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Geological Society, London, Special Publications; 1977; v. 7; p. 152;
DOI: 10.1144/GSL.SP.1977.007.01.17
© 1977 Geological Society of London

The Planes-San Antonio pyritic deposit of Rio Tinto, Spain: its nature, environment and genesis*

D. Williams, R. L. Stanton & F. Rambaud

Emeritus Professor of Mining Geology, University of London; formerly Geological Consultant to Companía Espanola Minas de Rio Tinto, Ltda, and Rio Tinto Patino, S.A.
Department of Geology, University of New England, Armidale, New South Wales, Australia
Unión Explosivos Rio Tinto, Madrid, Spain

The San Antonio stratiform cupriferous deposit of Rio Tinto, discovered in the early 1960s, is now being exploited as an eastward extension of the bedded pyritite sheet in the old Planes mine, which is there underlain by a stockwork feeder pipe.

The Planes pipe consists of sulphur-rich—copper-poor sulphide ore formed by the almost complete replacement of a stockwork conduit which traverses Lower Carboniferous felsic pyroclastic rocks. Overlying the pipe, and extending far beyond its margin, is the genetically related Planes-San Antonio cupriferous pyritite, a ‘sedimentary-volcanogenic’ deposit precipitated as a chemical sediment derived from sea-floor hot springs associated with the Lower Carboniferous volcanic island arc of southwest Iberia. Most of the pyritite bodies of Rio Tinto were deposited directly above or close to their underlying feeder stockworks. The Planes-San Antonio pyritite displays well-preserved stratification, slump structures and interdigitated tuff bands, and was mainly precipitated comparatively distant from the nourishing Planes stockwork.

Across-layer base-metal zoning in the Planes-San Antonio sheet is usually characterized by an increase in copper, and to a lesser extent in zinc and lead, towards the base of the deposit. Much of this enrichment may have been due to late hypogene processes involving the ascent of juvenile-meteoric solutions which leached base metals from the underlying volcanic pile and deposited them as chalcopyrite, sphalerite and galena by partial replacement of the lower part of the pre-existing pyrite-rich stratiform sulphide sheet.

A notable feature of the pyritite is the abundance of colloform and framboidal pyrite. The mean isotopic composition of the sulphide sulphur of the Planes stockwork ({delta}34S {approx} + 10{per thousand}) is distinctly heavier than that of the stratiform Planes-San Antonio pyrite ({delta}34 S {approx} + 2.7{per thousand}).

The cupriferous pyritic mass of Planes-San Antonio exhibits some of the features characteristic of pyritic deposits formed during the initial stages of arc development (‘Cyprus’ or ‘ophiolite’ type ores). It was actually formed, however, at a late stage of arc evolution, during a period of relative quiescence within the waning episodes of submarine explosive felsic volcanism, and it displays environmental traits analogous to those of the Japanese ‘Kuroko’ ores.


* Paper published in Trans. Instn Min. Metall. (Sect.B: Appl. earth sci.), 84, 1975, B73–82.