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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^1,, Michael Solomon², Carmen Conde³ and Baruch F. Spiro⁴

¹ Instituto Geológico y Minero de España, Azafranal 48, 37001 Salamanca, Spain
² Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania, Australia 7001
³ Instituto Geológico y Minero de España, Azafranal 48, 37001 Salamanca, Spain
⁴ Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom

Corresponding author: e-mail, f.tornos{at}igme.es

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 K₂O/Na₂O 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 ¹³C values between –31 and –25 per mil is probably the source of ¹³C-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 ¹⁸O and ¹³C compositions of +8 to +10 and –6 per mil, respectively (i.e., hydrothermal fluid and cooler seawater) but having isotopically light carbon (¹⁸O = –1 to 0 and ¹³C = –20). The massive sulfides have ³⁴S values between –33 and +4 per mil, similar to those of pyrite in the footwall and hanging-wall shale (–29 to +5). 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°C. A high proportion of these fluids would have reversed buoyancy, collecting in depressions to form brine pools. The ³⁴S values of the massive sulfides at Tharsis are consistent with the mixing of vent sulfur with sulfur depleted in ³⁴S and probably derived from the biogenic reduction of seawater sulfate. Biogenically reduced sulfur may have been supplied by H₂S leached from the footwall shale, biologic activity within the mounds, and biological reduction of sulfate diffusing downward from overlying oxic seawater. Sulfate is interpreted to have been reduced at temperatures in the range of 60° to 100°C, leading to the immediate precipitation of sulfide muds from the metal-rich fluids that would have filled the proposed brine pool. Early venting and mixing with oxic seawater probably formed the oxidized mineral assemblages at the base of the massive sulfides and led to the formation of the biogenic mounds. In this model, progressive venting led to the gradual displacement of seawater from the bottom of the basin and formation of a reduced brine layer. The existence of biogenic mounds and other indicators of biological activity such as the low ³⁴S compositions of the sulfides and the presence of burrows, suggest that biological activity was widespread and probably critical to the formation of the massive sulfide deposit at Tharsis.

The proposed genetic model may apply to the other shale-hosted, giant, massive sulfide deposits of the southern Iberian Pyrite Belt, such as Las Cruces, Aznalcollar-Los Frailes, Valverde, Lousal, Neves Corvo, and particularly Sotiel-Migollas.

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