Multiple-stage hydrothermal alteration in porphyry copper systems in northern Turkey: the temporal interplay of potassic, propylitic, and phyllic fluids

Abstract

The Bakircay and Ulutas Cu–Mo prospects represent the first occurrences of porphyry mineralization to be described in Turkey.Differences observed in the two prospects in terms of hydrothermal alteration (in particular, alteration overprinting), igneous textures, abundance of xenoliths, breccia phenomena, and style and intensity of fracturing may relate to different levels of exposure within a model porphyry system, Bakircay representing the deep root zone of such a system and Ulutas reflecting much higher levels close to the apex of such a system, or may simply reflect different levels of emplacement.The alteration assemblages present at the Bakircay prospect lend themselves to a geochemical study of the temporal variations in the hydrothermal fluids responsible for single- and multiple-stage alteration–mineralization. The chemical changes involved during single-stage potassic alteration are related to amphibole breakdown and the deposition of hydrothermal biotite (and chalcopyrite). These changes are manifested in light rare-earth element (LREE) enrichment and heavy rare-earth element (HREE) depletion reflecting the high K+ and Cl− activity of the hydrothermal fluids. During propylitic overprinting of potassic alteration changes in whole-rock geochemistry relate to the destruction of biotite (both igneous and hydrothermal) and the formation of chlorite, epidote, calcite, and apatite. These changes result in the loss of ail rare-earth elements (REE) due to increasing fluid/rock ratios and further changes within the HREE relating to zircon stability and the deposition of new mineral phases, e.g., epidote. Conversion of preexisting alteration types lo the quartz–sericite–pyrite ± rutile, calcite assemblages, typical of phyllic alteration, results in the loss of all elements not accommodated in these phases. The high fluid/rock ratios and low pH of the fluids cause progressive leaching of all REE, particularly the lightest (La and Ce).