Abstract

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% graphic) 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% graphic). Conodont colour-alteration-index values reported by others show no detectable variation in the same rocks, which indicates that graptolite-reflectance measurements are more sensitive to variations in thermal maturity, at least for overmature sediments, than conodont colour-alteration index.The anomalously high reflectance values of both the graptolites and the bitumen from the Howard's Pass (XY) property, particularly in samples from the upper Active Member, indicate that these rocks have experienced elevated temperatures compared with regional values. This study demonstrates that graptolite-reflectance measurements may be used to delineate thermal anomalies. The integration of graptolite-reflectance and biostratigraphic studies in lower Paleozoic sequences is potentially useful for determining episodes of high heat flow.

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