东秦岭中段板厂铜钼矿床S、Pb同位素对成矿物质来源的示踪作用

Tracer-S,Pb isotopes of the source of ore-forming materials of Banchang Cu-Mo polymetallic deposit in the East Qinling,China

东秦岭钼矿带是我国最重要的钼多金属成矿带,近年来在东秦岭板厂地区取得了新的找矿突破。本文利用区域成矿动力学、硫化物的硫、铅同位素组成研究,结合成矿地质特征,对板厂铜钼多金属矿床成矿物质来源进行示踪,并以板厂矿床为基点总结东秦岭钼矿带的稳定同位素时空分布特征。板厂矿床12件硫化物样品δ(34S)范围为1.2×10-3~5.7×10-3,平均值0.6×10-3,与地幔(0±3×10-3)硫同位素值相近;东秦岭钼矿床δ(34S)位素值有随成矿时代渐新而逐渐升高的趋势,印支期钼矿床(221~226Ma)δ(34S)同位素值偏负,燕山期第一阶段钼矿床(138~151Ma)δ(34S)值由"0"值向正值变化,燕山期第二阶段钼矿床(113~131 Ma)δ(34S)值较高(平均4.93‰)。板厂矿床深部硫化物206Pb/204Pb值为17.121~17.798,207Pb/204Pb值为15.369~15.433,208Pb/204Pb值为36.867~37.485,具有明显的低放射性成因铅特征,其铅同位素组成与华北克拉通南缘的类熊耳群和太华群相似,幔源特征明显;浅部硫化物铅同位素值相对较高,206Pb/204Pb值为18.266~18.392,207Pb/204Pb值为15.560~15.622,208Pb/204Pb值为37.611~38.438,反映了造山带混合铅特征。板厂矿床形成于中国东部构造体制转折阶段,深部构造体制重新调整导致地幔物质上侵以及壳幔混合物的重新熔融,岩浆沿着深大断裂上涌,并将一定规模的含矿流体运移至浅部,由于物理化学条件的变化以及浅部流体的混合,成矿流体在最终构造薄弱带沉淀Cu、Mo等金属,形成板厂铜钼多金属矿床。综上,在晚侏罗世-早白垩世,东秦岭地区地幔熔体活动强烈,板厂铜钼多金属矿床成矿物质来源以幔源为主,有少量壳源物质混入。

Qinling Mo ore belt is an important Mo-polymetallic metallogenic belt in China and ore prospecting breakthrough has been made in the belt.Based on regional metallogenic dynamics and sulfur and lead isotopes composition of sulfide and metallogenic geological characteris tics is traced the source of oreforming materials of Banchang Cu-Mo polymetallic deposit in the belt and the time-space zoning characteristics of stable isotopes in the East Qinling molybdenum ore belt is summarized on the data of Banchang deposit.The δ34S value of 19 sulfide samples from the Banchang deposit ranges from-1.2×10-3 to 5.7×10-3 with an average value of 0.6×10-3 similar to the sulfur isotope value of the mantle(0±3×10-3).The δ34S isotope value of molybdenum deposits in the East Qinling area gradually increased.The δ34S value of the Indosinian Mo deposits(221 to 226 Ma)is negative.The δ34S value of the first stage of the Yanshanian Mo deposit(138 to 151 Ma)changed from "0"value to positive value,and the δ34S value of the second stage of the Yanshanian Mo deposit(113 to 147 Ma)is high(average value of 4.93×10-3).The 206Pb/204Pb value of sulfides to depth of the Banchang deposit ranges from 17.121 to 17.798,207Pb/204Pb value ranges 15.369 to 15.433,and 208Pb/204Pb value ranges from 36.867 to 37.485 which are obviously characterized by of low radioactive lead,similarity to the Xionger Group and the Taihua Group in the southern margin of the North China Craton,and obvious characteristics of mantle lead.The Pb isotope values of the sulfide in the shallow is relatively high,206Pb/204Pb 18.266-18.392,207 Pb/204Pb value 15.560-15.622,208Pb/204Pb value 37.611-38.438 which is characteristic of mixed Pb isotopes of the mantle and crust of the rogenic belts.Banchang deposit was formed at the turning stage of tectonic system in eastern China.The readjustment of deep tectonic system resulted in the upwelling of deep mantle material and the remelting of crust-mantle mixtures.Magma upwelled along deep and large faults and move ore fluid to the shallow part.Due to the changes of physical and chemical conditions and the mixing of shallow fluids,ore-forming fluids precipitated Cu,Mo in the final weak tectonic zone.As a result,the Banchang Cu-Mo polymetallic deposit is formed.In summary,the mantle melt activity in the eastern Qinling orogen was intense during the Late Jurassic to Early Cretaceous.The main source of metallogenic materials in Banchang was mantle,with a small amount of crustal materials mixed in.