Formation Conditions of Copper-Sandstone and Copper-Shale Deposits

Abstract

The concept introduced here suggests a common provenance of copper origin for all cupriferous sandstones and shale-type deposits, the source of copper being terrigenous red-bed formations deposited under arid conditions. Copper was leached from these rock deposits by subsurface waters. Copper precipitation occurred at hydrogen sulphide barriers, among which two types are recognized: syngenetic (in unconsolidated sediments) and epigenetic (in lithified sediments). The latter are subdivided, according to the origin of their organic matter, into autochthonous and allochthonous groups. The decisive factor controlling the formation of sediment-hosted copper deposits with large reservers is the specific nature of the paleohydrogeological and geochemical environment.

Copper-sandstone and copper-shale deposits represent a highly specific group of deposits, distinguished by the following characteristic features: (1) invariable spatial association with red beds formed in an arid environment; (2) ore localization in grey sedimentary rocks in close proximity to the red beds; (3) consistent mineral composition and associations of major metallic minerals; (4) a zonal distribution pattern of the sulphides in the ore bodies.

Three hypotheses have been discussed by researchers for the origin of the metallic substances in this group of deposits: hydrothermal (endogenetic solutions), sedimentary (provenance area), and exogenetic-epigenetic (subsurface pore waters of arid red-bed formations). The validity of the above-cited hypotheses can readily be weighed: if it is true that they should provide a lucid understanding for the known spatial association of copper mineralization with arid red beds. This relationship, repeatedly verified by field observations, can be regarded as a well-founded empirical law; nevertheless, it is commonly ignored by the proponents of the hydrothermal hypothesis. A similarly adequate solution of the source problem has been offered by the supporters of the sedimentary hypothesis. According to the latter, advocated by Strakhov (1963), copper was leached from ore deposits of humid provenance by surface waters, and redeposited in arid regions in the form of carbonates that subsequently underwent conversion to sulphides during early diagenesis. However, the original copper carbonates have never been encountered. Consequently, it must be assumed that these carbonates were invariably precipitated only in those areas where the oxidizing environment in the sediments underwent a subsequent change to a reducing, hydrogen sulphide environment.

The general spatial association of copper mineralization with arid red-bed formations is feasibly explained by the epigenetic hypothesis, but from this viewpoint the characteristics of certain deposits, undoubtedly of sedimentary origin (for example, Mansfeld), has in past years remained incompatible. According to the epigenetic hypothesis, copper was leached from red beds by subsurface waters, and therefore its accumulation in marine sediments was regarded as inconsistent with this concept. In fact, the contribution of copper from bottom sediments to sea basin waters is quite natural, considering that influx to the seas is derived not only from the run-off of surface waters, but also from inflow of subsurface waters. If the waters of red-bed formations drained into the paleosea, then these waters could have been responsible for the copper supply. Hence, the most warranted hypothesis is that copper and associated metals were leached by subsurface waters from red-bed formations. In this respect the deposits under study are a group of closely related deposits, having in common the origin of their metal content.

Sulphide mineralization is usually restricted to grey sediments, occurring immediately adjacent to the red beds. This association indicates that the concentration of copper occurs at hydrogen sulphide barriers that are formed at the interface between sediments of markedly different characteristics. The distinctions between deposits are largely due to the conditions under which the hydrogen sulphide barriers are formed. According to the time of formation of the host rocks relative to mineralization, two types of hydrogen sulphide barriers can be distinguished: syngenetic and epigenetic types. The first occur in unconsolidated sediments, the second in lithified rock deposits.