Hydrochronology of a proposed deep geological repository for low- and intermediate-level nuclear waste in southern Ontario from U–Pb dating of secondary minerals: response to Alleghanian events

Abstract

U–Pb ages have been measured on secondary dolomite and silica cements in Cambrian sandstone at the base of an 840 m thick sub-horizontally bedded sedimentary sequence beneath the Bruce nuclear site in southern Ontario to document the history of fluid movement. Results show an average U–Pb age of 320 ± 10 Ma. The initial common Pb end member is slightly, but distinctly, enriched in 206Pb compared with that in older and younger calcite cements elsewhere within the sedimentary section. Combined with previous hydro-geochemical and fluid inclusion studies of the same rocks, the age is interpreted to record episodic migration of a saturated hydrothermal brine. Previously dated calcite cement in sub-horizontal fractures about 500 m higher in the stratigraphic section near the base of the Silurian sequence records similar U–Pb ages of 318 ± 10 Ma by laser ablation inductively coupled plasma mass spectrometry and 313 ± 1 Ma by isotope dilution thermal ionization mass spectrometry. We suggest that the subhorizontal fractures were generated by slumping that resulted from dissolution of underlying evaporite deposits. These ages overlap with the peak of plutonism in the Alleghanian mountains, which were being uplifted contemporaneously 500 km to the southeast. The results suggest the transport of hydrothermal brine from areas of crustal melting through the deep Cambrian sandstone aquifer, while at higher crustal levels meteoric water was also driven over equally large distances by hydraulic gradients from the Alleghanian mountains.