The manganese mineralization near Kato Nevrokopi, Drama, in the Falakron mountains, is hosted by the Precambrian to Early Paleozoic Upper Marble. The marble and the underlying Lower Schist unit were metamorphosed between the Late Cambrian and the Carboniferous and were extensively deformed during the Alpine orogeny. The Alpine deformation is considered to have occurred between the Early Cretaceous and the Oligocene. Granodiorites and rhyodacites of Oligocene age outcrop also in the area. The mineralization near Kato Nevrokopi is confined to intersecting Alpine fault zones of northeast and northwest trends. Negligible mineralization occurs into the schist, the richest orebodies being present as lateral extensions in the marble. Mineralization diminishes with increasing distance from the fault zones. The shape of the orebodies is irregular with individual offshoots being lenticular, pod-like or tabular. Sizes can be up to 50m in length, 20m in width and 5-10m in thickness. The boundaries between the orebodies and the marble are sharp and replacement and alteration phenomena around the orebodies include: dolomitization of the marble and clay mineral alteration. Two distinctly different mineralizing processes took place at Kato Nevrokopi: a) A hydrothermal process, characterized by mineral zoning in time and space, present as abundant rhodochrosite, ankerite, sphalerite, pyrite, pyrrhotine, chalcopyrite and marcasite in the stratigraphically deeper veins and "black calcite" (mixture of calcite and todorokite) and galena in the upper veins. b) A supergene process, in which the hydrothermal minerals in the veins were weathered. During this process, rhodochrosite in the veins was oxidized to MnO-gel and todorokite and in low altitude karstic cavities the mineral assemblage is: MnO-gel-nsutitechalcophanite. In high altitude karsts the mineral assemblage is: MnO-gel-nsutite-birnessite-cryptomelane-pyrolusite. During weathering Zn derived by dissolution of sphalerite was also mobile, being transported together with Mn to karstic cavities where it substituted for Mn in the Mn-oxides. The minerals pyrrhotine, marcasite, ankerite and birnessite are for first time recorded at Kato Nevrokopi. The lack of useable fluid inclusions severely limited the information on the composition of the hydrothermal fluids. From the mineral pair kaolinite-sericite, the pH of the hydrothermal fluids was estimated to have been between 3-4. Hydrothermal mineral precipitation took place mainly by reaction of the fluids with the marble and pH increase (cf., rhodochrosite) or by mixing of the hydrothermal fluids with near surface aquifers (black calcite). Thermodynamic data for the overall transformation of rhodochrosite to pyrolusite and the contemporaneous decomposition of chalcopyrite to azurite and malachite were used and the oxygen fugacity of the meteoric fluids was calculated to have ranged between 10-22 and 10-17. Mn-oxide precipitation during weathering took place either by oxidation of the primary veins (cf., MnO-gel, todorokite) or by reaction of the groundwaters with the marble in karstic cavities and precipitation of Mn-oxides as layers and encrustations above the local ground water table. Evidence from the whole rock geochemistry of the mineralized samples emphasizes the role of the thrust and fault zones as solution passageways and stresses their importance for the development of hydrothermal and supergene mineralization at Kato Nevrokopi. During weathering, downward percolation of C02-rich oxygenated meteoric waters within the veins, caused the breakdown and dissolution of sulphides and oxidation of rhodochrosite to Mn-oxides. The pH of these meteoric fluids was buffered by the dissolution of sulphides and the formation of karstic cavities was favoured by the high permeability induced by the occurrence of the thrust zone and the percolation of acidic meteoric waters through the marble. Chemical transport of MnZ+ to karstic cavities was possible in reduced meteoric waters at the beginning of weathering (pH"4-5), and as Mn(HCO3)2 in slightly alkaline groundwaters during advanced weathering (pH"6-8). Alkalies (K, Na) and alkaline earths (Ca, Mg, Ba, Sr) where leached away from the ore and the country rocks during weathering and the order of element mobility in the karstic cavities was: Na>K>MgaSr>Mn>As>Ca>Zn>Ba>Al>Fe>Cu>Cd>Pb. The Mn-oxide orebodies near Kato Nevrokopi are located in a northeast trending line parallel to the axes of major Alpine folds and significantly the rhyodacite volcanics outcrop also in a parallel fashion to this line providing evidence of an underlying pluton. This northeast trending line may therefore represent a zone of crustal weakness that was exploited by andesitic magma and subsequently by hydrothermal fluids. An average age of 33 Ma for the Kato Nevrokopi mineralization is provided. This age is similar to that of magmatism in and around the Drama area. It is therefore reasonable to conclude that the hydrothermal activity near Kato Nevrokopi and generally of the northeast Drama area was related to the Oligocene magmatism. On the basis of its age, style, morphology and genesis, the mineralization near Kato Nevrokopi is placed in the metallogenic province of N. Greece/S. Bulgaria which also includes the Madan hydrothermal Pb-Zn-Mn vein deposits and the Chalkidiki Pb-Zn deposits which have Mn-oxides in their upper parts.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:233390 |
| Date | January 1988 |
| Creators | Nimfopoulos, M. K. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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