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1.
The 7.1 Ma Broken Hills adularia-sericite Au–Ag deposit is currently the only producing rhyolite-hosted epithermal deposit in the Hauraki Goldfield of New Zealand. The opaque minerals include pyrite, electrum, acanthite (Ag2S), sphalerite, and galena, which are common in other adularia-sericite epithermal deposits in the Hauraki Goldfield and elsewhere worldwide. Broken Hills ores also contain the less common minerals aguilarite (Ag4SeS), naumannite (Ag2Se), petrovskaite (AuAgS), uytenbogaardtite (Ag3AuS2), fischesserite (Ag3AuSe2), an unnamed silver chloride (Ag2Cl), and unnamed Ag?±?Au minerals. Uytenbogaardtite and petrovskaite occur with high-fineness electrum. Broken Hills is the only deposit in the Hauraki Goldfield where uytenbogaardtite and petrovskaite have been identified, and these phases appear to have formed predominantly from unmixing of a precursor high-temperature phase under hypogene conditions. Supergene minerals include covellite, chalcocite, Au-rich electrum, barite, and a variety of iron oxyhydroxide minerals. Uytenbogaardtite can form under supergene and hypogene conditions, and textural relationships between uytenbogaardtite and associated high-fineness electrum may be similar in both conditions. Distinguishing the likely environment of formation rests principally on identification of other supergene minerals and documenting their relationships with uytenbogaardtite. The presence of aguilarite, naumannite, petrovskaite, and fischesserite at Broken Hills reflects a Se-rich mineral assemblage. In the Hauraki Goldfield and the western Great Basin, USA, Se-rich minerals are more abundant in provinces that are characterized by bimodal rhyolite–andesite volcanism, but in other epithermal provinces worldwide, the controls on the occurrences of Se-bearing minerals remain poorly constrained, in spite of the unusually high grades associated with many Se-rich epithermal deposits.  相似文献   

2.
Hypogene uytenbogaardtite, acanthite, and native gold parageneses have been revealed at the epithermal Yunoe gold-silver deposit, Magadan Region, Russia. Thermodynamic calculations in the system Si–Al–Mg–Ca–Na–K–Fe–Pb–Zn–Cu–Ag–Au–S–C–Cl–H2O were carried out at 25–400 °C and 1–1000 bars to elucidate the role of hydrothermal solutions in the formation of gold and silver sulfides. Several most probable scenarios for ore-forming processes in the deposit are considered: (1) interaction between cold and heated meteoric waters percolating along cracks from surface to depth and reacting with the host rock—rhyolite; (2) evolution of ascending postmagmatic fluid resulting in chloride–carbonic acid solution, which interacts with rhyolite at 100–400 °C; (3) stepwise cooling of hydrothermal ore-bearing solutions; (4) rapid cooling of ore-bearing hydrotherms on their mixing with cold surface waters. Rhyolite with Pb, Zn, Cu, Cl, S, Ag, and Au clarke contents was taken as an initial host rock. Calculations by model 3 showed the possible formation of uytenbogaardtite and petrovskaite at low-temperature stages. Gold and silver sulfides can be deposited during the mixing of ore-bearing acid chloride–carbonic acid hydrothermal solutions with surface alkaline waters.  相似文献   

3.
We carried out experiments on crystallization of Fe-containing melts FeS2Ag0.1–0.1xAu0.1x (x = 0.05, 0.2, 0.4, and 0.8) with Ag/Au weight ratios from 10 to 0.1. Mixtures prepared from elements in corresponding proportions were heated in evacuated quartz ampoules to 1050 ºC and kept at this temperature for 12 h; then they were cooled to 150 ºC, annealed for 30 days, and cooled to room temperature. The solid-phase products were studied by optical and electron microscopy and X-ray spectroscopy. The crystallization products were mainly from iron sulfides: monoclinic pyrrhotite (Fe0.47S0.53 or Fe7S8) and pyrite (Fe0.99S2.01). Gold–silver sulfides (low-temperature modifications) are present in all synthesized samples. Depending on Ag/Au, the following sulfides are produced: acanthite (Ag/Au = 10), solid solutions Ag2–xAuxS (Ag/Au = 10, 2), uytenbogaardtite (Ag/Au = 2, 0.75), and petrovskaite (Ag/Au = 0.75, 0.12). They contain iron impurities (up to 3.3 wt.%). Xenomorphic micro- (<1–5 μm) and macrograins (5–50 μm) of Au–Ag sulfides are localized in pyrite or between the grains of pyrite and pyrrhotite. High-fineness gold was detected in the samples with initial ratio Ag/Au ≤ 2. It is present as fine and large rounded microinclusions or as intergrowths with Au–Ag sulfides in pyrite or, more seldom, at the boundary of pyrite and pyrrhotite grains. This gold contains up to 5.7 wt.% Fe. Based on the sample textures and phase relations, a sequence of their crystallization was determined. At ~1050 ºC, there are probably iron sulfide melt L1 (Fe,S ? Ag,Au), gold–silver sulfide melt L2 (Au,Ag,S ? Fe), and liquid sulfur LS. On cooling, melt L1 produces pyrrhotite; further cooling leads to the crystallization of high-fineness gold (macrograins from L1 and micrograins from L2) and Au–Ag sulfides (micrograins from L1 and macrograins from L2). Pyrite crystallizes after gold–silver sulfides by the peritectic reaction FeS + LS = FeS2 at ~743 ºC. Elemental sulfur is the last to crystallize. Gold–silver sulfides are stable and dominate over native gold and silver, especially in pyrite-containing ores with high Ag/Au ratios.  相似文献   

4.
Gold and silver minerals from the Kupol epithermal deposit, Chukotka, were studied. A schematic sequence of mineral formation has been compiled. Specific mineral assemblages have been revealed in jarosite breccias: with native gold, uytenbogaardtite, fischesserite, acanthite, and native sulfur. We considered a possible mechanism of formation of gold and silver sulfides and selenides in volcanogenic deposits: They might have formed during solfatara and postsolfatara processes with the participation of volcanic gases or their condensates and sublimates as well as liquid sulfur (and selenium) and sulfuric hydrotherms produced under the interaction of volcanic gases with meteoric waters. The specific features of the Kupol deposit confirm the solfatara genesis of Au–Ag sulfides and selenides.  相似文献   

5.
Using the methods of electron spectroscopy of the surface and SEM–EDS, it is shown that native gold of the deposit related to the epithermal Au–Ag ore formation contains oxidized gold with an oxidation degree of Au (I) or higher on the surface. A thin layer (~15 nm) with high concentrations of Ag and S and an underlying SiO2-bearing layer with a thickness of ~30–60 nm play a protective role providing preservation of Ag and Au sulfides in the surface parts of the Au–Ag grains under the oxidizing conditions. S-rich marginal parts of native gold particles may be represented by solid solutions Ag2–xAu x S or (with a lack of S) by agglomerates of Ag n Au m S clusters. The formation of surface zoning in the nanoscale on the surface of native Au is abundant in nature and may be applied in prospecting.  相似文献   

6.
Gold–silver sulfoselenides of the series Ag3AuSexS2–x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray single-crystal analysis, compound Ag3Au1Se0.5S1.5 has the structure of gold–silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag3AuSe0.25S1.75 and Ag3AuSe0.75S1.25 also show trigonal symmetry. Compounds Ag3AuSeS and Ag3AuSe1.5S0.5 are structurally similar to the low-temperature modification of gold–silver selenide Ag3AuSe2 (fischesserite) with space group I4132. These data suggest the existence of two solid solutions: petzite-type cubic Ag3AuSe2–Ag3AuSeS (space group I4132) and trigonal Ag3AuSe0.75S1.25–Ag3AuS2 (space group R3c).It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5–4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol’cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver–gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS.  相似文献   

7.
New data on the mineral composition and the first data on the geochemical composition of ores of the Rogovik gold-silver deposit (Omsukchan ore district, northeastern Russia) have been obtained. Study of the regularities of the spatial distribution of ore mineralization shows that the deposit ores formed in two stages. Epithermal Au-Ag ores of typical poor mineral and elemental compositions were generated at the early volcanic stage. The major minerals are low-fineness native gold, electrum, acanthite, silver sulfosalts, kustelite, and pyrite. The typomorphic elemental composition of ores is as follows: Au, Ag, Sb, As, Se, and Hg. The content of S is low, mostly < 1%. Silver ores of more complex mineral and elemental compositions were produced under the impact of granitoid intrusion at the late volcanoplutonic stage. The major minerals are high-Hg kustelite and native silver, silver sulfosalts and selenides, fahlore, pyrite, chalcopyrite, galena, and sphalerite. The typomorphic elemental composition of ores is as follows: Ag, As, Sb, Se, Hg, Pb, Zn, Cu, and B. The content of S is much higher than 1%. The ores also have elevated contents of Mo, Ge, F, and LREE (La, Ce, and Nd). At the volcanoplutonic stage, polychronous Au-Ag ores formed at the sites of the coexistence of silver and epithermal gold-silver mineralization. Their specific feature is a multicomponent composition and a strong variability in chemical composition (both qualitative and quantitative). Along with the above minerals, the ores contain high-Hg gold, hessite, argyrodite, canfieldite, orthite, fluorapatite, and arsenopyrite. At the sites with strongly rejuvenated rocks, the ores are strongly enriched in Au, Ag, Hg, Cu, Pb, Zn, Ge, Se, La, Ce, Nd, S, and F and also contain Te and Bi. The hypothesis is put forward that the late silver ores belong to the Ag-complex-metal association widespread in the Omsukchan ore district. A close relationship between the ores of different types and their zonal spatial distribution have been established. In the central part of the Rogovik deposit, epithermal Au-Ag ores are widespread in the upper horizons, Ag ores are localized in the middle horizons, and rejuvenated polyassociation Au-Ag ores occur at the sites (mostly deep-seated) with ore-bearing structures of different ages.  相似文献   

8.
Chemical composition and mode of occurrences of (Au, Ag)Te2 minerals such as calaverite (AuTe2), sylvanite (AuAgTe4) and krennerite ((Au, Ag)Te2) in epithermal gold telluride ores from Suzaki, Kawazu and Teine are examined. In the ores from Suzaki, (Au, Ag)Te2 minerals occur in microbands of tellurides and fine quartz. The minerals in telluride bands change from krennerite, via calaverite‐native tellurium, to sylvanite, in the order of crystallization. A sample from Kawazu contains sylvanite and native tellurium with stutzite, hessite and tetradymite in the coarser gray quartz part. The Teine sample also contains sylvanite and native tellurium with barite and quartz. The peak patterns of XRD of calaverite, krennerite and sylvanite from Suzaki are almost identical to that of JCPDS 43–1472, JCPDS 8–20 and JCPDS 9–477, respectively. The Te, Au, Cu, and Ag contents of calaverite from Suzaki range from 56.4 to 57.9 wt.%, from 41.6 to 42.6 wt.%, from 0.28 to 0.45 wt.% and from 0.14 to 0.31 wt.%, respectively, corresponding to the formula Au0.97Ag0.01Cu0.02Te2. The Te, Au, Ag, and Cu contents of krennerite from Suzaki range from 59.6 to 61.4 wt.%, from 31.3 to 33.6 wt.%, from 4.91 to 6.13 wt.% and from 0.66 to 0.80 wt.%, respectively, corresponding to the formula Au0.71Ag0.22Cu0.05Te2 with Au and Ag ranging from 0.68 to 0.74 and from 0.20 to 0.25, respectively. The Te, Au, Ag, and Cu contents of sylvanite from Suzaki range from 61.5 to 63.4 wt.%, from 24.1 to 27.4 wt.%, from 10.0 to 12.5 wt.% and from 0.00 to 0.12 wt.%, respectively. The Te, Au, Ag, and Cu contents of sylvanite from Kawazu range from 62.7 to 63.3 wt.%, from 23.5 to 24.1 wt.%, from 12.0 to 12.5 wt.% and from 0.09 to 0.16 wt.%, respectively. The Te, Au, Ag, Cu and Fe contents of sylvanite from Teine range from 61.8 to 63.5 wt.%, from 23.6 to 24.7 wt.%, from 11.9 to 13.3 wt.%, from 0.01 to 1.65 wt.% and from 0.00 to 0.02 wt.%, respectively. The average formulae of sylvanite from Suzaki, Kawazu, and Teine are expressed as Au1.06Ag0.94Cu0.02Te4, Au1.00Ag0.95Cu0.02Te4 and Au1.01Ag0.95Cu0.06Te4, respectively. Judging from the mineral assemblages of these ores and other localities, Au–Te mineralization in the Japanese Islands can be divided into four types: native gold–calaverite at Date and Agawa, krennerite(?native tellurium) at Osore‐zan and Mutsu, sylvanite–native tellurium–hessite at Teine, Kawazu, Kobetsuzawa, and Kato, and polyminerallic assemblages at Suzaki and Kushikino. The pH–Eh diagram of aqueous tellurium species and tellurium minerals at 250°C indicates that (Au, Ag)Te2 minerals in epithermal gold telluride mineralization would have been formed under middle to low Eh and acidic (to intermediate) pH conditions. It is possible that dilute tellurium‐containing fluid would scavenge dilute gold.  相似文献   

9.
The thermodynamic simulation of the geochemical mobility of Au and Ag during their hydrothermal transfer and precipitation was conducted with regard for the formation of continuous nonideal Au-Ag solid solutions (or AuXAg1?X) alloys) and with the analysis of the effects of principal physicochemical parameters (temperature, pH, redox potential, and the concentrations of sulfide sulfur and chloride ions) on the solubility of Au-Ag alloys of various composition. Predominant Au and Ag complexes and the types of solutions were identified, which are characterized by different extractive ability with respect to these metals. A tendency in the variations in the Au/Ag ratio in the solid phase during the evolution of the hydrothermal process was revealed with regard for various conditions under which Ag sulfides can occur. The possible effect of the AuXAg1?X/solution ratio on the composition of the Au-Ag mineralization was demonstrated, and tendencies in the variations in the composition (fineness) of the alloys during their redeposition were identified.  相似文献   

10.
Here, we report the first documented occurrences of “invisible” gold and silver in seafloor sulfide deposits from an active hydrothermal system on the Central Indian Ridge. A detailed mineralogical and geochemical study of polymetallic sulfides from the Edmond vent field was conducted in order to identify controls on the distribution of precious metals. Bulk samples (N = 18) contain up to 18.7 ppm Au and 1450 ppm Ag, with average concentrations of 2.3 ppm Au and 218.9 ppm Ag. Among them, several Zn-rich chimney fragments and anhydrite-dominated ore samples have higher contents of precious metals than Fe-Cu-rich massive sulfides and silica-rich hydrothermal precipitates. Native gold grains are mainly associated with sphalerite, anhydrite, barite and Fe-oxyhydroxides. Abundant submicroscopic Au-Ag alloys tend to occur along grain boundaries between Cu-Fe sulfides and tennantite, or close to the rims of Fe-poor sphalerite. In contrast to primary electrum with high Ag/Au ratios, the absence of detectable silver in high-purity gold indicates that secondary Au enrichment has probably occurred after a direct co-precipitation with Zn-rich mineral assemblages upon cooling and mixing of vent fluids with cold seawater. A suite of late-stage Ag-rich phases, including argentotennantite, pearceite and acanthite, occur as crack-filling veinlets and patches in low-temperature fahlores, or as tiny inclusions enclosed by pyrite, chalcopyrite and colloform sphalerite. By using HRTEM combined with HAADF-STEM imaging, we have found out that silver is also present in significant quantities as discrete colloidal nanoparticles in tennantite. Minor native copper is closely associated with altered chalcopyrite, sphalerite and covellite, exhibiting signs of dissolution, recrystallization and reprecipitation. Extensive hydrothermal reworking resulted from a long history of high-temperature venting in this field, together with post-depositional supergene replacement processes (involving oxidation, leaching or coupled dissolution-reprecipitation mechanisms facilitated by a permeable porosity generated in primary Cu-Fe sulfides) are considered to be important for the remobilization and local reconcentration of early-formed precious metals, and may have been responsible for the formation of relatively coarse-grained native gold or silver within recrystallized massive sulfides and chimney debris.  相似文献   

11.
《Ore Geology Reviews》2011,43(1):32-46
Hydrothermal pyrite contains significant amounts of minor and trace elements including As, Pb, Sb, Bi, Cu, Co, Ni, Zn, Au, Ag, Se and Te, which can be incorporated into nanoparticles (NPs). NP-bearing pyrite is most common in hydrothermal ore deposits that contain a wide range of trace elements, especially deposits that formed at low temperatures. In this study, we have characterized the chemical composition and structure of these NPs and their host pyrite with high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), analytical electron microscopy (AEM), and electron microprobe analysis (EMPA). Pyrite containing the NPs comes from two types of common low-temperature deposits, Carlin-type (Lone Tree, Screamer, Deep Star (Nevada, USA)), and epithermal (Pueblo Viejo (Dominican Republic) and Porgera (Papua New-Guinea)).EMPA analyses of the pyrite show maximum concentrations of As (11.2), Ni (3.04), Cu (2.99), Sb (2.24), Pb (0.99), Co (0.58), Se (0.2), Au (0.19), Hg (0.19), Ag (0.16), Zn (0.04), and Te (0.04) (in wt.%). Three types of pyrite have been investigated: “pure” or “barren” pyrite, Cu-rich pyrite and As-rich pyrite. Arsenic in pyrite from Carlin-type deposits and the Porgera epithermal deposit is negatively correlated with S, whereas some (colloform) pyrite from Pueblo Viejo shows a negative correlation between As + Cu and Fe. HRTEM observations and SAED patterns confirm that almost all NPs are crystalline and that their size varies from 5 to 100 nm (except for NPs of galena, which have diameters of up to 500 nm). NPs can be divided into three groups on the basis of their chemical composition: (i) native metals: Au, Ag, Ag–Au (electrum); (ii) sulfides and sulfosalts: PbS (galena), HgS (cinnabar), Pb–Sb–S, Ag–Pb–S, Pb–Ag–Sb–S, Pb–Sb–Bi–Ag–Te–S, Pb–Te–Sb–Au–Ag–Bi–S, Cu–Fe–S NPs, and Au–Ag–As–Ni–S; and (iii) Fe-bearing NPs: Fe–As–Ag–Ni–S, Fe–As–Sb–Pb–Ni–Au–S, all of which are in a matrix of distorted and polycrystalline pyrite. TEM-EDX spectra collected from the NPs and pyrite matrix document preferential partitioning of trace metals including Pb, Bi, Sb, Au, Ag, Ni, Te, and As into the NPs. The NPs formed due to exsolution from the pyrite matrix, most commonly for NPs less than 10 nm in size, and direct precipitation from the hydrothermal fluid and deposition into the growing pyrite, most commonly for those > 20 nm in size. NPs containing numerous heavy metals are likely to be found in pyrite and/or other sulfides in various hydrothermal, diagenetic and groundwater systems dominated by reducing conditions.  相似文献   

12.
Mineralogical, fluid inclusion, and geochemical studies of precious metal mineralization within the Baimka trend in the western Chukchi Peninsula have been preformed. Porphyry copper–molybdenum–gold deposits and prospects of the Baimka trend are spatially related to monzonitic rocks of the Early Cretaceous Egdygkych Complex. Four types of precious metal-bearing assemblages have been identified: (1) chalcopyrite + bornite + quartz with high-fineness native gold enclosed in bornite, (2) low-Mn dolomite + quartz + sulfide (chalcopyrite, sphalerite, galena, tennantite-tetrahedrite) ± tourmaline with low-fineness native gold and hessite, (3) rhodochrosite + high-Mn dolomite + quartz + sulfide (chalcopyrite, sphalerite, galena, tennantite- tetrahedrite) with low-fineness native gold, electrum, acanthite, Ag and Au–Ag tellurides, and Ag sulfosalts, and (4) calcite + quartz + sulfide (chalcopyrite, sphalerite, galena) with low-fineness native gold, Ag sulfides and selenides, and Ag-bearing sulfosalts. Study of fluid inclusions from quartz, sphalerite, and fluorite have revealed that hydrothermal ores within the Baimka trend precipitated from fluids with strongly variable salinity at temperatures and pressures ranging from 594 to 104°C and from 1200 to 170 bar, respectively. An indicator of vertical AgPbZn/CuBiMo geochemical zoning is proposed. The value range of this indicator makes it possible to estimate the erosion level of the porphyry–epithermal system. The erosion level of the Baimka deposits and prospects deepens in the following order: Vesenny deposit → Pryamoi prospect → Nakhodka prospect → Peschanka deposit → III Vesenny prospect.  相似文献   

13.
Gold- and silver-containing pyrites of the Tikhii area at the Julietta deposit (Engteri ore cluster, Magadan Region) were studied by optical and scanning electron microscopy and electron probe microanalysis. One- or two-phase rounded microinclusions consisting of electrum (450-680%c) and/or galena or of petrovskaite and/or uytenbogaardtite, galena, and sphalerite have been found in early pyrites. Later As-pyrites (up to 2.6 wt.% As) contain multiphase xenomorphic microinclusions of acanthite, uytenbogaardtite, freibergite, argentotetrahedrite-tennantite, naumannite, petzite, selenopolybasite-selenostephanite, tellurocanfieldite, and other ore minerals localized in pores, cracks, and interstices. Pyrites that underwent hypergene alterations have rims and veinlets formed by acanthite, goethite, anglesite, plattnerite, and native silver. The presence of rounded ore mineral microinclusions and large pores in the early pyrites suggests the participation of volatiles in the mineral formation and the uptake of large amounts of impurities by pyrite under high-gradient crystallization conditions. The thermobarogeochemical studies of fluid inclusions in quartz have shown that the ore zone formed under boiling-up of hydrothermal medium-concentration NaCl solutions at 230-105 °C. The results of thermodynamic calculations evidence that Ag-Au-S-Se minerals formed under decrease in temperature and fugacity of sulfur (log1Q/S2 = -22 to -9) and selenium (log1Q/Se2 = -27 to -14) and change of reducing conditions by oxidizing ones in weakly acidic to near-neutral solutions.  相似文献   

14.
This paper is focused on the new data for geology, mineralogy, and geochemistry of stockworks consisting of steep and gentle quartz veins and veinlets forming a complex multilevel structure at the Rodion deposit. These stockworks range from 25 to 150 m in thickness. Average gold grade is 1.8 g/t. Ore minerals pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, and native gold are predominantly concentrated on the vein and veinlet walls. Thermal metamorphism caused by the intrusion of the Ulakhan granodiorite pluton is the important singularity of the deposit. The deposit ore is enriched in chalcophile microelements Au, Ag, As, Sb, Cu, Pb, Zn, and Bi as compared to the average composition of the upper crust and hosting Permian sequences. The enrichment factors range from a few to hundreds of times. Bi, W, Pb, Ag, and Na2O are positively correlated between each other and with Au. The highest correlation coefficient 0.59 is between Au and Bi. Au is negatively correlated with Ba, Li, Co, Ni, Mn, Ti, and Be. The stockwork ores were formed involving homogeneous low-saline (9.4–4.3 wt % NaCl equiv) substantially aqueous bicarbonate-chloride fluid at 275–330°C and 300–1840 bar fluid pressure. Fluid has a high concentration of CO2 (up to 349 g/kg of water) and is reductive (СО2/СН4 = 17–37.3). Na and Ca are the major cations in the fluid, whereas K and Mg are minor. In addition, many microelements were detected in the fluid: As, Li, Rb, Cs, Mo, Ag, Sb, Cu, Zn, Cd, Pb, U, Ga, Ge, Ti, Mn, Fe, Co, Ni, V, Cr, Y, Zr, Sn, Ba, W, Au, Hg, and REE. The results obtained are consistent with the metamorphic–magmatic formation model of orogenic gold–quartz deposits within the Yana–Kolyma belt.  相似文献   

15.
《International Geology Review》2012,54(12):1149-1165
The Gay deposit, situated in the Orenburg region, is identified with one of Russia's principal occurrences of pyrite (pyrite deposits are an important source of Russia's gold). It belongs to the west subzone of the Magnitogorsk synclinorium and occurs in Devonian rhyolite-basaltic volcanic rocks. The deposit comprises five large pyrite-chalcopyrite, pyrite-chalcopyrite- sphalerite, and pyrite orebodies. The supergene zone extends to 120-240 m below surface and consists of the following three subhorizontal zones (from bottom to top): the secondary sulfide enrichment, the leaching, and the oxidation zone (where ores are enriched in gold).

There are two levels of secondary gold enrichment in the weathering profile. The lower level, located in the leaching zone, corresponds with the level of water table fluctuations. The rich, flat-lying horizon (1.5-10.0 m) is composed of bedded, friable native sulfur-quartz ores; it contains 19.0-52.2 ppm Au and up to 389 ppm Ag. Native gold and silver halides (chlorargyrite, iodargyrite, and embolite) are the principal precious-metal minerals. Electrum, native silver, acanthite, and uytenbogaardtite constitute the minor ones. The upper level of the enrichment is located in the lower part of gossan. This bonanza is composed of hematite-quartz ochres. Gold concentration is 13.5 to 21.2 ppm. Native gold of high fineness and silver halides apparently are associated here with poorly crystallized iron oxides. The formation of supergene gold enrichments may result partly from residual concentration and partly from mobilization and reprecipitation of the precious metal. Rich horizons form by repeated gold redeposition in accordance with weathering and a gradual erosion surface lowering. The lower bonanza forms at first in the process of oxidation involving pyrite and native sulfur. Gold may be transported by complexes with metastable sulfur oxy-anions: sulfites, thiosulfates, or polythionates. The upper enriched horizon forms in the course of further evolution of the weathering profile in the stage of hematite recrystallizaiton and its transformation into goethite.  相似文献   

16.
The Laloki and Federal Flag deposits are two of the many (over 45) polymetallic massive sulfide deposits that occur in the Astrolabe Mineral Field, Papua New Guinea. New data of the mineralogical compositions, mineral textures, and fluid inclusion studies on sphalerite from Laloki and Federal Flag deposits were investigated to clarify physiochemical conditions of the mineralization at both deposits. The two deposits are located about 2 km apart and they are stratigraphically hosted by siliceous to carbonaceous claystone and rare gray chert of Paleocene–Eocene age. Massive sulfide ore and host rock samples were collected from each deposit for mineralogical, geochemical, and fluid inclusion studies. Mineralization at the Laloki deposit consists of early‐stage massive sulfide mineralization (sphalerite‐barite, chalcopyrite, and pyrite–marcasite) and late‐stage brecciation and remobilization of early‐stage massive sulfides that was accompanied by late‐stage sphalerite mineralization. Occurrence of native gold blebs in early‐stage massive pyrite–marcasite‐chalcopyrite ore with the association of pyrrhotite‐hematite and abundant planktonic foraminifera remnants was due to reduction of hydrothermal fluids by the reaction with organic‐rich sediments and seawater mixing. Precipitation of fine‐grained gold blebs in late‐stage Fe‐rich sphalerite resulted from low temperature and higher salinity ore fluids in sulfur reducing conditions. In contrast, the massive sulfide ores from the Federal Flag deposit contain Fe‐rich sphalerite and subordinate sulfarsenides. Native gold blebs occur as inclusions in Fe‐rich sphalerite, along sphalerite grain boundaries, and in the siliceous‐hematitic matrix. Such occurrences of native gold suggest that gold was initially precipitated from high‐temperature, moderate to highly reduced, low‐sulfur ore fluids. Concentrations of Au and Ag from both Laloki and Federal Flag deposits were within the range (<10 ppm Au and <100 ppm Ag) of massive sulfides at a mid‐ocean ridge setting rather than typical arc‐type massive sulfides. The complex relationship between FeS contents in sphalerite and gold grades of both deposits is probably due to the initial deposition of gold on the seafloor that may have been controlled by factors such as Au complexes, pH, and fO2 in combination with temperature and sulfur fugacity.  相似文献   

17.
广东陆丰硫铁矿床为一个中低温热液交代充填型脉状矿床。伴生有综合利用价值的Ｃｕ、Ａｕ、Ａｇ、Ｔｅ、Ｂｉ等元素。其中Ａｕ主要呈微细粒自然金、次显微金包裹在黄铁矿中，Ａｇ主要呈自然银、螺状硫银矿等独立矿物充填在脉石矿物中。Ｔｅ、Ｂｉ呈碲铋矿、硫盐矿物嵌布在黄铁矿或充填在黄铁矿微裂隙中。多数硫盐矿物为矿区首次发现。通过矿石物质组分的研究，查清了上述元素的赋存状态，为硫铁矿床的综合评价，有益组分的合理回收提供了依据  相似文献   

18.
The Ashanti belt, a world-renowned gold producing region in southwest Ghana, has received renewed attention in recent years. Most studies, however, focused on the major deposits situated along the Ashanti shear zone and in the adjacent Tarkwa Basin to the east, neglecting smaller yet important occurrences, such as the Pampe deposit which occurs few kilometers to the west of this shear zone, on the Akropong Trend. Nevertheless, investigating such simpler smaller-scale mineralizing systems is attractive, in that this can help shedding light on the processes that control gold deposition at the regional scale.At Pampe, gold endowment has been estimated at approximately half a million ounces with an average gold grade of 2.8 g/t. The mineralization is related to two sets of quartz veins; a first set (V1), which has a NE trend and is sub-parallel to the main foliation (S1), and a second set (V2), which crosscuts this foliation. The V2 veins have a NNW–SSE trend with local conjugate geometries indicating that they formed during NNW–SSE shortening, which, regionally, is linked to major orogenic gold deposition. Gold mineralization is systematically associated with sulfides, which occur disseminated in the vein walls and in the surrounding host rocks. The ore sulfide paragenesis consists of 1) a first generation of pyrite, which is associated with V1 veins; 2) a second generation of sulfides, consisting of an intergrowth of arsenopyrite and pyrite that crystallized contemporaneously with the formation of the second vein set; 3) a late phase of pyrite growth which occurs as overgrowths on phase-2 sulfides and formed during the waning stages of V2 emplacement. Invisible (sub-microscopic) gold was detected in all sulfide generations by LA-ICP-MS. The analytical profiles for the Au signal are mimicked by those for Pb, Cu, As, Ag, Te, and Bi. Invisible gold is thus interpreted to have precipitated within sulfides in the form of nanoparticles (colloidal gold alloys). Conversely, visible native gold grains were recognized exclusively in association with arsenopyrite from late V2 veins, either as inclusions or, more commonly, at the boundary with other sulfides, as well as in micro-fractures that crosscut the sulfides. Gold precipitation was likely induced by sulfidation of the wall rock during fluid–rock interaction. The Pampe deposit exemplifies the mineralization processes that took place at larger scale in neighboring world-class deposits such as Obuasi, Bogoso and Prestea.  相似文献   

19.
Gold mineralization at Chah Zard, Iran, is mostly concentrated in breccia and veins, and is closely associated with pyrite. Optical and scanning electron microscopy-backscattered electron observations indicate four different pyrite types, each characterized by different textures: porous and fractured py1, simple-zoned, oscillatory-rimmed, framboidal and fibrous py2, colloform py3, and inclusion-rich py4. Laser ablation ICP–MS analysis and elemental mapping reveal the presence of invisible gold in all pyrite types. The highest concentrations (161–166 ppm Au) are found in py2 and py4, which correlate with the highest As concentrations (73,000–76,000 ppm). In As-poor grains, Au concentrations decrease by about two orders of magnitude. Copper, Pb, Zn, Te, Sb, and Ag occur with invisible gold, suggesting that at least part of the gold occurs in nanoparticles of sulfosalts of these metals and metalloids. Gold distribution patterns suggest that only negligible Au was originally trapped in py1 from the initial ore fluids. However, most, if not all, Au was transported and deposited during subsequent overprinting hydrothermal fluid flow in overgrowth rims around the margins of the py2 and within microfractures of py4 grains. Oscillatory zonation patterns for Co, Ni, Sb, Cu, Pb, and Ag in pyrite reflect fluctuations in the hydrothermal fluid chemistry. The LA-ICP–MS data reveal that Cu, Pb and Ag show systematic variations between different pyrite types. Thus, Cu/Pb and Pb/Ag ratios in pyrite may provide a potentially powerful exploration vector to epithermal gold mineralization at Chah Zard district and elsewhere.  相似文献   

20.
The intermetallic compound Ni3Au and Au-Ni solid solutions (native nickel Au and Au-bearing native Ni) were found in the contact metamorphosed metalliferous sediments of the Triassic chert formation in Sikhote-Alin. The metalliferous rocks are characterized by high contents of Au, Ag, and PGE, as well as the presence of diverse minerals of precious metals. Nickel gold (Au0.91-0.88 Ni0.09-0.12) is found as grains and crystals (3 to 4 im in size) in Au-bearing cherts with hematite, which are conventionally defined as “itabir-ites” and in the altered siliceous rocks of the Dal’nerechensky district (the upper reaches of the Gornya River). The nickel gold is associated with copper Au, pure native Au (Au1.00), and Au-Ag and Au-Ag-Pb solid solutions (Au0.86/0.84Ag0.14_0.16 and Au0.78Ag0.19Pb0.03, respectively). The Au-bearing Ni is found in the metamorphosed carbonaceous mudstones and Au-bearing “itabirites” of the Shirokopadninsky area (Olgin-sky district). The Au content varies from 6.09 wt % (Ni0.98Au0.02) to 11.30 wt % (Ni0.96Au0.04) in some Au-bearing Ni grains (about 10—15 im in diameter) taken from the metamorphosed mudstones. The grains of Au-bearing Ni (Co0.001_0.00Au00.2 - 0.17 Ni 0.98_0.83) in the “itabirites” are also characterized by their heterogeneous composition and the fine impregnation of the Ni3 Au intermetallic compound (Ni2.99Au1.01 based on the microprobe analysis).  相似文献   

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