湖南瑶岗仙矽卡岩型白钨矿床成矿流体演化特征研究

Characteristics and evolutions of ore-forming fluids in the Yaogangxian skarn-type scheelite deposit,Hunan Province

湖南瑶岗仙超大型钨矿床位于南岭成矿带中段,主要由石英脉型黑钨矿矿脉和矽卡岩型白钨矿矿体组成。前人对瑶岗仙石英脉型黑钨矿矿体开展了较为详细研究,但对矽卡岩型白钨矿的研究则相对较少,有关其矿体特征、成矿过程及其与石英脉型矿化的成因联系尚不清楚。本文在矿床地质研究基础上,将瑶岗仙矽卡岩型钨矿床分为早期石榴子石-透辉石-白钨矿阶段(Ⅰ)和晚期碳酸盐-硫化物-白钨矿阶段(Ⅱ),并重点针对两个阶段的白钨矿开展了矿物学、元素地球化学、成矿流体地球化学研究,进而厘定了矽卡岩型白钨矿的成矿过程。结果显示,Ⅰ阶段白钨矿的Mo含量(1648×10^(-6)~3310×10^(-6))明显高于Ⅱ阶段白钨矿的Mo含量(816×10^(-6)~1725×10^(-6)),且I阶段白钨矿的稀土配分具明显的MREE和HREE亏损特征,指示早期矽卡岩阶段成矿流体具有相对高的氧化条件。两阶段的流体包裹体具有相似的δ^(18)O值(7.7‰~9.8‰和7.4‰~8.9‰)和δD值(-53‰~-60‰),表明成矿流体均主要来源于花岗质岩浆,而大气降水与岩浆流体的混合程度低于其他矽卡岩型钨矿床。白钨矿中的流体包裹体具有富CO_(2)和CH_(4)的特征,指示该成矿流体体系易于发生流体不混溶作用。白钨矿Ⅰ的成矿流体均一温度为229.1~377.3℃,盐度为1.8%~14.7%NaCleqv,白钨矿Ⅱ的成矿流体均一温度为187.4~294.5℃,盐度为1.2%~10.2%NaCleqv,指示成矿流体演化过程中流体温度和盐度逐渐降低。由此可见,瑶岗仙矽卡岩型矿床的成矿流体起源于花岗质岩浆,与石英脉型黑钨矿同源,总体上经历了两阶段钨矿的叠加成矿作用,流体不混溶作用对钨的聚集至关重要,在早期矽卡岩阶段形成了呈条带状的白钨矿矿体,在晚期矽卡岩退化蚀变阶段形成了被碳酸盐交代的白钨矿矿体。

Yaogangxian deposit,a famous W deposit in China,is located in in the middle Nanling metallogenic belt in Hunan Province.This deposit is mainly composed of the quartz vein-type wolframite and the skarn-type scheelite mineralization.Much attention of previous studies has been paid on quartz vein-type mineralization in Yaogangxian,however,the skarn-type mineralization is lack of detailed study,and as a result,its ore body and mineralization processes are not clear.This study focused on scheelite formed in the two-stage skarn-type mineralization to clarify its mineralization processes by using in-situ analytical techniques for the scheelite grains and its fluid inclusions.The skarn-type scheelite mineralization is classified into the garnet-diopside-scheelite stage(early stage)scheelite(Ⅰ)and the carbonate-sulfide-scheelite(late stage)scheelite(Ⅱ).The Mo content of scheeliteⅠ(1648×10^(-6)~3310×10^(-6))is significantly higher than that of scheeliteⅡ(816×10^(-6)~1725×10^(-6)),and scheeliteⅠshow a right-dipping chondrite-normalized REE pattern,with depleted MREE and HREE,implying that the early-stage is relatively oxidized.In addition,the fluid inclusions in both stages have similarδ^(18)O values(7.7‰~9.8‰and 7.4‰~8.9‰)andδD(-53‰~-60‰),which suggest that the ore-forming fluids of two stages might be mainly derived from magma.The involvement of meteoric water was probably extremely weak in the skarn-type mineralization.The fluid inclusion microthermometry show that the ore-forming fluids decreased in temperature and salinity from early(229.1~377.3℃and 1.8%~14.7%NaCleqv)to late stage(187.4~294.5℃and 1.2%~10.2%NaCleqv)with supersaturated CO_(2) and CH_(4).It suggests that fluid immiscibility would occur in the ore-forming fluid system.Therefore,we propose that the skarn-type scheelite mineralization of Yaogangxian deposit were mainly originated from the granitic magma,and generally experienced two stages of scheelite superposition mineralization.The fluid immiscibility was crucial to enrichment of W,where the banded scheelite mineralization was formed in the early skarn stage,and the disseminated scheelite mineralization was formed in the late skarn stage.