Xianzheng Guo

钴是我国关键金属矿产资源,主要产自岩浆型和热液型矿床中,其中矽卡岩型矿床是重要的钴矿床类型之一。长江中下游成矿带是我国重要的一条多金属成矿区带,产出大量的矽卡岩-斑岩型矿床,伴生钴、硒、碲等多种关键金属。鄂东南矿集区铁山矿床是该成矿带的一典型矽卡岩型铁铜矿床,铁矿石资源量超过1.6亿t,铜金属量67万t,伴生钴品位0.017%。然而,该矿床中关键金属钴的赋存状态不清、富集规律不明,不仅制约了其钴资源的开发利用,而且也限制了其钴成矿作用的认识。本文以铁山矽卡岩型铁铜矿床中的钴为研究对象,在矿床地质特征研究基础上,通过全岩地球化学、TIMA、LA-ICP-MS等测试分析手段,查明钴的赋存状态,包括独立矿物和类质同象替换两种,前者有硫铜钴矿和辉砷钴矿;后者则以类质同象形式进入黄铁矿、磁黄铁矿晶格替代铁元素。在矿石尺度和矿物尺度理清了钴的富集规律,其中磁黄铁矿矿石、含铜磁铁矿矿石和黄铁矿矿石中钴明显发生了富集,就矿石尺度而言,其全岩地球化学含量均大于200.0×10^-6,表明钴的富集与硫化物黄铁矿和磁黄铁矿密切相关;从矿物尺度而言,据LA-ICP-MS分析,黄铁矿和磁黄铁矿中Co含量分别介于292.3×10^-6~16054×10^-6、445.9×10^-6~497.9×10^-6。结合前人已有工作,综合探讨了钴的富集机制,认为铁山矿床成矿流体演化经历了矽卡岩阶段和硫化物阶段,温度降低和磁铁矿沉淀引起的流体Co/Fe比值升高、氧逸度降低是控制铁山矽卡岩铁矿钴独立矿物以及钴进入载体矿物富集沉淀的主要机制。本文研究对查明铁山矿床钴的矿化规律以及矿山钴资源评价和综合利用提供了依据,同时也丰富了矽卡岩型钴矿床成矿作用的认识。

碲作为稀散元素,很少形成独立矿床,主要以共伴生形式产出于多个类型矿床中,包括铜镍硫化物和铂族矿床、铁氧化物铜金(IOCG)矿床、块状硫化物(VMS)矿床、斑岩矿床、矽卡岩矿床、造山型金矿、卡林型金矿和浅成低温热液矿床等。研究表明,碲元素可以形成上百种碲矿物,除了自然碲之外,多与Au、Ag、Pb、Bi、Cu等形成碲化物,与S或者Se形成碲的硫化物或硒化物,也可以形成碲酸盐、硅酸盐、磷酸盐、硫酸盐等矿物;此外Te还可以以类质同象形式替换寄主矿物中的元素。在成矿带尺度、矿床尺度及其矿石中碲均表现出极不均匀的分布特征,与主矿种Cu、Au、Ag等具有成因关系。碲具有多来源特征,可以源自地幔,也可以是浅部壳源岩浆或是围岩地层提供。碲矿化一般发生在成矿的中晚阶段,流体可通过混合作用、水岩反应、沸腾作用等改变体系的物理化学条件(如pH值、硫逸度、氧逸度、碲逸度、温度等),导致流体pH值升高、硫逸度和氧逸度降低,碲逸度升高,这是诱发碲矿物富集和沉淀的主要机制。碲由于其受控成矿条件较为特殊,需要着重加强碲富集成矿的关键控制因素、成矿物质来源和富集沉淀机制的研究。

The Reshui porphyry Mo deposit is located in the East Kunlun orogenic belt (EKOB). Molybdenum mineralization is distributed in monzogranite and porphyritic monzogranite rocks, mainly presenting as various types of hydrothermal veinlets in altered wall rocks, and the orebodies are controlled by three groups of fractures. In this paper, we present the results of fluid-inclusion and isotopic (S and Pb) investigations of the Reshui Mo deposit. The ore-forming process of the deposit can be divided into three stages: an early disseminated molybdenite stage (stage 1), a middle quartz–molybdenite stage (stage 2) and a late quartz–polymetallic sulfide stage (stage 3). The alteration was mainly potassic and silicic in stage 1, silicic in stage 2, and sericitic and silicic in stage 3. Five types of fluid inclusions (FIs) can be distinguished in quartz phenocrysts and quartz veins, namely W, PL (pure liquid inclusions), PV (pure gas inclusions), C (CO₂ three-phase inclusions), and S (daughter mineral-bearing inclusions). The homogenization temperatures of fluid inclusions belonging to stages 1 to 3 are 282.3–378 °C, 238.7–312.6 °C and 198.3–228 °C, respectively. The fluid salinities at stages 1 to 3 are 4.65–8.14% NaCl eq., 4.34–42.64% NaCl eq., and 3.55–4.65% NaCl eq., respectively. The fluids of this deposit were generally moderate–high temperature and moderate–low salinity and belong to the H₂O–NaCl–CO₂ ± CH₄ system. The temperature and pressure changed considerably between stage 2 (high–medium-temperature) and stage 3 (low-temperature). The evidence for ore-forming fluids containing different types of coexisting inclusions in stage 2 and a decrease in the fluid temperature from stage 2 to stage 3 indicate that fluid boiling and fluid mixing were the main mechanisms of ore precipitation. The sulfide ³⁴S_V-CDT values range from 4.90‰ to 5.80‰, which is characteristic of magmatic sulfur. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values of the ore minerals are 18.210–18.786, 15.589–15.723, and 38.298–39.126, respectively. These lead isotopic compositions suggest that the ores were mainly sourced from crustally derived magmas, with minor input from the mantle. The fluid inclusions and S–Pb isotopes provide important information on the genesis of the Reshui porphyry Mo deposit and indicate that the Triassic has high metallogenic porphyry potential in the EKOB.