Structural setting, geochemistry, and temporal relationships of the lode gold deposits in the Star Lake-McLennan Lake area, northern Saskatchewan

Abstract

The gold deposits in the Star Lake-McLennan Lake area, northern Saskatchewan are epigenetic, shear zone-controlled lode gold deposits. The deposits are hosted in the calc-alkaline Star Lake and Island Lake plutons, which intrude the calc-alkaline and low-potassium tholeiitic volcanic rocks of the Central Metavolcanic Belt of the La Ronge Domain, the Paleoproterozoic Trans-Hudson Orogen.

The tectonic fabrics of the area define three phases of deformation. D1 structures include regional foliation with contained mineral lineation; isoclinal folds with axial surfaces parallel to the foliation; and foliation within the McLennan Lake Tectonic Zone that strikes parallel to the contact between the Central Metavolcanic Belt and the McLennan Group, and dips at shallower angles than the regional foliation. D2 structures are subvertical, mesoscopic shears and cataclastic zones; and associated steeply plunging mineral lineations. Asymmetric and rotated fabrics indicate that the direction of shearing on the D2 structures is at a high angle to the mineral lineation. D3 structures are dip-slip gold-bearing shears and shallow-dipping shears that are conjugate to the gold-bearing ones. D3b structures include asymmetric folds of early fabrics and associated shears and quartz veins, which indicate dextral movement on the McLennan Lake Tectonic Zone and the D3a gold-bearing shears. The gold deposits were emplaced into the D3 shears, synchronously with the D3a dip-slip shearing, and were subsequently deformed by the D3b deformation. The deformations occurred during and following the terminal continent-continent collision of the Trans-Hudson Orogen, and represent progressive strain in a transpressional regime.

The peak of regional metamorphism was coeval with the D1 deformation, and reached low to middle amphilobolite grade. Temperature and pressure of the regional metamorphism were approximately 545* to 575*C and 4.5 to 5.0 kbars. The P-T conditions of the D2 deformation were approximately 500* to 550*C and 3.0 to 4.0 kbars. The D3 deformation, with which the gold mineralization was associated, occurred at a temperature of 480* to 500*C and a pressure of 3.0 to 4.0 kbars.

The hydrothermal alteration associated with the McLennan Lake Tectonic Zone, the D2 structures and the D3 gold-bearing shears is characterized by enrichment of o^18O of silicate minerals in the deformed rocks relative to their precursors. The fluids that infiltrated the D2 and D3 structures were possibly of similar composition, and were enriched in Au, K, Rb, Ba, and Cu (+ Pb + Zn) relative to the wall rocks. The water/rock ratio associated with alteration was greater in the D3 shears than in the McLennan Lake Tectonic Zone and the D2 structures. The occurrence of gold mineralization in the D3 structures was probably related to a higher fluid flow.

Vein quartz from the D1 to D3 structures, with the exception of the Jasper deposit, defines a narrow range of oxygen isotopic composition of 10.0 to 11.5 per mil, with auriferous quartz showing minor o^18O enrichment relative to barren quartz. The auriferous quartz in the Jasper deposit is enriched in o^18O by up to 3 per mil relative to the Jolu, Rush Lake and Star Lake deposits. The calculated oxygen and hydrogen isotopic compositions of hydrothermal fluids that deposited the quartz veins in the D1 to D3 structures are 6.8 to 10.3% and -89 to -26%, respectively. The oxygen and hydrogen isotopic values are similar to fluids of metamorphic or magmatic origin, or to fluids that acquired isotopic equilibrium with metamorphic or magmatic rocks at high temperatures.

Hornblendes from the D1, D2 and D3 structures yield the same 40Ar/39Ar plateau ages (within error) of ca. 1750 Ma. The ages probably reflect the time of cooling through the closure of hornblende to argon loss at 500*C, and approximate the time of the D3 deformation. In conjunction with previous geochronological studies, the timing of hydrothermal activity linked to the formation of the gold deposits is constrained between 1807 Ma and ca. 1750 Ma. This postdates the emplacement of the Star Lake and Island Lake plutons (1855 to 1846 Ma), and the regional metamorphism (1820 to 1798 Ma).

The auriferous hydrothermal fluids were probably not of magmatic origin related to the emplacement of the host plutons. The fluids could have been generated by devolatilization of the lower levels of the crust during prograde metamorphism at the final stages of the Trans-Hudson Orogen. The absence of deep plumbing systems may account for the small size of the gold deposits with respect to Archean counterparts.