甘肃花牛山铅锌银矿床成因:来自原位S、Pb同位素及微量元素的约束

Genesis of the Huaniushan Pb-Zn-Ag deposit in Gansu:Constraints from in situ S,Pb isotopes and trace elements

甘肃花牛山铅锌银矿床位于中亚造山带中段的甘肃北山地区。本文在详细的野外观察和室内鉴定的基础上,将花牛山铅锌银矿床成矿阶段划分为石英-毒砂-黄铁矿(第Ⅰ成矿阶段)和石英-多金属硫化物(第Ⅱ成矿阶段)两个阶段;进一步将黄铁矿划分为三种类型,分别为第一种类型的胶状黄铁矿(Py0)、第二种类型的热液叠加交代特征的黄铁矿(PyⅠ)及第三种类型的热液黄铁矿(PyⅡ)。黄铁矿、磁黄铁矿的原位硫同位素研究表明,成矿从早到晚硫化物δ^(34)S值呈递增的规律,具有逐渐向岩浆硫演化的趋势;胶状黄铁矿δ^(34)S值为-9.37‰~-8.10‰,具有沉积(生物成因)硫的特征;成矿第Ⅰ阶段硫化物δ^(34)S值为-9.03‰~-7.03‰,成矿第Ⅱ阶段硫化物δ^(34)S值为-5.77‰~-4.88‰,成矿阶段具有沉积硫与岩浆硫混合来源的特征。黄铁矿、磁黄铁矿的原位铅同位素研究表明,成矿期硫化物的206Pb/^(204)Pb值、^(207)Pb/^(204)Pb值、^(208)Pb/^(204)Pb值以及μ、ω等铅同位素特征值组成范围较窄,铅源为与岩浆作用有关的壳幔混合来源,且与壳幔混合来源的晚三叠世花岗岩中长石铅及其控制的矽卡岩型金矿硫化物铅同位素组成类似。黄铁矿原位微量元素研究表明,胶状黄铁矿(Py0)Co/Ni和S/Se比值分别为0.004~0.34和3.43×10^(4)~34.84×10^(4),Se含量为1.558×10^(-6)~15.82×10^(-6),表现为沉积成因黄铁矿的特征。具有热液叠加交代特征PyⅠ的Co/Ni和S/Se比值分别为0.05~3.38、0.05×10^(4)~5.38×10^(4),Se含量为10.09×10^(-6)~1070×10^(-6),数值分布范围广,总体上有别于沉积成因黄铁矿,类似于热液成因黄铁矿的特征。热液黄铁矿(PyⅡ)的Co/Ni和S/Se比值分别为0.60-68.88、1.46×10^(4)~9.15×10^(4),Se含量为5.938×10^(-6)-35.91×10^(-6),表现为热液成因的特征。综上研究,认为花牛山矿床经历了南华纪-震旦纪沉积胶状黄铁矿形成期和晚三叠世岩浆热液成矿期,胶状黄铁矿在成矿过程中提供了部分硫,成矿金属物质主要来自晚三叠世岩浆成矿热液,并认为矿床成因为岩浆热液型矿床。

The Huaniushan Pb-Zn-Ag deposit in the Beishan area of Gansu Province is located in the middle segment of Central Asian Orogenic Belt.Based on detailed field observation and laboratory identification,the metallogenic stage of the Huniushan Pb-ZnAg deposit is divided into two stages:quartz-arsenopyrite-pyrrhotite(stageⅠ)and quartz-polymetallic sulfide(stageⅡ).The pyrite can be further divided into three types,namely colloidal pyrite of 1st type(Py 0),pyrite of 2nd type with the characteristic of hydrothermal superimposition(PyⅠ),and hydrothermal pyrite of the third type(PyⅡ).The study of in-situ sulfur isotopes of pyrite and pyrrhotite indicates that theδ34 S value of sulfide increases gradually in succession,and shows a characteristic trend of gradual evolution to magmatic sulfur.Theδ34S values of colloidal pyrites vary within a range of-9.37‰~-8.10‰,with characteristics of sedimentary(biogenic)sulfur;theδ34S values of the sulfides in the first and second stages of mineralization are within-9.03‰~-7.03‰and-5.77‰~-4.88‰,respectively.The metallogenic stage is characterized by the mixed source of sedimentary sulfur and magmatic sulfur.The study of in-situ lead isotope of pyrite and pyrrhotite indicates that the variation range of 206 Pb/204 Pb,207 Pb/204Pb,^(208)Pb/204 Pb and the characteristic values of lead isotopes such asμandωof sulfides during the mineralization period are narrow.The lead source is a crust-mantle mixed source related to magmatism.It is similar to the Late Triassic granite feldspar lead from the crust-mantle mixed source,and is also similar to the lead isotopic composition of the skarn-type gold sulfide ore controlled by the Late Triassic granite.The study of in-situ trace elements of pyrites indicates that the Co/Ni and S/Se ratios of PyO are 0.004~0.34 and 3.43×10^(4)~34.84×10^(4),respectively,and the mass fraction of Se is 1.558×10^(-6)~15.82×10^(-6),showing the characteristics of sedimentary pyrite;The Co/Ni and S/Se ratios of PyⅠwith the characteristic of hydrothermal superimposition are0.052~3.38 and 0.05×10^(4)~5.38×10^(4),and the mass fraction of Se is 10.09×10^(-6)~1070×10^(-6).These values are widely distributed,which are different from those of sedimentary pyrites and similar to those of hydrothermal pyrites.The Co/Ni ratio of PyⅡis within 0.60~68.88 with a mass fraction of Se varying within 5.938×10^(-6)~35.91×10^(-6),and a S/Se ratio of 1.46×10^(4)~9.15×10^(4),showing the characteristics of hydrothermal origin;PyⅠis a transitional type between PyO and PyⅡ,which exhibited the characteristic of hydrothermal superimposition.In conclusion,we inferred that the formation of Huaniushan deposit has experienced the pyrite depositional period of Nanhua-Sinian and the magmatic hydrothermal mineralization period of Late Triassic.Colloidal pyrite provide a few sulfur for mineralization,but the ore-forming metal materials mainly come from ore-forming hydrothermal fluid of the Late Triassic magmatic,and the origin of the deposit is considered to be magmatic hydrothermal deposit.