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3. 4.2 The Pechenga Greenstone Belt

Author

Melezhik, V. A., Hanski, E. J., Melezhik, Victor A., editor, Prave, Anthony R., editor, Fallick, Anthony E., editor, Kump, Lee R., editor, Strauss, Harald, editor, Lepland, Aivo, editor, and Hanski, Eero J., editor

Published
2013
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14. The petrology and genesis of the Paleoproterozoic mafic intrusion-hosted Co-Cu-Ni deposit at Hietakero, NW Finnish Lapland

Author

Konnunaho, J. P. (Jukka P.), Hanski, E. J. (Eero J.), Karinen, T. T. (Tuomo T.), Yann, Y. (Yann), Makkonen, H. V. (Hannu V.), Konnunaho, J. P. (Jukka P.), Hanski, E. J. (Eero J.), Karinen, T. T. (Tuomo T.), Yann, Y. (Yann), and Makkonen, H. V. (Hannu V.)

Abstract

Cobalt is a highly sought-after metal due to its economic importance in many hightech applications. It is mainly obtained from sedimentary-hosted Cu-Co deposits and magmatic Ni-Cu deposits as a by-product. We describe a recently discovered Co-enriched Cu-Ni deposit hosted by the Hietakero mafic intrusion in north-western Finnish Lapland. The intrusion contains gabbroic to pyroxenitic cumulates and was emplaced into a supracrustal strata composed mainly of mafic volcanic rocks, sulfur-bearing graphite schists and felsic volcanic interlayers. Hybrid rocks provide clear evidence for interaction of mafic magma with its country rocks. All these rocks have undergone intensive post-magmatic hydrothermal alteration by influx of H2O-, CO2- and Cl-bearing saline fluids, forming strongly scapolitized (±albitized) rocks and resulting in re-mobilization of sulfides and their metals. The Hietakero deposit is associated with pyroxenitic cumulates and hybrid rocks, with the sulfides (pyrrhotite, cobaltian pentlandite, chalcopyrite and pyrite) occurring as patches, brecciated to net-textured and vein to veinlet forms. Sulfur content is 6.1 wt.% on average, reaching 20 wt.% in some cases. Nickel and Cu tenors are low, but Co is elevated, resulting in very low Ni/Co of 2.6 compared to ‘classic’ Ni-Cu sulfide deposit globally. The average metal tenors are: 0.55 wt.% Ni, 1.07 wt.% Cu, and 0.23 wt.% Co. In-situ sulfur isotope analyses of sulfides from mineralized rocks yielded δ34S values from -2.0 to +4.5‰. In spite of strong post-magmatic alteration, we can conclude that the Hietakero cumulates were formed from a low-MgO and PGE-depleted basaltic magma, which developed in a staging chamber at a deeper crustal level. The magma produced compositionally different gabbroic to pyroxenitic rocks (i.e., high-Ti and low-Ti groups). Model calculations indicate that the relatively high Co tenor and low Ni/Co cannot be explained by an earlier phase of fractional crystallization or sulfid

Published
2018

15. Sulphide Segregation in Ferropicrites from the Pechenga Complex, Kola Peninsula, Russia.

Author

BRUÜGMANN, G. E., HANSKI, E. J., NALDRETT, A. J., and SMOLKIN, V. F.

Subjects
*SULFIDES, *IGNEOUS intrusions, *ROCKS, *LAVA, *CRYSTALLIZATION
Abstract

The Palaeoproterozoic Ni–Cu sulphide deposits of the Pechenga Complex, Kola Peninsula, occur in the lower parts of ferropicritic intrusions emplaced into the phyllitic and tuffaceous sedimentary unit of the Pilgujärvi Zone. The intrusive rocks are comagmatic with extrusive ferropicrites of the overlying volcanic formation. Massive lavas and chilled margins from layered flows and intrusions contain <3–7 ng/g Pd and Pt and <0·02–2·0 ng/g Ir, Os and Ru with low Pd/Ir ratios of 5–11. The abundances of platinum group elements (PGE) correlate with each other and with chalcophile elements such as Cu and Ni, and indicate a compatible behaviour during crystallization of the parental magma. Compared with the PGE-depleted central zones of differentiated flows (spinifex and clinopyroxene cumulate zones) the olivine cumulate zones at the base contain elevated PGE abundances up to 10 ng/g Pd and Pt. A similar pattern is displayed in intrusive bodies, such as the Kammikivi sill and the Pilgujärvi intrusion. The olivine cumulates at the base of these bodies contain massive and disseminated Ni–Cu-sulphides with up to 2 μg/g Pd and Pt, but the PGE concentrations in the overlying clinopyroxenites and gabbroic rocks are in many cases below the detection limits. The metal distribution observed in samples closely representing liquid compositions suggests that the parental magma became sulphide saturated during the emplacement and depleted in chalcophile and siderophile metals as a result of fractional segregation of sulphide liquids. Relative sulphide liquid–silicate melt partition coefficients decrease in the order of Ir > Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphide mass ratio) are high and of the order of 104–105, and they indicate the segregation of only small amounts of sulphide liquid in the parental ferropicritic magma. In differentiated flows and intrusions the sulphide liquids segregated and accumulated at the base of these bodies, but because of a low silicate–sulphide mass ratio the sulphide liquids had a low PGE tenor and Pt/Ir and Cu/Ir ratios similar to the parental silicate melts. During cooling the sulphide liquid crystallized 40–50% of monosulphide solid solution (mss) and the residual sulphide liquid became enriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. The Cu-rich sulphide liquid locally assimilated components of the surrounding S-rich sediments as suggested by the radiogenic Os isotopic composition of some sulphide ores (γOs > 100). Most of the massive and breccia ores represent mixtures of mss and residual Cu-rich sulphide liquid whereas chalcopyrite-rich veins formed when the Cu-rich sulphide liquids were squeezed out into the surrounding sediments. [ABSTRACT FROM PUBLISHER]

Published
2000

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