Ore Genesis of the Chuduoqu Pb-Zn-Cu Deposit in the Tuotuohe Area, Central Tibet: Evidence from Fluid Inclusions and C–H–O–S–Pb Isotopes Systematics
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The Chuduoqu Pb-Zn-Cu deposit is located in the Tuotuohe area in the northern part of the Sanjiang Metallogenic Belt, central Tibet. The Pb-Zn-Cu ore bodies in this deposit are hosted mainly by Middle Jurassic Xiali Formation limestone and sandstone, and are structurally controlled by a series of NWW trending faults. In this paper, we present the results of fluid inclusions and isotope (C, H, O, S, and Pb) investigations of the Chuduoqu deposit. Four stages of hydrothermal ore mineralization are identified: quartz–specularite (stage I), quartz–barite–chalcopyrite (stage II), quartz–polymetallic sulfide (stage III), and quartz–carbonate (stage IV). Two types of fluid inclusions are identified in the Chuduoqu Pb-Zn-Cu deposit: liquid-rich and vapor-rich. The homogenization temperatures of fluid inclusions for stages I–IV are 318–370 °C, 250–308 °C, 230–294 °C, and 144–233 °C, respectively. Fluid salinities range from 2.07 wt. % to 11.81 wt. % NaCl equivalent. The microthermometric data indicate that the fluid mixing and cooling are two important mechanisms for ore precipitation. The H and O isotopic compositions of quartz indicate a primarily magmatic origin for the ore-forming fluids, with the proportion of meteoric water increasing over time. The C and O isotopic compositions of carbonate samples indicate that a large amount of magmatic water was still involved in the final stage of mineralization. The S and Pb isotopic compositions of sulfides, demonstrate that the ore minerals have a magmatic source. On a regional basis, the most likely source of the metallogenic material was regional potassium-enriched magmatic hydrothermal fluid. Specifically for the Chuduoqu Pb-Zn-Cu deposit, the magmatic activity of a syenite porphyry was the likely heat source, and this porphyry also provided the main metallogenic material for the deposit. Mineralization took place between 40 and 24 Ma. The Chuduoqu deposit is a mesothermal hydrothermal vein deposit and was formed in an extensional environment related to the late stage of intracontinental orogenesis resulting from India–Asia collision. The determination of the deposit type and genesis of Chuduoqu is important because it will inform and guide further exploration for hydrothermal-type Pb and Zn deposits in the Tuotuohe area and in the wider Sanjiang Metallogenic Belt.Keywords:
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The Xiaokelehe porphyry Cu-Mo deposit in the Great Xing'an Range contains six stages of quartz-sulfide veins (V1 to V6) including potassic (V1 to V4), chlorite-illite (V5) and phyllic (V6). Two to three types of quartz were identified within each stage, of which the V2Q1, V3Q1, V4Q2, V5Q1 and V6Q2 are spatially associated with sulfides. Three types of fluid inclusions were identified in these veins, i.e., type I aqueous inclusions homogenized to liquid, type II aqueous inclusions homogenized to vapor, and type III aqueous inclusions containing halite and homogenized to liquid. Type I and type II primary inclusions occur in all stages, and type III inclusions are developed in V2 to V4 veins. Microthermometric data show that the maximum formation temperatures and pressures of V2-V6 veins of type I inclusions are 353 to 437 °C and 19.0 to 27.6 MPa, 309 to 415 °C and 15.0 to 30.1 MPa, 330 to 365 °C and 16.7 to 27.6 MPa, 243 to 351 °C and 13.9 to 18.9 MPa and 255 to 347 °C and 13.9 to 17.8 MPa, respectively, which show a decreasing trend. Oxygen isotope data show that the δ18OH2O values of V2-V4 veins (5.3 to 10.4 ‰) are consistent with typical magmatic values, whilst the δ18OH2O values of V5-V6 veins (0.2 to 4.6 ‰) are much lower than those of the magmatic water, which is due to involvement of meteoric water. The fluid inclusion microthermometry and O isotope data suggest an evolving magmatic-hydrothermal system with decreasing temperatures, pressures from V2 to V4 veins (potassic stage) to V5 (chlorite-illite stage) and V6 veins (phyllic stage), and an increasing incorporation of meteoric water in V5 and V6. The Xiaokelehe deposit has lower CO2 contents than the adjacent porphyry Mo deposits formed in the same post-collisional setting, which is mainly due to differences of mineralization depths and magma source. The formation of porphyry deposits with different Mo/Cu ratios highlights the diversity and complexity of the mineralizing systems in post-collisional settings, which has important implications for mineral exploration in such environments.
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Abstract. Primary fluid inclusions in quartz and carbonates from the Kanggur gold deposit are dominated by aqueous inclusions, with subsidiary CO 2 ‐H 2 O inclusions that have a constant range in CO 2 content (10–20 vol %). Microthermometric results indicate that total homogenization temperatures have a wide but similar range for both aqueous inclusions (120d̀ to 310d̀C) and CO 2 ‐H 2 O inclusions (140d̀ to 340d̀C). Estimates of fluid salinity for CO 2 ‐H 2 O inclusions are quite restricted (5.9∼10.3 equiv. wt% NaCl), whereas aqueous inclusions show much wider salinity ranging from 2.2 to 15.6 equivalent wt %NaCl. The 6D values of fluid inclusions in carbonates vary from ‐45 to ‐61 %, in well accord with the published δD values of fluid inclusions in quartz (‐46 to ‐66 %). Most of the δ 18 O and δD values of the ore‐forming fluids can be achieved by exchanged meteoric water after isotopic equilibration with wall rock by fluid/rock interaction at a low water/rock ratio. However, the exchanged meteoric water alone cannot explain the full range of δ 18 O and δD values, magmatic and/or meta‐morphic water should also be involved. The wide salinity in aqueous inclusions may also result from mixing of meteoric water and magmatic and/or metamorphic water.
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δ34S
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Abstract: The origin of mineralizing fluids responsible for the Hishikari vein‐type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (δD) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present δ values and the oxygen isotopic ratios (δ 18 O) previously reported by Shikazono and Nagayama (1993). The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500°C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450°C. The δD values were determined by mass spectrometer. The δD values of inclusion fluid obtained from quartz range from –61 to –114%. These are significantly lower than the δD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water. Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous δD values of lower than –100%. Such D depletion cannot be caused by simple oxygen‐shift of meteoric water or by contribution of magmatic volatiles. The δD values of water released from the shale samples of the Shimanto–Supergroup, a major host to the Hishikari veins range from –132 to –148%. Therefore, the anomalous δD values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.
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Abstract The Hongshi copper deposit is located in the middle of the Kalatage ore district in the northern segment of the Dananhu‐Tousuquan island‐arc belt in East Tianshan, Xinjiang, NW China. This study analyses the fluid inclusions and H, O, and S stable isotopic compositions of the deposit. The fluid‐inclusion data indicate that aqueous fluid inclusions were trapped in chalcopyrite‐bearing quartz veins in the gangue minerals. The homogenization temperatures range from 108°C to 299°C, and the salinities range from 0.5% to 11.8%, indicating medium to low temperatures and salinities. The trapping pressures range from 34.5 MPa to 56.8 MPa. The δ 18 O H2O values and δ D values of the fluid range from –6.94‰ to –5.33‰ and from –95.31‰ to –48.20‰, respectively. The H and O isotopic data indicate that the ore‐forming fluid derived from a mix of magmatic water and meteoric water and that meteoric water played a significant role. The S isotopic composition of pyrite ranges from 1.9‰ to 5.2‰, with an average value of 3.1‰, and the S isotopic composition of chalcopyrite ranges from –0.9‰ to 4‰, with an average value of 1.36‰, implying that the S in the ore‐forming materials was derived from the mantle. The introduction of meteoric water decreased the temperature, volatile content, and pressure, resulting in immiscibility. These factors may have been the major causes of the mineralization of the Hongshi copper deposit. Based on all the geologic and fluid characteristics, we conclude that the Hongshi copper deposit is an epithermal deposit.
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Gangue
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The Luming molybdenum deposit,located in the Lesser Xing'an Range-Zhangguangcai Range polymetallic ore-forming belt,is mainly hosted in monzogranite. According to mineral assemblages,alteration and crosscutting relationships of the veins,the mineralizing stages of the Luming molybdenum deposit can be divided into three: 1) potassium silicification disseminated mineralization stage; 2) silicified stockwork mineralization stage; 3) chlorite-carbonate stage. Aqueous,gaseous,CH4( CO2)-bearing and daughter mineral-bearing inclusions coexist in molybdenite quartz veins of Luming molybdenum deposit. The homogenization temperatures of aqueous inclusions are from 133 to 425℃,salinities of 1. 6% ~ 16. 1% Na Cleqv. Gaseous inclusions with homogenization temperatures of 243 ~ 500℃,salinities of 1. 2% ~ 14. 1% Na Cleqv. Daughter mineral-bearing inclusions with homogenization temperatures of 297 ~449℃,salinities of 38. 2% ~ 53. 1% Na Cleqv. CH4( CO2)-bearing fluid inclusions by laser Raman spectroscopic analysis confirmed that the components of bubble phase are dominated by CH4,a few containing a small amount of CO2,with homogenization temperatures of 334 to 437℃. δ34S range of 4. 5‰ ~ 5. 7‰,shows the sulfur comes mainly from magmatic hydrothermal ore-forming fluid. The hydrogen and oxygen isotope data fall near the magmatic water and drift to the meteoric water in the δD-δ18OH2Odiagram,indicating that ore-forming fluids in main mineralizing stages were magmatic water and mixed with a small amount of meteoric water. The trapping pressures of fluid inclusions are estimated to be 30 ~ 90 MPa,and consequently corresponding to a depth of 3 to 9km. The ore-forming fluid was initially a single H2O-Na Cl-CH4( CO2) supercritical fluid system with high temperature and medium salinity separated from the magma chamber. The ore-forming fluid boiling happened and multiple types of fluid inclusions captured due to reduced pressure and mixed with different fluids. With the continuous evolution of the fluid and the ore-forming temperature gradually reduced,the metallic minerals constantly precipitate and mineralization. From fluid inclusion study on the Luming molybdenum deposit,the ore-forming fluid may not originate from a single magmatic differentiation,but large-scale mixed fluids as well. The complex tectonic backgrounds also control the molybdenum mineralization.
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Inclusions in quartz and calcite from ores of the Baolun gold deposit are mainly fluid ones and plenty of CO2 inclusions are observed in quartz. The ore-forming fluids are of Na (K)-Cl type. The contents of the vapor phases CH4, C2H6, H2S, O2, N2 and Ar suggest a weak reducing environment. Cl-and Na+ are the predominant anion and cation in liquid phase, with minor SO42-, F-, Mg2+ and Ca2+. Au was transported as complexes of AuCl-2 and Au(HS)-2. The homogenization temperatures of fluid inclusions in quartz and calcite are principally in the range of 160-350℃, belonging to a medium temperature range. The δ18O and δD values for water in fluid inclusions range from -3.4 to +9.8 per mil and from -61 to -30 per mil respectively, implying a mixed origin of magmatic water and meteoric water. The δ18O values (+10.4 to +15.5 per mil) coincide with those of W, Sn, rare metal and REE deposits in South China genetically associated with granites of continental crustal type. Carbon and sulfur isotopic compositions of CO2 in fluid inclusions and pyrite in gold ores suggest that the carbon and sulfur as ore-forming materials in hydrothermal fluids were mainly derived from granitic magmas and some from Silurian strata or even older strata. It is concluded that the Baolun gold deposit is genetically related to Indosinian granitic magmatism.
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