurred in clear quartz, but was never massive. The fluids percolating within the granite were mainly aqueous-carbonic and reflect equilibrium with the metamorphic host rocks. They are very similar to those found in metamorphic environments. No evidence for the involvement of magmatic fluids was found.Stage 3 — intense microfissuring of the earlier vein infillings occurred, associated with the main episode of gold deposition. The P–T conditions were <100 MPa and <300°C based on aqueous fluid inclusions. Native gold and electrum crystallised together with sulphides (galena, chalcopyrite and bismuthinite), native Bi and sulphosalts (Pb–Bi–Ag dominated). The fractures frequently contain chlorite (± sericite) especially where they crosscut earlier sulphides (arsenopyrite).These processes and fluid types are similar in both the granites and metamorphic host rocks. Therefore, the gold ores appear to be the result of successive periods of fluid circulation, in this case related to the uplift of the Variscan basement in response to high heat flow and the intrusion of granites. Without exception, these fluids have been re-equilibrated with the metamorphic rocks. However magmatic fluids are absent; the granites thus act passively as heat engines for fluid circulation.     

Fluid flow, alteration and polysulphide mineralisation associated with a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium

In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone. The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts. It is concentrated in quartz–sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H₂O–CO₂–CH₄–NaCl–(KCl) composition and a minimum temperature of 250–320 °C. The δ¹⁸O values of quartz vary between +12.3‰ and +14.5‰ SMOW, and δD compositions of the fluid inclusions in the quartz crystals range from −65‰ to −35‰ V-SMOW. The δD and the calculated δ¹⁸O values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids. The δ³⁴S values, between +4.7‰ and +10.6‰ CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt.     

The Nassara gold prospect,Gaoua District,southwestern Burkina Faso

The Nassara-Au prospect is located in the Birimian Boromo Greenstone Belt in southwestern Burkina Faso. It is part of a larger mineralized field that includes the Cu–Au porphyry system of Gaoua, to the north. At Nassara, mineralization occurs within the West Batié Shear Zone that follows the contact between volcanic rocks (basalt and andesite) and volcano-sediments (pyroclastics and black shales) at the southern termination of the Boromo Belt. Gold is associated with pyrite and other Fe-bearing minerals that occur disseminated within the sheared volcanic and volcano-sedimentary rocks. In particular, highest grades are distinguished in alteration halos of small quartz–albite–ankerite veins that form networks along the shear zone. Here, pyrites are marked by As-poor and As-rich growth zones, the latter containing gold inclusions. Gold mineralization formed during D2_NA. Subsequent shear fractures related to D3_NA related are devoid of gold. Nassara is a classical orogenic gold occurrence where gold is associated to disseminated pyrite along quartz veins.     

Thin-skinned thrust mineralization in the Brasília fold belt: the example of the old Luziânia gold deposit

The Luziânia gold deposits in southern Goiás lie within the Late Proterozoic Brasília fold belt. The rocks that host the gold mineralization are a monotonous series of hydrothermally altered phyllites that have been subject to low grade regional metamorphism. The major controls on the gold mineralization are northeast trending and gently northwest dipping ductile-brittle, dextral-reverse shear zones associated with regional thin-skinned thrusting of the Canastra Group. From a preliminary fluid inclusion study it is deduced that low salinity, 7 eq. wt% NaCl, moderately dense, H₂O-CO₂ ± CH₄ ore fluids deposited gold at temperatures of 300 ± 75°C and pressures of 1.5 to 3 kb in the filling stage of the vein formation. Post-filling stage gold deposition probably occured by mixing of fluids at higher crustal levels (1.5–2 kb). During thrusting, prograde metamorphism released pore water which penetrated along thrust planes that acted as high permeability zones for the ponding and release, by hydraulic fracturing, of overpressured fluids. Later in the tectonic evolution and at shallower crustal levels, there was likely an incursion of near suface water into the fault zone.     

Gold mineralization paragenesis to tectonic structures in the Birimian of the eastern Dialé-Daléma Supergroup,Kédougou-Kéniéba Inlier,Senegal, West African Craton

ABSTRACT

New field and petrographic data from the Birimian of the Kolia-Boboti Basin in the Kédougou-Kéniéba inlier indicate two phases of gold mineralization related to Eburnean tectono-magmatic events. Syn- to late-tectonic (D₂/D₃) mineralization, controlled by stockwork sulphide-bearing quartz-chlorite and quartz-carbonate veins, is associated with fluid circulation related to magmatic intrusions. V₂ veins and disseminated assemblages are mainly composed of quartz + chlorite + pyrite and ± gold. Haematite and arsenopyrite are added in the sediments (pelite, greywake, sandstone, quartzite, and marble) and albite in the felsic rocks (andesite, rhyolite, granodirite, and diorite). V₃ veins assemblage is composed of quartz + carbonate + pyrite + chalcopyrite and ± gold. Pyrrhotite appears in the sediments (greywacke, quartzite, marble). Sericite, tourmaline, haematite, and magnetite are common in both V₂ and V₃ assemblages. The first sulphide-bearing quartz-chlorite assemblage is related to the hydrothermal activity of the Eburnean D₂ deformation, which was focused mostly along NW- to NNE-trending tectonic structures. The second sulphide-bearing quartz-carbonate assemblage is associated with hydrothermal activity during late Eburnean D₃ deformation, mainly located in NE- to E–W-trending tectonic structures. Gold is correlated with the abundance of sulphides (pyrite, chalcopyrite, arsenopyrite), and sulphide stockworks are more abundant in the veins sub-parallel (V_2b) or oblique (V_2c) to the N–S- to NNE-oriented S₂ foliation, as well as in the N45°–N90°-oriented V₃ veins. V₁ veins, which are related to D₁ Eburnean tectonics, are highly deformed (folded and boudinaged) and are poor in sulphides. The host structures of mineralization (V₂ and V₃ veins) represent the low- and medium-stress domains resulting from the Eburnean D₂ and D₃ tectonic phases, respectively. The intra-crystalline deformation of the quartz grains associated with these three vein types indicates relatively low temperatures. These different features suggest that most of the mineralization was associated with sulphides formed during the D₂ and D₃ Eburnean tectono-magmatic events dated around 2080 ± 0.9 and 2061 ± 15 Ma, respectively.     

Geology and inclusion studies on the genesis of the Baolun gold deposit in Hainan Province,South China

The late Triassic Baolun gold deposit hosted by Silurian phyllites is a large‐scale high‐grade gold deposit in Hainan Island, South China. The ores can be classified into quartz‐vein dominated type and less altered rock type. Three mineralization stages were recognized by mineral assemblages. The early stage, as the most important mineralization stage, is characterized by a quartz–native gold assemblage. The muscovite?quartz?pyrite?native gold assemblage is related to the intermedium mineralization stage. In late mineralization stage, native gold and Bi‐bearing minerals are paragenetic minerals. Microthermometry analyses show that the early mineralization stage is characterized by two types of fluid inclusions, including CO₂‐rich inclusions (C‐type) and aqueous inclusions (W‐type). C‐type inclusions homogenize at 276–335°C with an averaged value of 306°C and have salinities of 1.0–10.0 wt% NaCl equivalent (mean value of 4.9 wt% NaCl equivalent). W‐type inclusions homogenize at 252–301°C (mean value of 278°C) with salinity of 4.0–9.7 wt% NaCl equivalent (mean value of 7.4 wt% NaCl equivalent). In intermedium mineralization stage, C‐type and W‐type inclusions homogenize at 228–320°C (mean value of 283°C) and 178–296°C (mean value of 241°C), with salinities of 2.4–9.9 wt% NaCl equivalent (mean value of 6.5 wt% NaCl equivalent) and 3.7–11.7 wt% NaCl equivalent (mean value of 7.7 wt% NaCl equivalent), respectively. No suitable mineral, such as quartz or calcite, was found for fluid inclusion study from late mineralization stage. In contrast, only aqueous inclusions were found from post‐ore barren veins, which yielded lower homogenization temperatures ranging from 168–241°C (mean value of 195°C) and similar salinities (2.6–12.6 wt% NaCl equivalent with averaged value of 7.2 wt% NaCl equivalent). The different homogenization temperatures and similar salinities of C‐type and W‐type from each mineralization stage indicate that fluid immiscibility and boiling occurred. The Baolun gold deposit was precipitated from a CO₂‐bearing mesothermal fluid, and formed at a syn‐collision environment following the closure of the Paleo‐Tethys.