Journal of Earth Science  2019, Vol. 30 Issue (1): 109-120   PDF    
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Geology, Geochronology, and Hf Isotopic Composition of the Pha Lek Fe Deposit, Northern Laos: Implications for Early Permian Subduction-Related Skarn Fe Mineralization in the Truong Son Belt
Lin Hou1, Shusheng Liu1, Linnan Guo1, Fuhao Xiong2, Chao Li3, Meifeng Shi1, Qiming Zhang1, Siwei Xu1, Songyang Wu4    
1. Chengdu Center, China Geological Survey, Chengdu 610081, China;
2. Key Laboratory of Tectonic Controls on Mineralization and Hydrocarbon Accumulation of Ministry of Land and Resources, Chengdu University of Technology, Chengdu 610059, China;
3. National Research Center of Geoanalysis, Chinese Academy of Geological Science, Beijing 100037, China;
4. China University of Geosciences, Beijing 100083, China
ABSTRACT: The Truong Son metallogenic belt in central Laos and Vietnam is an important Fe-Cu-Sn-Au polymetallic ore district. The Pha Lek Fe deposit is closely related to Late Carboniferous-Early Permian Ⅰ-type granitic magmatism, and contains >50 Mt@45% to 50% of Fe ore. Ore minerals occur mainly as magnetite and hematite in the skarn alteration zone between a granitic pluton and metamorphosed Middle-Upper Devonian carbonates. The granitic pluton comprises granodiorite and granite, with zircon U-Pb dating indicating synchronous emplacement at 288.2±1.3 and 284.9±1.2 Ma, respectively. Zircons from these granitoids have εHf(t) values of 2.9-11.2 and relatively young TDM2 ages (< 1.0 Ga), indicating an origin by partial melting of depleted mafic crust or magma mixing. Previous studies have shown that these granitoids have high Y, Yb, and K2O contents, and low Sr and Na2O contents, which are interpreted as the melting of mafic continental crust. Pyrite of the main mineralization stage yields an 187Re/188Os-187Os/188Os isochron age of 287±17 Ma, indicating that mineralization is associated with Pha Lek granitic magmatism. A Late Carboniferous-Early Permian subduction-related skarn-type Fe mineralization model is proposed for the Pha Lek deposit. More evidence is needed to verify a hypothesis of volcanic overprinting during Late Triassic post-collisional extension.
KEY WORDS: Truong Son belt    Pha Lek Fe deposit    granitic intrusions    geochronology    Hf isotopic composition    

0 INTRODUCTION

The Truong Son metallogenic belt developed through subduction and collision between the Indochina and South China blocks, with four main Paleozoic–Early Mesozoic episodes of magmatism having been identified (Cheng et al., 2015; Shi et al., 2015; Lepvrier et al., 2008, 2004, 1997). Several Fe-Cu polymetallic deposits have been discovered and mined in the southwest of the belt, in close spatial relationships with Late Carboniferous–Early Permian igneous rocks (Wang et al., 2017b; Tran et al., 2015; Wu et al., 2015). However, for lack of regional correlations between mineralization and magmatism, our understanding of mineralization processes in the region is limited, with the potential for further exploration being unknown.

The Pha Lek Fe-dominant polymetallic deposit in the southwest of the Truong Son belt (Fig. 1) contains > 50 Mt @ 45% to 50% of Fe. Although it has been mined and studied for years, the genesis of the deposit remains in dispute. Some studies have proposed that it is typical skarn-type mineralization in association with Late Carboniferous–Early Permian Ⅰ-type granitic magmatism (Manaka et al., 2014, 2008; Zhao B et al., 2014; Liu et al., 2013; Wang et al., 2013; Mao, 2012; Zhou et al., 2012; Zhao H J et al., 2011). However, other studies have suggested two stages of mineralization, with second-stage Late Triassic volcanism-related mineralization being the more significant (e.g., Zhu et al., 2014). Both theories suffer from a lack of robust evidence of spatial-temporal links between the granitic pluton and mineralization, and further study is required to elucidate the origin of the Pha Lek deposit.

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Figure 1. Geological framework of the Pha Lek deposit. (a) Tectonic location of the Truong Son belt; (b) schematic geological map of the southwest part of the Truong Son belt (modified from 1 : 1 000 000 scale geological map of Laos); (c) geological map of the Pha Lek deposit (modified from the geological report of Pha Lek deposit), note the close spatial relationship between granitoid, skarns and ore bodies.

Here we use zircon U-Pb dating of the granitic plutons and pyrite Re-Os dating of the skarn ores to constrain the timing of magmatism and mineralization, and zircon Hf isotopic compositions to trace the origin of magmas responsible for mineralization. A genetic model is then proposed for Late Carboniferous–Early Permian skarn Fe mineralization in the southwestern Truong Son belt.

1 GEOLOGICAL SETTING

The Truong Son belt, also known as the Annamitic Chain, is bounded by the Song Ma suture to the north and Tamky-Phuoc Son suture zone to the south. It connects with the Dien Bien Phu-Loei suture, which is generally considered to be the western margin of the Indochina Block, and spans a distance of > 500 km from northeastern Laos, through central Vietnam, to the South China Sea (Fig. 1a, Roger et al., 2014).

Precambrian to Cenozoic strata are distributed throughout the Truong Son belt, and contain mainly continental or marine sediments. Precambrian, dominantly Proterozoic, medium-high- grade metamorphosed volcanic-sedimentary rocks such as granulite, gneiss, marble, and schist crop out locally in the northwest, northeast, southeast, and south of the belt. Early Paleozoic (Cambrian, Ordovician, and Silurian) marine sedimentary carbonates and clastic rocks are distributed in the north and east, with low degrees of metamorphism. Late Paleozoic rocks are widely distributed, including mainly interbedded continental and marine sediments such as carbonates, sandstone, shale, and mudstone. Tuff, rhyolite, andesite, and other volcanic- sedimentary rocks occur in Permian strata. Mesozoic strata developed mainly in the rifting or shearing basins of the Truong Son belt, including interbedded continental and marine sediments or continental sediments, with uppermost evaporite layers. Cenozoic continental sediments of sandstone, mudstone, and siltstone are distributed along the Mekong River.

Four main episodes of Paleozoic–Early Mesozoic magmatism have been identified in the Truong Son belt. Ordovician– Silurian (470–420 Ma) calc-alkaline volcanic rocks and magnetite-series granodiorite and monzonite granite (Mao, 2012) on the southwestern edge of the belt are generally considered to be related to bidirectional subduction of the Tamky-Phuoc Son oceanic plate and related processes occurring beneath the Truong Son terrane in the north and Kontum Terrane in the south. Late Carboniferous–Early Permian (300–280 Ma) Ⅰ-type granite, granodiorite, and diorite were emplaced in the southern Truong Son area (Qian et al., 2017, 2016a, b, c ) in association with syn-subduction lithospheric extension in response to north-dipping subduction of the Tamky-Phuoc Son oceanic plate. Late Permian–Middle Triassic (270–245 Ma) S-type granodiorite and monzonitic granite distributed in the south of the Truong Son belt represent closure of the Tamky-Phuoc Son Ocean, while synchronous Ⅰ-type granite and volcanic rocks in the north represent south-dipping subduction of the Paleo-Tethys Song Ma oceanic plate beneath the Truong Son Terrane (Wang et al., 2017a; Tran et al., 2014; Yan et al., 2006). A small number of alkaline intrusions in the northern Truong Son Terrane have Middle–Late Triassic ages (245–200 Ma) and were formed during closure of the Song Ma Ocean and collision of the South China and Indochina blocks.

2 PHA LEK GEOLOGY

The Pha Lek District is situated in the southwestern segment of the Truong Son belt, near the western Phuoc Son-Tamky Suture (Fig. 1a). Sedimentary rocks in the district are dominated by Paleozoic Silurian–Middle Devonian systems (Fig. 1c). The Silurian System, mainly in the south of the district, comprises grey, neritic facies, calcareous argillaceous siltstone and minor interbedded sandstone and shale, with a thickness of > 450 m. It is unconformably overlain by the Middle–Upper Devonian System (Fig. 2), which is widely distributed in the district and comprises: a 300–450 m thick lower sequence of grey argillaceous limestone, calcareous sandstone, and calcareous shale; a 270–320 m thick middle sequence of white to grey dolomitic limestone and limestone, locally interbedded with dark grey calcareous shale; and an upper 120–360 m thick sequence of thin limestone, argillaceous limestone, and minor argillaceous shale beds. Country rocks include a sequence of dolomitic and argillaceous limestone, siltstone, and grey to dark grey argillaceous shale of the Middle Devonian System (Figs. 3a, 3b, 3c).

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Figure 2. Section 38 of exploration line (modified from the geological report of Pha Lek deposit). Alteration styles are mainly controlled by the lithology of the wallrock.
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Figure 3. Field (a)–(d), hand specimen (e)–(l) and microscopy (m)–(x) photos describing the magmatism, alteration and mineralization of the Pha Lek deposit. (a) Macroscopical photo showing the open pit of No. D4 ore body of Pha Lek deposit; (b) obvious marbleization and weathering of the dolomite wallrock; (c) less obvious color changing of the clastic wallrock; (d) intrusive contact between the granite and wallrock; (e) hand specimen of granodiorite (Zk8206); (f) Hand specimen of granite (Zk4207); (g) hand specimen showing the strong bleaching of dolomite altered into marble; (h) hand specimen of hornfels; (i) Polished drill core showing intense prograde skarn alteration, coarse grain of garnet are largely crystallized during the process; (j) polished drill core showing retrograde skarn alteration (serpentine, calcite) overprint the former prograde skarn (diopside), in association with sulfides; (k) outcrop of layered / massive magnetite-hematite ore bodies; (l) hand specimen of limonite; (m) thin section of granodiorite, cross-polarized light; (n) thin section of granite, cross-polarized light; (o) magnetite coexisted with quartz, weathered into hematite, reflected light; (p) magnetite overgrown by lateral coarse cubic pyrite, reflected light; (q) hematite showing colloidal texture under microscope, reflected light; (r) enhedral garnet intergrown by quartz and calcite veins, transmitted light; (s) garnet intergrown with diopside, altered into quartz and calcite, cross-polarized light; (t) columnar texture of tremolite, cross-polarized light; (u) glaucophane, epidote intergrown with magnetite, cross-polarized light; (v) epidote intergrown with pyrite, cross-polarized light; (w) talc vein crosscutting chlorite altered carbonatite, cross-polarized light; (x) serpentine altered carbonatite, cross-polarized light. Dol. Dolomite; Mar. marble; Grt. garnet; Py. pyrite; Di. diopside; Srp. serpentine; Cal. calcite; Mgt. magnetite; Hem. hematite; Lm. limonite; Hbl. hornblende; Fsp. feldspar; Qtz. quartz.

The Pha Lek District hosts several NW-SE- and NE-SW-striking faults that control the distribution of igneous rocks and their related skarn alteration. These faults were originally reverse faults that became strike-slip faults. E-W-striking faults occur as secondary faults and crosscut the former faults and mineralization.

3 INTRUSIONS AND RELATED ALTERATION AND MINERALIZATION

Middle Devonian strata in the Pha Lek District are intruded by several phases of intrusive rocks including granodiorite, granite, diorite, and diabase. The intrusions are irregular in shape or oriented in a NW-SE direction along the faults. Granodiorite and granite occur mainly in the northwest and southwest of the deposit, often in direct contact with carbonate wall-rocks (Fig. 3d). It has been observed in drill cores that the granodiorite and granite are related to skarn formation and mineralization (Zhu et al., 2014). As for the diabase, we recognized some intruded in the granodiorite in the geological map of the mining area, suggesting it is later than the granodiorite. The diorite and quartz monzonite are only recorded in the exploration report, we have not found any in the field or drill cores, so their relationship with others is unknown.

The dominant granodiorite intrusion (Figs. 3e, 3m) is exposed as a batholith covering two-thirds of the total intrusion area. It comprises 45%–50% plagioclase, 15%–20% K-feldspar, 20%–25% quartz, 8%–10% hornblende, and 2%–3% biotite. Accessory minerals include magnetite, zircon, rutile, apatite, and titanite. The quartz monzonite porphyry is similar to typical magnetite-series intrusions, with an average alkali (Na2O+K2O) content of 7.10 wt.% and Na2O/K2O ratios of ~0.43 (Mao, 2012).

Granite (Figs. 3f, 3n) crops out mainly as stocks in the central part of the mining area, and displays medium- to fine-grained granitic textures. It comprises mainly 30%–40% K-feldspar, 20%–30% plagioclase, 20%–30% quartz, and minor biotite and amphibole.

Some intrusions have undergone moderate lateral hydrothermal alteration, with weakly developed chlorite and sericite locally replacing primary minerals. Endoskarn has also been recognized as an assemblage of garnet, diopside, and epidote, often associated with minor disseminated pyrite and chalcopyrite.

The Pha Lek deposit contains three dominant ore blocks (blocks D, E, and F) and ten major economic orebodies that occur in layers, as tabular and lens-shaped bodies hosted in skarn and hornfels (Figs. 1c, 2). Orebodies are 1–110 m thick and 70– > 1 600 m long, strike NE-SW or N-S, dip at 25º–45º to the SW, and contain an average of ~65 wt.% FeT. Massive and disseminated ores containing magnetite, pyrrhotite, chalcopyrite, pyrite, and/or hematite, limonite, and malachite, occur within orebodies, skarn, and calc-silicate hornfels (Figs. 3k, 3l, 3o, 3q). Low-temperature quartz and calcite veins hosting chalcopyrite-pyrite±pyrrhotite locally crosscut massive and disseminated ores (Figs. 3j, 3o, 3p).

Much of the sedimentary unit has been metamorphosed to marbles, calc-silicate hornfels, and skarns (Figs. 3b, 3c, 3g, 3h, 3i, 3j). Ore-related skarn alteration is characterized by a typical calc-magnesian-silicate mineral assemblage, dominated by prograde garnet, diopside, and olivine, and strongly developed retrograde tremolite, actinolite, epidote, talc, serpentine, and chlorite (Figs. 3i, 3j, 3r3x).

Based on field and petrographic observations, we have recognized three stages of skarn formation and ore deposition, with each stage partially replacing earlier stages: (1) pre-ore stage (garnet-forsterite-pyroxene-humite-wollastonite), (2) syn-ore stage (magnetite-sulfides-epidote-tremolite-actinolite-ophiolite- chlorite-flogopite), and (3) post-ore stage (hematite-malachite- sericite-calcite-quartz). Details of each stage are described in Mao (2012) and Zhu et al. (2014)'s work.

4 ANALYTICAL METHODS 4.1 Zircon U-Pb Dating

U-Pb dating and trace element analyses for zircon of granodiorite (sample Zk8206-B02) and granite (sample Zk4207-B07) involved laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Key Laboratory for Magmatism and Giant Ore Deposits (KLMGOD), Xi'an Center, China Geological Survey, China. Analyses were carried out with a GeoLas Pro laser system, an Agilent 7700x ICP-MS, and He carrier and Ar make-up gases. The laser spot diameter was 32 μm, and each analysis involved 10 s of background (gas blank) and 40 s of sample data acquisition with an Agilent Chemstation. Integration of background and analyte signals, drift corrections, and quantitative calibrations were performed using Glitter v. 4.4. Instrumental conditions and data acquisition procedures were similar to those described by Li et al. (2015).

Zircon 91500 was used as an external standard for the U-Pb dating. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected by linear interpolation. Uncertainties in sample results are based on those of measured standard values. Concordia diagrams and weighted-mean age calculations were conducted with Isoplot/Ex v.3 (Ludwig, 2003). Reference material NIST610 was used for calibration of the zircon trace element analyses, with Si used as an internal standard. NIST reference compositions were from the GeoReM database (http://georem.mpch-mainz.gwdg.de/).

4.2 In situ Zircon Hf Isotopic Analyses

In situ zircon Hf isotopic analyses were performed using a Geolas Pro laser ablation system coupled to a Neptune multi- collector ICP-MS at KLMGOD. Instrumental conditions and data acquisition procedures were similar to those described by Hou et al. (2007). Hf isotope analyses were conducted at sites that partially overlapped the U-Pb dating sites. The laser spot diameter was 32 μm, and He carrier and Ar make-up gases were used. Zircon GJ-1 was used as a reference standard and yielded a weighted-mean 176Hf/177Hf ratio of 0.282 030±0.000 040 (2σ).

4.3 Pyrite Re-Os Isotopic Dating

Pyrite samples for Re-Os dating were collected from open pit D4. Pyrite is euhedral-subhedral, closely intergrown with magnetite, and crosscut by post-ore-stage calcite veins. As such, the pyrite ages constrain the timing of mineralization.

Re-Os isotopic compositions of pyrite separates were determined at the Key Re-Os Laboratory, Chinese Academy of Geological Sciences, Beijing, China. The separates were prepared by digestion of powdered samples, with 185Re and 190Os spikes, in aqua regia in Carius tubes at 230 ºC for ~24 h. Os was separated by micro-distillation, and Re was extracted from the residue in an acetone-NaOH solution. Procedural details are provided by Li et al.(2010, 2009). Re-Os concentrations and isotopic compositions were determined using a Thermo Fisher Scientific Triton Plus MS in negative ion detection mode (Li et al., 2015). Instrumental mass fractionation of Os was corrected by normalization of measured 192Os/188Os ratios to a value of 3.082 71 (Nier, 1937). Blank analyses yielded procedural blanks of ~3 pg Re and 0.5 pg Os. Sulfide Re-Os isotope reference material JCBY, from the Jin Chuan Cu-Ni deposit, was used for quality control purposes. Isochron ages were calculated using ISOPLOT v. 2.90 (Ludwig, 2001).

5 RESULTS 5.1 Zircon U-Pb Geochronology

Granodiorite and granite zircon grains are colorless, prismatic, 80–120 μm long, and have aspect ratios of ~2 : 3. In cathodoluminescence (CL) images, they are characterized by oscillatory zoned rims and dark inherited cores (Fig. 4). Granodiorite and granite zircons have Th contents of 113 ppm–3 716 ppm and 107 ppm–3 053 ppm, U contents of 155 ppm–2 631 ppm and 179 ppm–3 051 ppm, and Th/U ratios of 0.56–1.55 and 0.59–1.11, respectively. The generally high Th/U ratios and observed morphological features are consistent with zircons of magmatic origin (Hoskin and Black, 2000).

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Figure 4. Zircon cathodoluminescence (CL) images of the two granitic intrusions from the Pha Lek deposit.

U-Pb dating results are presented in Table 1. Nine analyses of 20 zircon grains from granodioritic sample Zk8206-B02 yielded 206Pb/238U ages of 290–286 Ma, with a weighted-mean age of 288.2±1.3 Ma (MSWD=2.7; Fig. 5a). Fourteen analyses of 24 grains from granitic sample Zk4207-B07 yielded 206Pb/238U ages of 289–280 Ma, with a weighted-mean age of 284.9±1.2 Ma (MSWD=0.008 9; Fig. 5b).

Table 1 LA-ICP-MS zircon U-Pb ages of the Pha Lek felsic plutons
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Figure 5. (a) U-Pb Concordia and weighted mean ages for the granodiorite (Zk8206) from Pha Lek deposit; (b) U-Pb Concordia and weighted mean ages for the granite (Zk4207) from Pha Lek deposit; (c) εHf(t) vs. t diagram showing the evolution of Hf isotopic compositions of the two granitic intrusions from the Pha Lek deposit; (d) pyrite Re-Os age of the Pha Lek deposit.
5.2 Zircon Hf Isotopic Compositions

Hf isotopic analyses of selected zircon grains are provided in Table 2. Zircons with U-Pb ages of ca. 288 Ma from granodiorite sample Zk8206-B02 have initial 176Hf/177Hf ratios of 0.282 679–0.282 906, with εHf(t) values of 2.9–10.6 and TDM2 ages of 1 129–635 Ma (except for spot Zk8206-B02-14, with 176Hf/177Hf=0.283 224, εHf(t)=21.1, and a TDM2 age of 48 Ma). Zircons from sample Zk4207-B07, with crystallization ages of ca. 285 Ma, have initial 176Hf/177Hf ratios of 0.282 598– 0.282 914, with εHf(t) values of -0.07–11.16 and TDM2 ages of 1 313– 596 Ma (Fig. 5c).

Table 2 LA-ICP-MS Lu-Hf isotope compositions of the Pha Lek felsic plutons
5.3 Chalcopyrite Re-Os Geochronology

Concentrations of Re and Os and 187Re/188Os and 187Os/188Os ratios are presented in Table 3. Common Re and 187Re contents are relatively high at 6.32–43.32 and 6.33–27.24 ng/g, respectively. Common Os and 187Os contents are generally more than two orders of magnitude lower, at 0.002 2–0.020 8 and 0.019 2–0.136 1 ng/g, respectively. This leads to high 187Re/188Os (4 880–19 690), Re/Os (1 010–3 316), 187Os/188Os (24.2–94.4), and 187Os/Os (3.2–12.2) ratios, indicating a low-level, highly radiogenic pyrite source. The 187Re/188Os-187Os/188Os plot (Fig. 5d), which eliminates the influence of non-radiogenic 187Os, gives a Re-Os isochron pyrite age of 287±17 Ma, compared to single-mineral model ages of 300–286 Ma (Morelli et al., 2005).

Table 3 Pyrite Re-Os composition and model age of the Pha Lek deposit
6 DISCUSSION 6.1 Magmatic and Metallogenic Epoch

Previous geochronological studies of intrusions in the Truong Son belt (Shi et al., 2015; Zaw et al., 2014) have identified four phases of magmatism related to its tectonic evolution, including Late Carboniferous–Early Permian (300–280 Ma) felsic magmatism in the southern section of the belt resulting from SW-dipping subduction of the Song Ma oceanic slab, which induced porphyry-skarn-related Fe mineralization.

However, a reliable geochronological link between magmatism and mineralization has not previously been established. Here, zircon U-Pb ages of granodiorite (288±1.3 Ma) and granite (285±1.2 Ma), and the pyrite Re-Os age (287±17 Ma) of the main mineralization stage, are consistent and contemporaneous with Late Carboniferous–Early Permian flesic magmatism. Furthermore, mineralization is spatially close to the granodiorite and granite. The zoning of the alteration shows obvious rules from endoskarn, approximate exoskarn, to distal exoskarn. All these factors strongly support the close link between granitic magmatism and skarn mineralization in the Pha Lek deposit.

6.2 Genetic Model for Early Permian Magmatism and Mineralization

Zircon Hf isotopic compositions provide valuable information on magma sources and degrees of magma mixing during the formation of granitic rocks (Wang et al., 2014; Wu et al., 2007; Griffin et al., 2002). Geochemical studies to date have indicated that granitic intrusions in the Pha Lek District comprise Ⅰ-type granite, formed within a sub-arc environment. There has, however, been a lack of Hf isotopic data, which has limited studies of magma sources and their evolution.

Our results for granodiorite (288 Ma) and granite (285 Ma) of the Pha Lek deposit yielded relatively depleted Hf isotopic compositions. Zircon εHf(t) values are mostly in the range of 2.9–11.2, indicating the granitoids may be derived from partial melting of depleted mafic crust or magma mixing. Their TDM2 ages are relatively young (< 1.0 Ga), indicating origins related to melting of depleted basaltic oceanic crust, primitive underplating basaltic continental crust, or crust-mantle magma mixing. Previous studies (Qian et al., 2015; Zhu H P et al., 2014; Mao, 2012; Zhu D C et al., 2009) have reported relatively high Y and Yb, and extremely low Sr contents for granitic intrusions of the Pha Lek deposit, obviously different from typical adakites (Hoa et al., 2008). Furthermore, these granitic intrusions have high K2O and low Na2O contents, whereas subducted oceanic crust normally has Na2O/K2O ratios of > 1. We therefore consider that a model involving melting of mafic continental lower crust and mixing with depleted magma applies for the formation of the Pha Lek granitic intrusions.

The Pha Lek intrusions exhibit Hf isotopic compositions different from those of Triassic (256–234 Ma; Wang et al., 2016) granitic intrusions in the central Truong Son belt, which have εHf(t) values of -3 to -8 and TDM2 ages of 2.0–1.5 Ga, indicating they are derived from melts of ancient continental crust. The Truong Son belt had thus already entered a syn- or post-collisional stage during the Triassic, with melting of continental crust producing large amounts of S-type magma.

Combined with continental crust geochemical characteristics (e.g., high K2O and low Na2O contents, high Y and Yb with low Sr contents) and depleted Hf isotopic compositions, we propose that the parent magma of Pha Lek intrusions is derived from partial melting of mafic continental lower crust and subsequently mixing with depleted basaltic magma. Numerous studies show that the Late Carboniferous–Triassic magmatism throughout the Truong Son belt was closely related to subduction and closure of the Song Ma Ocean, and related collision between the South China and Indochina blocks. Our model (Fig. 6) suggests that during the Late Carboniferous– Early Permian, the subduction of Song Ma oceanic crust brought volatile-enriched fluids into the mantle wedge, inducing the LILE- and LREE-rich fluid metasomatism. Then, the mantle wedge underwent partial melting to form the depleted mafic magma. This magma subsequently underplated the lower crust, and produced partial melts of felsic composition. Inevitably, the interaction occurred between this hot, hydrous depleted mafic magma from the subduction zone and felsic crustal partial melts, just like the MASH model (melting-assimilation- storage-homogenization, i.e., Schwindinger and Weinberg, 2017; Richards, 2011). Because of intense crust-mantle magma mixing, the hybrid magmas were enriched in metals such as Fe, Cu, and Au, and volatiles such as H2O and CO2. When this enriched magma was transported into the upper crust along deep faults, it contacted carbonate rocks, where the resulting exchange metamorphosed wall-rocks into skarns, hornfels, and marbles, with precipitation of Fe, Cu, and Au (Fig. 6).

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Figure 6. A genetic model for the Pha Lek deposit. (a) Crust-mantle magma mixing process at the lower continental crust; (b) magmatic-hydrothermal alteration and mineralization of the Pha Lek deposit, note the different styles of alteration due to the difference of locations and lithology of the wall rocks; (c) a subduction-related mineralization model for the Pha Lek deposit.
6.3 Volcanism-Related Mineralization

Post-collisional extension, most likely due to break-off of oceanic crust and delamination of lower crust, occurred in the region during the Late Triassic. The upwelling of deep mantle magma caused crust-mantle-derived magma mixing, melting, and metamorphism of continental crust. Widely distributed intrusions of I- and S-type magma, and basaltic and rhyolitic bimodal volcanic rocks, may have caused overprinting of the Fe-Cu deposits.

Some Fe ore in the Pha Lek deposit displays structures and textures similar to volcanic iron ore (Fig. 3q), with some volcanic hematite even hosting magnetite clasts, suggesting overprinting by volcanic mineralization. However, attempts to obtain volcanic zircons from those ores have failed, and further geochemical evidence is needed to test this hypothesis.

7 CONCLUSIONS

(1) The two granitoids (granodiorite Zk8206 and granite Zk4207) from the Pha Lek District were emplaced at 288±1.3 and 285±1.2 Ma, respectively. Pyrite Re-Os ages (287±17 Ma) are consistent with the age of the granitic intrusions, indicating a spatio-temporal relationship between Late Carboniferous– Early Permian silicic magmatism and mineralization in the Pha Lek deposit.

(2) Zircon Hf isotopic data (εHf(t)=2.9–11.2), together with results of previous studies, indicate that the Pha Lek granitic intrusions were formed through melting of mafic continental lower crust and mixing with depleted mafic magma during SW-dipping subduction of the Song Ma oceanic slab under the Indochina Block.

(3) Our results support a Late Carboniferous–Early Permian subduction-related skarn mineralization model for the Pha Lek Fe deposit, although more information is needed to verify the hypothesis of overprinting by volcanic mineralization during Late Triassic post-collisional extension.

ACKNOWLEDGMENTS

This research was supported financially by the National Natural Science Foundation of China (Nos. 41402074 and 41502074), Applied Fundamental Research Funding of Sichuan Province, China (No. 2015JY0055), and the National Geological Survey Foundation of China (No. 121201010000150013). The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0864-7.


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