Mineralogical, chemical and micromorphological studies of the argillic alteration zone of the epithermal gold deposit Ovacik, Western Turkey: Tools for applied and genetic economic geology

The outward appearance of argillic alteration zones, forming a transition between the ore zone and the barren subvolcanic, volcanic and volcaniclastic host rocks may be rather dull. Using mineralogical and chemical methods may change our picture of a monotonous argillic alteration zone. Of particular interest are those techniques suitable for clay minerals (IR, XRD, and SEM-EDX), trace element analyses in the ppb range to decipher the chemical composition and micromorphological studies of the non-phyllosilicates zircon, pyrite, and barite and Fe-Ti compounds, present as accessory minerals in this alteration zone. A complex shallow hydrothermal system gave rise to a telescoping of four stages in the Ovacik gold deposit, Western Turkey. They reflect a change of the physical-chemical parameters of an epithermal system and furnish evidence of different lithogenetic and mineralogical processes telescoped into each other as the magmatic-hydrothermal system was operative under different geodynamic regimes during the Neogene in the Aegean region. The incipient stage 1 affected the andesitic parent rocks leading to a conversion of biotite into Fe-Mg chlorite and smectite between 300 degrees C and 250 degrees C. This type of argillitization followed the emplacement of the volcanic and volcaniclastic rocks and may be called as autohydrothermal with no direct effect on the ensuing gold mineralization. The formation of pure illite during stage 2 around 200 degrees C between pH 6 and 5 paved the way into the hydrothermal system by providing silica to create siliceous caps and layers in and around the deposit. The argillitization of stage 3 occurred in the wake of another heat pulse mirrored by the habit of zircon. Smectite-illite mixed layers formed in the T range between 200 degrees and 180 degrees C under moderately acidic conditions. While the phyllosilicate assemblage is subjected only to minor intracrystalline changes during this stage, non-phyllosilicate minerals significantly vary, reflecting latent-economic mineralizing processes driven by a variety of geodynamic processes during the Neogene. The Ag-Mo-Tl sulfide-arsenide association indicates a hidden porphyry system at depth during the Oligocene grading into a Carlin- or Alsar-like mineralization at a more shallow level which is conducive into the Au-silica mineralization mined at Ovacik. The REE-Ba-bearing mineralization, devoid of any gold, is representative of a post-subduction rift stage in the Aegean region during the Pliocene when alkaline and carbonatitic magma was vented. It marks the transition into stage 4 when a considerable drop to 130 degrees C along with decrease in the pH down to as low as 3 affected not only the mineralized parts of the epithermal system but also the barren parent rocks. Technical terms used to classify a shallow hydrothermal system as high sulfidation or low sulfidation, acidic or adularia-type are far from clarifying the situation in an area like that and might rather lead to conceal the true nature of a hydrothermal system. Therefore, latent economic systems related to particular geodynamic processes remain undetected whereas disentangling the telescoped argillaceous zones in terms of mineralogy, chemistry and micromorphology may not only broaden our knowledge of the origin of the deposit but direct our thoughts also to exploration targets hitherto drawn too little attention to in the area under consideration. (C) 2014 Elsevier B.V. All rights reserved.