川西北马脑壳金矿床成矿流体地球化学特征与性质

摘要

马脑壳金矿床是20世纪 80年代末期在川西北地区发现的一大型微细浸染型矿床，它赋存于中三叠统扎尕山组地层之中，矿体产出受北西向次级断裂构造的控制。矿床的形成经历了成矿前金初步富集、热液成矿作用原生矿石形成及表生氧化金次生再富集等三期主要成矿作用过程。热液金成矿作用可进一步划分为 (Ⅰ )黄铁矿毒砂石英； (Ⅱ )石英 (白钨矿 )辉锑矿； (Ⅲ )石英雄 (雌 )黄及 (Ⅳ )石英方解石等 4个矿化阶段 ,其中Ⅰ、Ⅱ阶段为金的主要沉淀富集成矿阶段。系统的流体包裹体研究表明，成矿前 (Ⅰ′ )及热液成矿Ⅰ～Ⅳ阶段石英中共发育液相、纯液相、含 CO2三相、富 CO2相及含有机质等 5种类型的原生流体包裹体。测温结果显示，Ⅰ′及Ⅰ～Ⅳ类石英中液相及含 CO2三相包裹体均一温度为 120～ 300 ℃，热液盐度为 0.5％～ 11.0％；包裹体成分分析结果表明，热液阳离子以 Na＋、 K＋及 Ca2＋为主，阴离子主要为及 Cl－，气相组分除 H2O外，尚含一定量的 CO2及 CH4等；热液 pH值为 6.7～ 7.2， Eh值为－ 0.85～－ 0.69 eV；成矿热液总体属中低温、低盐度、近中性和弱还原性的含有机质 Na＋ K＋ Ca2＋ Cl－体系类型。 H、 O同位素研究结果表明，成矿前热液主要来源于变质水和地层建造水，成矿期以来大气降水不断混入并逐渐占据优势。主成矿阶段成矿热液发生过明显的流体混合及相分离作用，对金的沉淀富集成矿起了重要作用。%Manaoke gold deposit is a large scale micro grained disseminated type gold deposit found in north western Sichuan Province in late 1980s. It is located in Middle Triassic Zadoishan Formation, and the occurrence of ore bodies was controlled by secondary northwestern striking faults. The forming of the deposit has experienced three main phases, i.e. (1) gold preenrichment, (2) the hydrothermal gold mineralization which resulted in the formation of primary ores, and (3) the re enrichment of gold by supergene oxidation process. The hydrothermal gold mineralization process can be subdivided into four stages, which are: I pyrite arsenopyrite quartz, Ⅱ quartz (scheelite) stibnite, Ⅲ quartz realgar (orpiment) and Ⅳ quartz calcite, among which stage Ⅰ and stage Ⅱ are the main ones for gold deposition. Systematic study on fluid inclusions shows that there are five main types of primary fluid inclusions present in quartz, such as two phase aqueous, one phase aqueous, CO2 bearing three phase, CO2 rich phase as well as organic matter containing etc. Results of microthermometric study show that the homogenization temperature ranges from 120 ℃ to 300 ℃ for two phase aqueous and CO2 bearing three phase inclusions in every kinds of quartz, and the estimated salinity of ore forming fluids ranges from 0.5％ to 11.0％ . The analytical results for composition of fluid inclusions show that Na＋ , K＋ and Ca2＋ are the main cations while and Cl－ are the main anions in the ore forming solutions, with definite quantities of CO2 and CH4 in addition to H2O for the vapor phase components. The pH value of the ore forming fluid range from 6.7 to 7.2 and Eh value ranges from － 0.85 to － 0.69 eV. So the ore forming fluids are of organic matter containing Na＋  K＋  Ca2＋   Cl－ type and featured by medium to low temperature, low salinity, nearly neutral and weakly reduction conditions. Studies on stable oxygen and hydrogen isotopes show that the pre ore fluids mainly originated from metamorphic and connate water, but since the mineralization episode meteoric water joined and gradually predominated fluid mixing and phase separation processes happened to the ore forming solutions during the main mineralization stages, and they played important roles for gold deposition and enrichment.