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ADVANCED INTERDISCIPLINARY EXPLORATION RESEARCH FOR BASE, PRECIOUS AND ASSOCIATED METALS IN POLYMETAMORPHIC TERRAINS NORTHERN GREECE.

Objective

BOTH AREAS HAVE CERTAINLY A MUCH LARGER MINERAL POTENTIAL BUT SEVERAL BASIC SCIENTIFIC PROBLEMS REMAIN TO BE RESOLVED IN ORDER TO FIND NEW GUIDELINES FOR EXPLORATION. THIS IS THE PRINCIPAL AIM OF THE PROJECT. THE DEVELOPMENT OF NEW CRITERIA FOR EXPLORATION WILL CONSIDERABLY IMPROVE THE EXPLORATION METHODOLOGY IN N.GREECE GIVING RISE TO NEW OREBODY DISCOVERIES.
A major mineral exploration programme has been conducted in the Drama County of northern Greece. This has resulted in the following geological scenario being suggested.
During a sedimentary stage, mixed chemical clastic sediments of the transitional zone, with sedimentary exhalative iron manganese horizons, and overlying limestones and dolomites of the upper marble unit were deposited in a back arc type basin.
After regional deformation, metamorphism, partial melting and nappe tectonism, in which the upper unit marbles are placed on top of the stratigraphically lower transitional zone rocks, block faulting, brecciation and supergenetic processes were triggered.
Meteoric solutions in an oxidizing environment, seeping down along fractures and breccias, infiltrating the transitional zone schists, and dissolving manganese bearing and associated phases on their way, were the mineralizing agents that deposit the manganese and accompanying elements in karst structures at lower structural levels. The minor concentrations of pyrite in the transitional zone schists were easily oxidized into sulphuric acid, which enhances the solubility of most elements involved.
The actual karst hosted manganese deposition presumably took place at the interface between cooling and rising hydrothermal fluids and downwards seeping meteoric solutions, possibly at groundwater level.

On the basis of the observedphenomena, the following prospection tools are proposed:
the degree of brecciation of marbles, with increasing matrix to fragment ratios towards the manganese ore;
the degree of silicification of marbles as an indicator for the presence of sulphide generating hydrothermal systems;
the increasing silver contents in altered marbles as a measure for the silver content in the manganese ore nearby.

Studies of the geochemistry of manganese, base metals and precious elements in Western Rhodope have led to the following conclusions:
the transitional zone hosted manganese iron mineralizations are presumably of non-volcanogenic, synsedimentary hydrothermal diagenetic origin;
there is no metamorphic gap between the manganese iron mineralizations and the surrounding wall rocks;
the transitional zone hosted manganese iron mineralizations are possible protores for the marble hosted manganese concentrations in higher stratigraphic levels, through a process of oxidation of manganese bearing carbonates;
the marble hosted manganese deposits show most of the features characteristic for supergene manganese deposits;
there is a metamorphic gap between the marble hosted manganese deposits and the host rock;
the marble hosted manganese deposits are surrounded by various types of breccias in the hanging wall and surrounding wall rock;
the sulphide mineralization at Pyrgi represents a shear zone controlled mineralization;
silicification seems associated with sulphide mineralization, whereas sideritization and limonitization seem associated with manganese mineralization;
manganese ore forming processes are clearly related to silver gold mineralization;
manganese oxide bearing pegmatite samples represent altered and mineralized pegmatites, thus excluding a genetic relationship between pegmatite intrusion manganese mineralization;
the Makriplagi are deserves a closer investigation;
The following regional exploration tools for the karst hosted manganese deposits are proposed:
the degree of brecciation of marbles, with increasing matrix to fragment ratios towards the manganese ore,
the degree of silicification of marbles as an indicator for the presence of sulphide generating hydrothermal systems;
the increasing silver and possibly yttrium content in altered marbles as an indication for precious elements in the manganese ore nearby.

The following local exploration to ols are proposed for use around existing mineralizations and small mines:
increasing manganese and calcium and decreasing magnesium content towards the karst hosted manganese deposits, the latter 2 presumably due to dedolomitization processes during mineralization, especially in the footwall;
increasing zinc, cadmium, barium and nickel content in the footwall, approaching the karst hosted manganese deposits.

One conclusion that can be drawn from the regional and local geochemical trends is that there appears to be a downward decreasing gradient represented by a number of elements (manganese, barium, yttrium and tin). This trend is probably the result of a seeping mechanism or drainage system, similar to a seeping mechanism that deposited the manganese in karst structures in a supergenetic environment.

Mineral samples from northern Greece have been examined for gold and silver content using beta autoradiography. New instrumental neutron activation analysis (INAA) data have also been found for the gold, arsenic and antimony contents of some of the samples. Beta autoradiography was used to examine samples prepared as polished thin sections and to identify and discrete gold or silver mineral grains.

2 styles of gold mineralization appear to be associated with these manganese rich samples. 1 style suggests that gold can be present in a very finely disseminated form, perhaps as intergranular coatings or even as a chemical precipitate in calcite. This form of mineralization is likely to be very difficult to characterize in thin section by scanning electron microscope (SEM) and optical microscope examination and is only likely to be recoverable on a commercial scale by subjecting the ore to a heap leaching process. A second style of mineralization contains a large number of discrete, often rounded, gold grains, apparently in the size range 1 to 4 um. On a commercial scale, these may be more amenable to recovery from ore samples by appropriate physical (flotation) processes.

Finally, application of the technique of beta autoradiography, has been essential to the success of this study in 2 respects. First, in suggesting the presence of finely disemminated gold below the resolving power of the SEM or optical microscope. Second, in demonstrating the presence of discrete gold (and silver) mineralization in samples that would otherwise be difficult to characterize due to the presence of numerous high reflectance and high atomic number accessory phases (including galena) that make the interpretation of optical microscope and SEM images very ambiguous.

Studies of geological samples from the Western Rhodope area of northern Greece have led to the following conclusions:
the basement gneisses represent a pseudocalcalkaline trend, irrespective of the degree of alteration;
only 4 out of 35 basement samples represent least altered rock compositions;
the rare earth element (REE) patterns of basement and transitional zone rocks are remarkably similar;
the transitional zone schists and gneisses host metamorphosed sedimentary exhalative iron manganese rich horizons, to be classified as mixed clastic chemical sediments, which are geochemically comparable to the banded iron manganese sediments at Thassos;
there is no metamorphic gap between the iron manganese mineralizations and the surrounding wall rocks;
the marble hosted manganese deposits bear all features characteristic to supergene manganese deposits;
there is a metamorphic gap between the marble hosted manganese deposits and the host rock;
it is quite likely that the transitional zone schists and gneisses, including the iron manganese rich sedimentary horizons, may have acted as source rocks for the carbonate hosted manganese concentrations higher up in the stratigraphy;
banded marbles are not significantly different from the Boz-Dag marbles, and presumably represent the more foliated equivalents of the Boz-Dog marbles;
manganese oxide bearing pegmatite samples represent altered and mineralized pegmatites, thus excluding a geneticrelationship between pegmatite intrusion and manganese mineralization;
trace element concentrations, microscopical data and field observations suggest that the area around Makriplagi is 1 of the potentially most interesting subareas

A petrologic and geochemical study has been carried out on selected rock types of the Drama County of northern Greece, including:
petrography of the ores and their host rocks in the area of Pirgi, Granite and the implications for the ore genesis;
petrographic study of the regional metamorphic rocks enclosed in the granite of Skaloti (investigation of contact metamorphic phenomena in order to establish the intrusive or anatectic character of the Skaloti granite;
study of the amphibolites (altered and unaltered) with emphasis on the petrochemistry of those in the vicinity of the mineralizations;
evaluation of the trace element data of the amphibolites, correlation with the major and rare earth elements, and determination of the geotectonic environment of formation of the protoliths.

The use of LANDSAT data, combined with other information such as that from geophysics and aerial photography, gives the geometry and spatial distribution of the fracture system in the Rhodope region. It is known that many of these fractures are major faults, fault zones or joint systems which control basin development, igneous activity and nature and kinematics of the fracture systems and their relation to the mineralization, an attempt has been made to relate the main fracture direction with field measurements and other relevant information.

Left lateral strike slip movements, as well as mineral veins, have been observed on north west to south east faults in Agistron, Stenolakos, Grammeni and Lagos areas. Also west north west faults are widely developed in the Madan area. 30 average strikes are about 290 degrees and are marked by intensively deformed and hydrothermally altered rocks. Faults of this group control the mineralization of Kato Thermes. North East faults of smaller dimensions also control the mineralization in the above area. Left lateral strike slip movements and veins also have been observed on north north west to south south east mineralized fracture zones, as in the Essimi area. Finally north west to south east and north east to south west tectonic unconformities are predominant in the Pangeon Mountain area. Generally, the 120-140 degree fractures appear to be extensional fractures and are observed as fillings or form major normal faults. The north to south fractures show various components of north north west extensional and left lateral movements, although some reactivation of these fractures under right lateral movement is seen locally. The 40-60 degree fractures are generally associated with reverse faults or dextral movements. This pattern of north east to south west extension is consistent with the plate tectonic models of the northern Aegean region.

The existence of LANDSAT thematic mapper (TM) linear structural systems with crosscut con sistently all the lithologies, including Tertiary basins of the Greek Rhodope region and Chalkidiki peninsula, independently of their trend, age and tectonic position, suggests that these fracture patterns may represent reactivated older structures or Tertiary structures superimposed on all units. It has been shown that the main fracture directions in the Mediterranean region and Central Europe represent reactivated older structures responding to isostatic movements. The same seems to apply to the study area.

Recent petrological investigations in west Rhodope reveal the nappe structure character of this domain. 2 tectonic units are recognised, on account of significant changes in metamorphic grade identified in closely lying rocks. The lower tectonic unit (LTU) consists from the base to the top of orthogneisses (possibly representing a pre-Alpine basement), an alternating pelitic schist marble series and a marble series starting with banded marbles and ending with massive marbles on the top. In the upper tectonic unit (UTU), the deepest lithologies are represented mainly by orthogneisses, pelitic gneisses, mica schists and migmatites and are overlain by an alternating marble mica schist amphibolite series and finally by massive marbles. The UTU is thrust over the LTU in a north south direction. The tectonic boundary proposed in the frame of the present work does not always coincide with the one set by previous researchers on account of tectonic criteria. Both tectonic units have been metamorphosed successively under high pressure, medium pressure and low pressure conditions, in the frame of a single metamorphic cycle. The peak metamorphic conditions of the high pressure and temperature metamorphism, as well as those prevailing during exhumation are different in the 2 tectonic units.

In the LTU conditions of 520 C and 12 kbar were estimated for the peak of the high pressure and temperature metamorphism, based on the maximum silicon content in the phengite of gneisses with the mineral assemblage phengite potassium feldspar albite and oligoclase quartz biotite, and the presence of the mineral paragenesis garnet (almandine 0.94) plus chloritoid plus chlorite in metapelites. The medium pressure stage of metamorphism is characterized by conditions of the transition zone between epidote-amphibolite and amphibolite facies. In metabasic rocks, this is indicated by the coexistence of magnesio (or tschermakitic) hornblende with albite or oligoclase plus chlorite. Coexisting albi te oligoclase occur also in the orthogneisses, the pelitic gneisses and the carbonate bearing schists. A slight temperature increase after the peak of the high pressure and temperature stage during exhumation is indicated by zonation of white micas showing an increase in titanium and sodium contents with a concomitant decrease in the celadonite component form the core to the rim, by the growth of magnesio (or tschermakitic) hornblende around tremolite and actinolite in metabasites and by garnet biotite geothermometry in metapelites.

In the UTU, high pressure metamorphism is indicated by the presence of eclogite relics in the metabasites. Estimation of the pressure and temperature conditions of the high pressure metamorphism in these rocks is uncertain. The petrography and mineral chemistry of metapelites suggest that the mineral association garnet plus biotite plus kyanite is relatedto the high pressure event and was probably formed under minimum pressure temperature conditions of 16 kbar and 680 C. Staurolite and sillimanite were formed subsequently during uplift.

Garnet plagioclase geobarometry in metapelites of west and central Rhodope, with the mineral assemblage garnet kynanite plagioclase quartz biotite muscovite, yielded pressures ranging between 8 and 10 kbar (for an assumed temperature of 670 C) which correspond to a posteclogitic stage of metamorphism. Garnet plagioclase geobarometry in sillimanite bearing rocks within the migmatite zone of the Echinos-Thermes area (central Rhodope), with the mineral assemblages garnet biotite kyanite sillimanite plagioclase potassium feldspar quartz, yielded pressures of 5.8 to 6.7 kbar. These values are in agreement with the petrographic data indicating that sillimanite is formed under lower pressure in a later stage during uplift.

The low grade, retrograde metamorphism of greenschist facies is well imprinted in the metapelites by the replacement of garnet by pale brown or green biotite with low titanium content , and in the gneisses by the change of brown biotite into greenschist.

Many granites that occur in orogenic belts, such as the Himalaya, the Hercynides of Caledonides, are thought to result form anatexis of the lower parts of overthickened thrust stacked crustal units, just preceding termination of the orogenic cycle. Numerous Tertiary granitoids occur in the Rhodope zone of northern Greece (and adjacent regions of Bulgaria), a belt regarded as an eastern extension of the Alpine system. These granites could also be regarded as syn or late tectonic crustal melts in that some have been emplaced as sheet like bodies along major Alpine thrust planes, and show a strong marginal foliation parallel to the pervasive Alpine compressional fabrics. In detail, however, the geochemistry of the granitoids, their timing and relationships with country rocks, suggest they have been emplaced in an extensional regime that is related to the collapse of the Hellenic Orogen and, more particularly, to the crustal thinning that the Aegean region has suffered since the mid Tertiary period. Rather than being products of crustal thickening, it is suggested that their petrogenesis is related to the elevation of thermal gradients during the extensional phase, which initiated thermal breakdown of hydrous minerals in the lithosphere and lower crust that had existed metastably during compression and the subduction of cool ocean lithosphere beneath the region in the late Mesozoic to early Tertiary period.

The major, trace and rare earth element data of the 30 Ma Stratoni granodiorite, combined with the presence of andensite and calcalkaline lamprophyre dikes, suggest that this Tertiary complex, that intrudes the amphibolite grade metamorphic Kerdilia formation, was formed in a subduction related setting. Neodymium strontium isotope data, oxygen, and probably lead, indicate that the Stratoni granodiorite was produced by mixing primarily of island arc type mafic liquids, with small contributions by siliceous, and isotopically more radiogenic, anatectic melts. The magma mixture that produced the Stratoni granodiorite is characterized by enrichments in a number of ore elements relative to the corresponding Clark values, thus classifying it to the specialized granites.

The Stratoni granodiorite, during emplacement and consolidation, produced an aureole which is represented by a skarn replacement ore system and the associated alterations. This ore system on the basis of lead, oxygen isotope and fluid inclusion data fits better to epigenetic rather than to syngenetic remobilized processes. At the early stages of this intrusive event materials and heat were transferred primarily from the granodiorite outward to the wall rocks. The late (24.2 Ma) hydrothermal fluids that produced the altered granodiorite acquired their more radiogenic nature, as regards the studied isotopes, via interaction probably with the isotopically evolved Kerdilia gneisses. The origin of these late fluids is currently under investigation whereas the nature of hydrothermal activity operating between 30 Ma and 24.2 Ma, as the intrusive system cooled, is expected to be revealed by ongoing more detailed studies.
THE WESTERN RHODOPE AND THE CHALKIDIKI AREAS HAVE A SIGNIFICANT MINERAL POTENTIAL WITH THREE MINES IN OPERATION.
THE MAIN AREAS TO BE INVESTIGATED IN THE PROJECT INCLUDE:

- THE TYPE, COMPOSITIONS AND NATURE OF THE GRANITOIDS AND OF THE COUNTRY ROCKS;

- THE EXTEND OF HYDROTHERMAL SYSTEMS INVOLVED AND THE NATURE AND COMPOSITION OF FLUIDS;

- THE SUITABLE STRUCTURAL FEATURES AND LINEAMENTS PROVIDING THE PERMEABILITY FOR EXTENSIVE HYDROTHERMAL CIRCULATION;

- FINALLY, DEVELOPMENT OF SIGNIFICANT EXPLORATION CRITERIA AT DIFFERENT SCALES TO ENABLE SELECTION OF AREAS OF HIGHEST MINERAL POTENTIAL WITHIN THE WESTERN RHODOPE AND CHALKIDIKI REGIONS.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Institute of Geology and Mineral Exploration
Address
70,Messogeion Street
11527 Athens
Greece

Participants (4)

Open University
United Kingdom
Address
Walton Hal
MK7 6AA Milton Keynes
UNIVERSITY OF KOBENHAVN
Denmark
Address

Kobenhavn
UNIVERSITY OF PATRAS AND ATHENS
Greece
University of Amsterdam
Netherlands
Address

Amsterdam