Mineral Chemistry of REE-Rich Apatite and Sulfur-Rich Monazite from the Mushgai Khudag , Alkaline Volcanic-Plutonic Complex , South Mongolia

The Mushgai khudag volcanic-plutonic complex consists of four REE mineralization zones: carbonatite zone, apatite zone, magnetite zone, and monazite zone. REE mineralization occurs within peripheries of alkaline magmatic rocks which consist of porphyritic syenite, microsyenite and quartz syenites. Three types of LREE-rich apatite can be found in the carbonatite, apatite, and monazite zones. Crystal-1 type of apatite exists as hexagonal prismatic shape and is mostly found in the apatite zone, and in syenite. Crystal-2 type of apatite can be exposed also at the apatite zone, and carbonatite zone as brecciated massive crystalline aggregate. Crystal-3 type of apatite demonstrates the compositional zoning texture with monazite as inter-zoning, and is only found in monazite zone. The LREE-bearing apatites from the Mushgai khudag complex are mostly fluorapatite to hydroxyl-bearing fluorapatite with variable REE content. Apatites from the monazite zone present individual sulfur-rich monazite grain, and are formed by comprehensive substitutions.


Introduction
The Mushgai khudag alkaline volcanic-plutonic complex with REE-Sr-Fe-F-P mineralization is located in South-Gobi, Mongolia.Russian and Mongolian Joint Geological Expedition discovered the Mushgai khudag complex in early 1970's.Since that, number of new studies about alkaline rocks, carbonatites, and REE mineralization the deposit has been completed [1]- [4].
Geological and geochemical surveys demonstrated a wide occurrence of ore-bearing rocks containing rare earth elements with economic grades, especially light lanthanides and also strontium and barium [5].The alkaline complex is composed of various types of LREE enriched rocks significantly apatite rocks and monazite-rich apatite rock.LREE-rich apatites contain up to 14 % LREE 2 O 3 , while monazite up to 68% LREE 2 O 3 (LREE 2 O 3 represent sum of La 2 O 3 , Ce 2 O 3 , Nd 2 O 3 , and Pr 2 O 3 ).
In this study, we discuss the chemistry of LREE-rich apatite crystals from the Mushgai khudag complex.Apatite from the Monazite zone, which has unusual crystal texture with high LREE composition, is main contribution of the study because such type of apatite with individual monazite is rarely found in naturally.Three types of apatite crystals have been examined.Crystal-1 is euhedral and sometimes associated with celestine grains, and crystal-2 is subhedral and brecciated, whereas crystal-3 has an unusual idiomorphic crystal that demonstrates compositional zoning texture and associated with sulfur-rich monazite grains.

Geological Background
The Mushgai khudag alkaline complex is associated with late Jurassic alkaline magmatic activity close to the Main Mongolian lineament along where Mesozoic rift structures have been developed.The Mushgai khudag alkaline complex is hosted in the Silurian-Devoniun Mandal-oboo Formation's turbidite sediments, lower Devoniun Orgol Formation's carbonate sediments, and mid Devoniun Bot-uuli Formation's volcanic sediments [10] (Figure 1). Figure 1.Geological map of the Mushgai khudag deposit (modified from exploration report, "Mongol gazar" LLC, 2010).
The carbonatite zone in central part of alkaline complex has approximately 1 km length and 500 meter width.The largest mineralized zone is composed of low-grade REE carbonatite rocks (total LREE 1.64 -2.11 wt.%).The Magnetite zone is composed of massive magnetite rocks, and low-grade LREE-bearing apatite veins occur in massive magnetite rocks's cracks (LREE up to 0.11 wt.%).The Apatite zone is composed of various type apatite rocks with high-grade LREE (5 -10 wt.%).Monazite zone composed of high-grade apatite rocks that contain LREE up to 10 wt.%.The apatite rocks from monazite zone demonstrate zoning apatite with individual sulfur-rich monazite grain (Figure 1).

Monazite
The monazite at Carbonatite and Monazite zone occur as fine particles.Sometimes very fine-grained monazite particles mostly associated with celestine at Carbonatite zone (Figure 3(e), Figure 3(f)).
The fine-grained subhedral monazite grains, associated with apatite, mostly occur as a rim of apatite crystals at Monazite zone (Figure 3(c), Figure 3(d)).Representative monazite compositions from the REE ores are given in Table 2.The monazite associated with fluorapatite or hydroxyl-bearing fluorapatite.The apatites show idiomorphic crystal grains, and compositional zoning texture.They have commonly LREE-poor core2 (dark area), LREE-rich core1 (moderate), rim1 (light areas).The apatites in the syenites, apatite zone and carbonatite zone have a high content of phosphate and calcium with depletion in Si, Na, and LREE, whereas apatite from monazite zone has a low content of P and Ca with increasing of Si, Na, and LREE (Figure 4).
The substitution of Si and/or Na from the apatite structure without coupled removal of (Y + REE) 3+ the resulting charge imbalance, due to the coupled substitution reactions in apatite from Monazite zone namely: ( ) Si Y REE P Ca Na Y REE 2Ca As pointed by Harlov (2013) and Pan (1997), the substitutions encouraged the removal of Ca 2+ and the subsequent growth of monazite and/or xenotime inclusions within the apatite grain body via dissolution-precipitation from the P 5+ available in the immediate vicinity of the growing crystal and (Y + REE) 3+ transported in from over a much broader area [4] [6] [13].
The intensive coupled substitutions with (Y + REE) 3+ contributed the nucleation of the individual sulfur-rich monazite between the rim1 and the rim 2 area in the apatite from Monazite zone (Figure 3(d)).
The monazite in the Mushgai khudag complex contains high S in nature (1.5 -14.72 wt.% SO 3 ), and enriched in Sr and Ca (up to 7.21 and ≤8.25 wt.% oxides, respectively).Our data show S and P, S, and LREE moderate negative trend (Figure 5(a), Figure 5(c)), whereas S and Ca 2+ , Sr 2+ divalent cations represent weak positive pattern (Figure 5(b)), implying that Ce 3+ + P 5+ = Ca 2+ (Sr 2+ ) + S 6+ scheme of substitution should be a major mechanism of incorporation of Ca, Sr and S in the composition of monazite from the Mushgai khudag complex, South Mongolia.

Discussion
The mobility of the rare earth elements in hydrothermal fluid is strongly depend on the relative concentrations of possible ligands, such as F − , Cl − , OH − , SO − , and CO − , and the pH of the solution.Also, hydrothermal fluid's temperature and pressure are important [17].
It has been reported that ( REE ∑ = 200 ppb) concentrations, in acid waters at Yellowstone hydrothermal system (USA) [18].REE content tends to be increased in Low-pH waters than neutral or alkaline solutions.Low-pH, sulfate-rich fluids with the highest REE concentrations in hydrothermal fluids have been found at the Valles-caldera ( REE ∑ = 339 ppb), [19].Moreover, Saline magmatic solutions with high-temperature can effective carry REE.Also, the solutions can transport a large amount of REE (1290 ppm) in the ancient hydrothermal system [20].Wood's (1990) experimental data and theoretical predictions suggest that the sulfate ions form relatively strong complexes with REE.At the temperatures ≥200˚C and in the absence of other ligands, sulfate complexes will be the dominant form of REE transport, even at free sulfate environment [21].
Nevertheless, Brookins (1989) noted that the REE with sulfate complexes is considerably important only in the absence of other potentially strong ligands.If strong complex-forming ligands such as fluoride, carbonate, or possibly the phosphate were present, the suggested sulfate complexes with REE would probably not be important because of the strong affinity of the hard REE ions for hard ligands such as F − , OH − , and 3 4 PO − , especially at higher temperatures [22].It has been suggested that the REE cations are classified as hard ions, as ions with a small size and high charge.Hard cations bind preferentially with hard ligands [23].In this study, we suggest sulfate ligand, which encouraged the rare earth elements mobility, based on the chemical characteristics of monazite-bearing apatite in the Mushgai khudag complex.It is no doubt that the hard ligand phosphate is abundant with a high alkaline condition of the first stage of magmatism in the Mushgai khudag complex.However, the hard ligands fluoride and carbonate were dominant with the carbonatite magmatism.In case, the sulfate ligand complexed with REE in the Mushgai khudag complex, it could be the other ligands abundance would have been minor.In addition, the pH condition should be low.Thus, the sulfate ligand complexing with REE may have been related to the last stage of hydrothermal activity (metasomatism) in the Mushgai khudag complex.Our result show that the monazite in the Mushgai khudag complex.Our result show that the monazite contains CaSO4 (anhydrite), SrSO 4 (celestine) component.In other words, the monazite was formed by sulfate-rich fluid induced metasomatism.The S 6+ versus LREE 3+ diagram show the S 6+ moderate positive correlation with LREE 3+ .The S 6+ content is increased with LREE 3+ increase in the apatite from the Mushgai khudag complex.Especially the sulfur-rich monazite forming (core2), (core1), (rim1) areas are strongly enriched in S content suggesting that the sulfur-rich monazite in the Mushgai khudag complex strongly depends on the sulfate ligand (Figure 6).
In the Mushgai khudag deposit, LREE-bearing minerals are dominantly fluorapatite, hydroxyl-bearing fluorapatite, and monazite.Fluorapatite and hydroxyl-bearing fluorapatite can be found in all mineralization zones, whereas monazite distributed limited area.S and Sr-rich individual monazite grains have been found in the rim of fluorapatite and hydroxyl-bearing fluorapatite in the samples from the Monazite zone.It can be explained that the Mushgai khudag rocks were affected by strong REE mineralization due to various stages of magmatic and hydrothermal segregations.REEs have a strong mobility in the last stage of metasomatism due to sulfate-rich fluid complexed with REE.Apatite from monazite zone shows diverse zoning texture, and individual high S content monazite grains are formed along the rim of apatite crystals, and is well described in the present study.

Table 2 .
Representative chemical composition of monaztite from the Mushgai khudag complex.
Note: Cations in crystal-chemical formulae normalized to 4 oxygen atoms.