The Alteration Chemistry of a Multi-Stage Volcanic Hosted Epithermal Gold Mineralization; Kısacık Case (Biga Metallogenic Province of NW, Turkey)

The Biga peninsula (NW, Turkey) hosts numerous polymetallic mineral deposits including skarns, porphyry Cu-Mo, vein type Pb-Zn, and epithermal Au–Ag. These mineralizations are mainly associated with widespread high-K magmatism in the Oligo-Miocene period. Of these the epithermal veins which have economic grade Au-Ag concentrations are generally found in volcanic rocks such as dacites, andesites, and pyroclastic rocks. The hydrothermal aureoles' interaction with highly permeable volcanic rocks caused voluminous alteration zones with potential precious metal enrichments. The Kısacık (Biga Metallogenic Province of NW, Turkey) area can be considered as a promising precious metal mineralization zone due to the high-intensity alterations observed in Miocene volcanic rocks (dacites and pyroclastic rocks). In the study area, the hydrothermal activity can also be seen in basement metamorphic (calcschists) and ophiolitic (basalts - dolerites) rocks. The identified hydrothermal alterations in the field are sericitic and argillic alterations with intense silicifications. The silicifications are mostly observed in cracks and joint fractures of volcanic rocks in the form of veinlets and cavity fills. Major oxide-Trace Element and ICP-MS Ag-Au-PGE’s analysis was performed on the samples taken from the field. The calculated alteration indexes (AI) are ranging between %82-99.2 and Chlorite-Carbonate-Pyrite Indexes (CCPI) values are between %73-97 according to major oxide values of altered volcanics. The Au content varies between 0.03 – 21.34 ppm. When the hydrothermal trend is examined in the AI-CCPI diagram, the altered high grade volcanics shows a chlorite-sericite-pyrite alteration trend. These implications are parallel with field observations. The less altered volcanics are located within the andesite/basalt field. Highly altered volcanics are located in the "ore center" area of the diagram. The calculated alteration index (AI&CCPI) values of the volcanic rocks are high, mainly caused by the removal of mobile cations such and a significant enrichment in immobile cations such as Al and Fe. In addition, a silica-carbonate alteration zone around ophiolitic rocks was observed and gold content of up to 0.16 ppm was detected in samples from this area which indicates a listwanite-type Au deposit.

Keywords: Alteration indexes, Miocene Volcanics, Hydrothermal Deposits, Biga Peninsula, Turkey