Discussion of “relationships between mineralization and silicic volcanism in the Central Andes” by P.W. Francis, C. Halls and M.C.W. Baker

Abstract

In their stimulating paper, Francis et al. (1983) present convincing evidence for the association of several Central Andean tin and copper vein/stockwork deposits with felsic volcanic domes, rather than with stratovolcanoes (ef. Sillitoe, 1973). They also reexamine the problem of the relationships between caldera formation (and voluminous ash-flow tuff eruption) and large-scale hydrothermal activity (see e.g., McKee, 1979; Sillitoe, 1980), concluding that protracted cooling histories of sub-caldera plutons may be reflected in the long time lags (1–10 m.y.) documented between caldera collapse and superimposed mineralization. They cite, inter alia, the El Salvador porphyry copper deposit, northern Chile (lat. 26°17′S) as revealing such a sequence of events, and provide LANDSAT evidence for the presence of an extensively dissected, ca. 15 km wide, caldera in the mine area. We consider the authors' case to be persuasive in general, but suggest that their argument regarding El Salvador is weakened by an apparent mis-reading of Gustafson and Hunt's (1975) brief description of the pre-mineralization geological evolution of the Indio Muerto complex. In particular, they conflate two distinct episodes of subaerial volcanism. Because Mercado (1978) also in part misinterprets the regional and local stratigraphic relationships in her 1 : 25,000 geological map of the area, there is considerable potential for confusion.