J. Cent. South Univ. (2016) 23: 1071-1084

DOI: 10.1007/s11771-016-0357-8

Re-Os isotopic data for molybdenum from Hejiangkou tungsten and tin polymetallic deposit in Chenzhou and its geologica significance

LIU De-bo(刘德波)^{1, 2}, YANG Liu(杨柳)³, DENG Xiang-wei(邓湘伟)^{1, 2}, DAI Xue-ling(戴雪玲)⁴,

WANG Xiong-jun(王雄军)^{1, 2}, CHONG khai yuen⁵, DU Gao-feng(杜高峰)⁶, WEI He-ping(魏和平)¹

1. Key Laboratory of Non-ferrous Metalloginic Prediction of Ministry of Education(Central South University), Changsha 410083, China;

2. Scholl of Geosciences and Info-Physics, Central South University, Changsha 410083, China;

3. Department of Resources Engineering, Hunan Engineering Polytechnic, Changsha 410000, China;

4. Hunan Provincial Geological Exploration Bureau, Changsha 410016, China;

5. Department of Geology, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia;

6. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry,Chinese Academy of Sciences, Guiyang 550002, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016                              

Abstract: Hejiangkou W-Sn-polymetallic deposit is a newly found deposit in Xitian ore field, one of the important and large scale W-Sn-polymetallic ore fields in the middle segment of Nanling metallogenic zone. Re-Os isotope dating was used on three molybdenite samples from Hejiangkou deposit to determine the ore forming period. The result is (224.9±2.6)Ma-(225±3.1)Ma and isochron age is (225.5±3.6)Ma. The field geological observations, geochronological data and optical petrography indicated that Hejiangkou deposit underwent multi-period of superimposed mineralization. It can be differentiated into three periods composed of six mineralization stages. The first period is the initial period for hydrothermal metasomatism and metal element enrichment during Indosinian Epoch. Further enrichment, strong brittle fracturing and hydrothermal metasomatism, remobilization and superimposition happened in the second period, during early Yanshanian. It is the major mineralization period of Hejiangkou deposit and can be subdivided into four mineralization stages, namely the skarn stage, oxide stage, high-temperature sulfide stage and low-temperature sulfide stage. And the third period is the mineralization period of a porphyry-skarn system related to the emplacement of the granite porphyry dyke. As minerogenic epoch of Hejiangkou deposit is similar with Hehuaping deposit, they show the possibility of Indosinian mineralization event in Nanling metallogenic zone. It can be an important perspective in any future mineral exploration in the same metallogenic zone.

Key words: molybdenite Rei-Os isotope age; ore-forming stage; Hejiangkou deposit; Xitian ore field

1 Introduction

Multi-period superimposed mineralization is found in certain ore deposits on the world and has been an important perspective in mineral deposit geology. Many famous doctrines and hypothesis were proposed. LOVERING [1] proposed the conception of deposits with a binary genesis; TU [2] proposed that mineralization could be superimposed and reconstructed; A polygenetic mineralization is model by CHEN [3]; XU and ZHU [4] reported a type of copper deposits with hydrothermal overprint; A hypothesis of superimposed mineralizing system and a polygenetic deposit model were proposed by ZHAI [5] and PERKINS [6]. More multi-period superimposition ore deposits were discovered successively. Examples including Baiyun Obo Fe-REE deposit, Dachang Sn-polymetallic deposit, Tongling Cu-polymetallic deposit, Jiaodong gold deposit, Mt. Isa polymetallic deposit in Australia [6], Au-U deposits in South Africa [7]. Multi-period structural and magma activities have always been considered one of important conditions of multi-period superimposed mineralization. Xitian ore field is located in the middle segment of Qinzhou-Hangzhou suture zone between Cathaysia plate and Yangtze plate. The suture experienced several opening and closure events, collision between continental plates and subduction of Pacific plate. These complicated structural histories provide a strong support for multi-period superimposed mineralization of suture zone.

Hejiangkou deposit includes two ore blocks, namely Hejiangkou and Longshan sections, of which, Hejiangkou ore section has been recently discovered in Xitian ore field as a hidden, medium-sized W-Sn- Cu-Zn polymetallic deposit (Fig. 1). Previous studies explored the geological characteristics of the deposits and granites and the tectonic setting of Xitian ore field [8-20]. Although some scholars noticed the multi-stage mineralization in the area, none of them has conducted systematic studies on any of those deposits. Contemporary studies in mineral deposit have advanced from qualitative descriptions of geological characteristics into the detailed quantitative or semi-quantitative research of ore-forming processes and mineralization evolution [21]. Based on detailed field geology and petrological studies of ore minerals and molybdenite Re-Os isotopic analysis, in combination with the results from previous studies, the authors conducted a systematic investigation of the ore-forming processes and proposed a multi-period inherited mineralization model. With the discovery of Indosinian ore-forming geochronology data,the possibility of Indosinian mineralization event in Nanling metallogenic belt is indicated that it can be a new perspective for future mineral exploration.

Fig. 1 Geological map of Xitian area, Eastern Hunan, China

2 Geological background

Xitian ore field is located at the junction of Chaling-Chenzhou-Linwu Fault (NE striking) and Xitian Fault (NW striking), and is a crucial part of the central segment of Nanling metallogenic zone within Qin Zhou-Hang Zhou suture zone. Similar to the other ore fields, such as Shizhuyuan, Huangshaping, Furong, Hehuaping, Xianghualing, Yaogangxian, Da’ao and Guposhan, it is a large scale and granite associated W-Sn-polymetallic ore field (Fig. 1).

The outcrops in Xitian ore field are dominated by the Devonian and Carboniferous neritic carbonate rocks, clastic rocks and clay rocks interlayered with littoral-marsh facies coal-bearing strata. Folds, faults and joints are strongly developed in the area due to the large-scale Indosinian-Yanshanian tectonic movements, providing important emplacement space for the later igneous rocks and also the transportation and storage of ore-forming fluids. Particularly Longshang syncline, Shaiheling syncline and Heshuxia syncline; sub-structures of Indosinian-aged 30°-50° striking synclinorium are controlling the occurrence of large scale deposits in this area. While NEE-strike, near NS-strike and NW-strike three sets of faults are playing important roles in ore mineralization. The Xitian granite complex outcrop occupied an area of 240 km²; it is dumbbell-shaped and distributed in NNW direction.

3 Geological characteristics of Hejiangkou deposit

3.1 Geology of mining area

Hejiangkou deposit is located in the northwestern limb of Longshang synclinorium and represents a monoclinic structure. The strike and dip angle of the strata are of 325° and 20°-30° SW, respectively. The outcropped lithologies in the area majorly are: Xikuangshan formation (D₃x) which is dominated by medium- to thick-bedded mica-bearing quartz sandstone interlayered with mudstone and banded limestone; Shetianqiao formation (D₃S) which is dominated by medium- to thick-bedded mica-bearing quartz sandstone interlayered with sandy mudstone, quartzite and siltstone; Qiziqiao formation (D₂q) which is composed of medium- to thick-bedded calcareous dolomite, marl, limestone interlayered with mudstone and sandy shale. Qiziqiao formation also is the most important ore-bearing stratum and it is later metamorphosed to a series of marble, marbleized limestone, diopside-bearing skarn and garnet-bearing skarn interlayered with calcareous- siliceous hornfels due to contact metamorphism and metasomatism.

According to the zircon SHRIMP U-Pb dating, muscovite ⁴⁰Ar-³⁹Ar dating and whole-rock Rb-Sr isochron dating conducted by LIU et al [9], the rocks in Xitian ore field were formed in three different periods. The first period was represented by the major body of Xitian granite complex that formed during Indosinian Epoch. The second period corresponds to the supplementary granite that formed during early Yanshanian Epoch. While the third period is represented by the veins formed in late Yanshanian Epoch. As a result of lithologies comparison and field observation, those three different periods of felsic rocks are observed distributed in Hejiangkou deposit.

The first-period granite occurred as a deep-seated hidden batholith (Figs. 2, 3 and 4) and regarded as the southwestern extension of Xitian granite batholith, which is dominated by grey medium- to coarse-grained biotite monzonitic granite. The second-period granite is represented by the stocks and apophyses of grey fine- grained two-mica granite and part of it outcropped on the periphery of the first-period granite batholith, where most of the second-period granite is beneath of the first-period granite. These two periods of granite can be differentiated not only through lithology but mineralization as well. The second-period granite intrusion into the limestone of Qiziqiao formation causing the latter strongly skarnized and mineralized; it exhibits strata-bound scheelite mineralization (Fig. 5(a)), Zn-Cu-Sn skarn-type mineralization and greisenization. The third-period granite is predominantly grey-pink granite porphyry dykes (actual rock-forming age unknown, Fig. 4).

A large-scale arc-shaped fault (F1) developed in the north of the deposit and it is a compressive shear fault. It started from somewhere outside the map, stretched through Xiaowuli and Mawangshan on the map and then extended out of the map in NE direction. The total length of F1 exceeded 2 km and a representing 1-4.5 m-width silicified fractured zone outcropped on the surface. The western segment of the fault has a NWW strike and dipping 75° towards SSW. While the middle segment shown a near EW strike and the eastern segment largely has a NE strike and dipping ~35° towards SE. A latter NEE-striking sinistral fault (F2) which has a strike of 70° and dipping 65°-75° towards NW crosscuts the western segment of F1. F2 outcrop is about 3 km long and 5-8 m wide on the surface.

Fig. 2 Corresponding morphology of granite batholith and orebodies in Hejiangkou segment

Fig. 3 Cross-section of No. 10A segment in Longshang mine

Fig. 4 Cross-section of No.3 segment in Hejiangkou mine

3.2 Orebody characteristics

3.2.1 Orebody types

1) Skarn-type orebodies

The skarn-type orebody is the largest orebody in the area and commonly occurs in the outer contact zones between the felsic intrusive rocks and the marble of the Qiziqiao formation. Based on the lithology of the associated felsic intrusion, three subtypes of mineralization are distinguished.

The first is the skarn-type Cu-Sn mineralization (with minor W mineralization) in the contact zone between the grey medium- to coarse- grained porphyritic granite and Qiziqiao formation The orebodies are discontinuous, thin-layered and lens-shaped (Figs. 5(b) and (c)), characterized by low Cu-Sn grades and enrichment of As (local concentrated of arsenopyrite, Fig. 5(d)). but locally contained disseminated chalcopyrite is lower than required cut-off grades. However, Fig. 2 shows pocket- like and lenticular-shaped high grade orebodies in the convex parts of the granite. The size and grade of the orebodies increased where the skarn contacted with the NEE-striking fault or where the skarn has been cross-cut by later dykes (apophyses) and quartz veins, indicating the superimpose and inheritance mineralization features (Figs. 3 and 6).

Fig. 5 Photographs of typical textures structures of deposit:

The second is the W associated with minor Sn mineralization that occurred in the skarn-type orebodies in the contact zone between the two-mica granite and Qiziqiao formation. The orebodies are rich in fluorite and local fluorite concentration is minable. The skarn-type Zn mineralization occurred away from the contact zone and its associated Cu mineralization is distributed along with the greisen orebodies (Figs. 5(e) and (f)).

The third is the skarn-type tungsten and tin orebodies occurred in the foot wall of the granite porphyry dyke. This type is included into the granophyre type orebodies for this study (Fig. 4).

2) Greisen-quartz (calcite, fluorite) vein type orebodies

This type of orebodies can be divided into two subtypes. The one is quartz veins within the granite or partly cutting into the wall rock. The ore bearing quartz veins contained wolframite and scheelite (with minor chalcopyrite and cassiterite). The width of single vein and vein-rich zone vary from 0.1 to 0.8 m and from 1 to 3 m, respectively. The quartz veins are 50 to 200 min length, mostly NEE-EW striking and dipping 60°-70° towards north. The other refers to the wolframite-, scheelite-, cassiterite- and/or chalcopyrite-bearing quartz vein within the strata. The width of single vein and vein-rich zone vary from 0.1 to 0.5 m and from 3 to 8 m, respectively. The veins distribution is controlled by the tensional joints in the hanging wall and it is NWW-striking and dipping 40°-50° towards SSW. Single vein length is roughly 100 to 200 m (Figs. 5(g)-(i)). The greisen-type orebodies are commonly associated with quartz-vein type or skarn-type orebodies. In the middle segment of tunnel 228 the wolframite and cassiterite-bearing quartz veins are present in the greisen dykes. The greisen dykes disappeared and only quartz veins are present in the top part. This observation is consistent with the magma- liquid immiscibility model by LIN et al [22] and is a direct evidence supporting the hypothesis that late magmatic fluids played a crucial role in mineralization of the area. The greisen-type orebodies are commonly found in the contact zone between later granite and Qiziqiao formation, and generally associcated with the skarn-type orebodies. These steep-sloped Sn-W-bearing greisen-type orebodies mainly occurred in the inner contact zone or the granite. They’re characterized by variable thickness and grades but relatively consistent length (Figs. 4, 5(e) and 6).

3) Granite porphyry dike type

A 335°-strike and 60°-70° SW-dipping grey-pink granite porphyry dyke outcrop is observed in the study area. The controlled dimension of dyke is about 2 km long and reaches a maximum depth of 850 m. A sinistral NEE-striking fault cross-cuts the dyke. The major alterations observed on the dyke are potassic alteration and silicification. Tin and tungsten mineralizations are distributed in the upper and lower parts, respectively. In the upper part of the dyke, the orebodies have higher grade. Porphyry-type mineralization occurred throughout the dyke and locally formed high-grade yet thick orebodies. Skarn-type W-Sn orebodies were formed in the contact zone between the dyke and Qiziqiao formation. The known maximum thickness of the orebodies is 7 m and average thickness of orebodies is around 2.7 m. (Fig. 5(j)).

4) Structurally altered rock type

The structural belt is NEE-striking (70°) and dipping 70° NW. Lens-shaped orebodies are distributed in the structural belt. As this type of orebodies is low-grade orebodies, hence detailed description is not given in this study. Nevertheless, the orebodies’ grade increases as the fault transects the skarn-type orebodies (Fig. 5(k)).

3.2.2 Cross-cutting relationships of different types of ore bodies

According to the observations on drillhole No. 301 and the stope in the middle segment of level No. 320 in the adit in Longshang, W-Sn-bearing quartz vein-type orebodies cross-cut and superimpose the skarn-type Cu-Sn-W orebodies and extend downwards into the granite. In the middle segment of level No. 288 in the adit in Longshang, the greisen-type orebodies overprint the thin-layered skarn-type orebodies. Sub-angular breccias (with variable sizes) of the early skarn are cemented by siliceous infillings in the fractural altered belt (Fig. 5(l)). As the later granite was intruded into the main granite batholiths, the ore grades of orebodies in the contact zone have increased (Fig. 6). The NEE- striking fracturs cross all the middle segments of Longshang district. The size and grade of the orebodies increase in and above the contact zones between the granite batholith and host rock. In the middle segment of tunnel 156 there are fluorite-bearing scheelite orebodies occurring above the late granite. In drillhole No. 10 of Hejiangkou district a late-stage fine-grained granite dike (apophysis?) cross-cuts Xitian supplementary granite and greisen-type orebodies form within the dyke. According to the drillhole observations, the thickness and grade of the orebodies increase near the contact zone and the late mineralized granite porphyry veins cross-cut the apophyses of the main-stage and early granites. In Hejiangkou district the drillholes of line 3 revealed that the late granite porphyry veins transected the main body of the granite batholith. The porphyry-type W-Sn orebodies formed in favorable locations within the veins for mineralization while the skarn-type W-Sn orebodies formed in the contact zone between the veins and host rock. The above-mentioned observations indicate: the skarn-type W-Sn-Cu mineralization in the main body of Indosinian granite represents the first period; the skarn- type W-Sn mineralization in the contact zones among the greisen-quartz veins, supplementary granite and strata represents the second period; the porphyry-type, skarn-type W-Sn mineralization and the greisen-type W-Sn mineralization belong to the third mineralization stage. The porphyry-type and skarn-type W-Sn mineralization are caused by the formation of granite porphyry, while the greisen-type W-Sn mineralization is caused by the alteration of the fine-grained granite veins.

Fig. 6 Plane-view map of segment 288 in Longshang mine

3.3 Ore textures

The mineralogy of Hejiangkou deposit is complicated and the major ore minerals included scheelite, cassiterite, chalcopyrite, sphalerite, pyrite, wolframite, arsenopyrite, pyrrhotite and magnetite. Molybdenite, bismuthinite and chiviatite are presented as minor ore minerals. The major gangue minerals included quartz, calcite, garnet, diopside, epidote, wollastonite, muscovite, fluorite, tourmaline, and spinel. The ore minerals are mainly anhedral to subhedral and granular. Replacement texture, interstitial texture, metasomatic relic texture and poikilitic texture are common. The ore structure in the deposit are predominantly disseminated and massive structures. Banded structure is also present. Emulsion droplet-like structure locally occurs. The detailed descriptions of all important minerals are given below.

Scheelite is the most abundant ore mineral in this deposit and was formed at all three different stages. The first generation of scheelite is commonly anhedral to subhedral, relatively medium-grained (~1 mm) and included early wolframinte and cassiterite (Fig. 7(a)). The first generation of scheelite is not abundant compared with the other two generations and is generally disseminated in the rocks. The second generation of scheelite is commonly present in the veins crosscutting the early chalcopyrite-rich skarn (Fig. 7(b)) or is strata- bound, locally forming marble-type fluorite-scheelite orebodies (Fig. 5(a)). This generation of scheelite is coarse-grained (up to cm) and abundant and has disseminated structure. The third generation of scheelite which commonly occurs in the granite porphyry dykes or the skarn in the foot wall of the granite porphyry dykes is generally 0.1 to 0.5 mm in grain size and is either disseminated or massive structured.

1) Cassiterite and wood-tin. Cassiterite (including wood-tin) is the second most abundant ore mineral and was formed at all three stages. The first generation of cassiterite is disseminated, subhedral to euhedral and wedge-shaped, commonly occurs in the thin-layered skarn, associated with chalcopyrite. The grain size of the first generation of cassiterite is < 0.1 mm. The formation of the first generation of cassiterite was later than wolframite but earlier than scheelite (Fig. 7(c)). Majority of the first generation of cassiterite is fractured and replaced and cross-cut by pyrite and chalcopyrite, due to later deformation. Compared with the first generation of cassiterite, the second generation of cassiterite hand specimens exhibited lighter color and is light brown to brown colors. The second generation of cassiterite mainly occurs in the greisen and skarn and quartz veins on the flanks as either anhedral granular single crystals or aggregates. The second generation of cassiterite is either disseminated or veinlet structures. The grain size of the second generation is variable, ranging from > 0.1 mm up to 1-2 cm. The second generation of casserite is commonly associated with wood-tin (Fig. 7(d)).

2) Chalcopyrite. It was Formed at two stages. Majority of chalcopyrite was formed at the first stage and is anhedral and densely disseminated to semi-massive structured. The grain size varies from 0.1 mm to 1 mm. Chalcopyrite mainly occurs in the contact zone between the main body granite batholith and host rock and is disseminated in the skarn-type chalcopyrite orebodies (cross section of line 10A in Fig. 2). Optical microscopic observation revealed that chalcopyrite cross-cuts and replaces wolframite, cassiterite, scheelite, tennantite and arsenopyrite (Fig. 7(e)). The second generation of chalcopyrite which is relatively rare and fine-grained (0.01-0.1 mm) is commonly anhedral, disseminated or present in veinlets. From microscope observation, the second generation of chalcopyrite transects and replaces early sphalerite (Fig. 7(f)).

Fig. 7 Typica ore textures and structures in Hejiangkou deposit:

3) Tennantite. It was only presented in the contact zone between the main granite body and host rock. The second generation has not been observed. Tennantite was formed earlier than chalcopyrite and is commonly included by chalcopyrite (Fig. 7(g)). Tennantite is generally anheral and the grain size varies from 0.05 to 0.5 mm.

4) Sphalerite. Two generations are present. The first generation of sphalerite is relatively clean, anhedral and disseminated. The first generation of sphalerite is replaced by late chalcopyrite (Fig. 7(f)). The second generation commonly contains chalcopyrite laminae formed due to exsolution. The majority of sphalerite crystals have a chalcocite rim. The second stage is the main precipitation stage for sphalerite. The thickness of the sphalerite orebodies can reach as deep as more than 10 m in the skarn near the greisen. The average grading is 10.5% (Fig. 7(f)).

5) Wolframite. Two generations occur in the deposit. The first generation of wolframite represents the earliest mineralization of the area and has been replaced by later cassiterite, scheelite and pyrite. The first generation of wolframite which is commonly fine- grained, anhedral and disseminated. It is fractured and cross-cut by later quartz veins (Fig. 7(i)). The second generation of wolframite occurs as tabular crystals or aggregates. The grain size of this generation generally ranged between 1 to 4 mm (up to 1 cm in some cases). This generation occurs in the skarn within the contact zones among greisen veins, quartz veins and two-mica granite and host rock. The optical properties of the second generation of wolframite is compositionally close to the end-member of ferberite (FeO>MnO). Wolframite is commonly associated with cassiterite, tourmaline, spinel and beryl and replaced early chalcopyrite (Fig. 7(e)).

6) Pyrite. Pyrite is one of the most abundant minerals and was formed in all ore-forming stages. Early pyrite is commonly replaced by chalcopyrite (Fig. 7(h)), whereas later pyrite is replaced early chalcopyrite and wolframinte. The latest pyrite is generally euhedral and replaces early chalcopyrite, pyrite, wolframite and scheelite (Fig. 7(j)).

7) Magnetite. Granular mainly occurs in the contact zones between wall rocks with the main granite batholith. It is surrounded and replaced by later chalcopyrite and pyrite.

8) Pyrrhotite. It only occurs in the contact zone or superimposed zone between early Yanshanian two-mica granite and host rock and predominantly formed during the second stage, pyrrhotite commonly replaced earlier chalcopyrite and pyrite. Generally as anhedral aggregates, the grain size vary from 0.01 to 0.1 mm (Fig. 7(k)).

9) Arsenopyrite. It either occurs in the contact zone between the main granite batholith and host rock, or in the quartz veins; It is associated with wolframite and cassiterite. It is replaced by later sphalerite, pyrite and chalcopyrite (Fig. 7(k)). The arsenopyrite is subhedral with a grain size ranging from 0.1 mm to 1 cm. Arsenopyrite is closely related to the quartz veins near the main contact zone.

10) Fluorite. Colors vary from white to light green to purple. It shows vitreous luster and is commonly translucent. Most fluorite is subhedral to euhedral, granular and occurs as veins between layers of marble in the skarn. Fluorite also occurs in the greisen and forms fluorite-rich greisen (Fig. 7(l)). Local fluorite veins cross-cut the strata. Fluorite is generally associated with scheelite or is included by scheelite. The formation of fluorite is closely related to the emplacement of the early Yanshanian two-mica granite during the second ore-forming period.

11) Molybdenite. It is very rare and only is observed in the granite batholith exposed in the middle segments of 320 and 228 tunnels (where the samples for Re-Os dating were collected). The dating results indicate that molybdenite was only formed in one stage and replaced by later chalcopyrite, sphalerite and pyrite. Also, later quartz veinlets have been observed to cross-cut the molybdenite ores (Figs. 7(m) and (n)).

12) Bismuthinite. It only occurred in the granite porphyry dyke and disseminated through the rock and replaced by later pyrite and sphalerite. The abundance of this mineral is low.

13) Chiviatite. It was formed in the second stage and commonly subhedral and scaly and in disseminated structure. It is commonly associated with euhedral cassiterite or sphalerite with emulsion droplet structure and distributed along quartz grains (Fig. 7(o)).

3.4 Ore-forming stages and paregenetic sequence

Similar to other W-Sn deposits in Nanling area [42], the rock-forming and ore-forming processes of the study area are continuous. The mineralization types in Hejiangkou deposit mainly include the hydrothermal mineralization in the skarn, greisen-quartz veins and granite porphyry veins related to the emplacement of felsic magma [20]. According to the field observations on the textures of the ore and veins exposed in the outcrops, drillholes and tunnels, petrographic analysis and Re-Os isotope dating results (see texts below), the ore-forming process of the primary mineralization can be divided into three periods (Fig. 8), (six stages in total). The first period was the initial mineralization period related to hydrothermal alteration in Indosinian (late Triassic, 225 Ma). The second period was early Yanshanian enrichment period (middle late Jurassic, 150 Ma with strong brittle fractures and hydrothermal metasomatism, remobilization and superimposition. The third period was the mineralization period of the porphyry-skarn system related to the emplacement of the granite porphyry veins.

Period (I): Due to the closure and collision of Tethys Ocean in the Indosinian epoch, this area was mainly in a compressional tectonic regime and S-type granites were formed in this period. According to previous studies [8, 13], the ages (1704.5 Ma, (1648±25) Ma, 1017.0 Ma, and 460.7 Ma) of the inherited zircon obtained from Indosinian granites in Xitian area indicated that the basement strata of Pre-Nanhuaian oceanic crust, ophiolite complex, intrusive igneous rocks and sea-floor sediments had experienced re-melting. The potential source for iron, sulfides, tungsten and tin is present in many horizons of those rocks [23]. During the emplacement of the S-type granites, fractures and faults were not fully developed. Therefore, no large-scale mineralization occurred during the first period [24]. Thin layered and low-graded skarn-type orebodies (or mineralization) were formed mainly in the contact zones between Xitian granite batholith and host rock. Only in the open space caused by structural stress (convex locations), lenticular or pocket-shaped and high-graded orebodies were formed (Fig. 2). The mineralization of this period is represented by chalcopyrite, pyrite, tin and tungsten-rich ores and molybdenite. The skarn-type Sn-W mineralization (orebodies) formed in this period was the result of the late remobilization of ore-forming material during early Yanshanian, which indicatded the inherited nature of the mineralization started in this area.

Fig. 8 Mineralization periods and paragenetic sequence of mineral assemblage in Hejiangkou deposit

Period (II): Due to the detachment and slab window opening in Nanling area during early Yanshanian (middle late Jurassic, 150 Ma), material from the asthenosphere was fluxed into the upper crust, resulting in South China basin-range tectonics and the formation of large-scale metallogenic district [25]. This period is also the main ore-forming period of Xitian metallogenic district, as indicated by the dating results by [9, 11-12, 26]. During this period, the upward movement and underplating of mantle-derived magma corresponded to the subduction of Pacific plate, a large number of granites (the supplementary granites) were formed through the crust-mantle interaction. In addition, a large amount of ore-forming material was concentrated and hydrothermal alteration caused large-scale mineralization and remobilization and re-enrichment of ore-forming material. Considering that the mineralization in the skarn and greisen were mostly coeval and the greisenization stage, oxide stage and high-temperature sulfide stage were overlapped, the second period was separated into four stages.The detailed descriptions are given below.

Stage (Ⅱ₁): skarn stage. The anhydrous skarn is dominated by garnet, diopside and wollastonite while the hydrous skarn by diopside, actinolite and epidote. Small-scale mineralization is represented by magnetite and wolframite.

Stage (Ⅱ₂): oxide-early greisenization stage. Wolframite, cassiterite, scheelite, magnetite, pyrite and minor molybdenite, pyrrhotite and arsenopyrite were precipitated from relatively high-temperature hydrothermal fluids. Gangue minerals included mica- group minerals, feldspar, minor quartz and fluorite.

Stage (Ⅱ₃): high-temperature sulfide-greisenization stage. Temperature decreased at this stage, resulting in abundant quartz and fluorite and minor chlorite, epidote, sericite and carbonate. Chalcopyrite, pyrrhotite, arsenopyrite, pyrite, molybdenite, bismuthinite and scheelite were precipitated. Due to the precipitation of quartz, a large number of greisen and quartz veins were formed.

Stage (Ⅱ₄): low-temperature stage. The system became low-temperature and abundant low-temperature minerals, such as calcite, dolomite, chlorite, sericite and quartz were formed. Sphalerite, pyrite, and matildite were precipitated in areas about 10 m away from the intrusion. It should be noted that the homogenization temperatures of primary fluid inclusions in scheelite can reach as low as about 200 °C, according to mircothermometric analysis. This implied the crystallization of scheelite might has last from stageⅡ₂ to stage Ⅱ₄.

Period (III): This period is characterized by emplacement of a large number of granite porphyry dikes, pegmatite dikes and fine-grained two-mica granite veins. The emplacement of these rocks was the latest but the ages have not been determined. The related orebodies belonged to a new orebody type in the area and are dominated by scheelite and cassiterite with minor chalcopyrite and bismuthinite.

It is also noted that there is potential mineralization related to the later altered rocks. The mineralization is represented by lense-shaped orebodies within the NEE- striking structural zone. The altered rocks exposed in the tunnels, but, showed weak mineralization. Considering that the mineralization in the contact zone between the NEE-striking structure and the granite batholith has been discussed above, this mineralization type is not describe in detailed.

4 Re-Os isotope dating

4.1 Sample collection

The molybdenite samples were all collected from the molybdenite-bearing porphyritic granite exposed in the middle segments of tunnels 320 and 288. The porphyritic granite samples are in grey coarse-grained granular texture and massive structure. The phenocrysts are predominantly quartz (35%-40%), plagioclase (25%-20%), K-feldspar (20%-25%), amphibole (5%), and biotite (5%). The matrix is dominated by quartz, plagioclase, K-feldspar, and amphibole. K-feldspar consisted of two types, orthoclase and microcline. Most plagioclase has been replaced by sericite and now is andesine (An = 35) (Figs. 3(a), 3(b)). Molybdenite is grey and disseminated throughout hand specimens. Locally well-developed fractures are observed and occurred as radiating aggregates. Molybdenite is associating with cassiterite in some cases.

Samples were prepared in the Key Laboratory of Metallogenic Prediction of Nonferrous Metals, Central South University. Samples were first pulverized to 180-250 μm and hand-picked under a binocular to get molybdenite samples with >99% purity. Then the samples were pulverized to 75 μm by using agate pestle and agate mortar.

4.2 Analytical method and results

Re-Os isotopic analysis was conducted at the Re-Os Isotope Laboratory, National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, referring to SHIREY and WALKER [27] for detailed descriptions of the analytical procedure. In summary, an X-series inductively coupled plasma-mass spectrometer (ICP-MS) manufactured by TJA, USA was used to determine the isotope ratios. For low Re and Os concentrations samples, a high resolution ICP-MS (HR-ICP-MS Element 2) manufactured by Thermo Fisher Scientific, USA was used to determine the isotope ratios. ¹⁸⁵Re, ¹⁸⁷Re, ¹⁹⁰Re, ¹⁸⁵Os, ¹⁸⁶Os, ¹⁸⁷Os, 188Os, ¹⁸⁹Os, ¹⁹⁰Os, and ¹⁹²Os were determined in the analysis. Isotopes ¹⁹⁰Re and ¹⁸⁵Os were used for monitoring the inferences over ¹⁹⁰Os and ¹⁸⁵Re, respectively. The blank Re, Os and ¹⁸⁷Os values were obtained by TJA X-series ICP-MS and are (0.0036±0.0006)×10^-9, (0.00397± 0.00063)×10^-9, (0.00015±0.00005)×10^-9, respectively. These values were much lower than the Re and Os concentrations in the standard and analyzed samples. Hence, it will not affect the analysis. The common Os values were calculated using the ¹⁹²Os/¹⁹⁰Os ratios obtained from ICP-MS analysis and the natural abundances of these two Os isotopes [28-29]. The uncertainties of the Re and Os concentrations were contributed from the weighing error of samples and diluent, calibration error of the diluent, calibration for isotopic fractionation during ICP-MS analysis, analytical error for the isotope ratios in the analyzed samples. The confidence level was 95%. The uncertainty for the model ages with a 95% confidence level also included in the contribution from the uncertainty of the decay constant.

The model age is calculated by the following equation:

                     (1)

where λ (¹⁸⁷Re decay constant) = 1.666×10^-11a-1 (relative uncertainty = 1.02% [30]). The results are given in Table 1.

Table 1  Re-Os isotopic data for molybdenum in Xitian ore field

The Os concentrations of the analyzed molybdenite samples are almost 0. Three samples exhibited similar model ages (224.9-225.5 Ma). Isoplot was used to generate the isochron based on the three age data points (Fig. 9). The obtained isochron age is (225.5±3.6) Ma with MSWD=0.0034, which is consistent with the model ages.

Fig.9 Re-Os isochron of molybdenite samples from the Hejiangkou deposit

5 Discussions

5.1 Mineralization ages of Xitian Ore Field

Many geochronology works for the rocks and mineral deposits in this area have been done previously. Due to the multi-period magmatism and complicated tectonic evolution, as well as different precision of analytical methods used by different authors, the mineralization age of this area is, however, still open for debation. In this study, the ages from different authors were obtained (Table 2) and a comparison was made by using the author’s data to distinguish the ages of the rocks and mineralization.

According to the geochronology data in Table 2, the study area has three major mineralization ages, namely Indosinian Epoch (from (215.7±3.3) to (230.4±2.3) Ma with the peak at 225 Ma), early Yanshanian (from (147±3) to (157.0±2.6) with the peak at 152 Ma) and late Yanshanian (from (141.6±4.1) to (145.09±0.52) Ma). CHEN et al [32] noticed that the age differences between granites in a single batholith composite in Qinzhou- Hangzhou suture zone could reach 10-20 Ma. The highly evolved granites are favorable for W-Sn mineralization.

Geochronology data from this Re-Os dating successfully filled in the mineralization age sequence and added Indosinian mineralization event into it. LI et al [31] suggested that consecutive W-Sn mineralization events developed in Nanling metallogenic belt are closely related to magma activities in that area. A few examples in Xitian ore field are found. For example, the Rb-Sr isochron of greisen-quartz vein orebodies in Early Yanshanian granite of Heshuxia W-Sn-polymetallic deposit is (150±2.7) - (160.2±3.2) Ma. Mineralization age of Indosinian granite in Hejiangkou deposit is (225.5±3.6) Ma. There are examples from other ore fields of the same belt as well. For example, rock formation age of granite in Wangxianling ore field is Indosinian [32]. The mineralization isochron of Hehuaping Sn deposit in afore mentioned ore field is (214±1.9) Ma. The relationship among granite and mineralization event and discovery of Indosinian mineralization age in Nanling metallogenic belt hinted the possibility of strong mineralization during Indosinian Epoch in the belt. This mineralization event can be one of the perspectives to look at in mineral exploration of this belt.

5.2 Ore-forming material source and metallogenic geodynamics

According to Refs. [33-34], the Re-Os isotopic system may provide critical information to the mixing degree of the crustal material. If the ore-forming material is derived from the mantle or is dominated by the material from the mantle, the Re concentration of molybdenite usually ranges from 10 to 1000 μg/g. If the ore-forming material is derived from the crust-mantle mixing, the Re concentration of molybdenite is only several tens μg/g. If the ore-forming material is only from the crust, then the Re concentrations of molybdenite are only several μg/g. The Re concentrations of the analyzed molybdenite samples in this study ranged from 12.21 to 118.1 μg/g, indicating that the ore-forming material was derived from the crust-mantle mixing during Indosinian Epoch. GUO et al [38] also provided the evidence to show that Indosinian granites in Nanling area were formed through the crust-mantle mixing. Molybdenite analyzed by LIU et al [9] contained 1106-2800 μg/g Re, indicating that the ore-forming material was derived from the mantle during early Yanshanian. These are consistent with the fact from previous studies that the orebodies formed during early Yanshanian are fluorite-rich [31, 36].

Table 2  Summary of geochronological studies on Xitian ore field

Many authors have studied the super large, large and medium-scale deposits within Qinzhou-Hangzhou suture zone [23, 39-41]. The exposed ophiolite zone, high-pressure blue schist, relocated ophiolite suites and inversed strata within the suture zone, in combination with the latest dating results, imply that Cathaynian plate and Yangtze plate experienced an assembly-break-up- assembly process. This hypothesis has been widely accepted. During the global wide plate collision and assembly, the South China continent was formed during the last collision event in Silurian [23].

The closure of Tethys Ocean and the continental collision around 250-220 Ma caused intensive deformation, metamorphism, magmatism and mineralization wthin South China craton. Xitian granite batholith was formed in such tectonic regime. The study area shifted from Tethys tectonic domain to Pacific tectonic domain around 160-190 Ma. Pacific plate which was moving west started to subduct into beneath of continent of Southeast China in middle Jurassic, which caused the reactivation and tension of Indosinian EW-striking faults and uprising of the asthenosphere due to decompression in the study area [42]. The emplacement of the supplementary granites occurred in this period, corresponding to the large-scale W-Sn- polymetallic mineralization around 160-150 Ma.

To date, the felsic veins (including the veins hosting the main mineralization at Xianghualing) in Nanling area have not been systematically studied. The ore-bearing felsic veins are the youngest product of the highly evolved magma at shallow level [43]. The emplacement of such veins indicated a later magmatic activity in which more mantle-derived material was involved at deep level. The granite porphyry veins exposed in the tunnels in Hejiangkou mine district revealed that there is a possible structural path for the upwelling mantle- derived material.

6 Conclusions

1) The Re-Os isochron age for Hejiangkou W-Sn-polymetallic deposit is (255±3.5) Ma, indicating that the deposit is formed in early Triassic and the formation of the deposit is related to Indosinian felsic magma emplacement. The ore-forming material is predominantly from the crust with little mantle-derived material. The deposit is formed at the same time as the orebodies in the contact zone between the batholith and host rock. The future prospecting should not only focus on the skarn- and greisen-hosted orebodies but also the Cu-Sn-W orebodies associated with Indosinian granite.

2) Hejiangkou deposit experienced three ore- forming periods. The first period is represented by the thin-layered skarn-type orebodies that are formed due to the emplacement of a crust-derived felsic magma in a compressional tectonic regime during early Triassic. The second period is represented by the skarn- and greisen-hosted orebodies related to the emplacement of a mantle-derived felsic magma during middle late Jurassic. The third period is represented by the orebodies hosted in the granite porphyry veins that were formed during the emplacement of the mantle-derived magma in a tensional tectonic regime.

3) The orebodies hosted in the granite porphyry veins are discovered for the first time. This discovery provided a new perspective for future prospecting in this area. Nevertheless, the mineralization of this type in Qinzhou-Hangzhou suture zone and Nanling region is not clearly understood and needs further study.

4) The emplacement of felsic magma played an important role in each ore-forming period. The late mineralization exhibited the inherited nature and superimposed of the early mineralization. The successful exploration of Hejiangkou hidden deposit implied that there could be more high-temperature hydrothermal deposits related to the felsic magmatism in Xitian ore field.

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(Edited by DENG Lü-xiang)

Foundation item: Project(41403035) supported by the National Natural Science Foundation of China; Project(13JJ4041) supported by Hunan Provincial National Natural Science Foundation, China

Received date: 2015-03-02; Accepted date: 2015-10-10

Corresponding author: WANG Xiong-jun, PhD, Lecturer; Tel: +86-13974838458; E-mail: wxjcsu@163.com