Metasomatic processes in the Lukkulaisvaara layered intrusion, Russia, and formation of low-sulfide PGE mineralization

Abstract

The Nadezhda ore-controlling structure and related rocks of the Lukkulaisvaara layered intrusion from the Oulanka plutonic group have been studied. Tensile deformation arising at the cooling stage at interfaces of rocks dissimilar in composition with different compressibility coefficients controlled the intensity of secondary processes and the high concentration of sulfide and platinum group minerals (PGM) along the perimeter of the fine-grained gabbronorite body in contact zones with country rocks. These zones were specific geochemical barriers. Magmas of supplementary injections could have been one of the sources of fluid affecting the rocks. Fe-Ni-Cu sulfides and PGM crystallized within the temperature range T = 800-350°C. Regional metamorphism did not exceed conditions of greenschist facies. Results of modeling of metasomatic processes indicate that decompression played the leading role in formation of the major secondary mineral assemblages: Czo-An10-Chl-Tr(Act) and Qtz-An70-Amph-Bt. The decreasing pressure under isothermal conditions resulted in a rise in solubility of metals in chloride solution and their transport into dilatable zones. The cooling model leading to the enrichment of rocks in potassium and then to its leaching is realized more seldom, resulting in a Qtz-Ms-Chl-Act-Czo mineral assemblage and eventually in Qtz-Chl veins. The change of P and T during pluton cooling resulted in changing of the direction of metasomatic processes and telescoping of different superimposed mineral assemblages: Amph(±Act)-Bt ± Chl 1 ± Qtz as an early one and Qtz-Czo-Chl2-Ab-Ms as a late one. The model mineral assemblages that match natural counterparts are as follows: (1) Qtz-Pl-Amph-Bt and Qtz-Pl-Chl-Bt(Ms) and (2) Tr(Act)-Chl-An10-Czo, Qtz-An10-Chl-Ms(Bt), and Qtz-Czo-Chl-An10-Ms. At the same time, the An40–50-Amph-Bt, Qtz-An40–50-Amph-Bt, Qtz-An10–20-Chl-Bt(Ms), An30–40-Chl-Tr(Act)-Bt, An30-Chl-Tr(Act), and An30-Tr (Act)-Bt assemblages are suggested to be intermediate and reflect the direction of metasomatic processes.