THERMODYNAMIC AND HYDROGEOCHEMICAL FORMATION CONDITIONS OF BROCHANTITE AS A CRYSTALLINE HYDRATE: A CASE OF THE UDOKAN COPPER DEPOSIT

Abstract

Brochantite Cu4SO4(OH)6 is an abundant secondary (mostly anhydrous) mineral in oxidation zone of copper deposits. Brochantite as a crystalline hydrate Cu15(SO4)4(OH)22 · 6.6H2O [8] (or Cu 4(SO4)(OH)6 · 1.65H2O) has been found for the first time at the Udokan cupriferous sandstone deposit. The mineral was defined as cryogenic, i.e., the permafrost phase. The formation of crystalline hydrate was accounted for by the deficiency of free water in frozen rocks and high copper concentration in the mineral- forming solution. Later on, thermodynamic prerequisites were also considered responsible for this process. In terms of thermodynamics, if Cu : H2O = 4 : 1.65, this mineral may only be precipitated from a solution with a Cu concentration of >8.56 kg/l. As is known, mineralization of the most concentrated brines does not reach 1 kg/l.