Lead isotope ratios in Niagara Escarpment rocks and galenas: implications for primary and secondary sulphide deposition

Abstract

Galenas occurring in vugs and veins in the dolomites of the Eramosa, Goat Island, and Gasport members of the Middle Silurian Lockport Formation in the Niagara Escarpment have a small but significant range in lead isotopic composition. These differences are recorded in specimens from exposures in the Niagara Gorge in the east to the Guelph quarry, 100 km to the northwest. The galena isotopic data fit linear trends on 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–206Pb/204Pb plots within experimental error. The most radiogenic ratios are anomalous, giving model ages (based on the growth curves of Zartman and Doe) up to 200 Ma younger than the most likely age of primary deposition of ~ 300 Ma. Sulphides currently being deposited from springs at the floor of the Vinemount quarry have a lead isotopic composition similar to that of some of the galena in the overlying dolomites. Rocks from several of the formations exposed at this site contain lead only slightly more radiogenic than the most radiogenic galenas. These results are consistent with the hypothesis of Haynes and Mostaghel that the modern sulphides result from local solution and redeposition of older sulphides. Previously published isotopic ratios for lead in the potassium feldspar constituents of ~ 300 Ma plutonic intrusives in the Appalachians cluster around the least radiogenic galena ratios and suggest that the lead in the Niagara Peninsula galenas has been remobilized from rock units in the Appalachian Basin. Grenville crystalline basement rocks do not appear to have contributed significantly to this lead. A single sample of galena from the Eramosa Member, exposed in the Ebel quarry in the Michigan Basin at the northwestern extension of the escarpment, has 207Pb/204Pb and 206Pb/204Pb ratios that are substantially more radiogenic than those in any of the galenas from the Niagara region of the Appalachian Basin and a 208Pb/204Pb ratio that lies significantly below the linear distribution defined by the Niagara region data set. We tentatively interpret this difference as reflecting derivation of the lead in the two basins from different crustal blocks.