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

Abstract

A combination of in situ laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) analyses guided by Scanning Electron Microscope-Back-Scattered Electron imaging (SEM-BSE) was applied to hydrothermal monazite from greisen veins of the Late Devonian, highly evolved, uraniferous Mount Douglas Granite, New Brunswick, Canada. Understanding the uraniferous nature of the suite and characterizing the hydrothermal system that produced the associated mineralized greisen veins were the main goals of this study. The uraniferous nature of the Mount Douglas Granite is evident from previous airborne radiometric surveys, whole-rock geochemical data indicating high U and Th (2-22 ppm U; 19-71 ppm Th), the presence of monazite, zircon, xenotime, thorite, bastnaesite, and uraninite within the pluton and the associated hydrothermal greisen veins, as well as anomalous levels of U and Th in wolframite, hematite, and martite within greisen veins. New U-Pb geochronology of hydrothermal monazite coexisting with sulfide and oxide minerals yielded mineralization ages ranging from 344 to 368 Ma, with most of them (90%) younger than the crystallization age of the pluton (368 ± 3 Ma). The younger mineralization age indicates post-magmatic hydrothermal activities within the Mount Douglas system that was responsible for the mineralization. The production of uraniferous greisen veins by this process is probably associated with the High Heat Production (HHP) nature of this pluton, resulting from the radioactive decay of U, Th, and K. This heat prolongs post-crystallization hydrothermal fluid circulation and promotes the generation of hydrothermal ore deposits that are younger than the pluton. Assuming a density of 2.61 g/cm 3 , the average weighted mean radiogenic heat production of the Mount Douglas granites is 5.9 µW/m 3 (14.1 HGU; Heat Generation Unit), in which it ranges from 2.2 µW/m 3 in the least evolved unit, Dmd1, up to 10.1 µW/m 3 in the most fractionated unit, Dmd3. They are all significantly higher than the average upper continental crust (1.65 µW/m 3 ). The high radiogenic heat production of the Mount Douglas Granite, accompanied by a high estimated heat flow of 70 mW/m 2 , supports the assignment of the granite to a 'hot crust' (>7 HGU) HHP granite and highlights its potential for geothermal energy exploration.

Effects of Metasomatism on Granite-Related Mineral Systems: A Boron-Rich Open Greisen System in the Highiş Granitoids (Apuseni Mountains, Romania)

“…During greisenization, epidote precipitation can trap much of the light REE [8], which is consistent with the observation that the epidote in the Highis , massive was enriched in the light REE toward allanite-(Ce) [12]. low some general rules [1- 3,5]. Deposition of the ore minerals typically starts with an Sn-W oxide phase, followed by common sulfides (e.g., porphyry Cu-Au-Mo-ore deposits), and then a late oxide phase characterized by Fe-oxides associated with carbonates [1- 9,36].…”

“…According to Ciobanu et al [12], this mineralization was related to the mafic suite or associated with the granite. On the one hand, however, the paragenetic sequences of greisen systems follow some general rules [1- 3,5]. Deposition of the ore minerals typically starts with an Sn-W oxide phase, followed by common sulfides (e.g., porphyry Cu-Au-Mo-ore deposits), and then a late oxide phase characterized by Fe-oxides associated with carbonates [1- 9,36].…”

“…However, mineralization that occurs millions of years after the emplacement of a pluton may become a reality. Mohammadi et al [5] and Liu et al [38] pointed out that there is another source of heat that can be involved in the postmagmatic hydrothermal activity associated with greisenization. The formation of uraniferous greisen veins can be associated with the high heat-producing (HHP) nature of a pluton, resulting from radioactive decay.…”

“…The formation of uraniferous greisen veins can be associated with the high heat-producing (HHP) nature of a pluton, resulting from radioactive decay. For example, based on the U-Pb geochronology of hydrothermal monazite, the time gap between the emplacement of the granite pluton (Mount Douglas Granite, New Brunswick, Canada) and the time of the youngest mineralization can be longer than 20 My [5]. The results of the finite element numerical modeling of Liu et al [38] indicate that an increase of 500 to 1000 m in magma thickness can prolong the suprasolidus lifetimes of HHP magmas by 50 to 74%.…”

“…Related late to postmagmatic metasomatic processes have a close relationship with the economical enrichment of volatile components such as B, F, and Cl, and metallic elements such as Li, Be, Mo, Sn, W, Bi, U, and rare earth elements (REE). The greisen ores contain a characteristic mineral assemblage comprising quartz, muscovite, tourmaline, topaz, and fluorite, usually adjacent to quartz-cassiterite-'wolframite' veins (see, e.g., [1][2][3][4][5][6][7][8][9]). The paragenetic sequences of greisen-related mineralizations appear to follow some general rules that could be widely applicable to most greisen deposits [1-3].…”