Zmiany hydrogeniczne w złożu miedzi "Lena" (Dolny Śląsk)

Jerzy Niśkiewicz, Czesław Skowronek


Hydrogenic change in copper deposits of the „Lena” Mine (Lower Silesia)

The copper deposits of the Lena Mine lie in the eastern part of the Leszczyna syncline which incorporates metamorphosed Old Paleozoic rocks and Permian-Mesozoic sedimentary rocks (Fig. 1). The deposits in question comprise alternating 'beds of marly limestone and marl of the Lower Zechstein age. The main ore minerals include chalcocite, bornite, chalcopyrite, and pyrite. These occur in minute xenomorphic and hypidiomorphic grains. Generally, the copper content is higher in the marls than in the marly limestones (the former are nearly 50 per cent richer in copper than limestones, Fig. 2). The area studied is cut by numerous faults trending generally NW—SE and the rocks are intensely jointed (Fig. 1 and 2B). In the study area (Fig. 1 and 2) rocks are coloured red along the plane of a small fault. The zones of reddened rocks are up to 25 cm in width. It has 'been also found that the zones are vertical and trend almost parallel to the fault plane. Outside the red zones there occur zones of „green” rocks up to 5 cm wide (these are grey on fresh surfaces but become green if oxidized). The unaltered deposits are also grey. The zones are rather sharply defined, owing to distinct changes in colour (Fig. 3, Pl. I and III). However, the .boundary between the zones becomes less distinct in parallel layered marls (Pl. II, Fig. 1). Near the fault, rocks are strongly jointed and displaced, some fissures being infilled by caleite (Pl. II, Fig. 2). Chemical investigations have demonstrated that the copper content diminished in the red rocks (which contain about 0.1 per cent Cu) in relation to grey rooks (unaltered rocks contain about 0.6 per cent Cu). On the other hand, the amounts of copper increased considerably in „green” rocks (up to about 2.0 per cent). The increase in copper content resulted from the migration of copper from the red to the „green” rokcs. The changes affected to varying degree both the marly limestones and the marls (Fig. 3). The microscopic investigations of red rocks have demonstrated that they contain only scattered grains of chalcopyrite as the only sulphide mineral. They also contain scarce pseudomorphs of goethite and lepidocrocite (?), mainly after pyrite. Moreover, carbonate minerals as well as clayey-carbonaceous matrix are stained red, most likely by submicroscopic grains of hematite. In the „green” rocks carbonate minerals do not exhibit any red colouring. These rocks, on the other hand, are characterized by a significant increase iin abundance of xenomorphic grains of chalcopyrite, chalcocite, and bornite. The minerals show a distinct zonal distribution, parallel to the boundaries of the zone of „green” rocks {Fig. 4). In the grey rocks (unaltered), chalcocite and chalcopyrite are accompanied by pyrite, the last mineral being unknown from both red- and „green” rocks. The investigations suggest that the belts of red and „green” rocks are in fact zones of oxidation and cementation which owe their origin to hypergenetic processes. In accordance with the genetic classification of such processes by A. E. Fersmann (A. Saukow, 1953) the modifications in ore mineralization found in these copper deposits resulted from hydrogenic processes. Descending waters penetrated the deposit along fault planes and at the same time resulted in horizontal infiltration into copper-bearing rocks. This caused hydrogenic changes in the deposits, which in turn resulted in the formation of oxidation and cementation zones. These developed symmetrically along both walls of the fault and parallel to it (Fig. 3). The parallel disposition of the zones, sharply defined boundaries between them as well as the regular zonation of ore minerals all suggest that the processes of infiltration and geochemical modification were accomplished slowly and uniformly. On the other hand, the processes caused neither essential changes in composition of carbonates and clayey carbonaceous matrix nor in structure and texture of the copper-bearing rocks.

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