Fluid inclusion and stable isotope evidence for mixing of magmatic - hydrothermal fluids with meteoric water in vein-type Cu-Au-Bi deposits, southern New Brunswick, Canada
Authors: Hanley J J , Tweedale F , Neyedley K , Sharpe R , Fayek M 
Published: June 2015

Vein-type Cu-Au-Ag-Bi mineralization in the Caledonian Highlands, southern NB, Canada, is hosted within quartz-carbonate-rich shear zones cutting felsic lithic tuffs, intermediate intrusives and interbedded felsic and mafic flows of the Neoproterozoic Broad River
Group. Mineralization in the veins consists of bornitechalcocite- hematite, coprecipitated with electrum and bismuthinite; ore minerals post-date quartz and REE-rich carbonates in the veins, with later supergene oxidation and hydration of the ores to cuprite-malachite. Wall-rock alteration is
characterized by albitization and paragonitization. Replacement of bornite by chalcocite- hematite indicates changes in fluid redox with mineralization progression.
Trails of secondary fluid inclusions in the quartz veins are two-phase liquid-vapour at room temperature. Homogenization occurs by vapour bubble disappearance between 150-270°C for all assemblages; individual assemblages show relatively narrow ranges (e.g., 173-191°C, n=22). Bulk
salinities from final ice melting range from 4 to 13 wt% NaCl eq. with individual assemblages showing similarly narrow ranges.
Stable isotope data (bulk separates, and in-situ by secondary ion mass spectrometry [SIMS]) for vein-stage quartz (d18Obulk = 13.7-15.1perthousand; d18OSIMS-qtz = 10.8±1.5 perthousand, 1c, n=32) and carbonate (d13Cbulk = -4.4 to -4.6 perthousand) combined with microthermometric data rule out
unmodified, heated seawater and meteoric water as the dominant fluid components, and suggest that the metal-bearing fluids were magmatic in origin or represented saline formation waters modified through fluid-rock interaction with the host volcanic rocks (calculated d18Ofluid ~ 6-7
perthousand). However, significant variations in d18OSIMS-qtz are observed within single quartz crystals across growth zones and in massive quartz texturally predating sulfides and gold (from as low as 8.2 perthousand to 14.8 perthousand in quartz enclosed entirely within bornite-chalcocite). This
indicates either (i) localized mixing of the metal-bearing fluid with low latitude meteoric water (calculated d18O = -1.0 to 0 perthousand), or (ii) fluctuations in fluid temperature during vein formation, with the lowest T portions of the vein associated with base metal-gold precipitation, or (iii)
both. The isotopic composition of coeval quartz-carbonate predict a crystallization/final equilibration T of vein-stage at ~250- 270°C; if inclusions are primary, then a maximum Ptrapping = ~1.5 kbar, based on the lowest T assemblages, is estimated.
Significant fluctuations in fO2, fluid temperature or fluid composition during vein precipitation highlight the importance of fluid mixing for mineralization. These characteristics, combined with the style of mineralization, link these deposits in NB genetically to much larger vein Cu deposits
worldwide (e.g., Churchill, Davis-Keays, and Mamainse Point, Canada; Inyati, Zimbabwe; Copper Hills, Australia; Messina, South Africa; Cornwall, UK).