Geology, mineral chemistry and petrogenesis of the Pipestone Lake anorthosite complex

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Jobin-Bevans, L. Scott
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The Pipestone Lake Anorthosite Complex is a 0.9 x 15 km layered Archean megacrystic (calcic) intrusion that is part of a suite of coeval megacrystic anorthosite bodies in the Cross Lake region, northwestern Superior Province, Manitoba. Bedrock geological mapping and diamond drill core studies have established a laterally continuous igneous stratigraphy comprising 11 zones, viz. (from south to north) the border (B), anorthosite 1 (A1), leucogabbro 1 (L1), melagabbro 1 (M1), leucogabbro 2 (L2), melagabbro 2 (M2), leucogabbro 3 (L3), melagabbro 4 (M4), leucogabbro 4 (L4), anorthosite 2 (A2), and transitional (T) zones. Four oxide-enriched subzones,--the South, Main Central, Disseminated, and North Contact--occur at, or near the base of the 3 melagabbro zones. Ilmenite and magnetite are constrained near their respective end-member compositions and have very low ($<$1 wt%) concentrations of Mn, Mg and Ca, which are considered deleterious to ore beneficiation. The range of anorthite contents of matrix plagioclase(An$\sb{31-79})$ are lower than plagioclase megacrysts (An$\sb{68-90}),$ suggesting crystallization from dissimilar liquids. There is a systematic decrease in the An content of matrix plagioclase cores upward through the intrusion. There are distinct and subtle changes at the lithologic boundaries between the L1-M1, L2-M2, and L3-M3 zones, that correspond with changes in the concentrations of Nb, Mn, Fe and Zn in ilmenite, the wt% V$\sb2$O$\sb3$ in ilmenite and magnetite, the anorthite content of matrix plagioclase cores, the whole-rock TiO$\sb2$, Fe$\sb2$O$\sb3$ and V$\sb2$O$\sb5$ contents, and the Fe$\sp{2+}$/Mg+Fe$\sp{2+}$ ratio in amphibole. (Abstract shortened by UMI.)