3-D MODELING OF WALL-ROCK GEOCHEMISTRY IN THE CLAN LAKE GOLD AREA, YELLOWKNIFE GREENSTONE BELT, NWT, CANADA: WHAT DOES POTASSIC ALTERATION TELL US?

Clan Lake is an area of gold exploration in the north end of the Yellowknife greenstone belt of western Canada. Gold occurs within an intermediate to felsic, metavolcanic-volcaniclastic complex of the Archean Banting Group (2.69-2.66 Ga). Ores are characterized by early arsenopyrite followed by pyrrhotite ± gold, associated with wall-rock sulfidation, silicification and potassic alteration. Alteration is pervasive, making it a challenge to identify an edge to the alteration zone in order to define the size of the mineralizing system. Oxygen isotope and lithogeochemical studies by Hansen (2013) determined that ore mineralization is a result of both metasedimentary- and metavolcanic-derived fluids, likely driven by heat from voluminous synmetamorphic granitic plutons. 3-D modeling defined a volume of rock with higher δ¹⁸O values that coincides with gold enrichment. However, plots of gold concentration versus elements associated with the observed types of alteration did not reveal a consistent correlation. The present study utilizes a numerical technique that was developed for alteration of volcanic rocks and applies it to the metavolcanic host rocks at Clan Lake. 3-D models were constructed of potassic alteration intensity by recasting whole-rock chemistry of more than 19,000 samples in terms of mineral phases associated commonly with potassic alteration. In the southwest portion of Clan Lake, gold enrichment is associated with rocks that have undergone moderate degrees of potassic alteration. These rocks have high δ¹⁸O values (~ 15‰ VSMOW), which may indicate a zone of enhanced porosity and permeability in which ore-bearing fluids reacted pervasively with wall rocks. In contrast, the central portion of the area contains a south-dipping zone of stronger potassic alteration, which is interpreted to represent more localized fluid flow along faults. In this zone, gold shows a spotty pattern of enrichment, with significant concentrations occurring frequently in rocks that display only weak potassic alteration. This may reflect preferential gold occurrence in quartz veins and/or remobilization of wall-rock-hosted gold by later fault-associated fluids. Though potassic alteration does not correlate directly with gold at Clan Lake, it does reveal valuable information about fluid flow pathways through time.