ALTERATION AND GEOCHEMICAL FOOTPRINTS OF GOLD MINERALIZATION AT RED LAKE MINE, ONTARIO

Gold mineralization in the Archean-aged Red Lake deposit (40 mt at 20 g/t) occurs in meter-wide zones within a much larger body of altered rock displaying distinct mineralogical and geochemical zoning.

Carbonate (ankerite) and biotite alteration occur for 5 km beyond the deposit limit with stronger alteration occurring in an elongated zone 4×2 km trending 340°. It is overprinted by chlorite and cut by abundant ankerite-quartz veins in the vicinity of the deposit (3×1.5 km). Garnet-magnetite-chlorite alteration occurs within 300-350 m of the ore zones, commonly at depths below 800 m, and grades inward and upward to quartz-andalusite assemblage. The latter gradually changes outward to pervasive sericite in an area >2×3 km. Gold mineralization postdates most of the above assemblages occurring in narrow ore zones of sulfide, silicification, biotite and muscovite alteration.

Spectral analysis (400-2500 nm) of 3000 samples from the mine and surrounding 10×10 km area indicate that chlorite composition changes markedly from Mg-rich to Fe-rich towards the deposit forming a distinct zone of Fe-rich chlorite 5×1 km overlapping the strong carbonate-biotite alteration. Composition of pervasive muscovite changes from K-rich to Fe-Mg rich, i.e., phengitic, towards the deposit. Alkali alteration index (K:Al vs. Na:Al) shows a zoning from distal Na-K, intermediate K to proximal Na-K-depleted alteration.

Whole rock analysis of 2200 samples indicate that anomalous concentrations (in ppm) of As (>250), Sb (>20) and W (>20) have the broadest halo around the deposit, 2×5 km, overlapping with the strong carbonate and Fe-rich chlorite footprints. Mo (>3.5), Te (>0.2), Tl (>1.5) and Ag (>0.5) have similar but more restricted halo occurring within 1 km of the deposit. Higher concentrations occur closer to the ore however geochemical halos become narrower at depth.

The Red Lake deposit records episodes of K-Ca alteration, which become progressively Fe-rich towards the deposit, followed by acid leaching generating a zoned pattern of alkali-depletion prior to the main veining stage and gold mineralization. Overlap of gold and pathfinder elements with the barren zoned alteration assemblages implies that they are linked to a large (>20 km²), structurally controlled, hydrothermal system which provides effective tool to vector towards ore.