Petrogenesis of Cu-Ag skarn mineralization in the Mackenzie Gulch area, Northern New Brunswick, Canada

Abstract

Northern New Brunswick and the adjacent Gaspé Peninsula of Québec host numerous contact metasomatic Cu–Au skarn deposits commonly in proximity to small Siluro–Devonian intermediate to felsic stocks and/or dikes. In this region, specifically northern New Brunswick, skarn occurrences are principally hosted within the late Ordovician to early Silurian Matapédia Group, which consists of thin-bedded, dark grey argillaceous limestone and calcareous siltstone. This thesis investigated Cu–Ag skarn occurrences at the McKenzie Gulch (MG) area in northern New Brunswick by integrating field observations, petrographic, petrochemical, electron-probe microanalysis (EPMA), laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), fluid inclusions, geochronology (U–Pb zircon and titanite), stable S isotopes, and radiogenic Pb isotopes with the main objective being to understand the petrogenesis of skarn occurrences and establish their relationship to the intrusive rocks. This investigation involved the evaluation of porphyry dikes that are spatially and temporally related to these skarn systems by examining potential parameters that are known to be associated with the genesis of intrusion-related deposits. These magmatic aspects include oxidation state of the magma, water content, P–T conditions, in addition to magma source characteristics, formation, petrochemistry, evolution and emplacement conditions. As a result, two suites of porphyry dikes were recognized in the MG area during this evaluation: (1) plagioclase–hornblende (P–H), and (2) quartz–plagioclase (Q–P) porphyry suites. These Middle Devonian (386.0 ± 2.3 Ma) intrusive rocks are mildly oxidized, I-type granitoids with calc-alkaline affinity that range in composition from granodioritic to tonalitic (with few granitic), based on their petrochemical features. These dikes also exhibit adakite characteristics, a geochemical feature that has been widely associated with many porphyry Cu (and skarn) systems. Petrochemical features and cathodoluminescence (CL) characteristics of quartz crystals from these suites of dikes suggest that two magma chambers were responsible for evolution of these intrusive rocks. The three types of quartz crystals with contrasting CL features have been attributed to at least three different environments of crystallization: 1) a higher temperature environment (up to 915 oC) as recorded by cores of phenocrysts with high Ti concentrations (up to 200 ppm); 2) a moderate to high temperature (≥ 700 ≤ 750 oC) environment as recorded by the transition zones within quartz phenocrysts and microphenocrysts; and 3) a low temperature environment in which groundmass quartz quench crystallized during hypabyssal dike emplacement. Studies of the mineral chemistry showed that composition of calc-silicates (i.e., garnets and pyroxene) have intimate relationship with the dominant metal of the mineralized skarns. These calc-silicate phases plot in their respective compositional fields of Cu-dominated skarns. The investigation conducted on fluid inclusion, geochronology, stable isotope, and mineralization supports field observations, which indicates that skarn formations at the MG area have an association with the intruding porphyry dikes and that this skarn is very similar to and shows no difference with the characteristics of other world-class, intrusion-related skarn systems. Geochronological studies based on U–Pb zircon and hydrothermal titanite indicates a coeval relationship between mineralization and dikes emplacement, suggesting that mineralization in the MG area is short lived or it occurred near the end of magmatic-hydrothermal activity.