The geochemistry of hydrothermal alteration at the Bakircay porphyry copper prospect, northern Turkey.
thesisposted on 19.11.2015, 09:03 by Richard P. Taylor
Isotopic data for the Bakircay granodiorite porphyry, give a Late Eocene age for the development of the porphyry copper system. They suggest a close temporal and genetic relationship between igneous and hydrothermal activity, and indicate that magmatic-hydrothermal fluids produced potassic alteration and that meteoric fluids enriched in radiogenic 87 Sr were responsible for propylitic alteration. The granodiorite porphyry is petrologically similar to porphyry copper-related intrusions from island arc and continental margin settings, which form a group with initial 87Sr/86Sr ratios of less than 0.7043, representing magmas produced in tectonic environments lacking any important component of old (i.e. Precambrian) continental material. 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 LREE enrichment and 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 all 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 pre-existing alteration types to the quartz-sericite-pyrite +- rutile +- calcite assemblage, 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).