西藏甲玛超大型矿床矽卡岩矿物组合及其分带模式

Skarn mineral assemblage and zonation pattern in the Jiama superlarge deposit,Tibet

甲玛矿床作为我国规模最大的矽卡岩型铜多金属矿床,其保存了矽卡岩形成演化及成矿作用的重要信息。前人在矿床地质、矿床地球化学等方面对该矿床开展了详细的研究,但对于矽卡岩矿物学及其分带模式还缺少深入系统的总结,制约了矿床模型的完善。为此,本文通过详细的野外地质编录、系统的光薄片镜下鉴定和电子探针分析,开展矽卡岩矿物组合、矿物成分及其分带特征研究。本次工作在矿床中识别出一套典型的钙质矽卡岩矿物,包括进变质阶段形成的石榴子石、辉石、硅灰石、符山石和退化蚀变阶段的绿帘石、绿泥石和角闪石等。矽卡岩矿物组合、化学成分及金属矿化在空间上具有明显分带性。硅灰石矽卡岩相对较独立,产于矽卡岩中下部与大理岩接触部位,主要赋存斑铜矿化。透辉石-石榴子石矽卡岩从斑岩接触带近端矽卡岩→中部带矽卡岩→远端矽卡岩具有明显分带现象:石榴子石/辉石含量比值具有大于20∶1→约10∶1→约5∶1的变化趋势;石榴子石颜色由红-棕色→棕-绿色→绿-淡黄色;石榴子石端元组分为Ad_(71.82)Gr_(26.52)Pyr_(1.61)→Ad_(62.09)Gr_(36.25)Pyr_(1.55)→Ad_(54.37)Gr_(44.40)Pyr_(1.46),钙铁榴石(Ad)含量呈现减小的趋势;矿化元素表现为Mo-Cu±Au±Ag→Cu±Mo±Au±Ag→Pb-Zn±Cu±Au±Ag的分带规律。矽卡岩矿物组合和化学成分特征对于成矿环境及矿化类型具有重要指示意义,运用矽卡岩分带模式可以有效指导找矿勘探。

As the largest skarn type copper polymetallic deposit in China,the Jiama deposit has preserved important information on skarn formation,evolution and mineralization.The geological and geochemical characteristics of the Jiama deposit have been studied by some researchers,but there is a lack of summary of skarn mineralogy and its zoning pattern,which impedes the completeness of the deposit model.In this paper,the mineral assemblage,mineral composition and zonation characteristics of skarn are studied through detailed field geological logging,systematic optical microscopy and electron probe analysis.A set of typical calcareous skarn minerals have been identified in the Jiama deposit,including garnet,pyroxene,wollastonite,vesuvianitae of the prograde stage and epidote,chlorite,amphibole of the retrograde stage.The mineralogy and mineral chemical compositions display an outward zonation with respect to the source porphyry.The wollastonite skarn is mainly present at the contact zone between skarn and marble,and hosts mainly bornite mineralization.From the proximal skarn to the intermediate skarn to the distal skarn in the garnet-pyroxene skarn,the garnet/pyroxene ratio varies from>20:1to~10:1to~5:1,the garnet color varies from red-brown to brown-green to green-yellow,the average composition of garnet varies from Ad_(71.82)Gr_(26.52)Pyr_(1.61)→Ad_(62.09)Gr_(36.25)Pyr_(1.55)→Ad_(54.37)Gr_(44.40)Pyr_(1.46),and the mineralization changes from Mo-Cu±Au±Ag to Cu±Mo±Au±Ag to Pb-Zn±Cu±Au±Ag ores,respectively.The mineral assemblage and chemical composition characteristics of skarn are important indicators of ore forming environment and mineralization type.The skarn zonation pattern can effectively guide ore prospecting and exploration.