Noble metals in the marginal rocks of the Bushveld Complex

Saved in:
Online Access: Get full text
doi: 10.2113/gsecongeo.77.6.1286
Authors:Sharpe, Martin R.
Author Affiliations:Primary:
Univ. Pretoria, Inst. Geol. Res. Bushveld Complex, Pretoria, South Africa
Volume Title:further issue devoted to the platinum-group elements
Volume Authors:von Gruenewaldt, Gerhard
Source:Economic Geology and the Bulletin of the Society of Economic Geologists, 77(6), p.1286-1295; Third international platinum symposium; a part of the South African Geological Society's Geocongress '81, Pretoria, South Africa, July 6-10, 1981, Gerhard von Gruenewaldt. Publisher: Economic Geology Publishing Company, Lancaster, PA, United States. ISSN: 0361-0128
Publication Date:1982
Note:In English. 35 refs.; illus. incl. 5 anal., 3 tables
Summary:Two major subdivisions of marginal rocks occur in the eastern Bushveld Complex. A pyroxenitic group borders the lower and lower critical zones, and a gabbroic group borders the upper critical and main zones. Three main facies of the gabbroic group can be recognized on the bases of field relations, petrography, and chemical composition. Five sampling sites were chosen that would best represent the varieties of marginal rocks by study of the field relations and bulk rock compositions of 184 previously analyzed samples. These sites were resampled and the rocks analyzed for major and trace elements and the noble metals (Ir, Rh, Pt, Pd, and Au). The field relations, petrography, and mineral and bulk rock composition of the samples are presented. The range in noble metal abundances is: Ir: 0.2 to 0.6, Rh: 3 to 12, Pt: 30 to 50, Pd: <10 to 20, and Au 10 to 40 ppb. The values are considerably higher than corresponding abundances in most basalts and chilled margins of other layered complexes--especially for Au. All marginal rocks have broadly similar chondrite-normalized noble metal distribution patterns, with enrichment in Rh and Au relative to Pt and Pd.Abundances of noble metals in a liquid from which the UG-2 chromitite layer and the Merensky Reef may have crystallized is calculated from marginal rock compositions, assuming a conical magma chamber. The liquid is composed of a ∼52/48 percent pyroxenitic/gabbroic mixture and has a composition of Ir: 0.41, Rh: 7.7, Pt: 40, Pd: 16, and Au: 21 ppb. Maximum distribution ratio values (Dsulfide liquidsilicate liquid) computed for the mixture, assuming no fractional crystallization, are between 1.8 and 26 times higher in the UG-2 (chromite) environment than in the Merensky Reef (silicate) environment. Distribution ratio values of Ir: 6,600, Rh: 1,100, Pt: 3,900, Pt: 4,000, Au: 480, and Cu: 480 were obtained for the Merensky Reef. With 50 percent fractional crystallization and incompatible behavior of the metals, the distribution ratios attain more realistic values of Ir: 3,300, Rh: 550, Pt: 1,950, Pd: 2,000, Au: 240, and Cu: 240. [K.A.R.]
Subjects:Composition; Economic geology; Gabbros; Geochemistry; Gold; Igneous rocks; Intrusions; Iridium; Layered intrusions; Major elements; Metal ores; Metals; Palladium; Platinum; Platinum group; Platinum ores; Plutonic rocks; Pyroxenite; Rhodium; Trace elements; Ultramafics; Africa; Bushveld Complex; Merensky Reef; South Africa; Southern Africa; Transvaal region; Abundance; Noble metals; Transvaal South Africa
Coordinates:S263000 S230000 E0302000 E0260000
Abstract Numbers:83M/1309
Record ID:1983005310
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists
Tags: Add Tag
No Tags, Be the first to tag this record!