Duchess tunnel, Latouche Island 8. Bald Eagle tunnel, Knight Island 9. Tunnel cutting five schistose ore-bearing zones, Knight Island 6 The important fact bearing on the distribution of the ore bodies is that they appear to be found only in the rocks of the Orca group and, indeed, for the most part in close association with the greenstone member. Mr. Grant and Mr. Higgins indicate that the copper was derived from the greenstones, a genesis which is entirely similar to that of most of the copper ores of the Chitina ® and White Riverd istricts.
Retrograde exchange of oxygen isotopes between minerals in igneous and metamorphic rocks by means of diffusion is explored using a finite difference computer model, which predicts both the zonation profile of 6"O within grains, and the bulk 6"O value of each mineral in the rock. Apparent oxygen isotope equilibrium temperatures that would be observed in these rocks are calculated from the 6lXO values of each mineral pair within the rock. In systems which cool linearly from a sufficiently high temperature or at a low enough cooling rate, such that the final oxygen isotope values are not dependent upon the initial oxygen isotope values ('slow cooling'), the apparent oxygen isotope temperature derived for a rock composed of a single mineral pair can be shown to be simply related to the Dodson closure temperatures (TJ for the two phases and the mode of the rock. Adding a third phase into a system which undergoes 'slow' cooling will cause the apparent temperature derived for the two minerals already present to differ from the simple relationship for a two-phase system. In some systems oxygen isotope reversals can be developed. If cooling is not 'slow', then the mineral 6"O values resulting from cooling will be partly dependent upon the initial temperature of the system concerned. The model successfully simulates the mineral 6'*0 values that are often observed in granitic rocks. Application of the model will help in assessing the validity of oxygen isotope thermometry in different geological settings, and allows quantitative prediction of the oxygen isotope fractionations that are developed in cooling closed systems.
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