我国铂族元素、钴和铬主要矿床类型的分布特征及成矿机制

Genetic classification, distribution and ore genesis of major PGE, Co and Cr deposits in China: A critical review

我国赋含铂族元素(platinum-group elements, PGE)、钴(Co)和铬(Cr)资源的主要矿床类型普遍与镁铁-超镁铁质岩有关,岩浆铜镍硫化物矿床提供了几乎全部的PGE储量和约一半的Co储量,蛇绿岩中的豆荚状铬铁矿矿床是Cr的主要来源.我国一直未发现与层状岩体有关的大型铂族元素和层状铬铁矿矿床,也缺乏全球Co储量占比很高的沉积层控型铜钴矿和红土型镍钴矿,唯一的大型铬铁矿矿床赋存在西藏罗布莎蛇绿岩中.本文介绍了我国目前已发现的PGE、Co和Cr主要矿床类型及其分布,并总结了近年来一些典型矿床在成矿元素赋存状态和富集机制方面的研究进展.在此基础上,从微-纳米尺度下PGE的赋存状态研究、镁铁-超镁铁质岩及相关矿床中Co的迁移富集机制、赋存状态及资源潜力、以及蛇绿岩豆荚状铬铁矿矿床中Cr迁移-富集机制的精细刻画等三方面提出了今后值得深入探索的问题.

Major platinum-group element(PGE), cobalt(Co) and chromium(Cr) deposits in China are commonly associated with mafic-ultramafic rocks regardless magmatic or hydrothermal origins. Nearly all PGE and half of Co resources in China are from magmatic Ni-Cu-(PGE) sulfide deposits hosted in mafic-ultramafic intrusions. Major resources of Cr are from podiform chromite deposits associated with the mantle sequences of ophiolites, including the Luobusha chromite deposit in Tibet and the Sartohay chromite deposit in Xinjiang. However, there is a lack of large PGE and chromite deposits related to layered mafic-ultramafic intrusions. Sedimentary rock-hosted stratiform Cu deposits and Ni-Co laterites are two dominant types of Co-bearing deposits among terrestrial Co resources and are sporadically distributed in China, although these are insignificant.Platinum-group elements from three Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions that formed within-plate settings account for 74% of the total PGE reserves of China. Among these, the Jinbaoshan and Yangliuping intrusions are parts of the Emeishan large igneous province(LIP) in SW China, and the Jinchuan intrusion emplaced in a rifting margin of the southern North China Craton. The Jinbaoshan intrusion hosts the only PGE-dominated deposit in China, but has not yet been mined due to the difficulty for access. In the Jinchuan and Yangliuping Ni-Cu deposits, PGEs are by-products. The Jinchuan intrusion is by far the largest PGE producer in China.Cobalt resources are from deposits associated with mafic-ultramafic rocks in China, including Ni-Cu sulfide deposits,Ni-Co laterites, volcanogenic massive sulfide(VMS) deposits and magmatic Fe-Ti oxide deposits, which make up 41% of total Co reserves of China. Major Ni-Cu sulfide deposits in both within-plate and convergent margin settings contain economic Co resources, such as the Jinchuan intrusion and those in the Emeishan LIP, and the Xiarihamu intrusion in the East Kunlun orogenic belt, the Jianchaling intrusion in a Neoproterozoic subduction zone in the northern margin of the Yangtze block and those in the central Asian orogenic belt. Ni-Co laterites that contain economic Co resource have been only discovered locally in Hannan, Yunnan and Taiwan. The Derni Cu-Co deposit in the A’nyemaqen suture zone of the East Kunlun orogenic belt is a special type of VMS deposits closely associated with ophiolite suites, which is also very rare worldwide. Magmatic Fe-Ti oxide deposits in the Emeishan LIP and hydrothermal iron oxide deposits in the East Tianshan orogenic belt have likely potential Co resources.Podiform chromite deposits in China occur in ophiolites, such as the Sartohay, Hegenshan and Suolunshan ophiolites in the central Asian orogenic belt, the Dadao Erji and Yushi ophiolites in the Qilian-Qinling orogenic belt, the Dongqiao ophiolite in the Bangong-Nujiang suture zone and Luobusha ophiolite in the Yarlung-Zangbo suture zone.Understanding the mechanisms of PGE enrichment in major Ni-Cu-(PGE) sulfide deposits such as the Jinchuan and Jinbaoshan intrusions has been a focus of studies in the past. The dissolution and re-precipitation processes that may lead to the enrichment of PGE and Co during hydrothermal overprints on magmatic Ni-Cu sulfide deposits have gained popularity.The origin of podiform chromite deposits is generally thought to be triggered by melt-mantle interaction and melt mixing.But it appears that such a traditional thought is challenged by the discoveries of various high-pressure and hydrous minerals that are trapped in chromite.Given PGE, Co and Cr are all strategic and critical metals to China, it is very important to conduct new exploration projects near active mines, such as those in the Emeishan LIP and the Jinchuan intrusion. However, additional experimental and mineralogical studies on the enrichment processes of PGE, Co and Cr are desired to make breakthroughs in order to better understand the behaviors of PGE, Co and Cr in magmatic-hydrothermal systems. Main issues that are worthy to be closely examined in the future include(1) micro-to nano-scale occurrence of PGE,(2) enrichment mechanism and occurrence of Co in mafic-ultramafic rocks and related ore deposits, and(3) mobilization and enrichment processes of Cr in the formation of podiform chromite deposits.