Abstract

The Santa Rita ore zone within the Fazenda Mirabela intrusion is an unusual example of high-tonnage magmatic Ni-Cu-PGE sulfide mineralization developed as a stratiform layer within the main cumulus sequence of a layered intrusion. The intrusion as a whole is about 4 × 2.5 km in outcrop dimensions and at least 3 km in original maximum stratigraphic thickness with an original boat-shaped morphology. The ore zone comprises a 50- to 200-m-thick interval containing up to 5 wt percent disseminated sulfides within a sequence of mostly unaltered harzburgites and orthopyroxenites. This orthopyroxene-dominated sequence lies between the dominantly olivine rich cumulates that comprise the lower half of the intrusion and the gabbroic cumulates that make up the upper half.

Sulfide tenor variations and silicate host-rock compositions have been derived from a very large database of continuous assays of more than 500 diamond drill cores. Consistent patterns of sulfide tenor—gradually decreasing Ni from over 20 to about 10 wt percent, near-constant Cu and gradually decreasing PGEs with cyclically varying Pt/Pd ratios—are found within the Santa Rita ore zone across the entire intrusion and display relatively little lateral variability. The position of the sulfide mineralization envelope within the harzburgite-orthopyroxenite package migrates to higher stratigraphic levels from the northern to the southern margin of the intrusion. Maximum grades and thicknesses correlate with the maximum thickness of orthopyroxene-bearing cumulates, asymmetrically disposed toward the southern margin of the intrusion.

Nickel tenors of the sulfides range from 15 to 25 wt percent, exceptionally high for rocks with relatively low values of MgO/(MgO + FeO) and are closer to the typical values observed in komatiite-hosted deposits, although Cu tenors (typically 5–8 wt %) are much higher than komatiite values. Sulfide PGE tenors range from less than 100 ppb to greater than 10 ppm of both Pt and Pd, with maximum values in S-poor rocks at the base of the sulfide-bearing interval. PGEs become significantly more depleted toward the top of the Santa Rita zone. We attribute these features, along with the localization of the sulfide-bearing interval at the contact between ultramafic and mafic cumulates, to mixing of an initially S-undersaturated, moderately Ni enriched resident magma with a relatively low Ni, PGE-depleted, significantly lower temperature replenishing magma charged with suspended sulfide liquid droplets.

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