- journal_title：The Canadian Mineralogist
- Contributor：James H. Crocket ; David M. Leng ; David J. Good ; William E. Stone ; Michelle S. Stone
- Publisher：Mineralogical Association of Canada
- Date：2005-
- Format：text/html
- Language：en
- Identifier：10.2113/gscanmin.43.5.1759
- journal_abbrev：Can Mineral
- issn：0008-4476
- volume：43
- issue：5
- firstpage：1759
- section：Phase equilibria, platinum-group minerals, and ore deposits
The Boston Creek ferropicrite (BCF), exposed near Kirkland Lake in the Abitibi Belt, Ontario, is a differentiated layered mafic–ultramafic unit of Archean age with a remarkably thick layer of spinifex-textured clinopyroxenite at its stratigraphic top, comprising about one-third of a total thickness of ~100 m. The spinifex layer was sampled at intervals of ~1 m. Relict spinifext-extured igneous clinopyroxene occurs in the basal two-thirds of the layer. Results of electron-microprobe analyses show that the clinopyroxene is Ca-rich augite with rim domains usually higher in Fe, Ca and Mn than the core. These variations are most prominent in the basal 10 m of the layer. An overgrowth- and embayment-textured clinopyroxene has much larger core-to-rim variations, with compositions plotting as diopside and hedenbergite. The compositional variations in the spinifex-textured clinopyroxene are attributed to reaction between clinopyroxene and trapped interstitial melt. A metamorphic origin is considered much less likely owing to the lack of evidence for mass transfer between wallrocks and the BCF, the probability that exchange reactions between pyroxene and trapped melt (now groundmass) are favored by magmatic temperatures, and the high Mn content in pyroxene, found in the basal 5 m of the spinifex layer. Whole-rock compositions exhibit prominent depletion trends for Mg, Ni, Co and Cr in the top 3 m of the layer, and weak but distinct enrichment trends for Ca, Al, K and Ti, features reflecting the degree of compatibility of these elements during clinopyroxene/melt fractionation. In the basal 10 m, distinct compositional reversals are evident in whole-rock profiles of Mg, Al, Cr and Mg#. These reversals are thought to reflect a significant change in magma composition, possibly due to magma recharge during the gabbro stage, whereby injection and mixing of relatively unfractionated magma with residual melt occurred. Local variations in PGE abundances, aspects of Re–Os isotope systematics, and detailed magnetic characteristics of the basal spinifex and the upper gabbro layers support this proposal.