湘南黄沙坪多金属矿床石榴子石地球化学特征及其对Cu与W-Sn复合成矿机理的指示

Geochemical characteristics of garnet in the Huangshaping polymetallic deposit,southern Hunan: Implications for the genesis of Cu and W-Sn mineralization

黄沙坪矿床位于钦杭成矿带与南岭成矿带的交汇部位,是湘南地区矽卡岩-热液脉型Cu多金属与矽卡岩型W-Sn多金属复合成矿作用的典型代表。为厘清区内Cu与W-Sn成矿物理化学条件以及成矿流体的性质,本文对黄沙坪矿床两类矽卡岩中的石榴子石开展电子探针和LA-ICP-MS原位微区主微量元素研究。结果表明,成W-Sn矿矽卡岩中石榴子石为钙铝榴石-钙铁榴石(平均And71. 3-Gro23. 8)固溶体系列,具有一致的稀土元素配分模式,轻重稀土分异不明显,负Eu异常显著,明显亏损La,∑REE含量与Fe^(3+)/(Fe^(3+)+Al)具有负相关关系,具有较高的U含量,表明石榴子石可能是在相对还原的环境下缓慢结晶形成的。石榴子石中W和Sn的含量随石榴子石中钙铝榴石的比例增加而减少,有利于W-Sn在晚期热液中富集成矿。与Cu矿有关矽卡岩中的石榴子石几乎为纯的钙铁榴石(平均And90. 1-Gro9. 2),具有低的U、∑REE含量,轻重稀土元素分异明显,相对富集轻稀土,具有明显正Eu异常,表明石榴子石具有较快的生长速度,成矿流体具有相对较高的氧逸度和Cl的含量,这种环境有利于Cu在岩浆热液中迁移和富集成矿。因此,早期成矿流体成分以及氧化还原条件的差异可能是导致黄沙坪矿床W-Sn与Cu成矿差异的重要原因。

Huangshaping deposit,located in the intersection between the Nanling W-Sn metallogenic province and the Qin-Hang Cu polymetallic belt,is a representative deposit in southern Hunan Province with skarn-vein Cu polymetallic and skarn W-Sn polymetallic superimposed mineralization. To clarify metallogenic condition and nature of the ore-forming fluid of the W-Sn and Cu mineralization,we present in-situ EPMA and LA-ICP-MS analysis of garnet from two kinds of skarn in the Huangshaping deposit. The garnet associated with W-Sn mineralization is andradite-grossular series（ average And71. 3-Gro23. 8） and have a consistent chondrite-normalized REE patterns with negative Eu anomalies and depletion in La. These garnets also display a positive correlation between ∑REE and Fe3＋/（ Fe3＋＋ Al）,and enrichment of U. These characteristics indicate that the garnets grow slowly in a relative reduced condition. The content of W and Sn show a positive correlation with the content of Al in garnet,which is favorable for the enrichment and mineralization of W and Sn. In contrast,garnet related Cu mineralization is almost pure andradite（ average And90. 1-Gro9. 2） with low content of U and ∑ REE. Chondrite-normalized REE patterns is characterized by enrichment of LREEs and positive Eu anomalies.These characteristics indicate that the garnets were formed rapidly from relative oxidized hydrothermal fluid with relative high Cl content,which is favorable for Cu mineralization. W-Sn and Cu superimposed mineralization in the Huangshaping deposit result from different hydrothermal fluid with different composition and redox state.