Ammonium silicates associated with sedimentary exhalative ore deposits: A geochemical exploration tool

The fixed-ammonium (NH₄⁺) concentration in sediments hosting sedimentary exhalative Zn-Pb-(Ag) sulfide deposits was examined at the Howards Pass, Clear Lake, Tom and Jason deposits in the Selwyn Basin, Yukon Territory, Canada. A few samples from the McArthur River deposit, Australia, were examined for comparison. Infrared spectrophotometry shows that NH₄⁺ concentrations in alkali-bearing mineral phases increase near ore bodies not diluted by the influx of abundant detrital material. In whole-rock samples from the Selwyn Basin, NH₄⁺ concentrations reach as high as 2000 ppm above ore, against a background of approximately 467 ± 70 ppm. In McArthur River's Emu Basin, the concentration of NH₄⁺ in two samples located adjacent to sulfides reaches over 2000 ppm whereas no NH₄⁺ was detected in any other samples taken from one core through the deposit. All samples for this study were taken from well below the weathered zone. X-ray diffraction of clay minerals does not indicate NH₄⁺ substitution for K⁺ in interlayer sites. Instead, a comparison of NH₄⁺ concentrations with X-ray diffraction estimates of major mineral constituents of the samples suggests that NH₄⁺ substitutes for K⁺ and Na⁺ in feldspars. The substitution is highest in the stratigraphic hanging wall at both the Howards Pass and Clear Lake deposits. At Clear Lake, high NH₄⁺ values (up to 1800 ppm) are found near stringer mineralization, presumably where the venting of hydrothermal fluids occurred. The Tom and Jason deposits contain low or background NH₄⁺ concentrations throughout the stratigraphic section, probably as a result of detrital dilution of the host sediments from slumping within a local graben coincident with mineralization. High concentrations of ammonia (NH₃) may be incorporated in hydrothermal solutions by breakdown of organic matter in the sedimentary pile. As hot, mineralizing fluids rise into a starved, stagnant basin of low pH and reducing conditions, NH₄⁺ would become the dominant nitrogen species at low temperature and may substitute for K⁺ or Na⁺ in authigenic minerals.