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A preliminary petrographic study of the Chilean nitrates

Published online by Cambridge University Press:  01 May 2009

Alison Searl  and
Sharon Rankin
Alison Searl
Affiliation:
School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.
Sharon Rankin
Affiliation:
School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K.
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Abstract

The nitrate deposits of the northern Atacama Desert occurin a narrow zone between the low-lying Pampa del Tamarugal to the east and the hills of the Coastal Range to the west. High purity nitrate ore occurs as stratiform seams about 20 cm thick, present at depths of 3–7 m below thedesert surface. The ore is hosted by a variety of lithologies and overlain by salt-cemented regolith. The ore is largely composed of nitratite (= sodanitre or Chilean salt peter) and halite with locally abundant humberstonite, polyhalite and mirabilite. Other salts present include Ca iodates and a variety of sulphate, borate and chromate minerals. Textural relationships can be used to deduce paragenetic sequences for individual salt samples and these combined to produce a generalized paragenetic web for the nitrate ore.These textural data can be combined with published solubility data to investigate the course of fluid evolution during ore genesis. The multiplicity of paragenetic relationships within the ore reflects the derivation of precipitating fluids from a variety of sources: westwards flowing Andean groundwater, coastal fogs, occasional rainfall and Andean-derived surface floodwaters. The unusual mineralogy of the nitrate ore reflects the extreme chemical evolution of the precipitating brines through multiple episodes of salt precipitation and remobilization during transport to the nitrate horizons. The formation of high purity nitrate ore appears to be the result of multiple phases of dissolution, reprecipitation and recrystallization, that have separated the highly soluble nitrate salts from less soluble salts in the overlying profile. Salts have largely accumulated through displacive growth, but some of the host silicate and carbonate lithologies have also undergonea small degree of salt replacement.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 3 , May 1993 , pp. 319 - 333
Copyright
Copyright © Cambridge University Press 1993

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