三江中段青海玉树吉龙沉积岩容矿脉状铜矿成矿作用研究

Metallogeny of Jilong sediment-hosted vein type Cu deposit in the central part of the ‘Sanjiang’ belt

吉龙铜矿位于青藏高原东、北缘三江造山带中段青海玉树地区,其矿化特征和三江南段兰坪盆地内沉积岩容矿脉状铜矿特征相似,对该矿床矿化特征和矿床成因的详细研究有助于理解区域沉积岩容矿脉状铜矿成矿规律,对区域找矿具有重要意义。详细的野外地质考察及室内显微镜下观察发现,吉龙矿床铜矿化以石英-碳酸盐-硫化物脉的形式发育在石炭系杂多群上段灰岩与碎屑岩的岩性分界面上,矿石矿物主要为黄铜矿+斑铜矿+黝铜矿+辉铜矿,脉石矿物以石英+方解石为主。方解石Sm-Nd同位素等时线年代学研究表明,吉龙铜矿形成于34Ma,在大地构造背景下和区域逆冲推覆构造活动间歇期的走滑断裂活动以及相伴随的钾质岩浆活动同期。主成矿阶段成矿流体仅发育富液相LV流体包裹体,流体表现出中温度(230~250℃)、低盐度(3%~6%NaCleqv)性质;δD V-SMOW值分馏较大(-154‰^-80‰),δ18O V-SMOW值分馏较小(7.72‰~10.30‰),反映了开放体系发生去气作用后的残余岩浆热液来源。硫化物沉淀阶段成矿热液S同位素组成一致(3.4‰~11.6‰),较岩浆来源硫分馏大,推测成矿还原硫主要来自赋矿灰岩地层封存硫酸盐的有机质热还原作用;铅同位素组成(206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值分别为18.668~18.707、15.657~15.792和38.645~38.873)同区域新生代钾质岩浆岩一致,推测成矿金属物质可能来自区域钾质岩浆作用。吉龙铜矿成矿模式可简要归纳为:走滑断裂→钾质岩浆活动,岩浆去气→富金属热液形成,热液运移→与富硫酸盐地层相遇,硫酸盐有机质热还原→还原硫产生,还原硫与金属物质结合→硫化物沉淀。综合对比吉龙铜矿与三江南段兰坪盆地内沉积岩容矿脉状铜矿发现,二者在成矿地质背景、矿床地质特征、成矿流体性质及来源、成矿物质来源上都具有极大相似性,推测吉龙铜矿为三江南段沉积岩容矿脉状铜矿在区域上向中北段的延伸,三江带上有望发现一条上千千米的沉积岩容矿脉状铜矿带。

Jilong Cu deposit, located in the Yushu area, Qinghai, which is in the central part of the ＇ Sanjiang＇ orogenic belt of the northeastern Tibetan Plateau, has similar mineralization characteristics with the sediment-hosted vein type Cu deposits in the Lanping basin in the southern part of the same belt. A study of this deposit is helpful to the understanding of ore-forming regularity of the regional sediment-hosted vein type Cu mineralization in the belt and also useful for the regional prospecting. Based on detailed field work and systematic thin section observation of ores, the authors indicate that the Cu mineralization in Jilong Cu deposit is produced as quartz-carbonate-sulfide veins and formed along a facies transition between clastic rocks and limestones in the Upper Zaduo Group, Carboniferous. Chalcopyrite, bornite, tennantite and chalcocite are the main ore minerals and quartz and calcite are the main gangue minerals. The Sm-Nd isochrone age of calcite from ore-forming stage is about 34Ma, indicating that the Cu mineralization formed at a regional geological background with active strike slipping faults and potassic magmatic activates. Only LV fluid inclusion rich in liquid occur in main ore-forming stage and it suggests a kind of hydrothermal fluid with middle temperature （230 - 250% ） and low salinity （3% - 6% NaCleqv）. The value of 8Dv.sMow （ -154‰- -80‰） is with big fractional distillation and the δ^18OV.SMOw （7. 72‰ - 10. 30‰） is with small fractional distillation, together of which indicate the ore fluids derived from magmatic hydrothermal fluid which has undergone degasification in open system. The δ34 S values of hydrothermal fluids in ore-forming stages are in unanimity （ 3.4‰ - 11.6‰）, which are with bigger fractional distillation than sulfur from magma. These δ34S values indicate reducing sulfur for ores came from thermal sulfate reduction, in which the sulfate belonged to the sulfate trapped in limestone in Carboniferous. The ranges of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb of chalcopyrite and calcite in main ore-forming stage are 18. 668 - 18. 707, 15. 657 - 15. 792 and 38. 645 - 38. 873, respectively. They are in unanimity with the Pb isotope composition of Cenozoic potassic igneous rocks in the region, indicating the metal for ores origin from regional potassic magmatic hydrothermal fluids. The moralization model of Jilong sediment-hosted vein type Cu deposit can be explained as followings： The strike slipping faults led to regional potassic magmatic activates. The magmatic hydrothermal fluid underwent degasification in open system and formed ore-forming fluid rich in metal. The ore-forming fluid moved along the regional thrust faults and strike-slip faults and finally met sediment strata rich in sulfate. The sulfate in the strata was thermal reduced by organic and produced reduced sulfur. The reduced sulfur mixed with metal in the ore-forming fluid and the sulfide finally precipitated. By comparing Jilong Cu deposit with sediment-hosted vein type Cu deposits in the Lanping basin in the southern part of ＇ Sanjiang＇ belt, the authors found that they are similar in geological background, mineralization geology, characteristics and origin of ore-forming fluid, and origins of sulfur and metals, and finally point out that there may be a 1000km-long sediment-hosted vein type Cu deposit belt in the ＇ Sanjiang＇ belt.