Minerageny of weathering crusts. Part two: Ores of nickel and cobalt, gold, rare metals

Abstract

Introduction: the weathering crusts of the world contain about 80 % of silicate nickel, about 10 % of cobalt, 1 % of gold and at least 10‒15 % of rare metals (REE). The reserves of the latter in the weathering crusts are calculated in total with the reserves in the parent rocks, so there are no exact data for each of them separately, and the given figure is approximate. A necessary condition for the accumulation of metals is the presence of a useful component in the parent rock. The content of the above metals in it varies from hundredths to 1‒2 % or more percent.

The research methodology and factual material are described in report one [1].

Results and discussion: unlike earlier accumulations of Al, Fe and Mn, deposits of silicate Ni and Co, developed in the weathering crusts on alpine-type ultrabasic rocks, appeared only in the Mesozoic. They were formed during periods of powerful crust formation with ages of the late Triassic-Early Jurassic and Early Cretaceous. Where the weathering crusts were not overlapped by marine sediments, the indicated time intervals could be longer, up to the end of the Cretaceous period. Mesozoic weathering crusts contain insignificant reserves of Ni and Co ores. They are concentrated mainly in the territory of Russia within the Urals and in Kazakhstan. The main part of Ni is confined to the zone of nontronite clays and the lower part of the ocher zone of weathering crusts. More than 80 % of the reserves of silicate nickel are associated with laterite Cenozoic weathering crusts. They are widespread in the territories of countries with a tropical climate. Weathering crusts is characterized by the presence of high-quality ores in zones of disintegrated serpentinites and saprolites. Under laterites, in the cuirass, Ni and Co are practically absent. The richest deposits are recorded on gentle slopes and in saddles of mountain spurs. A necessary condition for gold accumulation in weathering crusts is its increased content in the parent rocks. Linear and contact-karst zones are very favorable for the formation of increased concentrations of metal. The first rare accumulations of Au in weathering crusts are known in the Paleozoic. In the Mesozoic, their number increased sharply. In them, gold could accumulate in all zones of the weathering profile, but the highest contents are confined to illite-kaolinite clays of the hydrolysis zone. The main number of deposits and reserves of the metal are concentrated in lateritic weathering crusts of the Cenozoic in tropical countries. It is concentrated in saprolite, lithomarge and laterite cuirass, and its highest contents gravitate toward the upper part of the weathering profile. Gold can accumulate during the formation of bauxites. Such deposits are often complex gold-alumina. Tantalum and niobium accumulate in the form of independent minerals, gallium (in bauxites), and rare earth elements. The latter can be both in the composition of minerals and "ionic ores", in which they are sorbed by clay minerals. The remaining elements most often migrate from the weathering profile. Large reserves of REE are concentrated in the weathering crusts and the formations of their nearest demolition. Their carriers in the parent rocks are most often minerals of carbonatites, pegmatites, and granites. In the

weathering crusts, rare earth elements are concentrated in residual carrier minerals (monazite, zircon, orthite, xenotime, and others), as well as in "ionic" ores of clay eluvium, developed along parent rocks of different compositions. Powerful weathering crusts with REE were formed already in the Paleozoic on carbonatites (Eastern Siberia). In the Mesozoic, weathering crusts with ionic ores appeared on granitoids (China). Lateritic weathering crusts in tropical countries with numerous REE deposits are typical for the Cenozoic.

Conclusion: the formation of weathering crusts deposits of the considered metals was affected by the evolution of crust formation and associated ore genesis over time. Of particular importance was the emergence of vegetation on land in the Devonian, which resulted in a sharp intensification of weathering processes and the formation of hypergene deposits. The largest accumulations of the considered metals are confined to the Cenozoic.