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Nephrite in British Columbia Fraser, John Ross

Abstract

Nephrite is a compact, microfibrous variety of actinolite-tremolite in which bundles or tufts of minute fibers of the amphibole are twisted and thoroughly felted or interwoven with one another, producing a characteristic "nephritic" microstructure. In British Columbia, nephrite deposits, both in place and placer, are closely associated with a belt of alpine ultramafic rocks that extends for 1000 miles from the Hope area, east of Vancouver, northwestward to the Yukon border. The three major nephrite producing regions are the Bridge River - lower Fraser River area, the Takla Lake area and the Dease Lake area. The nephrite from British Columbia contains, in addition to essential tremolite, small amounts of chlorite, uvarovite, chrome spinel, diopside, talc, carbonate, sphene, phlogopite and pyrite. Grains of chrome spinel and uvarovite are usually visible in hand specimen. The colour of the majority of the specimens is yellowish green; this colouration is caused by the presence of iron in both the divalent and trivalent states. Polished surfaces of the nephrite have an average Vickers hardness of 950 Kg/mm² and an average Mohs hardness of 7. The average specific gravity is 3.00. The unit cell parameters of tremolite from British Columbia nephrite specimens are similar to those of nephritic tremolite from Siberia. X-ray diffraction data for the tremolite from these specimens are also in good agreement with those for nephritic tremolite from other localities. The nephrite specimens contain an average of 3.05 percent iron; small amounts of cobalt, nickel, manganese, copper, lead, zinc, chromium, titanium and vanadium are also present. Significant regional variations in the averages for iron, cobalt, manganese, copper, lead, zinc and vanadium are not observed when the specimens are grouped according to the area of origin; slight variations are observed in the average contents of nickel, chromium and titanium. The general similarity of the regional average values for these elements suggests that the nephrites have been formed in similar environments. At the O’Ne-ell Creek deposit in central British Columbia, nephrite occurs in a zone of tremolite-chlorite rock developed in serpentinite at the contact with metasomatically altered sediments. The nephrite has resulted from the metasomatic alteration, by addition of calcium and silica, of the serpentinite during the process of serpentinization. The calcium was derived from the pyroxenes contained in the original ultramafic rock; the source of the silica was the enclosing sediments. High concentrations of calcium and magnesium and relatively lower concentrations of sodium, iron, aluminum and silicon characterized the environment in which the nephrite formed. Calcium and sodium were perfectly mobile while the other elements were relatively inert. These conditions of mobility and concentration account for the fine fibrous nature of the nephritic tremolite. A temperature range of approximately 300°C to 500°C and a pressure in excess of 4 kilobars are suggested for the formation of the nephrite.

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