Devonian intrusions of southwestern New Brunswick, Canada: U-Pb geochronology, petrogenesis and genetic constraints on gold mineralization

Abstract

ABSTRACT The Clarence Stream intrusion-related gold deposit is hosted within metasedimentary and metavolcanic sequences of the Ordovician Cookson and Silurian Mascarene groups that were intruded by several phases of the Late Silurian to Early Devonian Saint George Plutonic Suite. Based on geochemical and geochronological data, the intrusive rocks in the deposit are categorized into three distinct suites, including gabbroic, granitoid, and Whittier Ridge Granite (WRG). The gabbroic suite is the same as the East Branch Brook metagabbro intrusion, which exhibits a subalkaline mafic composition fractionated from a depleted mantle in a suprasubduction zone setting. The granitoid suite shows the same geochemical characteristics and age as the metaluminous to slightly peraluminous Magaguadavic Granite and its fractionated phases. The WRG suite reflects characteristics of calc-alkaline, metaluminous to peraluminous I-type granites in the area, such as Sorrel Ridge and Mount Douglas granitic plutons. In situ U-Pb zircon geochronology yielded an age of 419.5 ± 3.8 Ma for a monzodiorite from the gabbroic suite. Porphyritic granite and alkali granite from granitoid suite yielded U-Pb zircon age of 402.4 ± 1.8 Ma and 394.3 ± 3.8 Ma, respectively. Geochronologic data from the WRG suite reflect two different crystallization ages of 383.8 Ma (382.4 ± 3.1 to 385.1 ± 4) and 372.4 Ma (367.4 ± 1.3 to 377.1 ± 1.9). Several pieces of evidence demonstrate that the gold mineralizing events occurred after the emplacement of the East Branch Brook metagabbroic dykes and are linked to the various intrusive phases of the nearby syntectonic Early Devonian Magaguadavic Granite. The absence of gold and sulphide mineralization in felsic dykes related to the WRG confirms that this plutonic body formed adjacent to the gold zones after the main gold mineralizing event.