新疆南天山阿沙哇义金矿床的成因与找矿启示:来自流体和硫化物成分的限定

Fluid and sulfide component constraints on the genesis and prospecting implications of the Ashawayi gold deposit,South Tianshan,Xinjiang,NW China

阿沙哇义金矿位于中国新疆南天山造山带,属于著名的中亚南天山锑-汞-金成矿带的东延部分。该矿床严格受断裂所控制,以浸染状黄铁矿化、毒砂化为特征。矿化可分为三个阶段:早期无矿或贫矿石英阶段,中期石英多金属硫化物阶段,晚期石英-碳酸盐阶段。其中,中期是主要成矿阶段。成矿流体气相成分以H2O为主,摩尔含量为75%~93%,其次为CO2,摩尔含量为6%~25%,其余为CH4、C2H6、H2S、N2和Ar;液相成分阳离子以Na+为主,含少量K+、Ca2+离子,阴离子以Cl-为主,SO2-次之;矿石的Au含量与其流体的CO2含量呈反相关,与K+含量呈正相关。硫化物成分分析结果表明:(1)围岩地层和矿石中的黄铁矿和毒砂是重要的载金矿物,黄铁矿Au含量为0~0. 09%,平均值0. 03%;毒砂Au含量为0~0. 28%,平均值0. 07%;(2)黄铁矿和毒砂Au含量与其自形程度没有明显的相关性;(3)环带状黄铁矿较均质结构黄铁矿具有更高的Au含量;(4)岩体中的黄铁矿几乎不含Au。在成矿构造环境、成矿流体特征及演化、金矿富集机制、成矿温压条件等方面,该矿床与世界上大多数造山型金矿显示出一致性,成矿类型应属于剥蚀程度较浅的造山型金矿。断层阀作用控制的断层愈合-破裂导致的流体不混溶作用是本区金富集、沉淀的最重要机制,但流体混合机制对金的富集沉淀也发挥了作用。黄铁矿、毒砂发育及较多的含炭物质三者共存是本区寻找富矿的关键标志。

The Ashawayi gold deposit is located in the eastern part of South Tianshan Sb-Hg-Au metallogenic belt which is one of the important sources of gold in China. This deposit is characterized by typical disseminated pyritization and arsenopyritization,and its distribution is controlled by faults. Three stages of hydrothermal alteration have been identied,including:( 1) the early stage with no mineralization or quartz-growth;( 2) the middle stage with occurrences of quartz polymetallic sulfide;and( 3) the late stage with quartz and carbonatization. Among them,the middle stage is the main period for mineralization. The gas phase compositions of the fluid from the middle stage ores are mainly H2 O( 75%~ 93%),and subordinately followed by CO2( 6%~ 25%),and the rests are CH4,C2 H6,H2 S,N2 and Ar. The cations are mainly Na+with minor K+and Ca2 +while the anions are mainly Cl-,followed by SO2-. The Au content in the ore is inversely correlated with the CO2 content,and is positively correlated with the K+content of the fluid from the ore. The analytical results of the sulfide compositions show that:( 1) The pyrite and arsenopyrite in the wall rocks and ores are mainly auriferous minerals. The Au contents in pyrite and arsenopyrite are 0 ~ 0. 09%( 0. 03% on average) and 0 ~ 0. 28%( 0. 07% on average),respectively;( 2) There is no obvious correlation between Au content and crystal morphology of the pyrite and arsenopyrite;( 3) Cyclic zonal pyrite contains higher Au contents than the pyrite having homogeneous structure;( 4) Au rarely occurs in the pyrite from magmatic rocks. Many factors,such as ore-forming structural environments,properties and evolution of ore-forming fluids,the mechanism of gold enrichment and the estimation for ore-forming P-T conditions,are similar with ones of most orogenic gold deposits in the world. Thus we suggest that the Ashawayi gold deposit is an orogenic gold deposit with relatively shallow denudation.The possible mechanism for mineralization is associated with the fault healing and fracturing which induced the immixing process of fluids. The fluid mixing mechanism also plays a role in gold enrichment and precipitation. As a result,we proposed that the development and coexistence of pyrite,arsenopyrite and carbonaceous material is the most favorable marks for the exploration of highgrade ores in the Ashawayi gold deposit.