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The relationship of destinezite to the acid sulfate alteration at the El Laco magnetite deposit, Chile

Francisco Velasco, Noelia de la Pinta, Fernando Tornos, Thomas Briezewski and Aitor Larranaga
The relationship of destinezite to the acid sulfate alteration at the El Laco magnetite deposit, Chile
American Mineralogist (June 2020) 105 (6): 860-872


Destinezite, ideally Fe (sub 2) (super 3+) (PO (sub 4) )(SO (sub 4) )(OH).6(H (sub 2) O), is found as nodular lumps in hematite-rich epiclastic sediments accumulated in small crater lakes on the slopes of El Laco volcano. These lumps are almost entirely dominated by fine-grained destinezite replacing earlier lipscombite, and associated with gray hematite. The crystal structure of destinezite has been re-examined to test for possible differences with respect to the earthy and poorly crystalline destinezite that forms by weathering in cave soils and mine waste dumps. The structural refinements confirm that the differences are minor. The El Laco destinezite was refined in space group P1 with a = 9.5828(2), b = 9.7440(3), c = 7.3302(3) Aa, and alpha = 98.796(3) degrees , beta = 107.985(3) degrees , gamma = 63.898(2) degrees , V = 584.50(4) A3) , and Z = 1. We measured by calorimetry the enthalpy and derived the entropy and the Gibbs free energy of formation of destinezite (-4051.7 + or - 4.3, -1518.5 + or - 20.0, and -3598.9 + or - 7.1 kJ/mol, respectively). This has allowed us to estimate the equilibrium constant in the temperature range 0-300 degrees C (log K = -27.97 + or - 1.1). We can predict that destinezite coexists with hematite over the range of conditions that typically encompass the steam-heated hydrothermal environments recognized at El Laco. The presence of destinezite along with hematite and the occurrence of alunite, jarosite, and variscite in the system provide evidence of intense hydrothermal alteration during the extrusion of Fe-rich melts at El Laco volcano. The mineral assemblage and the available geochemical data suggest that destinezite formed in hydrothermal conditions by "maturation" of an immiscible Fe-P-rich melt. An unknown Fe-P-O phase produced by this melt was first replaced by lipscombite and later, owing to intense sulfidation, by destinezite. This replacement took place in the vadose zone above the paleowater table by relatively cool (<150 degrees C) groundwaters acidified by oxidation of magmatic SO (sub 2) to aqueous sulfate in a steam-heated system. Our model precludes other modes of formation, such as devitrification of hypothetical Fe-P-S-O-rich melts ejected from the volcano and supergene crystallization.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 105
Serial Issue: 6
Title: The relationship of destinezite to the acid sulfate alteration at the El Laco magnetite deposit, Chile
Affiliation: Universidad del Pais Vasco, Departamento de Mineralogia y Petrologia, Bilbao, Spain
Pages: 860-872
Published: 202006
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 57
Accession Number: 2020-049551
Categories: Mineralogy of non-silicatesEconomic geology, general, deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, sketch map
S23°51'00" - S23°49'60", W67°30'00" - W67°28'60"
Secondary Affiliation: Universidad Complutense de Madrid, ESP, Spain
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202014
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