Petrogenesis of gold-related granitoid intrusions in southwestern New Brunswick, Canada

This thesis focuses on granitoid intrusions in southwestern New Brunswick (SWNB) with the objective of determining which granitoids are favorable for generating intrusion-related gold mineralization. The model of intrusion-related gold systems has been developed in the Tintina Belt of Alaska of USA and the Yukon Territory of Canada, which is different from porphyry copper-gold systems in fluid composition, hydrothermal alteration, mineral assemblages, and tectonic setting. The key factors controlling the origin of intrusion-related gold systems are poorly understood, which affects the development of gold exploration strategy. Recent discoveries of several gold deposits or occurrences in SWNB, a granitoid-dominated region well known for tin, tungsten, antimony, and molybdenum mineral resources, share some similarities with intrusion-related gold systems.;Two series of granitoids are recognized in SWNB: (1) a Late Devonian granitic series (GS), and (2) a Late Silurian to Early Devonian granodioritic to monzogranitic series (GMS). GS rocks are evolved I-type granites, whereas GMS rocks are oxidized to reduced I-type granitoids based on their petrochemical features. Gold appears to be associated with a particular phase in a granite suite, which is investigated by studies of such factors as magma sources, magmatic processes, redox conditions, and gold behavior during magmatic to hydrothermal evolution. Chemical composition of rock-forming minerals and relevant thermobarometric estimates suggest that magmatic equilibration among these minerals may have been disturbed by subsolidus or hydrothermal fluids of multiple generations. These fluids are approximated by the system H 2O-NaCl-CO2-CH4, similar to the fluids entrapped in Au-W-Mo-Sb ore deposits at Lake George, which are interpreted to be derived from exsolution of progressive cooling magmas at depth and may have been involved with circulating groundwater. The fluids bearing CH4 may play a role in lowering the redox of the intrusions.;Sulfur isotopes and sulfur content of the granitoids are determined to evaluate sources, magmatic processes, and redox conditions, indicating that GS and GMS granitoids are different in petrogenetic history. Selective-assimilation and fractional crystallization is a dominant process operating in GMS rocks that introduced external sulfur into intrusions under relatively low T (∼500°C), whereas degassing may be an important process controlling sulfur isotopes and contents of GS rocks. A magmatic equilibration model has been developed to estimate ratios of sulfate to sulfide in granitoids, which is linked to redox. Gold behavior is controlled by sulfide minerals (solid solutions) during magmatic evolution. Gold behaves incompatibly prior to sulfur saturation in a granitoid melt, so that gold increases in the residual melt with fractionation; thus the evolved phase within a granite suite may be a potential target for gold. Upon the appearance of sulfide minerals or liquid in magma, gold is incorporated into the sulfides, resulting in its depletion in the residual melt through crystal fractionation. Fluids evolved from the melt may have a low potential for gold mineralization. Nevertheless, gold in early sulfides is readily remobilized by low pH and high f(O2) hydrothermal fluids. If a significant amount of these gold-bearing fluids are concentrated in suitable settings, such as shear zones, faults, or hydrofractures, they may generate gold deposits. Indirectly, the factors controlling sulfur saturation in magmas are crucial for the formation of intrusion-related gold systems.