The geology of the Rakkurijärvi Cu-(Au) prospect, Norrbotten: a new iron oxide-copper-gold (IOCG) deposit in Northern Sweden

The Rakkurijärvi prospect consists of a group of mineralized magnetite and lithic breccias within the ca. 2.05- to 1.90-Ga Proterozoic supracrustal sequence of the Kiruna district, northern Sweden. Potentially economic grades of Cu and Au, largely in the form of chalcopyrite and other sulfide assemblages, are hosted in brecciated magnetite and metavolcanic rocks. The extent of the mineralization is currently open, both downdip and along strike. The deposit was discovered through an integrated geophysical and geochemical program focused on iron oxide-copper-gold (IOCG)–style mineralization. It is hosted by brecciated greenschist facies metavolcanic rocks within and adjacent to an east-northeast–trending shear zone. The dominant characteristics of the deposit are consistent with the IOCG class and include magnetite and lithic breccias hosted in a metavolcanic sequence, with matrices of albite, actinolite, and calcite surrounded by halos of sodic (albitescapolite) and potassic (scapolite-K-feldspar-biotite) alteration. A distinctive accessory mineral assemblage includes apatite, titanite, and allanite. The paragenesis and textural evolution of the deposit includes early Narich alteration accompanying massive magnetite alteration. The Na-rich alteration is overprinted by potassic alteration (also associated with magnetite), although the paragenesis is complex and multiple generations of both sodic and potassic alteration are recognized. Alteration of lithic clasts to magnetite confirms a metasomatic origin, as opposed to an orthomagmatic origin, for the magnetite mineralization. Re-Os analyses of two separates of molybdenite intergrown with magnetite, interpreted as cogenetic with the sulfide assemblage, yield mineralization ages of 1853 ± 6 and 1862 ± 6 Ma. Reconnaissance bulk-rock chemistry of the host volcanic rocks is consistent with an intermediate volcanic protolith, but much of the original character of the rocks is masked by albitization and incipient iron, sodic, and potassic alteration. The data also indicate significant element mobility during metasomatism and, in particular, the addition of Ti to the rock mass in biotite and as titanite. The compositions of secondary minerals are consistent with alteration and mineralization caused by highly saline fluids of relatively low F activity. The stable isotope characteristics of calcite, with δ18OSMOW ranging from 9.43 to 19.89 per mil and δ13CPDB ranging from –11.69 to +4.88 per mil, suggest that the fluids of the calcite and sulfide stage were derived from a magmatic source but had interacted extensively with local sedimentary and volcanic rocks.