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Composition of soils and ground waters at the Pampa del Tamarugal, Chile; anatomy of a fossil geochemical anomaly derived from a distant porphyry copper deposit

Matthew I. Leybourne and Eion M. Cameron
Composition of soils and ground waters at the Pampa del Tamarugal, Chile; anatomy of a fossil geochemical anomaly derived from a distant porphyry copper deposit
Economic Geology and the Bulletin of the Society of Economic Geologists (December 2006) 101 (8): 1569-1581

Abstract

The Pampa del Tamarugal in N Chile is a gravel plain, part of the hyperarid Atacama desert. Soil geochemical surveys over the Pampa using partial extraction analyses identified a large, >100-km (super 2) geochemical anomaly for Cu. Subsequent analyses showed unusually high concentrations of elements that are associated with copper porphyry deposits, Mo, As, and Se. The soils are saline (NaCl) and contain high concentrations of Br and I, which are abundant in ground waters from N Chile. The anomaly lies W of one of the worlds most important copper porphyry districts, which includes the Chuquicamata, Radomiro Tomic, and El Abra deposits. Follow-up drilling in the centre of the anomaly, through approximately 300m of piedmont gravel cover, revealed barren andesitic basement rocks. The soil anomalies were caused by groundwater reaching the surface, and then evaporating. Both ground waters and soils share a common element assemblage, with higher concentrations of Mo, As, and Se than Cu. The former dissolve as anions in ground water, which can travel far from their source, whereas dispersion of Cu (super 2+) is restricted by adsorption. The soil anomaly is interpreted to be a "fossil" anomaly, preserved by the hyperarid climate. To the S the anomaly is cut by an alluvial fan; another nearby fan has been dated at approximately 7 Ma. To the W the anomaly was eroded in the flood plain of the Rio Loa. Prior to the present approximately 50-m-deep incision of the Rio Loa, the water table in the area of the anomaly may have been higher than the present depth of 33 to 55m, facilitating capillary rise of water to the surface. It is suggested that the source for the anomaly was the El Abra deposit, 75 km E and updrainage from the anomaly.


ISSN: 0361-0128
EISSN: 1554-0774
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 101
Serial Issue: 8
Title: Composition of soils and ground waters at the Pampa del Tamarugal, Chile; anatomy of a fossil geochemical anomaly derived from a distant porphyry copper deposit
Affiliation: GNS Science, Ocean Exploration, Lower Hutt, New Zealand
Pages: 1569-1581
Published: 200612
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 34
Accession Number: 2007-021272
Categories: Economic geology, geology of ore depositsGeneral geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
S22°11'60" - S21°53'60", W69°40'00" - W68°54'00"
Secondary Affiliation: Eion Cameron Geochemical, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
Update Code: 200707
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