Gold contents of the Akarşen copper deposit, E-Pontides, Turkey

Transcription

1 Gold contents of the Akarşen copper deposit, E-Pontides, Turkey N.Özgür lnstitutfür Angewandte Geologie der Freien Universität Berlin, Germany ABSTRACT: The Akarşen deposit belongs to the East Pontic metallotect and is associated With a subvolcanic formation of an Upper Cretaceous island arc volcanism. The main deposit is linked to a 200-m-thick altered pyroclastic sequence of Senonian age. The ore mineral paragenesis consists of pyrite, chalcopyrite, sphalerite, galena, tennantite, arsenopyrite, and gold. The microscopical identification of gold in pyrite, chalcopyrite, and internal areas of both minerals throughout the entire East Pontic metallotect is a novelty. The altered host rocks are representing Au values between and 8 ppm enriched in the silicic alteration zones. It may be attributed to hydrothermal remobilization of the Au contents of the host rocks. The predominance of gold in the Akarşen deposit indicates a genetic relationship with the copper porphyries of the island arc type. 1 INTRODUCTION The Akarşen deposit belongs to the East Pontic metallotect which represents a volcanic island arc system hosting a great number of base metal occurrences and extends over an area of more than 350 km E-W and 60 km N-S. A systematic sampl ing of the deposit has been performed in connection with a project on lithogeochemical indicators for base metal exploration. As a supplement to the results of our investigations (i.e. Schneider et al. 1988), the aim of this study was to represent the Au contents in altered host rocks and ore minerals. 2 GEOLOGIC SETTING The deposit is located in the northeastern part of Turkey (Fig. 1) and belongs to the East Pontic metallotect representing a mobile belt between the Pontic and Anatolia microplate. The succession of rocks of the metallotect consists of an about 3000-m-thick volcanic sequence which might be divided into three cycles (Özgür & Schneider 1988). The first cycle comprises a volcanic pile deposited between Early Jurassic and Upper Cretaceous. It is represented by initial basaltic activity which changes progressively to felsic lava flows and thick pyroclastics in the middle and upper part. The second cycle starts with volcanic breccias, tuffs, and marine sediments overlain by andesitic lava flows, followed by limestones (Maastrichtian). The last cycle displays a basal sequence of marine sediments which are overlain by basaltic and andesitic lava flows representing Tertiary volcanic activity. 1

2 Fig. 1 Geological sketch map of the Akarşen deposit. 1=mineralization, 2=dolerite, 3=sandstone and tuffite, 4=limestone and marl, 5=altered dacitic pyroclastic host rock, 6=main faults, generally vertical movements, 7=location of the samples from the surface, 8=location of the deep drilling hole, and 9=investigated area. The main deposit occurs within the strongly altered dacitic pyroclastics of the first volcanic cycle. It is associated with a 200-m-thick felsic sequence of Senonian age. The intense host rock alteration in the investigated area can be divided into an initial stage of phyllic alteration surrounded by a argillic alteration zones and a late stage of silicification. 3 THE AKARŞEN DEPOSIT The ore mineral assemblage of the Akarşen deposit consists of predominantly pyrite and lesser chalcopyrite, sphalerite, galena, tennantite, covellite, and arseno pyrite. In many polished sections, small inclusions of native gold were detected. The deposit is composed of (a) disseminated ore with varying Cu contents ranging from 0.1 to 0.5 percent, (b) stockwork ore with average Cu contents between 1.0 and 3.0 percent, and (c) small ore lodes with Cu contents up to 10 percent. The recoverable ore reserves are estimated at metric tonnes with an average content of 3.2 percent Cu, 2.7 percent Zn, 0.25 percent Pb, 50 ppm Ag, and 4 ppm Au. 4 SAMPLING AND ANALYTICAL METHODS Various rock and ore samples have been obtained from altered mineralized zones; 17 rock and 4 ore samples from the surface and 1 rock sample from deep drilling holes (Fig. 1 and Tab. 1). In addition to microscopical investigations, Au, As, and Sb in host rocks were determined by instrumental neutron activation at the Hahn-Meitner Institut fur Kernforschung, Berlin, with a 2

3 precision better than ± 9 %. In the same way, pyrite and chalcopyrite samples have been analyzed at the Max-Planck Institut fur Kernforschung, JUlich and Mainz, FRG. Cu has been analyzed by atomic absorption spectrometry at the Institut fur Angewandte Geologie, Freie Universität Berlin, with a routine precision better than ± 5%. The chemical composition of gold inclusions was analyzed with ARL SEMQ electron microprobe at the Institut für Mineralogie, Freie Universität Berlin, (Özgür 1990). The shape of gold and other minerals has been investigated by scanning electron microscopy (SEM) and EDX-system at the above mentioned institute. Table 1. Various rock and ore samples from the Akarşen deposit with Cu, As, Sb, and Au contents (see for locations Fig. 1) (n.d.=not determined). 5 RESULTS The altered host rocks of the Akarşen deposit exhibit gold contents ranging from to 8.4 ppm and representing a background value of ppm. Au distribution in the altered dacitic member displays anomalous areas with values of 0.15 ppm and locally values greater than 2.22 ppm (Fig. 2 and Tab. 1). Some sectors show local economic concentrations. The enrichment of Cu, Sb, and As displays a close correlation with the anomalous Au grades (Fig. 2). Ore microscopical investigations indicate that native gold often occurs as inclusions with a diameter of approximately 25 µm in pyrite, chalcopyrite, and internal areas of both 3

4 minerals (Fig. 3). The gold seems to belong to the same mineral assemblage as chalcopyrite. Electron microprobe investigations reveal remarkable high gold contents. The gold contains around 21 wt % Ag (Özgür 1990). According to determination by scanning electron microscopy (SEM), the native Au occurs in pyrite as a shape of cube (Özgür 1990). Aditionally, arsenopyrite could be detected as a new mineral phase within the above mentioned ore mineral paragenesis of the Akarşen deposit which can be designated as a Autracer. Fig. 2 Sampling profiles crossing the Akarşen deposit. Arrows and numbers indicate sampling locations. 1=mineralization, 2=dolerite, 3=sandstone and tuffite, 4=limestone and marl, and 5=altered dacitic pyroclastic host rock. Fig. 3 Native gold (Au) at the boundary of pyrite (py) and chalcopyrite (cpy), polished section, plane polarized light, oil immersion, x

5 6 CONCLUSIONS The Akarşen deposit can be assigned to a subvolcanic formation associated with an Upper Cretaceous island arc volcanism. The mineralization is related to a distinct member of a 200-m-thick altered dacitic sequence and may be assigned as stratabound in a wider sense. The chemistry of the volcanic rocks (Schneider et al. 1988) and sparsely and only locally intercalated marine sediments into the thick volcanic rocks as well as shallow water facies of the limestone lenses indicate the development of the island arc which could be proved by the presence of island arc tholeiites in the neighbouring ore deposit of Murgul (Özgür 1985). As a pyroclastic member, the host rock has been altered and mineralized duriny a late stage of the dacitic volcanism by ascending hydrothermal fluids. The shape of the orebody with its characteristic geometry of concentric alteration zones implies a relationship to localized zones of ascendent hydrothermal fluids. According to microscopical observations and REE patterns (Dieterle 1986), there are two types of alteration, an early distal one of phyllic and argillic zones and a later silicic alteration in a central zone. The first phase of mineralization (disseminated) corresponds to the early alteration stage of phyllic and argillic zones. The second and third phase of mineralization (stockwork and small ore lodes) is related to the second stage of alteration. The Au distribution within the orebody displays anomalous areas represented by values of ppm and more than 8 ppm. Particularly remarkable is the distribution of higher Au values which are locally linked to silicic alteration zones. The enrichment of gold in silicic alteration zones could be attributed to hydrothermal remobilization of the Au contents of the host rocks (Özgür & Palacios 1990). Au inclusions probably could be incorporated into the lattice of pyrite and chalcopyrite (Hilmy & Osman 1989). Furthermore, the ore microscopical gold observation throughout the entire East Pontic metallotect could be assigned as a novelty. The precipitation of native gold in an initially saturated fluid is an effect of decreasing temperature, boiled down to about C, and decreased concentration of reduced sulfur 5

6 (Krupp & Seward 1990). In comparison with hydrothermal Au deposits of Marche- Combrailles in Central France (Boiron et al. 1989), the system is related to a fault zone containing dacitic pyroclastics, deep source, phyllic alteration, the mineral assemblage of pyrite and arsenopyrite (Au, As, Sb), C, aqueous fluids and low salinity. Ag displays a similar behaviour. The range of Ag contents within the gold in the observed polished sections, %, is responsible for the white yellow brilliance of native gold according to ore microscopy (Özgür 1990). The genetic interpretation of the Akarşen deposit can be corroborated by the predominance of gold over molybdenum in pyrite and chalcopyrite. Therefore, it could be regarded as a further indication to an island arc position of the mineralization (Horton 1978). Moreover, the elevated Au values probably are indicating a genetic relationship of the Akarşen deposit, as reported by Özgür & Schneider (1988), with the copper porphyries of the island arc type. REFERENCES Boiron, M.C., Cathelineau, M. and Trescases, J.J Conditions of gold-bearing arsenopyrite crystallization in the Villeranges basin, Marche-Combrailles shear zone, France: A mineralogical and fluid inclusion study. Econ. Geol. 84: Dieterle, M Zur Geochemie und Genese der schichtgebundenen Buntmetall- Vorkommen in der Ostpontischen Metallprovinz/NE Türkei. Ph.D.thesis, Freie Universität Berlin, 112p. Hilmy, H.M. and Osman, A Remobilization of gold from a chalcopyritepyrite mineralization Hamash gold mine, Southeastern Desert, Egypt, Mineral. Deposita 24: Horton, D.J Porphyry-type coppermolybdenum mineralization belts in eastern Queensland, Australia. Econ. Geol. 73: Krupp, R.E. and Seward, T.M Transport and deposition of metals in the Rotokawa geothermal sysem, New Zealand. Mineral. Deposita 25: Özgür, N Zur Geochemie und Genese der Kupferlagerstätte Mugul, E-Pontiden/ Türkei. Ph.D.thesis, Freie Universität Berlin, 139p. Özgür, N. and Schneider, H-J New metallogenetic aspects concerning the copper deposit of Mugul, NE Turkey. Soc. Geology Applied to Mineral Deposits, Spec. Pub. 6: 6

7 Özgür, N. and Palacios, C.M Geochemical proximity indicators of the volcanogenic copper deposit, East Pontic metallogenetic province, NE Turkey. Maden Tetkik ve Arama Enstitüsü (Ankara/ Turkey), Bull. 111 (in press). Özgür, N Gold contents of the Akarşen copper deposit, E-Pontides/ Turkey. Isparta Mühendislik Fakültesi (Isparta/Turkey), Bull. 5 (in press). Schneider, H-J., Özgür, N. and Palacios, C.M Relationship between alteration, rare earth element distribution, and mineralization of the Murgul copper deposit, Northeastern Turkey. Econ. Geol. 83: