湘东北横洞钴矿床钴的富集机制: 来自黄铁矿的微区结构、成分和硫同位素证据

世界上典型热液脉型钴矿床, 如五元素(Ag-Bi-Co-Ni-As±U)矿床, 因其高的钴品位(高达8%)而具有重要的经济价值和研究意义, 该类矿床以自然元素和砷化物组合为特征。不同于五元素矿床, 湘东北横洞热液脉型钴矿床(中型, 品位约0.04%)的矿石矿物组合为黄铁矿+黄铜矿+闪锌矿+方铅矿+磁黄铁矿, 目前对该矿床中含钴矿物的特征及钴的富集机制尚不清楚。本文在详细的矿相学工作基础上, 采用EPMA、EBSD、LA-ICPMS和LA-MC-ICPMS等多种分析方法, 对其黄铁矿开展了精细结构、成分和硫同位素分析。将黄铁矿划分为PyI、PyII和PyIII三个世代。其中, PyII是Co的重要载体, 常呈丝带状、不规则状或韵律环带交代PyI或呈细粒状产出, 其Co含量高达52141×10^-6, 明显高于其他世代黄铁矿。黄铁矿中Co与Fe具有较好的负相关性, 表明Co主要呈类质同象的形式置换Fe存于黄铁矿晶格中。而黄铁矿中高的Co/Ni比值(1.05~393)和Se含量(10.69×10^-6~129×10^-6)则暗示了其热液成因。此外, PyII与PyI之间具有不规则港湾状接触界面、突变的化学成分以及相近的晶体取向, 这些暗示了富钴的PyII系含钴流体与围岩及早阶段黄铁矿快速反应的产物, 溶解再沉淀的动力学过程是控制PyII的形成机制。不同世代黄铁矿的δ³⁴S_V-CDT值接近, 变化范围为-13.12‰~-8.70‰, 与围岩地层的硫同位素特征接近。结合湘东北结晶基底高的Co含量(30.4×10^-6~72.3×10^-6)和以往流体包裹体显微测温结果, 认为在晚侏罗世-早白垩世期间, NE-ENE向的长沙-平江断裂带走滑剪切活动使得深源热液流体从结晶基底连云山岩群活化萃取了金属钴, 富钴热液沿走滑断裂向上运移至成矿部位, 再因压力周期性变化引起流体发生不混溶作用进而分离, 导致钴金属络合物失稳, 最后钴沉淀富集成矿。通过与世界上典型热液脉型钴矿床成矿特征的对比, 提出流体低的盐度和低砷、高硫的特征是造成横洞钴矿床矿物组合简单的主要控制因素。

Cobalt enrichment mechanism in the Hengdong cobalt deposit,Northeast Hunan Province: Evidence from texture,chemical composition and sulfur isotopic composition of pyrite

The typical hydrothermal Co deposits in the world, as represented by the five-element (Ag-Bi-Co-Ni-As±U) vein type Co deposit, have great economic potential and research significance on account of their high metal grades (up to 8%). Native elements and arsenides are common in these high-grade hydrothermal Co deposits. The medium-scale Hengdong Co deposit (with an average Co grade of 0.04%) in Hunan Province has an ore mineral assembalge of Co-rich pyrite, chalcopyrite, sphalerite, galena and pyrrhotite. In order to characterize the Co-bearing minerals and examine the Co enrichment mechanism of the deposit, a detailed study on the texture, chemical composition and sulfur isotopic composition of pyrite in its ores was carried out by using combined EPMA, EBSD, LA-ICPMS and LA-MC-ICPMS methods. The pyrites in this deposit can be divided into three generations, i.e., PyI, PyII and PyIII, among which the PyII is the most important Co-bearing mineral, and it always occurs as ribbon-like, oscillatory or uneven zones replacing PyI or as fine grains. The Co contents of PyII are up to 52141×10-6, which is remarkably higher than those of PyI and PyIII, indicating that PyII is the major Co carrier of the deposit. The negative correlation between Co and Fe of the pyrite indicates that Co should substitute for Fe in the lattice of the pyrite. The high Co/Ni ratios (1.05~393) and Se contents (10.69×10-6~129×10-6) of the pyrite indicate that it is of a hydrothermal origin. The sharp interface (sometimes with a bay shape) between PyII and PyI, togther with the contrast chemical variation and similar crystal orientation of PyII and PyI, disclosed that the dissolution-precipitation mechanism during the rapid interaction of the Co-bearing fluid with the wall rocks and early-formed pyrite was responsible for the formation of the Co-rich PyII. The three generations of pyrite have δ34SV-CDT values varying from -13.12‰ to -8.70‰, which is consistent with that of the host rocks of the Lengjiaxi Group. Based on the high Co concentrations (30.4×10-6~72.3×10-6) of the basement rocks in the Northeast Hunan Province, together with previous microthermometric measurements of fluid inclusions, we proposed a new model of the Co enrichment in the deposit as follows: during the strike-slip faulting of the Changsha-Pingjiang fault zone in the period from the Late Jurassic to Early Cretaceous, the deep-sourced fluids that leached Co from the basement (i.e., the Lianyunshan Group) progressively migrated upward along the NE-ENE fault zone; then the cyclic pressure release happened during the fault-zone movements may have led to the fluid immiscibility and vapor phases separation from the liquid phases, thus the Co should subsequently precipitate from the liquids due to the destability of Co complex. Compared with the typical hydrothermal vein-type Co deposits elsewhere in the world, the features of the ore-forming fluids with low salinity and low As but high S compositions may explain the mineral assemblage of the Hengdong Co deposit.