New lithological approach for the 3D Geological modeling of the Upper Cretaceous Sakdrisi Gold Copper Epithermal deposit in Bolnisi District, Lesser Caucasus, Georgia

The Bolnisi mining district in Southern Georgia belongs to the central segment of the Tethyan orogenic and metallogenic Belt. It is located in the eastern extremity of the Turkish Eastern Pontides and the northern part of the Jurassic-Cretaceous sedimentary-volcanic Somkheto-Karabagh belt of the Lesser Caucasus.  The Late Cretaceous (~87–71 Ma) bimodal explosive volcanism in this region resulted in mafic and felsic rock types, the latter being a major host of the ore deposits and prospects, and being defined locally as the felsic Mashavera and Gasandami suites. The Late Cretaceous Sakdrisi gold-copper epithermal deposit is a major deposit in the Bolnisi district where different projects are going on. The Late Cretaceous sequences in this region are subdivided into six volcanogenic suites and corresponds to Cenomanian and Maastrichtian in age.

To better understand what controls the mineralization and the distribution of ores in the deposit, a 3D geological model have been built (in the Leapfrog Geo program) considering the distribution of lithological unit boundaries and structures at the Sakdrisi (Sakdrisi 4 and 5) epithermal deposit, based on outcrop observations, detailed mappings, and interpretations of drill core data.  The ore types and their distribution in different levels between the Sak4 and Sak5 have always been a matter of question. Our new lithological approach, which included genetic naming of rocks, allowed us to perform correct paleovolcanological reconstructions together with structure in context and obtain a complete representation in the 3D model of the displacement of these stratigraphic units and the distribution of mineralization within them. The following lithological units were identified on Sakdrisi deposit: the upper non-mineralized volcano-sedimentary complex (UVSC) - Ignimbrite (IGN) and cross-cutting andesite-basaltic (AN_BA), and rhyodacite (RHD) dikes and lower mineralized (LVSC) volcano-sedimentary complex – pumice tuff (PT), massive fine-grained tuff (MFT), pumice tuff with the transition to fine-grained intervals (PTTI), layerd tuff (LT), ignimbrite like tuff (IGNT). The upper (UVSC) and lower (LVSC) complexes are separated by the thrust fault zone (central fault) including the sedimentary formation (SF) which is also non-mineralized. The central fault represents the upper limit of the mineralized zone. The lower base of the mineralization zone in Sak.4 is observed in some drill holes in layered tuff where the gypsum veins occur and also in ignimbrite-like tuffs. The explosive breccia pipes cross-cut all these lower mineralized lithological units on Sak.4 and Sak.5.

The high-grade ore zones are mainly localized in massive fine-grained tuffs, which are brittle and strongly silicified and of course connecting with the explosive breccia in matrix and also in the clasts. These tuffs easily succumbed to fractures developed by faults and fractures together with the explosive breccias, creating a favorable environment for the movement of ore-forming fluids and, consequently, the precipitation of metals. Two major trends of faults are recognized in the Sakdrisi deposit where dominant in the SE and NE directions. Both lithological and structural control on mineralization was convinced in the Sakdrisi deposit.