中甸红山矽卡岩铜矿稳定同位素特征及其对成矿过程的指示

Stable isotopic compositions of the Hongshan skarn copper deposit in the Zhongdian area and its implication for the copper mineralization process.

红山铜矿床位于三江地区义敦岛弧南端的中甸弧,是在晚三叠世甘孜-理塘洋盆向西俯冲过程中形成的一个中型规模的矽卡岩矿床。通常,矽卡岩体就是铜矿体或铜矿化体,主要呈似层状、层状、脉状及透镜体状产于大理岩与角岩接触带或局部在角岩中,未见其与侵入岩直接接触。通过对不同成矿阶段所形成的石榴石、磁铁矿、磁黄铁矿、黄铁矿、黄铜矿、方解石等典型矿物以及大理岩的稳定同位素特征研究,发现矽卡岩的最主要组成矿物石榴石的δ^(18)O_(SMOW)范围为6.2～8.3‰,反映了矽卡岩可能直接继承隐伏斑岩体的氧同位素组成。根据磁铁矿的氧同位素组成(5.5～7.1‰)所计算的磁铁矿化阶段成矿流体的δ^(18)O_(SMOW)为13.1～14.7‰(400℃)或12.5～14.1‰(500℃),暗示有富集δ^(18)的 CO_2溶入到成矿流体中。硫化物的δ^(34)S_(CDT)范围4.45～6.20‰,说明矿床具有高度均一的硫源,并且在硫化物的结晶沉淀过程中,流体中硫同位素分馏很弱。由此推测主成矿期成矿流体的δ^(34)S_(ΣS)为5.6±0.6‰。矿床中的大理岩的δ^(13)C_(PDB)为2.0～2.2‰,δ^(18)O_(SMOW)为24.0～24.8‰,说明大理岩是由海相碳酸盐岩经重结晶作用而成。成矿晚期阶段形成的方解石脉的δ^(13)C_(PDB)范围是-2.4～1.7‰,δ^(18)O_(SMOW)范围是16.3～22.4‰,表明其 C 和 O 主要来源于大理岩。总之,我们推测红山矽卡岩很可能主要是由中酸性岩浆浅成侵位时局部同化碳酸盐围岩所形成的一种富含钙质成分的次生岩浆就位于碎屑岩与碳酸盐岩之间的构造薄弱带冷凝固结而成,矽卡岩型矿化与深部斑岩型矿化具有共同的成矿物质和成矿流体来源。

The Hongshan Cu deposit is located in the Zhongdian area which lies on the southern part of the Yidun island arc. It is a medium sized skarn copper deposit formed during the westward subduction of the Ganzi-Litang oceanic basin in Late Triassic. Generally, the skarn bodies are copper orebodies or copper mineralized bodies occurred in stratoid, bedded, veining and lentoid forms mainly in the contact between marble and hornfels with minor in hornfels. No direct contact between the skarn and intrusive bodies has been observed. The stable isotopic compositions of various kinds of minerals including garnet, magnetite, pyrrhotite, pyrite, chalcopyrite, calcite, which were formed in different stages, and marble have been analyzed respectively. The results indicate that the δ^18OSMOw values of the garnet, major mineral component of the skarn, are varied from 6. 2‰ to 8. 3‰, indicating the oxygen isotopic compositions of skarn could inherited those of the concealed porphyry body. The oxygen isotopic compositions of magnetite have been used to calculate the δ^18OSMOw values of the ore-forming fluid at magnetitization stage. They are 13.1 - 14. 7‰ at assumed temperature of 400℃ or 12. 5 - 14. 1‰ at assumed temperature of 500℃ respectively. This indicates that the δ^18 O-enriched CO2 was dissolved into the ore-forming fluid. The δ^34SCDT values of sulfides, varying from 4.45‰ to 6. 20‰, indicate that the sulfur of the sulfides in the deposit is from a highly homogeneous source, with very weak sulfur isotopic fractionation in the process of sulfide crystallization and precipitation in the ore-forming fluid. Thus, it is suggested that the δ^34S∑s value of the ore-forming fluid of the main ore-stage is 5.6 ± 0. 6‰. The δ^13CPDB value of 2. 0 - 2. 2‰ and the δ^18OSMOW value of 24. 0 - 24. 8‰ for the marble in the deposit indicate that the marble was re-crystallized from the oceanic carbonate. The δ^13CPDB value of -2. 4 - 1.7%0 and the δ^18OSMOW value of 16. 3 -22. 4‰ for the calcite vein formed in the late ore-stage indicate that the carbon and oxygen of calcite vein are mainly sourced from the marble. In general, it is suggested that the Hongshan skarn could be formed in the lithologieal transition zone between the elastic rocks and carbonate with the crystallization of a SiO2, Al2O3, Fe2O3 , CaO-rich secondary magma which was formed through the assimilation of carbonate by the underneath porphyry magma. It is also believed that the ore-forming materials and the ore-forming fluid of the skarn mineralization in the Hongshan deposit are congenetic to those of the porphyry copper mienralization underneath.