Rubidium as a guide to ore in Chilean porphyry copper deposits

Abstract

Hypogene mineralization in porphyry copper deposits is characteristically associated with zones of potassic alteration. The mineralization and hydrothermal alteration are thought to be produced by an aqueous phase which has separated from a crystallizing magma. Due to the similarity in geochemical characteristics of K and Rb, Rb can be incorporated in K minerals that crystallize from a magma. The Rb ion is only 0.14 Aa larger than the K ion, but this size difference results in a higher bonding energy between K and O than between Rb and O. Therefore, when K minerals crystallize from a magma, K is preferentially removed with respect to Rb. The Rb tends to concentrate in the residual melt. If an aqueous phase separates from a magma during late stages of crystallization, it should have relatively higher Rb concentrations and low K/Rb ratios. High Rb concentrations and low K/Rb ratios might therefore be expected in rocks that have undergone potassic alteration.Wide zones of potassic alteration are closely associated with the orebodies at the Chilean porphyry copper deposits in the El Teniente (Braden), Rio Blanco, and Los Bronces (Disputada) mines. Therefore, the distribution of Rb and its relationship to ore and alteration at the three deposits is determined. As K replaces Ca in plagioclase, and Sr is geochemically related to Ca, the distribution of Sr is also determined. Background values for Rb and Sr are difficult to determine, as all accessible parts of the intrusions near the deposits are hydrothermally altered. Background values for intrusive rocks were obtained from 17 samples of unaltered intrusive bodies not associated with porphyry copper deposits.The following conclusions, though based on the study at El Teniente, Rio Blanco, and Los Bronces, may also be applicable to other porphyry copper deposits.1. Rocks that have undergone potassic or sericitic alteration have a two- to threefold increase in Rb and low K/Rb ratios. As these types of alteration characteristically accompany mineralization in porphyry copper deposits, Rb distribution may be useful to locate favorable alteration zones for porphyry copper exploration. The highest anomalies are found in mafic host rocks, which have low Rb background values, though anomalies can also be detected in rocks as siliceous as quartz monzonite.2. A twofold increase in Rb in the propylitic zone at El Teniente produces an anomaly which extends more than 400 meters beyond the margin of the orebody. No corresponding increase in K was observed. The Rb anomaly represents an enlarged target enclosing the zones of alteration commonly associated with porphyry copper mineralization.3. Weathering does not affect the distribution of Rb in rocks from the leached capping at El Teniente. Due to this immobility of Rb, weathering of the potassic zone in porphyry copper deposits should not destroy the Rb anomaly. The distribution of Rb could be a useful tool to locate potassic alteration zones in areas of tropical weathering, for example, in the southwest Pacific and in Central America.4. Addition of Rb and removal of Sr during hypogene alteration can produce high Rb/Sr anomalies which extend out through all of the zones of hydrothermal alteration. However, care must be taken in interpreting the significance of Rb/Sr anomalies because they may simply indicate extensive removal of Sr during normal weathering processes.5. Contouring the Rb values may be useful for quantitatively mapping the extent of K metasomatism.6. Anhydrite is readily dissolved and removed by acid meteoric waters. In deposits where anhydrite is an abundant hypogene alteration product, Sr depletion can be used as a quantitative guide to the degree of supergene activity.