湖北蛇屋山金矿床含金碳酸盐岩风化成矿过程

Weathering and Laterites Formation Processes of Gold-Bearing Carbonate Rocks in the Shewushan Gold Deposit, Hubei Province

为阐述含金碳酸盐岩的风化成土过程,以湖北蛇屋山红土型金矿床为例,系统开展了元素地球化学和矿物学研究。研究发现,从风化壳腐泥层→杂色粘土带,碱金属、FeO、MnO、有机碳(Morg)、稀土元素(REE)和高场强元素(HFSE)等元素,以及钾长石、斜长石和黄铁矿等矿物的含量逐渐降低。质量平衡计算表明,风化过程中Si、Fe、Al发生了不同程度的迁移,溶解在流体中的碱金属亦会在风化壳特定层位富集,形成自生粘土矿物。研究认为,碳酸盐岩风化成土与金迁移富集均受风化强度制约。早期氧化阶段,风化较弱,以黄铁矿氧化和碳酸盐岩溶解,Ca、Mg大量淋失为特征,环境显中碱性,金以硫代硫酸盐络合物形式迁移。晚期水解阶段,风化较强,强烈的水-岩相互作用,导致不溶硅酸盐矿物水解,伴随着硅酸盐矿物中K、Na等碱金属的进一步淋失和高岭石的生成;最终,风化彻底时,粘土矿物中SiO2和Al2O3发生分离,形成典型的红土。此阶段,环境从中碱性缓慢过渡到中酸性,金以胶体或氯化物络合物形式迁移。金沉淀富集主要与表生矿物吸附和岩-土界面附近被大量Fe2+、Mn2+还原有关。

To reveal the weathering processes of gold-beating carbonate rocks and their constraint on gold enrichment, this paper, taking the Shewusban lateritie gold deposit in Hubei Province as an example, presents a systematic study of elemental geochemistry and mineralogy. The results show that from the saprolite layer to its relatively upper mottle layer, the contents of FeO, MnO, organic carbon, REE, alkali metals, HSFE and- a group of minerals including K-feldspar, plagioclase and pyrite have dramatically declined accompanied by the increase of weathering strength. Mass balance calculation suggests that Si, Fe and A1 are leached and nfigrated to different extents, and dissolved alkali metals can also be re-precipitated in the weathering crust, forming authigenic clay minerals. The study indicates that both weathering of carbonate rocks and gold enrichment are constrained by weathering strength. In the early weak-weathered oxidation phase, pyrite and carbonate dissolved, and Ca and Mg were largely leached. During this stage, the ore-forming fluids were intermediate-alkaline, and thus gold transported as the complex of thiosulfate. Whereas, in the late high weathered hydrolyzation phase, strong water-rock reaction resulted in the hydrolyzation of undissolved silicate, further leaching of alkali metals and formation of kaolinite. Finally, in the phase of complete weathering, SiO2 and Al2O3 in silicate separated, forming typical laterites. During this stage, the pH value of the ore-fluids was slowly changed into medium-acidic properties. As a result, gold transported as colloid or the complex of chloride. Commonly, gold was leached down and en- riched in the lower part of the weathering crust, especially, absorbed by supergene minerals and deoxidized by Fe2＋ and Mn2＋ at the rock-soil boundaries.