Rhyolite
C D
0 500 1000 1500 2000 2500 3000 3500 0 1000 2000 3000 4000 R1 R2 Rhyolite Rhyodacite Dacite Alkali RhyoliteA (Na2O+K2O) F (FeOt) M (MgO) Tholeiite Calc-alkaline
Figure 3.25: A - Zr/TiO2 versus Nb/Y discrimination diagram (after Winchester & Floyd, 1977). B - TAS classification diagram illustrating the compositional range for the Neves Corvo samples. C - R2 versus R1 discrimination diagram (after de la Roche, 1980). R1 (4Si- 11(Na+K)-2(Fe+Ti)) and R2 (6Ca+2Mg+Al). D - FeO versus MgO versus total alkalies discrimination diagram (after Irvine & Baragar, 1971).
Fiamme-rich facies samples. Larger symbol corresponds to the large fiamma.
Rhyolite facies samples.
Chapter 3 - The VS Complex at Neves Corvo 3-56
probably had very low primary permeability, have been relatively unaffected by alteration, as indicated by D. Rosa et al. (2004) for the Albernoa area, also in the Iberian Pyrite Belt.
On the de la Roche (1980) compositional classification diagram (Fig. 3.25C), the studied samples fall within the rhyolite field or on the rhyolite/dacite boundary. The exception is the sample of the dacite facies association that plots in the dacite field, although near the boundary between the two fields. The de la Roche et al. (1980) classification diagram is based on cation proportions of the entire major element composition and avoids some of the problems of the Winchester and Floyd (1977) scheme (e.g. Rollinson, 1993). This approach was considered successful for classifying felsic samples of the VS Complex of the Albernoa area (D. Rosa et al., 2004). The de la Roche et al. (1980) classification (Fig. 3.25C) also correlates well with the phenocryst mineralogy of the Neves Corvo samples.
Using both classification schemes (TAS and de la Roche), samples from the fiamme-rich facies association and the rhyolite facies association are rhyolites and the single sample from the dacite breccia facies is a dacite. This dacite sample plots in the dacite field on the Winchester and Floyd (1977) diagram.
In the AFM diagram of Irvine & Baragar (1971), the Neves Corvo data plot within the calc-alkaline field (Fig. 3.25D).
3.5.3 Summary
The geochemical data reveals three different compositional groups that clearly relate to different facies associations (Table 3.5). The two most abundant volcanic facies associations at Neves Corvo, the fiamme-rich facies association and the rhyolite facies association, are both rhyolitic, and both occur below the massive sulfide deposits. The dacite facies association is less abundant than the other two facies associations and was not identified near the massive sulfide orebodies.
Classification of rock types in the early stages of mining was in some cases incorrect. For example, the volcanic rocks associated with the Zambujal orebody were thought to be mafic. Reassessment of that classification by the mine staff suggested a more felsic composition that has been confirmed by this study. The Zambujal volcanic rocks contain abundant quartz and feldspar phenocrysts and the sample analysed (SQ4 - 490.00 m) is rhyolitic, indistinguishable from other samples of the rhyolite facies association.
Chapter 3 - The VS Complex at Neves Corvo 3-57
Table 3.5: Distinctive geochemical characteristics of the principal volcanic facies associations of Neves Corvo.
Lithology and facies
association Characteristics
Rhyolite, fiamme-rich facies
association Highest: TiO2 (from 0.14-0.23 wt %) Rb (211 ppm) Zr/Nb (14.7) Lowest: P2O5 (0.06 wt %) Nb (7.6 ppm) Zr/TiO2 (~787) Rhyolite, rhyolite facies
association Highest: Al2O3/TiO2 (~130) P2O5 (0.1-0.6 wt %) Lowest: Zr/Nb (5.4-7.9) TiO2 (0.07-0.13 wt %) Zr (70-109 ppm) Th (4.8-9.3 ppm) Rb (46 ppm on average, but one sample has 192 ppm) Dacite, dacite facies
association Highest: Zr/TiO2 (~1064) Zr (217 ppm) Nb (19.2 ppm) Th (21.1 ppm) Nd (65.4 ppm) Lowest: Ti/Zr (~5.6) Al2O3/TiO2 (~77)
3.6 Evolution, facies architecture and water depth at Neves Corvo
3.6.1 Environment of deposition and water depth
A submarine depositional environment for the Neves Corvo succession is evident from the fossil assemblage and the occurrence of massive sulfide deposits (Oliveira et al., 2004; Korn, 1997; Herzig and Hannington, 1995; Barrie and Hannington, 1999). The nature of some of the volcanic and sedimentary facies also supports this interpretation. The two fiamme breccia units (fiamme breccia A and fiamme breccia B) and the graded fiamme sandstone units have an internal organisation that indicates deposition from water-supported volcaniclastic gravity currents (Lowe, 1982). The large volume of hyaloclastite also suggests emplacement in a subaqueous setting (e.g. Pichler, 1965). The outsize fiamme in the fiamme mudstone facies are interpreted to be water-settled.
The water depth is not tightly constrained but was clearly below storm wave base, given the presence of planktonic ammonoids, the absence of traction current structures and the great thickness of massive black mudstone. The massive to graded, very thick tabular bedforms in the fiamme breccia A and B units and the turbidite-like nature of the graded fiamme sandstone units are also consistent with a below-wave-base depositional settings (Lowe, 1982). In modern oceans, storm wave base varies from 10 to 200 m deep (Johnson and Baldwin, 1996).
3.6.2 Evolution and facies architecture of the Neves Corvo volcanic succession
Three main volcanic events generated the volcanic succession at Neves Corvo. Each volcanic event has a well defined, distinct age. Prior to the volcanic activity, the Neves Corvo depositional basin was
Chapter 3 - The VS Complex at Neves Corvo 3-58
a site of submarine suspension sedimentation recorded by thick black mudstone intercalated with quartzite (PQ Group). The first late Famennian unit above the PQ Group is black mudstone from the Corvo Formation (Fig 3.26).
Late Famennian volcanism
The first volcanic event occurred in the late Famennian and was explosive, rhyolitic and recorded by the fiamme-rich facies association (coarse fiamme breccia, fine fiamme breccia, polymictic-lithic breccia, fiamme-crystal sandstone facies and fiamme mudstone facies). Fiamme breccia A and fiamme breccia B units are two distinct, rhyolitic, explosive-eruption-fed gravity-current deposits (Fig. 3.27). These two units occur in the southern part of the area, and their finer grained and thinner equivalents (graded fiamme sandstone units) occur to the north. Hence, the gravity currents were probably sourced from one or more vents to the south. The abundance of pumice lapilli (now fiamme) and absence of subaerially derived lithic clasts are consistent with the source vents being submarine (e.g. McPhie and Allen, 2003; Kano et al., 1996).
Relvas (2000 unpub.) has identified growth faults that limit the “tin corridor”, in the central part of the Neves Corvo mine. These faults may have controlled the deposition of most of the massive tin ore, the sedimentation of some clastic units and probably the basin topography. The clastic units inside this zone are typically thicker than outside it (e.g. Relvas, 2000 unpub.). These depressed zones may have controlled the distribution of the volcaniclastic, gravity-driven currents, therefore influencing the occurrence of the fiamme-rich deposits in the basin.
The polymictic sandstone facies is compositionally distinct from the fiamme-crystal sandstone facies and was intersected in drill-holes SE22A, SRN968 and SD6. The polymictic sandstone facies may be a relatively distal facies probably sourced from an explosive eruption(s) at a vent with uncertain location (Fig. 3.28). Alternatively, these facies may have formed by remobilisation and resedimentation of previously formed, unconsolidated debris. Distinguishing primary pyroclastic deposits from resedimented pyroclastic deposits that were temporary stored can be very difficult, or in some cases impossible (Cas and Wright, 1991; McPhie et al., 1993; White et al., 2003).
Deposition of the Corvo Formation continued during and after these volcanic events elsewhere in the basin, as lateral equivalents of the volcanic units or covering them.
Early Strunian
Early Strunian strata are absent in the Neves Corvo area (Oliveira et al., 2004). This stratigraphic hiatus seems to be restricted to the Neves Corvo area (Oliveira et al., 2004) and although of uncertain significance, may reflect local tectonic events perturbing the sedimentation.