Abstract

Age determinations by the total-rock Rb-Sr method of several suites of felsite from the White Pine area of Michigan indicate dates ranging from 978 + or - 40 m.y. to 1,100 + or - 25 m.y. A single specimen of felsite from Government Peak of the Porcupine Mountains was dated at 1,042 + or - 32 m.y. Assuming that the Porcupine Mountains are an anticline, this date sets an upper limit to the time of deposition of the overlying Nonesuch Shale. Another upper limit is provided by dates of 1,107 and 1,180 m.y. for two felsite pebbles from the lower sandstone unit of the Parting Shale member exposed in the old Nonesuch Mine.Nine samples of mineralized and unmineralized rock from the basal section of the Nonesuch Shale exposed in the mine workings of the White Pine Mine were analyzed. These samples form a good co-linear array in coordinates of Rb 87 /Sr 86 and Sr 87 /Sr 86 . The apparent age, calculated from the slope of the isochron, is 1,075 + or - 50 m.y. The initial Sr 87 /Sr 86 ratio is 0.7080 + or - 0.0008. The apparent age of the Nonesuch Shale is interpreted to be slightly greater than the time of deposition because of the probable incorporation of inherited Sr 87 into the sediment at the time of deposition.The isotope composition of three lead samples extracted from chalcocite of the ore body at White Pine was found to be anomalous. This suggests that the lead in the ore was mixed with radiogenic lead and favors an epigenetic or late diagenetic rather than a syngenetic origin for the copper sulfide in the basal portion of the Nonesuch Shale.

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