Genesis and Preservation of a Uranium-Rich Paleozoic Epithermal System with a Surface Expression (Northern Flinders Ranges, South Australia): Radiogenic Heat Driving Regional Hydrothermal Circulation over Geological Timescales

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Brugger, et al., reported in 2011 that the surface expressions of hydrothermal systems are prime targets for astrobiological exploration, and fossil systems on Earth provide an analogue to guide this endeavor. The Paleozoic Mt. Gee–Mt. Painter system (MGPS) in the Northern Flinders Ranges of South Australia is exceptionally well preserved and displays both a subsurface quartz sinter (boiling horizon) and remnants of aerial sinter pools that lie in near-original position.

The energy source for the MGPS is not related to volcanism but to radiogenic heat produced by U-Th-K-rich host rocks. This radiogenic heat source drove hydrothermal circulation over a long period of time (hundreds of millions of years, from Permian to present), with peaks in hydrothermal activity during periods of uplift and high groundwater supply. This process is reflected by ongoing hot spring activity along a nearby fault. The exceptional preservation of the MGPS resulted from the lack of proximal volcanism, coupled with tectonics driven by an oscillating far-field stress that resulted in episodic basement uplift. Hydrothermal activity caused the remobilization of U and rare earth elements (REE) in host rocks into (sub)economic concentrations.

Radiogenic heat-driven systems are attractive analogues for environments that can sustain life over geological times; the MGPS preserves evidence of episodic fluid flow for the past ~300 million years. During periods of reduced hydrothermal activity (e.g., limited water supply, quiet tectonics), radiolytic H₂ production has the potential to support an ecosystem indefinitely. Remote exploration for deposits similar to those at the MGPS systems can be achieved by combining hyperspectral and gamma-ray spectroscopy.

The surface expression of terrestrial volcanogenic epithermal systems—including geysers and hydrothermal pools—is among the most spectacular signs of ongoing geological activity on Earth (e.g., Yellowstone National Park, New Zealand, Kamchatka Peninsula. These systems provide direct insight into ore-forming processes. Most importantly, they support diverse ecosystems and may have played a direct role in the development of life on Earth, given that they provide sources of water, heat, and chemical energy (e.g., molecular hydrogen, sulfur, reduced carbon) conducive to sustaining life. Hence, surface expressions of hydrothermal systems are a prime target for exploration for life in the Solar System Evidence of silica-rich hydrothermal deposits linked to martian volcanic activity has been reported recently.

On Earth, production of H₂ by water radiolysis may fuel 10% of the metabolic respiration in oceanic sediments where organic-fueled respiration is lowest hence, radiolytic H₂ has the potential to support an ecosystem over very long periods. Hydrothermal activity at Mt. Gee–Mt. Painter caused the remobilization of U to form deposits with grades up to several weight percent uranium. Such concentrations may evolve into sustained nuclear reactions early in the planet’s history, which would contribute energetic and chemical conditions conducive to the appearance of life.

Given its Late Paleozoic age, the MGPS should contain both macro- and microfossils. Indeed, the modern expression of the hydrothermal system, the Paralana Hot Springs, contains abundant microbiota. However, neither macrofossils nor unambiguous microfossils have yet been reported from the MGPS. The authors attribute this to the fact that most of the system has been eroded, with only a core preserved; this core is characterized by extensive, multiple silicification, which may be responsible for obliterating morphological evidence of fossils. A study of the organic content (for molecular biomarkers) in the MGPS would be of great interest for understanding the preservation of biosignatures in fossil epithermal systems.