Geochemistry of alunite from the Pesovets epithermal system, Central Srednogorie Zone, Bulgaria

The chemical composition of alunite and APS minerals from well expressed advanced argillic alteration zone of Pesovets epithermal system is studied coupled with the alunite isotope geochemistry. Alunites from the Pesovets are mixed K-Na to natroalunites with Na/K ratio varying from 1:1.5 to 4:1. There are always small admixtures of P, Ca, Sr, Ba and LREE, mostly up to 0.1 apfu, sometimes up to 0.2 apfu. Rare mixed alunite–APS phases are also observed. APS minerals are scarce in alunite zone but they are found in kaolinite advanced argillic altered rocks. APS minerals are solid solutions (s.s.) between svanbergite and woodhouseite with Ca/Sr ratio from 0.7 to 2.3. Ca-rich phases correspond to woodhouseite while Sr-dominate phases are close to svanbergite. There are permanent admixtures of Ba, K, Na, and less LREE, F and Cl. LA-ICP-MS study of alunite from Pesovets shows permanent presence of P, V, Ga, Sr, Sn, Ba, La, Ce, Pb. An increase of Sn, V, Ga, Pb, La, Ce content in alunite in the direction to the highest parts of the system is registered.  Sulphur and oxygen stable isotope data suggest magmatic-hydrothermal origin of alunite with low contribution of external fluid at temperature around 250 °C and high H₂S/SO₄ ratio between 2 and 5. Our study gives reason to conclude that the Pesovets epithermal system has a potential for development of high-sulphidation epithermal gold mineralization, which can be proved with modern exploration technics.