Genetic mineralogy of pyrite from Jindoushan gold deposit, Yantai, Shandong Province
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In-seam pyrite distributions, inorganic and organic associations, and effects of seam floor paleotopography on these distributions were examined in the Springfield Coal, Indiana. Macroscopic and microscopic pyrite types were oxidized in an electrograph, and pyrites with differing oxidation rates were subjected to x-ray diffraction analysis to measure relative crystallinity. Paleotopographically high portions of the seam tended to be thinner and contain more macroscopic lenticular and cleat filling pyrite than lower portions. Microscopic pyrite was concentrated in the inorganic (mainly clay) layers in the seam. Other pyrite-associated minerals were calcite and siderite. Highly oxidizing pyrite exhibited small crystal size (less than 10 microns) and low crystallinity. Moderately oxidizing pyrite occurred as 15 to 25 micron crystals with higher crystallinity than the higher oxidizing type, and weakly oxidizing pyrite had the largest crystal size (greater than 25 microns), and the highest degree of crystallinity.
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Abstract The Z imudang gold deposit is a large C arlin‐type gold deposit in the S outhwest G uizhou P rovince, C hina, with an average Au content of 6.2 g/t. Gold is mainly hosted in the fault zone and surrounding strata of the F 1 fault and P ermian L ongtan F ormation, and the ore bodies are strictly controlled by both the faults and strata. Detailed mineralogy and geochemistry studies are conducted to help judge the nature of ore‐forming fluids. The results indicate that the Au is generally rich in the sulfides of both ores and wall rocks in the deposit, and the arsenian pyrite and arsenopyrite are the main gold‐bearing sulfides. Four subtypes of arsenian pyrite are found in the deposit, including the euhedral and subhedral pyrite, framboidal pyrite, pyrite aggregates and pyrite veins. The euhedral and subhedral pyrite, which can take up about 80% of total pyrite grains, is the dominant type. Au distributed unevenly in the euhedral and subhedral pyrite, and the content of the Au in the rim is relatively higher than in the core. Au in the pyrite veins and pyrite aggregates is lower than the euhedral and subhedral pyrite. No Au has been detected in the points of framboidal pyrites in this study. An obvious highly enriched As rim exists in the X ‐ray images of euhedral pyrites, implying the ore‐forming fluids may be rich in As . The relationship between Au and As reveals that the Au may host as a solid solution ( Au + ) and nanoparticles of native gold ( Au 0 ) in the sulfides. The high C o/ N i ratio (>1) of sulfides and the enrichment of W in the ores all reflect that the gold‐bearing minerals and ore‐forming process were mainly related to the hydrothermal fluids, but the magmatic and volcanic activities cannot be neglected. The general existence of Au and As in the sulfides of both ores and wall rocks and the REE results suggest that the ore‐forming fluids may mainly be derived from the basin itself. The enrichment of T l suggests that the ore‐forming fluids may be enriched in Cl . The Ce and Eu show slightly or apparently negative anomalies, which means the ore fluids were probably formed under reducing environment. The Y / H o ratios of ore samples fluctuate around 28, implying the bicarbonate complexation and fluorine were both involved in the ore‐forming process. Combined with the previous studies and our results, we infer that the ore‐forming fluids enriched Au , As , HS − and halogen ( F , Cl ) were derived from the mixture of reducing basinal fluids and magmatic or volcanic hydrothermal fluids.
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Pyrites are widely distributed in marine sediments, the morphology of which is applied as a proxy to infer the redox conditions of bottom water, and identify diagenetic stages and hydrocarbon leakage activities. In this review, the methods used for the morphological study of pyrite are summarized. The textural and size characteristics of euhedral pyrite and pyrite aggregates, as the formation and evolution mechanism of pyrite are discussed for their significance in reconstructing the geochemical environment. The morphological study of pyrite includes shape observation, size estimation, and surface feature analysis. Scanning electron microscope and optical microscope are the main methods for morphological observation; transmission electron microscope and scanning tunneling microscope are applicable to observe nanoscale morphological structures and crystal growth on the crystal surface, and X-ray computed tomography is capable of measuring pyrite size distribution at the scale of a micrometer. Under the marine sedimentary condition, the single crystal of pyrite appears in cube, octahedron, dodecahedron, and their intermediates, the size of which ranges from several nanometers to more than 100 µm. The morphology of euhedral pyrite is controlled by temperature, pH, the chemical composition of interstitial water, etc., and might have been experienced in later reformation processes. The pyrite aggregates occur as framboid, rod-like, fossil-infilling, etc., characterized by the comparatively large size of several microns to several millimeters. It is found that certain textures correspond with different formation mechanisms and geochemical environments. Particularly, under special geological conditions, for instance, the methane leakage and/or decomposition of gas hydrate, pyrite is anomaly enriched with morphological textures of massive framboid cluster, rod-like aggregates, etc., and framboid is found with a large mean diameter (>20 µm) and standard deviation (>10 µm). These typical features can be employed to ascertain the position of the paleo sulfate methane transition zone (SMTZ).
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The massive sulfide ores of the Pobeda hydrothermal fields are grouped into five main mineral microfacies: (1) isocubanite-pyrite, (2) pyrite-wurtzite-isocubanite, (3) pyrite with minor isocubanite and wurtzite-sphalerite microinclusions, (4) pyrite-rich with framboidal pyrite, and (5) marcasite-pyrite. This sequence reflects the transition from feeder zone facies to seafloor diffuser facies. Spongy, framboidal, and fine-grained pyrite varieties replaced pyrrhotite, greigite, and mackinawite “precursors”. The later coarse and fine banding oscillatory-zoned pyrite and marcasite crystals are overgrown or replaced by unzoned subhedral and euhedral pyrite. In the microfacies range, the amount of isocubanite, wurtzite, unzoned euhedral pyrite decreases versus an increasing portion of framboidal, fine-grained, and spongy pyrite and also marcasite and its colloform and radial varieties. The trace element characteristics of massive sulfides of Pobeda seafloor massive sulfide (SMS) deposit are subdivided into four associations: (1) high temperature—Cu, Se, Te, Bi, Co, and Ni; (2) mid temperature—Zn, As, Sb, and Sn; (3) low temperature—Pb, Sb, Ag, Bi, Au, Tl, and Mn; and (4) seawater—U, V, Mo, and Ni. The high contents of Cu, Co, Se, Bi, Te, and values of Co/Ni ratios decrease in the range from unzoned euhedral pyrite to oscillatory-zoned and framboidal pyrite, as well as to colloform and crystalline marcasite. The trend of Co/Ni values indicates a change from hydrothermal to hydrothermal-diagenetic crystallization of the pyrite. The concentrations of Zn, As, Sb, Pb, Ag, and Tl, as commonly observed in pyrite formed from mid- and low-temperature fluids, decline with increasing crystal size of pyrite and marcasite. Coarse oscillatory-zoned pyrite crystals contain elevated Mn compared to unzoned euhedral varieties. Framboidal pyrite hosts maximum concentrations of Mo, U, and V probably derived from ocean water mixed with hydrothermal fluids. In the Pobeda SMS deposit, the position of microfacies changes from the black smoker feeder zone at the base of the ore body, to seafloor marcasite-pyrite from diffuser fragments in sulfide breccias. We suggest that the temperatures of mineralization decreased in the same direction and determined the zonal character of deposit.
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