辽东砖庙矿区硼矿床的海相蒸发成因--来自硼、硫、碳同位素的证据

摘要

Zhuanmiao Mg-borate ore bodies exhibit layered or lenticular structure in the wall rocks of serpentinized marbles, which are located in the lower strata of volcanic-sedimentary Lieryu Formation in Kuandian County of eastern Liaoning Province. The authors applied LA-MC-ICP-MS in-situ technology to analyze the B isotopic composition of borate ores in the Zhuangmiao Mg-borate ore district, and systematically studied the S and C stable isotopic compositions of the ores and marbles. Theδ11BNIST SRM-951 values of Mg-borate ores vary from 12.6‰ to 13.9‰, showing marine evaporative characteristics; the δ34SV-CDT values of Mg-borate ores and serpentinized marble vary from 11.6‰to 24.3‰, suggesting marine sedimentary characteristics;(3) theδ13CV-PDB values of serpentinized marble vary from -4.6‰ to -0.5‰, while those of marble vary from 4.1‰ to 4.6‰, exhibiting positive carbon isotopic anomaly of Paleoproterozoic marine carbonates. Therefore, the ore-forming environment of the Mg-borate deposit in Zhuanmiao area was probably an alternate cycle of marine sedimentation and volcanism of the littoral facies, while a suite of homochronous Mg-rich ultrabasic volcanic rocks covered the borate deposits and preserved the borate ores during hydrothermalism and metamorphism at late stages, and formed a unique rock unit of borate minerals, carbonates and ultrabasic minerals.%辽宁砖庙硼矿区的硼矿体呈层状或透镜状赋存于辽河群里尔峪组火山-沉积建造下部的蛇纹石化大理岩中，内部含有大量镁橄榄岩包裹体。本研究利用LA-MC-ICP-MS技术对砖庙硼矿区内的硼矿石硼同位素进行了微区原位分析，对矿石及大理岩围岩的硫、碳稳定同位素进行了系统研究。硼矿石的δ11BNIST SRM-951为12.6‰~13.9‰，具海相蒸发沉积特征；硼矿石和大理岩的δ34SV-CDT为11.6‰~24.3‰，具海相沉积成因特征；矿体上下层位中蛇纹石化大理岩的δ13CV-PDB为-5.0‰~-0.5‰，部分未蛇纹石化大理岩的δ13CV-PDB为4.1‰~4.6‰，具有古元古代海相碳酸盐岩特有的碳同位素正异常现象。据此提出，砖庙矿区的硼矿床可能形成于海相蒸发沉积和火山喷发旋回交替的滨海环境，随后同期喷发的超基性火山岩覆盖于海相蒸发沉积成因硼矿体之上，保护了易溶的硼酸盐矿物，经后期变质和热液改造，形成目前独特的硼酸盐矿物，碳酸盐岩与超基性岩岩石组合。