Composition of the Marginal Rocks and Sills of the Rustenburg Layered Suite, Bushveld Complex, South Africa: Implications for the Formation of the Platinum-Group Element Deposits

A chilled margin of komatiite and Mg-rich basaltic andesite in the western Bushveld Complex, South Africa

“…It has a broadly similar composition as the komatiite, but slightly higher Cr (1844 ppm) and Ni (584 ppm) contents, and mostly slightly Rudnick and Fountain (1995), except for Nb (Barth et al 2000). B1 samples are from Barnes et al (2010), Clapham samples are from Wilson (2012), komatiite represents median of Al-undepleted komatiites with <0.3 % S from GEOROC. Tielines indicate AFC of upper crust by komatiite, with tick marks denoting proportion of crystallization higher incompatible trace element contents (27 ppm Zr, 0.98 ppm Nb, 0.75 ppm Th), with the exception of the LREE.…”

“…The Bushveld 1 (B1) magma underlies the Lower Zone, B2 underlies the Upper Critical Zone, and B3 underlies the Main Zone. Strontium and Nd isotopes for these suites were determined by Harmer and Sharpe (1985) and Curl (2001), and platinum-group elements (PGE) by Davies and Tredoux (1985) and Barnes et al (2010). The present paper deals with the B1 suite, previously described as having MgO contents of up to 16.5 wt% (average ~13 wt% MgO) and high levels of compatible trace elements (up to ~1500 ppm Cr, ~500 ppm Ni), but at the same time also high SiO 2 contents (50-57 wt%) and incompatible trace elements (e.g., up to 23 ppm La).…”

“…In order to better understand the petrogenesis of this unusually voluminous and economically 1 3 57 Page 2 of 22 important event, it is critically important to know the composition of the parent magmas to the mafic-ultramafic cumulate rocks. Fortunately, the Bushveld Complex is one of few mafic-ultramafic intrusions where the parent magma composition could be studied in considerable detail, based on a suite of fine-grained, relatively unaltered sills exposed in the floor of the Complex (Davies et al 1980;Cawthorn et al 1981;Sharpe 1981;Barnes et al 2010). Chilled margins also occur, albeit relatively rarely, and their study has only begun relatively recently (Wilson 2012).…”

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex, South Africa: evidence for extensive melt migration within a sill complex

“…The primitive mantle (PM) Th/Sm ratio was taken from McDonough and Sun (1995). The Pt/Pd ratio for B1 was taken from Barnes et al (2010) …”

“…To estimate the mineral proportions expected to crystallise from a patch of trapped liquid in equilibrium with cumulus orthopyroxene in a typical LZ pyroxenite, we conducted fractional crystallisation modelling, assuming a Bushveld B1 magma from Barnes et al (2010), which has undergone fractional crystallisation up to the point where orthopyroxene compositions match the ones observed in the LZ. Subsequently, the residual liquid trapped in the interstitial space of the cumulus mineral framework crystallised in local equilibrium between phases under closed-system conditions.…”