Development of framboidal pyrite in the Upper Permian marly limestone of the NE-Hungarian Darnó Hill

Abstract

The NE Hungarian Darno Hill has a complex geology; it is built up by an accretionary melange complex, which contains Permian, Triassic and Jurassic sedimentary and magmatic blocks too. The succession can be well correlated with the NW Dinarides, and evidences of the different evolutionary stages of the Neotethyan Ocean (rifting, marginal basin opening, closure) were described too. Several ore indications are known from this area, but their genesis was by far not well reconstructed, mostly because of the complexity of the geological background. The present study deals with one of these indications found in deep drillings. A framboidal pyrite bearing Permian-Triassic marly limestone series was investigated, which was described earlier as a possible analogy of the Polish-German copper shales ( Kupferschiefer ). Based on detailed microscopic, EPMA and whole rock geochemical analyses, it was found, that besides the framboidal pyrite, euhedral pyrite overgrowths (both contain high amount of Au), disseminated euhedral and anhedral pyrite (which do not form framboids or overgrowths), anhedral chalcopyrite, galena and sphalerite were also found, causing a slightly enriched total metal content of 100-200 ppm. The minerals were formed under reducing, anoxic marine conditions, in more steps; after the synsedimentary processes, sulphides formed during early diagenetic/hydrothermal processes too. The found characteristics are typical to the weakly mineralized type of the Kupferschiefer .