Formation of Chlor- and Fluor-Apatite Layered Intrusions

R. Grant Cawthorn
Department of Geology, University of the Witwatersrand, Private Bag 3, PO Wits 2050, Republic of South Africa

Abstract: Apatite in most igneous intrusions has a high Cl/F ratio. However, chlor-apatite has been reported in the lower portions of the Bushveld and Stillwater Complexes. This has been used as evidence supporting the early separation of a Cl-rich discrete hydrous fluid in these intrusions. Mineralogical evidence is presented here to demonstrate that the Bushveld Complex, at least, formed from a nearly anhydrous magma, and did not release a hydrous fluid before apatite began to crystallize. It is suggested that apatite in the earliest cumulates equilibrated with trapped interstitial liquid, which converted it from the typical F-rich composition of cumulus apatite to a Cl-rich composition. This is an analogous process to that in which cumulus mafic minerals may become more Fe-rich on cooling and reaction with interstitial liquid.

Keywords: apatite • layered intrusions • Cl/F ratio • Bushveld Complex

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 299-306; DOI: 10.1180/minmag.1994.058.391.12
© 1994, The Mineralogical Society
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