Manganese mineralisation near Kato Nevrokopi, Drama, Greece
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
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
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
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.