The Saqisan Mine — a paleokarst uranium deposit, South China

Abstract

The Saqisan uranium deposit, located in Guangxi Province, South China, is hosted mainly by a dark-gray limestone, solution-breccia unit, and paleocavern-fill sedimentary rocks. The deposit is the largest known breccia-hosted uranium deposit in China and was initially described as of synsedimentary shallow-marine origin. However, recent open-pit operations at the Saqisan mine have uncovered new exposures that necessitate a thorough change in the genetic interpretation of the deposit. The new exposures suggest that the occurrence of Saqisan ores in sediment-filled solution breccia and paleokarst cavities of limestone is in part similar to that of solution-collapse breccia pipe-type uranium deposits in Arizona, United States. The interconnected paleokarst passages and caverns were filled with consolidated, mineralized, clastic sedimentary rocks. Most of the orebodies were deposited in the interconnected paleokarst passages and caverns. Two stages of primary uranium mineralization are recognized. The early stage (I) mineralization is the most important for uranium as it accounts for 60% of Saqisan's total uranium tonnage. Stage I mineralization formed low-grade ore. The late stage (II) mineralization, which accounts for the remaining 40% of Saqisan's total uranium tonnage, is hosted by strongly silicified, foliated, and mylonitized host limestones. The uranium in stage I ores predominantly occurs as micro-sized pitchblende in the clay-rich breccia matrix. The uranium in stage II ores mainly occurs as pitchblende veinlets, 0.1–2 mm wide, and as disseminated pitchblende grains ranging from less than 0.1 to 1 mm in size. Sulfur isotope data indicate that biogenic pyrite in the host carbonate may have been the major source of sulfur in the mineralizing fluids. Oxygen and carbon isotope data show that most of the participated CO 2 and HCO 3 − during stage I mineralization was derived from bicarbonate dissolved out of the marine host carbonate. Oxygen and hydrogen isotope data combined with determinations of temperature and salinity for the inclusion fluids of stage II veinlet quartz show that the ore-bearing fluids of stage II mineralization are apparently devoid of CO 2 and CH 4, and were originally saline, meteoric ground water. These basinal brines, which were from connate waters trapped in the Paleozoic carbonates, are believed to have flowed along aquifers of the basinal sequence and up preexisting subvertical faults to produce the observed style of mineralization by chemical interaction with host carbonates. It is postulated that the high-grade uranium ore of stage II is the product of multiple, superimposed reconcentration of uranium from the early synpaleokarstic ore (stage I) by low- to middle-temperature (110–251°C) uranium-enriched hydrothermal brines driven by a regional heating event, such as the late Yanshanian tectonism. The heat source for the Saqisan hydrothermal system could have been related to conversion of mechanical energy into heat during the late Yanshanian regional tectonic deformation.