Mineral Chemistry of the Eagle Lake Granite Porphyry, Southwestern New Brunswick, Canada: Implications for Cu-Mo-Au Mineralization

Abstract

Abstract The oxidized I-type Eagle Lake Granite stock in southwestern New Brunswick, Canada, is texturally divided into porphyritic and equigranular phases. The porphyritic granite consists of phenocrysts (i.e., plagioclase, K-feldspar, quartz, and biotite) and microcrystalline groundmass with minor magnetite–ilmenite, titanite, apatite, and zircon. The equigranular phase has a similar primary mineral assemblage to the porphyritic phase. Their common magnetite-ilmenite-titanite assemblage reflects co-crystallization (magnetite series) from a magma imparting some redox control. However, these granite phases show minor potassic to propylitic alteration mineral assemblages with very minor sulfides, suggesting localized fluid–rock reaction. The composition of plagioclase varies between albite and oligoclase, and K-feldspar is orthoclase commonly displaying considerable turbidity. The An% versus Al/(Ca+Na+K) data indicate that these feldspars are slightly aluminous, reflecting cryptic alteration. Biotite is rich in Fe, plotting near the boundary of primary and re-equilibrated biotite; these biotites formed at temperatures ranging from 670 to 725 °C, based on Ti-in-biotite thermometry. Secondary biotite grains are also locally evident, formed from magmatic-hydrothermal fluids. Secondary fine-grained biotite associated with fine-grained magnetite-pyrite indicates potassic alteration and related Cu±Mo±Au mineralization. Some of these various biotites are partially altered to chlorite at ∼301–361 °C. Like biotite, apatite occurs as both igneous and hydrothermal phases. Based on the concentration of F (4.21–2.90 wt.%), all these apatites are fluorapatites with content of light rare earth elements about 7000 ppm and Eu/Eu* = 0.16.