西天山阿希金矿成矿地球化学特征研究

阿希金矿为1990年代在西天山发现的产于晚古生代陆相火山岩中的大型金矿床.容矿围岩为下石炭统大哈拉军山组中基性-中酸性火山岩,以钙碱性为主,少量碱性系列岩石,具有富硅、钠,贫铝、钾、镁、钙成分和里特曼指数偏高的特点;岩石稀土元素含量中等,属于轻稀土富集型,分馏程度较高;容矿火山岩以K、Rb、Ba、Th等大离子元素富集和Ti、Y、Yb、Sc等亏损为特征,接近于板内过渡型玄武岩系列岩石;容矿岩、近矿蚀变岩和矿石皆富集有Au、Ag、As、Sb、Bi等低温组合元素,且表现出渐进富集之特点;火山岩和矿石中铅具壳幔混合源特征;含金石英脉锶同位素初始值与区内容矿火山岩、矿化有关的蚀变矿物十分接近,均小于0.709而大于0.705;成矿流体属于K⁺[Na⁺]-SO₄^2-[Cl^-]型,具有低温(120~180℃)、低压(7.2~21.8 MPa)、低盐度(2.4%~4.5%,NaCl)、富含CO₂、N₂、O₂等特征;流体pH值中等(5.7~8.8),E_h值-0.88~+1.18 eV,成矿环境弱还原;金矿石中黄铁矿δ³⁴S值0.95‰~10.51‰,以深源火山成因硫为主;氢氧同位素值域较宽,反映流体以大气降水为主.总之,阿希金矿具备了浅成低温热液系统金矿的基本地球化学特征,但在成矿时代和成矿背景等方面,有别于经典同类矿床.

STUDY ON THE METALLOGENIC GEOCHEMISTRY OF AXI GOLD DEPOSIT IN WESTERN TIANSHAN MOUNTAINS

One of the most important prospecting achievements in Xinjiang recently is the discovery of the large-scale Axi gold deposit,which occurs in Late Paleozoic continental volcanic rocks in West Tianshan Mountains.The deposit is hosted by the acid-intermediate to intermediate-basic volcanic rocks of Lower Carboniferous Dahalajunshan Formation.The mineralization is closely related to the volcanism and volcano-geothermal system.The hosting volcanic rocks,which belong to calc-alkaline and alkaline series,are characterized by rich silicon and sodium while lacking of potassium,aluminum,magnesium and calcium.The host rocks with medium contents of rare earth elements (ΣREE = 41.59×10-6 ～135.83×10-6 ) belong to LREE enriched type (LREE =29.05×10-6-95.37×10-6 ) with fairly high REE fractionation. And the host rocks with features of enriched macro-ion elements such as K,Rb,Ba and Th while lacking of Ti,Y,Yb,Sc etc. are similar to the intraplate transitional basaltic rocks. The low-temperature elements of Au,Ag,As,Sb and Bi are enriched in host rocks,near-ore altered rocks and ore in a gradual enrichment process,showing the genetic relationship between the host rocks and ore bodies.With 206Pb/204Pb of 17.5269 -18.6644,207Pb/204Pb of 15.1073-16.0108 and 208Pb/204Pb of 37.3670-38.1958,the lead-isotope contents of hosting volcanic rocks and ores show their crust-mantle mixed origin.The strontium-isotope initial ratios of Au-bearing quartz vein,hosting volcanic rocks and the metallogenesis-related altered minerals are very closed from 0.705 to 0.709,suggesting the close genetic relation between the metallogenesis and volcanism.The δ34S of pyrite of Au-bearing ore ranges from 0.95‰to 10.51‰,indicating that the sulfur isotope compositions related to Au ore-forming are mainly from deep volcanism.The ratio ranges of hydrogen and oxygen isotopes are wide ( δ18O from 5‰to 13‰,δD from -59.504‰to -115.20‰),suggesting the meteoric character of the ore-forming fluid,which belongs to K+[Na+]-SO42-[Cl-]type with low-temperature (120-180℃),low-pressure (7.2-21.8 MPa),low-salinity (NaCl,2.4%-4.5%) and enriched CO2,N2,O2 etc.In general,Axi gold deposit has the basic geochemical characters of the classic epithermal ones,but with its own features on ore-forming epoch,background and so on.