Geology and geochemistry of base metal deposits

Abstract

The geochemical character and principles of distribution of the elements in igneous rocks are summarized. The geology and geochemistry of the base metals are then discussed in turn. Chromium and titanium are small lithophile ions related in size and distribution to ferric iron; thus they are concentrated in early magmatic differentiates associated with ultrabasic and basic rocks respectively. The thiophile elements nickel, copper, cobalt, zinc, and lead are concentrated in sulphide bodies. Immiscible sulphide deposits can separate only from ultrabasic and basic magmas because of the solubility of sulphur in silicate melts; hence the only thiophile elements that concentrate in the sulphide segregations are copper, nickel, and cobalt which are strongly concentrated in basic rocks and magmas. Nickel sulphide occurs almost entirely in these magmatic sulphide ores because of the small size of the nickel ion, but the somewhat larger cobalt and copper ions may be concentrated in hydrothermal as well as magmatic sulphide deposits. The hydrothermal deposits may be typed according to whether they are copper or copper-cobalt, copper-zinc, zinc-copper-lead, or lead-zinc deposits. Each type is associated with specific compositions of igneous rocks or magmas in which the elements are most concentrated because of their geochemical character and associations. Many of the principles outlined can be used in assessing deposits and areas for exploration and development.