江西相山铀矿田深部多金属矿化成矿流体来源:流体包裹体He-Ar同位素证据

江西相山铀矿田西部地区实施的铀矿科学深钻3号孔在深部–700 m发现大量铅锌银多金属矿化脉,垂向上呈"上铀下多金属"的分布特征。本文对深部多金属矿化主成矿阶段的黄铁矿、闪锌矿、方铅矿进行了流体包裹体He和Ar同位素组成测定,结果表明,相山深部多金属矿化主成矿阶段的黄铁矿、闪锌矿和方铅矿具有较一致的He、Ar同位素组成,矿物内流体包裹体的~3He含量范围为4.55×10^–14～1560×10^–14 cm^3·STP/g,~4He含量范围为1.05×10^–7～525×10^–7 cm^3·STP/g,~3 He/~4He为2.98×10^–7～4.96×10^–7,即0.21～0.35 Ra,介于地壳(0.01～0.05 Ra)和饱和大气水(1 Ra)之间,明显低于地幔的~3 He/~4He值(6～9 Ra),⁴⁰Ar浓度为0.969×10^–7～157×10^–7 cm^3·STP/g,⁴⁰Ar/³⁶Ar值为303.7～573.9,略高于饱和大气水的Ar同位素组成,结合矿石和蚀变岩石中Li元素富集特征,本文认为相山西部地区深部多金属成矿流体以地壳流体和大气降水为主,成矿流体富Li元素是He同位素明显高于地壳的原因,可能存在少量幔源流体的加入。另外,通过对比前人研究的该地区铀矿化矿石中黄铁矿He-Ar同位素组成,并结合前人对该地区幔源组分明显参与铀成矿的认识,本文认为多金属矿化和铀矿化成矿流体可能并非同一来源。

Origin of Ore-forming Fluids in the Deep Polymetallic Mineralization in Western Xiangshan Uranium Ore Field,Jiangxi Province,China:Constraints from He-Ar Isotopes

The Xiangshan uranium ore field,the largest hydrothermal uranium ore field in China,is hosted in the Early Cretaceous felsic volcanic rocks.In recent years,a discovery of hidden lead-zinc-silver mineralization in deep reach was achieved through drilling exploration in the west part of the Xiangshang uranium orefield.This paper provides He and Ar isotope data of fluid inclusions in pyrite,sphalerite,and galena from the lead-zinc-silver veins of main mineralization epoch.Isotopic abundance and ratios of He and Ar found in inclusion fluids show that the concentrations of 4 He vary widely from 1.05×10^-7 cm^3·STP/g to 525×10^-7 cm^3·STP/g while 3 He from 4.55×10^-14 cm^3·STP/g to 1560×10^-14 cm^3·STP/g.The R/Ra ratios of fluid inclusions are in the range of 0.21-0.35(where Ra is the 3 He/4 He ratio of air=1.39×10^-6)with a mean value of 0.29,broadly similar to that of the crust.The concentrations of 40 Ar range from 0.969×10^-7 to 157×10^-7 and the 40 Ar/36 Ar ratios range from 303.7 to 573.9,which are slightly higher than those of the atmospheric argon.On the basis of abundant neutrons in uranium-enriched felsic volcanic rocks and enriched lithium in ore veins and altered rocks,we propose that lithium is probably the main reason which lead to the higher 3 He/4 He ratios due to significant 3 He production via 6 Li(n,α)3 H(β)3 He reactions.The helium and argon isotope compositions of fluid inclusions suggest that the ore-forming fluids of the deep polymetallic mineralization in the western Xiangshan orefield were mainly derived from the crust and meteoric water with input of minor mantle component.Referring to previous research on mantle-derived fluids and deep-sourced uranium mineralization,it can be assumed that the ore fluids of the two types of deposits were from different sources.