四川轿顶山钴矿床黄铁矿LA−ICP−MS微量元素原位测试及其对矿床成因的制约

LA−ICP−MS trace element analysis of pyrite from Jiaodingshan cobalt deposits in Sichuan,and its constraints on the ore genesis

轿顶山钴矿床位于四川盆地西南缘,其矿床成矿物质来源及矿床成因鲜有研究,由于Co元素具有亲铁和亲硫的双重特性,使其在硫化物中富集明显。以轿顶山钴矿床中的黄铁矿为研究对象,在黄铁矿显微结构研究基础上,利用LA−ICP−MS对黄铁矿微量元素进行原位测试分析,为探讨矿床成因提供制约。根据黄铁矿的显微组构特征,可将轿顶山钴矿床中的黄铁矿划分为3个世代。PyⅠ为原始沉积型黄铁矿,包括少量半自形状黄铁矿(PyⅠ-a)、胶状黄铁矿(PyⅠ-b)及莓状黄铁矿(PyⅠ-c),其Co/Ni值基本上都小于1(平均值0.53),成矿元素(Co)平均含量99.8×10^(−6)。PyⅡ受构造变形及热液叠加作用影响,常具有交代残余结构(PyⅡ-a)及变形重结晶结构(PyⅡ-b),其Co/Ni>1(平均值1.31),成矿元素(Co)平均含量为1060.8×10^(−6);PyⅢ为细粒散砂状及细粒胶状集合体形式,强金属光泽,并与硫钴矿、硫镍钴矿、黄铜矿共生,Co/Ni平均值为2.05,成矿元素Co平均含量为10453.5×10^(−6),PyⅢ中Co、Ni元素可能以硫钴镍矿的微细物包裹体形式存在于黄铁矿内部。综合分析认为,早期沉积型PyⅠ成矿元素背景值较高,受后期构造变形-热液叠加改造作用影响,PyⅡ中成矿元素逐渐富集,而主成矿期PyⅢ为后期热液对PyⅡ进一步叠加改造,最终形成了现有的钴矿床。

The Jiaodingshan cobalt deposit is situated in the southwestern of Sichuan basin,and the source of the ore materials and fluids is still under debated.Cobalt is enriched in vulcanized species because of its double properties of iron and sulfur affinity.In order to analyse the genesis of the deposit,this study investigates the pyrite microstructure and the geochemical compositions of pyrite based on LA−ICP−MS.The results show that three generations of pyrite were identified according to its microstructure characteristics in Jiaodingshan cobalt deposit.The first generation(pyriteⅠ)is identified as original sedimentary pyrite,including colloform pyrite,subhedral pyrite and framboidal pyrite.The pyriteⅠis characterized by low Co/Ni ratio(<1 and the average ratio is 0.53)and low Co content(99.8×10^(−6)).Due to the influence of tectonic deformation and hydrothermal superimposition,the pyriteⅡoften presents the structure with metasomatic relict texture(pyriteⅡ−a)and recrystallization(pyriteⅡ−b),which is characterized by high Co/Ni ratios(Co/Ni>1,the average ratios is 1.31)and middle Co content(1060.8×10^(−6)).PyriteⅢexists in the form of fine sand−like and fine colloidal aggregates,along with strong metal luster.Such pyrite is often associated with linnaeite,siegenite and chalcopyrite,with the ratios of Co/Ni higher than 1(the average ratios is 2.05)and the Co content being 10453.5×10^(−6).The Co and Ni contents in PyⅢperhaps represented by siegenite inclusions enclosed in pyrite.Overall,the early sedimentary pyrite I with high ore−forming element background value may be affected by tectonic deformation−hydrothermal superposition transformation in the later period,resulting in further enrichment of ore−forming elements(Co)in pyriteⅡ.While the main ore−forming period of PyⅢwas hydrothermal superimposed on PyⅡ,and finally formed the existing cobalt deposit.