Aluminium phosphate-sulphate minerals in the Chelopech Cu-Au deposit: Spatial development, chemistry and genetic significance

Aluminium phosphate-sulphate (APS) minerals of the svanbergite-woodhouseite (Sv-Wh) solid solution series have been determined in the advanced argillic and sericitic zones of alteration, as well as along with the Cu-As-S ore mineralization in the Chelopech high-sulphidation epithermal deposit. In the advanced argillic zone, Sv-Wh phases are documented down to a depth of 2000 m as known from 2 deep drill-holes. On the Earth surface (elevation 750 m) these minerals are associated with quartz, dickite, kaolinite, alunite, pyrite and anatase. From level 450 m to level 250 m, the alteration assemblage is composed of quartz, dickite, kaolinite, nacrite, pyrite, Sv-Wh and anatase. At deeper levels, diaspore, pyrophyllite, alunite and zunyite occur together with Sv-Wh. The temperature of formation of the advanced argillic alteration assemblages is considered to range from less than 200°C in the upper levels to more than 300°C in the deepest levels of the hydrothermal system. In the sericitic zone, these minerals are associated with quartz, illite, pyrite, halloysite, anatase and apatite. The relationships with apatite indicate that at least a part of Sv-Wh phases are formed by dissolution and replacement of apatite in a low pH environment. As a part of the Cu-As-S ore mineralization, these minerals are closely associated with enargite, pyrite, tennantite, chalcopyrite and barite, which suggest similar Eh-pH conditions of their formation. The Sv-Wh phases with general formula (Ca,Sr)Al3(PO4,SO4)2(OH,H2O)6 range in composition from high Ca- to high Sr-varieties without reaching pure end-member composition. Minor amounts of Ba and K complete their chemistry. The crystals have clear chemical zoning due to substitution among Sr and Ca. Svanbergite-woodhouseite phases and zunyite are described for the first time in the Chelopech Cu-Au deposit