Secondary dispersion of transition metals through a copper-rich bog in the Cascade Mountains, British Columbia

Horizontal and vertical variations of copper, cobalt, iron, manganese, molybdenum, nickel, zinc, organic carbon and pH were studied in a small bog close to a known copper-mineral occurrence in the foothills of the Cascade Mountains, British Columbia This bog consists of up to 3 m thickness of moderately decomposed, water saturated, fetid organic material underlain by glacial till that almost completely covers the contact between copper-mineralized Nicola Group volcanic rocks and porphyry dykes. Soils with more than 16% organic carbon and 0.1% Hl-reducible sulphur are enriched in copper, cobalt, nickel, zinc and molybdenum. Sympathetic relationships between nickel and zinc and between cobalt and copper are demonstrated by correlation analysis of metal data. Metals generally increase down organic soil profiles, but fall sharply in the till except at the western end of the bog where small areas of concealed till have up to 0.57 copper and 100 ppm molybdenum. Iron and manganese are generally higher in the till than in organic soil although these metals are locally very abundant in near surface fibrous organic material. Reducing, subsurface bog waters generally have higher dissolved iron, manganese and organic carbon, but lower copper contents than do surface waters. However, several subsurface water samples from the area underlain by copper-rich till contain up to 1 ppm copper. Copper is also very abundant in springs water flowing from a probable fault zone west of the bog; in seepages draining humic gleysolic soils surrounding the west side of the bog and in acid, semi-stagnant surface water. Small, irregularly shaped grains of pyrite, chalcopyrite, covellite, native copper and framboidal pyrite are scattered throughout the organic soils. Copper sulphide and native copper grains are restricted to two areas at the eastern and western ends of the bog occurring between 1 and 3 m depth. Framboidal pyrite, however, has a wider spatial distribution in organic soils than the copper and copper-iron sulphide mineral grains. Copper and iron are principally derived through oxidation of sulphides,disseminated in the underlying volcanic rocks, by circulating ground water which then discharges into the bog along concealed fault zones. Ground water,- percolating through reduced till beneath organic soils and through humic gleysolic soils,dissolves cobalt, nickel, zinc, manganese, iron and molybdenum which then migrate through the bog as simple ions, complex ions or soluble metal-fulvate complexes. A major proportion of the dissolved copper, cobalt, nickel and zinc is probably immobilized by adsorption and complexing to solid humic and fulvic acid fractions in the soil. Authigenic copper and iron sulphides also form through reaction of metals with sulphide ions produced from biogenic sulphate reduction. Stability relationships between copper and iron minerals indicate that the grain textures reflect changes in Eh, pH, sulphide ion activity, metal ion activity and possibly dissolved organic carbon abundance. Hydrous oxides of iron and possibly manganese form close to the bog surface where metal-rich solutions discharge into the oxidizing environment. Molybdenum is also concentrated in the acid fibrous organic layer due to immobility of the acid molybdenate ion. Abundant copper may be adsorbed from the metal-rich surface water by plants and is then bound to proteins forming the cell-wall membrane. This form of copper is relatively stable and the metal will only be released from the association during advanced organic diagensis.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat