Abstract
A detailed ore microscopic study strengthened by fire assay data of Al Wajh stream sediments (Wadi Al Miyah, Wadi Haramil and Wadi Thalbah) in northwestern Saudi Arabia shows economic concentrations of gold in the silt fraction (40-63 [mu]m). However, particles of extremely fine "dusty" gold (<=40 [mu]m in size) were also identified in most stations as independent grains. The maximum gold content in the samples of Wadi Al Miyah is 13.61 wt%, which is reported for the heavy fraction (<40 [mu]m). Maximum gold content in the heavy fractions of Wadi Haramil stream sediments amounts 6.90 g/t Au in a relatively coarse fraction (63-125 [mu]m). It appears that the most fertile heavy fraction in gold among the analysed samples are those from Wadi Thalbah that have the highest index figure, which makes the placer gold in them more profitable from the economic point of view. The gold content in the heavy fractions of samples from Wadi Thalbah is economically high lying in the range 6.27-28.83 g/t Au, except for a sample collected at the upstream with 0.77 g/t Au. Al Wajh stream sediments (including the beach light and black sands) are also rich in Fe-Ti oxides, rutile and zircon, whereas monazite and thorite are much lesser. Mineral chemistry of magnetite indicates a distinct titanomagnetite variety (with 3.85 wt% TiO sub(2)) which is consistent with the ore microscopic investigation. The titanomagnetite is V- and Cr-free, which indicates derivation from a more felsic source than a mafic one. No traces of U were found in zircon that sometimes bears up to 2.74 wt% Hf sub(2)O sub(3). Chemical analyses of monazite show typical common contents of rare earth elements such as La, Ce, Nd and Sm. Thorite is either U-free or uranothorite varieties where the latter contains up to 31.79 wt% UO sub(2). One of the U-free thorite grains is Y-bearing and contains 7.13 wt% Y sub(2)O sub(3).