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Middle Miocene Climatic Change in the Atacama Desert, Northern Chile: Evidence from Supergene Mineralization at La Escondida

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Middle Miocene Climatic Change in the Atacama Desert, Northern Chile: Evidence from Supergene Mineralization at La Escondida Middle Miocene climatic change in the Atacama Desert, northern Chile: Evidence from supergene mineralization at La Escondida CHARLES N ALPERS* 1 ^c^d^c rr nnTWTTArr / Department of Geology and Geophysics, University of California, Berkeley, California 94720 ubUKuh. H BKIMHALL) ABSTRACT during the middle Miocene. This climatic des- Chile (Fig. 1). (We use "Atacama region" to iccation caused termination of significant refer to the area of northern Chile and adjacent Geochronology and paleotopographic re- supergene copper-sulfide enrichment at La Peru, Bolivia, and Argentina which makes up construction of the porphyry-copper deposit Escondida and elsewhere in the Atacama re- the present Atacama Desert, including the three at La Escondida, Chile, are used to calculate gion and preservation of surficial features, in- northernmost political "Regiones" of Chile: long-term erosion rates and to deduce the cluding the ash horizons and the leached Tarapaca, Antofagasta, and Atacama.) Second, timing of Tertiary climatic change for a por- capping. we discuss the present climatic setting in the tion of the Atacama Desert region. Hypogene Middle Miocene climatic desiccation in Atacama region and propose a mechanism for hydrothermal alteration and protore mineral- northern Chile and southern Peru was prob- middle Miocene desiccation. Third, we explain ization at La Escondida took place between ably related to a pronounced decrease in why the formation and preservation of super- 33.7 ± 1.4 and 31.0 ± 1.4 Ma based on K-Ar temperature of coastal waters supplied by an gene-enriched, base- and precious-metal depos- dating of hydrothermal biotite and sericite. ancestral Humboldt Current and an increase its are logical consequences of severe, long-term Supergene weathering and copper-sulfide en- in upwelling intensity as the Antarctic ice cap climatic desiccation. richment processes were active from 18.0 ± became established at approximately 15 to 13 The effects of chemical transport due to surfi- 0.7 to 14.7 ± 0.6 Ma based on K-Ar dating of Ma. The Central Andes Cordillera, which cial processes have long been of major impor- supergene alunite, distinguished from hypo- now provides a rain shadow protecting the tance to economic geologists because of the gene alunite by grain size, color, and sulfur Atacama region from precipitation from the well-documented enrichment of certain base isotopic composition. Reworked lenses of east, must have attained at least half its pres- and precious metals under supergene (weather- volcanic ash in the vicinity of La Escondida ent elevation prior to about 15 Ma to have ing) conditions (Emmons, 1917; Anderson, provide useful time-stratigraphic markers at played a similar role in the middle Miocene. 1955). The term "supergene," literally "from 8.7 ± 0.4,6.5 ± 0.2, and 4.2 ± 0.2 Ma within above," refers to the low-temperature effects of present soil profiles. INTRODUCTION chemical weathering; whereas the term "hypo- Long-term average rates of erosion are de- gene," or "from below," refers to the relatively termined by these age dates and quantitative Geochronologic studies of multiple hydro- high-temperature effects (above -50 °C) of hy- calculation of eroded leached capping thick- thermal events and subsequent chemical weath- drothermal alteration. Weathering profiles in ness at La Escondida using mass-balance ering of sulfide ores permit deduction of ore deposits are also of general scientific interest analysis of geochemical profiles coupled with long-term rates of geochemical, geomorphic, because they provide unusually well exposed an estimate of unmineralized lithocap thick- and paleoclimatic processes. Of particular utility settings at which to observe the interrelated ef- ness based on alteration petrology and fluid- in studying the weathering history of mineral- fects of tectonics, climate, hydrology, and geo- inclusion geobarometry at similar deposits. ized systems are methods which illuminate the morphology (Chivas and others, 1983). The The observed trend of decreasing long-term timing, direction, and magnitude of major transi- complex interrelationships between rates of ero- average erosion rates with time is consistent tions in rates of surficial processes occurring sion, uplift, precipitation, infiltration, and verti- with arid to semiarid conditions in the early over geological time spans. cal movements of the ground water table have Miocene changing to hyperarid conditions The purpose of this paper is threefold. First, important consequences for the development of supergene-enrichment zones in sulfide-ore de- we present evidence for the timing and magni- posits (Bateman, 1950; Titley, 1982; Brimhall tude of Tertiary climatic changes in the Atacama •Present address: U.S. Geological Survey, M.S. 420, and others, 1985; Alpers, 1986), as well as for 345 Middlefield Road, Menlo Park, California 94025. region from a detailed study at La Escondida, Additional material for this article (three appendices) may be obtained free of charge by requesting Supplementary Data 8821 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 100, p. 1640-1656,9 figs., 3 tables, October 1988. 1640 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/10/1640/3379676/i0016-7606-100-10-1640.pdf by guest on 28 September 2021 MIDDLE MIOCENE CLIMATIC CHANGE, CHILE 1641 point to past periods of higher rainfall during the m.y.)1 includes El Salvador (Quirt and others, Paleogene and early Miocene, including dissec- 1971; Gustafson and Hunt, 1975), Quebrada tion of pediplain surfaces (Mortimer, 1973, Bianca (Hunt and others, 1983), and Potrerillos 1980) and erosion of the upper portions of hy- (Olson, 1984). The slightly younger group (36c pogene porphyry-copper deposits of Paleogene to 31c m.y.) includes Chuquicamata, El Abra, age (Sillitoe, 1981; this study). Mocha (Quirt and others, 1971; Ambrus, 1977; One of the outstanding problems regarding Soto, 1981), Potrerillos (Olson, 1984), and La ore genesis is to define the optimal conditions for Escondida ( Alpers, 1986). supergene enrichment in terms of the climatic, Neogene volcanism in the Andes Cordillera is hydrologic, and tectonic settings. Specifically, represented by a thick accumulation of andesitic one would like to know if there is anything spe- to dacitic lavas interfingered with dacitic to rhy- cial about the tectonic or climatic history of the olitic ignimbrites (Lahsen, 1982a; Gardeweg Atacama region that may have led to formation and Ramirez, 1985). In different portions of the and preservation of such highly enriched ore Andes, the oldest known volcanic rocks range in deposits. We address this problem by determin- age between about 26c and 10c m.y. (Clark and ing the timing and magnitude of Tertiary paleo- others, 1967; Scholl and others, 1970; Noble climatic transitions in the Atacama region from and others, 1974; Baker, 1977; Baker and Fran- a detailed study of long-term average erosion cis, 1978; Lahsen, 1982b; Coira and others, Salar 7 rates at La Escondida, a large porphyry-copper 1982). In the Andes Cordillera of the Antofa- „MANTOS . de / deposit in northern Chile (Fig. 1). ^BLANCOSJ gasta Region from approximately 22°S to 26°S Mntofagasta "ly^AtacaT In this study, we present evidence for middle (Fig. 1), the oldest known volcanic rocks have been dated at about 24 Ma (Boric and others, •24° ESCONDID/V Miocene climatic desiccation in the Atacama re- gion from radiogenic and stable isotopic data, 1987). Salar de v plus geomorphic observations from La Escon- The magmatic arc in northern Chile and ad- Punta Negra dida. Supergene weathering at La Escondida jacent Argentina, Bolivia, and Peru first became was responsible for development of a thick, active in its present position, 250 to 450 km east ^Taltal well-preserved, leached capping zone and an ex- of the present Peru-Chile trench axis at about 26 tensive underlying "blanket" zone of copper- Ma, during the late Oligocene. It has been sug- • 26° x EL SALVADOR sulfide enrichment (Brimhall and others, 1985; gested that the onset of volcanism at about 26 to 9f POTRERILLOS Alpers, 1986; Alpers and Brimhall, 1989). Al- 20 Ma in the Andes Cordillera and Pre- though the supergene copper deposit at La Es- Cordillera was related to rotation of the Pacific Copiapo condida is of exceptional tonnage and grade plate, on the basis of the physiography of the 70° (Anonymous, 1985), mining had not yet com- Hawaiian-Emperor chain (Clague and Jarrard, menced at the time of this study, providing a 1973; Noble and others, 1974). The dramatic Figure L Location map of northern Chile, relatively undisturbed natural laboratory for the eastward migration of the locus of magmatic showing principal copper deposits and other study of supergene processes. activity in the Atacama region during the Oligo- localities mentioned in text. cene may correlate with the cessation of sea- REGIONAL GEOLOGIC SETTING floor spreading along the northwest-trending Pacific Farallon rise and the subsequent initia- Eocene to Holocene Magmatic and tion of rapid spreading along the north- Hydrothermal Activity northwest-trending Galapagos rise (Boric and residual enrichment in lateritic profiles (Brimhall others, 1985; Pilger, 1984). and Dietrich, 1987). The magmatic arc between 22°S and 29°S in The Atacama region of northern Chile hosts northern Chile has undergone a stepwise east- Tertiary Uplift, Erosion, and Supergene many spectacular examples of supergene-metal ward migration, from a position near the present
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