History of the fluids associated with the lode-gold deposits, and complex U-PGE-Au vein-type deposits, Goldfields Peninsula, Northern Saskatchewan, Canada
Rees, Matthew Ian
The varied mineral deposit types of the Beaverlodge area, Northern Saskatchewan, indicate that the rocks have undergone a complex and protracted fluid history. All of the deposits studied are physically hosted in a variably metamorphosed sedimentary sequence, the Aphebian-aged Murmac Bay Group (complex Au-PGE-U vein-type deposits, e.g. Nicholson, Fish Hook), or within granites "intrusive" into the Group (lode-gold deposits, e.g. Box, Athona, Frontier). The complex Au-PGE-U vein-type deposits, and other minor mineralizations, are genetically associated with later, overlying sedimentary sequences such as the Late Aphebian to Helikian-aged Martin and Athabasca Groups. The succession of sedimentary basins was probably controlled by the virtually continuous tectonic evolution of the area prior to, and during, the Thelon-Talston (ca. 2.0 Ga), and Hudsonian (ca. 1.8 Ga) orogenies. The first fluid event identified is F1, which occurred at ca. 1.97 to 1.95 Ga, when the mine granites were formed as nearly in-situ melts in the presence of a "metasomatic" fluid. The origin of the event is attributed to the burial of the Murmac Bay Group, presumably in a subduction-related setting, during the Thelon-Talston orogeny at ca. 2.0 to 1.9 Ga. δ¹⁸O values of the granite minerals indicate an isotopic closure temperature for this event of 550 to 650°C, and that the "metasomatic" fluids may have been related to magmatic activity that occurred at deeper structural levels in the subduction zone. The development of a regional foliation, D1, may have occurred during the earlier stages of this event as well. The Thluicho Lake Group may be the remnants of a turbidite basin that developed in the back-arc area of the subduction zone. The next fluid event documented is that which produced lode-gold quartz vein mineralization, associated with D2 deformation. This event is recorded in the veins by Type I and Ia fluid inclusions, and by the stable isotopic compositions of vein minerals as fluid events F2 and F3 (Which are likely parts of the same overall fluid event). This fluid, at least during the late stage of vein development, is suspected to have been associated with gold deposition, and is indicated to have had minimum temperatures of about 300°C, and minimum pressures of 2.5 to 3.0 kbars. As the fluid was C0₂-bearing, and the inclusion populations appear to indicate pre-entrapment immiscibility, these temperatures and pressures are trapping conditions. This is confirmed by some stable isotopic fractionations between quartz and chlorite gangue minerals that yield temperatures of 300 to 400°C. The minerals were in equilibrium with a fluid of metamorphic origin. Rb/Sr ages of vein minerals indicate they are at least 1.84 Ga, and probably between 1.855 and 1.84 Ga, suggesting that the D2 folding, and the retrograde metamorphic fluid that likely caused the lode-gold mineralization, were related to uplift during early Hudsonian collision. The source of the retrograde fluids is not clear, but the fluids may have been derived from the dehydration of material that was subducted during the Thelon-Talston orogeny, as the high ⁸⁷Sr/⁸⁶Sr ratios of the vein minerals, as well as feldspar from the Mine granites, imply that F1 to F3 fluids were derived from, or interacted with significantly older (i.e. deeply buried) crustal material. The fluid represented by Type II inclusions is of an uncertain origin, but clearly post-dated the lode-gold veins (1.84 Ga), and pre-dated the Au-PGE-U mineralization (ca. 1.75 Ga?). This fluid event appears to have been regionally extensive, however, and may be related to events in the Beaverlodge area that are thought to have occurred ca. 1.78 to 1.74 Ga, such as regional albitization, or the simple vein-type uranium deposits. This age of approximately 1.75 Ga is often referred to as the "late overprint", suggested to be related to the later stages of the Hudsonian Orogen. The "late overprint" is also recorded by feldspars from the Box mine granite, which yield an Rb/Sr age of ca. 1.77 Ga. The Martin basin may have started to form at this time, due to epeirogenic fault movement during the waning stages of the Hudsonian Orogen, and this fluid event may also be related to early diagenesis within the basin. The next fluid event, F4, is recorded by primary fluid inclusions in the complex AU-PGE-U vein-type deposits, which contain fluids of a moderately high salinity (28 to 36 wt % NaCl eq.). A similar fluid is found in secondary Type IV inclusions from the lode-gold veins, and in fluid inclusions from the "sponge rock" zones that have locally altered the mine granites. Stable isotopic equilibration temperatures of vein minerals from the Nicholson and Quartzite Ridge deposits indicate that the fluid was at a temperature of 100 to 120°C. The age of this fluid event may be as old as ca. 1.8 to 1.7 Ga, because the depleted δD value of the fluid, at approximately -90 permil, implies a high latitude for the area. The area was only located at the required high latitudes during two periods of time. For this reason, it is possible that Au-PGE mineralization, such as the Nicholson and Quartzite Ridge deposits, may have formed from diagenetic basinal fluids derived from, or influenced by, the Martin basin, or an unrecognized "proto-Athabasca" basin. Whether or not uranium accompanied the gold and platinoids, or was later overprinted on pre-existing Au-PGE mineralization is not presently known. F5 is the youngest fluid event, recorded by the late U-bearing comb veins. The fluids were of a high salinity (40 wt. % NaCl eq.) , and stable isotopic equilibrium fractionations between vein minerals indicate a temperature of approximately 175°C. The calculated stable isotopic values of the fluid are similar to those of the diagenetic fluids in the Athabasca basin between ca. 1.6 and 1.0 Ga, during which time the unconformity-type and complex vein-type uranium deposits were formed. As mentioned above, this may be the time during which uranium mineralization overprinted the AU-PGE mineralization, forming "hybrid" complex Au-PGE-U vein-type deposits. An early stage of the F5 fluid event is represented by Type III secondary inclusions in the lode-gold veins. The moderate salinity of the fluid (10 to 15 wt. % NaCl eq.) is similar to fluid inclusions observed in quartz overgrowths from the Athabasca basin, where the fluid is interpreted to be of an early diagenetic origin.