小红石砬子铅锌(银)矿床成因:来自S-Pb同位素和元素特征的证据

The Genesis of the Xiaohongshilazi Pb-Zn(Ag)deposit:Evidence from S-Pb isotopes and elemental characteristics

小红石砬子铅锌(银)矿床是吉中-延边铜镍多金属成矿带中具有代表性的中型矿床,前人关于该矿床类型划分的观点有火山成因块状硫化物型(Volcanogenic Massive Sulfide,VMS)、中-低温热液脉型、喷流沉积-热液叠加改造型,这三种观点的争论严重制约了对该矿区及区域成矿和找矿的认识。本文选取该矿床中金属硫化物开展原位微量和S-Pb同位素分析,赋矿围岩开展全岩S同位素分析。闪锌矿Fe-Zn含量(4.48%~10.33%、53.67%~62.12%)与中温热液矿床一致(3.00%~10.00%、50.00%~60.00%,成矿温度约200℃),Zn/Cd比值(53.68~76.75)同样指示中低温成矿作用(高温:Zn/Cd>500;低温:Zn/Cd<250),Ga/Ge比值在lg(Ga/Ge)-T图解中,对应温度约为100~180℃,上述温度范围与前人测定的流体包裹体均一温度基本吻合(90~250℃)。闪锌矿Zn/Cd的比值(53.68~76.75)与Cd含量(7577×10-6~11266×10-6)特征与密西西比河谷型(Mississippi Valley-Type,MVT)矿床一致;黄铁矿Fe/S比值(0.76~1.29,平均值为0.89)接近典型MVT矿床的理论值(0.88)。金属硫化物δ34 S V-CDT值介于-9.6‰~-0.5‰之间,总体偏负;赋矿围岩δ34 S V-CDT值介于-8.5‰~4.2‰之间,两者重叠范围较大(主要集中于-9.0‰~-2.0‰),说明围岩提供了主要的还原硫,而古老基底可能提供了部分较轻的硫。金属硫化物Pb同位素比值为206 Pb/204 Pb=18.30~18.39、207 Pb/204 Pb=15.61~15.63、208 Pb/204 Pb=38.38~38.44,指示铅为混合来源,成矿物质可能主要来自上地壳和造山带,模式年龄计算显示上太古界乌拉山群变质岩可能为铅的初始来源。综上所述,小红石砬子铅锌(银)矿床地球化学特征与MVT型矿床基本一致,地质特征略有差别,因此推测其为类MVT型矿床。

The Xiaohongshilazi Pb-Zn(Ag)deposit is a representative medium-sized deposit within the Jizhong-Yanbian copper-nickel polymetallic metallogenic belt,previous views for deposit types mainly include Volcanogenic Massive Sulfide(VMS)-type,medium-low temperature hydrothermal vein type,and exhalative deposits overprinted by hydrothermal fluid,which seriously restricts the understanding of mineralization and prospecting within orefield and the in region.In this paper,in-situ elemental and S-Pb isotopic analyses were conducted for metal sulfide and whole-rock S isotope analysis for ore-bearing wall-rocks.The Fe-Zn contents(4.48%~10.33%,53.67%~62.12%)of sphalerite is consistent with those of medium-temperature hydrothermal deposits(3%~10%,50%~60%,mineralization temperature about 200℃),the Zn/Cd ratios(53.68~76.75)also indicate medium-low temperature mineralization(high temperature:Zn/Cd>500;low temperature:Zn/Cd<250),and the corresponding temperature for Ga/Ge ratios of sphalerite in lg(Ga/Ge)-T diagram is about 100~180℃,the aforementioned temperature ranges are basically consistent with the homogenization temperature of fluid inclusion reported in previous literatures(90~250℃).The Zn/Cd ratios(53.68~76.75)and Cd contents(7577×10-6~11266×10-6)of sphalerite are in agreement with the characteristics of Mississippi Vally-Type(MVT)deposit;the Fe/S ratio of pyrite(0.76~1.29,the average value is 0.89)is close to the theoretical value for typical MVT deposits(0.88).Theδ34 S V-CDT values of metal sulfide range from-9.6‰to-0.5‰,and are generally negative;and theδ34 S V-CDT values of the ore-bearing wall-rocks are from-8.5‰to 4.2‰,there is a large overlap(mainly concentrated in-9.0‰to-2.0‰),which indicates that the wall-rocks provide reduced sulfur,and the ancient basement may provide some light sulfurs.Pb isotope ratios of metal sulfide are 206 Pb/204 Pb=18.30~18.39,207 Pb/204 Pb=15.61~15.63,208 Pb/204 Pb=38.38~38.44,which imply that lead is mix sourced,the ore-forming materials probably originated from the upper crust and orogenic belts.The model age calculation shows that the metamorphic rocks in the Upper Archean Wulashan Group may be the initial source lead.To sum up,the geochemical characteristics of the Xiaohongshilazi Pb-Zn(Ag)deposit are basically consistent with the MVT-type deposit,yet there are little differences in geological characteristics,thus it is speculated as an MVT-like deposit.