新疆阿希低硫型金矿床流体地球化学特征与成矿机制

Geochemistry of Ore Forming Fluid and Metallogenic Mechanism of Axi Low-Sulfidation Gold Deposit in Xinjiang,China

位于新疆西天山的阿希金矿为一赋存于古生代陆相火山岩中的冰长石—绢云母型(低硫型)浅成低温热液金矿床,其围岩为下石炭统安山质火山岩和火山碎屑岩。矿体呈脉状产出,严格受古火山机构外围的环形断裂带控制。成矿作用分为石英—玉髓状石英脉阶段、石英脉阶段、石英碳酸盐脉阶段、硫化物脉阶段、碳酸盐脉阶段,形成的矿石有石英脉型和蚀变岩型两类。成矿作用阶段形成的流体包裹体主要有三类:液相水溶液包裹体、富液相气液两相水溶液包裹体和富气相的气液两相水溶液包裹体,其中以前两类为主。三类流体包裹体在矿石中同时出现,说明它们形成于非均一的流体介质条件,矿床形成时流体发生了沸腾作用。成矿流体冰点温度一般为-0.3--2.3℃,相应盐度为0.48%-3.75%NaCleq,平均1.85%NaCleq;冰点峰值温度位于-0.4--1.6℃,相应盐度为0.66%-2.63%NaCleq。流体包裹体均一温度为121-335℃,平均209℃;均一温度峰值为140-240℃,计算得到流体密度为0.73-0.96,成矿深度小于700m。成矿流体的氢、氧同位素变化范围小,δDSMOW,H2O=-98‰--116‰,δ18OSMOW,H2O=-0.55‰-1.65‰;碳同位素δ13CPDB,方解石=2.6‰-4.9‰,δ34SCDT=-4.0‰-3.1‰,平均值为δ34SCDT=-0.45‰,表明成矿流体主要为循环的大气降水,成矿物质主要来自赋矿围岩火山岩及基底岩石。含金石英脉中冰长石、叶片状石英和硫化物集合体的出现,以及气相比例和均一温度变化很大的液相、富液相和富气相的水溶液包裹体的共生,说明流体的沸腾作用是引起成矿流体中矿质发生沉淀富集的主要成矿机制;对于蚀变岩型矿石,其成矿以水岩交代反应为主,成矿作用过程中流体处于近中性pH值环境。

Axi gold deposit is an adularia-sericite （high-sulfidation） type epithermal gold deposit hosted in Lower Carboniferous andesitic volcanic rocks and pyroclastics in western Tianshan, Xinjiang. The ore bodies occur in vein form, and are strictly controlled by the circular faults of palaeocaldera margin. The mineralization can be divided into five main stages, that are quartz-chalcedony vein stage, quartz vein stage, quartz-carbonate vein stage, sulfide vein stage and carbonate vein stage, and two types of ore are formed, including quartz vein and altered rock. The main ore minerals are native gold, pyrite, marcasite, arsenopyrite and gange minerals include quartz, chalcedony, siderite, calcite, sericite, adularia, etc. There three types of fluid inclusions formed in major mineralization stages were recognized：Ⅰliquid aqueous inclusion; Ⅱliquid-rich two-phase fluid inclusion; and Ⅲgas-rich fluid inclusion. The former two predominate. The ice melting temperatures of fluid inclusions range from -0.3- -2. 3℃, the corresponding salinities are 0. 48% - 3.75% NaCleq, averaging 1.85% NaCleq, and the peak ice melting temperatures range from -0.4--1.6℃, the corresponding salinities are 0. 66%-2. 63% NaCleq. The range varies from 121- 335℃, averaging 209 ℃, and the peak temperatures range from 140- 290℃. Based on the the salinities and homogenization temperatures, we measured the fluid density is 0. 73-0. 96, and the ore-forming depth is less than 700 m. ore-forming fluid has a narrow range of oxygen and hydrogen isotopic composition δDSMOW,H2O=-98‰--116‰,δ^18OSMOW,H2O=-0.55‰-1.65‰, and the carbon isotope δ^13Celcite,ODB 2.6‰-4.9‰the sulfur isotope δ34SCDT=-4.0‰-3.1‰（ average -0. 45‰）. It is indicated by the stable isotope data hat ore fluids come mainly from circulating meteoric water, the ore solutes are mainly from volcanic rocks osting the deposit and basement rocks. Appearance of adularias and bladed quartz and sulfides aggregates and coexisting type Ⅰ, type Ⅱ and rpe Ⅲ inclusions with greatly varied of vapor/liquid rations and homogenization temperatures suggest that boiling is the main factor causing the deposition of ore solutes. While for altered rock type ores, waterrock interaction may be major cause for ore solutes precipitation. The ore-forming fluid was close to neutral pH during major mineralization stages.