Abstract
Granites are assumed to be the main source of heavy rare-earth elements (HREEs), which have important applications in modern society. However, the geochemical and petrographic characteristics of such granites need to be further constrained, especially as most granitic HREE deposits have undergone heavy weathering. The LC batholith comprises both fresh granite and ion-adsorption-type HREE deposits, and contains four main iRee (ion-adsorption-type REE) deposits: the Quannei (QN), Shangyun (SY), Mengwang (MW), and Menghai (MH) deposits, which provide an opportunity to elucidate these characteristics The four deposits exhibit light REE (LREE) enrichment, and the QN deposit is also enriched in HREEs. The QN and MH deposits were chosen for study of their petrology, mineralogy, geochemistry, and geochronology to improve our understanding of the formation of iRee deposits. The host rock of the QN and MH deposits is granite that includes REE accessory minerals, with monazite, xenotime, and allanite occurring as euhedral inclusions in feldspar and biotite, and thorite, fluorite(–Y), and REE fluorcarbonate occurring as anhedral filling in cavities in quartz and feldspar. Zircon U–Pb dating analysis of the QN (217.8 ± 1.7 Ma, MSWD = 1.06; and 220.3 ± 1.2 Ma, MSWD = 0.71) and MH (232.2 ± 1.7 Ma, MSWD = 0.58) granites indicates they formed in Late Triassic, with this being the upper limit of the REE-mineral formation age. The host rock of the QN and MH iRee deposits is similar to most LC granites, with high A/CNK ratios (>1.1) and strongly peraluminous characteristics similar to S-type granites. The LC granites (including the QN and MH granites) have strongly fractionated REE patterns (LREE/HREE = 1.89–11.97), negative Eu anomalies (Eu/Eu* = 0.06–0.25), and are depleted in Nb, Zr, Hf, P, Ba, and Sr. They have high 87Sr/86Sr ratios (0.710194–0.751763) and low 143Nd/144Nd ratios (0.511709–0.511975), with initial Sr and Nd isotopic compositions of (87Sr/86Sr)i = 0.72057–0.72129 and εNd(220 Ma) = −9.57 to −9.75. Their initial Pb isotopic ratios are: 206Pb/204Pb = 18.988–19.711; 208Pb/204Pb = 39.713–40.216; and 207Pb/204Pb = 15.799–15.863. The Sr–Nd–Pb isotopic data and TDM2 ages suggest that the LC granitic magma had a predominantly crustal source. The REE minerals are important features of these deposits, with feldspars and micas altering to clay minerals containing Ree3+ (exchangeable REE), whose concentration is influenced by the intensity of weathering; the stronger the chemical weathering, the more REE minerals are dissolved. Secondary mineralization is also a decisive factor for Ree3+ enrichment. Stable geology within a narrow altitudinal range of 300–600 m enhances Ree3+ retention.
Highlights
Rare-earth elements (REEs) production is currently limited to a small number of large deposits (e.g., Bayan Obo, Baotou, China; Mountain Pass, San Bernardino County, CA, USA; Mount Weld, Laverton, Australia; Lovozero, Russia [2,3,4,5]), by-products, or deposits enriched with specific elements of current high demand, such as Dy, Tb, and other heavy REEs (HREEs, e.g., the so-called ion-adsorption-type REE deposits in weathered granite in southern China [6])
The present study focused on the geochemical characteristics of host granites in the LC area with a view to (i) determine the magma source of the host rock of Ion–adsorption-type REE (iRee) deposits; (ii) distinguish REE minerals in the host rock; (iii) elucidate the geodynamic mechanism of the host rock generation; and (iv) explain the REE mineralization process in weathering profiles
In granodiorite lc2-j2, accessory minerals are ilmenite, sphene, apatite, and zircon with minor monazite and xenotime; in the medium–coarse-grained biotite granites lc4-j1 (Figure 3d,e,g,h), allanite is associated with monazite, apatite, and zircon, fluorapatite contents are higher, and monazite, thorite, and zircon occur as anhedral crystals filling fissures (Figure 3b,c); in the medium–fine-grained biotite granite of REE mineral are present, such as allanite(–Y), fluorite(–Y), and REE fluorcarbonate (Figure 3f,i)
Summary
Rare-earth elements (REEs) are currently a focus of global attention because of geopolitical controls on their supply, which have led to them being included in current lists of critical metals. SW China, and has an altitudinal range of 1500–2000 m In this area, four comparable deposits have been identified recently in the LC granite: the QN, Shangyun (SY), Mengwang (MW), and Menghai (MH) iRee deposits. REEs occur isomorphously in the lattices of minerals such as feldspar, biotite, and apatite, or independently in minerals such as allanite, monazite, and xenotime [12] Dissolution of these minerals is the first stage of the formation of iRee deposits, with clay being the predominant secondary mineral in which REEs are accommodated [7,24,25,26]. The present study focused on the geochemical characteristics of host granites in the LC area with a view to (i) determine the magma source of the host rock of iRee deposits; (ii) distinguish REE minerals in the host rock; (iii) elucidate the geodynamic mechanism of the host rock generation; and (iv) explain the REE mineralization process in weathering profiles
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