The subeconomic, Gümüshane (GMS) porphyry copper-gold system was discovered 1990 by the exploration company Anglo-Tur Madencilik A.S. in the Eastern Pontids of Turkey and represents the first Au-Cu occurrence of this type found in this country. This mineralisation is located within the Eastern Black Sea Metallogenic Province associated with an E-W trending, Jurassic - Eocene volcanic arc complex. The gold-rich, copper porphyry-type system developed in the Early Eocene (52.3Ma) and is related to the emplacement of a composite intrusive stock of calc-alkaline quartz-diorites and tonalites into an Upper Cretaceous to Eocene volcano-sedimentary sequence. Results of drilling have shown that mineralisation is essentially restricted to the GMS stock and contains 20Mt minable ore at an average grade of 0.25% Cu, 0.45g/t Au and 65ppm Mo. The composite GMS stock and the surrounding area were mapped and sampled for mineralogical and geochemical investigation. K-Ar ages were obtained on one basalt body and four plagioclase-amphibole±quartz - phyric intrusions which have a similar mineralogy and texture to the igneous units of the composite GMS pluton. 7 specimens of the GMS complex and 36 of the country rock were analysed for major and trace elements by the XRF method. Furthermore, the concentration of the rare earth elements in 38 GMS and 23 country rock samples was determined by a solution ICP-MS technique and ⁸⁷Sr/⁸⁶Sr and/or ¹⁴³Nd/¹⁴⁴Nd in 24 representative igneous rocks were measured by TIMS. The source, the nature of the parental magmas and the magmatic evolution of the igneous country rocks and the composite GMS system were investigated and the effects of alteration on the major and REE composition of the hydrothermally altered rocks of the composite GMS pluton were examined. The attempt to date the sedimentary formations in the area by palynological techniques and fitting of the ⁸⁷Sr/⁸⁶Sr measured on four limestone samples to the ⁸⁷Sr/⁸⁶Sr versus age curve of seawater was unsuccessful. The composite GMS stock consists of at least twelve individual, plagioclase-amphibole ± quartz-phyric intrusive units which are geochemically very similar, but only five intrusions are likely to be related by fractional crystallisation of one and the same magma. Only four of these intrusions were hydrous enough to cause secondary boiling and associated fracturing, alteration and mineralisation on cooling. The alteration assemblages were examined by the study of more than 200 thin sections under the microscope and applying the XRD technique to over 50 bulk rock samples and over 100 clay fractions. On the basis of crosscutting relations between different veins and overprinting of different alteration assemblages, the period of hydrothermal alteration/mineralisation of the pluton can be subdivided into early (K-silicate, CAS, propylitic), intermediate (chloritic, CAS) and late (phyllic, phyliic-argillic, argiilic) stage phases. Furthermore, a limited skarn and sporadic zones of calc-silicate minerals developed in the carbonate host rock. During the early stage, a K-silicate assemblage (biotite, albite, K-feldspar) formed in the core and the deeper parts of the system and is followed outwards into the periphery by a zone of CAS (chlorite, albite, sericite, anhydrite) and finally propylitic (chlorite, albite, epidote) alteration. During the intermediate stages of alteration, chloritic alteration overprinted the K-silicate assemblage in the centre of the system and CAS alteration advanced onto the core. During the late stage, phyllic (sericite/illite, quartz, pyrite), phyliic-argillic (sericite/illite, clay, quartz) and argiilic (kaolinite, quartz) alteration developed mainly in the periphery of the core and in those parts of the system which were most intensely stockworked by early veins. K-silicate alteration is associated with an opaque mineral assemblage including essentially chalcopyrite-pyrite-magnetitemolybdenite, CAS and chloritic alteration with chalcopyrite-pyrite-magnetite-hematite-moiybdenite and phyllic, phyllic-argillic and argiilic alteration with pyrite-molybdenite ± chalcopyrite. The evolution of the hydrothermal fluids responsible for alteration and mineralisation on cooling can be tracked by the observation of the stable mineral assemblages and may be modelled in an a(K⁺/H⁺) versus a(Na⁺H⁺) diagram for the Na₂O-K₂O-Al₂O₃-SiO₂-H₂O silicate system and a f(S₂) versus f(O₂) diagram for the Fe-Cu-sulphide-oxide system. A fluid inclusion study showed that two high temperature fluids were present in the system. Type I inclusions contain halite-bearing, metal rich, hypersaline (33-55wt% NaClequiv) fluids and type II inclusions accommodate saline, metal-bearing, gas-rich fluids (< 24.9wt% NaClequiv)- The homogenisation temperature of the type II inclusions suggests that the minimum temperature of trapping may have exceeded 594°C. The dense, hypersaline fluid (type I) was probably responsible for most of the Au and Cu mineralisation, whereas the low-density, vapour phase (type II) may have been responsible for most of the Mo transport and deposition. A third type of fluid is recorded in two-phase, liquid-rich inclusions (type III) and has a variable salinity, mainly < 3wt% NaClequiv, and a homogenisation temperature < 380°C. This fluid was probably responsible for the late stage phyllic, phyllic-argillic and argiilic alteration and has a mixed magmatic and meteoric origin. The homogenisation behaviour of the type I brine inclusions suggest that the depth of emplacement may have exceeded 5km, the lack of greenschist facies minerals in the surrounding volcanogenic country rock however rather supports a maximum depth around 4km. Comparing the tectonic setting and the mineral occurrences in the Pontids with its eastern (Lesser Caucasus) and western (Sredno-Gorie province) extensions, the author concludes that there exists a potential for the discovery of further Au-rich, porphyry-type copper systems in the Eastern Pontids. These however are essentially restricted to Tertiary intrusions which were emplaced as a result of arc reversal on continental collision in the Tertiary.