Mineralogical, textural, and chemical thermodynamic constraints of the Crawford Ni-Co-(PGE) deposit, Abitibi greenstone belt, Superior Province, Ontario, Canada

Abstract

The 2704 ± 0.88 Ma Crawford Ni-Co-(PGE) deposit, located 50 km north of Timmins, is hosted in a mafic-ultramafic intrusion emplaced within volcanic rocks of the Stoughton-Roquemaure assemblage in the Abitibi greenstone belt. The intrusion shows progressive stages of serpentinization, and nickel mineralization occurs dominantly in the dunitic section of the intrusion as disseminated to patchy net-textured assemblages containing pentlandite, heazlewoodite, and awaruite. The deposit is dissected into two bodies (the western body and the eastern body) by a Proterozoic N-NW sinistral strike-slip fault. An assumed Archean fault cuts the eastern body parallel its dunitic-peridotitic core. The main goal of the project was to document the effects of serpentinization and extract mineralogical and geochemical parameters from present mineral assemblages to better understand mineralization processes and assist in locating and evaluating similar deposits elsewhere. Ninety-one samples were collected from eight drill holes based on core-logging and preliminary petrography. The suite of samples collected includes dunite, peridotite, pyroxenite, and gabbro. However, the focus of the work was on the dunitic samples since those are the intervals hosting the Ni mineralization. The mineralogy and textures were characterized using optical microscopy and SEM-EDS. A subset of samples was analyzed by EPMA. The results show that the dunites were almost monomineralic adcumulates consisting of subhedral olivine, typically containing less than 5% pyroxene, chromium spinel, and primary sulfides. Samples from the East zone are completely serpentinized, whereas samples from the Main-West zone range from completely serpentinized near a fault to approximately 20% towards the western margin of the intrusion and away from a fault. Serpentinization also affected accessory minerals. Samples from the Main-West zones record the progressive destruction of primary olivine and pentlandite during serpentinization and the appearance of secondary heazlewoodite and awaruite. Completely serpentinized samples contain heazlewoodite, awaruite, and secondary magnetite, but only minor relict pentlandite. Native copper was observed in some samples and was used to assess changes in sulfur and oxygen fugacity during serpentinization. Olivine composition could be extracted from samples where primary olivine was preserved. On average, olivine grains are Fo88 ± 1 and have 2,800 ± 350 ppm Ni (average range: 2,200 to 3,380 ppm). Nickel-bearing mineral composition yields Fe4.6Ni4.1S8 for pentlandite (i.e., with an Fe:Ni ratio close to 1:1), stoichiometric heazlewoodite (Ni3S2), and Ni2.5Fe for awaruite. Rare primary Cr-spinel (Mg# 0.53 ± 0.08; Cr# 0.67 ± 0.02) is mostly preserved but secondary magnetite is also present. The mineralogical changes indicate loss of sulfur during serpentinization, mobilization of Fe, Ni, and Cu, and the release of Ni previously locked in the olivine structure. The relatively high Ni content in olivine and nearly monomineralic nature of the dunite layers indicate that the Ni liberated from olivine during serpentinization could become part of the Ni in secondary Ni-bearing minerals. However, as expected, the original (magmatic) pentlandite content largely controls the bulk nickel content in the system. Desulfurization of primary pentlandite, linked to the effects of serpentinization, results in formation of secondary Ni-bearing minerals and increasing in the Ni:S values in the target rocks.