Application of “Alteration indices” at the Elatsite porphyry-copper deposit: a reliable geochemical tool for identification of hydrothermal alteration zones at porphyry deposits

The Elatsite porphyry-copper deposit is one of the largest and well-studied deposits in Bulgaria. The mineralization and hydrothermal alteration are related to Late Cretaceous magmatic activity associated with the Late Alpine evolution of the area. The three of the most common alteration types are K-silicate, K-silicate-sericitic and quartz-sericitic alteration. The three alterations have complicated relationships and in most cases are superimposed on each other, which makes field studies and petrographic observations difficult. The geochemical data for altered rocks can provide useful information and further be used as a geochemical tool for accurate determination of the alteration. One way of representing this geochemical data is by calculating the „Alteration indices“. These indices represent ratios of the major elements oxides and here we present data for three Alteration indices (AI 1, AI 2 and CCPI). Our results suggest that the increase of the AI 1 and the decrease of the AI 2 and CCPI for the K-silicate, K-silicate-sericitic and quartz-sericitic alterations compared to the unaltered rocks are related to the gain of K2O and MgO and the loss of CaO and Na2O during the alteration process. The graphical construction of juxtaposition of two Alteration indices provides a useful tool for accurate distinction of the three most common alteration types of the Elatsite PCD.