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Economic Geology; April 1989; v. 84; no. 2; p. 229-255; DOI: 10.2113/gsecongeo.84.2.229
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Paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at La Escondida, Atacama Desert, northern Chile

Charles N. Alpers, and George H. Brimhall

Univ. Calif., Dep. Geol. and Geophys., Berkeley, CA, United States

Quantitative limonite mapping within the leached capping of the porphyry copper deposit at La Escondida, Chile, permits reconstruction of the paleohydrologic and chemical evolution of a well-developed supergene ore-forming system. The mineralogy, textures, and relative abundance of supergene limonite minerals (hematite, goethite, and jarosite) are used to reconstruct the former ratio of pyrite to chalcocite and the preoxidation copper grade based on empirical limonite sulfide correlations (after Locke, 1926; Blanchard, 1968; and Loghry, 1972). Estimates of preoxidation copper grades in surface exposures and tops of drill holes at La Escondida are significantly lower than actual copper grades in the underlying enrichment blanket at depth. This apparent inconsistency is explained by a progressive increase in the copper grade of the sulfide enrichment blanket as it descended to its present location in response to a descending water table. This systematic trend of reconstructed grades of supergene-enriched copper sulfide increasing with depth offers the first quantitative proof of cumulative downward enrichment in a supergene profile, as proposed by Locke (1926). The consistent trends of limonite mineralogy and abundance in vertical profiles indicate that water table descent at La Escondida was relatively continuous in space, although not necessarily in time.Evidence for cumulative downward enrichment in vertical profiles through leached capping allows quantitative analysis of chemical mass balance in dynamic supergene systems. Slopes of linear regressions for profiles of reconstructed enriched copper grades vs. depth indicate lateral fluxes into or out of a given vertical profile. This method provides independent verification of conclusions from a previous study of copper mass balance at La Escondida (Brimhall et al., 1985) which showed that lateral fluxes of copper were a significant factor during supergene leaching and enrichment.

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