Abstract

Samples from the Porphyry deposit and the Shoot zone prospect of St. Andrew Goldfields Ltd. in Taylor Township near Matheson, Ontario, have been dated by several different techniques and utilized as a test of the use of Pb-isotope measurements in determining the time of mineralization in gold deposits of the Abitibi greenstone Belt. Clear and abraded zircons from an altered "sulfidic porphyry" unit yield a well-defined age of 2697.3 ± 1.3 Ma, indicating that the original intrusive rock unit containing these zircons was either latest synvolcanic or earliest syntectonic. Larger "bulk" samples of zircon from the same unit contain many altered and cracked grains, and yield an age of 2682 ± 4 Ma, close to the peak of syntectonic igneous activity. Pb/Pb isochrons determined from sulfide samples in mineralized material from the Taylor "porphyry zone" yield a two-stage model age of 2663 ± 17 Ma, and suggest that mineralization postdates the syntectonic granitoids. These Pb-isotope data are compared with isotope ratios determined on samples from the Dome mine. For these latter samples, the isotopic ratios indicate that an earlier mineralization event was reset at 2266 ± 49 Ma, suggesting to us that the sulfides, and hence gold mineralization, were remobilized at this later time. It is proposed that this remobilization is responsible for a significant benefaction of the gold ore and may make the difference between a mineable orebody and an uneconomic prospect. This time of remobilization corresponds well with some Rb/Sr dates in the Abitibi Province and may represent a previously unrecognized, but significant hydrothermal event. Rb/Sr ages on volcanic units yield ages of 2520–2580 Ma, consistent with similar ages in the surrounding area. They may represent cooling following a thermal event associated with the intrusion of the latest granitic plutons. A minor hydrothermal event at ~1600 Ma seems to have reset the Rb/Sr system in some micas and affected some pyrite samples, resulting in the formation of late carbonate and hematite.

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