滇西北铜厂沟Mo-Cu矿床成矿流体和成矿物质来源:矽卡岩矿物学与稳定同位素证据

The source of ore-forming fluids and materials in the Tongchanggou Mo-Cu deposit,northwestern Yunnan,China: Constrains from skarn mineralogy and stable isotopes

义敦地体位于三江特提斯成矿域中北段,晚三叠世和晚白垩世斑岩-矽卡岩型Mo-Cu多金属成矿作用强烈。铜厂沟Mo-Cu矿床位于义敦地体最南端,是近年来该区新探明的Mo-Cu矿床之一,已探明资源量142.5Mt。矽卡岩在铜厂沟矿区广泛出露,是该矿区最主要的赋矿岩石。根据矿物组合及共生关系,可将矽卡岩划分为石榴子石矽卡岩、透辉石矽卡岩和透闪石矽卡岩三种类型。通过详细的地质填图和钻孔岩心编录,发现铜厂沟矿区矽卡岩矿物组合受花岗闪长斑岩体与大理岩空间分布的控制:(1)由大理岩向外依次发育透辉石矽卡岩→透闪石矽卡岩→石榴子石矽卡岩;(2)由浅至深,石榴子石粒度逐渐变大;③矿化与透闪石、绿帘石等退化蚀变矿物密切相关,矿体多形成于外接触带。矽卡岩中最主要的矿物是石榴子石,多呈自形粒状或粒状集合体产出,颜色较深,均质性,以钙铝榴石为主(62.2%~78.3%),其次为钙铁榴石(16.7%~34.2%),少量锰铝榴石、铁铝榴石和镁铝榴石,属于钙铝榴石-钙铁榴石固溶体系列(Gro62-78And17-34Spe+Pyr+Alm2-6)。石榴子石Fe^(2+)/Fe^(3+)比值变化范围为0.00~0.20,平均值为0.06,指示石榴子石形成于酸性的氧化环境。石榴子石的δ^(18)OSMOW变化范围为5.2‰~9.5‰,反映矽卡岩可能直接继承斑岩体的氧同位素组成;金属硫化物具有较为均一的S-Pb同位素范围(δ^(34)S(CDT)=-0.7‰~1.4‰;^(206)Pb/^(204)Pb=18.332~18.694,^(208)Pb/^(204)Pb=38.454~39.088,^(207)Pb/^(204)Pb=15.588~15.663),表明成矿流体和成矿物质均来源于壳源的长英质岩浆。

Yidun Terrane is located in the northern-central segment of Sanjiang Tethys metallogenic domain,developing massive Late Triassic and Late Cretaceous felsic magmatism and related porphyry-skarn Mo-Cu-polymetallic mineralization. The Tongchanggou deposit is a newly discovered porphyry-skarn type Mo-Cu deposit in the southern Yidun Terrane,with proven reserves of 142. 5Mt.Skarn extensively exposed in the Tongchanggou district,with skarn-type ore being the most important ore type. Based on the mineral assemblages and paragenesis,Tongchanggou skarn can be divided into garnet skarn,diopside skarn and tremolite skarn. Detailed adit mapping and drill-hole logging suggest that the skarn mineralogy at Tongchanggou is controlled by the spatial distribution of granodiorite and marble： 1） it develops diopside skarn → tremolite skarn → garnet skarn upward or downward from the marble,2） the grain size of garnet turns to be larger from top to bottom along the drill-hole,and 3） Mo-Cu mineralization is associated with retrograde altered minerals such as tremolite and epidote,thus orebody usually develop in the outer contact zone between granodiorite and marble. The dominant garnet at Tongchanggou occurs as euhedral grain or granular aggregates,and is dark and isotropical. The compositions of garnet are mainly grossularite（ 62. 2 - 78. 3）,followed by andradite（ 16. 7 - 34. 2）,with minor spessartite,almandine and pyrope.The Fe-（2＋）/Fe-（3＋） ratios of garnet from Tongchanggou deposit are varying from 0. 00 to 0. 20,with an average value of 0. 06,indicating a relatively oxidative environment for the formation of garnet. The δ-（18）OSMOWvalues of garnet are ranging between 5. 2‰ and 9. 5‰,indicating that skarn might have directly inherited the oxygen isotope compositions of granodiorite. Additionally, sulfides are characterized by uniform S-Pb isotopes,with δ^34（ CDT）values of 0. 7‰ - 1. 4‰,-（206）Pb/-（204）Pb ratios of 18. 332 - 18. 694,^208Pb/^204Pb ratios of 38. 454 - 39. 088,and ^207Pb/^204Pb ratios of 15. 588 - 15. 663,indicating that the ore-forming fluids and materials of Tongchanggou deposit originated from crust-derived felsic magmas.