The distribution of base metals and platinum-group elements in magnetititeand its host rocks in the Rio Jacaré Intrusion, Northeastern Brazil

Sá, J. H. S., Barnes, Sarah-Jane, Prichard, Hazel Margaret and Fisher, Peter Charles 2005. The distribution of base metals and platinum-group elements in magnetititeand its host rocks in the Rio Jacaré Intrusion, Northeastern Brazil. Economic Geology 100 (2) , pp. 333-348. 10.2113/gsecongeo.100.2.333

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Abstract

Anomalously high Pt and Pd values have been found in three magnetite bodiesin the Rio Jacaré intrusion of northeastern Brazil. The intrusion hosting thesemagnetite bodies consists predominantly of pyroxenite and gabbro. One magnetitebody occurs in the Lower zone and two in the Upper zone of the intrusion. Thesebodies contain approximately 0.04 percent Ni, 0.1 percent Cu, 0.18 percent S, 1ppb Ir, 3 ppb Rh, 160 ppb Pt, 120 ppb Pd, and 37 ppb Au. They are much richer inplatinum-group elements (PGE) than the surrounding silicate rocks, and there aresignificant correlations among all of the PGE and between PGE and Ni. Howeverthe correlations between PGE and Au, Cu, and S are much weaker than correlationsbetween Au, Cu, and S. In the magnetite bodies palladium-rich minerals, especially bismuthides andantimonides, are the most abundant platinum-group minerals (PGM). In most casesthese occur with interstitial silicates or within silicate inclusions inmagnetite and ilmenite grains and are associated with Co-bearing pentlandite andin a few cases with Co-Ni sulfarsenides and arsenides. Sperrylite (PtAs2)is the most abundant Pt mineral and is associated with silicates interstitial tomagnetite and ilmenite grains and sometimes with Co-Ni arsenides. At sites wherethe igneous mafic minerals have been altered to amphiboles, sperrylite may bealtered to Pt-Fe alloys. Other alloys present include Pd-Sn-Cu, Pt-Cu, Pt-Ni,and Pt-Au. It is suggested that Ni and PGE were concentrated in the magnetite bodies bythe coprecipitation of a small quantity of sulfide with the magnetite. These PGE-bearingbase metal sulfides subsequently exsolved PGM. The association of Pd mineralswith base metal sulfides and the small variation in the Pt/Pd ratio (ca. 1.4)suggests that the PGE have not been extensively remobilized in the magnetitite.In contrast, the strong correlation between S, Cu, and Au suggests that, inaddition to the redistribution of S, it is likely that Cu and Au wereremobilized. It is not possible to say whether the redistribution of sulfur wasdue to late magmatic fluids dissolving S or the later metamorphic events. The association of PGE enrichment with magnetite layers in the Rio Jacaréintrusion contrasts with that of the Bushveld, Stillwater, Great Dyke, and MunniMunni Complexes. In these complexes PGE-enriched layers or reefs are found inthe lower third of the complexes and the oxide associated with the reefs ischromite. Magnetite-bearing layers, which form from an evolved magma in theupper parts of the intrusions, are generally barren of PGE because, at the timeof magnetite crystallization, the PGE had already precipitated either insulfides or PGM. However in a number of intrusions (e.g., Rincon del Tigre,Skaergaard, Stella, and Rio Jacaré) the upper magnetite-bearing portion of theintrusion shows PGE enrichment. This enrichment is rarely associated withvisible sulfides but suggests a possible new target for PGE exploration.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > QE Geology
Publisher:	Society of Economic Geologists
ISSN:	0361-0128
Last Modified:	03 Oct 2022 08:21
URI:	https://orca.cardiff.ac.uk/id/eprint/10107

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