Genesis of the Moly Brook molybdenum-copper deposit, Newfoundland, Canada: Geological and geochemical constraints on fluid evolution, links with adjacent tungsten mineralization, and the role of Late Devonian granitic intrusions
Lynch, Edward P.
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An integrated geological, structural and geochemical study of the Moly Brook Mo-Cu deposit, southern Newfoundland (Canada), is presented. The role of subordinate leucogranite as progenitor to the mineralizing components (e.g. fluids, metals, sulfur) is assessed. Re-Os molybdenite dating constrains the timing of Mo-Cu-(W) mineralization and establishes a temporal framework for Late Devonian granite-hydrothermal systems in this sector of the northern Appalachian orogen. The Moly Brook deposit comprises a c. 0.8 x 1 km, c. N- to NNE-trending vein network that transects the c. 412 Ma Burgeo Intrusive Suite [BIS]. At the deposit, BIS wall rocks are moderately to intensely deformed and contain structures consistent with cataclasis within an c. E-W-trending deformation zone. Four wall rock subunits (megacrystic, intermediate, mafic, granitic) reflect igneous heterogeneities and variable responses to episodic deformation-metasomatism and alteration. Compared to distal (less altered) equivalents, BIS wall rocks display geochemical characteristics reflecting episodic modification by early pervasive calcic ± sodic metasomatism and subsequent vein-related potassic ± phyllic alteration associated with Mo-Cu mineralization. Overall, the vein network has a sheeted conjugate form, with local stockwork or irregular zones also occurring. Individual veins mainly strike c. NNW – NNE and are steeply c. E- or W-dipping. Subordinate c. E-W-striking veins also occur and exploit older structural fabrics. Vein characteristics, geometries and growth textures (e.g. crustiform banding) indicate the network formed under dominantly extensional conditions during a single phase of multi-step (cyclical) hydrofracturing. During veining, the maximum effective principal stress (σ’1) had a sub-vertical orientation, while fluctuating fluid pressures produced variable effective differential stress (σ’3 ≥ σ’2) which influenced vein orientations and formation modes. Paragenetically early, nonplanar, sulfide-bearing veins also suggest mineralization initially occurred at relatively high temperatures within a deeper setting and mark a ductile-brittle transition. A systematic relationship between vein frequency and Mo and Cu grades is not observed at Moly Brook, suggesting these metals were decoupled within the hydrothermal system. Nine vein types, formed during three successive stages (I to III), are recognised at Moly Brook. Stage I comprises amphibole-chlorite ± biotite (Type 1) and rare quartz-Kfeldspar veinlets (Type 2). Stage II (main sulfide stage) is divided into three sub-stages. Stage IIA (Mo-rich) comprises aplite-sulfide-molybdenite (Type 3), quartz-molybdenite (Type 4) and quartz-molybdenite-chalcopyrite-pyrite (Types 5) veins associated with variable K-feldspar + hematite ± magnetite alteration (potassic-ferroan assemblage). Stage IIB (Mo-poor) comprises quartz-chalcopyrite-pyrite (Type 6) and quartzmuscovite- fluorite ± chalcopyrite (Type 7) veins, mainly associated with sericite + pyrite ± fluorite ± quartz haloes (phyllic/greisen assemblage). Stage IIC (Pb ± Au association) comprises quartz-pyrite-galena veins (Type 8) associated with weak sericite alteration. Finally, barren Stage III calcite-fluorite veinlets (Type 9) and breccia zones crosscut all other vein types and represent a late, retrograde fluid infiltration event. Re-Os molybdenite dating of Stage IIA veins (Types 4 and 5) yielded overlapping ages ranging from 381.4 ± 1.6 to 379.9 ± 1.7 Ma (± 2σ), which have a weighted mean age of 380.9 ± 0.8 Ma (n = 4). Additionally, Type 5 equivalent veins crosscutting metamorphic rocks further south (closer to the Grey River tungsten prospect) yielded Re-Os ages of 381.9 ± 1.6 and 380.6 ± 2.0 Ma, which are similar to the Moly Brook dates. Combined, the mean weighted age for the six samples is 381.0 ± 0.7 Ma and represents the best estimate for the timing of Mo-Cu mineralization. These results confirm the epigenetic character of the deposit (with respect to the host BIS), the contemporaniety of Mo-Cu and W mineralization, and support a causative link with felsic magmatism which overlaps in both space and time with the deposit forming event. Fluid inclusions in Stage IIA-B quartz veins comprise carbonic, aqueous-carbonic [AC] and aqueous types that record the progressive evolution of the hydrothermal system. During Stage IIA veining, an initial high temperature (> 500°C), moderate salinity (c. 5 – 12 wt% NaCl eq.), supercritical fluid (approximate H2O-CO2-NaCl-KCl ± CO32- system) underwent volatile phase immiscibility and cooling between c. 520 and 250°C without major dilution by external fluids. Later fluid pulses (Stage IIB) record similar cooling trends (c. 460 – 240°C), a bulk loss of CO2 and fluctuating salinities (with a general trend towards marginally lower salinities). Isochore modelling of AC fluid inclusions in a Type 4 vein indicate initial trapping pressures of 1.8 to 2.2 kbars (c. 7 – 8 km lithostatic equivalent) and confirm a relatively deep (plutonic) setting. Vein sulfides have δ34S values ranging from +4.3 to +8.5‰ (n = 27) and indicate a consistent supply of 34S-enriched magmatic sulfur, derived from a mainly infracrustal (metaigneous) source, was involved in the mineralization. Undeformed leucogranite dykes at Moly Brook are similar to Late Devonian leucogranites in the broader Grey River-south coast area. Field relationships and petrographic features support a temporal and causative link between the dykes and Stage II veining and mineralization. Compositionally, the dykes are weakly peraluminous, high-silica granites enriched in K, Na, Rb, U and Th, with elevated K/Na, Rb/Sr, Ta/Zr and Mo/Cu ratios. Conversely, they are depleted in Fe, Ca, Mg, Ba, Sr and Ti, and have low Nb/Ta, Zr/Hf, LaN/YbN and (Eu/Eu*)N ratios. Whole-rock ɛNd(380Ma) values for Moly Brook-Grey River leucogranites range from -0.6 to -1.8 (n = 8), while Nd-TDM model ages range from 1.2 to 1.5 Ga (n = 4). These data, combined with trace element and sulfur isotope systematics, suggest parental magmas formed by anatexis of Ganderian continental crust that was compositionally modified by earlier Acadian-cycle subduction-collision processes. A possible direct or indirect contribution from juvenile lithospheric mantle material cannot be excluded however. Melt migration along translithospheric deformation zones would have facilitated extended magma fractionation ± assimilation and the transfer of mineralizing components from a deeper source area to the uppermost continental crust. Several lines of evidence support a genetic link between the Moly Brook Mo-Cu deposit and the Grey River tungsten prospect. These include; the structural similarity and continuity of both vein networks, their similar vein types and parageneses, overlapping hydrothermal ages from both areas, comparable fluid regimes, and inferred causative links with granitic intrusions. Combined, their geological features bear the hallmarks of a larger granite-hydrothermal system with an apparent north-south zoned metal distribution. Overall, the character of the Moly Brook-Grey River system and other analogous Mo ± W ± Cu mineralized areas in southeast Newfoundland, coupled with Re-Os age constraints for these systems, highlight a focused Givetian-Frasnian (c. 387 – 377 Ma) granitoid-metallogenic epoch in this sector of the northern Appalachians.
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