Ammonium geochemistry in mineral exploration—a comparison of results from the American cordilleras and the southwest Pacific

Abstract

Ammonium concentrations in the host rocks to circum-Pacific epithermal precious metal and exhalative base metal deposits vary from 0 to 25,000 ppm. High ammonium concentrations in mineralizing fluids are probably derived from the breakdown of organic matter in sediments or sedimentary rocks within the hydrothermal system. Shales are particularly good ammonium source rocks. Ammonium substitutes for K and other alkali elements in crystal lattices. Host rocks with K-bearing minerals therefore have the highest potential for the development of ammonium haloes. Both surface and subsurface rocks can contain high concentrations of ammonium and substantial haloes have been found associated with precious metal veins, particularly in the hanging wall. Such haloes are well developed at mines in Mexico, Bolivia and Peru and ammonium geochemistry thus provides a potentially useful exploration tool, especially for vein-type epithermal precious metal deposits.