Volcanic Stratigraphy, Alteration, and Sea-Floor Setting of the Paleozoic Feitais Massive Sulfide Deposit, Aljustrel, Portugal

Abstract

The Feitais deposit is one of six polymetallic, volcanic-hosted massive sulfide deposits located adjacent to the town of Aljustrel in the Iberian Pyrite Belt of southern Portugal. The deposit occurs on the upright limb of the Feitais anticline, within the upper part of the regional Volcano-Sedimentary Complex of Devonian to Mississippian age and near the contact with the overlying Culm Flysch Group. It is stratigraphically underlain by felsic tuffs and flows and overlain by a unit of feldspar-bearing felsic tuff that passes upward into chert, then argillite, and finally graywacke turbidites. The massive sulfide lens is about 1,000 m in length, 500 m wide, and up to 100 m thick. Although mainly fine-grained pyrite, it commonly has a Cu-rich base that passes upward through a low-grade pyritic zone to a Zn-Pb–enriched upper portion. Sulfide minerals include sphalerite, galena, chalcopyrite, tetrahedrite, and minor arsenopyrite. Copper stockwork veins in rhyolite underlie the thicker sections of the deposit and appear to be spatially associated with growth faults that controlled basin geometry. These faults also influenced the distribution of tuffaceous and cherty (exhalative) units that overlie the sulfides. In terms of mined plus reserve ores, the Feitais deposit was estimated in 1998 to contain 54.5 Mt grading 3.72 percent Zn, 0.42 percent Cu, 1.2 percent Pb, and 44 g/t Ag (Leistel et al., 1998). Recent drilling has identified a new Zn resource in the upper part of the massive sulfide lens consisting of a measured and indicated resource of 15.22 Mt grading 6.00 percent Zn, 0.21 percent Cu, 1.85 percent Pb, and 67.4 g/t Ag; and a deeper Cu resource straddling the lower part of the massive sulfide lens and the underlying stockwork mineralization and consisting of an indicated resource of 4.87 Mt grading 0.88 percent Zn, 2.12 percent Cu, 0.24 percent Pb, and 13.5 g/t Ag. The deposit is open downplunge to the northwest for 200 m. It is truncated by the northeast-striking, steeply dipping Represa fault. The deposit is offset right laterally about 500 m along this fault, north of which it is called the Estacao deposit. Application of lithogeochemical methods to 170 new and 80 published analyses of the Feitais host rocks led to the chemical identification of five main felsic rock types that have distinctive immobile element signatures. These signatures are maintained even where the rocks are strongly altered. The deposit is underlain mainly by a thick (>150 m) unit of rhyolite A, the most fractionated of the felsic rocks. Rhyolite X occurs structurally below rhyolite A in two deep footwall holes. Rhyolite B occurs mainly downdip of the massive sulfide lens but also is present as meter-scale intervals near the base of the sulfide lens. The deposit is overlain stratigraphically by 5 to 30 m of tuffaceous rhyolite C, which is followed by 2 to 20 m of Fe-bearing chert. A centimeter-scale crystal tuff bed with a distinctive felsic composition overlies the cherts and forms a marker horizon that can be traced at least 700 m along the strike of the deposit. The massive sulfide lens becomes thicker downdip to the northeast, then terminates laterally against an uplifted block of footwall rhyolite A capped by rhyolite B. Field relationships and isopach data suggest that an asymmetrical sea-floor graben bounded by a synvolcanic fault was filled by massive sulfides, followed by rhyolite C, then chert, with the chert also extending laterally beyond the graben margin. Footwall and hanging-wall felsic volcanic rocks are altered to chloritesericite-pyrite-quartz-carbonate assemblages and contain stockwork chalcopyrite-pyrite-quartz-carbonate veins. Hydrothermal alteration is strongest in the upper 50 m of rhyolite A below the orebody but extends at least 200 m into the footwall along the main synvolcanic fault zone. Rhyolite C above the orebody is also strongly altered. These areas have experienced significant additions of Fe and Mg, with variable gains or losses of Si. Barium is enriched for up to a few tens of meters above and laterally from the orebody. Dominating the central part of the Aljustrel area is a quartz-feldspar (QF) porphyry that shows primary enrichment in Fe, Ti, and V relative to the rhyolites. At Feitais, this lithology was only encountered deep in the footwall, structurally below rhyolite X. No andesitic or mafic extrusive rocks have been identified. The QF porphyry is chemically unrelated to the four rhyolite types. The rare earth element (REE) patterns of the rhyolites show moderate enrichment in the light REE, with near-flat middle to heavy REE. The rhyolites are of tholeiitic to transitional affinity, with moderately high Zr (180–380 ppm) but low Nb (7–15 ppm) contents. These collective features are consistent with a rift-related, continental-margin setting, behind an arc, for the rhyolite volcanism.