Evolution of Precambrian REE mineralization

Abstract

Archaean BIF of the world generally contain only minor amounts of REE (20–30 ppm or less), but some of the post-Archaean iron-bearing and other formations of China contain several thousand ppm or even up to several percent of total REE. The following types of Proterozoic REE mineralization can be distinguished: 1.(1) carbonate-hosted Fe-REE deposits;2.(2) carbonate-hosted REE mineralization;3.(3) REE-Fe or REE-Fe-B mineralization in metamorphosed—migmatized volcanic and volcano-sedimentary rocks;4.(4) REE-Fe mineralizations in clastic sedimentary beds; and5.(5) REE in phosphorite deposits.Large quantities of REE in sea water begin to precipitate only when the CO2 content of the atmosphere drastically decreases, because the REE may occur in carbonate and phosphate minerals or isomorphically replace Ca2+ in oceanic sediments. As a CO2-N2 atmosphere probably prevailed prior to the Proterozoic, the high CO2 partial pressure not only restricted the precipitation of REE-bearing carbonates, but also promoted the appearance of easily soluble H2PO3−. Hence, REE-rich carbonates and phosphates scarcely occur in large amounts in Archaean sediments. Only with the decrease of CO2 in the early Proterozoic did REE-bearing dolomite, REE carbonate and phosphate minerals begin to appear in noticeable quantities. The high concentration of Fe, REE and P in Proterozoic sediments seems to have evolved with geologic time. The peak of Fe accumulation appeared in the early Proterozoic, the mid-Proterozoic was favourable for REE, while large P concentrations appeared in the late Proterozoic and early Cambrian.In all of the above-mentioned types of REE mineralization, the REE distribution patterns are highly fractionated with significant light rare earth enrichment. Early Proterozoic deposits display distinct positive or no Eu anomalies, whereas middle and late Proterozoic ores show negative Eu anomalies.