The Rio Tinto deposit is a giant volcanogenic massive sulfide deposit (VMS) that contains more than 500 Mt of pyrite-rich massive sulfides and more than 2 Gt of mineralized stockwork. Three broad lithostratigraphic groups occur in the regional stratigraphy: the phyllite-quartzite group, the volcano-sedimentary complex, and the Baixo Alentejo Flysch Group. These three major packages reflect the evolution of a depositional environment from a stable platform to deposition in pull-apart continental basins during oblique subduction and collision and coeval synorogenic flysch sequence.
The volcano-sedimentary complex, which hosts massive sulfide mineralization at Rio Tinto, can be divided into four major units: (1) the Mafic Siliciclastic Unit, (2) the Lower Sedimentary Unit, (3) the Felsic Unit, and (4) the Upper Sedimentary Unit. The Felsic Unit is further subdivided based on new U-Pb zircon geochronology into three distinct subunits. Felsic Unit I (ca. 356 Ma) includes dome complexes dominated by rhyodacite and reflects the onset of felsic magmatism in the region. Felsic Unit II (ca. 352–348 Ma) represents the main interval of volcanic activity, also dominated by rhyodacite domes and related aprons, and is associated with widespread VMS mineralization. Felsic Unit III (ca. 340 Ma) reflects a late pulse of rhyolitic volcanism. Massive sulfides occur as two different styles of mineralization: (1) replacive ores as discordant pipes hosted by glass-rich felsic rocks and enclosed by a large zone of stockwork-like mineralization and (2) overlying shale-hosted exhalative mineralization in small anoxic basins, probably formed during the collapse of the volcanic domes of Felsic Unit II in the Middle-Late Tournaisian.
New lithogeochemical data illustrate two types of mafic rocks in the Mafic Siliciclastic Unit: a basaltic andesite and a high–Ti-Zr basalt, both of tholeiitic affinity. Using immobile element ratios (heavy rare earth elements [HREEs], Al, Y, Zr, and Ti) of the Felsic Unit, fundamental differences have been recognized between the subunits. The unmineralized Felsic Unit I is characterized by high Zr content (225–300 ppm) and a pronounced Eu negative anomaly, and probably represents the most fractionated rocks. Felsic Unit II is characterized by Zr values between 50 and 200 ppm. The low Zr values of the mineralized unit contrast with the typically high Zr values of the felsic rocks related to volcanogenic massive sulfides elsewhere and, at a regional scale, can help to discriminate potentially fertile domes from barren volcanism.