A comprehensive geological, petrological, and geochemical evaluation of the Voisey's Bay NI-CU-CO sulfide deposit: an integration of empirical data and process mechanics

Huminicki, Michelle Anne (2007) A comprehensive geological, petrological, and geochemical evaluation of the Voisey's Bay NI-CU-CO sulfide deposit: an integration of empirical data and process mechanics. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis consists of three manuscripts, which report results on: 1) methodologies for systematic ore characterization utilizing the Voisey's Bay Ni-Cu-Co magmatic sulfide Ovoid deposit of northern Labrador as an example; 2) genesis of Voisey's Bay ores, focusing on the Ovoid deposit; and 3) discovery of a platinum-group element (PGE) occurrence in the vicinity of the Southeast Extension of the Voisey's Bay Ovoid deposit. -- The first manuscript defines, describes, and characterizes mineralogical and geochemical zonation of the Voisey's Bay massive sulfide Ovoid deposit to determine relationships between macroscopic ore textures, mineralogy, and chemical compositions of the sulfides. The emphasis is on development of algorithms to predict ore mineralogy derived from bulk chemical assays. A comparison to results from mineral liberation analysis indicates that the algorithms are very robust. The results show that the Ovoid deposit is strongly zoned from pyrrhotite-rich margins to a chalcopyrite-pentlandite-rich transitional zone and a magnetite-rich core. The TYPE I inner magnetite-rich ore has elevated Pb, Pt, and Pd relative to the remainder of the Ovoid (which correlate with high magnetite and moderate chalcopyrite) with the exception of a very central zone that has high chalcopyrite and Zn but lower Pb, Pt, and Pd relative to the remainder of the Ovoid. TYPE II ore is elevated in Pd and Zn, which corresponds to the chalcopyrite-pentlanditerich zone. Lead, Zn, Pt, and Pd are not enriched in the pyrrhotite-rich TYPE III outer ore zone relative to the remainder of the Ovoid. -- The second manuscript integrates mineralogical data and bulk assay data to quantitatively describe and model the emplacement (including physical/mechanical controls/processes) and crystallization history of the sulfide parent magmas of the Ovoid deposit and associated mineralized zones. Various models for ore upgrading (R, N, L factors) and crystallization (equilibrium, fractional, and partial fractional crystallization) were evaluated to determine which was involved in producing observed Ni and Cu tenors. Parental and derivative silicate magma compositions that may have been responsible for the ores were evaluated. The major conclusion is that the Ovoid deposit formed at an R factor of -150 from common basalt parent magmas and differentiated by partial fractional crystallization, trapping 30-40% residual liquid, mainly in the core of the ore body. Some of the residual liquid escaped to form disseminated mineralization. -- The third manuscript reports the first occurrence of platinum group-minerals (PGM) at Voisey's Bay, with grades of 1.95g/t Pt, 1.41g/t Pd, and 6.59g/t Au. Most of the PGE are present as discrete minerals hosted by disseminated base-metal sulfides in a hornblende gabbro dyke near the Southeast Extension of the Ovoid deposit. The PGM were determined to have a primary magmatic origin. Geological and geochemical relationships suggest that the dyke is associated with the main troctolite rocks that host the Ovoid, indicating a similar magma source. The PGM are related to a highly differentiated sulfide liquid that formed intermediate solid solution (ISS), which was derived by crystallization of monosulfide solid solution (MSS) from a sulfide melt. The sulfide melt was similar to that which formed the Ovoid deposit. The depletion of Pb and Pt in the center of the Ovoid may indicate loss of this material to the surroundings.

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