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

This study investigates geochemical controls on sediment metal distribution s for three lakes and groups of lakes adjacent to sub-economic porphyry Mo prospects in the Endako region of central British Columbia. Tatin Lake, Han son Lake and the Counts Lakes are within Middle Jurassic felsic intrusive r ocks 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-oxi c 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 mineralizat ion. Sediments at Tatin, Hanson and the Counts Lakes contain elevated media n 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 t he locations of adjacent quartz-molybdenite veins and their dispersed remna nts in soil and till. Anomaly patterns are interpreted to be of hydromorphi c origin, with Mo transported to lake basins in sub-surface ground water an d, to a lesser extent, surface waters prior to complexing by organic-rich s ediments. Most of the highest Mo concentrations in Tatin Lake are in the we st end of the lake (up to 14 ppm), in near-shore shallow-water organic sedi ments near Mo-Cu mineralization and in profundal sediments of the westernmo st sub-basin. There is a close association between the distribution of Mo a nd Lot. Elevated Mo concentrations (11-18 ppm) are closely associated with centre-lake profundal sediments in Hanson Lake. Sediment geochemical patter ns in the Counts Lakes are strongly influenced by proximity to Mo mineraliz ation, 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, 6 0, 33, 38, 56, 54, 83 ppm) exceed regional background by 3-80x. Elevated ce ntre-basin Mo concentrations of at least 12 ppm reflect the presence of adj acent porphyry Mo mineralization. Centre-lake sediments do not however alwa ys have the highest Mo concentrations, due to variations in Mo content betw een sub-basins of the same lake and to the presence of near-shore Mo zoning patterns downslope and down drainage of mineralization. Centre-lake sampli ng of every lake and sub-basin is recommended for regional geochemical surv eys. For follow-up surveys, near-shore sediments should also be sampled, pa rticularly at base of slope and adjacent to drainage inflows, to delineate any lateral zoning patterns which may infer a general direction toward buri ed mineral deposits. (C) 2000 Elsevier Science B.V. All rights reserved.