Evaluation of crystallization and emplacement conditions of the McKenzie Gulch porphyry dykes using chemistry of rock-forming minerals: Implications for mineralization potential

Abstract

Electron microprobe analyses of rock-forming minerals in the Middle Devonian (386.2 ± 3.1 and 386.4 ± 3.3 Ma) intermediate to felsic porphyry dykes of the McKenzie Gulch (MG) area, in northern New Brunswick, Canada are used to constrain important intensive parameters, such as oxygen fugacity (fO2), temperature, pressure, dissolved water (H2Omelt) content, and emplacement conditions. Biotite compositions plot just above the nickel-nickel oxide (NNO) buffer at fO2 values of ~−12.22, which together with the presence of magnetite indicate oxidizing conditions during crystallization. On a FeOtotal–MgO–Al2O3 discrimination diagram, biotite compositions plot in the calc-alkaline field, consistent with petrochemical observations, whereas in terms of log(XF/XOH) versus log(XMg/XFe), biotite plots in the moderately contaminated I-type field, suggesting possible interaction of these magmas with host rocks during their evolution.Pressure and temperature conditions of the magma estimated from aluminum-in-hornblende barometry and amphibole–plagioclase thermometry suggest that hornblende phenocrysts in these rocks crystallized at temperatures between ~700° and 810 °C and pressures between 4 and 6 kbar. Depth estimates based on these pressures suggest that these magmas resided at depths between ~13 and ≥19 km prior to final emplacement as dykes in the upper crust. The estimated water (H2Omelt) content of the magmas that fed the dykes indicates that they were highly hydrous (up to ~5.0 wt%), consistent with the presence of phases such as hornblende and biotite. High magmatic water content is a prerequisite for the formation of magmatic-hydrothermal ore deposits and can be used to assess the prospectivity of these slab failure magmatic suites for porphyry and skarn mineralization.