Geochemistry of advanced argillic altered rocks in the area of Breznik, Western Srednogorie Unit (Bulgaria)

The distribution and behaviour of major and trace elements in the main zones of hydrothermal alteration (propylite, sericite and advanced argillic) in the area of Breznik Town have been studied, coupled with the chemical composition of alunite group minerals and their isotope geochemistry. The content of Sr decreases in propylite and especially in sericite rocks, and increases significantly in advanced argillic altered (AAA) rocks, which is connected with the formation of aluminium phosphate-sulphate (APS) minerals. Rubidium has high content in sericitic rocks, whereas the element is almost depleted in AAA rocks. The Rb/Sr ratio increases in sericitic rocks and strongly decreases in AAA rocks. Zirconium, Ti, Cr, V, Ga, Hf, Nb, Ta, Th, U and partially Sn and Ba have comparatively inert behaviour during alteration. Manganese, Zn, Ni, Co, Cs and Y are mobile in different degree, with a trend toward complete extraction in the most altered rocks. Arsenium concentrates in AAA rocks, while Li is concentrated only in kaolinite AAA rocks. LREE are comparatively inert in AAA rocks, while MREE and HREE are very mobile, especially in alunite rocks. Alunite has Na-K composition, with permanent admixtures of Ca, Sr, Ba and LREE. APS minerals themselves are woodhouseite and svanbergite-woodhouseite solid solutions (s.s.). Stable isotopes of alunite are characteristic for magmatic-hydrothermal alunite, whereas δ³⁴S of jarosite is intermediate between δ³⁴S of alunite and sulphides, which confirms its supergene origin. All these features define a high-sulphidation epithermal environment, and a possible porphyry mineralization at deeper part of the system is suggested.