云南建水水草冲铜矿矿床地质特征及成矿物质来源

Geological Characteristics and Ore-Forming Material Sources of the Shuicaogou Copper Deposit in the Jianshui Area of Yunnan Province

本文通过研究云南建水水草冲铜矿地质特征、围岩玄武岩和矿石矿物的微量元素、S、C、O同位素的组成,示踪了该矿床的成矿物质来源和性质。研究结果表明,云南建水水草冲铜矿位于滇东南哀牢山变质体北侧,容矿围岩为峨眉山玄武岩,矿体受北东向逆断层及其次级断裂控制,延深大于延长,形状不规则,矿体内多夹灰岩角砾及矿化玄武岩角砾,矿石矿物主要为黄铜矿,矿化蚀变以硅化、碳酸盐化、绿泥石化为主。水草冲铜矿黄铜矿与黄铁矿稀土元素具有相同的稀土元素组成与含量,ΣREE平均为1.901×10-6,富集轻稀土元素,轻重稀土元素比(LREE/HREE)平均为8.188,(La/Yb)N值平均为10.259;δEu值平均为0.977;Ce呈现弱负异常,δCe值平均为0.808。黄铁矿中的Co/Ni比值为0.835,显示水草冲铜矿床为中温热液矿床;黄铁矿的Y/Ho比值平均为38.23,表明水草冲铜矿床的成矿流体为混合来源。结合黄铁矿的稀土微量元素特征,推断水草冲铜矿床的成矿流体是Cl多于F的流体。S的来源复杂,黄铁矿δ34SCDT(‰)的变化范围(18.3~29.4)与海水硫酸盐(海相蒸发岩)的δ34S值接近或略低;黄铜矿δ34SCDT(‰)变化范围是-0.5~10.1,表明有深部硫的参与。δ13CV-PDB(0.9‰~2.6‰)、δ18OV-SMOW(14.3‰~15.9‰)反映了方解石流体可能为热液循环萃取沉积岩地层混合形成。综上,推测成矿物质为后期构造热液萃取围岩(玄武岩和灰岩等)所得,其中铜主要来自于玄武岩本身,而硫的来源以地层硫为主。

This work is based on REE and S isotopic compositions of copper ores and basalts to trace the sources of metallogenic materials and ore forming fluids in the Shuicaochong copper deposit,Jianshui,Yunnan. The REE of pyrite is as low as 1. 901 × 10-6on average,with relatively high LREE and low HREE. The LREE /HREE ratio(8. 188) and the(La /Yb)Nratio(10. 259) indicate that the pyrite belongs to a LREE-rich type. The REE distribution patterns show weak negative Ce anomalies with average values δCe(0. 808) and δEu(0. 977),respectively. The Co /Ni ratio(0. 835) of pyrite suggests that the Shuicaochong copper deposit is of the medium temperature type and the Y /Ho ratio(38. 23) shows that the ore-forming fluids are magmatic hydrothermal fluids mixed with formation water. Such features as REE,LREE,HFSE,Hf /Sm,Nb /La and Th /La,together with Co /Ni and Y /Ho ratios of pyrite and chalcopyrite,reveal that the ore-forming fluid of the Shuicaochong deposit originated from the Cl-rich mixed water. The δ34SCDTvalues of pyrite ranging from 18. 3‰ to 29. 4‰ are close to or slightly lower than those of the seawater sulfates and the δ34SCDTvalues of chalcopyrite are from-0. 5‰ to 10. 1‰,indicating that part of sulfur might come from a deep source. In a word,it can be speculated that the ore-forming material is late tectonic hydrothermal with extraction(Emeishan basalt and limestone,etc.) of surrounding rock,and the copper comes from the basalt proper while the sulfur of the mineralizing fluids might be primarily derived from stratum sulfur.