JultikaUniversity of Oulu repository

The Palaeoproterozoic Johan Petersen Intrusive Centre (JPIC) formed as a continental arc at the North Atlantic Craton margin during the formation of the ca. 1.9 Ga Nagssugtoqidian Orogen (NO) in SE Greenland. Magmatic Ni-Cu deposits in the coeval Trans-Hudson and Svecofennian Orogens (THO and SFO) make the NO the primary target for Ni-Cu deposits in Greenland. Using mapping, petrography, geochemistry, and multiple sulphur isotope geochemistry, this thesis deals with the formation of the JPIC and related mineralisation, and compares it with previously discovered Ni-Cu mineralisation in the NO and Trans-Hudson and Svecofennian Orogens. The JPIC formed from an early series of dioritic to granitic magmas intruding into paragneiss under granulite facies conditions. A mafic series of gabbroic rocks intruded into both the paragneiss and the earlier felsic series rocks. Late mafic-ultramafic sheets and magmatic breccias intruded predominantly into the mafic series rocks. Two episodes of intrusion by mafic-ultramafic stringers happened when the felsic and mafic series rocks were semi-solid. Late granite likely formed from back-veining of residual melts. Uplift during orogenesis caused retrograde amphibolite facies metamorphism, and late hydrothermal alteration of both intrusive rocks and their mineralisation took place under temperature conditions corresponding to amphibolite- to mid greenschist facies. The mineralised rocks in the JPIC occur as contact mineralisation between the intrusive rocks and the adjacent paragneiss, as mineralisation in the stringers mingling with the mafic series rocks, and as accumulated sulphide liquids in the later sheets and breccias. In the contact style mineralisation, sulphides are disseminated, network-like or semi- massive and show S isotopic compositions indicating a Palaeoproterozoic sedimentary sulphur source. Ni and Cu contents in the sulphide phase are low. Sulphides in the mineralised stringers and mafic-ultramafic sheets and breccias are disseminated to network-like, and have mantle-like isotopic compositions of sulphur. The ultramafic sheets and breccias have higher Ni-Cu contents and tenors and currently form the primary targets. The JPIC mineralisation is similar to that in the Kotalahti-Vammala belt of the SFO, and although sub-economic at the moment, possible existence of deeper-seated sulphides would give the area an upside potential for more discoveries.