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

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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.