Summary:
The Krumovgrad region is located within the Eastern Rhodopes which comprises the eastern portion of a large metamorphic complex. Basement rocks in the Ada Tepe area consist of Precambrian and Paleozoic metasediments, gneisses, and amphibolites. The basement is unconformably overlain by Paleogene conglomerates, sandstones, siltstones and limestones of the Krumovgrad group that were deposited during rapid uplift of the metamorphic core complex.
At Ada Tepe, gold and silver mineralization is predominantly hosted within the Shavar Formation proximal to the unconformable listric fault contact or detachment with the underlying basement rocks of the Kessebir-Kardamos core complex. Sedimentary rocks within the Shavar Formation typically form laterally discontinuous lenses ranging from chaotic breccias to conglomerate to inter-bedded pebbly sandstone, siltstone, and marl to marl-argillite.
The dominant structure at the Ada Tepe mine is a “detachment fault” that separates the metamorphic basement rocks from the overlying mineralized sedimentary rocks and forms a 10° to 15° north dipping lower structural bounding surface to the deposit.
The Ada Tepe mine is a low sulphidation epithermal gold-silver deposit. High gold grades in association with electrum-bearing open-space fill colloform-banded and lattice-bladed silica-carbonate-adularia veins and hydrothermal breccias and the presence of sinter, suggest proximity to the paleosurface and a low sulphidation character.
Mineralization at Ada Tepe is subdivided into two types, based on the geometry and style of the mineralized zone, as follows:
• Initial stage of mineralisation hosted by a massive, shallow-dipping (15° north) siliceous body forming the hangingwall to the detachment and defining the contact between the core complex and overlying sedimentary rocks. This mineralisation is termed the “Wall Zone” by local geologists and displays multiple stages of veining and brecciation.
• Second phase of mineralisation represented by steep dipping veins that exhibit textures indicative of formation within an epithermal environment. These veins have a predominant east-west strike, crosscut the shallow-dipping siliceous Wall Zone mineralisation, and extend upwards into the sedimentary breccia unit above the Wall Zone. This phase of mineralisation has been locally termed the “Upper Zone”.
The initial stage Wall Zone mineralisation is interpreted to be associated with early silica flooding and relatively low gold grades. However, regions of the Wall Zone through which well-developed Upper Zone vein mineralisation passes are typically thicker, more intensely brecciated and contain epithermal vein and hydraulic breccia infill textures and associated high gold grades that are not present in regions where Upper Zone vein mineralisation is absent. These thick strongly continuous regions of high-grade Wall Zone mineralisation generally thin and diminish in grade away from and between regions of well-developed Upper Zone vein mineralisation.
Typical epithermal textures present at the Ada Tepe deposit include the following:
• Crustiform and colloform banding;
• Chalcedonic banding;
• Bladed silica replacement textures after carbonate;
• Compositionally zoned crystals;
• Hydraulic breccia textures;
• Late-stage carbonate veins.
The textural style and grade of mineralisation at Ada Tepe, high grades in association with open-space fill textures, such as bladed silica replacement after carbonate (i.e. evidence of boiling), hydrothermal breccias and also the presence of sinter material, suggests proximity to the paleo surface and a lowsulphidation nature of mineralisation.
The Ada Tepe deposit is approximately 600 m long along strike (north-south), and 300–350 m wide (east-west). The dominant structure at the Ada Tepe deposit is the detachment structure that separates the Kessebir core complex rocks (basement) from the overlying sedimentary rocks, which forms a 10–15° north-dipping lower structural bounding surface to the deposit. The deposit is bound to the north and south by approximately northeast-southwest striking, steep dipping faults.
Mapping and structural data from diamond drill core indicate that the latter stages of movement along the detachment structure at Ada Tepe post-dates emplacement of epithermal mineralisation and were responsible for translation of the hanging wall sediments (and deposit) towards 030°. Consequently, any basement feeder structures are interpreted as lying to the south and may have been eroded or are concealed beneath later graben sediment fill.
Gold precipitated in veins where the mineralisation observed macroscopically consists mainly of silica, quartz, carbonate (calcite, manganese-rich), adularia, and some opaque minerals as chalcopyrite and pyrite.
The veins occur in two domains: the “Wall Zone” and the “Upper Zone”.
The term “Wall Zone” describes a siliceous body shallowly dipping 15° north, forming the hangingwall of the detachment and defining the contact between the core complex and the overlying sedimentary rocks. The “Upper Zone” is the rest of the sedimentary cover above the Wall Zone.
The veins appear as a series of east-west sub-vertical structures which extend through the Wall Zone and upwards to the Upper Zone. A series of north-south gently dipping sub-horizontal veins form the Wall Zone.