Micromorphologies and sulfur isotopic compositions of pyrite in sandstone-hosted uranium deposits: A review and implications for ore genesis

Abstract

Serving as an adsorbent and a reductant for the dissolved U, pyrite is abundant and commonly found associated with U minerals in sandstone-hosted U deposits. Different nucleation and growth rates of crystals, sulfur sources and precipitation mechanisms (i.e., biogenic or chemical processes) result in a variety of micromorphologies and a wide range of δ34S values of pyrite under low-temperature conditions. A compilation of 793 published δ34S values ranging from -72.0 to +142.8‰ for pyrite mainly consisting of framboidal, euhedral and cement, and a series of evolved morphologies (i.e., early framboidal, followed by euhedral overgrowth, and finally cements) from sandstone-hosted U deposits across the world is presented here. For the different geographical locations of deposits (e.g., China, USA, Australia and Kazakhstan), the different ages of host sandstones (e.g., Triassic, Jurassic, Cretaceous and Paleogene) and the different morphologies of pyrite (e.g., framboidal pyrite and euhedral + cement pyrite), the distribution characteristics of δ34S values are distinct. Four case studies with comprehensive analyses of micromorphologies, in situ sulfur isotopic compositions as well as trace element contents are discussed to illustrate that pyrite has been employed successfully as a redox proxy to evaluate the genetic model of sandstone-hosted U deposits. The data also implies that during the translation of redox interface, the changes of geochemical characteristics well documented in micro-scale pyrite are responsible for U mineralization.