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Secondary arsenic minerals from the Złoty Stok As-Au abandoned mine (SW Poland)

Siuda R., Macioch A.

Geological Quarterly

2018

Vol. 62, No. 4

925--940

Secondary arsenic minerals (SAM) formed recently in abandoned adits of the former Au-As mine at Złoty Stok (SW Poland) constitute two assemblages. The first consists of two types of scorodite, pitticite, kaňkite, hörnesite, picropharmacolite and minor amounts of jarosite and gypsum. Formation of the Fe arsenates took place under acidic conditions (pH ~3-4) as a result of löllingite, arsenopyrite and pyrite oxidation. Hörnesite and picropharmacolite crystallized as products of interactions between acidic arsenic-rich pore solutions with Mg-Ca carbonates from rocks that surround the ore mineralisation. The interaction of carbonates with acid pore solutions caused a rapid increase in pH that reached neutral or weakly alkaline values. The chemical compositions of hörnesite and picropharmacolite correspond well to their ideal compositions: (Mg3.17Ca0.07)Ʃ3.24(AsO4)1.90 8H2O and Ca4.31Mg0.92(HasO4)1.91[(AsO4)1.99(SO4)0.01]Ʃ2.00 11H2O, respectively. The second assemblage of SAM comprises exclusively the Mg-enriched erythrite [(Co1.66Mg1.03Ni0.28Ca0.05Zn0.02)Σ3.03(AsO4)1.99× 8H2O)] – annabergite [(Ni1.48Mg0.94Co0.66Ca0.12Fe0.01Zn0.01)S3.20AsO4)1.92× 8H2O] series. These minerals crystallized from slightly acidic (pH ~5–6) to neutral media. Dissolution of SAM and other secondary phases (e.g., schwertmannite) causes the release of arsenate and sulphate ions into mine waters. These ions can be reduced under anaerobic conditions by different strains of bacteria. The product of this proces is orpiment.