Origins of Molybdenum, Nickel, Gold and Platinum-Group Metals in Cambrian Black Shales of Southern China

在中国南方，下寒武统黑色页岩中赋存一种罕见的金属组合，包括钼、镍、铂族金属和金。这些金属特别集中在大量硫化物的薄层和透镜中。Fan等人（1973;1978）报告了页岩和伴生块状硫化物层的数值，这些数值超过2% Mo、2% Ni、3% Zn、700 ppb Au、200-400 ppb Pt、200-400 ppb Pb和50 ppb Ir。在一些情况下，金属价值等于或接近常规矿床中的矿石品位。例如，在贵州省遵义附近的一个矿山，钼正在从一个页岩为主的结核硫化物床中提取。这是世界上唯一一个从页岩中开采这种战略金属的地方。黑色页岩型镍钼铂金矿床分布于中国南方10个省份（Chen etal ., 1982），可能是一种具有巨大经济潜力的金属资源。成矿作用包括通常以两种不同组合出现的金属：即典型的多金属热液矿床（Zn、Au、Mo）和与基性火成岩（Ni、Pt、Pd）伴生的火成岩/气成岩矿床。考虑到Chen （1988a, b）描述的场关系和结构特征，多金属硫化物透镜体和床层很有可能是寒武纪典型的火山成因海底喷出物。在拟议的研究中，将寻求支持这一假设的其他证据。然而，考虑到钼和某些其他矿石成分在碱性溶液中的特殊地球化学行为，矿石金属也可能是分两个阶段引入的：(1)海底温泉早期析出含贵金属结核状铁镍硫化物；(2)在现代风化过程中，沉积了大量的钼和其他金属，形成了壮观的“塔形”喀斯特地貌（前寒武纪晚期深层风化对金属的动员也可能是一个因素）。贵州、云南、湖南寒武纪页岩硫化物透镜体发育塔式岩溶地形关键地点样品的金属分布、结构关系、流体包裹体和同位素研究将有助于通过确定中国南方页岩中是否存在两个阶段的矿床沉积来区分这些可能性。对现代风化带和古裂缝内及其附近的寄主岩和围岩进行分析，以评估表生对烃源层中钼等元素的运移可能产生的影响。

In southern China an unusual association of metals including molybdenum, nickel, platinum group metals and gold occurs in Lower Cambrian black shales. These metals are especially concentrated in thin beds and lenses of massive sulfides. Fan et al. (1973; 1978) report values for shales and associated massive sulfides beds exceeding 2% Mo. 2% Ni, 3% Zn, 700 ppb Au, 200-400 ppb Pt, 200-400 ppb Pb and 50 ppb Ir. In several cases metal values are at or near what would be ore grades in conventional deposits. For example, at one mine near Zunyi, Guizhou province, molybdenum is now being extracted from a shale-hosted nodular sulfide bed. This is the only place in the world from which this strategic metal is recovered from shale. Black shale-hosted Ni- Mo-Pt-Au deposits can be traced across ten provinces in southern China (Chen et al., 1982) and may constitute a metal resource of great economic potential. The mineralization includes metal which normally occur in two distinct associations: i.e., classic polymetallic hydrothermal deposits (Zn, Au, Mo) and igneous/pneumatolitic deposits associated with mafic igneous rocks (Ni, Pt, Pd). Considering the field relations and textural features described by Chen (1988a, b), there is good possibility that the polymetallic sulfide lenses and beds were developed as classic volcanogenic submarine exhalites during the Cambrian. Additional evidence in support of this hypothesis will be sought during the proposed study. However, taking into account the peculiar geochemical behavior of molybdenum and certain other ore constituents in alkaline solutions, it is also possible that the ore metals were introduced in two stages: (1) early precipitation of precious metal-bearing nodular Fe-Ni sulfide exhalites from submarine hot springs, followed by (2) late supergene precipitation of significant amounts of Mo and other metals during modern weathering while the spectacular "tower" karst topography of southern China was formed (Mobilization of metals by deep weathering during the Late Precambrian may also be a factor). Studies of metal distributions, textural relations, fluid inclusions and isotopes for samples from key locations in Guizhou, Yunnan and Hunan where sulfide lenses occur in Cambrian shales in the midst of well-developed tower karst topography, will help to distinguish between these possibilities by establishing whether or not there were two stages of ore deposition in the shales of southern China. Analyses of host and country rocks in and near the modern weathering zone and ancient fractures will be used to assess the probable impact of supergene mobilization of Mo and other elements from source beds.