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 Rhodope massif near Kato Nevrokopi in the Drama region, Northern Greece. The Mn-oxide mineralization has developed by weathering of continental hypogene rhodochrosite-sulphide veins. The vein mineralization is confined by tectonic shear zones between marble and metapelites, extending laterally into the marble as tabular, pod or lenticular oreshoots (up to 50 m ?? 20 m ?? 5-10 m). Supergene oxidation of the hypogene mineralization 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 zones confirm the role of thrusts and faults as solution passageways and stress the importance of these structures in the development of hydrothermal and supergene mineralization at Kato Nevrokopi. Three zones are recognized in the insitu supergene veins: (A) a stable zone of oxidation, where immobile elements form (or substitute in) stable oxide mineral phases, and mobile elements are leached; (B) a transitional (active) zone in which element behavior is strongly influenced by seasonal fluctuations of the groundwater table and variations in pH-Eh conditions; and (C) a zone of permanent flooding, where variations in pH-Eh conditions are minimal. Zone (B) is considered as the source zone for the karst cavity mineralization. During weathering, meteoric waters, which were CO2-rich (PCO2 ~10-3.8 to 10-1.4) and oxygenated (fO2 ~10-17 for malachite), percolated downward within the veins, causing breakdown and dissolution of sulfides and marble, and oxidation of rhodochrosite to Mn-oxides. Karst cavity formation was favored by the high permeability along thrust zones. Dissolved Mn2+ was transported into karst cavities 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). Mn4+-oxide precipitation took place by fO2 increase in ground waters, or pH increase by continuous hydrolysis and carbonate dissolution. In the well developed karst setting, some mobility of elements occurred during and after karst ore formation in the order Na>K>Mg>Sr>Mn>As>Zn>Ba>Al>Fe>Cu>Cd>Pb.
Keywords: Ore deposits, Geology, Geochemistry, Vein mineralization, Supergene oxidation
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Summary: The Geological Society has its own quarterly journal called Exploration
and Mining Geology, for the publication of Canadian and international papers on
applied aspects of mineral exploration and exploitation, including mineral
deposit geology, geochemistry, and geophysics, mining geology, mineral resource
appraisal and estimation methods, environmental geology, and case histories. The
editor of the journal is Jeremy P. Richards
Publication: Exploration & Mining Geology
Issue: 2
Volume: 6
Year: 1997
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<BODY>The mechanism by which stratiform ore deposits of PGE (platinum-group
elements) are laid down in ultramafic igneous complexes is not completely clear.
Early studies by the author and coworkers on numerical analyses of the
convective cooling of low-Rayleigh-number magma chambers...
Publication: Exploration & Mining Geology
Author(s): ALAN RICE: Departments of Geology and Physics, Rhodes University, Grahamstown 6140, South Africa
Keywords: PGE, Ultramafic igneous complexes, Magma chambers, Suspended
Issue: 2
Volume: 6
Year: 1997
Web Page
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<BODY>The Campo Morado precious-metal-bearing, volcanogenic massive sulfide
deposits occur in a lower Cretaceous, bimodal, calc-alkaline volcanic sequence
in a major northerly trending belt in the Guerrero Terrane in northeastern
Guerrero, Mexico. During upper Cretaceous to Early...
Publication: Exploration & Mining Geology
Author(s): JIM OLIVER, JOHN PAYNE and MARK REBAGLIATI: Farallon Resources Ltd. Vancouver, British Columbia, Canada V6C 2V6
Keywords: Precious metals, Sulfide deposits, Volcanic, Mexico, Minerals
Issue: 2
Volume: 6
Year: 1997
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Summary: The Croft diorite in central England belongs to a suite of Caledonian igneous rocks collectively known as the South Leicestershire diorite complex. Although the intrusions occupy separate outcrops, they are linked at depth to form a single pluton, buried beneath a cover of Triassic sediments (Le Bas, 1972; 1982, Allsop and Arthur, 1983). Both the Caledonian diorites and the overlying sub-Triassic unconformity have been affected by a complex history of alteration and mineralization which can...
Publication: Exploration & Mining Geology
Author(s): K. PEARSON and C.A. JEFFREY: Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, U.K.
Keywords: Caledonian igneous rocks, Intrusions, Deuteric effect, Sodic enrichment, Laumontite veins, Analcime veins, Calcite veins, Mineralization
Issue: 2
Volume: 6
Year: 1997
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Summary: Detailed petrographical and geochemical studies have been carried out on the igneous and sedimentary rocks in the Omai area, part of the Paleoproterozoic Barama-Mazaruni greenstone belt, Guyana. The stratigraphic succession at Omai begins with basalts, associated with mafic ultramafic intrusives and poorly-sorted conglomerates; these are overlain by andesites and quartzfeldspar porphyries, with pelites and tuffaceous sediments at the top. The volcanic-sedimentary succession was intruded by a...
Publication: Exploration & Mining Geology
Author(s): GABRIEL VOICU, MARC BARDOUX, LUC HARNOIS: Département des Sciences de la Terre, Université du Québec à Montréal, Montréal, QC, Canada H3C 3P8 and ROBERT CRÉPEAU:
Omai Gold Mines, Georgetown, Guyana, South America
Keywords: Petrographical studies, Geochemical studies, Guyana, Basalts, Mafic-ultramafic
intrusives, Conglomerates
Issue: 2
Volume: 6
Year: 1997
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Summary: Most large skarn deposits are zoned in both space and time relative to
associated intrusions. Zonation occurs on scales from kilometers to micrometers,
and reflects infiltrative fluid flow, wallrock reaction, temperature variations,
and fluid mixing. The most spectacular examples of skarn zonation usually occur
at the skarn-marble contact, where transitions between monomineralic bands can
be knife sharp. Other small-scale examples occur in zoned veins and individual
mineral crystals....
Publication: Exploration & Mining Geology
Author(s): LAWRENCE D. MEINERT: Department of Geology, Washington State University, Pullman, Washington 99164-2812, U.S.A.
Keywords: Skarn deposits, Skarn zonation, Mineral exploration
Issue: 2
Volume: 6
Year: 1997
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