|dc.description.abstract||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.||en_IE