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Formation of the Tharsis massive sulfide deposit, Iberian pyrite belt; geological, lithogeochemical, and stable isotope evidence for deposition in a brine pool

Fernando Tornos, Michael Solomon, Carmen Conde and Baruch F. Spiro
Formation of the Tharsis massive sulfide deposit, Iberian pyrite belt; geological, lithogeochemical, and stable isotope evidence for deposition in a brine pool (in A special issue devoted to continental margin massive sulfide deposits and their geodynamic environments, Stephen J. Piercey (prefacer), Jan M. Peter (prefacer) and James K. Mortensen (prefacer))
Economic Geology and the Bulletin of the Society of Economic Geologists (February 2008) 103 (1): 185-214

Abstract

The giant Tharsis massive sulfide deposit is one of the largest shale-hosted orebodies in the Iberian Pyrite Belt. It consists of several ore lenses located in the lowermost VSC and overlying a thick siliciclastic sequence Phyllite-Quartzite Group. The ore lenses are up to 1,500 m long, 130 m thick, and tectonically stacked within shale with rare sandstone layers that have been dated as late Strunian. Most of the massive sulfides are monotonous fine-grained pyrite. In the footwall of the orebody there are some siderite-rich facies (carbonate ore) consisting of laminated and brecciated sulfides and siderite that form mounds. We interpret these to be biogenic mounds formed proximal to the vents, probably by accumulation of thermophila archea (bacterial mats) and the products of their erosion. Stockwork veins occur in shale beneath most of the massive sulfide lenses. At the base of the ore lenses there are relics of laminated iron oxides, sulfates, and marcasite, which are partially replaced by pyrite. Seven kilometers west is the San Jorge mine, where small pyrite lenses occur in shale probably at the same stratigraphic position as Tharsis. The shale stratigraphically below the massive sulfides has a chemical composition mainly consistent with its formation in oxic to suboxic conditions and is characterized by low Mo contents (<50 ppm), low V/Cr ratios (<2), and high Fe but variable Mn contents. Shale directly related to the massive sulfide formation is characterized by high V/Cr ratios (>4) and low Mn contents indicative of deposition in an anoxic setting. A few meters above the massive sulfides the shale again shows evidence of deposition under oxic to suboxic conditions. The deposition of the massive sulfides was synchronous with a major geochemical change at the basin scale, involving a major increase in the Ca and base metal contents, S/total organic carbon, V/Cr, and K (sub 2) O/Na (sub 2) O ratios of the shale, probably related to the onset of the volcanism and hydrothermal activity. Shale that hosts the massive sulfides is also characterized by an over maturation of the organic, plankton-derived compounds that are not observed in the shale away from the orebodies. The destruction of this organic matter having delta (super 13) C values between -31 and -25 per mil is probably the source of (super 13) C-depleted carbonate. The siderite in the carbonate ore and the ankerite within the alteration zone have isotopic signatures that can be interpreted as the product of mixing of fluids having delta (super 18) O and delta (super 13) C compositions of +8 to +10 and -6 per mil, respectively (i.e., hydrothermal fluid and cooler seawater) but having isotopically light carbon (delta (super 18) O = -1 to 0 per mil and delta (super 13) C = -20 per mil). The massive sulfides have delta (super 34) S values between -33 and +4 per mil, similar to those of pyrite in the footwall and hanging-wall shale (-29 to +5 per mil). Stockwork vein sulfides, however, range from -4.5 to +1.9 per mil. Sedimentary textures in sulfides, the stratiform morphology, asymmetric distribution of wall-rock alteration, an indication of widespread biogenic activity and lack of evidence of major replacement in the hanging wall or laterally, show that the massive sulfides were deposited on the sea floor. The lack of evidence for the rubble mounds that characterize modern black smoker systems and the lack of barite and of oxidized facies suggest sulfide deposition occurred in an anoxic basin from a diffuse venting system. Fluid inclusions in stockworks of other massive sulfide deposits in the Iberian Pyrite Belt have salinities ranging from 3 to 12 wt percent NaCl equiv and homogenization temperatures up to 350 degrees C. A high proportion of these fluids would have reversed buoyancy, collecting in depressions to form brine pools. The delta (super 34) S values of the massive sulfides at Tharsis are consistent with the mixing of vent sulfur with sulfur depleted in (super 34) S and probably derived from the biogenic reduction of seawater sulfate. Biogenically reduced sulfur may have been supplied by H (sub 2) S leached from the footwall shale, biologic activity within the mounds, and biological reduction of sulfate diffusing downward from overlying oxic seawater.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 103
Serial Issue: 1
Title: Formation of the Tharsis massive sulfide deposit, Iberian pyrite belt; geological, lithogeochemical, and stable isotope evidence for deposition in a brine pool
Title: A special issue devoted to continental margin massive sulfide deposits and their geodynamic environments
Author(s): Tornos, FernandoSolomon, MichaelConde, CarmenSpiro, Baruch F.
Author(s): Piercey, Stephen J.prefacer
Author(s): Peter, Jan M.prefacer
Author(s): Mortensen, James K.prefacer
Affiliation: Instituto Geologico y Minero de Espana, Salamanca, Spain
Affiliation: Laurentian University, Department of Earth Sciences, Sudbury, ON, Canada
Pages: 185-214
Published: 200802
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
Meeting name: Geological Association of Canada-Mineralogical Association of Canada-Society of Economic Geologists meeting
Meeting location: Vancouver, BC, CAN, Canada
Meeting date: 200305May 2003
References: 135
Accession Number: 2008-089758
Categories: Economic geology, geology of ore deposits
Document Type: Serial Conference document
Bibliographic Level: Analytic
Annotation: Includes two appendices
Illustration Description: illus. incl. sect., strat. col., 2 tables, geol. sketch maps
N37°00'00" - N38°00'00", W07°19'60" - W07°00'00"
Secondary Affiliation: Geological Survey of Canada, CAN, CanadaUniversity of British Columbia, CAN, CanadaUniversity of Tasmania, AUS, AustraliaNatural History Museum, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200828
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