Ore mineralogy and formation conditions of the Pirunkoukku gold occurrence (Finland)

The Pirunkoukku gold occurrence is localised within intensely altered felsic to intermediate volcanic rocks in the contact zone of a granite massif belonging to the Central Finland complex. A chalcopyrite association and an arsenopyrite association represent the two major mineral assemblages. The chalcopyrite association is dominantly composed of chalcopyrite, pyrrhotite, magnetite, and marcasite, as well as minor and rare minerals including Ag-bearing chalcopyrite, cubanite, native bismuth, joseite-B, matildite, schapbachite, hessite, molybdenite, arsenopyrite, sphalerite, electrum, auroan silver, an Ag₄Sb phase, and a Ni-Ag-Sb phase. Arsenopyrite and loellingite are the dominant minerals in the arsenopyrite association. Minor and rare minerals present in this association include chalcopyrite, electrum, maldonite, aurostibite, the Au_0.53Bi_0.47 phase, bismuth tellurides, and native bismuth. The arsenopyrite association formed in a temperature range of 550–270 °C. Due to the presence of cubanite, a formation temperature of 250–300 °C is estimated for the chalcopyrite association.