PACROFI VI - Electronic Program

Tectonism, mesothermal gold, and fluid evolution in the Paleoproterozoic Glennie Domain of the Trans-Hudson Orogen, Saskatchewan, Canada

Durocher, Kyle E.1, Kyser, T. Kurt2, Ansdell, Kevin M1 and Delaney, Gary D.3

  1. Department of Geological Sciences, 114 Science Place, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, S7N 5E6.

  2. Department of Geological Sciences, Queen's University, Kingston, Ontario, Canada, K7L 3N6.

  3. Saskatchewan Geological Survey, 1914 Hamilton Street, Regina, Saskatchewan, Canada, S4P 4V4.

The characterization of fluid events in terms of physiochemical and temporal relationships plays an important role in deciphering the P-T-t history of a lithotectonic terrane. This paper presents results of a multi-disciplinary study using fluid inclusion microthermometry, stable and radiogenic isotope systematics and mineral compositions to constrain the P-T-t evolution of the Glennie domain (GD) of the Trans-Hudson Orogen (THO) and probable fluids associated with mesothermal gold mineralization.

The GD is a distinct lithotectonic terrane within the THO and hosts approximately 80 gold showings, including one of Saskatchewan's operating gold mines, the ca. 300,000 oz. Seabee Mine. This terrane comprises narrow, arcuate, arc-derived volcanic and sedimentary rocks, separated by arc-derived granitoid plutons. The exposed Glennie domain is bounded to the east by the early-ductile, late-brittle Tabbernor Fault, to the south by Paleozoic sedimentary cover, and to the west by the Stanley Shear Zone. The GD is one of several distinct lithotectonic elements of the 1.7-1.9 Ga THO, a collage of variably deformed and metamorphosed volcanic, plutonic, and sedimentary rocks, which separate the Archean Superior Province to the southeast from the Archean Hearne Province to the northwest.

Existing and new geochronological data have delineated four pulses of granitoid plutonism in the Glennie domain. Suite A comprises felsite and dioritic dykes which range in age from 1886 to 1856 Ma. Most granitoid rocks belong to Suite B, which consists of tonalitic and granodioritic plutons intruded between 1846 and 1859 Ma. Suite C comprises small granodioritic to granitic intrusions emplaced between 1830 and 1836 Ma. Syntectonic to post-tectonic pegmatites and aplogranites were emplaced between 1817 and 1760 Ma and comprise Suite D.

Four episodes of deformation have been documented in the GD, termed D1 through D4. Peak metamorphic conditions were synchronous with D3. The timing of three episodes has been constrained by U-Pb dating of granitoid plutons from the suites mentioned above. D1 occurred sometime prior to ca. 1860 Ma, D2 between ca. 1830 and 1825 Ma, and peak metamorphism and D3 sometime after ca. 1825 Ma. U-Pb and Pb-Pb zircon ages from elsewhere in the Trans-Hudson Orogen suggest that peak metamorphism took place at ca. 1790-1815 Ma, consistent with what is observed in the Glennie domain. Metamorphic grade generally increases from upper greenschist facies in the south to granulite facies in the north, where many of the greenstone belts have been highly deformed and migmatized.

Two areas of the GD have been the focus of this study, namely the Santoy Lake area within the Pine Lake Greenstone Belt, in the northern portion of the domain, and the Brownell Lake area, within the Brownell-Wapawekka Lakes Greenstone Belt in the southern portion of the domain. Both areas are underlain by arc-derived supracrustal assemblages, later intruded by granitoid plutons of varying ages. Garnet-biotite and GASP thermobarometry yield peak metamorphic conditions of ca. 660oC and 7.2 kbar in the Santoy Lake area, and ca. 545oC and 5.5 kbar in the Brownell Lake area. Ar-Ar ages of biotite and hornblende from both areas suggest a cooling rate in the Brownell Lake area of approximately 4oC/Ma between ca. 1805-1760 Ma, and approximately 1oC/Ma between 1760 and 1700 Ma. The cooling history at Santoy Lake is approximately 4oC/Ma between ca. 1805-1720 Ma.

Gold mineralization in the Santoy Lake area is spatially associated with shear-hosted quartz-diopside-calcite veins. Shear zones and associated veins are temporally related to the pre-peak metamorphic D2 event, and these structures, up to 25 metres wide and 600 metres in length, are in places folded by D3 style deformation. Gold mineralization typically occurs along the vein-wallrock contact as disseminations or within late fractures in arsenopyrite. Gold is paragenetically late relative to other sulfides at Santoy Lake, and is often associated with tellurides, chalcopyrite, sphalerite and galena. Mineralization is difficult to follow from drilling, but grades are as high as 143 g/t Au over a two metre interval at Gold Zone 2. The location of Au may be related to post-vein remobilization during later deformation/thermal/fluid events. Oxygen isotope equilibration temperatures of vein mineral pairs and arsenopyrite geothermometry indicate temperatures between ca.440-620oC, and calculated d18O fluid compositions of 6-8.5 per mil. These veins, emplaced prior to peak metamorphism, may have seen one or more later fluid events, thereby altering the primary isotopic compositions of mineral constituents. Therefore, vein thermometry data likely represent re-equilibration temperatures and not the original vein emplacement conditions. Rb-Sr isotope data on alteration biotite suggest a re-equilibration age of ca. 1800 Ma.

Gold mineralization in the Brownell Lake area is found within shear-hosted quartz-arsenopyrite microveinlets in volcanic rock and along arsenopyrite-bearing foliation surfaces within granite-hosted shear zones. These structures are related to a late stage of the peak metamorphic D3 event, and have not been affected by a later regional deformational episode. Gold is paragenetically early relative to most sulfides within the quartz-arsenopyrite veinlets. Shear zones are significantly smaller in dimension and gold grade lower from those observed at Santoy Lake. Structures which host mineralization are typically no larger than 100 metres long and 10 metres wide, with grades typically less than 10 g/t Au. Arsenopyrite geothermometry yields temperatures of between 400-550oC. Gold deposition probably occurred within this temperature range, based on the overlap in timing of deposition of gold and arsenopyrite. Vein minerals have oxygen isotope temperatures between ca. 375-575oC and calculated d18O fluid compositions of 8.5-10.5 permil. A Rb-Sr alteration biotite model age of approximately 1785 Ma, and is the best estimate for the timing of gold mineralization in the area.

The recognition of primary vs. secondary inclusions in variably-deformed quartz veins is problematic, especially since early inclusions were likely affected by later deformation and associated fluid events, particularly in the Santoy Lake area. Most inclusions are small, averaging about 5-10 microns in diameter, although some are as large as 30 microns in length. Inclusions can be divided into 2 principle types: (1) H2O-NaCl and (2)H2O-CO2-NaCl.

At Santoy Lake, both inclusion types are present as isolated, possibly primary inclusions. Relative timing of these early inclusions is difficult to discern. Type (1) primary inclusions have a high degree-of-fill (90-98), ice-melting temperatures (Tmice) between -0.5 to -8.2oC, which correspond to salinities of 0.8-12.0 wt% NaCl equiv., and homogenize to the liquid phase at temperatures (Th) between 125 and 219oC. Type (2) primary inclusions have CO2 volume fractions of 0.3 and 1.0, decrepitate prior to total homogenization, have bulk densities of 0.73-0.95, and CO2 homogenization temperatures (ThCO2) of 9.3-23.7oC. Clathrate melting temperatures (TmClath) range from 8.4 to 10.0oC which correspond to salinities of 0.02 to 3.2 wt% NaCl equiv. Type (1) and (2) inclusions are found in secondary planes of various relative ages. Type (1) secondary inclusions at Santoy Lake have a high degree-of-fill (90-98), Tmice values between -0.1 and -8.3oC, and salinities of 0.0-12.1 wt% NaCl equiv. These inclusions homogenize to the liquid state between 143-222oC, and possess bulk densities of 0.85-1.0. Type (2) secondary inclusions are pure CO2, possess ThCO2 values between 23.1-23.6oC, and have bulk densities of 0.73-0.74.

At Brownell Lake, two inclusion types are present, with only Type (1) inclusions being apparently primary in origin. These primary inclusions are two-phase, have a high degree-of-fill (88-95), and Tmice values of -0.6 to -7.4oC, with corresponding salinities of 1.0-11.1 wt% NaCl. Type (1) primary inclusions homogenize between 183-232oC and have bulk densities of 0.83 to 0.93. Inclusion types (1) and (2) can be found as secondary inclusions. Type (1) secondary inclusions are three-phase, with degrees-of-fill between 90 and 97. Daughter mineral dissolution temperatures (TmNaCl) of 169-200oC correspond to salinities of 30.4-31.8 wt% NaCl equiv. These inclusions homogenize to the liquid phase between 107-159oC. Two-phase secondary aqueous inclusions are also found. These inclusions have degrees-of-fill between 85 and 97. Thier small size makes salinity measurements difficult, with one value of 4.4 wt% NaCl equiv. These inclusion homogenize between 162-305oC, and have bulk densities between 0.67-0.94. Type (2) secondary inclusions have CO2 volume fractions of 0.6-1, ThCO2 values of 13.7-14.7, and TmClath values of 7.4-7.5, corresponding to salinities of 4.8-5.0 wt% NaCl equiv. and bulk densities of 0.83-0.91.

Secondary and earlier-formed inclusions at Santoy Lake are similar in thier physiochemical characteristics. These trapped fluids are of similar densities and salinities and suggest that either a similar fluid was trapped during multiple fluid events or earlier formed inclusions were modified by later low salinity fluids during peak and subsequent episodes of metamorphism and deformation. Using quartz-mineral oxygen isotope geothermometry and calculated fluid inclusion isochores of earlier formed inclusions, corrected trapping pressures fall over the wide range of 2 to 9 kbar, suggesting that some of these inclusions have undergone post-entrapment density modification. At Brownell Lake, primary inclusions are characterized by thier low salinity relative to the common 3-phase aqueous, high salinity secondary inclusions. Primary inclusions at Brownell Lake possess a range of corrected trapping pressures of 3 to 5 kbar, and may represent the actual depth of emplacement of the quartz-arsenopyrite veinlets.

Similarities and contrasts between gold camps in the two areas are evident. Veins which host mineralization were emplaced during different stages of the tectonic evolution of the GD. The calculated oxygen isotopic compositions of fluids from both areas are distinct and consistent with fluids in systems characterized by low water/rock ratios. Oxygen isotopic compositions from both areas straddle the accepted values of magmatic- and metamorphic-derived fluids, although fluid sources are difficult to constrain based on oxygen isotope data alone. In both areas, gold showings occur within 400 metres of conglomeratic/pelitic rocks and the devolatilization of these units may have mobilized gold into the structures which host gold mineralization.