Ulaan Del Zr-Nb-REE deposit , Lake Zone , Western Mongolia

The Ulaan Del deposit is located in the Lake Zone, Western Mongolia. In the area, middle-late Devonian alkali dykes of the Khalzan Complex are hosted in the middlelate Cambrian granodiorite-tonalite of the Togthohiinshil Complex. The alkali dykes of the Khalzan complex comprise mediumto fine-grained syenite, microsyenite, syenite-porphyry and trachyte, trachyrhyolite, and trachyandesite. The dykes are replaced to silica, sericite, albite, fluorite and are brecciated. They crosscut by quartz and quartz-carbonate veinlets. The dykes contain zircon (>0.19% Zr) with a total of rare earth elements oxides >0.1%. The host rocks of the Togtokhiinshil complex are mid-K, metaluminous, Itype granite, depleted in HFSE. Based on geochemical and mineralogical data, economic REE mineralization is concentrated in syenite and syenite porphyry of calc-alkaline high K to shoshonite series of Atype granite, emplaced at within a plate setting. Syenite dykes are enriched in REE. Ore minerals are zircon, apatite, sphene, monazite, xenotime, synchysite, parisite, fluorite and REE complex minerals, pyrite, rutile and limonite. Magmatic, metasomatic and hydrothermal processes significantly contributed to the formation of Zr, Nb, REE and Y mineralization at the Ulaan Del deposit.


INTRODUCTION
The Ulaan Del deposit is located southwest of Khyargas Lake within the Lake Island arc terrane (Lake Zone) of the Northern Mongolian domain (Fig. 1). The Lake terrane contains several intact ophiolites (Khantaishir, Bayannuur, Geriin Nuruu and Agardagh), calcalkaline basalt, andesite (522 ± 13 Ma, Sm-Nd; Kovalenko et al., 1995Kovalenko et al., , 1996, rhyolite, volcanoclastic rocks intruded by Late Cambrian -Ordovician syn-and post-tectonic diorite, granodiorite and granite (497 Ma, Kroener et al., 2007) covered by clastic sediments. Geological investigation of the region, including the Ulaan Del deposit area started in the early 1980's by 1:200000 scale geological mapping (Samozvantsev et al., 1982) and an airborne geophysical survey on the scale of 1:50000 (Gavrilov et al., 1987(Gavrilov et al., , 1989(Gavrilov et al., and 1990. In the early 1990's Minin et al. discovered anomaly A-657 (later named Shar Tolgoi) during an airborne geophysical survey on the scale of Original article Mongolian Geoscientist 1:50000 and explored it with a few drill holes and trenching (Fig. 2). From the exploration, the geological probable reserves were assessed at REE bearing ore of 19,900 tonnes of Ta 2 O 5 , 19,000 tonnes of Nb 2 O 5 and 1.4 Mt of ZrO 2 (Minin et al., 1991). Since 2009, geological prospecting and exploration works in the Ulaan Del deposit area (Fig. 2) were carried out by Geo-Info LLC, Mongolia, As a result, inferred and probable (C+P) reserves estimated 6.1 Mt of ore at 0.16% ∑TREO, 0.33% ZrO 2 , 0.05% Nb 2 O 5 and proved by the Mineral Resources Professional Council of Mongolia in 2018 (Oyunbat et al., 2018). The Ulaan Del deposit comprises swarm stockwork-like alkaline dykes with Zr-Nb-REE mineralization (Fig. 2). This study discusses geology, petrography and geochemistry of the mineralized alkaline dykes at the Ulaan Del and includes a mineralogical study of REE bearing minerals.  (Badarch et al., 2002) REGIONAL GEOLOGY OF THE LAKE ZONE The Lake Zone represents one of the main tectonic domains of the Central Asian Orogenic Belt (CAOB) (Badarch et al., 2002). A number of the arc-related magmatic complexes distributed in the Lake Zone, Western Mongolia, which are considered as a result of exclusively oceanic crust subduction (Badarch et al, 2002, Kovalenko et al., 2004, Rudnev et al. 2012. This magmatism recorded the whole history from Late Proterozoic to Early Paleozoic including the formation of oceanic crust, subduction processes and arc-related magmatic activity (Sojeono et al., 2016, Rudnev et al., 2009) and post-orogenic magmatism (Yashina, 1982 Alkaline dykes at the Ulaan Del deposit area extend over 300-500 m along the strike with variable widths (0.5-2 m). The dykes show diverse composition from andesite to granite and syenite with micro to porphyritic texture. The dykes sometimes brecciated along strike and small breccia pipes can found within the area (Fig. 3).
A geological map of the "Central" mineralized area of the Ulaan Del deposit shown as an example (Fig. 4)  crosscut with quartz-carbonate-iron oxide thin veinlets. Medium to coarse-grained granodiorite -tonalite, diorite and quartz monzonite of the Togtokhiinshil complex commonly distributed in the Ulaan Del area, represent the second phase of the Complex. REE mineralization associated with alkaline dykes of the Khalzan complex. The high-grade Zr-REE mineralization associated with potassium feldspar enriched or albititic rocks with iron oxide, fluorite-(carbonate) alteration. The main minerals of the REE are xenotime, zircon and parisite-synchysite, complex REEbearing minerals, monazite and apatite.

METHODS AND MATERIALS
More than 1000 rock samples collected, including channel and core samples. The sampling interval was about 25-40 m, but near the mineralized, more fractured and altered parts, the sampling interval narrowed. Samples designated with their drill hole identifier followed by the depth in meters. The mineralogical and chemical composition of the rocks, alteration, REE-bearing minerals and their spatial and temporal distributions where determined by various laboratory methods. About 65 selected samples from various dykes investigated for their texture and mineral composition. Petrographic and mineralogical microscopic studies carried out at the Geoscience Center, Mongolian University of Science and Technology. Totally 12 drill core and rock samples from metasomatically altered alkaline dykes and host quartz diorite selected for whole rock geochemistry. Major oxides of SiO 2 , TiO 2 , Al 2 O 3 , FeO, Fe 2 O 3 , CaO, MgO, Na 2 O, K 2 O, MnO, P 2 O 5 , CO 2 (in wt.%), and trace elements of Ba, Nb, Sr, Ta and Zr (in ppm) determined by using XRF and rare earth elements La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu (in ppm) analyzed by ICP-MS at the Central Geological Laboratory of Mongolia. Mineral identification using an X-ray diffraction (XRD) instrument has done at the laboratory of the Physics Department, Mongolian Academy of Sciences, and Tomsk University, Russia. The instruments operated at 40 kV voltage and a current 30 mA. The data measured in the 2σ range from five to 80 o step width of 0.02 o and measuring time 0.60 sec per step. The relationship of zircon and xenotime was determined using an electron probe microanalyzer (EPMA), with mineral liberation analysis (MLA) and alteration and REE minerals by energy-dispersive spectra (EDS) analysis and microscopic studies carried out in GTK, Finland.

Petrography
The Khalzan complex alkali dykes mostly represented by quartz syenite and microsyenite are fine to medium-grained pinkish, ranging from pale to reddish in color. Sometimes they are weakly brecciated and mainly replaced by albite, sericite-muscovite, carbonate, iron oxides and pyrite. The main rock forming minerals are plagioclase, K-feldspar, altered dark minerals and quartz. Phenocrysts are 0.1-0.8 mm, and groundmass is 0.05-0.6 mm in size (Fig. 5).
Plagioclase grains are 0.01x0.02 mm in size or small grains in interstitial spaces, and it associated with muscovite and sericite in volcanoclastic rocks. Plagioclase is mostly fresh and zoned, sometimes weakly replaced by secondary sericite and brecciated.
K-feldspar grains are 0.1-0.8 mm, sometimes up to 5 mm in size. Perthitic feldspar grains are 1-2 mm and are forming plate-like crystals. Twinning is very common with albite microgrowths ( Fig. 6). Albite is fresh, prism-like, with muscovite micro-flakes and contains apatite, zircon and fluorite as inclusion. One of the distinguishable features is the abundance of highly ordered microcline and albite established by XRD analysis and verified by optical microscopy. The highly ordered microcline forms the main part of perthitic alkali feldspar grains in trachytic or alkaline syenite (Fig. 7).
Goethite is pale gray, zoned and replaced with hydrogoethite from the edge and enveloped by muscovite flakes (Fig. 8).
Nests and lenses of iron-oxidized carbonates developed evenly within the dykes and host rutile, fine grains of zircon and apatite. Apatite grains as fine, prism like, irregular, isometric are observed as aggregates along microfractures, mostly along the edge of potassium feldspar Accessory minerals. Sphene grains have prolonged shape, 0.01-0.06 mm in size, show dark brown color and occur very often. Zircon (0.03-0.05 mm) dipyramidal, occurs as nests and micrograins. Monazite is (0.01-0.1 mm) pale, rectangle or plate like grains with a dark frame. Apatite is (0.02-0.125 mm) clear, forming hexagonal, short prism like grains. Synchysite is (0.02-0.06 mm) clear, pale to creamy brown, thick and short prism like grains and associated with dark framed grains or ore minerals. Fluorite (0.5 mm) is sometimes clear with a purple tint, filling the veinlets, spaces and fractures in the rocks. Typical fluorite crystals and xenomorth grains are associated with zircon, synchysite and xenotime (Fig. 9).

Geochemistry
Whole rock geochemical data are in Table 1, and REE data are summarized in Table 2. The alkali dykes are fine-to medium grained, often porphyritic, weakly brecciated and affected by metasomatic alteration followed by mineralization. In the TAS diagram (Fig. 10) the Khalzan Complex rocks range from sublalkaline  gabbro to monzonite and syenite, whereas the Togtokhiinshil Complex represented by diorite to granodiorite (Fig. 10). The Khalzan Complex rocks are high-K to shoshonite series (Fig. 11), with high Al 2 O 3 ( Table 1) and ASI of <1.1, whilst the Togtokhiinshil Complex rocks plot in the low to medium-К field (Fig. 11) Fig. 12).

Mineralogy
Detailed EDS, EPMA and MLA analyses carried out to investigate the alteration and REE bearing minerals. Selected parts of the altered rocks and accessory minerals were analyzed for their chemical composition by the REE spectrum distribution method. K and Na, silicified and iron oxidized (Fig. 13). Yttrium detected in different grains together with phosphorus spectra that indicate xenotime. Zircon is associated with xenotime and it is edging or framing xenotime in the same grains (Fig. 14). The REE distribution patterns showed the Ce, La, F, Y, P and Zr spectra. The La-Ce-F spectra are in the same grains that may indicate the composition of synchysite and parisite (Fig.  15). REE distribution patterns are a responsive indicator for the formation of magmatic and metasomatic rocks.

The genesis of REE-bearing rocks in the Ulaan Del area
The results of the petrographical, geochemical, and mineralogical investigations show that the REE mineralization bearing rocks of the Ulaan Del deposit is metasomatically altered syenite and syenite porphyries of the Khalzan Complex. There is a tendency that REE-bearing quartzalbite metasomatism was generated "selectively". It means that metasomatic alteration has not developed in all rock types in the area. Primarily, it was affecting only syenitic rocks in the outer contact zones. There are many hypotheses on syenite genesis like partial melting of the metasomatic altered mantle (Lynch et al., 1993;Platt, 1996), alkali and alkaline basaltic magma differentiation (Thorpe and Tindle, 1992;Civetta et al., 1998;Ronga et al., 2010), melting of floor basalts (Sisson et al., 2005;Bonin, 2007;Jahn et al., 2009), and magma mixing with crusts (Marks and Markl, 2001). The Togtokhiinshil Complex rocks are formed from depleted LIL and HFS sources. Therefore, it has no relation to the REE mineralization. However, it has noted that the Togtokhiinshil Complex includes an alkaline gabbro. Another proposition is that syenitic rocks may have generated from a primary enriched magma source and REE enrichment was due to subsequent metasomatism.

Origin of mineralization
The genesis of HFSE and REE mineralization associated with the peralkaline rocks is not well understood whether the mineralization is magmatic, hydrothermal, or a combination of the two (Dostal, 2016). The extensive fractional crystallization played a significant role for rare metal enrichment. However, the mineralization is typically associated with altered alkaline intrusions and the enrichment is related to hydrothermal activities. Volatiles played an important role in the evolution of highly evolved peralkaline rocks. The parent melts of the peralkaline rocks were generated under relatively dry and reduced conditions when these magmas undergo extensive fractionation accompanied by aqueous fluids in very late crystallization history (Dostal, 2016). At these advanced stages of fractionation, the aqueous fluids escape from a magma highly enriched in incompatible elements such as REE, HFSE, Th and U containing significant amounts of halogens, particularly fluorine (Dostal, 2016). Thus the hydrothermal fluids, which are enriched in all these elements, can play an important role in concentrating HFSE, REE, Th, and U as well as during autometasomatism. It appears that both magmatic and hydrothermal processes are likely to have contributed to the formation of the rare metal deposits of peralkaline rocks. The deposits related to layered silicaundersaturated syenite complexes display various indications of crystal accumulation associated with layers rich in REE-bearing minerals (Ilimaussaq Complex). On the other hand, in pegmatites like at Strange Lake, intrusions at Khalzan Buregtei, and felsic dikes at Bokan Mountain, there are no obvious indications of crystal accumulation. The primary magmatic mineralization is probably related to the crystallization of highly fractionated, latestage, fluorine-rich peralkaline magma in pegmatites, minor intrusions, and dikes. At the Bokan Mountain deposit the mineralization is associated with altered zones and composed of secondary minerals, which pseudomorphically replaced primary minerals. The secondary hydrothermal processes played an important role. All these deposits typically show imprints of hydrothermal processes that led to the remobilization of REE, HFSE, U, and Th and their redeposition in the nearby mineralized sites as secondary phases. The secondary processes not only redistributed but also enriched REE in the mineralized zones. During remobilization, LREE moved greater distances than HREE inferred that the remobilization took place at low temperatures (<350°C). The metasomatism at the Ulaan Del area expressed in albititization of syenite and microsyenite (trachyte, trachyrhyolite). The albititized rocks have elevated by REE mineralization.

Magmatic-metasomatic -hydrothermal processes played significant role in formation of REE mineralization at the Ulaan Del deposit
The Ulaan Del REE deposit is located within the Lake Zone, where the well-studied Khalzan Buregtei Nb-ZR-REE deposit is also located. The genesis and ore formation of both Ulaan Del and Khalzan Buregtei deposits may be similar, regarding their spatial and close locality and the Zr-REE mineralization type. According to the research works conducted at the Khalzan Buregtei deposit, enrichment in Zr, Hf, Nb, Ta, REE, Y-HFSE was related to both magmatic and hydrothermal processes (Kempe et al., 2014). But, the magmatic and hydrothermal processes developed during different and distinct events. Several distinct hydrothermal processes resulted in the formation of heterogeneous metasomatic rocks strongly enriched in HFSE. For example, a fluid, rich in carbonate and silica caused the development of Zr, Nb, and LREE rich ores in calcite-bearing alteration assemblages (Kempe et al., 1995(Kempe et al., , 1996(Kempe et al., , 1997(Kempe et al., , 1999(Kempe et al., and 2014. Late alteration by F-rich fluids resulted in some remobilization of HFSE and significant local enrichments of HREE and Y in the ores. The interplay of all processes resulted in the formation of a complex, economic Zr-Nb-REE mineralization at the Khalzan Buregtei (Kempe et al., 2014). Petrographical, mineralogical and geochemical data and a comparison with the Khalzan Buregtei deposit suggested that Zr-Nb-REE mineralization of the Ulaan Del deposit may have been related to magmatic-metasomatichydrothermal processes during multiple events of alkaline magmatism.

CONCLUSION
REE-bearing dykes at the Ulaan Del deposit are silicified and albititized syenite and microsyenite (trachyte, trachyrhyolite). Geochemically the dykes of the Khalzan Complex are presenting calc-alkaline high-K to shoshonite series, with ASI 0.6-1.1, related to A -type rocks formed within plate setting. Syenitic rocks are REE enriched and contain HREE and LREE mineralization. REE ore bearing minerals at the Ulaan Del deposit are zircon, apatite, sphene, monazite, xenotime, synchysite, parisite, fluorite, REE complex minerals and also pyrite, rutile and limonite. REE grades are not even but with constant >0.19% Zr and REE >0.1%.