Graptolites as indicators of regional maturity in lower Paleozoic sediments, Selwyn Basin, Yukon and Northwest Territories, Canada

Variations in reflectance and bireflectance of graptolites and bitumen are documented from lower Paleozoic rocks of Howard's Pass (XY) Zn–Pb deposit and adjacent portions of the Selwyn Basin. Two types of graptolite morphologies are recognized in these rocks: a nongranular type, comprising blocky and lath-shaped varieties, and a granular type. The nongranular graptolites generally display higher reflectance and bireflectance than granular graptolites. Bitumen (epi-impsonite to meso-impsonite) is common in these rocks and displays a wide range of reflectance and bireflectance values.At the Howard's Pass (XY) property, graptolite-reflectance values display a slight increase with depth, and anomalously high values (7.46–10.25% ) are documented in the sulphide-rich, upper Active Member. In surrounding regions of the Selwyn Basin, remote from Howard's Pass, graptolite-reflectance values are lower (3.02–6.26% ). Conodont colour-alteration-index values reported by others show no detectable variation in the same ...