Summary:
The formation of the Efemçukuru gold deposit in western Anatolia coincided with Miocene extension, magmatism and hydrothermal activity including the formation of several other significant gold-rich porphyry and epithermal deposits in the region. In detail, Efemçukuru can be classified as an intermediate sulfidation epithermal vein deposit due to its high-base metal content and the Mn-rich nature of the veins, analogous to intermediate-sulfidation epithermal veins of Mexico. The dominantly NE dipping Efemçukuru veins formed within faults that had east side down normal-dextral (right lateral) shear sense.
The epithermal veins at Efemçukuru contain multiple ore shoots with zoned mineral and metal distributions and a complex paragenesis. The 2 km long Kestanebeleni vein hosts the major gold resource at Efemçukuru and comprises four ore shoots: SOS, MOS, NOS and KBNW.
The SOS outcrops for 500 m and dips between 45° to 70° to the northeast. The shoot has a down dip extent of approximately 500 m. The southern half of the shoot consists mostly of a single vein whereas in the central part of the SOS, the vein splays at depth. A second larger splay is developed nearer the surface, close to the mid-point of the SOS, and continues for approximately 230 m to the north end of the shoot. Where the SOS consists of a single vein, its thickness generally ranges from 3 m to 5 m but can locally reach more than 10 m. Gold mineralization is not generally distributed across the whole vein, but more typically occurs as high grade (> 50 g/t × m) discrete zones within the vein that have a steep northerly dominant plunge and shallow to moderate southerly secondary plunge. Where the vein breaks into splays, the splays are generally narrower, with thicknesses of 1 to 2 m. Additionally, significant stockwork type mineralization is locally present between the vein splays where they cut hornfels. Limited amounts of stockwork mineralization occur where the vein hanging wall consists of phyllite, but these instances are more restricted in size and continuity.
In the MOS area, the Kestanebeleni vein strikes 320° over approximately 230 m and has an average dip of 60° to 65° to the northeast. High-grade gold mineralization in the MOS forms a steeply plunging central shoot and is hosted entirely within hornfels. The shoot has a narrow surface expression of approximately 3 m, but expands with depth to more than 20 m. A single splay diverges from the main vein in the central part of the shoot and an extensive zone of stockwork mineralization occurs between the two veins and extends into the hanging wall. The stockwork zone is traceable along strike for approximately 75 m and is best developed between 550 m and 600 m elevation, with some extensions above and below these elevations.
The NOS is separated from the MOS by an inflection that coincides with an additional 25° westward bend in the strike of the Kestanebeleni vein. Here the vein strikes approximately 300° and the vein flattens to a 45°dip. The NOS outcrops intermittently for approximately 200 m along this bearing before it pinches out. Another inflection occurs between the NOS and KBNW, with the latter striking nearly east-west over a 300 m strike length before reverting to a 300° strike orientation. KBNW is typically narrower than the other three main shoots (< 3 m) and comprises two vein splays for much of its length.
The Kestanebeleni vein has a distinct mineralogical zonation with the proportions of Mn-silicate and carbonate and sulfide vein material varying across the vein system. Mn-rich vein assemblages are most abundant in the upper portions of the SOS, whereas the sulfide content of the MOS and NOS, particularly at depth, is much higher. The increased sulfide content coincides with an increase in Pb and Zn in the MOS and NOS and the Ag / Au ratio is typically higher at depth and on the periphery of the veins.
Mineralization
Two major, broadly NW-SE striking epithermal vein systems, namely Kestanebeleni and Kokarpinar, occur at Efemçukuru. They have strike extents of approximately 2 km and 4 km respectively. Both vein systems cut the rhyolite dikes, calc-silicate alteration and unaltered phyllite and schist. At surface, the veins are up to 5 m wide and occur as either multi-phase, brecciated, banded crustiform-colloform or as massive quartz-rhodochrosite-rhodonite-sulfide veins. Stockwork veins are commonly associated with the major epithermal veins and occur both in the footwall and hanging wall, as well as between vein splays, in zones of at least 3 m wide. The stockwork zones manifest as thin (< 3 cm) Fe-oxide-stained quartz veinlets cutting strongly to moderately illite and smectite altered phyllites over widths. Minor (< 2%), fine-grained disseminated pyrite characterizes the stockwork zone. The bulk of the epithermal veins at surface contain Mn- and Fe-oxide coated, banded quartz-rhodochrosite-rhodonite with pyrite-galena-sphalerite. Evidence that the major veins exploited fault zones includes the common presence of lithified cataclasite proximal to the veins and clasts of cataclasite within the veins. Furthermore, the vein-hosting fault network was likely coeval with the calc-silicate alteration event because calc-silicate (epidote) partly cements the cataclasite and the distribution of the hornfels broadly maps out the fault-vein network.
In detail, the two main veins have complex geometries with multiple shoots and splays. The Kestanebeleni vein is divided into several ore shoots: SOS, MOS, NOS and KBNW. The southern portion of Kestanebeleni system, including the SOS and MOS, dips moderately to steeply (45-70°) to the east-northeast. In the northern zones at the NOS and KBNW, the vein has a more moderate dip (55-60°) to the north-northeast (20° to 25°). In the footwall to the Kestanebeleni vein occur two similarly oriented but narrower veins called the Bati veins. The Kokarpinar vein has a more consistent northwesterly strike and dips moderately to the northeast. In the furthest northwest and southeast extent of Kokarpinar the veins display a complex horsetail geometry with numerous splays. The Kokarpinar vein is also divided into four ore shoots: Kokarpinar South (KPS), Kokarpinar Middle (KPM), Kokarpinar North (KPN), Kokarpinar Northwest (KPNW).
The thickest vein segments typically coincide with areas where the controlling fault system has a more northerly strike and steeper dip. Northwest-striking fault segments lack veins or have thin vein material. Such patterns are consistent with an oblique east side down normal-dextral (right lateral) shear sense on the overall fault-vein network, with northwest striking segments accommodating a dominantly right lateral component. North-striking steeply dipping segments accommodating dominantly normal and extensional shear sense leading to preferential vein development.