Nickel sulphide mineralization in the lick fork prospect, Virginia, U.S.A.

The Lick Fork prospect shows appreciable nickel sulphide mineralization in a narrow, steeply dipping, layered sequence of metamorphosed hornblende peridotite, gahhro and hornblende gahhro, intrusive into the Pedlar Formation (charnockitic pyroxene granulite) of the Virginia Blue Ridge Complex (Precambrian). Rocks of the intrusion have equilibrated to granulite- facies metamorphism, as indicated by pervasive polygonal texture of silicate aggregates, development of biotite- and hornblende-preferred orientation, compositional equilibration of the silicate minerals, the Mg-Fe2+ (orthopyroxene-dino- pvroxene) distribution coefficients (average 0.57), (MgO + FeO + Fe20 3)/Al20 3ratios in orthopyroxenes, and Ti contents and AI(i\)/A I(\i) ratios of hornblendes. The sulphides, consisting of pyrrhotite-pentlandite (with up to 4 wt% Co)-chalcopyrite-pyrite-marcasite-violarite- bornite—mackinawite, are virtually restricted to the ultramafic unit of the intrusion and constitute 20-30% of the samples of hornblende peridotite studied. Although the basal accumulation of the sulphides may suggest magmatic segregation, sulphide- silicate textures typical of sulphide liquid immiscibilitv are not present, even in the sparsely mineralized gahbro units. On the other hand, the transgressive and relict textures are strongly suggestive of replacement of the silicate minerals by sulphides. These include chaleopyrite ± pyrrhotite ± pentlandite (violarite) ± pyrite ±magnetite veins crosscutting silicates, sulphide penetration into silicates at silicate-sulphide contact zones, rounding of silicate grains against sulphide matrix, and islands of silicate minerals (olivine, hornblende, pyroxene, biotite) in sulphide masses. Metamorphic mobilization of pre-existing sul­ phides is a possibility, but involvement of post-metamorphic, hydrothermal fluids is suggested by the following features: paucity of annealing and deformation textures in the sulphides, replacement of metamorphic and deformed silicates, lack of differential sulphide mobilization and the common occurrence of sulphides along grain boundaries and triple junctions of poly gonal silicate aggregates. The consistent association between sulphides and hydrothermal alteration products of silicates suggests that they are cogenetic. The hydrothermal fluids may have merely mobilized and concentrated the pre-existing, disseminated, magmatic sulphides, or there may have been significant addition of sulphides during this process. This question cannot be resolved unequivocally with the existing data.