热液金矿成矿作用地球化学研究综述

Geochemistry of hydrothermal gold mineralization

文章总结了热液金矿成矿地球化学研究进展,其中包括含金热液来源、控制岩浆热液中金浓度的因素、金在热液中的富集和沉淀机制以及热液金矿成矿地球化学环境。含金岩浆演化早期,磁铁矿、磁黄铁矿和钛铁尖晶石结晶分异会影响岩浆演化形成热液型金矿的能力。与金成矿有关的地热流体中w（Au）为1×10-9~80×10-6,岩浆热液中w（Au）在1×10-6~4×10-6之间变化。金主要以Au-Cl和Au-HS络合物形成运移富集,当热液体系的压力较高（〉20 MPa）时,Au-HS或Au-Cl络合物在热液气相中富集。流体混合、沸腾、不混溶和水岩反应等引起的热液物理化学条件变化会导致金络合物的溶解度降低,发生金矿化。在含非晶质As2S3和Sb2S3溶胶的热液中,As2S3和Sb2S3溶胶可以吸附金硫络合物并携带其随热液运移,Au最终随着溶胶沉淀而沉淀。在硫逸度较低〔logf（S2）〈-12〕而砷浓度较高的热液中,As可以增大Au的溶解度,As溶解度降低引起含砷矿物和金矿物沉淀,形成含砷黄铁矿-毒砂-自然金/银金矿或自然砷-自然金/银金矿矿物组合。在温度高于自然铋（271.4℃）或铋碲化物（富Bi端266℃,富Te端413℃）熔点的热液中,自然铋和铋-碲化物以熔融状态与热液共生,并吸附热液中的Au,当热液温度降低至Au-Bi（241℃）、Au-Bi-Te（235℃）熔体的熔点时,结晶形成自然铋-自然金-黑铋金矿或自然铋-自然金-铋碲化物组合。含金成矿热液沿着有利热液通道运移过程中,与围岩反应导致围岩发生硅化、绢云母化、碳酸盐化以及硫化等蚀变。中-碱性热液体系中,绢云母化会导致金矿化发生。热液蚀变矿物组合反应了成矿体系的物理化学条件,可以用于推断金矿成矿地球化学环境。

Recent advances in the study of hydrothermal gold mineralization are reviewed in this paper,which include the source,transportation form and precipitation mechanism of gold.Pyrrhotite and magnetite fractionated from gold-bearing mantle-derived magma will decrease the potential for forming magmatic hydrothermal gold deposits.Gold is always transported as complex species,such as AuCl-2, AuCl-4,Au（HS）-2,Au（HS）0,HAu（HS）0 and Au2（HS）2S2-.The complex species of gold will concentrate in vapor when the pressure of the hydrothermal system is higher than 200×105 Pa.The change of hydrothermal physicochemical conditions such as temperature,composition and pressure induced by fluid mixing,boiling,immiscibility and water-rock reaction is the major mechanism leading to the decomposition of gold complex and the formation of gold deposits.Noncrystalline As2S3 and Sb2S3 can serve as gold scavengers in the hot spring.Gold will precipitate with the deposition of noncrystalline As2S3 and Sb2S3.In hydrothermal fluid with low sulfur fugacity（logf（S2）-12） and high arsenic concentration,arsenic plays an important role in increasing gold solubility.Crystallization of As-bearing mineral induces the precipitation of gold,and forms the arsenian pyrite-arsenopyrite-native gold/electrum or native arsenic-native gold/electrum assemblage.Native bismuth or bismuth-telluride melt coexisting with hydrothermal fluid is also an important gold scavenger in hydrothermal fluid with temperature higher than 271.4℃（melting point of native bismuth） or 400℃（melting point of bismuth-telluride）.The decrease of temperature is the most effective mechanism for gold precipitation.Gold co-precipitates with native bismuth or Bi-telluride,and forms the native bismuth-native gold-maldonite or native gold-native bismuth-Bi-telluride mineral assemblage. These mechanisms make it possible for the gold undersaturated hydrothermal fluid to form economic gold deposits.The hydrothermal geochemistry of gold mineralization can be reconstructed based on mineral assemblages formed during alteration.