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The genesis of the giant Dajiangping sedex-type pyrite deposit, south China

Wenhong Johnson Qiu, Zhou Meifu, Li Xiaochun, Anthony E. Williams-Jones and Yuan Honglin
The genesis of the giant Dajiangping sedex-type pyrite deposit, south China
Economic Geology and the Bulletin of the Society of Economic Geologists (September 2018) 113 (6): 1419-1446

Abstract

The Late Devonian Dajiangping pyrite deposit in South China is a giant sediment-hosted stratiform deposit and contains >200 Mt of pyrite ores with an average grade of 30 wt % S. The orebodies are hosted in carbonaceous siltstone and black shale and consist mainly of laminated pyrite ore. Small euhedral pyrite crystals (pyrite 1) and larger subhedral pyrite crystals (pyrite 2) are the main ore mineral and occur as laminated aggregates of subhedral to euhedral grains. Anhedral pyrite crystals (pyrite 3) associated with pyrrhotite are present in the host rocks and are interbedded with the ore layers. Mass-balance calculations reveal that the pyrite ores have Al-normalized Ca, Mg, Mn, Si, Na, K, and organic carbon contents similar to the host sedimentary rocks, precluding substantial carbonate dissolution and a stratiform replacement origin for the ores. Pyrite 1 and 2 crystals in the laminated ores have delta (super 34) S values ranging from -28.7 ppm to 23.6 ppm, and mostly lower than -16 ppm. Such highly negative values indicate that the sulfur in these ores was sourced from bacterially reduced seawater sulfate in an open system. Pyrite 1 and 2 crystals within a single laminated ore sample have similar delta (super 34) S values. The mostly negative sulfur isotope composition implies that the pyrite crystallized above to just below the seawater-sediment interface by consuming aliquots of H (sub 2) S supplied by bacterial sulfate reduction (BSR) in sulfate-dominated shallow muds. Pyrite 3 crystals have mostly positive delta (super 34) S values, ranging from -3.2 to 42.8 ppm with a mean of 5.7 ppm. There are also large ranges of delta (super 34) S values within a single sample, for example, from 12.1 to 42.8 ppm in sample YF0204. Such positive and highly heterogeneous delta (super 34) S values likely resulted from thermochemical sulfate reduction (TSR) and anaerobic oxidation of methane (AOM) coupled with sulfate reduction in a tightly closed system of deep sediments that experienced strong Rayleigh fractionation during diagenesis. We propose that the laminated pyrite ores of the Dajiangping deposit formed by sedimentary exhalation in an anoxic seawater column. Hydrothermal fluids leached Fe from the metamorphic basement underlying the stratiform orebodies, as indicated by the similar Pb isotope compositions of the laminated ores and metamorphic basement, and vented onto the sea floor. Mixing of Fe (super 2+) from the exhaled metalliferous fluids with H (sub 2) S from the ambient anoxic seawater resulted in the precipitation of pyrite 1 and 2 crystals above and immediately below the sea floor, respectively, and the formation of laminated ores. This was followed by crystallization of a small proportion of pyrite 3 in the pores within the deep sediment pile.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 113
Serial Issue: 6
Title: The genesis of the giant Dajiangping sedex-type pyrite deposit, south China
Affiliation: University of Hong Kong, Department of Earth Sciences, Hong Kong, China
Pages: 1419-1446
Published: 201809
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 82
Accession Number: 2019-002014
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols., 5 tables, geol. sketch maps
N23°11'60" - N34°30'00", E112°45'00" - E114°04'60"
Secondary Affiliation: McGill University, CAN, CanadaNorthwest University, CHN, China
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2019
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