Reply to D. Large's comment

Abstract

The relationship between the original magma type and exhalative mineralization has been discussed by many workers. In deposits such as Meggen, Rammelsberg, Mr. Isa and McArthur River the presence of minor acid tuffaceous units may be a result of volcanism genetically associated with mineralization or, alternatively, could be quite fortuitous. With reference to the Iberian pyritite deposits, the relationship to ca!c-alkaline volcanism is much clearer. The pyrJtite deposits in the Volcanic Siliceous ComFlex (VS) are associated with a differentiated suite of mafic to acid volcanics with intercalated clastic and chemical sediments. At Rio Tinto, the VS sequence comprises a basal sequence of thick mafic-intermediate volcanics overlain by felsic volcanics with pyritite mineralization at the top of the felsic pile (Palomero, 1977). Similarily at La Zarza and Aljustrel, the mineralization is at the top of a thick sequence of felsic volcanics (Strauss and Madel, 1974; Carvalho et al., 1976) whereas the mineralization at Tharsis is intimately associated with the acid Tharsis I volcanic sequence. The rock type names keratophyre and spilite are unfortunately generally very loosely applied and the data of Aye (1974) shows that VS acid volcanics display a continuous variation from sodic to potassic rhyolite. Classification of rocks such as keratophyre or spilite give little indication of the calc-alkaline or tholeiitic affinity of these rocks and SiO~ vs. FeO/MgO, CaO vs. FeO/MgO and AFM plots of the VS volcanics (Fig. I) shows that they are dominantly calc-alkaline with minor tholeiitic units. In the Iberian pyrite belt and many other areas, a bimodal suite of calc-alkaline and tholeiitic volcanics is present and one magma type is intimately associated with mineralization. For example, Descarreaux (1973) has shown that the volcanics of the Abitibi Belt in Canada are eharacterised by rocks belonging to both the calc-alkaline and tholeiitic magma series and the stratiform Cu-Zn mineralization is associated with calc-alkaline rocks (predominantly rhyolite).