PETROCHEMICAL CHARACTERISTICS OF LATE PALEOZOIC MAGMATIC ROCKS OF THE MANDAKH AREA, SOUTHEAST MONGOLIA

The late Paleozoic magmatic rocks are widely distributed in the Mandakh area which is located in the Gurvansaikhan and Manlai terrains, where porphyry Cu deposits occur. In this paper we discuss petrochemical features and mineral assemblages of magmatic rocks in the Mandakh area. Furthermore, we compared petrochemical characteristics of magmatic rocks in the Mandakh area with host magmatic rocks of the Tampakan deposit (Philippines), Cerro Colorado deposit (Chili) and negative criteria of Cu deposits (Japan) due to try to characterize potential of the porphyry copper deposit related to magmatic rocks in Mandakh area. Geochemical features of magmatic rocks in Mandakh area are calc-alkaline, magnetite-series, I-type and similar to adakite type. The Devonian intrusive rocks comprised of syenite and syenogranite, while the Carboniferous intrusive rocks consist of granodiorite, monzodiorite, quartz-monzonite and hornblende granite. Devonian magmatic rocks are more alkaline in composition. Although, Devonian and Carboniferous magmatic rocks are slightly different from each other. Comparing with bonanza copper deposits in the world, they are possible to host porphyry mineralization.


INTRODUCTION
Mongolia lies within the Central Asian Orogenic Belt (CAOB) which is enormous and composed of a multiplicity of terranes including ancient island and continental arcs, ophiolites, passive continental margins, Precambrian continental blocks and high-T/low-P metamorphic zones. Its largest continental crust development stage occurred during the Phanerozoic (Badarch et al., 2002;Windley et al., 2007;Yarmolyuk et al., 2008;Mihalasky et al., 2015). South Mongolia has become a major survey area for unraveling the Paleozoic tectonic evolution (Fig. 1). Badarch et al. (2002) describes the Gurvansaikhan island arc terrane (Fig. 2a) which lies in the central part of the southern domain in Mongolia. This island arc terrane is predominantly composed of Devonian to Carboniferous island arc volcanic rocks, but also includes sporadic Ordovician and Silurian volcanics, as well as Ordovician to Carboniferous sedimentary rocks, and are extensively intruded by voluminous Permian-

Mongolian Geoscientist
Carboniferous granitoids in the south (Badarch et al., 2002). Geological setting, magmatism and mineralization of the South Mongolia arerelatively well studied, but there is considerable debate regarding thePaleozoic-Mesozoic tectonic evolution, especially in the Mandakh area (Yakubchuk, 2002;Blight et al., 2010a,b). Previous researchers (Blight et al., 2010a) have not agreed about the age of some intrusive bodies, and the tectonic setting. For instance, the Budar pluton and the Nariin Khudag monzonite bodies (Blight et al., 2010a) mapped as the same intrusive bodies but have been assigned different geological age (Fig. 2b). On the other hand, important mineral deposits have been discovered in the South Mongolia during the last decade and consequently, the Southeast Gobi is a major mineral exploration province (Yakubchuk, 2002;Seltmann and Porter, 2005;Porter, 2016). The Kharmagtai-Khongoot-Oyut and the Tsagaan Suvarga Cu-Mo porphyry deposits were dated as Middle Carboniferous to Early Permian, and Late-Devonian to Early Carboniferous respectively (Rodionov et al., 2003). Devonian to Carboniferous volcanic and volcanogenic sedimentary rocks of Unduruud (D 1-2 uu), Alagbayan (D 2-3 ab), Ikhshankh (C 1 is), Gunbayan (C 1 gb), Tsokhiot (C 1 ch), Dushiinovoo (C 2 do) and Sainshandkhudag (C 2 ss) Formations widespread in the Mandakh area, which are all differ in lithological characteristics. Also, Devonian to Carboniferous granitic plutons such as Tsagaan Suvarga (D 1 c), Bronze Fox (C 2 ), Mandakh (C 2 m), Mogoit (C 2 mo) and Shuteen (C 2 ) are classified in the area. Detailed research works have been done for the Nariin Khudag, Bronze Fox, Mandakh plutons (Blight et al., 2010a, Enkhjargal et al., 2016 and Shuteen complex (Batkhishig and Iizumi, 2001;Batkhishig et al., 2010), besides other granitic plutons and Formations are not studied well.
Our study is focusing on petrochemistry of Devonian and Carboniferous magmatic rocks which are widely distributed in the Mandakh area. Moreover, we try to characterize potential of the porphyry copper mineralization in the study area, comparing geochemical characteristics of magmatic rocks to the Tampakan deposit (Philippines) and Cerro Colorado deposit (Chili) areas. They are gigantic representative porphyry deposits in the world.  Lower Carboniferous Ikhshankh Formation (C 1 is) consists of marine deposited tuff, conglomerate, sandstone, siltstone, and limestone,with a total thickness of about 2100-2700 m. The Formation can be divided into upper and lower members, and fossils of fauna and flora are locally observed. Hovan et al. (1984) reported a Lower Carboniferous (Tournasian-Visean) age for the Ikhshankh Formation, based on brachiopod fossils. The Tsokhiot (C 1 ch) Formation occurred in west southern part in Mandakh area (Fig. 2). This Formation consist of andesite, basaltic andesite and basalt. The Dushiinovoo Formation (C 2 do) outcrop stretches from the northeast to southeast, and intruded by the Mandakh, Shuteen, Budar and Nariin Khudag intrusions (Fig.2). It consists of mostly andesite, andesite porphyry and associated pyroclastics with a total thickness of about 3000 m. In some places, dacite and basalt are also occurred. Pennsylvanian age was confirmed by fossils, and Rb-Sr whole rock and mineral ages yielded 336±24 Ma, 315.85±0.02 Ma and 319±0.03 Ma (Batkhishig et al., 2010(Batkhishig et al., , 2016. The Sainshandkhudag Formation (C 1 ss) occurs in the southeastern part of the Mandakh area (Fig. 2). Volcanic rocks of the Formation comprise of basalt to dacite in composition and are associated with pyroclastic rocks. It has contacted to Devonian Tsagaan Suvarga intrusive and Carboniferous Tsagaansuvarga Formation by north-east trending faults. Intrusions: Devonian Tsagaan Suvarga pluton occurs in the southeastern part of the Mandakh area. The pluton is relatively well studied by previous researchers, because of hosting Cu-Mo porphyry deposit. The Tsagaan Suvarga pluton is covered by Carboniferous Tsagaansuvarga and Sainshandkhudag Formations in northwestern and south-eastern parts. Pluton is composed of monzonite to granodiorite, metaluminous with ASI values ranging from 0.97 to 1.14, I-type and the Rb-Sr and Sm-Nd isochron indicates a 338±14 Ma and 364 Ma ages respectively (Tungalag et al., 2014). The Carboniferous magmatism is represented by Mandakh complex which includes several plutons such as the NariinKhudag, Bronze Fox and Shuteen. The Shuteen pluton, which yields a Rb-Sr whole -rock age of 321.5±9 Ma (Iizumi and Batkhishig, 2000), and is comprised of monzonite, monzodiorite, diorite, hornblendebiotite granodiorite, granite, hornblende granite, granodiorite, and syenite, and porphyritic stocks and dikes (Batkhishig et al., 2010). The Bronze Fox pluton consists of gabbrodiorite to granite series intruded into the Ikhshankh Formation. Blight et al., (2010b) identified U-Pb age of 333.2±0.6 Ma for the biotite-hornblende granodiorite. The Mogoit pluton is occurred as an isometric body in southeast part of the study area and intruded into the Sainshandkhudag Formation. The Mogoit pluton consists of biotite granite, granodiorite, monzonite and diorite. Fourth,the powder from the rocks filled a PVC ring on the dies, which was pressed by apiston cylinder 3124-00. In addition, we used ethanol and MilliQ for cleaning between each sample and following stages of preparation for X-ray fluorescence. We have analyzed with calibrated using standard samples provided by Geological Survey of Japan (JA-3). Thirty-six igneous rock and thirty volcanic rock samples were analyzed using standard XRF techniques at Graduate School of Environmental Studies in Tohoku University, Japan (Yamasaki, et al., 2002).

Petrography of volcanogenic Formations in the Mandakh area
Total 56 representative samples from the Unduruud (3), Ikhshankh (3), Tsokhiot (4), Dushiinovoo (16) and Sainshandkhudag (28) are studied for petrography and the results are summarized in Table 1. Porphyritic texture is common in all volcanogenic rocks, and phenocrysts are largely replaced by chlorite, sericite and clay minerals. Samples of the Unduruud and Dushiinovoo Formations contain more opaque minerals than the samples of other formations. comprised of granodiorite and granite with porphyritic medium to course-grained texture. These rocks consist mostly of euhedral plagioclase (Pl) and K-feldspars (Kfs) which altered to sericite and clay minerals. The rocks of the Mandakh, Mogoit and the Bronze Fox plutons are similar to each other in terms of petrography. Those have porphyritic texture, mineral composition (abundant plagioclase, K -feldspar, quartz (Q), hornblende, biotite (Bt), rare clinopyroxene (Cpx)), and fine to medium grained quartz in the groundmass. However, the Mandakh pluton rocks have more alkalic mineral composition than rocks of the Mogoit and the Bronze Fox plutons. The Tsagaan Suvarga pluton comprises of mainly K-feldspar, which largely altered to sericite and clay minerals, and a few calcite minerals. All rocks of the Bronze Fox, the Mogoit, the Mandakh and the Tsagaan Suvarga plutons contain ore minerals, and the pyroxene is replaced by hornblende and biotite.

Geochemistry
Major and trace element composition of volcanic and plutonic rocks are shown in Table 3 and 4, respectively. All samples of volcanic and plutonic rocks are plotted in AFM diagram and are related to calc-alkaline series (Fig 3). Magmatic rocks of the Mandakh area show a wide range of compositions for both silica and alkalis. Rock composition is ranging from diorite to granodiorite and granite. Whereas rocks of the Devonian Tsagaan Suvarga pluton show more alkaline characteristics than Carboniferous rocks, they are plotted in quartz monzonite field (Fig. 4A). Volcanic rock show less evolved trend. Devonian Unduruud Formation rocks are more mafic (basalts), whereas Carboniferous field and the tholeiitic field after Kuno, 1968. Formations are ranging from mafic to felsic in composition (basalt-andesite-rhyolite). Permian Argalant Formation rocks are more alkaline, and plotted in trachyandesite and trachyte fields in terms of TAS diagram (Fig.  4B). All plutonic rocks from the Mandakh area are plotted in I-type granite field in SiO 2 vs Al/K+Na+Ca diagram (Fig. 5). Furthermore, the geochemical features of the Mandakh area are compared with plentiful of igneous rocks in giant porphyry copper deposits (Tampakan, Cerro Colorado) in order to characterize potential of porphyry copper mineralization in the Mandakh area (Fig. 5,8,9). As 11 3 55 4 3 1 2 1 1 2 15 6 8 8 Pb 16 6 16 25 10 6 6 6 3 3 7 10 5 10 Th 4 3 17 19 1 1 1 3 1 1 1 1 1 1 Table   3.        (Pearce and Cann, 1973) Samples of plutonic rocks in the Mandakh area show enriched values of LILE and depleted values of HFSE (Fig. 6). Positive anomalies of K, Rb, Ba, and negative anomalies of Nb, P, Ti and Y; which indicate subduction environment. Samples from Bronze Fox pluton show two different patterns in LILE, some are enriched in K, Rb and Ba, the others are depleted in K and Rb (Fig. 6). Andesitic rocks of the Dushiinovoo Formation are enriched in LILE, whereas dacite and rhyolites give some depletion on LILE, for instance K, Rb and Ba. The rocks of Shuteen pluton and Dushiinovoo Formations, as well as other magmatic rocks in the Mandakh area are distinguished as adakite-like rocks, by the previous researchers (Batkhishig et al., 2010;Blight, 2010a). Therefore, we also try to show how our new data plot in the Y vs Sr/Y diagram , comparing with data from the Cerro Colorado and Tampakan area. Samples show differentiation trend from ADR to adakitic rocks (Fig. 8). Plutonic rocks are all characterized as Volcanic Arc Granite in terms of tectonic discrimination diagram (Fig. 9).

Petrochemical differences of Devonian and Carboniferous rocks in the Mandakh area
Geochemical features of Devonian Tsagaan Suvarga pluton are slightly different than Carboniferous rocks. The Tsagaan Suvarga pluton rocks are calc-alkaline, I-type and alkali series (Na 2 O+K 2 O>9.8%), Sr 505-973 ppm, Y 7-9 ppm, Nb>5 ppm, Rb>87 ppm. Porphyry systems related to Devonian magmatic rocks in South Mongolia mostly host economic deposits such as Tsagaan Suvarga and OyuTolgoi. Carboniferous magmatic rocks distributed in the Mandakh area are characterized by calcalkaline series, I-type, silica saturated SiO 2 ≥50 %, high Al 2 O 3 ³12.9%, Na 2 O+K 2 O>5 %, contain Sr 300-800 ppm, Y ≤ 25 ppm, Nb≤12 ppm, Rb≤150 ppm which are indicating adakitic features and are formed within volcanic arc tectonic setting (Table 5). Carboniferous plutonic and volcanic rocks show co-magmatic features, formed during the Carboniferous subduction zone developed in South Mongolia. Carboniferous plutonic rocks are commonly host copper porphyry systems (Shuteen, Oyut-Ulaan and Kharmagtai), but economic deposits are not yet discovered.

Porphyry copper mineralization potential in Mandakh area related to Devonian and Carboniferous magmatic rocks
The Devonian and Carboniferous magmatic rocks have porphyritic texture and contain plagioclase, K-feldspar, quartz, hornblende, biotite, rare clinopyroxene phenocrysts. All rocks of the Bronze Fox, the Mogoit, the Mandakh (Carboniferous) and the Tsagaan Suvarga (Devonian) plutons contain ore minerals, and the pyroxene replaced to hornblende and biotite. Richards (2011) considered the presence of hydrous phenocryst phases and arc magmatic suites with high Sr/Y ratios are indeed prospective for porphyry Cu+/-Mo+/-Au deposits. The biotite within magmatic rock in this study area is greenish brown, implying the magnetite-series (Ishihara, 1998). These magmatic rocks were affected by hydrothermal alteration. However, samples of Tsagaan Suvarga pluton contain calcite, carbonate minerals and show pinkish red color, implying Devonian rocks are more alkalic than Carboniferous plutonic rocks. Furthermore, the porphyry copper deposits are closely related to subduction system. However, there are two types of subduction system depending on porphyry copper deposit in the world. One is rich in porphyry copper deposits

Plagioclase is ubiquitous
Plagioclase is common all the rocks and hornblende.

Amphibolite is common; a common assemblage is Pl&Amp
Pyroxenes are to Hornblende.
Plagioclase is common all of the rocks and also hornblende. Tectonic setting What is tectonic setting?

I-type
Volcanic arc granite Adakite like Adakite like Table 5. Carboniferous magmatic rocks of the Mandakh area compared to adakitic signature such as Chili, Indonesia and Philippines. Another is poor in porphyry copper deposits, which is Japan. Sillitoe (2018) mentioned some reasons why Japan has lack of porphyry copper deposits and further impossible geological factors for large and high-grade porphyry copper deposits. As he classified that lack porphyry copper deposits have formed in association with convergent plate margin settings by slab rollback and consequent crustal extension, volcano-plutonic complexes generated during caldera flare-ups, arc processing chemically reduced crustal profiles, ilmenite-series intrusion, arc having few and locally high Sr/Y igneous suites, fractionated intrusions of rhyolitic (granitic) composition, metallogenic belts or provinces dominated by rhyolite-associated VMS deposits or lowsulfidation epithermal Au deposits and steep subduction (Sillitoe, 2018). Based on comparison in Table 9, the geochemical features of the late Devonian and Carboniferous magmatism in Mandakh area, are considered much different from these negative criteria of the location of porphyry Cu deposits. In addition, the Devonina Tsagaansuvarga deposit (Cu-Mo) and the Carboniferous Shuteen mineralized area (Cu-Au) are indication possibility of porphyry copper deposits formation in the study area (Table 7).
Geochemical data of Devonian and Carboniferous igneous rocks of the Mandakh area compared with igneous rocks of giant porphyry copper deposit Tampakan and Cerro Colorado (Fig. 5, 8 and 9). The Tampakan deposit is a large copper and gold ore body located in the south of the Philippine which is classic example of the island arc (adakite) tectonic setting (Rohrlach, 2002;Cooke et al, 2005), the Cerro Colorado (Cu 0.81%, Mo 0.005%) occurs in the Chili that is good example of the continental margin setting (Tsang et al, 2018;Cooke et al, 2005). Geochemical features of Devonian to Carboniferous igneous rocks in the Mandakh area are close to rocks in the Tampakan deposit but differ from rocks in Cerro Colorado deposit (Fig.5, 8 and 9; Table 7). It means that Devonian to Carboniferous magmatic rocks in the Mandakh area somehow related to porphyry copper system and I-type adakite which has chemical characteristics of high Sr/Y ratios. Defant and Drummond (1990) identified a suite of rocks called "adakites" that are inferred to be the product of melting of the subducted slab, which generated by melting of eclogitic basaltic crust. Tungalag et al (2014) identified that whole-rock geochemical characteristics of quartz monzodiorite and quartz monzonite of the Porphyry Cu-Mo, Cu-Au deposits and occurrences are associated with typical calcalkaline metaluminous, oxidized with a fO 2 above the FMQ+2 (fayalite-magnetite-quartz buffer), I-type, magnetite series granitoids (Sillitoe, 2018;Hattori, 2018), which is dominated granitoid type in Mongolia. The petrochemical features of magmatic rocks of the Mandakh area are similar to above characteristics of magmatic rocks which contain potential porphyry mineralization.

CONCLUSION
The late Paleozoic magmatic rocks are broadly distributed in the Mandakh area located in the Gurvansaikhan and Manlai terrains with porphyry Cu deposits. We have investigated petrochemical features and mineral assemblages of Devonian and Carboniferous magmatic rocks in the Mandakh area and compared with magmatic rocks in giant porphyry copper deposits in the world. We summarized the following remarks: -Intrusive rocks in the Mandakh area are composed of syenite, granite, quartz monzonite and granodiorite, while volcanogenic rocks consist of dominant basalt, andesite, dacite and their tuffs. Carboniferous magmatic rocks from the Mandakh area are silica saturated (SiO 2 =50-73 %), have high Al 2 O 3 12.9-19.4%, Na 2 O+K 2 O 4-12%, Sr 300-800 ppm, Y£25 ppm, Nb 3-6 ppm, and Rb≤150 ppm, and are hydrothermally altered.
-Igneous rocks are calc-alkaline, magnetiteseries, I-type and are close to adakite type. All -Similarity of petrographical and geochemical characteristics of magmatic rocks in the Mandakh area to those magmatic rocks in giant porphyry deposit area in the world could indicate a good possibility for new discovery of giant porphyry copper deposit, like Tampakan which related to adakite type magmatism. It implies that magmatic rocks source in Mandakh area may be melting subduction slab.