Petrogenesis and geochronology of the bronze fox porphyry Cu-Au deposit: Implications for the geodynamic evolution of the Gurvansaykhan Island arc, southern Mongolia

ABSTRACT A diversity of Devonian-Carboniferous volcanic arcs and granitoid intrusions are exposed in the Gurvansaykhan arc terrane of southern Mongolia. Several giant porphyry Cu-Au-Mo deposits are associated with intrusions within these arcs. The Bronze Fox deposit located in the central part of the Gurvansaykhan arc terrane is an example of a potentially economic deposit with affinity of adakite-like granitoid porphyry (i.e. 428 Mt grading 0.26% to 0.30% Cu for up to 2,437 Mlb of copper and 0.84 Moz gold). The genesis and magmatic evolution of the causative rocks of the Bronze Fox porphyry Cu-Au hosting rocks are poorly understood, and different models are still under debate. In the current study, we present new field observations and petrographic, geochemical, and geochronological data to describe the magmatic evolution and geodynamic settings. In addition, the mineral chemical data are reported from some selected rock-forming minerals to identify crystallization conditions of the studied rock suites by employing geothermobarometric methods. Zircon chronological data reveal the emplaced age at 331 ± 3.4 Ma for the volcanic lava, followed by mineralized granodiorite and monzodiorite with intruded ages of 326.5 ± 3.4 Ma and 326.5 ± 2.7 Ma, respectively. The studied intrusive and extrusive rocks are high-K-alkaline and metaluminous, characterized by high Mg#. They are enriched in large ion lithophile elements (Sr and Ba) and light rare earth elements (LREEs) and depleted in high field strength elements (Nb, Ta, and Ti) and heavy rare earth elements (HREEs). The highly mineralized granodiorite intrusion yields zircon crystallization temperatures ranging from 631.8 to 779.4°C, and the monzodiorite intrusion, which contains relatively low-grade mineralization, has a wider range (599.0 to 803.4°C). The formation pressures of the former range from 0.5 to 0.2 kbar, which is equivalent to a depth of 1.38 km based on amphibole geochemical calculations. The results of mineral chemistry indicate that the unmineralized diorite unit was formed under medium pressure (~50 MPa) and high temperature (~750°C) at depths of 6.8 to 7.5 km. Similarities in geochemistry and petrographic features suggest that the igneous rocks probably originated from a similar magma source (i.e. from the melting of ancient oceanic crust and sediments) but experienced different degrees of partial melting or mixing with lower juvenile crust melts. The estimated fO2 values obtained by amphibole chemistry in the granodiorite and quartz diorite of the Bronze Fox copper-gold mineralization range from −13.0 to −11.8 and −13.2 to −10.8, respectively, confirming that these igneous rocks probably crystallized under relatively oxidized conditions above the nickel-nickel oxide (NNO) buffer. Such results yield an overall range between NNO +2.6 and NNO +1.4, which are characterized by high oxygen fugacities and high Ce4+/Ce3+ ratios that reflect oxidizing magma, thus indicating quite promising potential for porphyry Cu-Au mineralization in the region. Based on the findings of this work, we suggest a new tectonic model of arc magmatism that is a response to the subduction of the Paleo-Asian Ocean in the early Carboniferous period.


Introduction
Mongolia is situated in the southern margin of the Central Asian Orogenic Belt (CAOB) (Jahn et al. 2000;Badarch et al. 2002;Windley et al. 2007), the Altaid orogenic collage (Sengor and Natalin 1996) or the Central Asian Orogenic Supercollage (Yakubchuk 2004;Yakubchuk et al. 2005).It extends from Kazakhstan in the west to northeastern China in the east and is bounded by the Siberian Craton in the north and the Tarim and North China Cratons in the south (Li et al. 2022) (Figure 1a).The CAOB amalgamated several segments, including continental blocks, Island arc terranes, and accretionary complexes and ophiolites, during the late Neoproterozoic to the Permo-Triassic (Windley et al. 2007;Kröner et al. 2010), including large volumes of juvenile crust that formed in Palaeozoic subduction accretion zones (Jahn et al. 2004;Yarmolyuk et al. 2007Yarmolyuk et al. , 2008)).The orogenic belt hosts numerous prolific porphyry-style systems, which were emplaced between the Ordovician and Jurassic in association with a number of Island arcs that formed within the Paleo-Asian Ocean.The most economic deposits were formed during the late Devonian (e.g.Oyu-Tolgoi porphyry deposit) and late Carboniferous (e.g.Kalmakyr, Dalneye, and Konrad porphyry deposits).Several studies (e.g.Blight et al. 2010a;Lehmann et al., 2010) have demonstrated that the Island arc and related mineral deposits formed during the Devonian Carboniferous periods in southern Mongolia.Explanations supporting this hypothesis are insufficient due to a lack of geochemistry and geochronology studies of Carboniferous magmatism.Precise emplacement age and geochemistry studies for granitoid-related ore deposits can provide significant insights into the relationship between magmatic activity and Cu mineralization.
In this study, we provide new fieldwork, microscopic observation, whole-rock geochemical data, mineral chemical analyses, and in situ zircon U Pb age dating of the Bronze Fox deposit.This paper aims (a) to better understand the formation mechanism of the Bronze Fox pluton and coeval volcanic rocks and (b) to constrain the geodynamic framework that facilitated the evolution of the Bronze Fox Cu-Au (Mo) deposit.

Regional geology
The Bronze Fox porphyry Cu-Au deposit is located in the middle to late Palaeozoic Gurvansaykhan Island arc  Jahn et al. 2000;Badarch et al. 2002;Windley et al. 2007); b) Simplified map of southern Mongolia, Central Asian orogeny belt (Badarch 2005); c) Gurvansaykhan Island Arc terrane, showing its relationship to the adjacent Palaeozoic rocks and major occurrences of porphyry copper deposits (Ekhjargal and Jargalan 2016).

Geology of the bronze fox porphyry Cu-Au deposit
The Bronze Fox porphyry Cu-Au (Mo) deposit is located in the Gurvansaykhan arc terrane, Southern Mongolia (Tomurtogoo et al. 2000;Badarch et al. 2002Badarch et al. , 2005;;Windley et al. 2007).The drill-hole logging data indicate that the lower part of the deposit mainly comprises the early Carboniferous (Tournasian-Visean age) Ikh-Shankh formation, which is composed of marine sediments, tuffs, and conglomerates with a total thickness of approximately 2100-2700 m (Hovan et al. 1982(Hovan et al. , 1984)), followed by the middle-late Carboniferous Gunbayan volcanic-sedimentary formation (Figure 2a), which consists of andesite, dacite, trachy-dacite, trachy-rhyolite, and tuffs with sandstone and limestone beds.
The entire sequences were later intruded by numerous granitoids, where the Bronze Fox intrusive bodies include granite, granodiorite, quartz monzodiorite, monzodiorite, diorite, quartz diorite, and tonalite porphyry (Figure 2b).Blight et al. (2010a) obtained a U-Pb zircon age of 333.6 ± 0.6 Ma for the Bronze Fox granodiorite intrusion.The last phase of igneous activity in the investigated area is represented by different mafic and intermediate dikes.

Sampling description and petrography
The drill-hole lithologic information reveals that the studied medium-to coarse-grained Bronze Fox granitoid intrusion was emplaced mainly through volcanic units of the lower Carboniferous Gunbayan volcano-sedimentary formations (Figure 3).This intrusive suite comprises alkaline granite, granodiorite, quartz monzonite, monzodiorite, quartz diorite, and tonalite, which are divided into two main units (i.e.granodiorite and diorite) (Figures 4a,b,c,d,and e), along with a small number of stocks and late and early mafic to alkaline dikes.The lower Carboniferous Gunbayan Formation is the most widespread sequence in the Bronze Fox drill hole section.It consists essentially of andesitic lava (Figure 4g), mineralized trachyandesite dikes (Figure 4f), and unmineralization andesite dikes (Figure 4h).The host monzonite and monzogranite contain many microgranular enclaves of diorite that are ovoid, ellipsoidal, lenticular, or irregular in shape and range in size from 2 to 20 cm (Figure 5a).

Granodiorite unit
Granodiorite: Medium-to fine-grained granodiorite composed of plagioclase (40-45 vol %), K-feldspar (20-25 vol %), quartz (10-15 vol %), amphibole (7-10 vol %), and biotite (0-5 vol %).Plagioclase has been altered to sericite to varying degrees, from strongly to moderately, up to 1.2 mm in size.K-Feldspar ranges from irregular shreds of interstitial grains to large poikilitic masses, commonly engulfing other constituents.Quartz is particularly abundant in these rocks.Mafic minerals are represented by amphibole and biotite, which are strongly altered to epidote and chlorite.Opaque minerals are dominated by chalcopyrite, pyrite, and molybdenite (Figure 5a).It is the main host rock of copper-gold mineralization in the study area.In the mineralization zone, the rock was strongly altered into argillic, sericitic, and propylitic mineral assemblages.
Monzodiorite: This monzodiorite is composed of medium-to fine-grained phenocrysts embedded in a holocrystalline to relatively equigranular groundmass (i.e. porphyritic texture).Plagioclase (40-45 vol %), K-feldspar (30-35 vol %), amphibole (10-15 vol %), biotite (5-7 vol %), augite (3-5 vol %), and quartz (10-15 vol %) are the major constituents.Accessory minerals include calcite, chalcopyrite, pyrite, apatite, and zircon.Secondary minerals are actinolite, calcite, epidote, and chlorite.Euhedral plagioclase is up to 2 mm in size.Amphibole is commonly found as irregular grains that corrode and engulf other constituents (i.e.K-feldspar crystals).Within this rock unit, grid twining and patchy perthite are fairly common.Quartz is equal in abundance to both plagioclase and K-feldspar.Biotite is found as irregular, ragged crystals.Alteration is mainly pervasive within this stock.Evidences of intensive alteration are widely observed in the monzodiorite intrusion, especially in the central part of the Bronze Fox deposit.For instance, plagioclase feldspar grains are converted to sericite minerals associated with the replacement of biotite by chlorite.Additionally, the amphibole crystals have been partially to wholly altered to biotite (Figure 5).This rock unit is considered the second main host formation for Cu-Au mineralization.
Although these crystals are altered, primary textures are relatively well preserved; for example, these minerals have subhedral tabular and rounded shapes and are characterized by several typical zoned twinning with Carlsbad law, up to 1.5 cm in size.The groundmass mainly involved feldspar and quartz (15-20 vol %) with smaller amounts of amphibole and biotite.The accessory mineral assemblages are primarily composed of zircon, apatite, rutile, chalcopyrite, pyrite, and magnetite (Figure 5c).The tonalite porphyry unit of the Bronze Fox deposit contains a very low grade of copper mineralization compared with other rock types within the investigated area.

Mafic-intermediate lava:
The volcanic lava is dominated by basaltic andesite and andesite rocks.The samples collected from drill holes display strong macroscopic fracture patterns and a high degree of alteration (Figure 4g).Porphyritic basaltic andesite samples usually   contain phenocrysts of euhedral-subhedral plagioclase (20-30 vol %), euhedral-subhedral amphibole (15-25 vol %), and calcite-filling vesicles (3-5 vol %), with a small number of accessory grains, including apatite, rutile, and zircon (Figure 5g).The whole sequence subsequently experienced low-grade regional metamorphism (i.e. a partial degree of lower greenschist facies).Due to secondary minerals having largely replaced the primary igneous minerals (Figure 5g), we concentrated our investigation on the whole-rock geochemical data for more accurate results.
Intermediate dikes: Early-stage or mineralized trachyandesite porphyritic dikes are characterized by finegrained, dark grey to black greenish, weakly porphyritic texture.Phenocrysts are rare, occurring as small rounded grains (3-5 vol%).They are composed of biotite (3-5 vol %), amphibole (35-40 vol%), plagioclase (35-40 vol%), and quartz (3-5 vol%).Quartz phenocrysts are up to 1 to 1.2 mm across and form up to 10% by volume of the rock.Plagioclase phenocrysts are dominantly tabular crystals (up to 4 mm across) (Figure 5f).On the other hand, late-stage or unmineralized dikes have a porphyritic texture and contain phenocrysts of calcite, quartz, and plagioclase as well as accessory and opaque minerals.The fine-grained groundmass consists of strongly altered plagioclase and opaque minerals (mostly iron oxides).Calcite phenocrysts are up to 10 mm in diameter, while subhedral amphibole is moderately altered to chlorite that is up to 6 mm across (Figure 5h).

Major and trace elements
The ALS Geochemistry Laboratory in Guangzhou, China, carried out whole-rock major and trace element analyses for ten (10) samples.Before the analyses, samples were crushed in a steel jaw crusher and then powdered in an agate mill to a grain size of 74 µm.The detailed methodologies for major element compositions are as follows: Loss of ignition (LOI) was determined after igniting sample powders at 1000°C for 1 hour.A calcined or ignited sample (0.9 g) was added to 9.0 g of lithium borate flux (Li 2 B 4 O 7 -LiBO 2 ), mixed well, and then fused in an auto fluxer at 1050 and 1100°C.A flat molten glass disk was prepared from the resulting melt.A Panalytical xios Max X-ray fluorescence (XRF, Panalytical, Almelo, The Netherlands) instrument was used to analyse this disk with an analytical accuracy of ca.ICP MS was used to measure the trace element compositions (Perkin Elmer Elan 9000, Perkin, Waltham, MA, USA), with an analytical accuracy of better than 5%.Detection limits, defined as 3 s of the procedural blank, for some critical elements are as follows (ppm): Th (0.05), Nb (0.2), Hf (0.2), Zr (2), La (0.5), and Ce (0.5).The accuracy and precision of the data are better than 5% for major elements and 10% for trace elements based on analytical results and replicate analyses of international standard reference material (SRM) (Liu 1996).

U Pb zircon dating
Zircons were separated from three samples (KCC-41, KCC-43, and KCC-46) using conventional heavy liquids and magnetic separation.The grains were subsequently handpicked with a binocular microscope and mounted on adhesive enclosed in epoxy resin, polished down to near half sections, and then photographed under transmitted light.The internal structures were examined using the cathodoluminescence (CL) imaging technique at the CAS Key Laboratory of Crust-Mantle Materials and Environments, University of Science and Technology of China (USTC), Hefei, China.Most grains chosen for dating are free of inclusions based on the CL images and transmitted light microscope.The spots were selected to represent igneous cores and areas of recrystallization.
In situ zircon U Pb dating was performed using laser ablation (LA-ICP MS) at the CAS Key Laboratory of Crust-Mantle Materials and Environments, USTC.The GeoLas Pro laser ablation system is equipped with a 193-nm excimer ArF laser (spot size of 32 µm with a laser repetition rate of 10 Hz) used in connection with a quadrupole ICP MS (Agilent 7700e).For U/Pb calibration, standard zircon 91,500 was analysed once per four sample measurements in succession to calibrate the mass discrimination and elemental fractionation.The Pb/U ratios were calculated offline with an in-house MS Excel spreadsheet of LaDating@Zrn (http://icpms.ustc.edu.cn) from time-resolved raw data.Common lead corrections were conducted by using the Excel programme ComPbCorr#3_181 (Andersen 2002).Uncertainties in individual analyses are reported at the 1σ level.The concordia plots and weighted mean calculations were created using the Isoplot program, version 4.15 (Ludwig 2003).Zircon trace element analyses were performed synchronously using NIST-610 glass as an external calibration and stoichiometric Si as an internal standard.
During the time-resolved analysis of minerals, the contamination resulting from inclusions and fractures was monitored by different elements, and a corresponding part of the signal was excluded.The results were processed using the macro program La-TEcalc.xlsmwritten in Excel spreadsheet software.

Electron probe microanalysis (EPMA)
The chemical compositions of amphibole, biotite, and plagioclase were selected for a total of approximately 30 points for study by using an electron probe microanalyzer (EPMA).Before the analyses, all thin polished sections were carbon-coated, and natural standards were used for calibration.

Major and trace elements
A total of 60 representative samples were collected from the host rocks of the Bronze Fox deposit.Such samples were collected from the cores of drill holes and surface outcrops.Ten (6 plutonic and 4 volcanic) samples were selected to analyse for major and trace element concentrations, and the results are listed in Supplementary Data (Table 1 (Winchester and Floyd 1977) are employed to classify the intrusive and extrusive rocks of the Bronze Fox deposit (Figures 6a and 6b).The resultant diagrams of the studied rock units are similar to those of the Shuteen complex (Batkhishig  Peccerillo and Taylor (1976), and the dashed line is from Middlemost (1985).All literature data on the Shuteen complex and Bronze Fox intrusive are from Batkhishig et al. (2010).Granodiorite units (KCC43, KCC46), diorite units (KCC44, KCC47, KCC48, KCC49), and volcanic rocks (KCC41, KCC42, V10, KCC45).
et al. 2010) and are in agreement with the petrography examination of these rocks.Based on the A/NK versus A/CNK and K 2 O versus SiO 2 diagrams (Figures 6c and d), the Bronze Fox intrusive rocks are mainly classified as metaluminous and high-K calc-alkaline series.Additionally, the studied lavas have aluminium saturation index (ASI) ratios of < 1.1, indicating that their compositions are equivalent to those of andesitic rocks.
On the Harker diagrams, major elements of Bronze Fox and Shuteen intrusive rocks roughly define continuous variation trends (Supplementary Figure 1), resulting in all these plutonic rocks probably coming from a single magma chamber but evolving to different degrees.The REE and trace element dataset can be found in the Supplementary Data (Table 1).The rocks of the Bronze Fox intrusive and extrusive rocks are enriched in Sr and depleted in Y and Yb.When plotted on a primitive mantle-normalized (Sun and McDonough 1989) spider diagram (Figures 7a and c), the rocks are enriched in high field strength elements (HFSEs; e.g.Nb, P, Zr, Ti, Ta, and Pb).Therefore, the REE elements show high (La/Yb) N ratios ranging from 3.36-9.04,displaying listric REE distribution patterns, with fractionated LREE-MREE and negligible Eu anomalies (Figures 7b and d) that define the characteristics of the Shuteen complex and Bronze Fox volcanic rocks with similar arc volcanic rocks.Both the Bronze Fox and Shuteen intrusive rocks have positive Pb, Sr, and Zr anomalies and negative Rb, Nb, Ta, and Ti anomalies (Figure 7a), typical of lower continental crust.The Bronze Fox and Shuteen rocks are both characterized by high Sr/Y ratios and low Y contents and are classified as adakites (Figure 9a).In addition, those rocks have slightly higher MgO contents than experimental melts of metabasalt with MORB compositions at given SiO 2 contents (Figure 9d).Their Mg# values are in the overlapping area of experimental oceanic crust melts (1-4 GPa) and modern Island arc adakites (Figure 9d).

U Pb zircon age dating
Zircon grains of the intrusive rocks (i.e.granodiorite and monzodiorite) and andesitic lava from the Bronze Fox   Sun and McDonough (1989); LCC data are from Rudnick and Gao (2003).All literature data on the Shuteen complex are from Batkhishig et al. (2010) deposit are generally prismatic, colourless, transparent, and euhedral crystals under the optical microscope.Cathodoluminescence (CL) images show microscale oscillatory zonation (Supplementary Figure 2), implying a magmatic origin for these grains.The age dating results of the zircon U Pb analyses are listed in the Supplementary data (Table 2).

Intrusive rocks
The zircon grains separated from granodiorite (KCC-43) and monzodiorite (KCC-46) are colourless or pale yellow, euhedral prismatic crystals, ranging from 90 to 200 μm in length and 35 to 75 μm in width, and exhibit numerous narrow oscillatory zones (Supplementary Figures 2b  and c).Among the thirty zircon grains (30 spots) selected from the granodiorite sample (KCC-43), 15 were concordant with concordant age of 326.5 ± 3.4 Ma and a weighted average age of 326.1 ± 5.1 Ma (Figure 8b).The zircon grains from this unit have consistent Th and U contents with Th/U ratios over 0.65, indicating a magmatic origin (Hoskin and Schaltegger 2003).
Thirty zircon grains (30 points) have been dated from the monzodiorite sample (KCC-46) (Supplementary Data Table 2).Except for a discordant age, the other 17 analyses have concordant 206 Pb/ 238 U and 207 Pb/ 235 U ratios within the analytical precision, and most of the grains display similar ages.They yield a crystallization age of 326.3 ± 2.7 Ma for this rock.The measured 206 Pb/ 238 U ratios give a weighted mean age of 327.04 ± 4.7 Ma (Figure 8c).The average concentrations of Th and U are 38.68 and 61.58 ppm, respectively; therefore, they are interpreted as magmatic zircons.

Extrusive rocks
The crystal habit of analysed zircons from the andesite sample (KCC-41) is dominated by subhedral to euhedral and exhibits weak zoning or uniform internal textures.Most of them are approximately 70-150 µm in size.They display large variations in Th (12.31-295.7 ppm) and U (25.4-338.8ppm) concentrations, with Th/U ratios mostly greater than 0.6, indicating a magmatic origin.The analysed spots were highly concordant (≤ 5% discordance) and defined a concordia age of 331.0 ± 3.4 Ma, which is consistent with the weighted mean 206 Pb/ 238 U ages of 330.9 ± 5.4 Ma, indicating a magmatic crystallization age (Figure 8a and Supplementary Figure 2a).

Zircon chemistry and Ti thermometer
The chondrite-normalized REE patterns of zircons from the Bronze Fox complex are depleted in LREEs and enriched in HREEs (not shown), with pronounced positive Ce anomalies and slight Eu negative anomalies, suggesting that they are all magmatic zircons.The calculated Eu/Eu* and Ce 4+ /Ce 3+ values (Zhang et al. 2017a) range from 0.05 to 0.78 and from 22.37 to 2757.69 for andesite, from 0.23 to 0.94 and from 12.8 to 542.2 for granodiorite, and from 0.13 to 0.9 and from 11.4 to 9056.7 for quartz diorite, respectively (Supplementary data Table 3).Zircon crystallization temperatures are calculated by the thermometry of Watson et al. (2006).Ti-in-zircon temperatures vary from 643.81 to 773.77°C with an average of 713.5°C for andesitic lava, while the analysed grains from the intrusive units yield zircon crystallization temperatures of ~ 598.91-803.35°Cwith an average of 714.28°C.

Plagioclase
Plagioclase provides a unique window into the genesis of calc-alkaline igneous rocks.All analysed spots in plagioclase crystals from the granodiorite host unit are andesine (Supplementary Figure 3a; Supplementary Data Table 4) with weak zoning textures.In contrast, plagioclase from the quartz diorite intrusions displays chemical zoning, which may record the history of magma evolution (Supplementary Figure 3a).Their EPMA analyses show that almost all crystals have compositions ranging from andesine to oligoclase (Supplementary Figure 3b).
The detailed investigation from the rim to the core for the studied plagioclases is given in Supplementary Figure 3e and Supplementary Data Table 4. Plagioclase that occurs in granodiorite is moderately altered to sericite and forms very weakly zoned domains.The anorthite mol % contents of the core range from 18.98 mol% to 24.64 mol%, showing an increasing trend in general.In addition, the plagioclase phenocrysts of quartz diorite are composed of two oscillatory zoned domains around a core and outer rim.The An (anorthite) contents of the core range from 37.93 mol% to 39.33 mol %, showing an increasing trend in general, but those of the oscillatory zoned domains show a decrease of 44.22 mol% from the inner portion to 41.66 mol% in the rim area (Supplementary Figure 3a and c).
To rule out alteration, we searched for microcrystalline alteration phases within the excitation volume of EPMA analyses by plotting SiO 2 versus CaO+Na 2 O+K 2 O wt% (Supplementary Figure 5a).Data from the study area mostly fall along the tie line between albite and anorthite, defining the affinity of intermediate mineralization porphyries (with SiO 2 < 65 wt%).Furthermore, these points show moderate to high An values (>10%).The variations in plagioclase contents (Supplementary Figure 5b) and strong excess Al (but relatively low An), which occurs within discrete concentric zones, strongly suggest magmatic processes (Williamson et al. 2016).
Plagioclase from fertile and barren systems can be effectively discriminated on a diagram of anorthite An % versus Al/(Ca+Na+K) (calculated on the basis of atoms per formula unit (a.p.f.u.); Supplementary Figure 5b), where the black lines join the pure composition between albite (NaAlSi 3 O 8 ) and anorthite (CaAl 2 Si 3 O 8 ) endmembers.In particular, the black line, denoting Al* = ((Al/Ca+Na+K)-1)/0.01An)= 1, separates analyses from fertile systems, which contain excess Al (Al*>1), from those that are barren Al*<1 (Supplementary Figure 5b).The cause of excess Al provides important constraints on porphyry copper deposit formation (Williamson et al. 2016).
The studied samples from the granodiorite of the Bronze Fox Cu-Au-Mo deposit are largely plotted in the fertile field, while the quartz diorite samples show scattered distributions in and around the borderline between fertile and barren fields (Supplementary Figure 5b).In this regard, all crystals closely resemble plagioclases formed in intermineralization porphyries.

Biotite
Many biotite crystals are found as disseminated grains in the selected thin sections from the Bronze Fox intrusive rocks.A small amount of biotite is found as narrow and long grains that mainly occur as porphyritic texture.The results of the analysis of 9 biotite grains reveal high  wt.%) and  wt.%) contents (Supplementary Data Table 5).These crystals are mainly plotted in the magnesio-biotite and phlogopite fields (Supplementary Figure 3 f), showing the chemical characteristics of the equilibrated primary biotites (Supplementary Figure 3d).

Amphibole
The chemical compositions of selected amphibole from the Bronze Fox porphyry copper deposit are summarized in Supplementary Data Table 6.The amphibole grains are generally euhedral phenocrysts and elongated and columnar crystals.A total of 6 crystals were analysed in amphiboles from the studied intrusive samples (Figure 5a-c).They show calcic amphibole compositions on the B(Na) versus B(Ca+Na) classification diagram (Leake et al. 1997) (Supplementary Figure 3e), with Ca ranging from 1.60 to 1.87 a.p.f.u.(Supplementary Data Table 6).The amphiboles from granodiorite intrusion are magnesio-amphibole, while those from quartz diorite rock are distributed along the boundary between magnesio-amphibole and tschermakite fields (Supplementary Figure 3 g).Generally, amphibole has SiO 2 contents ranging from 43.86 to 52.19 wt.%, and TiO 2 contents range from 0.10 to 1.12 wt.%.

Age of Bronze Fox porphyry Cu-Au deposit
The porphyry copper deposits of the Gurvansaykhan Island arc terrane in southern Mongolia are related to late Devonian-early Carboniferous arc magmatism (Batkhishig et al. 2010;Wainwright et al. 2011).A summary of isotope dates from associated porphyry deposits of Gurvansaykhan arc magmatism is presented in Figure 11 and will be discussed in the following paragraphs.
Many researchers have concluded that the peak magmatic activity related to the intensively mineralized porphyry Cu-Au deposits of the Gurvansaykhan Island arc terrane predominantly occurred during the early Carboniferous, e.g.Bronze Fox (dated at 333.6 ± 0.6 Ma; (Blight et al. 2010a)), Shuteen (dated at 325.5 ± 1.0 Ma (Blight et al. 2010a)), Zogdor (dated 327-330 Ma; (Davaasuren et al. 2021)) and Nariin Hudag (dated 333.0 ± 0.5 Ma; (Blight et al. 2010a)).This study provides the first robust ages (zircon age dating) for the Bronze Fox syn-mineralization intrusions that were emplaced at 326.5 ± 3.4 Ma for granodiorite and 326.5 ± 2.7 Ma for monzodiorite (Figure 12).More specifically, the timing of granitoid magmatism in the Bronze Fox porphyry Cu-Au deposit is in strong agreement with that proposed in previous research on the surrounding porphyry deposits in the Gurvansaykhan Island arc terrane.Furthermore, the recorded age falls within the time interval in which the Shuteen porphyry (10 km away, Figure 2) deposit was formed (Blight et al. 2010a).
In the current study, the extrusive and intrusive units are genetically similar.Although zircon U Pb dating yields a concordia age of 331.0 ± 3.4 Ma for andesite, which is slightly older than the mineralized intrusions (with an overlapping error range of 3.4 Ma), the field relationship indicates that the andesite represents an early fractionated sequence of the parent magma and that both volcanic and plutonic rocks occurred at/ around the same time interval (i.e.early Carboniferous, Figure 11).Additionally, petrographic studies show that all the studied rocks define a single progressive differentiation trend, from andesite to diorite, monzodiorite, granodiorite, and finally tonalite (Figures 6, 7 , and 9).Furthermore, geochemically, the extrusive and intrusive units that host copper mineralization in the investigated area are notably similar (Figures 6 and 7).
Based on the above discussion, we suggest that the ore system at the Bronze Fox and Shuteen complexes was generated from the same magma source due to their spatial proximity, chronological similarity, and geochemical characteristics (Figure 12).

Petrogenesis and affinity of the adakitic magma
The primitive mantle-normalized spider diagram (Figures 7a and c) of the host units suggests a typical subduction-related origin, as indicated by the enrichment of LILEs, the depletion of HFSEs, and the negative troughs of Nb, Ta, and Ti.This is also consistent with the tectonic discrimination diagram illustrated in Figure 10b  and c, where the studied igneous rocks were emplaced in the late stage of Island arc evolution.As the Island arc evolves towards maturity, the mantle wedge becomes successively hydrated and oxidized due to the influx of water, sulphur, and fluid-mobile elements from the subducting oceanic slab.This process has been estimated to have lasted over ∼10 Ma from the nascent Island arc.
The studied samples from the granodiorite unit, which are mainly plotted in the adakite field (Figures 9a and b), are also characterized by high Sr and low Y contents with high Sr/Y ratios, which are typical characteristics of adakite.Given their moderate to high SiO 2 contents and moderate to low MgO contents (Figure 9c), they are identified as high-silica adakites (HSA), according to the scheme presented for low-silica adakites (LSA) and HSA (Defant and Drummond 1990;Martin et al. 2005).HSA rocks are the direct result of melting subducted hydrated basalt followed by variable peridotite interaction as they ascend through the mantle wedge (Martin et al. 2005), with their MgO and Mg# radically elevated (Rapp et al. 1999;Prouteau et al. 2001) (Figure 9c and d).Adakitic magmas, whether derived directly from partial melting of the subducted oceanic slab (including MORB and marine sediments) or lower continental crust mafic rocks, show low Mg# (<40) regardless of melting degrees (Rapp et al. 1999).The studied host units have Mg# values varying from 46.8 to 47.1, which are obviously higher than those of experimental oceanic crust melts and are projected in the field defined by modern Island arc adakites (Figure 9d), suggesting mantle interaction during the ascent of adakitic magma (Rapp et al. 1999).In addition, the elevated Cr and Ni contents of these adakites (up to 123-141 and 8.52-17.1 ppm, respectively; Supplementary Data Table 1) provide further evidence for mantle interaction.Although the studied rocks from the diorite host unit are also characterized by high Sr and low Y contents and high Sr/Y ratios and were plotted in the area of low-silica adakite (Figure 9c), they do not meet the required geochemical signatures of LSA (Moyen 2009).
The ratios of Hf/Yb and Th/U of zircons are also consistent with an evolving magmatic system from early amphibole-dominated fractionation to amphibole plus apatite and titanite (Supplementary Figure 4 h).As melt evolves to more silicic compositions, the concentrations of U, Th, and Y generally increase, and Th/U ratios typically decrease in zircon (Claiborne et al. 2006(Claiborne et al. , 2010;;Gagnevin et al. 2010)  mineral, crystallizing from the ferromagnesian phase and subsequently removing Y from the melt; thus, the Hf/Y values increase (Moore and Carmichael 1998;Gagnevin et al. 2010;Richards et al. 2012;Loucks 2014;Large et al. 2018).The mineralized granodiorite has higher Hf/Y and Th/U ratios than quartz diorite and andesite rocks.As the melts evolved into higher felsic compositions, less amphibole crystallized and the amounts of apatite and titanite increased (Supplementary Figures 4 g and h).This crystallization mineral assemblage cannot reflect magma mixing for granodiorite.
The strong negative Nb anomalies in the Bronze Fox rocks (Figures 7a and c) indicate that residual rutile (± amphibole) was represented in the source.Similar to other adakites/adakitic rocks, these features of the Bronze Fox adakites can not only be caused by partial melting of the subducted oceanic slab (Kay 1978;Defant and Drummond 1990;Stern and Kilian 1996) but also may be caused by lower continental crust (LCC) melting (Atherton and Petford 1993;Petford and Atherton 1996;Chung et al. 2003;Huang et al. 2008).Furthermore, adakites/adakitic rocks produced by the partial melting of oceanic or continental crust can still be distinguished in some critical geochemical features and signatures (Liu et al. 2010;Ling et al. 2011;Sun et al. 2012;Deng et al. 2016;Zhang et al. 2018).First, partial melting of the oceanic crust with plagioclase could produce low K 2 O/ Na 2 O adakitic melts, whereas partial melting of the mafic LCC under high pressure could produce high K 2 O adakitic magmas with high K 2 O/Na 2 O ratios (Xiao and Clemens 2007;Liu et al.).The high K 2 O/Na 2 O ratios of LCC-derived (i.e.C-type) adakitic rocks could be ascribed to elevated K 2 O/Na 2 O in the protolith (Rapp et al. 2002;Xiao and Clemens 2007), a low degree of partial melting (Sen and Dunn 1994;Huang and He 2010;Qian and Hermann 2013), or very high-pressure melting (Xiao and Clemens 2007).The source characteristics could not be the dominant factor in the difference between the K 2 O/Na 2 O ratios of the two types of adakites/adakitic rocks, as suggested by Deng et al. (2019).However, as LCC is usually dry, the degree of partial melting for LCC is lower than that of the hydrous oceanic crust at a given P T condition (Deng et al. 2019).Subsequently, low-degree partial melting of the dry eclogitic LCC at high pressure (in the absence of amphibole) can produce adakitic melts.These melts would have high K 2 O contents and K 2 O/Na 2 O (~1) ratios (Huang and He 2010), as K is highly incompatible during partial melting (Nash and Crecraft 1985) (Qian and Hermann 2013), especially at very high pressure with an omphacite-bearing eclogite residue.The relatively high Na 2 O and low K 2 O contents and thereby low K 2 O/Na 2 O ratios (<0.7) of the Bronze Fox adakites are not consistent with the model of LCC melting.These features are in strong agreement with those of oceanic slab-derived adakites (Figure 9e).The oceanic adakites with sodium enrichment are commonly the result of partial melting of the MORB compositions with residual amphibole (Defant and Drummond 1990;Rapp and Watson 1995;Martin et al. 2005).Second, a positive correlation between (La/Yb) N and Sr/Y ratios should be observed in adakitic rocks produced by partial melting of thickened or delaminated LCC to various degrees with garnet amphibolite or eclogite residue, as observed in those from Dabie Mountain and adjacent areas of the South Tan-Lu Fault (STLF), eastern China (Liu et al. 2010;He et al. 2011;Ling et al. 2011;Sun et al. 2012).Under such melting conditions, both Y and Yb are compatible, whereas Sr and La are incompatible in amphibole-or garnet-bearing and plagioclase-free residues (Defant and Drummond 1990;Rapp and Watson 1995;Moyen 2009).In contrast, adakites generated by oceanic crust melting in modern subduction zones worldwide show Sr/Y -(La/Yb) N decoupling (Liu et al. ;Sun et al. 2012), probably due to low La contents and (La/Yb) N ratios of the subducted oceanic crust, as the average (La/Yb) N ratio of MORB is 0.8 (Sun and McDonough 1989), compared with an average of 5.3 for the LCC (Gao et al. 2004).The Bronze Fox studied rocks are also characterized by low (La/Yb) N but high and variable Sr/Y ratios, which are similar to the Shuteen complex (Figure 9e) and comparable to arc adakites from modern subduction zones.No clear correlation between (La/Yb) N and Sr/Y ratios is found in the studied samples from the Bronze Fox deposit, suggesting the decoupling of Sr/Y and (La/Yb) N .The different trends defined by Bronze Fox adakites and LCC-derived adakitic rocks further exclude the LCC melting model.Therefore, Bronze Fox adakites were most likely produced by partial melting of a subducted slab in the garnet stability field.
The diorite units have low SiO 2 contents, indicating that they were derived from mantle melting.As mentioned before, they show high Sr contents, low Y contents, and relatively high Sr/Y ratios, which are different from those of normal Island arc rocks (Figure 9b).Although they are mainly plotted in the field of LSA (Figure 9c), they do not meet other geochemical features of LSA (Martin et al. 2005;Moyen 2009), e.g. more fractionated REE patterns and lower HREE contents relative to HSA and high TiO 2 contents (> 3 wt.%), etc.Thus, it is concluded that the dioritic rocks were not formed from adakitic melt-metasomatized mantle.Therefore, the parental magma was likely generated from the subarc mantle that was metasomatized by Si-Al-rich fluids released from a subduction slab.The resultant intermediate melt was characterized by high Sr contents and less fractionated REEs relative to the slab melts.
Experimental studies show that partial melting of metamorphosed MORB with MgO of 6.59 wt.% produces melts with low MgO (< 3 wt.%)and low Mg# (< 50) (Rapp et al. 1999).However, adakites in modern Island arcs may have higher MgO contents and Mg#, coupled with high Cr and Ni contents, which probably reflects the interaction between primitive slab melt and mantle peridotite (Defant and Drummond 1990;Rapp and Watson 1995;Moyen 2009).Because of the high Cr and Ni concentrations of olivine or pyroxene, the addition of even a small amount of mantle peridotite to adakitic melts can significantly enhance the Ni and Cr contents, with Mg# increasing, without obvious SiO 2 depletion (Rapp et al. 1999;Huang et al. 2008).In contrast, the Bronze Fox intrusions have MgO contents and Mg# values that are comparable with those of the Shuteen complex at given SiO 2 contents (Figure 9d), plotting in the area between experimental metabasalt and modern Island arc adakites.Bronze Fox granitoid intrusions also have slightly higher Cr (26-100 ppm) and lower Ni (15.9-55.9ppm) contents, which are also similar to the Shuteen complex (Supplementary Figure 4c).These slightly higher Mg# values, low Ni contents, and slightly higher Cr contents suggest that the Bronze Fox and Shuteen complex may have negligible interactions with mantle materials (Figure 10a).In addition, the negative correlation of Ni and Mg# for the Bronze Fox and Shuteen complexes does not support the different degrees of mantle interaction but could reflect the source partial melting process.

Geothermobarometry
The mineral chemistry of amphibole and feldspar has been widely used to constrain the temperature and pressure during magma crystallization (Blundy andHolland 1990, 1992;Robie et al. 1995;Anderson and Smith 1995 ;Ridolfi et al. 2010;Aysal 2015).As noted by many previous researchers, magma emplaced at shallow crustal levels (mostly 1-2 kb or 3 to 6 km) has the greatest potential to form economic porphyry deposits (Muntean and Einaudi 2000).At a shallow level, volatiles can be readily exsolved from the magma, which is critical to the formation of magmatic-hydrothermal deposits, and metals can be partitioned from the melt into exsolved hydrothermal fluids (e.g.Candela and Holland 1984;Cline and Bodnar 1991;Pokrovski et al. 2008).In the present research, the biotite Ti versus Mg/(Mg+Fe) diagram was used (after Henry et al. 2005a) (Supplementary Figure 4c) to estimate the crystallization temperature.The results illustrate that the biotite mineral originated at relatively high temperatures (i.e.700 to 750°C).This evidence bears to witness that similar results were calculated from the amphiboles.Such temperatures are highly analogous to those estimated from zircon thermometers (average 714°C for Bronze Fox granitoid intrusions).In addition, the physical-chemical parameters of amphibole were calculated by a spreadsheet provided by Ridolfi et al. (2010), and the results imply that the amphibole from the host rock of Bronze Fox porphyry deposits was expected to form in two different phases/stages.The amphibole phenocrysts formed at medium (~50 MPa for quartz diorite) to high pressure (~190 MPa for granodiorite) with absolutely high temperatures (~750°C for quartz diorite and ~870° C for granodiorite) (Supplementary Figure 4d).Based on these calculated parameters, the phenocrysts of amphibole were most likely crystallized at depths of 6.8 to 7.5 km for quartz diorite and 1.0 to 1.6 km for granodiorite.Ridolfi et al. (2010) identified that the Al VI in amphibole is mainly sensitive to the water content in the parent magma source.Following this method, the calculated water content of melt at the time of amphibole crystallization was approximately 5.4-6.0 wt.% for quartz diorite (which represents the melt in the deep magma chamber) and approximately 3.5 wt.% to 3.6 wt.% for granodiorite (which describes the melt after emplacement) (Supplementary Figure 4f; Supplementary Data Table 4).Assuming that the pressure temperature conditions were derived from amphibole chemistry, we infer that the magma emplacement process must have been rapid.This assumption is also supported by the widespread porphyritic texture in quartz diorite rock.When the H 2 O-rich magma intruded through the deep magma chamber to the shallow crust, the reduction in pressure suddenly led to water exsolution from melt (Zhang et al. 2015).Therefore, identifying the water content of the melt during magma emplacement may represent a key parameter to determine the potential fertility of intrusive emplacement of porphyry copper deposits.

Magma source
The chemistry of amphibole and biotite can be used to distinguish magma sources.The data from amphibole phenocrysts in granodiorite from the Bronze Fox area fall in the crust source field, while amphibole phenocrysts from quartz diorite plot in the crust-mantle mixed source in the TiO 2 versus Al 2 O 3 diagram (Supplementary Figure 4a).The same result is observed from the studied biotite crystals of both rock types (i.e.granodiorite and quartz diorite) in TFeO/(TFeO+MgO) against the MgO diagram (Supplementary Figure 4b).The data support the assumption that the magma was derived from the partial melting of subducted oceanic crust by incorporating/mixing with lower crustal components (i.e.juvenile crust) during the crystallization process (Batkhishig et al. 2010;Dolgopolova et al. 2013).On the other hand, magma mixing may play a significant role in the process of magmatic evolution and mineralization formation (Zhang et al. 2015).Plagioclase provides a unique window into the genesis of calc-alkaline rocks, as it is relatively affected by subsolidus re-equilibration (Grove et al. 1984) and records subtle changes in magma composition (Blundy and Shimizu 1991;Williamson et al. 2016).In addition to studying the complex zoning in plagioclase crystals (which is widespread in monzodiorite rock and weakly observed in granodiorite rock), the data of analysed plagioclase grains from the Bronze Fox porphyry deposit are mostly distributed between oligoclase and andesine fields.Furthermore, the An contents have different variations from core to rim on zoned plagioclase, indicating magma recharge and mixing (Supplementary Figures 3a and c).

Implication for Cu-Au mineralization
Most porphyry copper deposits worldwide are formed in association with subduction-related calc-alkaline magmas, and they spatially occur in magmatic arcs (Sillitoe 1972;Cooke et al. 2005).In the Bronze Fox area, adakite magmatism associated with Cu-Au porphyry deposits has been documented by several authors (Batkhishig et al. 2010(Batkhishig et al. , 2014;;Gerel et al. 2013).Given the high initial Cu content of oceanic crust and high oxygen fugacity of slab melt, adakite produced by partial melting of subducted oceanic crust is especially favourable for producing copper mineralization (Ballard et al. 2002;Liang et al. 2006;Zhang et al. 2013Zhang et al. , 2017aZhang et al. , 2017b;;Deng et al. 2016;Wang et al., 2021).Oxygen fugacity (fO 2 ) is the critical factor controlling the abundance and oxidation state of sulphur in magmas, which directly controls the partitioning and transport of Cu and Au in magmas before mineralization.Additionally, high oxygen fugacity is also beneficial to extracting additional sulphur from the source region to the magma in the form of sulphate during the partial melting process, leading to the liberation of extra chalcophile elements into the oxidized parent magma (Sun et al. 2004).The precipitation and accumulation of Cu and Au in a porphyry deposit is usually accompanied by sulphate reduction and corresponding oxygen fugacity fluctuation, which is attributed to magnetite crystallization (i.e.ferrous iron oxidation (Sun et al. 2004(Sun et al. , 2014;;Liang et al. 2009).The initial Cu content of the magma is another important parameter, which makes the slab melt and is considered the best candidate for porphyry Cu mineralization (Sun et al. 2015;Zhang et al. 2017b).As the Cu content of MORB (~100 ppm) is more than three times higher than that of the primitive mantle or the continental crust (McDonough and Sun 1995;Rudnick and Gao 2003;Sun et al. 2003), adakitic melts produced by the partial melting of subducted oceanic crust (under high oxygen fugacities, ΔFMQ> +1.5) normally have higher initial Cu contents (> 500 ppm), which are probably easily enriched at shallow depths to form porphyry Cu deposits.Therefore, several methods are reported for calculating the oxygen fugacity of parent magma, such as the ratios of certain elements from whole rock chemical data (such as Fe 3+ /Fe 2+ ratios) (Blevin 2004) or the concentration of some rare earth elements (Eu and Ce) from chemical analysis of host rocks or some significant minerals (i.e.zircon, magnetite, etc.) (Ballard et al. 2002).
Given the existing geochemical data for magmatic plagioclase, which is a dominant mineral in calc-alkaline rocks from fertile porphyry-associated and other barren 'nonporphyry' magmatic systems worldwide.Furthermore, plagioclase from fertile systems is distinct in containing excess aluminium, which is similar to the findings of this work and the previous research carried out on plagioclase from the fertile La Paloma and Los Sulfatos copper porphyry systems in Chile (Williamson et al. 2016).Most likely, these chemical signatures of plagioclase can be explained as an exploration indicator for porphyry copper deposits.
It can be assumed that the Bronze Fox granitoids are typical of adakitic rocks derived from oceanic crust, and a Cu-fertile source may have therefore existed during the partial melting process.The Ce +4 /Ce +3 ratios of Bronze Fox intrusions estimated through the zircon Ce anomalies (Ballard et al. 2002), coupled with high Eu/Eu* values (Supplementary Figure 5e), are similar to other Cu-bearing adakites or adakitic rocks in the giant Oyu-Tolgoi Cu-Au-Mo deposit (which is also located within the Gurvansaykhan Island arc terrane).Our data show that contrasting amphibole compositions were formed in the deep magma chamber, reflected by amphiboles from granitoids that have a greater range of variation (0.28-0.37), compatible with amphiboles formed after magma emplacement.Based on the amphibole Fe/(Fe+Mg) vs. Al IV diagram (Supplementary Figure 5c) (Anderson and Smith 1995), it can be interpreted that amphiboles in the monzodiorite rocks crystallized under high fO 2 conditions.Their fO 2 is higher than the expected value in the magma chamber under the same conditions.Amphibole compositions have also been used to calculate fO 2 by following the method of Ridolfi et al. (2010).The results show that log fO 2 ranges from NNO +1.4 to +2.6.From the magma chamber to the emplacement depth, the log fO 2 of magma shows an obvious increase (from average NNO +1.4 to +2.5) (Supplementary Figure 5d).The Fe 3 + /Fe 2+ ratios of some minerals (i.e.amphibole, zircon, magnetite, etc.) is a robust indicator of the fO 2 of magma (Zhang et al. 2015).The Fe 3+ /Fe 2+ ratio of amphiboles in our study is markedly high, with most values in the range of 0.64-3.23.This confirms a high fO 2 for the host rocks of the Bronze Fox Cu-Au deposit.Furthermore, the above mentioned geochemical characteristics are highly consistent with the fact that the Bronze Fox granitoid intrusions host high-potential Cu-Au deposits in the Southern Mongolia Metallogenic Region.

Conclusions
In the present work, we report a comprehensive geological investigation (i.e.field work, petrographic studies, geochemical analyses, microprobe mineral chemistry, zircon age dating) of the Bronze Fox Cu-Au deposit to shed new insights into the magmatic evolution during the early Carboniferous period.The findings of this contribution have been compared with other case studies from the surrounding region (i.e. the Shuteen complex).Based on the analyses and discussion, we draw the following conclusions: (1) The parental magma of the Bronze Fox granitoid (adakites) most likely originated from partial melting of a descending oceanic slab under amphibolite-facies conditions, modified by interaction with mantle and lower crustal material during ascent, to be ultimately emplaced in an intraoceanic Island arc setting.
(2) The mineralized granitoids were largely formed at the same time (i.e.326.5 ± 3.4 Ma for granodiorite and 326.5 ± 2.7 Ma for monzodiorite).This age is slightly younger than the age of the intermediate phase of volcanic activity in the region, which occurred at ~ 331 ± 3.4 Ma (andesite rock).
(3) The magma crystallization temperature and pressure of each of the individual intrusives differ: the mineralized granodiorite was emplaced under high pressure and very high-temperature conditions compared to the unmineralized quartz diorite of the Bronze Fox deposit.(4) The mineral chemistry data (i.e.amphibole, biotite, and zircon) can be interpreted to suggest that the granodiorite intrusions at Bronze Fox have a higher potential to host economic mineralization.(5) High Ce 4+ /Ce 3+ ratios (11.42-9056.7)are associated with the Bronze Fox deposit, suggesting that the host granitoids were generated from an oxidizing parent magma by fractional crystallization, and with other data, suggest the potential for a large porphyry Cu deposit within the Gurvansaykhan Island arc terrane.

Figure 2 .
Figure 2. a) Regional geology map of the Bronze Fox (modified as Buyandorj and Buburuu 2014; Boldbaatar et al. 2017) b) Geology map of the Bronze Fox Cu-Au deposit (this study).

Figure 3 .
Figure 3. Schematic stratigraphic column of the Bronze Fox area, South Mongolia (modified after Buyandorj and Buburuu 2014).

Figure 4 .
Figure 4. Field view of lithological units in the Bronze Fox area; a) granodiorite of drilling core, b) monzodiorite of drilling core c) outcrop rock of tonalite porphyry, d) outcrop of quartz diorite.e) Outcrop of diorite.f) Trachyandesite dike with mineralization that presents as disseminated and veining chalcopyrite and pyrite, g) andesite of drilling core, h) unmineralized andesite dike.

Figure 7 .
Figure 7. REE and trace element distribution diagrams for the Bronze Fox intrusive and volcanic rocks.a) Chondrite-normalized REE plots of representative samples from the Bronze Fox volcanic rock; b) primitive mantle-normalized spider diagram.Chondrite and primitive mantle data for normalization, N-MORB, E-MORB and OIB data are all from .Sun and McDonough (1989); LCC data are fromRudnick and Gao (2003).All literature data on the Shuteen complex are fromBatkhishig et al. (2010)

Figure 8 .
Figure 8. Concordia diagrams, probability distribution plots, and weighted mean ages for the Bronze Fox intrusive and volcanic rocks, southern Mongolia (MSWD: mean square weighted deviation).
The analyses were conducted at the CAS Key Laboratory of Crust-Mantle Materials and Environmental Sciences, USTC.The contents of Na 2 O, MgO, Al 2 O 3 , SiO 2 , TiO 2 , Cr 2 O 3 , MnO, FeO, K 2 O, and CaO for minerals were obtained with a ).They are characterized by a wide range of major element compositions, e.g.SiO 2 contents(48.45-66. . In mafic to intermediate magmas under hydrous conditions, amphibole is the dominant Figure 11.This study and previously published radiometric ages for the Bronze Fox deposit (Batkhishig et al. 2010; Blight et al. 2010a; This study).