Auriferous Lodes Research Articles

Gold-Bearing Sperrylite: Occurrence and Significance in Minas Gerais, Brazil

Abstract One alluvial grain of gold, recovered from a placer deposit in the platiniferous Au-Pd belt of Minas Gerais, at Fazenda Limeira, is distinguished by inclusions, between approximately 2 and 10 µm across, of sperrylite. Field-emission electron-microprobe analyses indicate amounts of Au of up to about 3.4 wt.% at the expense of As, without affecting Pt, the amount of which remains stoichiometric, i.e., 1.0 atom per formula unit (1.0 apfu Pt). Transmission electron microscopy of sperrylite foils, extracted by focused ion beam (FIB), shows no gold nanoparticles and thus provides evidence for Au-bearing sperrylite. The finding of Au-bearing sperrylite represents a Au-rich example of invisible Au in sulfide and arsenide minerals. Regionally, the occurrence of gold-hosted inclusions of Au-bearing sperrylite is in line with the highly oxidized fluid environment of the auriferous lodes that characterize the platiniferous Au-Pd belt of Minas Gerais.

Elucidating Pathfinding Elements from the Kubi Gold Mine in Ghana

X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) are applied to investigate the properties of fine-grained concentrates on artisanal, small-scale gold mining samples from the Kubi Gold Project of the Asante Gold Corporation near Dunwka-on-Offin in the Central Region of Ghana. Both techniques show that the Au-containing residual sediments are dominated by the host elements Fe, Ag, Al, N, O, Si, Hg, and Ti that either form alloys with gold or with inherent elements in the sediments. For comparison, a bulk nugget sample mainly consisting of Au forms an electrum, i.e., a solid solution with Ag. Untreated (impure) sediments, fine-grained Au concentrate, coarse-grained Au concentrate, and processed ore (Au bulk/nugget) samples were found to contain clusters of O, C, N, and Ag, with Au concentrations significantly lower than that of the related elements. This finding can be attributed to primary geochemical dispersion, which evolved from the crystallization of magma and hydrothermal liquids as well as the migration of metasomatic elements and the rapid rate of chemical weathering of lateralization in secondary processes. The results indicate that Si and Ag are strongly concomitant with Au because of their eutectic characteristics, while N, C, and O follow alongside because of their affinity to Si. These non-noble elements thus act as pathfinders for Au ores in the exploration area. This paper further discusses relationships between gold and sediments of auriferous lodes as key to determining indicator minerals of gold in mining sites.

Geochemistry of Tourmaline from the Laodou Gold Deposit in the West Qinling Orogen, Central China: Implications for the Ore-Forming Process

The Laodou gold deposit, located in the West Qinling Orogen of central China, is a newly recognized intrusion-related gold deposit. It consists of auriferous quartz-sulfide-tourmaline and minor quartz-stibnite veins that are structurally controlled by fault zones transecting the host quartz diorite porphyry. Two types of tourmaline were identified in this study: Type 1 tourmaline occurs as quartz-tourmaline nodules within the quartz diorite porphyry, whereas type 2 tourmaline occurs as quartz-sulfide-tourmaline veins in auriferous lodes. Here, we present a major and trace element analysis by electron microprobe and laser ablation inductively coupled plasma mass spectrometry on these two types of tourmaline. Both tourmaline types fall into the alkali group, and are classified under the schorl-dravite solid solution series. The substitutions of FeMg–1, FeAl–1, AlO((Fe, Mg)(OH)) –1, and X-site vacancyCa–1 are inferred by the variations of their major element compositions. Field and mineralogy observations suggest that type 1 tourmaline is a product of the late crystallization process of the quartz diorite porphyry, whereas type 2 tourmaline coexists with Au-bearing arsenopyrite and is crystallized from the ore-forming fluids. Their rare earth element compositions record the related magmatic hydrothermal evolution. The Co and Ni concentrations of the coexisting type 2 tourmaline and arsenopyrite define a regression line (correlation coefficient = 0.93) with an angular coefficient of 0.66, which represents the Co/Ni ratio of the tourmaline and arsenopyrite-precipitating fluids. This value is close to the Co/Ni ratios of the host quartz diorite porphyry, indicating a magma origin of the ore-forming fluids. The substitution of Al3+ by Fe3+ in both tourmaline types shows that type 1 tourmaline approaches the end member of povondraite whereas type 2 tourmaline occurs in opposite plots near the end member of Oxy-dravite, reflecting a more oxidizing environment for type 2 tourmaline formation. Moreover, the redox-sensitive V and Cr values of type 2 tourmaline are commonly 1–2 orders of magnitude higher than those of type 1 tourmaline, which also suggests that type 2 tourmaline forms from more oxidizing fluids. Combined with gold occurrence and fluid properties, we propose that the increasing of oxygen fugacity in the ore-forming fluids is a trigger of gold precipitation.

Geology, mineralogy and geochemistry of the Shangxu orogenic gold deposit, central Tibet, China: Implications for mineral exploration

Shangxu is an orogenic gold deposit within the Bangong-Nujiang suture zone, central Tibet, China. It is hosted by turbidite sedimentary rocks of the Jurassic Mugagangri Group. The host rocks were metamorphosed to subgreenschist facies prior to being hydrothermally altered adjacent to mineralization. Hydrothermal minerals in wall rocks consist of muscovite, carbonate, sulfides and chlorite. Whole rock geochemistry indicates hydrothermal alteration is characterized by the introduction of K2O, CO2 and S, and leaching of Na2O. Increasing 3 K/Al and decreasing Na/Al alteration indexes approaching the ore correlate to muscovitization. CO2 concentration reflects the degree of carbonation, which in conjunction with molar (Mn + Fe)/(Fe + Ca + Mg + Mn) and CO2/CaO can be used to distinguish siderite from ankerite in carbonate alteration. Mass transfer analysis shows gains in As, Au, CO2, W, Ca, Cd, Ni, Cr, Sr, Rb, K and losses in Na, Pb, Cu during hydrothermal alteration. In the muscovite alteration zone, muscovite within shear zones (Ms1) shows an increase in Fe, Mg and Na towards the ore, whereas metamorphic muscovite along cleavage planes and in pressure shadows (Ms2), and hydrothermal muscovite (Ms3) change in composition from phengitic (Mg, Fe) distal to mineralization, to paragonitic (Na) adjacent to gold. In carbonate alteration, the composition in carbonate phases changes from early magnesian siderite to ankerite, and then to calcite and dolomite during the evolution of the hydrothermal system. Siderite spots in host rocks and ankerite in veins, wall rock or its replacement of siderite are all enriched in Fe, Mn and depleted in Mg from proximal to distal alteration zones. Pyrite is a dominant mineral in sulfide alteration of the Shangxu deposit. The earliest framboidal pyrite (Py1) formed during diagenesis, and overprinted by Py2, which is recrystallized pyrite associated with burial metamorphism. Py3 rims and enlarges previous pyrite aggregates, or recrystallizes into cubic grains with cracks and quartz pressure shadows during deformation. Py4 is ore-related euhedral pyrite, disseminated near gold, and formed in hydrothermal stage. Vein pyrite (Py5) coexists with marcasite, and fills cracks in quartz veins. Hydrothermal alteration minerals form overlapping alteration haloes surrounding the main auriferous lodes. Distal muscovitization, carbonatization and proximal sulfidation constitute a progressive alteration front from gold. The alteration envelope is tabular in shape, following the strike and dip of the mineralization, and tends to be the widest around the thickest parts of the ore, such that the occurrence and thickness of the alteration envelope can be used as an approximate guide to the size of gold mineralization. Shangxu shares many diagnostic features with turbidite-hosted orogenic gold deposits, such as Bendigo, Reefton and Meguma districts, in deposit geology, alteration mineralogy and lithogeochemistry.

Nature of ore forming fluids, wallrock alteration and P-T conditions of gold mineralization at Hira-Buddini, Hutti-Maski Greenstone Belt, Dharwar Craton, India

Gold mineralization at Hira-Buddini deposit in Hutti-Maski Greenstone Belt of Dharwar Craton, India is associated with quartz veins and hydrothermal alteration formed during a brittle-ductile deformation episode. The vein minerals (quartz and plagioclase) exhibit microstructures of dynamic recrystallization/intracrystalline deformation. Estimated temperature and pressure condition during mineralization is around 500 °C and 2.05–4.36 kbar, respectively. Carbonic inclusions are the most abundant type of fluid inclusions found in quartz grains of mineralized veins. Such abundance of carbonic inclusions must be due to preferential loss of water from original fluid inclusions during intracrystalline deformation of quartz. Rock-fluid interaction led to enrichment of SiO2 (in the form of quartz veins), K, Pb, S and LOI in the auriferous lode. It is inferred that the hydrothermal fluids involved in gold mineralization were of both metamorphic and magmatic origin. Deposition of gold has been attributed to phase separation as well as wall rock sulfidation.

Field geology, geochronology, and isotope geochemistry of the Luyuangou gold deposit, China: Implications for the gold mineralization in the eastern Qinling Orogen

Gold deposits are closely related to accretionary processes. Gold mineralization in eastern China, however, is usually > 100 Ma younger than associated collisional orogeny and partly controlled by extensional tectonics. Being controlled by interbedded detachment faults, the Luyuangou hydrothermal deposit is a typical mineralization under extensional tectonic. We present mineral deposit geology, tectonic geology, LA‐ICP MS zircon U–Pb age data, and Sr‐Nd‐Hf isotopic data for the Luyuangou ore vein and porphyritic dyke, with the aim of elucidating the metallogenesis. The morphological features and ages of zircons from the Luyuangou auriferous lode are similar to the anatectic zircons from migmatite within the Qinling Group (a metamorphic complex unit) of the North Qinling Terrane, indicating that the North Qinling Terrane may be one of the provenances. Although the major age peak of 453 ± 13 Ma (MSWD = 3.3, N = 12) cannot constrain the exact metallogenic timing, the data imply a possible link between the Luyuangou metallization and the North Qinling Terrane. Meanwhile, the weighted‐mean age of post‐ore porphyritic dyke postdates the Luyuangou gold mineralization at 121.1 ± 1.5 Ma (MSWD = 0.21, N = 14). The Sr‐Nd‐Hf isotopes of the Luyuangou auriferous lodes and post‐ore porphyritic dyke exhibit prominent differences from those of the Taihua Group and Xiong'er Group and are similar to those of the Qinling Group, implying that the Qinling Group is likely the main ore‐forming provenance of the Luyuangou gold deposit. The S‐Pb isotopes of the main mineralization stage of the Luyuangou gold deposit indicate a distinct deep‐seated source. According to comprehensive analysis, it is proposed that, during the early Mesozoic, the large‐scale continental collision between the Yangtze and the North China Craton involved the underthrusting of the North Qinling Terrane beneath the North China Craton that led to intracontinental folding, thrusting, and thickening without inducing extensive mineralization or magmatic activity. During the late Mesozoic, the subduction of the Pacific Plate induced the instability of the thickened continental crust and the development of the regional extensional structures, large‐scale crust–mantle magmatism as well as deep‐seated or mantle‐derived fluids dissolving materials from the Qinling Group that precipitated at the interbedded detachment faults.

Platiniferous gold–tourmaline aggregates in the gold–palladium belt of Minas Gerais, Brazil: implications for regional boron metasomatism

The platiniferous gold–palladium belt of Minas Gerais, Brazil, forms an approximately 240-km-long, roughly north–south-trending domain that includes numerous auriferous lodes and platiniferous alluvium. The belt transects two Precambrian terranes, the Quadrilatero Ferrifero in the southern part, and the southern Serra do Espinhaco in the northern part. Both terranes were overprinted by regional fluid flow that led to tourmalinisation, with or without hematitisation, and precious-metal mineralisation. Here, we report the occurrence of coarse-grained gold–tourmaline aggregates and integrate recently obtained ages and tourmaline boron-isotope values published elsewhere. One type of aggregate is unique because it has patches that are close to stoichiometric PdPt, in which gold content varies from 2.5 to 33.5 at.%. The gold–tourmaline aggregates seem to be the ultimate expression of the boron metasomatism.

Auriferous Lode of Hira-Buddini Gold Mine, Hutti-Maski Schist Belt, Dharwar Craton: Mineralogy, Alteration, Types and Mechanism of Vein Emplacement

Abstract: The auriferous lode in the Hira-Buddini deposit is confined to the sheared contact of amphibolite and felsic metavolcanic rock. Gold mineralization in the deposit is associated with sub-horizontal, sub–vertical, irregular and with few conjugate veins. These veins were emplaced during deformation in a ductile-brittle regime as inferred from the megascopic features and microstructures of the vein minerals. Fluid pressure was higher than the sum of the minimum principal stress and lithostatic load as well as the tensile strength of the shear zone. Crack-seal process appears to be the mechanism of vein formation. The microstructures of the vein minerals indicate a temperature of ~500ºC during the vein emplacement. In the auriferous lode, amphibolite and felsic metavolcanic rock have been subjected to intense alteration by the ore fluid with development of biotite-chlorite-tourmaline-calcite and muscovite (sericite)-chlorite-calcite-feldspar-biotite assemblages, respectively. Both the altered rocks contain significant amount of pyrite and chalcopyrite with native grains of gold and silver. Post-dating the fluid activity associated with gold mineralization, there is another stage of fluid activity manifested by the calcite veins and micro-veinlets.

Detrital zircon without detritus: a result of 496-Ma-old fluid–rock interaction during the gold-lode formation of Passagem, Minas Gerais, Brazil

Zircon and xenotime occur in tourmaline-rich hydrothermal pockets in the auriferous lode of Passagem de Mariana, a world-class gold deposit. Zircon grains show pristine oscillatory zoning, but many of them are altered, exhibiting porous domains filled with graphite. Uranium–Pb dating of zircon, using in-situ laser ablation–inductively coupled plasma–mass spectrometry, yields ages between 3.2 and 2.65 Ga, which match those for detrital zircon of the footwall quartzite of the > 2.65-Ga-old Moeda Formation. Discordant analyses point to zircon-age resetting during the Brasiliano orogeny at ca. 500 Ma. This interpretation is supported by U–Pb dating of euhedral xenotime immediately adjacent to altered zircon within the same tourmaline pocket. The xenotime grains give a Concordia age of 496.3 ± 2.0 Ma, which is identical to that determined for monazite of a quartz–hematite vein-type deposit (i.e., jacutinga lode) in the region (Itabira), another important mineralisation style of gold. The occurrence of relatively abundant inherited detrital zircon, but absence of rock fragments in the tourmaline pocket investigated here, implies that detrital material was completely replaced by tourmaline. The graphite overprint on the altered detrital zircon attests to a reducing fluid, which was likely formed by fluid–rock interaction with carbonaceous phyllite of the Batatal Formation, the host rock of the Passagem lode.

Manganese ore in New Zealand

Abstract Manganese ore in New Zealand can be divided into: (a) manganese gangue in auriferous quartz lodes; (b) manganese pockets or masses associated with contemporaneous “Red Rocks” in Permian–Cretaceous sediments; and (c) manganese pockets or lenses associated with metamorphosed “Red Rocks” in regionally metamorphosed schists. All the manganese mined has come from Permian–Jurassic rocks in (b), the characteristic features of the deposits being their patchiness and their close association with “Red Rocks” that are intercalated as scattered subordinate masses in the dominant greywackes and argillites. The “Red Rocks” consist of spilitic lavas, jaspers, cherts, volcanic argillites and carbonate, and the manganese is considered to have been precipitated during the submarine volcanism. The deposits do not lend themselves to large scale mining but selectively worked by small parties, as is done in the Otau region, they can contribute to New Zealand's mineral wealth and could supplement imports if ever there ...

Hydrothermal fluid source constrained by Co/Ni ratios in coexisting arsenopyrite and tourmaline: the auriferous lode of Passagem, Quadrilátero Ferrífero of Minas Gerais, Brazil

The auriferous lode of Passagem de Mariana is characterised by abundant tourmaline, which is intergrown with arsenopyrite. Spot measurements using laser ablation–inductively coupled plasma–mass spectrometry show that Co and Ni are the most abundant trace elements in the arsenopyrite (45–538 ppm Co, 246–828 ppm Ni), with Co/Ni ratios consistently <1. The coexisting tourmaline also has Co/Ni <1, with Co and Ni contents that are ~2 orders of magnitude lower than those in the arsenopyrite. The Co/Ni ratios of tourmaline and arsenopyrite are tightly distributed along a positive linear trend, the angular coefficient of which represents the Co/Ni of the hydrothermal fluid from which these minerals precipitated. The fluid Co/Ni ratio is close to the average Co/Ni value for the upper continental crust. In conjunction with the abundance of lode tourmaline and its B-isotope data (from the literature), the Co/Ni ratios of tourmaline and arsenopyrite fingerprint a continental evaporitic source of B.

Epigenetic BIF-hosted gold lodes at the Abu Marawat area, Eastern Desert, Egypt: integrated mineralogical, structural control and fluid inclusion studies

Numerous massive and sheared, milky quartz veins cut a sequence of Neoproterozoic island arc metavolcanic/volcaniclastic rocks and related banded iron formation (BIF) at the Abu Marawat area, central Eastern Desert of Egypt. Sulphide-bearing quartz veins and related hydrothermal breccia masses display a range of textures including sheared, boudinaged and recrystallised quartz, quartz with open space filling and microbreccia, implying a complex history of crack-seal processes characterising the relationship between mineral deposition and a major N–S-trending shear zone, during a late brittle–ductile deformation event that affected the area at ∼550 Ma. Gold-base metal mineralisation is associated with brecciation and fracturing of the iron ore bands, close to silicified shears and related quartz veins. The auriferous quartz lodes are characterised by the occurrence of visible pyrite-chalcopyrite±pyrrhotite±sphalerite±galena mineralisation. Gold is refractory in pyrite and chalcopyrite, but rare visible gold/electrum and telluride specks were observed in a few samples. Hydrothermal alteration includes selectively pervasive silicification, pyritisation, sericitisation, carbonatisation that are confined to a delicate set of veins and altered shears, and a more widespread propylitic alteration assemblage (quartz+||chlorite+pyrite+calcite±epidote). The predominance of hydrated silicate phases (i.e. chlorite and sericite) within and adjacent to the shear zones points toward fixation of H2O during the shear zone development, while CO2 was consumed to form carbonate in halos around the chlorite-dominated zones, leading to variations in the H2O/CO2 ratio of the ore fluid with progression of the hydrothermal alteration.Ore textures, including hydrothermal quartz intimately associated with replacement of magnetite by pyrite, suggest the introduction of ore fluids via epigenetic conduits into the iron-rich wallrocks. Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (1·4–6·7 wt-% eq. NaCl) aqueous solution that coexisted with a carbonic fluid. Evidence for fluid immiscibility during ore formation includes variable liquid/vapour ratios in inclusions along individual trails and bulk inclusion homogenisation into liquid and occasionally to vapour at comparable temperatures. The trapping conditions of intragranular aqueous-carbonic inclusions approximate 264–378°C at 700–1300 bar. Similar temperature estimates have been obtained from Al-in-chlorite geothermometry of chlorite associated with sulphides in the mineralised quartz veins. Fracturing enhanced fluid circulation through the wallrock and related BIF, allowing reaction of the S-bearing ore fluid with iron oxides. This caused formation of pyrite, a rise in Eh and concomitant Au precipitation, which was enhanced by fluid immiscibility as H2S partitioned preferentially into the carbonic phase. The ore fluids have probably originated from felsite intrusions, whereas Abu Marawat gold deposit and the post-Hammamat felsites is assumed in view of their communal relationship with the N–S fault/shear trend and high geothermal gradient calculated from the fluid inclusion data.

Intraplate magmatism in Central Asia and China and associated metallogeny

Large areas of central Asia and mainland China have been affected by intraplate anorogenic magmatism, linked to mantle activity from the Palaeoproterozoic to present day. This magmatism is manifested by layered mafic–ultramafic intrusions, flood basalts, bimodal volcanic rocks, dyke swarms, I- and A-type granitoids, alkaline complexes, carbonatites and kimberlites. In this paper we discuss a selection of these thermal events, namely East Asia Mesozoic–Cenozoic, Siberian Traps, Emeishan, Tarim (Bachu), Yakutsk and South China. Mineral systems that are associated with the Asian intraplate magmatic events include Ni–Cu–PGE in mafic–ultramafic intrusions, porphyry Cu–Mo and skarn deposits, polymetallic veins and auriferous lode deposits generally hosted in granitoids, rare earths and rare metals in alkaline complexes and diamondiferous kimberlites. The anorogenic intraplate igneous rocks originate from mantle sources or mixed mantle and crustal sources and are related, directly or indirectly, to deep mantle plumes or to asthenospheric upwellings that occur in response to lower crust and subcontinental lithosphere delamination linked to collision and subduction processes.

Structural and lithological controls on the high-grade Hangingwall Reef quartz – gold veins, Stawell, Victoria

The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites and jogs in which the auriferous quartz veins formed. Results from 3D numerical modelling show that dilation along the Stawell Fault provided the conduit for fluid flow.

Fluid evolution in the Kolar Gold Field: evidence from fluid inclusion studies

Gold mineralization in the Kolar schist belt of the Dharwar craton occurs dominantly in the form of a sulfide-poor Au-quartz lode (the Champion lode exposed in the Mysore and other mines) and sulfide-rich auriferous lodes (from the Nundydroog mine). Fluid inclusion microthermometric experiments were conducted on primary inclusions in quartz intimately associated with Au-mineralization. Homogenization studies on aqueous-biphase (L + V), aqueous polyphase (L + V+ halite) and aqueous-carbonic (LCO2± VCO2 + Laq) inclusions from the Champion lode furnish a temperature range of 120 to 420 °C. Freezing of aqueous biphase inclusions and dissolution of halite in the aqueous polyphase inclusions provide salinity of 5 to 50 wt.% NaCl equivalent. Fluid inclusion thermobarometry from the total homogenization of aqueous-carbonic inclusions and from intersecting isochores of coeval pure-carbonic (LCO2± VCO2) and pure-aqueous inclusions constrain the P-T path of evolution of the fluid in the Champion lode. Gold precipitation was likely to have been brought about in response to a sharp fall in pressure with attendant unmixing of liquid-CO2 from the parent H2O-CO2 fluid of possible metamorphic origin. This would imply transportation of gold by some pressure-sensitive complex such as the Au-carbonyl. Fluid characteristics are different in the sulfide-rich auriferous lodes, as indicated by the virtual absence of the CO2-bearing and the halite-bearing inclusions. The fluid evolution path, as evident from the crude positive colinearity of temperature and salinity, is due to mixing of a low (≤200 °C) temperature-low saline (≤7 wt.% NaCl equivalent) fluid with a high temperature (≥400 °C)-high saline (≥50 wt.% NaCl equivalent) fluid. The lack of CO2 and association of Au with sulfides indicate a different mode of gold transport, as chloride or bisulfide complexing, deposition of which was possibly brought about by fluid mixing.

Stratigraphic and structural controls on gold mineralization in the Pilgrim's Rest goldfield, eastern Transvaal, South Africa

Since 1872, approximately 185 metric tons of gold and a significant amount of silver have been won from numerous stratiform ore bodies within the Pilgrim's Rest goldfield. The epigenetic stratiform ore bodies are found primarily in the Malmani Dolomite Subgroup, a laterally extensive Lower Proterozoic carbonate platform succession. A strong relationship exists between the presence of stratiform ore bodies and environment of deposition and structural deformation. Analysis of the host sediments indicates deposition in a retrogradational, followed by an aggradational, sequence that comprises supratidal (sabkha) siliciclastic sediments to midshelf carbonates. The retrogradational and aggradational sequences are characterized by numerous higher frequency transgressive and regressive cycles. Mineralization is, with few exceptions, restricted to shallow-water siliciclastic sediments; deeper water carbonates are barren. More importantly, siliciclastic mudstones and sandstones have acted as zones of detachment for thin-skinned thrust-fault deformation, resulting in the development of conformable passageways in which fluids concentrated their auriferous lodes. Evidence of shallow hinterland-dipping duplex, antiformal stack, and imbricate thrust systems abounds in several underground and field exposures. The gold-silver emplacement model for the Pilgrim's Rest goldfield infers fluids derived from a deep-seated magmatic source. Oxygen-isotopic compositions of the mineralizing fluids, recalculated from δ 18O quartz (+ 12.1‰ to + 19.5‰), point to a magmatic source and subsequent mixing with saline, evolved formation water. A magmatic origin is also supported by the high homogenization temperature gradient displayed by the fluid inclusions (100°C/km). The Bushveld Igneous Complex (Rustenburg Layered Suite) to the west of Pilgrim's Rest is thought to be the source of the auriferous hydrothermal solutions, and its emplacement was the driving mechanism of thrust-fault deformation in the Pilgrim's Rest goldfield.

The Coolgardie Goldfied, Western Australia: district-scale controls on an Archaean gold camp in an amphibolite facies terrane

Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west compressional far-field stress regime, but with local heterogeneous stress orientations. This occurred after an earlier period of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase + calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels.

Multiphase methane-rich fluid inclusions in gold-bearing quartz as illustrated at Pontal (Goias, Brazil)

AbstractIn the Pontal auriferous lode, dominant saccharoidal quartz is associated with oligoclase, biotite, hornblende, tremolite-actinolite, sulphides (less than 2%), and disseminated native gold. Four main types of fluid inclusion have been distinguished based on their habit, distribution and spatial relation-ship with gold particles. Type S are primary multiphase large sized (100 to 200 µm) inclusions with homogenization temperatures (V + L → L) between 350 and 450°C. They contain siderite and/or calcite and graphite-like microcrystals as daughter phases. Commonly associated with these inclusions are tiny (50 to 100 µm) solid inclusions of biotite or actinolite. In most of these inclusions, only CH4 has been detected in the vapour phase. However some noticeable exceptions were observed (CO2/CH4 ratio near 0.85). Type C inclusions are later than gold and occur disseminated in quartz or along trails that crosscut quartz grain boundaries. They may contain nahcolite daughter crystals. CO2/CH4 ratios range from 0.0 to 0.5. Homogenization temperatures vary from 150 to 300°C. Type V are mainly gaseous CH4-H2O inclusions. They may occur as small-sized inclusions directly associated with gold particles. Type L are aqueous two-phase inclusions of late secondary origin.The scattering of the CO2/CH4 ratios could be related to fluctuations of the oxygen fugacity that triggered gold precipitation at the time of trapping. These variations of fO2 with time could reflect unbuffered fluid-rock interaction with respect to redox conditions during quartz deposition.Finally, gold deposition is interpreted to have occurred at elevated temperature (500°C) and pressure, compatible with boundary conditions between greenschist and amphibolites facies.

Gold mineralization in the Hutti mining area, Karnataka, India

Economic deposits of gold associated with sulfides occur in the Precambrian (Dharwar) rocks of the Hutti-Muski schist near Hutti village (16 degrees 12' N, 70 degrees 39' E) in the northern part of Karnataka State and have been exploited since 1904. The principal mine has produced about 22,500 kg of gold. The average yield, which was 19.49 g per ton of ore in 1920 has now decreased to an average of about 6.3 g per ton. The auriferous lodes, confined to the schist belt composed of metabasalts and massive chert bands, comprise vein quartz with parallel to subparallel, narrow, linear layers of chlorite-biotite schist. The entire schist belt has been intruded by granite and aplite in its western and northern parts. Of nine essentially parallel gold-quartz-sulfide lodes only six are being mined. The lodes are localized along narrow zones of highly sheared chlorite-biotite schist. Structurally the schist belt appears to represent an original isoclinal fold, with its axis striking north-northwest-south-southwest, which has suffered later cross folding. There is evidence of at least three periods of folding. The area is also traversed by a major postmineralization fault trending N 70 degrees W-S 70 degrees E. On a regional scale ore mineralization exhibits stratigraphic, lithologic, and predominantly structural controls. Mineralogically, the ores are composed of gold, sulfides, and oxides. The gangue minerals are quartz, chlorite, biotite, sericite, actinolite, hornblende, tourmaline, calcite, siderite, and ankerite. Textural studies indicate that in the first stage, rutile, pyrite containing inclusions of gold, arsenopyrite, specularite, quartz, and carbonates were formed. During the second stage, pyrrhotite, chalcopyrite, sphalerite, galena, and gold were deposited. During the last stage, pyrite, calcite, and quartz were formed. Further, the studies also suggest that gold deposition occurred mainly during the second stage. The gold occurs in the native state and contains silver up to a maximum of 9 percent, copper up to 0.05 percent, and traces of iron, nickel, and cobalt. Genetically, the gold lodes appear to have been formed by remobilization of the metals present in the mafic volcanic rocks during late periods of tectonic disturbance.

Certain problems of auriferous lodes and placers

Origins and positions of gold fields and ores, particularly of placers, in the Yana-Kolyma area and elsewhere in the Northeast, as indicated by the morphostructural analysis and the behavior of gold in endogenic and exogenic processes. – V.P. Sokoloff.