GEOLOGY, GEOCHEMISTRY, AND ORIGIN OF THE CONTINENTAL KARST-HOSTED SUPERGENE MANGANESE DEPOSITS IN THE WESTERN RHODOPE MASSIF, MACEDONIA, NORTHERN GREECE

Economic Mn-oxide ore deposits of commercial grade occur in the Rhodop e massif near Kato Nevrokopi in the Drama region, Northern Greece. The Mn-oxide mineralization has developed by weathering of continental hy pogene rhodochrosite-sulphide veins. The vein mineralization is confin ed by tectonic shear zones between marble and metapelites, extending l aterally into the marble as tabular, pod or lenticular oreshoots (up t o 50 m x 20 m x 5-10 m). Supergene oxidation of the hypogene mineraliz ation led to the formation of in-situ residual Mn-oxide ore deposits, and secondary infills of Mn-oxide ore in embryonic and well developed karst cavities. Whole rock geochemical profiles across mineralized zon es confirm the role of thrusts and faults as solution passageways and stress the importance of these structures in the development of hydrot hermal and supergene mineralization at Kato Nevrokopi. Three zones an recognized in the insitu supergene veins: (A) a stable zone of oxidati on, where immobile elements form (or substitute in) stable oxide miner al phases, and mobile elements are leached; (B) a transitional (active ) zone in which element behavior is strongly influenced by seasonal fl uctuations of the groundwater table and variations in pH-Eh conditions ; and (C) a zone of permanent flooding, where variations in pH-Eh cond itions are minimal. Zone (B) is considered as the source zone for the karst cavity mineralization. During weathering, meteoric waters, which were CO2-rich (P-CO2 similar to 10(-3.8) to 10(-1.4)) and oxygenated (fO(2) -10(-17) for malachite), percolated downward within the veins, causing breakdown and dissolution of sulfides and marble, and oxidatio n of rhodochrosite to Mn-oxides. Karat cavity formation was favored by the high permeability along thrust zones. Dissolved Mn2+ was transpor ted into karst cavities in reduced meteoric waters at the beginning of weathering (pH similar to 4-5), and as Mn(HCO3)(2) in slightly alkali ne groundwaters during advanced weathering (pH similar to 6-8). Mn4+-o xide precipitation took place by fO(2) increase in ground waters, or p H increase by continuous hydrolysis and carbonate dissolution. In the well developed karst setting, some mobility of elements occurred durin g and after karst ore formation in the order Na>K>Mg>Sr>Mn>As>Zn>Ba>Al >Fe>Cu>Cd>Pb. (C) 1998 Canadian Institute of Mining, Metallurgy and Pe trolem. Published by Elsevier Science Ltd. All rights reserved.