14:45 〜 15:00
[HRE13-04] Mineral assemblages and their implication to forming the Wigu Hill Carbonatite Dykes-related Rare Earth Element (REE) Deposits, Kisaki Area, Eastern Tanzania
キーワード：Wigu Hill Carbonatite, Burbankite pseudomorph, REE fluorcarbonates, Goyazite
Carbonatite deposits are essential for research as they contain critical metals like niobium and rare earth elements. REE minerals such as bastnaesite, monazite, apatite, and parisite have been found in the Wigu Hill Carbonatite. However, the formation mechanism of these minerals is not well understood. This study aims to describe the mineral assemblages and their implications for forming the REE deposits by integrating petrography, XRD, XRF, and SEM-EDX techniques.
Fieldwork observations identified two sets of carbonatite dykes that trend NW-SE and NE-SW; the former dykes dominate in the study area. The dykes measure 1-8 m thick, dip nearly vertical 89°. The dykes intrude the country rocks that are comprised of feldspathic and quartzo-feldspathic gneisses.
The studies on the carbonatite dykes revealed a range of minerals including REE fluorcarbonates (bastnaesite, synchysite, and parisite), REE phosphates (monazite, fluorapatite, goyazite), burbankite, carbonates (calcite, dolomite, ankerite, and strontianite), oxides (magnetite, chromite, hematite, pyrolusite, and quartz), sulfate (barite and celestine), silicate (zircon), fluorite, and sulfides (pyrite and sphalerite). Zircon, magnetite, and chromite were observed in NE-SW carbonatite dykes, while parisite, celestine, and fluorite were observed in NW-SE carbonatite dykes. Other minerals are commonly observed in both carbonatite dykes while abundances vary. The presence of zircon, magnetite, and chromite in NE-SW carbonatite dykes suggests early crystallization from primary carbonatite melts.
Synchysite crystals formed early during magmatic conditions, showing syntaxial crystal growth and dissolution texture. Dissolution of synchysite was followed by crystallization of bastnaesite. Monazite fills cracks and fractures in fluorapatite and hematite, replaces synchysite, and appears in both stages of formation during magmatic and hydrothermal conditions. Other minerals such as burbankite pseudomorphs, pyrolusite, hematite, barite, celestine, and sulfides suggest hydrothermal conditions and high oxygen fugacities whereby soluble minerals (such as burbankite) are destabilized while stabilizing oxide and its subordinate minerals. The samples are classified chemically as magnesio-carbonatite.
Wigu Hill Carbonatite dyke-related REE deposits formed through (1) early crystallization of magnetite, chromite, and zircon (2) evolution of carbonatite primary melts, simultaneous concentration and crystallization of dolomite, REE-fluorcarbonates, REE-phosphates, and burbankite, and (3) hydrothermal processes resulted to brecciation which leads to the interaction of the carbonatite with external fluids and subsequently precipitation of late-stage mineral assemblages like monazite, barite, and celestine.
Fieldwork observations identified two sets of carbonatite dykes that trend NW-SE and NE-SW; the former dykes dominate in the study area. The dykes measure 1-8 m thick, dip nearly vertical 89°. The dykes intrude the country rocks that are comprised of feldspathic and quartzo-feldspathic gneisses.
The studies on the carbonatite dykes revealed a range of minerals including REE fluorcarbonates (bastnaesite, synchysite, and parisite), REE phosphates (monazite, fluorapatite, goyazite), burbankite, carbonates (calcite, dolomite, ankerite, and strontianite), oxides (magnetite, chromite, hematite, pyrolusite, and quartz), sulfate (barite and celestine), silicate (zircon), fluorite, and sulfides (pyrite and sphalerite). Zircon, magnetite, and chromite were observed in NE-SW carbonatite dykes, while parisite, celestine, and fluorite were observed in NW-SE carbonatite dykes. Other minerals are commonly observed in both carbonatite dykes while abundances vary. The presence of zircon, magnetite, and chromite in NE-SW carbonatite dykes suggests early crystallization from primary carbonatite melts.
Synchysite crystals formed early during magmatic conditions, showing syntaxial crystal growth and dissolution texture. Dissolution of synchysite was followed by crystallization of bastnaesite. Monazite fills cracks and fractures in fluorapatite and hematite, replaces synchysite, and appears in both stages of formation during magmatic and hydrothermal conditions. Other minerals such as burbankite pseudomorphs, pyrolusite, hematite, barite, celestine, and sulfides suggest hydrothermal conditions and high oxygen fugacities whereby soluble minerals (such as burbankite) are destabilized while stabilizing oxide and its subordinate minerals. The samples are classified chemically as magnesio-carbonatite.
Wigu Hill Carbonatite dyke-related REE deposits formed through (1) early crystallization of magnetite, chromite, and zircon (2) evolution of carbonatite primary melts, simultaneous concentration and crystallization of dolomite, REE-fluorcarbonates, REE-phosphates, and burbankite, and (3) hydrothermal processes resulted to brecciation which leads to the interaction of the carbonatite with external fluids and subsequently precipitation of late-stage mineral assemblages like monazite, barite, and celestine.