Crystallization and destabilization of eudialyte-group minerals in peralkaline granite and pegmatite: a case study from the Ambohimirahavavy complex, Madagascar

Eudialyte-group minerals (EGM) are very common in highly evolved SiO2-undersaturated syenites and are characteristic minerals of agpaitic rocks. Conversely, they are extremely rare in peralkaline granites, with only a handful of EGM occurrences reported worldwide. Here, we study two new examples of EGM occurrence in two types of peralkaline pegmatitic granites from the Cenozoic Ambohimirahavavy complex, and assess the magmatic conditions required to crystallize EGM in peralkaline SiO2-oversaturated rocks. In the transitional granite (contains EGM as accessory minerals) EGM occur as late phases and are the only agpaitic and major rare-earth element (REE) bearing minerals. In the agpaitic granite (contains EGM as rock-forming minerals) EGM are early-magmatic phases occurring together with two other agpaitic minerals, nacareniobsite-(Ce) and turkestanite. In these granites, EGM are partly-to-completely altered and replaced by secondary assemblages consisting of zircon and quartz in the transitional granite and an unidentified Ca-Na zirconosilicate in the agpaitic granite. Ambohimirahavavy EGM, as well as those from other peralkaline granites and pegmatites, are richer in REE and poorer in Ca than EGM in nepheline syenites. We infer that magmatic EGM are rare in SiO2-oversaturated rocks because of low Cl concentrations in these melts. At Ambohimirahavavy, contamination of the parental magma of the agpaitic granite with Ca-rich material increased the solubility of Cl in the melt promoting EGM crystallization. In both granite types, EGM were destabilized by the late exsolution of a fluid and by interaction with an external Ca-bearing fluid.

Hydrothermal metasomatism of a peralkaline granite pegmatite, Khaldzan Buragtag massif, Mongolian Altai; complex evolution of REE-Nb minerals

The low-temperature hydrothermal alteration of certain rare-metal minerals is recorded in a quartz-epidote metasomatite from the Tsakhirin Khuduk occurrence in the Khaldzan-Buragtag Nb-REE-Zr deposit, Mongolian Altai. A peralkaline granitic pegmatite was metasomatized by hydrothermal fluids released from associated intrusions, with the formation of, inter alia, chevkinite-(Ce), fergusonite-(Nd) and minerals of the epidote group. The textural pattern indicates recrystallization and coarsening of these phases. Later, low-temperature alteration by fluids resulted in the chevkinite-(Ce) being replaced by complex titanite-TiO2 -cerite-(Ce)-hingganite-hydroxylbastnasite-( Ce) assemblages. Calcite formed late-stage veins and patches. The hydrous fluids were poor in F and CO2 but had high Ca contents.

Geophysical interpretation of U, Th, and rare earth element mineralization of the Bokan Mountain peralkaline granite complex, Prince of Wales Island, southeast Alaska

A prospectivity map for rare earth element (REE) mineralization at the Bokan Mountain peralkaline granite complex, Prince of Wales Island, southeastern Alaska, was calculated from high-resolution airborne gamma-ray data. The map displays areas with similar radioelement concentrations as those over the Dotson REE-vein-dike system, which is characterized by moderately high %K, eU, and eTh (%K, percent potassium; eU, equivalent parts per million uranium; and eTh, equivalent parts per million thorium). Gamma-ray concentrations of rocks that share a similar range as those over the Dotson zone are inferred to locate high concentrations of REE-bearing minerals. An approximately 1300-m-long prospective tract corresponds to shallowly exposed locations of the Dotson zone. Prospective areas of REE mineralization also occur in continuous swaths along the outer edge of the pluton, over known but undeveloped REE occurrences, and within discrete regions in the older Paleozoic country rocks. Detailed mineralogical examinations of samples from the Dotson zone provide a means to understand the possible causes of the airborne Th and U anomalies and their relation to REE minerals. Thorium is sited primarily in thorite. Uranium also occurs in thorite and in a complex suite of [Formula: see text] oxide minerals, which include fergusonite, polycrase, and aeschynite. These oxides, along with Y-silicates, are the chief heavy REE (HREE)-bearing minerals. Hence, the eU anomalies, in particular, may indicate other occurrences of similar HREE-enrichment. Uranium and Th chemistry along the Dotson zone showed elevated U and total REEs east of the Camp Creek fault, which suggested the potential for increased HREEs based on their association with U-oxide minerals. A uranium prospectivity map, based on signatures present over the Ross-Adams mine area, was characterized by extremely high radioelement values. Known uranium deposits were identified in the U-prospectivity map, but the largest tract occurs over a radioelement-rich granite phase within the pluton that is likely not related to mineralization. Neither mineralization type displays a well-defined airborne magnetic signature.