PYRRHOTITE-BEARING ULTRAMAFIC DIKES IN THE BUNKERVILLE DISTRICT, VIRGIN MOUNTAINS, SOUTHERN NEVADA

The Bunkerville district features regionally unusual Ni-PGE mineral deposits and prospects, associated with a swarm of variably metamorphosed Paleoproterozoic mafic to ultramafic dikes. Within this dike swarm, we have identified four major protolith rock types: augite-hornblende gabbro; gabbro-related hornblendite; olivine hornblendite (OHB); and altered OHB, some pyrrhotite-rich. Gabbro and related hornblendite are far more abundant than OHB.

Gabbro-related hornblendite is a minor, gradational facies of some gabbro dikes; and evidently formed by crystal accumulation. In contrast to OHB, gabbro and gabbro-related hornblendite only locally contain phlogopite, lack primary pyrrhotite, and are only indirectly associated with mineralization. Whole-rock levels of Ni, Cu, Pd, and Pt are basaltic.

OHB dikes are mainly fine- to medium-grained olivine, olivine-clinopyroxene, and clinopyroxene hornblendite, typically with accessory phlogopite; and subordinate phlogopite hornblendite, and clinopyroxenite. All have accessory pyrrhotite. These ultramafic dikes contain ≈ 44 % SiO2, 22 % MgO, 0.1–0.5 % S, 200 µg/g Cu, 900 µg/g Ni, and 30 ng/g Pd and Pt.

Altered OHB is characterized by abundant phlogopite, secondary amphibole, serpentine, and/or carbonate. Some, but not all, of these altered rocks contain a few to as much as 20 % pyrrhotite, plus other sulfide minerals. Pyrrhotite forms veinlets and diffuse patches or clusters, intergrown with phlogopite, amphiboles, and serpentine. Alteration and abundant pyrrhotite are largely restricted to OHB dikes, and appear to be related but to some extent independent.

Outcrop evidence indicates that at least some phlogopitization of OHB owes to reaction between OHB dikes and fluids derived from the quartzofeldspathic gneisses they intrude. We suspect that formation of abundant pyrrhotite was caused by interaction of ultramafic magma with silicic country rocks.

Thus, our field and geochemical data establish the petrologic affinities of Ni-PGE mineralization: the chief or sole antecedent is OHB dikes. Such dikes are actual or potential protore, and have been explored for Ni or PGE several times in the past century. On the other hand, presence of the much more common dikes of gabbro (and derivative amphibolite) alone does not necessarily indicate Ni-PGE mineralization.