ALBERT

Description: Shallow magmatic-hydrothermal systems are characterized by steep gradients in temperature and pressure, and because the fluid is of low density and highly compressible, the solubility of ore minerals in these systems varies considerably as a function of both temperature and pressure. We use novel pressure- and temperature-dependent experimentally derived thermodynamic data to geochemically model the transport and deposition of Au, Ag, and Mo by vapor and low- to intermediate-density supercritical fluids in the context of Au and Mo porphyry and Au-Ag epithermal ore formation. The results show that there is a strong compositional control on the Au/Mo ratio of the parental ore fluid, which can explain Au-Mo zoning in porphyry ore deposits and the formation of Au-rich and Mo-rich subtypes. Gold solubility reaches a maximum between 320 °C and 500 °C, depending on the fluid density, whereas Mo and Ag concentrations decrease with decreasing temperature and pressure. These differences in mineral solubility help explain the fractionation of Au from Ag and Mo and the preferential mobilization of Au into sites of epithermal ore deposition. Application of this modeling of metal solubility in vapor-like fluids offers an important opportunity for understanding individual ore-forming hydrothermal systems and determining the limiting factors for metal enrichment.

Description: Maureen is the largest among several U (-Mo-F) prospects occurring along a Late Devonian unconformity in the Georgetown area, northern Queensland, Australia. Mineralization is structurally controlled by the intersection of steep east-west fractures with an unconformity between a Proterozoic basement and a Paleozoic cover sequence of continental sedimentary rocks and abundant rhyolitic volcanic rocks. The mineralogical composition of high-grade ore (pitchblende + Fe-rich molybdenite + arsenopyrite + arsenian pyrite + fluorite + dickite + chlorite + goyazite ± graphite or hematite), postlithification brecciation, and quartz dissolution indicate a strong chemical gradient and disequilibrium during mineralization between the reduced basement and the largely oxidized cover sequence. Elemental and mineral zonation centered to faults, and fractures cutting the unconformity, indicates locally reducing and quartz-dissolving conditions during the formation of tabular U-Mo orebodies, which are surrounded by a halo rich in fluorite. A detailed petrographic study, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of quartz- and fluorite-hosted fluid inclusions provide concentrations of ore-forming components in fluids related to hydrothermal uranium mineralization. Three fluids were involved in the mineralizing process at Maureen: a likely oxidized saline (L3) fluid, an aqueous (L1) fluid, and a CH 4 -bearing carbonic vapor (cV). The oxidized saline (L3) fluids trapped in fluorite and dickite-bearing hydrothermal quartz veinlets crosscutting detrital quartz show the highest average concentrations of U (10–47 ppm), Mo (489–888 ppm), and As (318–777 ppm), with element ratios close to those of high-grade mineralized rocks. We suggest that aqueous (L1) fluids mixed in the cover sequence with oxidized saline fluids (L3) to precipitate fluorite as a distal halo surrounding the east-west fractures. Their mixture, an oxidized moderately saline fluid (L2), reacted with methane-bearing carbonic vapor (cV) to precipitate U and Mo by reduction, close to the intersection of the fractures with the unconformity. Geochemical analysis of coexisting vapor and liquid assemblages indicate mixing of oxidized saline fluids with reduced carbonic vapors to be the driving force for uranium precipitation. The Maureen deposit shares essential geologic characteristics and hydrothermal processes with Proterozoic unconformity-related U deposits, but probably owes its unusual element association of U with (equally redox sensitive) Mo and abundant F to a source region in the volcano-sedimentary cover sequence that was enriched in these elements, due to the presence of highly fractionated, easily leachable, and oxidizable felsic volcanics.

Description: Rare earth elements (REE) include the lanthanides (La–Lu), Y, and Sc which are critical elements for the green energy transition. The REE show a decrease in ionic radii with increased atomic numbers, which results in a so-called lanthanide contraction systematically affecting crystal structures and mineral properties. Here we present a compilation of reference Raman spectra of ten REE sesquioxides (A-, B- and C-type), five REE hydroxides, eight xenotime-structured REE phosphate endmembers and two solid solutions, seven monazite-structured REE phosphate endmembers and two solid solutions and seven rhabdophane endmembers with up to five Ce1−xLREEx rhabdophane solid solutions (LREE = La–Gd). Raman mode assignment is based on a detailed literature review summarizing existing analytical work and theoretical calculations and systematic trends observed in this study by analyzing different REE-bearing solids. The wavenumbers of the main REE-O Raman band systematically increase with decreasing ionic radii forming discrete linear trends within isostructural mineral groups, that can be used to estimate the REE-O mode in other solids with known REE-O coordination numbers. Photoluminescence using 266 nm, 532 nm and 633 nm excitation laser wavelengths for REE-bearing oxides, hydroxides, anhydrous and hydrous phosphates is also presented providing a new framework for identifying REE-phases in phosphate-bearing natural mineral deposits.