Metallogenic Mechanism of Typical Carbonate-Hosted Uranium Deposits in Guizhou (China)

Abstract

Research on topics such as geological–tectonic evolution, metallogenic models of deposits (gold, mercury, lead, zinc, etc.), and ore-forming fluids’ evolution has been conducted in Guizhou. However, few studies have been conducted on uranium (U) deposits (especially carbonate-hosted U deposits). Moreover, the relationship between hydrocarbon fluids and U-mineralization has not been addressed at all. Typical carbonate-hosted U deposits (including some ore spots) in Guizhou Province have been investigated through close field work, petrography, mineralogical, micro-spectroscopy, organic geochemical and C isotope studies. The central part of the U-ore body is often black (the black alternation zone) at the outcrop, and its sides are gray and gray-brown (the gray alternation zone); the color gradually becomes lighter (black to gray) from the center of the ore body out to the sides. Petrographic observations, microscopic laser Raman spectroscopy, and infrared spectroscopic and scanning electron microscope analyses have indicated that U-minerals (pitchblende and coffinite), pyrite and “black” organic matter (OM) are closely co-dependent, with the OM having the typical characteristics of bitumen. Large light oil fluid inclusions were found in gray alternation rocks (besides the U-ore body) with strong light blue fluorescence properties, indicating that hydrocarbon fluids and U-minerals may came from the same U-bearing hydrocarbon fluids. The values of the 13C isotope value, a biomarker of OM and trace elements, REEs in U-ores, were found to be similar here to those in the local paleo-petroleum reservoir, indicating that the bitumen may originate from the deeply intruding paleo-petroleum reservoir. The precipitation of U is related to the cracking differentiation of hydrocarbon fluids. As result, the carbonate-hosted U-mineralization in Guizhou is neither of a sedimentary diagenesis type, nor of a sedimentary diagenesis superimposed leaching hydrothermal transformation type, as have been described by previous scholars. To be exact, the U deposit is controlled by fault and hydrocarbon fluids, and so it can be defined as a structural hydrocarbon–carbonate-type U deposit. A new U-mineralization model was proposed in this study. Here, U, molybdenum, and other metals were mainly found in the black rocks in the lower stratum (presumably Niutitang Formation), having migrated together with hydrocarbon fluids in the form of tiny mineral inclusions. The hydrocarbon fluids (containing some brine) caused cracking and differentiation upon entering the fracture zone, at which point the ore-forming materials (U, pyrite, and other metals) were released and precipitated.