湘西沃溪金锑钨矿床燕山期幕式成矿作用:来自原位白钨矿U-Pb定年与黄铁矿元素-同位素的约束

2024.Episodic mineralization at Yanshannian in the Woxi Au-Sb-W deposit,western Hunan,South China:Constraints from in-situ scheelite LA-ICP-MS U-Pb geochronology and element-isotope of pyrite

沃溪矿床位于江南造山带中段,是一座同时具有金、锑、钨大规模成矿的大型金矿,近年的研究揭示其主要形成于燕山期,但该矿的成矿物质源区及矿床成因尚存争议。基于此,本文利用SEM、EPMA、LA-MC-IF-ICP-MS等原位分析技术,对该矿床多世代黄铁矿的结构、成分及同位素组成和成矿期白钨矿的U-Pb年代学进行了研究。在沃溪矿床中,识别出3个世代5种类型黄铁矿(Py),即成矿前Py1,成矿第二阶段Py2和第三阶段的Py3a、Py3b和Py3c。黄铁矿微区成分及围岩蚀变特征指示,成矿期Au、As和Cu耦合,Au主要以固溶体(Au+)的形式存在于Py3b的晶格中;As-取代S-进入黄铁矿是促进晶格金富集的主要因素;而强烈的水-岩反应是导致成矿期热液黄铁矿Co/Ni<1的原因。黄铁矿的原位微量元素和S-Pb同位素组成(δ34 S V-CDT值:Py2为-4.33‰~-1.61‰,Py3a为-6.36‰~3.59‰,Py3b为-4.36‰~3.59‰,Py3c为-2.21‰~-0.65‰;206 Pb/204 Pb、207 Pb/204 Pb、208 Pb/204 Pb值:Py2分别为18.029、15.618和38.653,Py3a分别为18.047~18.341、15.594~15.698、38.781~39.394)及其在环带结构中的变化,指示第二及第三成矿阶段流体性质存在显著差异。成矿第三阶段白钨矿的LA-ICP-MS U-Pb定年结果为132.7±7.9Ma(1σ,n=31,MSWD=0.43)。结合前人研究成果,我们认为沃溪金锑钨矿床主要形成于燕山期148~140Ma的黑钨矿(少量白钨矿、黄铁矿和金)和~130Ma的金-锑-白钨矿两阶段幕式成矿作用。早阶段成矿为下伏基底岩石重熔岩浆演化出的相对高温的还原性流体;晚阶段金-锑-白钨矿成矿以脉冲性幔源流体为主,金主要具地幔来源,而锑和钨可能来源于壳源与幔源流体混合演化出的富矿热液,成矿的脉动性可能与深部断层阀机制有关。金的主要沉淀机制为水岩反应和硫化作用,流体混合是辉锑矿与白钨矿的沉淀机制。富集交代的岩石圈地幔活化和古太平洋板块俯冲后撤、拆沉导致的岩石圈大规模伸展是江南造山带中段(湖南)燕山晚期金多金属大规模成矿的关键因素及构造背景,区域上具有进一步寻找陆内弧后伸展背景金多金属矿床的潜力。

The Woxi deposit,located in the central Jiangnan Orogen,is a large gold deposit with significant antimony and tungsten mineralization.Recent research indicates that the deposit was formed primarily at Yanshanian,but the source of the mineralization materials and ore genesis are still controversial.By using SEM,EPMA,and LA-MC-IF-ICP-MS,this study analyzed the structures,elemental compositions and isotopic components of multi-stage pyrites,as well as U-Pb geochronology of scheelite.Within the Woxi deposit,three generations of five types of pyrite(Py)have been identified,i.e.,pre-mineralization Py1,mineralization stage 2 Py2,and mineralization stage 3 Py3a,Py3b,and Py3c.The in-situ analysis of pyrite and the alteration characteristics of whole rocks indicate that,during mineralization,Au,As,and Cu were coupled,with Au mainly existing in the crystal lattice of Py3b as a solid solution(Au+),As substitution for S in pyrite was the primary factor promoting lattice gold enrichment,while strong water-rock reactions were responsible for the low Co/Ni ratios in the mineralizing stage.The in-situ trace elements and S-Pb isotopic compositions of the different generations of pyrite(δ34 S V-CDT values:Py2 range from-4.33‰to-1.61‰,Py3a range from-6.36‰to 3.59‰,Py3b range from-4.36‰to 3.59‰,Py3c range from-2.21‰to-0.65‰;the values of 206 Pb/204 Pb,207 Pb/204 Pb and 208 Pb/204 Pb:Py2 are 18.029,15.618 and 38.653 respectively,and Py3a are 18.047~18.341,15.594~15.698 and 38.781~39.398,respectively)and their variations in the zoned textural indicate significant differences in fluid properties and source areas during the second and third stages of mineralization.LA-IF-ICP-MS U-Pb dating of the third-stage white scheelite yields an age of 132.7±7.9Ma(1σ,n=31,MSWD=0.43).Combined with the previous studies,our study revealed that the Woxi Au-Sb-W deposits went through two stage mineralization at Yanshanian,i.e.,ca.148~140Ma wolframite(with minor scheelite,pyrite and gold)and ca.130Ma gold,anitimony and scheelite deposition.The earlier stage was formed in a relatively high-temperature reducing fluid produced by partial melting of underlying basement rocks.The later stage Au-Sb-W mineralization is dominated by pulsed mantle-derived(magma)fluids,and its metal may mainly come from the ore-rich hydrothermal solution evolved by the mixture of crust-and mantle-derived fluids.The pulsation of mineralization and fluid pressure fluctuation may be related to fault valve mechanism.The main precipitation gold mechanism is sulfidation and water-rock reaction,while fluid-mixing leads to deposition of stibnite and scheelite.The reactivation of enriched and metasomatic.Our research shows that the large-scale lithospheric extension caused by the episodic subduction and detachment of the lithospheric mantle enriched and metasomatic in multiple stages and the ancient Pacific plate is the key factor and tectonic background for the large-scale gold(polymetallic)mineralization in Woxi and other gold deposits in Jiangnan Orogen during Late Yanshanian.