DISCUSSION MINERALIZATION IN MOBILE BELTS

Sir: Dr. Gilmour's views on copper mineralization (1) are compatible in principal with metallogenic oncepts presented by the writer (3), but some details warrant discussion and his Group 5, massive sulfide deposits in post-orogenic rhyolitic rocks (p. 451), is questioned. The nature of the host rocks and their role in the history of geosynclinal-orogenic belts is mentioned (p. 450) but the importance of orogenic history is not emphasized in Gilmour's presentation. The use of examples from various geosynclinal belts and Precambrian Shields further detracts from the chrono-logical theme. It seems too much to ask of the reader to grasp the tectonic setting and regional geology for a world-wide variety of deposits. Dr. Gilmour mentions that "similar classifications could also be devised for deposits of other metals than copper " (p. 450). His proposed classifica-tion notes some sulfides associated with copper minerals but omits molybdenite, for example, which seems to be restricted to skarn and porphyry copper deposits and is lacking in eugeosynclinal environments representative of th