T216 Advances in Mineral Chemistry for Petrogenesis and Exploration of Mineral Deposits

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

MODELLING THE ROLE OF CARBONATE ASSIMILATION ON THE STABILITY OF SPINELS: A FOCUS ON FLATREEF CHROMITES, BUSHVELD COMPLEX, SOUTH AFRICA

Paper number: T216-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

The Critical Zone-correlative Flatreef of the northern limb of the Bushveld Complex is emplaced in direct contact with footwall quartzite, shales, dolomite and BIF of the Transvaal Supergroup. There has been recorded petrological and geochemical evidence for magma-footwall interaction in the footwall assimilation zone (FAZ) but its influence on ore formation remains Assimilation of carbonate material can have a significant effect on the crystallization path and mineral assemblages of contaminated magmas and could potentially result in an increase in oxygen fugacity thus triggering crystallization of spinels1,2. While many researchers have examined the effect of wall-rock contamination on spinels of the Bushveld Complex, no work has attempted to measure the recorded fO2 and how their stability and composition varies with contamination.

Assimilation-fractional crystallization modelling of Critical Zone (CZ) melt with ~30 % dolomite predicts a shift in the liquid line of descent towards assemblages dominated by spinel+olivine+clinopyroxene±plagioclase. The results correspond to assemblages found in the FAZ, which comprise high proportions of disseminated spinels compared to least-contaminated units. The Cr-spinels of the FAZ have low contents of Cr (~20 wt.%) and high V (~1300 ppm), compared to Cr spinels in least contaminated units that have, on average, higher Cr (~40 wt.%) and lower V (~1100 ppm). The V/Cr ratio in spinels from the FAZ correspond to a shift in fO2 from NN0-0.3 to NNO+1.

Application of a simple mixing model of CZ parental melt with 30 % dolomite, predicts a drop of Cr in spinels from 44 to ~20 wt.% and an increase in V from 1100 to 1300 ppm. Therefore, a key finding of this study is that local dolomite assimilation has a potential to induce fO2 (up to NNO+1.5) thus triggering abundant spinel crystallization. However, the solubility of Cr in the melt will increase with contamination in the melt, thus producing Cr-poor spinels.

  1. Ganino, C.et al. (2008).Miner. Depos. 43, 677–694
  2. Hill, R. & Roeder, P. (1974).J. Geol. 82, 709–729

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-355594

© Copyright 2020 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.