西昆仑喀来子钡-铁矿床地质特征、时代及成因探讨

Geological characteristics,formation age and genesis of the Kalaizi Ba-Fe deposit in West Kunlun

喀来子大型钡-铁矿床位于新疆西昆仑塔什库尔干地区,矿体赋存于原"布伦阔勒群"变火山-沉积岩序列中。作为西昆仑地区独有的钡-铁矿床,其矿石具条纹-条带状构造,矿石矿物以磁铁矿和重晶石为主。矿区出露围岩主要为变泥质碎屑沉积岩(含石榴石二云石英片岩),并夹少量变火山熔岩(黑云斜长变粒岩和黑云斜长片麻岩等)。矿体夹层黑云斜长片麻岩LA-ICP-MS锆石U-Pb年龄显示原火山熔岩的形成年龄为(537.2±6.4)Ma,从而间接约束了喀来子钡-铁矿床的成矿年龄。依此,本文建议"原古元古界布伦阔勒群"应分离出一套寒武纪(火山)沉积-成矿系列。矿石中磁铁矿氧同位素(δ18 O=5.0‰~9.2‰)、硫酸盐(δ34S=39.3‰~41.4‰)和黄铁矿硫同位素(δ34S=17.0‰~23.5‰)特征表明,喀来子钡-铁矿床为经历细菌还原作用(BSR)的热液喷流沉积型矿床(SEDEX)。具体成因机制可能为:在早寒武世半封闭的海水环境中,富含铁钡的海底热液沿断裂等构造喷流至海底,与周围温度较低的海水混合导致温度、pH和氧逸度等因素发生突变,从而促使相关矿物沉淀。其中,重晶石为热液流体中钡与经BSR作用后残余的SO_4^(2-)反应生成,黄铁矿为含铁热液与SO_4^(2-)还原产物H_2S相结合形成。当H_2S消耗完全后,富铁热液在弱氧化条件下沉淀于海底形成磁铁矿。

Located in Taxkorgan area of western Kunlun Mountains, the large-scale Kalaizi Ba Fe deposit is hosted in the metamorphic volcanic sedimentary sequences of previously defined ＂Bulunkuole Group＂. As a unique Ba-Fe deposit in western Kunlun Mountains, the ore minerals mainly consist of magnetite and barite, and the ores are characterized by banded or laminated structure. While the wallrocks consist predominantly of meta-argillaceous clastic rock （garnet-bearing two-mica quartz schist）, which is interlayered with magnetite- barite ore and a small amount of meta-acd volcanic rocks （biotite plagiogneiss and biotite plagioclase granulhe）. LA-ICP-MS zircon U-Ph dating age of biotite pIagiogneiss limits the ore-forming age of the Kalaizi Ba-Fe deposit to ca. 537.2± 6.4 Ma, which also suggests that the previously defined ＂Bulunkuole Group＂ can isolate the Cambrian volcano sedimentary metallogenic series. Oxygen isotope compositions of magnetite samples, sulfur isotope compositions of sulfate minerals and pyrite show that the ore deposit was marine hydrothermal-exhalation sedimentary type deposit （SEDEX） that was affected by bacterial sulfate reduction. So the specific mineralization mechanism may be as follows： in the Early Cambrian semi-restricted marine basin, when barium and iron-bearing hydrothermal fluids were migrated and encountered with sea water by fault, the minerals precipitated because of the changes in temperature, pH and oxygen {ugacity. To summarize, barite formed by mixing of barium bearing hydrothermal fluids and sulfate-bearing pore waters. Pyrite precipitated where irombearing hydrothermal fluids encountered pore waters that had been charged with HzS by bacterial sulfate reduction. And after the H2S was consumed, magnetite precipitated directly by suboxic iron-bearing hydrothermal fluids in the sea floor.