Hydrothermal alteration and mineralization in the Neves-Corvo volcanic-hosted massive sulfide deposit, Portugal. I. Geology, mineralogy, and geochemistry

The Neves-Corvo volcanic-hosted massive sulfide deposit is one of the largest (>300 Mt) and richest known deposits of the Iberian pyrite belt. The deposit is unique because of its extremely high Cu and Sir grades (45 Mt at 6% Cu plus 4.5 Mt at 12% Cu and 2.2% Sn). More than 300,000 metric tons (t) of tin metal is contained in several types of tin mineralization that grade from 500 ppm to 60 percent Sri. The deposit consists of five orebodies, including Corvo, the focus of the present study. The volcanic, volcaniclastic, and sedimentary lithofacies that directly host the Neves-Corvo mineralization consist of a rhyolitic dome-cryptodome-hyaloclastite complex. Facies interpretation indicates that volcanic activity occurred at moderate water depth (below storm-wave base) in variably subsiding basins of different orders. Several syn- and postore modification events included gravity-driven mass transport processes and subsequent low-angle thrusting and asymmetric detachment folding, which shaped the present deposit. The ore geology and geochemistry, and the textural, mineralogical, and geochemical aspects of the ore-forming process in the Corvo orebody collectively suggest that ore formation resulted from a long-lived hydrothermal system, including two main mineralizing events: an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulfide ore generation. Two spatially independent stockworks fed these genetically related mineralizing systems. The early deposition of stringer and massive cassiterite ores took place almost exclusively in the tin corridor-a structural alignment bounded by synvolcanic faults. Textural analysis and geochemical data both indicate minimal fluid-rock interaction during ascent of the tin-bearing fluid. The massive and semimassive cassiterite ores at Corvo most likely formed by direct venting into seawater and/or emplacement in water-laden (unlithified) sediments and early hydrothermal products that mantled the footwall volcanic sequence. Stringer and massive cassiterite precipitation is thought to have been a product of a short-lived episode in the overall ore-forming system. The vein-filling mineral assemblage in the cassiterite stockwork (quartz, cassiterite, pyrite, donbassite, kaolinite, chamosite), coupled with whole-rock and mineral geochemical data, indicate that the tin-bearing fluid had a low pH and relatively high temperature (similar to 380 degrees C). The massive sulfide-related hydrothermal alteration in Corvo is essentially strata bound. The permeability contrasts in the uppermost portion of the footwall succession played an important role in controlling the fluid flow. The hydrothermal alteration is zoned and consists of an inner chloiite/donbassite-quartz-sulfide-(sericite) core that grades outward into successive enveloping K-sericite-quartz-sulfide and Na-sericite-quartz-sulfide halos. Textural evidence, from the mine to the microscopic scale, and mass-balance considerations indicate that extensive silicate replacement in the coherent volcanic rocks of the footwall sequence and disseminated replacement mineralization in the uppermost volcaniclastic and/or sedimentary units were major mechanisms of massive sulfide deposition at Corvo. Alteration mineralogy and geochemistry indicate that the Neves-Corvo ore fluids were hotter and more acidic than typical ore-forming fluids of the Iberian pyrite belt. The geochemical characteristics of the tin and copper mineralization also differ from the ores of typical deposits of the Iberian pyrite belt and imply the involvement of additional metal sources, possibly magmatic, in the ore-forming system. Copper enrichment due to mechanical and fluid-assisted tectonometamorphic remobilization processes accounts for the formation of extremely high grade ore shoots in the deposit.