Hydrothermal fluid evolution at the Maisie gold occurrence, Menneval, New Brunswick

Abstract

The Maisie gold occurrence, Menneval, New Brunswick, is hosted by Middle to Late Ordovician, fine grained clastic sedimentary rocks of the Whites Brook Formation, located within the Aroostook–Percé Anticlinorium. The Maisie occurrence consists of veins containing two generations of quartz (laminated and massive), hematite, pyrite, and trace chalcopyrite and galena. Electrum has a close spatial relationship with hematitized pyrite, and is late stage infilling vugs and fractures. “Hot” cathodoluminescence imaging of vein quartz is consistent with low grade metamorphism. Stable oxygen isotope analysis of quartz by SIMS shows relatively homogeneous [delta superscript 18]O[subscript V-SMOW] values between 12.3 and 18.8‰ (Fig. 15a). Estimated initial fluid [delta superscript 18]O[subscript V-SMOW] composition was between 7.0 and 18.5‰, consistent with both sedimentary-derived metamorphic waters and magmatic fluid sources, and inconsistent with meteoric or formation water. Fluid inclusion microthermometry of rare, two phase (L+V at 20[degrees]C) secondary inclusions gave an average temperature of homogenization of 170.3 [plus or minus] 30.8[degrees]C (1σ, n = 150), and an average salinity of 2.8 ± 2.2 wt% eq. NaCl (1[sigma], n = 150). Inclusion petrography (an abundance of low density vapour inclusions with rare two phase parental fluids) and isochoric data suggests rapid decompression and boiling, a potential mechanism for gold deposition. Laser Raman spectroscopy shows that minor methane and nitrogen are present in the vapour phase. The common occurrence of electrum with hematite suggests an interaction with oxidized meteoric waters, but the stable isotope and fluid salinity data do not support this idea. Maisie is considered here to represent an epizonal-orogenic Au occurrence. Rutile found within the veins shows enrichment in tin, tungsten, and rare-earth elements, and high niobium/tantalum ratios. These characteristics do not occur in rutile found within adjacent host rocks, providing a potential indicator mineral for regional exploration.