The occurrence and spatial distribution of cobalt in Longqiao iron deposit in Luzong Basin, Anhui Province

Abstract

中国东部的长江中下游、莱芜、邯邢和临汾等矿集区中发育大量的矽卡岩型铁矿床，其中普遍伴生钴，部分矿床估算伴生钴资源量大于1万t，达到中型钴矿床的规模。已有研究表明，中国东部富钴矽卡岩型铁矿床的成矿时代相同，成矿特征相似，均为中国东部130Ma大规模成矿作用的产物，反映较大规模的铁钴成矿作用受统一的动力学背景控制。钴与其他伴生组分一样，其工业价值主要不取决于矿石中钴的含量，而取决于钴的赋存状态，因此开展矽卡岩型铁矿床中钴的赋存状态和分布规律研究具有重要的理论和实践意义。龙桥矽卡岩型铁矿床位于长江中下游成矿带，是目前发现的成矿带内伴生钴含量最高的矿床。矿床主矿体长1000m，成矿岩体位于矿体中部，黄铁矿分布范围广，距离岩体中心不同距离均有分布，是研究矽卡岩型铁矿床中钴赋存状态和空间分布规律的理想对象。本次研究在龙桥铁矿床勘探巷道-370m平面自西向东采集了9件含黄铁矿样品，黄铁矿主要呈浸染状-细脉状交代磁铁矿。研究表明，龙桥铁矿床中钴主要赋存在黄铁矿和磁铁矿中，其次为独立钴矿物（主要为辉砷钴矿）。黄铁矿的LA-ICP-MS微区微量元素分析结果表明，龙桥黄铁矿中Co（0.019×10^-6~5639×10^-6）、Ni（0.025×10^-6~5798×10^-6）和As（0.46×10^-6~14526×10^-6）的含量均具有较大的变化范围，黄铁矿总体上具有边缘富钴、核部贫钴的特征，黄铁矿边部的钴含量是核部的100~1000倍。通过对LA-ICP-MS测试数据以及时间分辨率剖面的逐一对比，推测Co主要以类质同象形式替代Fe进入黄铁矿晶格。由于钴在黄铁矿中分布极不均匀，不同空间位置采集的黄铁矿样品中Co含量平均值和变化范围没有明显规律。本文通过类比，提出中国东部矽卡岩型铁矿床中钴矿物可能是普遍存在的，但以前受限于分析测试手段未能发现；如何在选矿过程中分离富集钴矿物和富钴黄铁矿，是矽卡岩型铁矿床硫精粉中钴回收利用的关键。;A large number of skarn-type iron deposits are developed in the Middle-Lower Yangtze River, Laiwu, Hanxing and Linfen areas in eastern China. The average grades of cobalt in those iron deposits are 0.01%~0.03%, and some iron deposits are estimated to contain more than 10,000 tons associated cobalt resources (medium-sized cobalt deposit). Previous studies have shown that those Co-rich skarn iron deposits in eastern China have the same metallogenic age and similar metallogenic characteristics, which are the products of large-scale mineralization event at 130Ma in eastern China, indicating that the large-scale Fe-Co mineralization is controlled by same mineralization process. According to previous studies, independent cobalt minerals are very rare in skarn iron deposits, and pyrite is the main Co-bearing mineral, but the occurrence and spatial distribution of cobalt in skarn iron deposit is still unknown. The Longqiao skarn iron deposit with average 0.01% of cobalt is located in the Middle-Lower Yangtze River metallogenic belt. The main ore body of the deposit is 1000m long, and the ore-forming intrusion is located in the middle of the ore body. Pyrite is widely distributed in different distances from the intrusion to distal. Therefore, it is an ideal object to study the occurrence and spatial distribution of cobalt in the skarn iron deposit. In this study, 9 pyrite-bearing samples were collected from west to east in the -370m level of Longqiao iron deposit. Pyrite is mainly disseminated-veined in magnetite ore. LA-ICP-MS analysis results show that the contents of Co (0.019×10^-6~5639×10^-6), Ni (0.025×10^-6~5798×10^-6) and As (0.46×10^-6~14526×10^-6) in pyrite varied widely. The highest and lowest Co contents differ by several magnitudes. Through comparison of LA-ICP-MS analysis data and time-resolution profile, it is inferred that there are no Co-rich mineral inclusions in pyrite, and Fe²⁺ in pyrite is mainly replaced by Co²⁺ in the form of isomorphism. The Co content in the same pyrite grain often changes by 2~3 orders of magnitude, and cobalt can be abnormally enriched in a single pyrite grain. Independent cobalt minerals such as pyroarsenite are also developed in the deposit, indicating that independent cobalt minerals maybe widely exist in skarn iron deposits in eastern China. Due to the limitations of analysis and testing methods, those cobalt minerals has not been reported before. The mean and variation range of Co content in pyrite samples at different spatial locations have no obvious regularity, and there is no particularly Co-rich ore body in the deposit. According to the distribution characteristics of cobalt in pyrite, how to separate Co-rich and Co-poor pyrite in the process of beneficiation is the key to the recovery and utilization of cobalt in skarn iron deposit.