安徽汞洞冲角砾岩型铅锌矿床成矿作用过程:来自矿床地质、流体包裹体和C、H、O、S同位素的证据

Ore-forming process of Gongdongchong breccia type Pb-Zn deposit, Anhui: Evidences from geology, fluid inclusions and isotopes of C, H, O and S

汞洞冲铅锌矿床位于大别成矿带的东段,是该区内重要的角砾岩型多金属矿床。矿体受角砾岩体控制,赋存于早古生代佛子岭岩群诸佛庵组云母石英片岩和千枚岩之中。矿床经历他形石英-绢云母-黄铁矿阶段(Ⅰ)、自形石英-铁锰镁碳酸盐-多金属硫化物阶段(Ⅱ)和方解石-绿泥石-黄铁矿阶段(Ⅲ),其中Ⅱ阶段为最主要的铅锌沉淀阶段。流体包裹体岩相学、显微测温、激光拉曼综合研究表明:Ⅰ阶段主要发育富CO_2包裹体(均一温度为307～354℃,盐度(NaCleq)为0.6%～5.6%(质量分数))和含子晶多相包裹体(均一温度为323～377℃,盐度为38.2%～45.3%);Ⅱ阶段主要发育气液相体积比变化很大的气液两相水溶液包裹体,及少量含CO_2的包裹体,均一温度为249～315℃,盐度(NaCleq)为2.9%～6.9%;Ⅲ阶段主要发育气液两相水溶液包裹体,均一温度为242～280℃,盐度为1.4%～5.0%,其中Ⅰ阶段流体发生了沸腾作用。H、O同位素测试结果表明:Ⅰ阶段硅化细粒石英流体包裹体显示岩浆水来源,而Ⅱ阶段晶簇石英的流体包裹体则存在有大气水混入的特征。成矿流体由中高温、高盐度、富CO_2的岩浆水向低温、低盐度、贫气富水的大气水方向演化。C-O同位素测试结果表明,与铅锌等金属共生的白云石中δ^(13)C_(V-PDF)值为-4.6×10^(-3)～-1.2×10^(-3)之间,相对变化较小,δ^(18)O_(SMOW)值为7.1×10^(-3)～10.2×10^(-3),显示岩浆碳酸岩来源。金属硫化物的δ^(34)S_(V-CDT)值变化范围很窄,在2.5×10^(-3)～4.5×10^(-3)之间,也显示深源硫的特征。综合分析表明:汞洞冲铅锌矿床为一热液隐爆角砾岩型矿床,成矿流体和成矿物质主要来自深部的岩浆热液,矿床经历了隐爆作用和减压过程,使得流体发生了沸腾作用,此时气液相开始分离,CO_2不断逃逸,成矿金属在残存的高盐度液相中富集,随后大气降水沿着隐爆作用所产生的裂隙加入热液中,流体混合使得体系盐度大幅降低,金属络合物失稳,最终铅锌大量沉淀。

The Gongdongchong Pb-Zn deposit located in Jinzhai County, Anhui Province, China, is one of the important breccia type lead-zinc polymetallic deposits in the east end of Qinling-Dabie metallogenic belt. Ore bodies are controlled by file breccia, hosted in Early Palaeozoic mica-quartz schist and phyllite of Zhufo＇an Formation, Foziling Group. The ore-forming processes of Gongdongchong deposit can be divided into fflree stages, namely file anhedral quarctz-sericite-pyrite stage （ I ）, file euhedral quartz-carbonate-polymetal sulfides stage （ II ） and file calcite-chlorite-pyrite stage （III）, among which, the stage （II） is the major mineralization stage. Studies of fluid inclusions show fflat file fluid inclusions trapped in stage （ I ） including two-phase inclusions, as evidenced by the coexistence of CO2-rich（C-type） （Homogenization temperatures of 307-354 ℃, Salinity（NaCleq）： 0.6%-5.6% mid multi-phase（S-type） inclusions with daughter minerals （Homogenization temperature of 323-377 ℃, Salinity of 38.2%-45.3%. The fluid inclusions of stage （ II ） are two-phase inclusions, as identified by file coexistence of L1- mid L2-type fluid inclusions; Ll-type inclusions homogenized is 249-315 ℃, with salinities of 2.9%-6.9%. The fluid inclusions formed in stage（III） are dominated by vapor-liquid two phase inclusions, with file homogenization temperatures rmlging from 242 ℃ to 280℃ and salinities between 1.4% and 5.0%. The fluid-boiling is evidenced by divergent-phase homogenizations of fluid inclusions with contrasting salinities at similar temperatures in stage （ I ）. The hydrogen and oxygen isotopes composition in quartz grains from different stages show fflat ore-forming fluid for stage （ I ） is of magmatic origin, and is mixd by the meteoric water in stage （II）. The ore-forming fluid is characterized by medium-high temperature, high salinity and CO2-rich, and then evolved into low temperature, low salinity and CO2-release from early to late stage. The carbon mid oxygen isotope composition in file dolomite in file deposit range from -4.6× 10^- 3 to 1.2×10^- 3 mid from 7.1 × 10^- 3 to 10.2× 10^- 3, respectively, which is similar to fflose in magmatic carbonatite. The values of 83%V-CDT in major sulfides have narrow variable range, from 2.5 × 10 ^-3 to 4.5× 10^- 3, indicating that file sulfur is derived from file mantle. All the data presented show that Gongdongchong Pb-Zn deposit belongs to file cryptoexplosion breccia-type deposit. The ore forming fluid and metallogenic materials come from magma in depth. The fluids boiling after cryptoexplosion and decompression result in gas-release. The metallogenic elements, such as Pb, Zn and Cu, concentrated in file solution with high salinity. The ore-forming hydrothermal solution migrated upwards along file tensional cracks with the sharply salinity decreasing by mixture with meteoric water, finally ore-forming materials are precipitated.