LA-ICPMS trace elements of pyrite from the super-large Dulong Sn-Zn polymetallic deposit, southeastern Yunnan, China

Abstract

滇东南马关都龙是一个以锡锌为主，共-伴生铟、铜、铅、钨、铁、银等多种元素的锡锌多金属超大型矿床。虽然前人从矿物学、矿床地球化学、年代学等不同角度开展了较多的研究，该矿床锡锌多金属矿化为燕山晚期岩浆热液活动的产物已是不争的事实，但关于该矿床是否存在热水沉积作用及其与锡锌多金属成矿作用的关系依然存在较大争议。本文选取都龙矿区广泛存在的黄铁矿作为主要研究对象，在矿相学基础上利用LA-ICPMS对不同阶段黄铁矿的微量元素组成开展了系统的研究。野外及显微鉴定结果表明，矿区存在四种类型（期次）的黄铁矿，即：鲕状黄铁矿Py1；穿切或交代Py1的细脉状黄铁矿Py2；与闪锌矿等硫化物共生的自形黄铁矿Py3；包裹早期黄铁矿或闪锌矿等硫化物的他形黄铁矿Py4。LA-ICPMS分析结果表明，该矿床黄铁矿中富集多种微量元素，其中Co、Ni、As、Ge等元素以类质同象的形式存在黄铁矿晶格中，而其余元素多以显微矿物包体形式赋存于黄铁矿中。上述四期黄铁矿微量元素组成存在较大差别，Py1相对富集Zn和As，而其余微量元素含量较低，Co与Ni含量较低，Co/Ni比值远低于1.00，其微量元素组成与典型沉积作用形成黄铁矿基本一致；Py2与Py1具有相似的微量元素组成特征，其Co/Ni比值接近Py1变化范围；Py3和Py4除富集Zn、As外，Mn、Co、Ni、Cu、Sb、Pb、Bi元素含量也相对较高，其Co/Ni比值相对较高，多大于1，与典型岩浆热液型黄铁矿微量元素组成相似，而与沉积型黄铁矿差异明显。结合各阶段黄铁矿产出地质特征，对比不同类型黄铁矿微量元素组成，本研究认为：Py1鲕状黄铁矿为热水沉积作用形成；Py2为Py1变质改造形成的细脉状黄铁矿，其微量元素继承了Py1；Py3为岩浆热液活动形成的自形黄铁矿；Py4为岩浆热液活动晚期形成的他形黄铁矿，Ag和Bi组成作为区分不同成因类型黄铁矿的化学指标的潜力。矿区早期沉积作用形成鲕状黄铁矿过程可能为后期成矿作用提供了部分硫源及少量Zn等成矿物质，海西-印支期区域变质改造作用对矿区成矿作用影响不大，而燕山晚期岩浆热液活动才是矿区锡多金属大规模成矿作用的主导因素。;The Dulong Sn-Zn polymetallic deposit, located in the southeastern Yunnan Province, is a super-large Sn-Zn polymetallic deposit with by-products of In, Cu, Pb, W, Fe, Ag, etc. Many studies on mineralogy, ore deposit geochemistry and chronology have been carried out on this deposit, confirming a genetical relationship of its Sn-Zn mineralization to the Late Yanshanian magmatic activity. However, whether the deposit underwent exhalative sedimentary Sn-Zn mineralization is still controversial. Based on detailed petrographic observation, the LA-ICPMS trace element compositions of pyrite that widely occur in the Dulong deposit are investigated systematically in this paper. Field and microscopic observations show that four types of pyrite have been identified in Dulong, including the oolitic pyrite (Py1), fine vein pyrite (Py2) crosscutting and/or replacing Py1, euhedral pyrite (Py3) coexisting with sphalerite and other sulfides, anhedral pyrite (Py4) enclosing early generated pyrite and sphalerite. LA-ICPMS analysis results suggest that pyrite is enriched in Co, Ni, As and Ge, which are mainly incorporated into the pyrite lattice in the form of isomorphism. Other elements occur mostly as mineral micro-inclusions in pyrite. The trace element compositions of the four types' pyrite exhibit significant difference from each other: Py1 is enriched in Zn and As but depleted in Co, Ni and Cu with low Co/Ni ratios (<1.0), which share similar features with typical sedimentary pyrite; Py2 is similar in its trace element compositions and Co/Ni ratios to those in Py1; while Py3 and Py4 have elevated Zn, As, Mn, Co, Ni, Cu, Sb, Pb and Bi contents with high Co/Ni ratios (mostly >1.0), which is consistent with that of euhedral pyrite formed by magmatic-hydrothermal activity rather than sedimentary pyrite. Comparing with the trace element compositions of pyrite in different types, we consider that Py1 is of sedimentary origin, Py2 that inherits the trace element features of Py1 is formed by metamorphic reworking of Py1, while Py3 and Py4 are formed by late magmatic-hydrothermal activity. Ag and Bi in pyrite display potential probobilities as a geochemical indicator to distinguish pyrite of different genetic types. Oolitic pyrite formed in the depositional diagenesis stage may provide some sulfur and a small amount of Zn for the later Sn-Zn mineralization. The regional metamorphic reworking in the Hercynian to Indosinian displays a limited effect on the Sn-Zn mineralization, while Late Yanshanian magmatic-hydrothermal activity is the controlling factor for the formation of the super-large scale Dulong Sn-Zn polymetallic deposit.