MINERALOGY, FLUID INCLUSION MICROTHERMOMETRY, AND THE GENESIS OF ORE-BEARING FLUIDS IN THE WEST OF QALEH-ZARI (CU-AU-AG MINE) ANOMALY ZONE, KUDKAN MINERALIZATION, KHORASAN, IRAN
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The Kudkan mineralization is one of the promising areas for Cu-Mo porphyry and related epithermal mineralization in north part of Lut Block in the Central Iran. Based on surficial geochemical studies in this area, there are four anomalous zones. The most important zone is zone 1 that is located in the west of Qaleh-zari mine. In this district gold, silver, and base-metal mineralization occurs in breccia and veins associated with Tertiary volcanic host (andesitic, trachyandesitic, basaltic andesite and related tuff) of Eocene age. Microthermometric measurements on quartz-hosted fluid inclusions indicate that mineralization formed between 109 - 429°C, from a weak to moderately saline hydrothermal fluid (0.2–9.7wt. %NaCl equiv). First ice-melting temperatures were determined to be between (−0.2 and−6.3 °C) indicate that the fluids mostly contained NaCl and KCl. Coexisting liquid-rich and vapor-rich fluid inclusions in quartz provide evidence for boiling in ore-stage breccia and veins, consistent with the presence of hydrothermal breccia and chalcedonic quartz. Metal (base and likely precious) deposition in this zone is inferred to have been largely caused by boiling, although fluid mixing and/or wall rock reactions may also have occurred. After rising to a depth of between (1510- 189 m), the fluid boiled causing deposition of fine-grained quartz and sealing of the hydrothermal conduit. Episodic boiling in response to alternating silica sealing and hydraulic brecciation was responsible for ore deposition. Pressure for mineralization is estimated to be between <20- 350 bar, with a density of fluids of 0.86 -0.91 g/cm3. Main alteration types consist of sericitization, argillization, and silicification. Hydrothermal alteration zones are well-developed and zoned around mineralized veins and host rocks with abundant sulfide. Metals may have precipitated due to the destabilization of bisulfide and chloride complexes, caused by boiling-off of H2S to vapor, whereas the dilution and/or cooling of hydrothermal fluids led to the precipitation of base metals. Results show that the source of mineralization is mixed magmatic-meteoric fluids and thus represents later stages of hydrothermal activity. Mineralization in this district are low to intermediate sulfidation systems, rich in Cu-As-Pb-Zn-Fe-S and locally Au-Ag assemblages related to intrusive Cu-Mo bodies postulated at depth.Keywords:
Breccia
Ore genesis
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Argillic alteration
Molybdenite
Magmatic water
Cassiterite
Hypogene
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Fluid inclusions in the subeconomic porphyry Cu-Mo-Au system at Nevados de Famatina and closely associated high-sulfidation epithermal Cu-Au-(Ag-As-Sb-Te) veins at La Mejicana, northwest Argentina, were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment. Field geology, vein petrography, fluid inclusion microthermometry, and single-inclusion microanalysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were combined to determine the evolution of pressure, temperature, and ore metal concentrations in the fluids. Cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations. Aqueous liquid inclusions of ~5 wt percent NaCl equiv salinity trapped in quartz-sericite-pyrite (QSP) veins between 360° and 325°C contain unusually high concentrations of Cu (400–5,000 ppm), As (~200 ppm), Sb and Te (both up to 100 ppm), and Au (several ppm) along with other ore-forming elements. These veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor, and high-sulfidation epithermal veins with high Au grades, which are preserved along a high ridge adjacent to the porphyry stock. Low-density vapor and hypersaline liquid (i.e., brine) inclusions were trapped in early-formed quartz of the porphyry stockwork veins, between 450° and >600°C. The brine contains lower Cu and Au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins; the latter may be equivalent to a parental fluid. The low- and intermediate-density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz-sericite-pyrite veins. Fluid inclusions and geologic time relations indicate that the transitional quartz-sericite-pyrite veins were the channelways for metal-rich aqueous liquids that generated the high-sulfidation epithermal Cu-Au deposit. These mineralizing liquids are interpreted to have evolved by continuous density increase (“contraction”) from a low-salinity, S-rich magmatic fluid of low to intermediate density. This vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic-hydrothermal complex, and probably underwent minor brine separation prior to reaching the present level of exposure. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent.
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The Igoudrane mine with a total production of 700,000 t of ore grading 485 g/t Ag is currently one of the most productive mines in the Imiter district of the eastern Anti-Atlas in Morocco. The silver-rich ± base metal deposit occurs dominantly as vein- and hydrothermal breccia-hosted orebodies at the interface between the lower Ediacaran turbidites of the Saghro Group and the unconformably overlying, dominantly felsic volcanic, and volcaniclastic rocks of the late Ediacaran Ouarzazate Group. Higher-grade ores are lithologically hosted by the uppermost organic-rich black shale unit and structurally controlled by the intersection of subvertical NW- and NE-trending fault systems. Ore-related hydrothermal alteration includes, in order of decreasing abundance, carbonatization, silicification, sericitization, and chloritization. Three primary paragenetic stages of veining and associated silver ± base metal mineralization have been recognized: (1) early pyrite + quartz + Ag-bearing sulfides and sulfosalts; (2) main Ag-bearing sulfides and sulfosalts + calcite ± fluorite ± dolomite; and (3) late quartz + calcite + base-metal sulfides (galena, sphalerite, pyrite, chalcopyrite). Irrespective of the ore stage, the dominant Ag-bearing ore minerals are Ag-Hg amalgam, argentite, freibergite, acanthite, polybasite, pyrargyrite, and proustite. Fluid inclusion data show a trend of decreasing temperatures with time, from the main silver stage (Th = 180 ± 12 °C) to late base-metal stage (Th = 146 ± 7 °C), consistent with fluid mixing, cooling, and/or dilution. The coexistence of aqueous-rich and vapor-rich fluid inclusions together with variations in bulk salinity (NaCl + CaCl2) of the mineralizing fluids during the main silver stage, at similar temperatures, indicate that boiling and subsequent degassing occurred during the main ore-forming event due to a pressure decrease. Calculated δ18Ofluid values along with REE+Y and Sr isotope constraints suggest that the ore-forming fluids originated from a predominantly magmatic source, although incursion of meteoric waters during collapse of the hydrothermal system could have contributed to deposition. The post-ore, base-metal quartz-carbonate-dominated mineralization was deposited from dilute Ca-Na-Cl-bearing fluids at temperature below 150 °C. Overall, fluid–rock interaction with the black shales along major faults and thin permeable horizons, boiling-degassing—with subsequent fluid mixing, cooling, and/or dilution—were the main mechanisms of silver deposition.
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