The Kupferschiefer; an overview with an appra
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摘要

The Kupferschiefer, a thin (<4 m) bed of marine bituminous marl of Upper Permian (Zech-stein) age, occurs over a large area of north-central Europe and has, in certain areas, been exploited for silver and some base metals, notably copper, since medieval times. It has been regarded as the type example of a shale-hosted, strata-bound sulfide deposit and theories regarding the origins of Kupferschiefer mineralization have exerted considerable influence on theories of ore genesis.The Kupferschiefer sediments were deposited following a rapid marine transgression over an area that had been subject to a very long period of arid to semi-arid conditions. In many areas the Kupferschiefer overlies red-bed sediments (Rotliegende), but in others it overlies Carboniferous sandstones and marls or the bleached and reworked equivalents of earlier rocks (Grauliegende and Weissliegende). Within the Kupferschiefer, lithologieal variations can be related to detailed paleogeography. Overall a euxinic, sapropelic facies predominates, but changes occur in the region of paleohighs where more carbonate and clastic-rich facies are developed. Strong evidence exists for the development of chemical stratification in the Zech-stein sea (from which the Kupfersehiefer was deposited) with oxidizing conditions in the upper part and reducing conditions in the lower part. Everywhere the Kupferschiefer grades upward into overlying dolomitie limestones.Although large areas of the Kupferschiefer contain only average concentrations of base and precious metals compared to other shales and marls, in certain areas the concentrations reach ore grade. Historically, the Mansfeld district (SE Harz Mountains) was important for copper and silver mining, but at present-day mining is undertaken only in the Spremberg-Weisswasser area (East Germany) and in Lower Silesia (Poland). The ores in such regions contain sulfides of Cu, Pb, and Zn and may be enriched in a variety of other elements, notably V, Mo, U, Ag,As, Sb, Bi, and Se; Cd, Tl, Au, Re, and the platinum-group metals are also reported; lateral and vertical zoning of Cu, Pb, and Zn may be observed; and in some areas, a reddening of the rocks adjacent to ores (Rote Fule facies) is a useful exploration guide. The Kupfersehiefer in Poland, in two contrasting regions in Germany (the Lower Rhine basin and the Hessian depression) and in England (where it is termed the Marl Slate) are compared and provide evidence for four types of mineralization.The first (and oldest) is a weakly mineralized type exemplified by the English Marl Slate. Average base metal content of this type is approximately 100 ppm. Detailed mineralogieal, geochemical, and isotopic studies indicate that the mineralization is synsedimentary; these studies have enabled a model to be developed in which precipitation of the various mineral phases can be related to stratification of the early Zeehstein sea and oscillations in water and oxic-anoxic boundary levels.The second is an average mineralization involving base metal content at the 2,000-ppm level. In this ease, the study of German examples indicates the important influence of strata underlying the Kupferschiefer, as stressed by subdivision into a basin type that overlies thick Rotliegende sediments, and a schwellen type that overlies Paleozoie basement. The two subtypes differ in mineralogy and overall base metal ratios, indicating the importance of underlying strata as a source of metals. The relationship between barium concentrations in the Kupferschiefer and barite mineralization in underlying rocks is also clear from geochemical (including Sr isotope) studies of German examples. The evidence points to an early diagenetic origin for the average mineralization, with sulfur derived bacteriogenically interacting with low-temperature solutions deriving metals largely from immediately underlying rocks.The third is an ore mineralization where the average base metal concentration reaches approximately 3 percent. Zonation is clearly developed, as is the association of the Rote Faeule facies with mineralization, although the detailed zoning patterns are more complex than commonly believed. The ore mineralization type is generally restricted to the regions representing the margins of Rotliegende basins, and models of origin associated with late diagenetic processes and the introduction of metal-rich brines (possibly associated with basin compaction) are in line with the geologic and geochemical evidence. Preliminary fluid inclusion data on strata associated with the mineralization point to temperatures of approximately 120 degrees C, and organic maturation studies can also be used to support models involving introduction of oxidative metal-rich brines during late diagenesis. Again, sulfur isotope data point to fixation of the metals by sulfur derived from bacterial reduction of sulfate. The fourth mineralization associated with the Kupferschiefer is a much later (postdiagenetic) structure-controlled mineralization (Ruecken) involving Co, Ni, Ba, As, and Ag phases, genetically distinct from the types mentioned above and of probable hydrothermal origin. Other possible episodes of mineralization can be identified on a local scale; in many cases insufficient data are available to assess their genetic significance.The Kupferschiefer is a deposit that appears to be the product of a variety of mineralizing processes influenced by the environment of deposition of the host rock and the underlying geology, but there are many unifying features; notably, that the bulk of the evidence still points to fixation of metals as sulfides by bacteriogenic processes.

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