Dr J. Milsom has written as follows—A number of speakers in referring to the Ok Tedi, Frieda or Yandera ore bodies, had regarded mainland New Guinea as an island arc environment. The geological setting in all these cases is that of a continental margin, and if their mineralogical affinities are with the deposits in nearby island arcs, then the reason must be found in the existence of some other factor, which might be purely geographical.

Furthermore the New Guinea porphyrys, and particularly Ok Tedi, which has been dated at 1·1Ma present difficulties for any theory which proposes a straightforward relation between porphyry coppers and subduction. It is not possible to envisage active subduction beneath Ok Tedi at the time of mineralisation; the irregular zone of intermediate earthquakes that exists at present beneath the New Guinea Highlands is, if of subduction origin at all, the result of a late Miocene or Cretaceous event in a proposed but still unproven history of arc reversals and collisions.

Papua New Guinea's porphyry deposit on Bougainville lies above a well defined Benioff zone in the Solomons island arc, but this arc is not a simple one. Its complexities and the reported existence of a sediment filled trench along the northeastern flank have led to the suggestion that there was formerly subduction from the Pacific side. The numerous and important porphyry coppers of the Philippines are also in an area which has at different times overlain subduction zones of opposite polarities and I was greatly in-

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