Re-Os geochronological constraints on the mineralizing events within the Mount Pleasant Caldera: implications for the timing of sub-volcanic magmatism

Abstract

The Mount Pleasant granite-related polymetallic deposit, located on the southwestern margin of the Late Devonian to Early Carboniferous Mount Pleasant Caldera Complex in southwestern New Brunswick, contains a significant resource of tin, tungsten, molybdenum, zinc, indium, and bismuth. The Caldera Complex comprises Intracaldera, Exocaldera, and Late Caldera-Fill sequences and associated subvolcanic granitic rocks. Three granitic phases of the Mount Pleasant Granitic Suite (Granite I, II, and III) are recognized in the vicinity of the Mount Pleasant deposit and are interpreted to be fractionates of the more regionally exposed McDougall Brook Granitic Suite. Granite I and Granite II are associated with tungsten-molybdenum-bismuth, and tin-zinc-indium mineralization, respectively. Despite extensive research within the Caldera Complex, the exact age of mineralization at Mount Pleasant has never been firmly established. An inferred age of 363 ± 2 Ma was based on the proposed synchronicity of the U-Pb dated Bailey Rock Rhyolite of the Exocaldera Sequence with that of the undated McDougall Brook Granitic Suite, which intrudes the Intracaldera Sequence. Here, we present Re-Os dating of two molybdenite samples associated with the tungsten mineralization related to Granite I at the Fire Tower Zone, that constrain the initial onset of mineralization at Mount Pleasant to be between 369.7±1.6 Ma and 370.1±1.7 Ma. The new Re-Os ages clearly indicate that the McDougall Brook Granitic Suite, which pre-dates mineralization, must be at least seven million years older than the Bailey Rock Rhyolite, whose type-section is located within the Exocaldera Sequence. A re-examination of the gradational relationship between the McDougall Brook Granitic Suite and purported rocks of Bailey Rock Rhyolite within the Intracaldera Sequence revealed that the latter should instead be assigned to the Seelys Formation. Thus, deposition of the polymetallic mineralization likely took place contemporaneously with caldera collapse and an early phase of resurgent doming in response to degassing of the magma chamber rather than being coincident with erosion of the volcanic edifice as inferred from previous modeling of the eruptive history at Mount Pleasant.

Geological relationships and laser ablation ICP-MS U-Pb geochronology of the Saint George Batholith, southwestern New Brunswick, Canada: implications for its tectonomagmatic evolution

“…McLeod (1990) recognized and named the poorly exposed John Lee Brook Granite as a distinct pluton during his mapping of the Saint George Batholith. It is a mediumgrained, equigranular, garnetiferous, two-mica granite, underlying an area of about 50 km 2 along the northern margin of the batholith just south of the Late Devonian Mount Pleasant Caldera (Thorne et al 2013). The 40 Ar / 39 Ar muscovite age of 384 ± 7 Ma from the John Lee Brook Granite together with the reported presence of white microgranitic veins cutting the adjacent Magaguadavic Granite, dated by U-Pb on zircon at 396 ± 1 Ma (Bevier 1990), suggested that the former was younger than the latter (McLeod 1990).…”

Structural setting and age of the Partridge Island block, southern New Brunswick, Canada: a link to the Cobequid Highlands of northern mainland Nova Scotia

“…The Mount Douglas Granite, young- (Bevier 1988). The associated Mount Pleasant caldera complex includes felsic volcanic rocks emplaced in a post-collisional, transtensional regime following the Early Devonian Acadian Orogeny, and is likely of similar age having erupted between 370 ± 2 and 363 ± 2 Ma (Thorne et al 2013). These rocks are at least in part older than the Sheldon Point rhyolite of the Taylors Island Formation, reported herein at 360 +6/-5 Ma.…”

U–Pb Geochronology of Hydrothermal Monazite from Uraniferous Greisen Veins Associated with the High Heat Production Mount Douglas Granite, New Brunswick, Canada

“…The highly fractionated peraluminous Mount Douglas Granite (MDG), composed of three units (Dmd1, Dmd2, and Dmd3; Figures 1 and 2), has characteristic features of High Heat Production (HHP) Granites, in which the granites have elevated concentrations of K 2 O, Rb, LREE, U, and Th. They [22] and Mohammadi et al [29]). The MDG is divided into units Dmd1, Dmd2, and Dmd3 (modified after McLeod [17]).…”

“…The crystallization age of the Bocabec Gabbro was taken from Clarke et al [30]. Mount Pleasant's molybdenite Re-Os geochronology was taken from Thorne et al [22]. LA ICP-MS U-Pb zircon and monazite geochronology of the Utopia Granite, Jake Lee Mountain Granite, Wellington Lake Granite, John Lee Brook Granite, and Magaguadavic Granite were taken from Mohammadi et al [29].…”

“…LA ICP-MS U-Pb zircon and monazite crystallization ages of the MDG were taken from Mohammadi [15]. [22] and Mohammadi et al [29]). The MDG is divided into units Dmd1, Dmd2, and Dmd3 (modified after McLeod [17]).…”