DIAGENESIS, STRATIGRAPHY, PALEOHYDROLOGY, AND LEAD ISOTOPE STUDY OF THE PROTEROZOIC SIBLEY GROUP, WESTERN ONTARIO

The Proterozoic Sibley Group is composed of sandstones, red-colored mudstones, and thin limestones that could potentially contain economic mineralization if there has been sufficient basin-scale fluid flow. In particular, there is potential for uranium mineralization to occur at the unconformity between the Proterozoic Sibley sediments and the underlying Paleoproterozoic metasedimentary and Archean granitic basement rocks. We examined numerous drill cores and outcrop exposures to determine the stratigraphic relationships, mineral paragenesis, and diagenesis in order to deduce the paleohydrologic evolution of the Sibley Basin. In addition, to determine if the Sibley Basin has the potential to host unconformity-type uranium mineralization in particular, we analyzed the lead isotopic compositions of twenty-one samples.

Petrographic studies indicate that high temperature silicate minerals, such as microcline, plagioclase, and biotite are commonly well preserved in the Sibley Group.  This level of preservation indicates minimal rock-fluid interaction in the basin. Carbonate cement is widespread, occurred relatively early in the burial history, and essentially sealed off many paleo-aquifers by blocking pore space.

Twenty-one sandstones were analyzed for their lead and trace element compositions by HR-ICP-MS. The majority of the drill hole samples have common ²⁰⁶Pb/²⁰⁴Pb ratios that range between 19 and 22, with the exception of drill hole SB-102, which contains radiogenic ratios that range between 25 and 49.  These radiogenic ²⁰⁶Pb/²⁰⁴Pb values correlate strongly with phosphate, and anomalous uranium concentrations consistent with a source for the radiogenic lead from insitu decay of a U-Th-phosphate phase, such as monazite or apatite and not scavenging of radiogenic lead from a uranium deposit.

Based on our results, the rocks that fill the Sibley Basin have experienced limited water-rock interaction suggesting little basin-scale movement of potential ore-bearing fluids. The lead isotope analyses indicate that where there has been minor water-rock interaction, these fluids have not come into contact with a uranium-rich source.