滇西北铜厂沟矽卡岩−斑岩型钼铜矿床中石榴子石地球化学、U−Pb 年代学及地质意义

梁丰; 王艳军; 赵严; 李凯旋; 任志; 毛金伟; 陈涛亮; 冷成彪

doi:10.12401/j.nwg.2023142

滇西北铜厂沟矽卡岩−斑岩型钼铜矿床中石榴子石地球化学、U−Pb 年代学及地质意义

Geochemistry, U−Pb Chronology and Geological Significance of Garnet in the Skarn−Porphyry Molybdenum Copper Deposit in Tongchanggou, Northwest Yunnan Province

摘要

摘要: 滇西北铜厂沟矽卡岩−斑岩型Mo−Cu矿床是扬子地块西缘新发现的大型钼铜矿床。矽卡岩在铜厂沟矿区广泛发育，是矿区最主要的赋矿岩石。笔者以铜厂沟矽卡岩中石榴子石（Grt）为研究对象，利用电子探针和LA−ICP−MS原位技术开展成分和年代学测试分析。根据矿物镜下特征，将矿区石榴子石分为早期（Grt I）和晚期（Grt II）两个世代。Grt I为浅黄色−黄白色、无环带石榴子石，Grt II为褐色−红褐色、具有明显环带的石榴子石。两期石榴子石的SiO₂（34.41%～38.45%）、CaO（32.7%～35.25%）、Al₂O₃（6.64%～12.57%）和FeO（12.03%～22.63%）含量指示，二者均属于钙铁榴石−钙铝榴石系列（And_34-64Gro_34-58）。Grt I和Grt II均呈轻稀土和重稀土亏损、中稀土富集的“驼峰型”配分模式。相比于Grt I，Grt II具有更低的U含量和变化范围更大的Eu异常，分别指示晚期矽卡岩矿化流体氧逸度较高和pH变化较大。Grt II中显著的环带结构及变化较大的主量成分指示晚期石榴子石化经历了更为强烈的水岩反应，与pH值变化较大的特征一致。因此，铜厂沟两期石榴子石记录了矽卡岩体系由相对封闭到开放、逐渐氧化的过程。此外，Grt I原位LA−ICP−MS U−Pb 年代学分析获得的年龄为（85.4±5.6）Ma （MSWD=0.91），直接限定铜厂沟矽卡岩的形成时代为晚白垩世。铜厂沟矽卡岩作用与矿区内斑岩体侵位及钼矿化时代在误差范围内一致，表明铜厂沟Mo−Cu矿床记录了完整的斑岩−矽卡岩矿化过程。结合区域构造演化历史，提出铜厂沟及区域上同时代的Cu−Mo−W等多金属矿床是晚白垩世碰撞后斑岩热液体系的产物。

Abstract: The Tongchanggou skarn−porphyry Mo−Cu deposit, located in the northwest Yunnan Province, is one of the newly−discovered large molybdenum−copper deposits in the western margin of the Yangtze Block. Skarn is widely exposed in the Tongchanggou deposit and is the most important ore−bearing body in the deposit. In this paper, both EPMA and LA−ICP−MS technology have been used to analyze the major and trace element compositions and U−Pb isotopes of garnet grains from the Tongchanggou skarn. Based on microscopic observation, the garnet in the Tongchanggou deposit can be divided into two generations: the early, light yellow to yellowish white, zoning−free to weakly zoning garnet (Grt I) and the late, brown to reddish brown, strongly zoning garnet (Grt II). The results of EPMA show that the two-generation garnet samples have SiO₂ contents of 34.41%～38.45%, CaO contents of 32.7%～35.25%, Al₂O₃ contents of 6.64%～12.57% and FeO contents of 12.03%～22.63%, indicating both the two generations belong to the andradite−grossularite series (And_34-64Gro_34-58). Both Grt I and Grt II have similar “hump−type” rare earth element (REE) pattern with enrichments in middle REE and depletions in light and heavy REE. Relative to Grt I, Grt II have lower U concentrations and more obvious δEu anomalies, indicating it formed under higher oxygen fugacity (f_O2) and more variable pH conditions. Meanwhile, stronger oscillatory zoning within Grt II than Grt I, together with more variable major elements, indicate more intensive water/rock interaction. The two-generation garnet of the Tongchanggou deposit record its an evolving hydrothermal system, accompanying with increasing oxygen fugacity and water/rock interaction degrees. In addition, LA−ICP−MS U−Pb dating results of Grt I yield an age of (85.4±5.6) Ma (MSWD= 0.91), which directly constrains the Tongchanggou mineralization age. Within uncertainties, this age is coeval with emplacement of porphyry intrusions and timing of Mo mineralization in the Tongchanggou area, indicating that the Tongchanggou skarn mineralization consists of an important part of the porphyry−hydrothermal mineralization system. Combined with regional tectonic evolution, it's proposed that the late Cretaceous Tongchanggou and regional coeval Cu−Mo−W deposits are products of post−collisional porphyry−hydrothermal system.

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