湘南黄沙坪多金属矿床花岗斑岩的矿物化学及其对矽卡岩白钨矿成矿的指示意义

黄沙坪多金属矿床是湖南最大的铅锌生产基地,并且在与矿床内花岗斑岩接触的矽卡岩带产有隐伏的大型矽卡岩型白钨矿和中型规模的辉钼矿。钨-钼矿化的时代为晚侏罗世,与矿床内花岗斑岩侵入时代一致。然而,已有研究认为,由于该花岗斑岩规模很小,矽卡岩型白钨矿的成矿热液应来自深部岩浆房而非此花岗斑岩。为此,我们对花岗斑岩进行了仔细的镜下观测,并且对其中的副矿物和黑云母以及矽卡岩中的白钨矿进行了电子探针成分分析,应用原位LA-ICP-MS方法测定了矽卡岩中白钨矿的稀土元素含量,试图对白钨矿矿化的物质和流体来源提供确切的证据。通过研究,首次在矿床内花岗斑岩中发现了与未蚀变黑云母伴生的黑钨矿和铌铁矿,表明花岗斑岩至少在岩浆结晶作用晚期或岩浆-热液过渡阶段早期就已发生钨的矿物富集,为确定花岗斑岩是控制钨矿化的成矿岩体提供了依据。此外,发现花岗斑岩中的黑云母(属铁叶云母)含有极高的氟含量(3%),指示其应形成于富含氟的高分异岩浆。研究进一步揭示,矽卡岩中白钨矿的轻稀土元素配分模式与花岗斑岩十分一致,而重稀土元素则显著亏损,而且Eu的含量较花岗斑岩更为富集。这暗示形成白钨矿的成矿流体应直接来自花岗斑岩,即:在早期无水矽卡岩阶段,石榴子石的沉淀导致流体中的重稀土亏损而Eu相对富集;白钨矿随后再从这种流体中沉淀。此外,白钨矿的Eu含量与Sm、Gd含量具有负相关关系,表明Eu的分配是相对独立的行为,主要以Eu2+存在,从而指示沉淀白钨矿的流体具有还原的性质。结合前人的研究成果及本文所提供的新证据,我们认为,形成矽卡岩型白钨矿的钨和成矿热液应来自高分异且富F的花岗斑岩,而所需的钙则可能来自于碳酸盐围岩,即矿床内花岗斑岩应是形成钨钼矿床的物质来源,驱动热液活动的能量来源,和寻找隐伏钨矿床的重要找矿标志。

Mineral geochemistry of granite porphyry in Huangshaping pollymetallic deposit, southern Hunan Province, and its implications for metallogensis of skarn scheelite mineralization

The Huangshaping deposit, located in southern Hunan Province, is one of the largest polymetallic ore deposits in this region and is unique for its metals combination of W-Mo-Pb-Zn and the occurrence of significant reserves of all these metals. Previous research suggested that the scheelite mineralization is predominated by the deep magma chamber rather than the granite porphyry because of the limited volume of the latter, even though they have close spatial and temporal relationship. In this paper, wolframite and columbite are found coexisting with unaltered biotite in granite porphyry, and their chemical compositions are determined by electron microprobe analysis. Additionally, in situ LA-ICP-MS analysis is performed to obtain the REEs contents of scheelite in order to provide new evidence for the materials and fluids sources of the scheelite mineralization. Wolframite and columbite can be the direct evidence that the granite porphyry dominates the W mineralization, and also suggest that W-bearing minerals might concentrated at the late magmatic stage or early hydrothermal stage, the high F contents in biotite (>3%) indicate that the hydrothermal fluids were derived from high evolved and F-rich magma. The REEs patterns of scheelites are consistent with granite porphyry, except that the former are HREEs-depleted but rich of Eu. This suggests that the materials and fluids for scheelite mineralization are derived from granite porphyry, and the precipitation of garnet in the anhydrous skarn stage results for the fluids depleted of HREEs but rich of Eu, and finally scheelite is crystallized in these fluids. At the same time, the Eu contents in scheelites from the Huangshaping deposit are of negative correlation compared with the contents of Sm and Gd, which indicates the Eu must exist as Eu²⁺ in the hydrothermal fluids, meaning that the fluids is possibly reduced. Combining with previous research, the new evidence provided in this paper suggests that the material (W) and fluids for the scheelite mineralization were derived from high evolved and F-rich granite porphyry while the Ca must come from dissolution of the wallrock limestone, and the granite porphyry is a proxy to explore the hidden W mineralization in Huangshaping deposit.