西天山阿希金矿成矿地球化学特征研究

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

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+-SO42-Cl-型,具有低温(120～180℃)、低压(7.2～21.8MPa)、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),Eh值-0.88～+1.18eV,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.

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 （∑REE = 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 δ^34S 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 （ δ^18O 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^＋]- SO4^2- [C1-] type with low-temperature （120-180℃ ）,low-pressure （7.2 -21.8 MPa）, low-salinity （NaC1, 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.