Origin, timing, and fluid characteristics associated with paleoproterozoic Jasper lode-gold deposit, Saskatchewan Canada
Date
1994
Authors
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Degree Level
Masters
Abstract
The Jasper mesothermal gold deposit, in northern Saskatchewan,
Canada, is situated within the Paleoproterozoic La Ronge domain of
the Trans-Hudson orogen. The deposit occurs within a northeasterly
striking, and steeply northwest-dipping, mylonitic shear zone which
cross-cuts the Island Lake pluton having a zircon Pb-Pb age of 1855
+/- 8Ma. Gold mineralization, which is hosted by subvertical quartz
veins that occupy the most strained portions of the shear zone,
occurs in late fractures and as particles within recrystallized
quartz. The gold mineralized zones are 2-3 metres wide and plunge
steeply to the northeast. Mineralized veins have a complex
paragenesis wherein recrystallized quartz, muscovite, sphalerite,
galena, and chalcopyrite are paragenetically associated with gold.
Obvious alteration of the granitic hostrock adjacent to the veins
is lacking except for slight ¹⁸O-enrichments of wallrocks up to 20
metres from the veins.
Quartz veins which occur peripheral to the most-strained
regions of the shear zone are discontinuous bands with little to no
subgrain development and do not contain gold. The δ¹⁸O values of
this barren quartz average 11.1 per mil, whereas the δ¹⁸O values of
quartz comprised of at least 50% subgrains, and associated with
gold mineralization, average 12.9 per mil. Oxygen isotope
fractionation between quartz and coexisting muscovite associated
with gold indicate gold mineralization occurred at ca. 300°C.
Calculated δ¹⁸O and δD values for the ore-forming fluid are
compatible with fluids derived from, or which have interacted with,
igneous or metamorphic rocks at low water/rock ratios. The initial
⁸⁷Sr/⁸⁶Sr ratio of the auriferous fluid is 0.7028, which is similar
to the surrounding igneous rocks, and implies that the strontium
was likely derived by leaching of Paleoproterozoic rocks similar in
age and strontium isotope composition to those presently exposed on
surface.
Two-phase secondary aqueous fluid inclusions outline and
dominate healed fracture planes in both barren quartz veins and in
the less deformed portions of mineralized quartz veins. The fluids
in these inclusions have low salinities, generally not exceeding 7wt% NaCl equivalent, and homogenization temperatures of 150°-200°C.
Fluid inclusions within dynamically recrystallized quartz tend to
concentrate at quartz subgrain boundaries along with gold, are
vapour-rich, have complex gas and cation compositions,
homogenization temperatures between 300°C and 375°C, and show
characteristics indicative of fluid-phase immiscibility.
The age of gold mineralization was close to 1720 Ma, as
indicated by Ar-Ar systematics of gold-associated muscovite, and
Rb-Sr ages of muscovite and tourmaline. The 1720 Ma age indicates
gold mineralization occurred ca. 100 million years after peak
regional metamorphism, initial development of the shear zone and
quartz vein emplacement and the cessation, at ca. 1820 Ma, of
collision in this section of the Trans-Hudson orogen.
These data are consistent with a model wherein earlier quartz
veins were reactivated and partially recrystallized during a post-
tectonic, gold-depositing fluid event. Gold distribution was
regulated by the development of fracture-controlled permeability
and by H₂O-CO₂ phase separation of a low salinity, CO₂-rich,
metamorphic fluid at 300°C to 375°C, which was derived from a source
similar to the igneous rocks spatially associated with the deposit.
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Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geological Sciences