Low-sulfide PGE ore in the Volchetundra gabbro-anorthosite pluton, Kola Peninsula, Russia

Abstract

The internal structure of the Volchetundra gabbro-anorthosite massif is considered, including localization of low-sulfide PGE mineralization and its mineralogy. The Volchetundra massif 24 km long and 0.5–4.0 km wide occupies the middle part of the Main Range complex, which extends for 75 km in the nearly meridional direction. The main and marginal zones are distinguished in the massif. The marginal zone 20–400 m wide extends along the entire eastern contact of the massif and is primarily composed of mediumgrained meso- and leucocratic norite, gabbronorite, plagioclasite, and less fequent orthopyroxenite. The main zone consists of coarse-grained leucogabbro and gabbronorite with an anorthosite zone in the axial part of the massif. The PGE mineralization of the Volchetundra massif is distinctly subdivided into two types substantially differing in localization, mineralogy, geochemistry, and economic importance. Mineralization of the first type is localized in the marginal zone and characterized by the highest resource potential. Mineralization hosted in the main zone belongs to the second type. The PGE ore of marginal zone is spatially and genetically related to the pyrite-pentlandite-chalcopyrite-pyrrhotite sulfide mineralization (1–5%) in the form of fine inequigranular interstitial disseminations, and less frequent larger grains and pockets localized within two ore zones each up to 2 km in extent. The thickness of separate mineralized layers varies from 0.5 to 3.0 m and up to 45 m in bulges. The average Pt + Pd grade is 1.37 gpt at Pd/Pt = 3.1. The mineralization of the second type has been penetrated by boreholes. Separate intersections do not correlate with one another and are limited in extent both along the strike and down the dip. The PGE mineralization is related to finely dispersed pentlandite-pyrite-pyrrhotite-chalcopyrite sulfides, sulfide emulsions, and less abundant stringer-disseminated sulfide ore. The orebodies vary from 2 to 7 m in thickness. The average Pt + Pd grade is 1.61 gpt; Pd/Pt = 1.3. The PGE mineralization includes 22 mineral species. PGE sulfides (cooperite-braggite-vysotskite; laurite and erlichmanite in insignificant amounts) are predominant. Bismuthotellurides (moncheite-kotulskite-merenskyite) and arsenides (sperrylite, palladoarsenite, arsenopalladinite, atheneite) are subordinate in abundance. In addition, sulfoarsenides (platarsite, hollingworthite), tellurides (telargpalite, sopcheite, keithconnite, melonite, hessite), paolovite, and Pt-Fe alloy have been identified. An admixture of native gold and electrum occur constantly.