Geology, alterations and mineral chemistry of the Tjårrojåkka Fe-oxide Cu-Au occurrences, northern Sweden

Abstract: The Tjårrojåkka area is located about 50 km WSW of Kiruna, northern Sweden, and hosts one of the best examples of spatially related Fe-oxide Cu-Au occurrences (the Tjårrojåkka-Fe and Tjårrojåkka-Cu). The bedrock, depositional environment and tectonic evolution of the area were studied through petrological, geochemical and geophysical-petrophysical investigations. The bedrock is dominated by intermediate and basic extrusive and intrusive rocks. The intermediate andesites and basaltic andesites are cut by diabases which acted as feeder dykes for the overlying basalts. The intrusive rocks range from gabbro to quartz-monzodiorite in composition. The area is metamorphosed to epidote-amphibolite facies and has been affected by scapolite, K-feldspar, epidote and albite alteration that is more intense in the vicinity of deformation zones and mineralisations. Based on geochemistry the andesites and basaltic andesites are similar to the Svecofennian Porphyrite Group intermediate volcanic rocks, but have also features common with the intermediate volcaniclastic unit in the underlying Kiruna Greenstone Group. Chemically the basalts and diabases have the same signature, but cannot directly be correlated with any known basaltic unit. Some of the samples have characteristics comparable to the basalts of the Kiruna Greenstone Group. Whether the volcanic sequence at Tjårrojåkka represents the Porphyrite Group or is part of the greenstones could not be unequivocally determined without geochronological data. Three events of deformation have been distinguished in the Tjårrojåkka area; the first one involving NW-SE compression creating NE-SW-striking steep foliation corresponding with the strike of the Tjårrojåkka-Fe and Cu bodies, followed by the creation of an E-W deformation zone. Finally a second compressional event resulted in folding and the formation of a NNW-SSE striking and gently dipping structure possible related to thrusting from SW. The Tjårrojåkka apatite-magnetite ore (52.6 Mt of iron ore @ 51.5% Fe) is a blind ore consisting of a massive magnetite core surrounded by an ore- breccia containing low-grade Cu-mineralisation. Apatite, amphiboles and carbonate occur disseminated and as veins within the massive ore and in the wall rock. The Tjårrojåkka-Cu mineralisation is located 750 m from the Tjårrojåkka-Fe and contains 3.23 Mt ore @ 0.87% Cu. The main ore minerals are chalcopyrite and bornite occurring both disseminated and in veinlets. Minor pyrite, molybdenite and gold have also been observed. The host rock has been affected by strong albite, scapolite, amphibole and K-feldspar alteration. The alteration assemblages at Tjårrojåkka are highly variable with several of the alteration minerals occurring in several generations and settings, and with multiple reactivations of already existing veins and overlapping alteration stages indicating a complex, long history of fluid activity in the area. Similarity in alteration minerals and paragenesis in the iron and copper mineralisation is described in terms of whole rock geochemistry, mineral chemistry and paragenesis. This may partly be explained by the common host rock to the mineralisations, but indicates also similarities in fluid composition. Within the massive magnetite ore apatite, tremolite and carbonate veinlets fill fractures probably formed during cooling of the magnetite body. The wall rock has been affected by extensive pervasive albite and plagioclase alteration. Scapolite occurs locally as porphyroblasts and later veins. The albitised and scapolitised rocks are overprinted by pervasive K-feldspar alteration and veins of K-feldspar + Mg-hornblende ± titanite ± quartz ± magnetite ± sulphides along the foliation. Epidote is common in veins together with K-feldspar. Allanite occurs as an accessory mineral associated with epidote, otherwise REE-minerals are rare. Carbonate and zoelites were the last phases to form in vacancies. The area between the apatite-iron and copper bodies is strongly albite + magnetite altered. The footwall of the copper body is characterised by pervasive albite alteration spatially associated with magnetite and apatite veins cut by later carbonate veinlets. Scapolite (porphyroblasts and veins) is formed in an early stage in the hanging wall overprinted by pervasive K-feldspar alteration. Amphiboles (tschermakites, Mg-hornblende and actinolite) occur in several generations as porphyroblasts, in veins on its own, or together with K-feldspar ± titanite ± quartz ± carbonate ± chalcopyrite ± bornite. Epidote, REE- carbonate, zeolites and fluorite are the latest alteration phases in the copper mineralisation. Ba, Cl, S and F are enriched in the alteration minerals in the Tjårrojåkka occurrences. Barium-rich varieties of K-feldspar (max. 3.5% BaO) occur in the Cu-mineralised breccia surrounding the apatite-magneitie body indicating high concentrations of Ba in the hydrothermal fluids. Absence of sulphate in the fluids probably caused the formation of Ba-feldspars instead of barite. Scapolite shows a trend with more Cl-rich varieties around the magnetite body gradually getting more SO3 and CO2-rich in the Cu-mineralisation. The presence of accessory barite in the copper mineralisation also indicates that the SO3 content in the fluids were higher than in the iron ore. The biotites are rich in Ti while Cl and F contents are more moderate and do not show great variation in different parts of the systems. All amphiboles are Ca-rich ranging from tschermakites, Mg-hornblende to actinolite and tremolite. The apatites are F-dominate with higher Cl content in the apatite- iron ore than in the copper occurrence. Overall the alteration minerals related to the apaite-iron ore are more rich in Cl and Ba than the ones in the Cu-mineralisation that show higher contents of F, SO3 and CO2.