黔西南灰家堡金矿田热液方解石稀土元素与C-O-Sr同位素地球化学特征

为探讨灰家堡金矿田矿化剂和成矿物质来源,系统开展了热液方解石的稀土元素和C-O-Sr同位素研究。结果表明,热液方解石的稀土元素具有"上凸型"配分模式,低的(La/Sm)N值(0.003~0.580),较高的(Gd/Yb)_N值(1.21~15.73),Eu正异常,显示成矿流体的壳源特征;产于龙潭组(P3l)的矿体中的方解石δ13CPDB为-9.3‰~1.7‰(均值为-3.93‰);产于构造蚀变体(SBT)的矿体中的方解石δ¹³CPDB为-7.8‰~3.3‰(均值为1.04‰),C-O同位素组成介于地幔碳酸岩与海相碳酸盐岩之间,在δ¹³CPDB-δ¹⁸OSMOW图解上呈近水平分布,与矿体全岩及围岩碳同位素组成相似,表明成矿流体的C可能具有多来源特征。成矿流体与围岩之间的水-岩反应并未改变矿石的碳、氧同位素组成,流体-岩石相互作用不是导致热液方解石沉淀的主要机制,而更可能与CO₂的脱气作用有关;方解石的⁸⁷Sr/⁸⁶Sr值接近碳酸盐岩围岩、明显高于幔源物质的平均值,表明成矿热液中的Sr可能主要来自碳酸盐岩围岩。

Rare Earth Elements and C-O-Sr Isotopic Geochemical Characteristic of Hydrothermal Calcites of the Carlin-Type Gold Deposits in the Huijiabao Orefield,Southwestern Guizhou,China

In order to constrain the origin of mineralizers and ore-forming materials in hydrothermal fluids involved in the gold mineralization of the Huijiabao orefield, We here have systematically carried out geochemical studies of rare earth elements(REE) and C-O-Sr isotopes of hydrothermal calcites of the Carlin-type gold deposits in the Huijiabao Orefield. The results show that the hydrothermal calcites have MREE-enriched REE patterns, low(La/Sm)N values ranging from 0.003 to 0.58, relatively high(Gd/Yb)_N values ranging from 1.21 to 15.73, and positive Eu anomalies, indicating the characteristics of crustal originated ore-forming fluids. The δ¹³CPDB values of calcites from the ore-bearing Longtan Formation(P3l) vary from-9.3‰ to 1.7‰(average-3.93‰);whereas those of calcites of the structure-controlled hydrothermal altered rocks(SBT)vary from-7.8‰ to 3.3‰(average 1.04‰). The C-O isotopic compositions of hydrothermal calcites are plotted in a near horizontal band between those of mantle carbonatite and those of marine carbonate on the δ¹³C-δ¹⁸O diagram. The similarity of C isotopic compositions of hydrothermal calcites to those of the primary carbonate rocks and wallrocks of the orebody indicates that C in the ore-forming fluid may have multi-source characteristics. The water-rock reaction between the ore-forming fluid and wallrocks had not changed the carbon and oxygen isotopic compositions of ores. The precipitation of hydrothermal calcite is more likely resulted from the degassing of CO₂ rather than the water-rock reaction. The ⁸⁷Sr/⁸⁶Sr values of hydrothermal calcites are close to those of carbonate wall rock, but are significantly higher than the average value of mantle-derived materials, indicating that the Sr in the ore-forming hydrothermal fluid could be mainly sourced from the carbonate host rocks.