| 西藏昌都赵发勇溶洞控矿MVT铅锌矿床地质特征与矿床成因 |
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| 关键词:MVT (Mississippi Valley-type) Pb-Zn deposit karst-controlled type Zhaofayong Pb-Zn deposit characteristics genesis thrust-fold belt Sanjiang metallogenic belt Tibet |
| 基金项目:国家重点研发计划(编号: 2016YFC0600306);国家自然科学基金项目(编号: 41773042; 41922022; 41773043; 41772088; 41472067);自然资源部深地动力学重点实验室自主研究课题(编号: J1901-6);国际地质对比计划(编号: IGCP-662) |
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| Characteristics and Genesis of the Zhaofayong Karst-controlled MVT Deposit in the Changdu Region, Tibet |
| The Sanjiang metallogenic belt on the Tibetan Plateau is an important polymetallic metallogenic belt in China. The sediment-hosted Pb-Zn deposits in this belt not only have huge reserves of resources, but also have a great potential for scientific research, because they represent a new type of MVT Pb-Zn deposits developed in the thrust-fold system of the collisional orogenic belt. The Zhaofayong lead-zinc deposit is located in the Changdu area in the middle part of Sanjiang metallogenic belt. Its Pb-Zn mineralization is developed in the Cenozoic regional thrust nappes and is controlled by the paleokarst cave structures related to the thrust faults. This deposit presents a new deposit style among the thrust-fold hosted MVT Pb-Zn deposits and can be called karst-controlled style. Therefore, systematical geological mapping and detailed genetic study of the Zhaofayong deposit have been performed and a new karst-controlled model for the MVT deposits in the thrust-fold system has been built. The Pb-Zn mineralization in the Zhaofayong deposit occurs in the hanging wall and footwall of a thrust fault, and orebodies, mainly produced in tubular shape, are distributed in the paleokarst caves in limestones of Permian Licha Formation and Triassic Jiapeila Formation. Breccia, massive and crust are main ore structures, and galena and sphalerite are main ore minerals. The main ore-forming process in the Zhaofayong deposit can be divided into Period I (sulfide period) and Period II (sulfide-carbonate period), and between them a compressing deformation existed. A kind of LV inclusions rich in liquid occurred in Period I. Microthermal testing work suggests that the main hydrothermal fluids in this period had low temperature (130~140℃) and high salinity (23%~24% NaCl eq.) characteristics. However, some data indicate that some of the fluids had different characteristics in that they had higher temperature (as high as ~400℃) but lower salinity (as low as ~8% NaCl eq.). the liquid parts of the hydrothermal fluids in Period I belonged to a complex Mg2+-Na+-K+-SO2– 4-Cl–-F–-NO– 3 system. Combined with the H-O isotope composition (–147‰~ –94‰ for δDV-SMOW and 1.25‰~13.62‰ for δ18Ofluid) and the ratios of the ion content (14~287 for Cl/Br and 4~148 for Na/Br) for the hydrothermal fluids and the C-O isotope composition of calcite (–2.4‰~5.1‰ for δ13CV-PDB and 15.1‰~27.4‰ for δ18OV-SMOW), two resources of hydrothermal fluids were recognized, i.e., ① low-temperature, high salinity basin brine from the regional paleogene basins and/or trapped evaporated seawater in the strata and ② low to middle-temperature, middle to high-salinity regional fluid composed of evaporated-concentrated seawater cocooned in strata and metamorphic water released from metamorphic rock. The δ34S values of sulfide for the two periods are negative (–15.1‰~ –1.6‰), and those of barite are positive (10.8‰). Compared with the δ34S values of the regional gypsum, the sulfur isotopes suggest that reducing S came from biological sulfate reduction derived from the basin brine descending from overlying Paleogene basin and/or evaporated-concentrated seawater cocooned in strata, and mixing of two kinds of fluids, which were local fluid rich in reducing sulfur and regional fluid rich in metal, caused the deposition of mineralized minerals. The 206Pb and 207Pb compositions of the sulfides (18.703 0~18.966 2 for 206Pb/204Pb and 15.638 9~15.687 2 for 207Pb/204Pb) in the two periods are similar to those of the regional limestone; however, the 208Pb compositions of the sulfides (38.554 8~38.924 0 for 208Pb/204Pb) are different from those of the regional limestone. This information indicates that the metals came from metamorphic basement and limestone (and clastic rock) caprock distributed in the orogenic belt. Based on the geological characteristics and genesis of the Zhaofayong deposit, combined with the information from the regional MVT lead-zinc deposits controlled by karst caves, the authors propose a metallogenic model of karst-controlled MVT lead-zinc deposit in the thrust-fold system: ① Regional thrusting and nappe activities led to the uplift of local limestone strata in the deposit and paleokarst caves were formed with shallowing strata, forming the most favorable trap space in the thrust fault. The brine and/or evaporative concentrated seawater in the infiltrating Paleogene basin converge in the trap space and formed a local fluid reservoir rich in reducing sulfur under the action of bacterial reducing sulfur. ② Regional extrusion continued, metamorphic water and sealed water were released to become regional fluids, and regional fluids migrated over a long distance in deep detachment systems to form exotic fluids rich in metals and halogens; ③ After thrusting and stress relaxes, extensional faults were developed into open space, and exotic fluids migrated upward along the open space. Metal-rich and sulfur-rich fluids met, forming cylindrical lead-zinc orebodies in the most advantageous ancient karst caves. |
| LIU Ying-chao,YANG Zhu-sen,YU Yu-shuai,MA Wang,YUE Long-long,TANG Bo-lang.2019.Characteristics and Genesis of the Zhaofayong Karst-controlled MVT Deposit in the Changdu Region, Tibet[J].Acta Geoscientica Sinica,40(6):853-870. |
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