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Abstract

For the strata-bound lead and zinc ore deposits of Mežica and Bleiberg the following origin can be accepted: hydrothermal, syngenetic or hydatogene in the meaning of the word according to syngeneticists. The following facts favor a syngenetic origin: the ores occur in horizons at certain distances from a characteristic bed; they have a regional extent; mineral composition differs strongly from that of normal hydrothermal deposits; geopetal fabrics suggest a syngenetic origin. The geopetal fabric can be observed in fossils (syngenetic) and in vugs, occurring generally in carbonate rocks (epigenetic). Lead is isotopically older than the enclosing rocks.

Arguments against a syngenetic origin are the lenticular form of the ore bodies and the zonal arrangement of individual minerals in some deposits (Topla). There is also a problem in the co-existence of carbonates and sulfides at the same EH and pH. There are regional differences in the Pb: Zn ratio which increases from west to east. The relation of marcasite towards the economic minerals is not favorable (Kherzet Youssef, Algeria). Faults along the ore bodies contain mineralization with the same texture as the strata-bound ores. Aureoles of metals surround the ore bodies. The hydatogene origin of the deposits is very doubtful, as a selective solution of sulfides should take place with respect to carbonates. Secondary dolomite never shows a bedded structure; hence it does not replace the primary stratiform sulfide ores but rather forms irregular vein systems following fissures parallel to the main faults. The absolute age of the lead speaks for regenerated ore deposits as proposed by the late Professor Schneiderhöhn (11). The rhythmic fabric could be observed not only in the strata-bound mineralization but in fragments of the ore in the fault ore bodies posing a question whether this is a true geopetal texture or only a replacement of rhythmic carbonate beds. So more of the facts favor a hydrothermal origin for these two deposits.

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