The pedogenesis of tin- and sulfide-lode mineralization at True Hill, southwestern New Brunswick

Abstract

The subvolcanic True Hill granite porphyry in southwestern New Brunswick is genetically related to the Beech Hill series of granites of Devono-Carboniferous age. Three True Hill (TH) greisenised granite porphyry cupolas host Bi-Sn-Mo-W mineralized zones that have similarities to the nearby W-Mo-Bi orebodies at Mount Pleasant (MP).Tin-bearing lodes (0.03 to 0.66 wL % Sn) are developed along faults and fractures that cut both mineralized (Bi-Sn-Mo-W) and unmineralized True Hill granite porphyry and enclosing metasedimentary rocks of the Waweig Formation. Within these lodes, particularly the central Main lode, two types of mineralization are present, an early chlorite-sulfide assemblage (Stage 1) and a later hematite-phengite-cassiterite assemblage (Stage 2) that replaces, in part, the earlier Stage 1 assemblage. The Stage 1 assemblage consists of Fe-rich septechlorite, pyrite, low-Fe sphalerite, magnetite, chalcopyrite, galena, Pb-Bi sulfides, tennantite, arsenopyrite and native Ag. Fe-Mg-Mn metasomatism responsible for chloritization was coincident with leaching of alkali and alkali-earth elements during feldspar hydrolysis. The Stage 1 assemblage (chl-sulfide) was formed at low temperature (<200°C) and low oxygen fugacity. The Stage 2 assemblage includes hematite, phengite, cassiterite and monazite with minor late quartz and fluorite. This assemblage was deposited at slightly higher temperatures (200p-300pC), higher oxygen fugacity (above hematite-magnetite buffer) and at moderate pH 's (<musc-Kfs buffer). The overprinting of Stage 2 assemblages on portions of Stage 1 locally produces complex textures within the lodes at True Hill. The composition of both Stage 1 and 2 assemblages in the lodes is controlled by the (i) host rock composition and (ii) the mineralizing hydrothermal fluids. The deposition of Sn and LREE from Stage 2 fluids formed cassiterite and monazite along the reaction front between Stage 2 assemblages and chlorite-sulfide (Stage 1) and granite. Deposition of cassiterite probably occurred in response to (i) an increase in the Fe-Cl complexing at the expense of Sn-and LREE-C1 complexes during interation with Stage 1 assemblages, (ii) an increase in pH due to the feldspar hydrolysis in the granite and (iii) cooling of the fluid by the host rock. Although uneconomic, tin-sulfide lodes at True Hill have similarities with lodes developed within the North Zone of Mount Pleasant (New Brunswick) and Cornwall (England). However, the association of hematite and phengite with cassiterite mineralization is unique to this deposit.Le porphyre granitique subvolcanique de True Hill (Nouveau-Brunswick meridional) est lid par sa genese au cortege granitique devono-carbonifere de Beech Hill. Trois coupoles de porphyre granitique de True Hill (TH) alter6 en greisen renferment des zones min£ralis6es en Bi-Sn-Mo-W et qui montrent des similitudes avec les gisements voisins de W-Mo-Bi presents k Mount Pleasant (MP).Des faisceaux filoniens porteurs d 'dtain (0,03 a 0,66% pds. Sn) s'installent le long de failles et de cas...

Petrochemical evolution of subvolcanic granitoid intrusions within the Late Devonian Mount Pleasant Caldera, southwestern New Brunswick, Canada: comparison of Au versus Sn-W-Mo-polymetallic mineralization systems

“…Clearly, hydrothermal breccias, related pervasive alteration and mineralization (silicifi cation, chloritization, and greisenization) in GRI and GRII (Kooiman et al 1986) refl ect pressure fl uctuations of fl uids associated with the granitic magmas in the Mount Pleasant deposits. These processes are similar to those that operated in the adjacent True Hill biotite porphyritic granite intrusions (Lentz et al 1988;Lentz and McAllister 1990;Lentz 1994;Lentz and Gregoire, 1995), suggesting a comparable genesis for all Sn-W-Mo-Bi mineral deposits in the region.…”

“…More detailed petrochemical investigations and reliable age determinations (e.g. U-Pb zircon or monazite) are required to characterize the petrogenetic linkages among these granitoids, although they generally have been thought to be related temporally, spatially, and thus genetically (Lentz et al 1988; Lentz and McAllister 1990;McLeod 1990). …”

Petrochemical discrimination of evolved granitic intrusions associated with Mount Pleasant deposits, New Brunswick, Canada

“…Based on these calculations (Table 5), the NZ granites have a greater tin potential than the FTZ granites; the PG has the highest ranking in the latter. The THG, 3 km west of Mount Pleasant, is poorest although still a tin-bearing granitoid, as confirmed by its moderate-size bodies of mineralisation (Lentz, 1994;Lentz and McAllister, 1990). In the NZ, the facies GI, GII and GIII have an increasing tin metallogenetic potential and because it is based only on one analysis, the authors can merely suggest that the potential of the ETZG may be even higher than that of GIII (Table 5).…”

Trace element composition and U-Pb ages of cassiterite from the Bolivian tin belt

“…Pollard et al 1987 andHeuschmidt et al 2002) showing the postulated ideal location of the studied deposits. Granite-hosted and granite cupolas: Moore and Howie (1979), Lentz and McAllister (1990), Plimer et al (1991), Murciego et al (1997), Gorelikova et al (2004Gorelikova et al ( , 2006, Abdalla et al (2008), Neiva (2008) 2019), Hulsbosch and Muchez (2019). Migmatiticrelated: Murciego et al (1997), Jiang et al (2004).…”