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

L. B. Williams, H. Zantop, R. C. Reynolds

Research output: Contribution to journalArticlepeer-review

20 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)125-141
Number of pages17
JournalJournal of Geochemical Exploration
Issue number1-2
StatePublished - Oct 1987
Externally publishedYes

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Economic Geology


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