Geochemistry and origin of a new-type of Zr-Nb-Y-REE deposit in highly-evolved alkali granite, Chungju area, South Korea

<p>In the Chungju area, Korea, highly enriched Zr-Nb-Y-REE deposits occur in subhorizontal layered Paleozoic granitic rocks (331±1.5 Ma), which can be divided into layered alkali granite, alkali aplite, and pegmatite. The rocks are mainly composed of alkali feldspar, quartz, and microcrystalline zircon. The ubiquitous zircon is the distinctive feature of the alkaline rocks, which plot with within plate granite, anorogenic granite, and ultrapotassic rocks, and show very similar REE patterns. Alkali aplite has especially high total rare earth elements and negative europium anomalies compared to the layered alkali granite. The Zr-Nb-Y-REE mineralization occurs as zircon-magnetite bands that are associated with several REE minerals. Repeated graded textures of layered alkali granites with interlayered Zr-Nb-Y-REE mineralization can be explained by gravity accumulation in the late magmatic stage. The compositions of zircons plot between the late magmatic and hydrothermal fields. The REE patterns of zircon-rich mineralization shows slightly negative slopes, whereas zircons show positive slopes. This can be explained by the HREE being strongly partitioned into zircon grains from the melt. Zircons with low total REE contents show high positive Ce anomalies. Although zircon analyses were conducted on one sample from a small area, it shows variable Ce anomalies and TREE, which indicates the zircons crystallized under conditions of rapidly changing oxygen fugacity, as the REE contents of zircon are related to the oxygen fugacity of the melt. The limited Th/U ratios of zircons indicate that they crystallized during a simple magmatic event, and were not affected by hydrothermal alteration and metamorphism. Here we suggest a flotation, aggregation, and gravity accumulation model can explain settle down of microcrystalline zircon and magnetite grains in fluid rich alkaline melt. This is the first report on highly evolved alkali granite that associated with Zr-Nb-Y-REE mineralization. The features displayed in these deposits have important implications for the evolution of alkali magmas.</p>

Britholite Group Minerals from REE-Rich Lithologies of Keivy Alkali Granite—Nepheline Syenite Complex, Kola Peninsula, NW Russia

The Keivy alkali granite-nepheline syenite complex, Kola Peninsula, NW Russia, contains numerous associated Zr-REE-Y-Nb occurrences and deposits, formed by a complex sequence of magmatic, late-magmatic, and post-magmatic (including pegmatitic, hydrothermal, and metasomatic) processes. The REE-rich lithologies have abundant (some of economic importance) and diverse britholite group minerals. The REE and actinides distribution in host rocks indicates that the emanating fluids were alkaline, with significant amounts of F and CO2. From chemical studies (REE and F variations) of the britholites the possible fluid compositions in different lithologies are proposed. Fluorbritholite-(Y) and britholite-(Y) from products of alkali granite (mineralized granite, pegmatite, quartzolite) formed under relatively high F activity in fluids with low CO2/H2O ratio. The highest F and moderate CO2 contents are characteristic of fluid from a mineralized nepheline syenite, resulting in crystallization of fluorbritholite-(Ce). Britholite group minerals (mainly fluorcalciobritholite and ‘calciobritholite’) from a nepheline syenite pegmatite formed from a fluid with composition changing from low F and high CO2 to moderate F and CO2. An extremely high F content is revealed for metasomatizing fluids emanating from alkali granitic magma and which affected the basic country rocks. The dominant substitution scheme for Keivy britholites is REE3+ + Si4+ = Ca2+ + P5+, showing the full range of ‘britholite’ and ‘calciobritholite’ compositions up to theoretical apatite.

Bands of Zircon, Allanite and Magnetite in Paleozoic Alkali Granite in the Chungju Unit, South Korea, and Origin of REE Mineralizations

High-grade Zr–Nb–Y–rare earth element (REE) mineralization occurs as zircon–allanite–magnetite bands in layered Paleozoic alkali rocks which intruded the Gyemyeongsan Formation of the Chungju unit, South Korea. The mineralization period and genesis have been controversial. We investigated the petrological and mineralogical properties of the newly discovered zircon–allanite–magnetite bands and the geochronological properties of zircon within the bands in the alkali granite. We analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The repeated quartz–feldspar-rich layers in the alkali granite show grain-sized grading textures and equilibrium igneous textures. Magnetite and allanite grains in these layers varied in size and exhibited isolated, aggregated, and coalesced textures. In addition, the settling texture of zircon grains onto the other minerals was observed. These observations could reasonably be explained by the process of gravitational accumulation during the solidification of magma. The 206Pb/238U ages obtained from zircon from the zircon–allanite–magnetite-rich layer and the alkali aplite were 331.1 ± 1.5 Ma and 334.5 ± 8.9 Ma, respectively. Therefore, we suggest that the Zr–Y–Nb–REE mineralization developed in the alkali rocks and the Gyemyeongsan Formation in the Chungju unit were formed by fractional crystallization of alkali magma and hydrothermal fluids which evolved from alkali magma fractional crystallization, respectively. The correlation between alkaline granite and REE mineralization found in this study could be used as a tool for REE exploration in other regions where the permeable geological unit is intruded by the alkali granite.

Geochemistry, Zircon U–Pb Geochronology, and Lu–Hf Isotopes of the Chishan Alkaline Complex, Western Shandong, China

Mass alkaline magmatic activities in Western Shandong during the late Mesozoic controlled the mineralization processes of gold and rare earth element (REE) polymetallic deposits in the region. The Chishan alkaline complex is closely associated with the mineralization of the Chishan REE deposit, which, as the third largest light REE deposit in China following the Baiyenebo (Inner Mongolia) and Mianning (Sichuan) deposits, is considered a typical example of alkaline rock mineralization throughout the North China Craton. To determine how the Chishan alkaline complex and REE deposit interact with each other, a systematic study was conducted on the petrology, rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes of the quartz syenite, and alkali granite contained in the Chishan alkaline complex. The results reveal that the deposits feature similar geochemical characteristics typical of an alkaline rock series—both are rich in alkali, high in potassium, metaluminous, and poor in Ti, Fe, Mg, and Mn. In terms of REEs, the deposits are strongly rich in light REEs but poor in heavy REEs, with weak negative Eu anomalies. In terms of trace elements, they are rich in large ion lithophile elements Ba, Sr, and Rb but poor in high field-strength elements Nb, Ta, and Hf. Zircon LA-ICP-MS U–Pb dating indicated that the quartz syenite and alkali granite formed in Early Cretaceous at 125.8 ± 1.2 Ma and 127.3 ± 1.0 Ma, respectively; their εHf(t) values are −22.67 to −13.19, with depleted model ages (TDM) ranging from 1296 Ma to 1675 Ma and crustal model ages (TDMC) of 2036–2617 Ma. The Chishan alkaline complex originated from partial of the EM I-type (enriched mantle I) lithospheric mantle with assimilation of ancient crustal materials. The complex is of the same origin as the REE deposit, and developed in an extensional setting that resulted from plate subduction and lithospheric thinning and upwelling in the eastern area of the North China Craton.