Biotic-Abiotic Radial-Acicular Crystal Fan Fabrics with Rosickýite at the Angel Springs Tufa-Travertine Deposit in Southern British Columbia

Abstract

The tufa-travertine deposit at Angel Springs developed through the interaction of thermal waters with bedrock of the Proterozoic Okanagan Gneiss, which extends along the eastern margin of the Okanagan Valley fault system in south-central British Columbia. This warm spring is proximal to Miocene plateau basalt flows that cut the Okanagan Gneiss bedrock eastward of the Okanagan Valley. The carbonate deposit, which is widely referenced as tufa, consists of a tufa and an overlying travertine. These texturally and petrographically different deposits developed in response to a shift in the groundwater pathway wherein the surface discharge transitioned from multiple vents of the tufa to a single downslope vent pool of the overlying travertine. Lower interval tufa fabrics are characterized by networks of fenestral-type porosity supported by a framework of curvilinear stromatolitic sheets of calcite that resulted from the partial coalescence of sub-horizontal arrays of calcite-encrusted cyanobacteria. These tufa fabrics aligned with the strong stream flow emanating from multiple vents. In contrast, radial crystal fan fabrics of the upper interval travertine developed with the transition from biotic calcite encrustations of upward branching cyanobacteria to the abiotic development of radial crystal fans. Length-long mouldic porosity tubules were occluded as the abiotic sheaf- and fan-shaped crystal growth surfaces expanded radially with rapid degassing. Core areas of the radial crystal arrays coalesced and were partially recrystallized, resulting in spheroidal crystalline fabrics that preserve inclusion-delineated concentric growth surfaces. Syntaxial overgrowths of radially aligned acicular crystals rapidly expanded from these core areas of coalesced calcite, resulting in the formation of abiotic radial-acicular and starburst crystal arrangements. The biotic–abiotic transition resulted in the rapid growth of abiotic crystal fans as the dominant deposition process of the upper interval travertine fabrics, but not in the lower interval tufa fabrics. Activities of sulfate-reducing bacteria in the sulfate-rich thermal water of both intervals resulted in precipitation of rosickýite, the monoclinic polymorph of S, as nanocrystalline globular protuberances on surfaces of calcite crystals. This is the first recorded occurrence of rosickýite in a tufa-travertine deposit and the first recognition of this mineral in Canada.