Chemical composition of rock-forming minerals in gold-related granitoid intrusions, southwestern New Brunswick, Canada: implications for crystallization conditions, volatile exsolution, and fluorine-chlorine activity

Abstract

Chemical composition of rock-forming miner- als in Appalachian Siluro-Devonian granitoid intru- sions, southwestern New Brunswick, was systematically determined by electron microprobe. The mineral chem- ical data together with petrographic examination was used to test magmatic equilibration and to constrain crystallization conditions, volatile exsolution, and fluo- rine-chlorine activity of fluids associated with these intrusions. Mineralogical distinction between Late Silu- rian to Early Devonian granodioritic to monzogranitic series (GMS) and Late Devonian granitic series (GS) rocks is evident, although both are subsolvus I-type to evolved I-type granitoids. Oxidized to reduced GMS rocks consist of quartz, plagioclase (An>10), K-feld- spar, biotite, apatite, titanite, zircon, monazite, ± hornblende, ± pyroxene, ± magnetite, ± ilmenite, and ± sulfide. GS rocks comprise quartz, K-feldspar, pla- gioclase (An<10), mica group minerals, zircon, mon- azite, apatite, sulfide, ± ilmente, ± magnetite, ± topaz, ± columbite, and ± xenotime. Inter-intrusion and intra-intrusion variations in mineral chemistry are in- terpreted to reflect petrogenetic processes (e.g., assim- ilation and fractional crystallization) during granitoid evolution. Although magmatic equilibration among rock-forming minerals are disturbed by subsolidus hy- drothermal processes, GMS rocks appear to have higher magmatic temperatures, variable levels of emplacement, a range of fO2 (i.e., reduced intrusions 10 � 16.7 � 10 � 13.4 and oxidized intrusions 10 � 14.0 � 10 � 10.5 bars), and rela- tively low fHF/fHCl ratios (10 � 3.0 � 10 � 1.0 ) in exsolved fluids, compared to GS rocks. Reduced GMS intrusions bear higher gold potential and thus may be prospective targets for intrusion-related gold systems.