REE mineralisation within the Ditrău Alkaline Complex, Romania: Interplay of magmatic and hydrothermal processes

The Ditrău Igneous Complex (north-east Romania) is a tilted Mesozoic alkaline intrusion (~19 km diameter), with enrichments in rare earth elements (REE), niobium, and molybdenum. It has the potential to contribute to a secure and sustainable European REE mining industry, ensuring supply security for these critical metals. The complex comprises a sequence of ultramafic rocks, alkali gabbros, diorites, syenites, nepheline syenites and alkali granites. These units have been significantly modified by sub-solidus interaction with late-stage magmatic fluids and are cut by secondary mafic dykes. The complex was subsequently cut by REE-mineralised carbonate-rich veins. Geochemical and petrological data, including apatite mineral chemistry, from the alkaline igneous rocks, dykes and veins within the Ditrău Complex, have been used to assess the interplay of magmatic processes with late-stage magmatic and hydrothermal fluids, and the effects of these processes on element remobilisation and concentration of critical metals. Only limited critical metal enrichment was achieved by magmatic processes; the REE were preferentially incorporated into titanite and apatite in ultramafic cumulates during primary crystallisation, and were not enriched in evolved magmas. A hydrothermal system developed within the Ditrău Complex magma chamber during the later stages of magmatic crystallisation, causing localised alteration of nepheline syenites by a sodium-rich fluid. Mafic dykes subsequently acted as conduits for late stage, buoyant potassic fluids, which leached REE and HFSE from the surrounding syenitic rocks. These fluids percolated up and accumulated in the roof zone, causing the breakdown of nepheline to K-rich pseudomorphs and the precipitation of hydrothermal minerals such as zircon and pyrochlore within veins. REE mineralisation within the Ditrău Complex is hosted in the latest hydrothermal phase, mineralised carbonate-rich veins, which cross-cut the complex. Monazite is the main REE-bearing phase, it crystallised from a late REE- and carbonate-rich fluid with pH controlled REE deposition

The age of HP metamorphism in the Gran Paradiso Massif, Western Alps: A petrological and geochronological study of "silvery micaschists"

We present a petrological and geochronological study of “silvery micaschists” that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P–T conditions and age of the Alpine high-pressure metamorphism. The studied “silvery micaschists”, which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U–Th bearing accessory minerals: florencite->monazite->allanite. Thermobarometric calculations indicate that the Mg-chlorite+talc+chloritoid+allanite peak assemblage was stable at P=1.9–2.7 GPa and T=515– 600 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7±1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4 ±0.9 Ma, implying a minimum duration of ~2–4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ~1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges

REE mineralisation within the Ditrău Alkaline Complex, Romania: Interplay of magmatic and hydrothermal processes

The Ditrău Igneous Complex (north-east Romania) is a tilted Mesozoic alkaline intrusion (~19 km diameter), with enrichments in rare earth elements (REE), niobium, and molybdenum. It has the potential to contribute to a secure and sustainable European REE mining industry, ensuring supply security for these critical metals. The complex comprises a sequence of ultramafic rocks, alkali gabbros, diorites, syenites, nepheline syenites and alkali granites. These units have been significantly modified by sub-solidus interaction with late-stage magmatic fluids and are cut by secondary mafic dykes. The complex was subsequently cut by REE-mineralised carbonate-rich veins. Geochemical and petrological data, including apatite mineral chemistry, from the alkaline igneous rocks, dykes and veins within the Ditrău Complex, have been used to assess the interplay of magmatic processes with late-stage magmatic and hydrothermal fluids, and the effects of these processes on element remobilisation and concentration of critical metals. Only limited critical metal enrichment was achieved by magmatic processes; the REE were preferentially incorporated into titanite and apatite in ultramafic cumulates during primary crystallisation, and were not enriched in evolved magmas. A hydrothermal system developed within the Ditrău Complex magma chamber during the later stages of magmatic crystallisation, causing localised alteration of nepheline syenites by a sodium-rich fluid. Mafic dykes subsequently acted as conduits for late stage, buoyant potassic fluids, which leached REE and HFSE from the surrounding syenitic rocks. These fluids percolated up and accumulated in the roof zone, causing the breakdown of nepheline to K-rich pseudomorphs and the precipitation of hydrothermal minerals such as zircon and pyrochlore within veins. REE mineralisation within the Ditrău Complex is hosted in the latest hydrothermal phase, mineralised carbonate-rich veins, which cross-cut the complex. Monazite is the main REE-bearing phase, it crystallised from a late REE- and carbonate-rich fluid with pH controlled REE deposition.NERC funded Warwickshire Geological Conservation Group: Holloway Award British Geological Survey: BUFI funding EURARE project, funded by the European Community’s Seventh Framework Programme under grant agreement no. 30937