Distribution and dispersion of molybdenum in lake sediments adjacent to porphyry molybdenum mineralization, central British Columbia

Abstract This study investigates geochemical controls on sediment metal distributions for three lakes and groups of lakes adjacent to sub-economic porphyry Mo prospects in the Endako region of central British Columbia. Tatin Lake, Hanson Lake and the Counts Lakes are within Middle Jurassic felsic intrusive rocks of the Francois Lake plutonic suite. The objectives are to determine: (i) the extent to which lake sediment geochemistry reflects the presence of adjacent porphyry Mo prospects; (ii) distribution patterns of Mo and other elements in lake sediments, and (iii) the effectiveness of lake sediments as a regional Mo exploration medium. Bottom waters of Tatin Lake are anoxic below 15–17 m depth and a range of limnological regimes, from eutrophic to oligotrophic, are present in three sub-basins. Mo–Cu mineralization occurs northwest of the lake. Hanson Lake is thermally unstratified, with sub-oxic to anoxic conditions occurring locally below 4–5 m depth. A Mo–Cu showing is present about 2.5 km south of the lake, and there is an extensive area of elevated Zn–Pb–Cu in soils to the north. The Counts lakes are a group of small, mostly eutrophic, lakes and ponds downslope of known Mo mineralization. Sediments at Tatin, Hanson and the Counts Lakes contain elevated median Mo concentrations of 8 ppm (max: 23 ppm), 7 ppm (max: 55 ppm) and 42 ppm (max: 165 ppm), respectively, relative to regional background of 1–2 pm. Mo distribution patterns are influenced by proximity to mineralization, basin morphology, limnological variations and the high organic matter content of centre-basin sediments. The distribution of Mo in lake sediments reflect the locations of adjacent quartz–molybdenite veins and their dispersed remnants in soil and till. Anomaly patterns are interpreted to be of hydromorphic origin, with Mo transported to lake basins in sub-surface ground water and, to a lesser extent, surface waters prior to complexing by organic-rich sediments. Most of the highest Mo concentrations in Tatin Lake are in the west end of the lake (up to 14 ppm), in near-shore shallow-water organic sediments near Mo–Cu mineralization and in profundal sediments of the westernmost sub-basin. There is a close association between the distribution of Mo and LOI. Elevated Mo concentrations (11–18 ppm) are closely associated with centre-lake profundal sediments in Hanson Lake. Sediment geochemical patterns in the Counts Lakes are strongly influenced by proximity to Mo mineralization, with the highest Mo concentrations (max: 165 ppm) in the westernmost ponds downslope of known prospects. Centre-basin results from Tatin Lake (12, 7–10, 23 ppm), Hanson Lake (11–12 ppm) and the Counts lakes (49, 160, 60, 33, 38, 56, 54, 83 ppm) exceed regional background by 3–80 x . Elevated centre-basin Mo concentrations of at least 12 ppm reflect the presence of adjacent porphyry Mo mineralization. Centre-lake sediments do not however always have the highest Mo concentrations, due to variations in Mo content between sub-basins of the same lake and to the presence of near-shore Mo zoning patterns downslope and down drainage of mineralization. Centre-lake sampling of every lake and sub-basin is recommended for regional geochemical surveys. For follow-up surveys, near-shore sediments should also be sampled, particularly at base of slope and adjacent to drainage inflows, to delineate any lateral zoning patterns which may infer a general direction toward buried mineral deposits.