Types, geological features and geodynamic significances of gold-copper deposits in the Kanggurtag metallogenic belt, eastern Tianshan, NW China

Abstract

Occupying the middle of the central Asia Paleozoic accretionary and collisional orogenic belt, the eastern Tianshan area has a great economic potential due to Au-Cu mineralization during syn- and post- orogenic events. In the Kanggurtag Au-Cu metallogenic belt, three major types of gold deposits have been recognized: ductile-shear-zone-hosted gold deposits, magmatic hydrothermal gold deposits, and epithermal gold deposits. Four kinds of copper deposits have also been identified recently: the porphyry-type, the skarn-type, the magmatic type, and volcanic/sedimentary-type. Tectonically, the development of these late Paleozoic gold and copper deposits was closely associated with the subduction and collision of the ancient Tianshan ocean that intervened between the Tarim craton and the Siberian block. In the early to mid-Carboniferous, N-dipping subduction beneath the Dananhu arc generated magmatic intrusions, leading to formation of the porphyry Cu deposits. The magmatic front migrated southward to form the Yamansu arc upon the Kanggurtag accretionary wedge. In the latest Carboniferous to early Permian, during the closure of the ancient Tianshan ocean, large mafic-ultramafic complexes were emplaced, resulting in several magmatic copper-nickel deposits. Gold deposits of the shear-zone-type are controlled by the Kanggurtag ductile shear zone, which is related to collisional orogenesis. The epithermal gold deposits are associated with extensional tectonics and post-tectonic volcanic activity. The tectonic settings, geological features, and temporal and spatial distributions of these different types of gold and copper deposits reflect, to a great extent, the accretionary and collisional tectonics that occurred between the northern margin of the Tarim block and the southern margin of the Siberian block.

Introduction

The Kanggurtag gold-copper metallogenic belt occupies a key position in the middle segment of the Central Asia orogenic belt. The belt area extends EW for approximately 600 km and NS for 60–100 km (Fig.1). Over 95% of the gold-copper deposits and prospects in the east Tianshan region are concentrated in the Kanggurtag belt. These deposits constitute an important gold-copper metallogenic potential and a gold-copper-producing base in China. Since the Kanggur gold deposit was discovered in 1988 (Ji et al. 1994), a series of gold deposits, such as the Xitan, Kangxi, Xifenshan, Dadonggou and Mazhuangshan deposits have been discovered in this belt. The recent discovery of the very large porphyry-Cu deposit Tuwu-Yandong is the most important breakthrough for metal exploration in Xinjiang, NW China. These gold-copper deposits occur on the northern margin of the Tarim plate and the southern margin of the Siberian plate, to form an EW-trending, gold-copper metallogenic belt. Systematic study of this mineralization belt will shed some light on the interaction of these two tectonic regimes in Central Asia.

Fig.1

A. Tectonic sketch map of the west China; B. Tectonic setting and distribution of major gold and copper deposits in the Kanggurtag metallogenic belt of eastern Tianshan. Tectonic divisions are simplified from Ji et al. (1994) and Xiao et al. (2004). ① Kumishi fault/suture; ② Southern central Tianshan fault; ③ Weiya fault; ④ Kushui fault; ⑤ Kanggurtag fault; ⑥ Qincheng fault; ⑦ Xingxingxia fault. The deposits numbered 1–29 are listed in Table 3

The tectonic evolution of the eastern Tianshan is of key importance to unravel Paleozoic tectonics of Central Asia, but its accretionary and collisional orogenic processes are still under discussion (Coleman, 1989; Massakovsky et al. 1993; Sengör et al. 1993; Sengör and Natalin, 1996). Cu-Au mineralization can help to reconstruct the paleogeography (Bierlein et al. 2002), thus providing important constraints on the nature and age of the subduction and collision events. This paper presents an investigation of the basic characteristics of the different types of gold and copper deposits in this area, and discusses their geological settings and distribution in space and time. A preliminary genetic model for these gold and copper deposits is also proposed.

Geological setting of eastern Tianshan

Located in the convergent area between the Siberian and Tarim blocks, the eastern Tianshan region had a complex tectonic evolution (Coleman, 1989; Xiao et al. 1990; Windley et al. 1990; Allen et al. 1992; Sengör et al. 1993; He et al. 1994; Gao et al. 1998; Buckman, 2001). The east Tianshan orogenic belt can be divided into four tectonic units from north to south: the southern margin of Siberian block, the Kanggurtag accretionary zone, the central Tianshan terrane, and the northern margin of Tarim plate (Ji et al. 1994; Ma et al. 1993, 1997). The host rocks of the Kanggurtag gold-copper ore belt are of the Qiershan Formation, the Xiaorequanzi Formation and the Aqishan Formation. These volcanic rocks were formed in the Kanggurtag accretionary arc, from early Devonian to early Carboniferous time. The the Kanggurtag complex collided with central Tianshan and the Qiugemingtashi-Huangshan (Kanggurtag) ductile shear zone was formed at the end of the Carboniferous-early Permian (290–280 Ma, Ma et al. 1997; Li et al. 1998).

The Kanggurtag metallogenic belt is composed of the Dananhu arc, the Kanggurtag accretionary complex and the Yamansu island arc (Ji et al. 1994; Liu et al. 2001). A series of EW- to ENE-trending faults are well developed and include the Kanggurtag fault, the Yamansu fault, the Kushui fault, the Weiya fault, the Southern Central Tianshan fault and the Kumishi fault (Fig.1) (Ji et al. 1994; Ma et al. 1997). The strata in the area are mainly Devonian-Carboniferous in age, with subordinate Permian rocks. The Devonian rocks, exposed north of the Kanggurtag fault and south of the Weiya fault, are composed of neritic intermediate-basic and intermediate-acid volcanic rocks and clastic sedimentary rocks with carbonates. The Carboniferous rocks are extensively distributed, consisting predominantly of intermediate-basic volcanic rocks and clastic sedimentary rocks with a small amount of limestone. Controlled by the Kanggurtag fault and Weiya fault, the Permian rocks are mainly distributed in the Yamansu island arc, and are clastic rocks with carbonates and basic to acid volcanic rocks.

The regional strata of the study area include the following lithostratigraphic units (Table1): (1) the Qiershan Formation (Devonian); (2) the Xiaorequanzi Formation (Lower Carboniferous); (3) the Aqishan Formation (Lower Carboniferous); (4) the Kushui Formation (Carboniferous); (5) the Gandun Formation (Carboniferous); and (6) the Wutongwuozi Formation (Carboniferous). The stratigraphic relationships among these formations are not clear because they occur as tectonic slabs and are in fault contact with each other. Based on their compositional characteristics and lithological combinations, we believe that these rocks represent typical accretionary arc assemblages including an accretion complex (e.g. Kushui, Wutongwuozi and Gandun Formations) and magmatic-arc rocks (Aqishan Formation, Xiao et al. 2003).

Table 1 Lithostratigraphic units of Kanggurtag region in the eastern Tianshan mountains

In the Kanggurtag Paleozoic accretionary complex, there is a large-scale ductile shear zone called the Quigemingtashi-Huangshan shear zone. The ductile shear zone trends EW, dipping N, and is over 500 km in length and 10–30 km in width. The ductile shear zone is composed of a series of mylonites and mylonitized rocks. Deformation fabrics are well developed, including S-C fabrics, stretching lineations, and rotational porphyroclastic structures. Cleavage planes are N-dipping with a dip-angle of 70^o–75^o. The typical metamorphic mineral assemblage contains actinolite, biotite and chlorite, and indicates a metamorphic grade of the lower greenschist facies. The formation temperature and depth of the ductile shear zone is estimated to be 350^o–500^oC and 15–20 km, respectively (Ma et al. 1997; Yang et al. 1998).

The Quigemingtashi-Huangshan ductile shear zone formed during the process of collisional orogenesis between the Tarim block and the Siberian (Angara) block (Rui et al. 2002a). A series of gold-copper deposits in the Kanggurtag metallogenic belt are controlled by the ductile shear zone (Ji et al. 1994; Zhang, 1999; Rui et al. 2001).

Some intrusions and subvolcanic rocks were intruded in eastern Tianshan, including mainly early Carboniferous-Permian (350–270 Ma) granite, plagiogranite porphyry, tonalite, quartz porphyry, dacite porphyry, and mafic-ultramafic bodies. They are related to the gold-copper mineralization.

Types of gold and copper deposits

The gold and copper deposits now being mined along the Kanggurtag metallogenic belt of eastern Tianshan can be divided into seven types according their ore geneses (Table2):

Table 2 The main deposit types of the Kanggurtag Au-Cu metallogenic belt in the eastern Tianshan mountains

(1):

Epithermal gold-bearing quartz veins hosted by late Paleozoic volcanics and subvolcanics. Examples include the Xitan, Mazhuangshan and Harla gold deposits.

(2):

Ductile-shear-zone-type gold deposits. Two subtypes include those: (I) hosted by carboniferous volcanics (examples are the Kanggur and Matoutan gold deposits); (II) hosted by carboniferous turbidites (examples are the Yuanbaoshan and Qishitan gold deposits).

(3):

Magmatic hydrothermal Au-bearing quartz veins hosted by late Carboniferous-Permian granites. Examples include the Xifengshan, Jinwuzi and Mahuanggou gold deposits.

(4):

Porphyry copper deposits hosted by Devonian-Carboniferous volcanic rocks, diorite-porphyrite and plagiogranite porphyry. Examples include the Tuwu, Tuwudong, Yandong and Sanchalu porphyry Cu-Mo deposits.

(5):

Magmatic deposits hosted by mafic-ultramafic rocks. Examples include the Huangshan, Huangshandong, Huangshannan and Xiangshan Cu-Ni deposits.

(6):

Volcanic sedimentary deposits hosted by volcanic sedimentary rocks. An example is the Xiaorequanzi Cu-Zn deposit.

(7):

Skarn-type copper deposits. Up to the present, the Weiquan copper deposit is the only known example of a skarn-type copper deposit in east Tianshan.

Geologic features of gold and copper deposits along the Kanggurtag metallogenic belt

Basic geologic features of the gold and copper deposits along the Kanggurtag metallogenic belt of Eastern Tianshan are listed in Table3.

Table 3 Basic geologic features of selected gold and copper deposits of the Kanggurtag metallogenic belt in eastern Tianshan

Epithermal type gold deposit

Xitan gold deposit

The Xitan (or Shiyingtan) gold deposit, located in the western part of the Kanggurtag ore belt (Fig.1), was the first epithermal gold deposit discovered in the Yamansu volcanic arc. Estimated gold reserves are 10 tonnes grading approximately 12.5 g/t Au and 20 g/t Ag. The deposit is closely associated with listric faults related to paleo-calderas (Fig.2A). In the Xitan gold ore area, the main Au-bearing quartz veins are controlled by circular fractures, ENE- and ESE-trending faults, and interlayer fissures.

Fig.2

A. Regional geology map of the Xitan gold deposit, eastern Tianshan; B. Geological map of the Xitan ore area

The mineralization is hosted in Permian andesite (290–270 Ma, Rb-Sr isochron age, Zhang, 1999), ignimbrites, and volcanic breccia, which overlie the widespread Carboniferous andesitic and dacitic volcanic arc terrane. A granophyre lies beneath the host andesite and also crops out 300 m to the southeast of the deposit. Although only 2 km south of the Kanggurtag ductile shear zone, the volcanic host rocks only show brittle features and are unmetamorphosed. More than ten quartz veins have been identified on the surface and four of these are being mined (No. 1, 3, 4, and 5; Fig. 2B). The largest ore vein (No. 3) averages 20 m in width, and 150 m in length, and extends down-dip for >200 m. The ore consists of quartz, calcite, pyrite, chalcopyrite, native gold and electrum. Additionally, adularia occurs associated with quartz, mainly in veins. The low sulfide content (<5%) is one of the characteristics of the Xitan deposit.

Silica occurs as mosaic-textured microcrystalline quartz, cryptocrystalline chalcedonic quartz, and, rarely, opal. Adularia is observed with quartz in open-space-filling veins and veinlets, as colloform-banded aggregates with chalcedonic quartz, or as fine-grained mosaic-textured aggregates associated with microcrystalline quartz. Clay minerals, such as kaolinite and illite, and, to a lesser extent, montmorillonite, occur as fine-grained clusters and matted aggregates within a siliceous matrix. Sericite was observed as fine-grained aggregates and rare veinlets in microcrystalline quartz.

The mineralization is thought to have occurred in four stages: The first mineralization stage consist-eds of a barren gold massive quartz vein; the second stage is characterized by deposition of fine-grained banded quartz, sulfides and precious metals; the third stage commonly produced gold-bearing crustiform-banded quartz veins; the fourth stage produced barren gold calcite-quartz veins.

The fluid inclusions in quartz show characteristics of simple inclusion type, low temperature (150–250^oC), low salinity (0.8–6.7 wt% NaCl equ.) and pressure (3.5×10⁵–50×10⁵Pa), low CO₂/H₂O ratios (0.001–0.005) and Na⁺/K⁺ ratios (0.20–0.29). The data of δD (-119.4 to -81.7‰) and δ¹⁸O (-1.65 to -12.74‰) in fluid inclusions show that ore fluid of the Xitan gold deposit originates from meteoric water (Zhang et al. 2000). It was concluded that the Xitan gold deposit is a kind of adularia-sericite type epithermal gold-silver deposit, according to Heald et al.’ s classification (Heald et al. 1982).

Mazhuangshan gold deposit

The Mazhuangshan gold deposit is located in the border area of the Xinjiang and Gansu provinces, in the eastern part of the Kanggurtag ore belt. The Lower Carboniferous Mazhuangshan (Aqishan) Formation strata are composed of acidic volcanic breccia and intermediate-basic lava and are the predominant outcrops in this area. The gold orebodies were controlled by the superimposition of a volcanic basin on an island arc. The volcanic rocks are a set of subvolcanics, consisting of quartz porphyry, feldspar quartz porphyry, rhyolite porphyry, granodiorite porphyry, diabase veins and cryptoexplosive rocks, which are dated at 301–303 Ma by Rb-Sr isochron methods (Li et al. 1998).

Ore bodies are vein-shaped and made up of quartz veins and silicic alteration zones, and mainly trend EW, NW and NS. At present, geologic exploration has shown that there are more than 20 gold ore bodies, with an average Au grade of 7.4 g/t. The width of the ore bodies varies from 0.8 to 13.1 m, whereas the total length can reach 600 m. The ore exhibits features of cataclastic, crushed, and metasomatic textures, massive, banded, vein and disseminated structures. Wall-rock alteration associated with the mineralization includes mainly pyritization, silicification, sericification and carbonatization.

The dominant ore minerals are native gold, native silver, argentite, pyrite, arsenopyrite, chalcopyrite, galena, christophite and pyrrhotite; the main gangue minerals are quartz, calcite, sericite, chlorite and albite. Native gold shows granular, dentritic and irregular forms, with a grain size of 0.015–0.06 mm in diameter; gold often occurs associated with pyrite, quartz, galena and chalcopyrite.

Three mineralization stages may be distinguished (Chen et al. 2000): an early volcanic hydrothermal mineralization stage, a main hydrothermal mineralization stage and a superimposed hydrothermal mineralization stage. Homogenization temperatures of fluid inclusions vary between 120°C and 300°C. The salinity of fluid inclusions average 15 wt % NaCl equ.. The data of δD (-123.5 to -70.8‰) and δ¹⁸O (-7.3 to +8.4‰) in fluid inclusions suggest that the ore fluids of the gold deposit consist predominantly of meteoric water, and subordinately of magmatic water (Chen et al. 2000).

Shear zone type I: hosted in volcanics

The Kanggur gold deposit

The host rock of the Kanggur gold deposit is a calc-alkalic volcanic rock series consisting of andesite, dacite, tuff and subvolcanic rocks. A tonalite located near the ore area plays an important role in ore-forming process of the Kanggur gold deposit. The gold ore bodies were distributed in a transform zone that exhibits strong ductile to weak deformation. The ore-controlling structure is an EW-trending normal brittle-ductile shear zone (Fig.3A). The gold ore bodies are composed of Au-bearing altered rocks and a few Au-bearing sulfide-quartz veins. The Au reserve is estimated to be up to 20 tonnes. At present, the ore deposit is mined by the local government with considerable profit.

Fig.3.

Local geology of the Kanggur gold deposit, eastern Tianshan, in A. plan view (modified from Ji et al. 1994 and Ma, 1998) and B. cross section (modified after Zhang, 1999)

The Kanggur gold deposit (or Kanggur VI) comprises three offsets (VI-1, VI -2 and VI -3) which are veined or lenticular bodies located in the southern brittle-ductile shear zone. The ore body VI -1 is 1000 m long, 1.05–10.26 m thick (averaging 3.95 m), trending 85^o, dipping 355^o N with an angle of 70^o–75^o. In the dip direction, the explored ore bodies extend over 600 m below the surface (Fig.3B). There is a clear vertical zonation: gold is enriched in the upper, lead and zinc in the middle, and copper in the lower part of the orebodies.

The ore grade is proportional to the thickness of ore body. The ore contains 3.2–40.5 g/t Au (averaging 8.0 g/t), 0.3–1.6% Cu (averaging 0.76%), 0.5–14.6 g/t Ag , 0.5–2.1% Pb, and 0.7–3.7% Zn. The Au/Ag ratio of the ore averages 2.16. Principal metallic minerals in these ores include native gold, electrum, pyrite, galena, sphalerite, and chalcopyrite as well as minor quantities of pyrrhotite, arsenopyrite and bornite, etc. The gangue minerals include mainly quartz, chlorite, magnetite and sericite, with lesser amounts of calcite, dolomite, siderite and barite.

Ores are characterized by euhedral, subhedral, cataclastic and metasomatic textures. The ores are of veinlet, disseminated and brecciated structures. According to mineral assemblages and alteration intensity, from ore body to wall rock, hydrothermal alteration is divided into three zones: silicification, chloritization, pyritization and magnetization zone (major ore body); pyrite-phyllic rock and/or sericite alteration zone; sericite-chlorite alteration zone.

Based on the mineral assemblage and the crosscutting relationship of veins, five mineralization stages were identified. The first stage featuring formation of pyrite-phyllic rock, is mineralogically represented by native gold, pyrite, sericite and quartz; the second stage, representing the main ore-forming stage, is characterized by a native gold-quartz-pyrite-magnetite-chlorite assemblage; the third mineralization stage features native gold-quartz-pyrite veins; the fourth stage contains Au-bearing polymetallic sulfide-quartz veins; the fifth stage is composed of sulfide-free quartz-carbonate veins. The first two stages were related to ductile-brittle deformation of the shear zone, and are named the dynamic metamorphic hydrothermal epoch. The later three stages were related to intrusive processes of tonalite and brittle activity of the shear zone, and are called the magma-hydrothermal-mineralization epoch. On the basis of studies of geological characteristics, the ore-forming ages of the Kanggur gold deposit were determined by Rb-Sr and Sm-Nd isochron methods. The results show that the ore-forming ages of the Kanggurtag gold deposit are 290–254 Ma (Zhang et al. 2002).

The δ³⁴S compositions of sulfide in the ore range from -0.9‰ to +3.3‰, similar to that of the volcanic and subvolcanic rocks (-3.21 to +1.3‰), indicating that the sulfur isotope composition is characterized by unfractionated mantle sulfur. The fluid inclusions in quartz show characteristics of complex inclusion type, middle temperature (180–340^oC), high salinity (8.5–27.7 wt% NaCl equ.) and pressure (100×10⁵–1000×10⁵Pa), high CO₂/H₂O ratio (0.023–0.166) and Na⁺/K⁺ ratio (2.70–6.68). The δ¹⁸O values of the ore fluid vary between +3.83 and -4.63‰ and the δD values between -46 and -66‰. The δ¹⁸O vs. δD diagrams indicate that the ore fluids were derived mainly from dynamic metamorphic water and meteoric water (Zhang et al. 1999).

Matutan gold deposit

The Matutan gold deposit (or Kanggur VIII) occurs 3 km east of the Kanggur (VI) ore area. The host rock, controlling structure and orebody occurrence are similar to that of the Kangur deposit. But there are obvious differences in mineral assemblage, mineralization stage, mineralization types and zonation. The ore grade of Au ranges from 2.2 g/t to 120 g/t with an average of 9.59 g/t. The ore type is mainly auriferous magnetite-chlorite-altered rock. The lack of Au-bearing sulfide-quartz veins differentiate this deposit from the mineralization features of the Kanggur (VI) gold deposit. Other characteristics of the Matutan gold deposit are the same as of the Kanggur deposit.

Shear zone type II: hosted in turbidites

Qishitan gold deposit

The Qishitan deposit is located 10 km north of the Kanggur mine (Fig.1), and is hosted in Lower Carboniferous turbidite greywacke and sandstone of the Kushui Formation, which developed in a typical accretionary arc setting (Kerrich and Wyman, 1990; Kerrich and Cassidy, 1994). All of these host rocks have been strongly fractured and sheared, varying from incipient mylonite and mylonite to ultra-mylonite. Gold mineralization is structurally controlled by the EW-trending ductile shear zone. At present, tens of orebodies consisting mainly of gold-bearing mylonite lenses have been delineated, of which the Q1, Q2 and Q3 orebodies are of larger extent. The length of the orebodies varies from 300 to 800 m, and the width from 1 to 12 m. The ore grade of Au ranges from 1.2 g/t to 10.5 g/t, averaging 4.5 g/t.

Magmatic hydrothermal vein type

Xifengshan gold deposit

The Xifengshan deposit is spatially and temporally related to a late Variscan granite (Rb-Sr isochron age 284 ±13 Ma, initial ⁸⁷Sr/⁸⁶Sr value 0.7061). The granite was emplaced into early Carboniferous volcanics and tuff of the Aqishan Formation. The deposit consists of 34 Au-bearing quartz veins which are confined to the granite. Main veins trend in an EW direction with a gentle dip-angle (20–30°), and secondary veins trend NE and NW with a steeper dip-angle (60–70°). It has been concluded that the Au-bearing quartz veins were controlled by primary radiate and ring fissures.

At present, eight ore veins have been discovered, of which No.1 is the largest (Fig.4). Individual orebodies consisting of Au-bearing quartz veins vary from 100 to 300m in length and 0.3 to 2.5 m in width. The ore grade of Au is 10–15 g/t. The metallic mineral assemblage of the gold ore consists of native gold, pyrite, galena and sphalerite. Quartz, calcite, and sericite are the predominant gangue minerals. Gold occurs, associated with pyrite, as inclusions, interparticle gold, or intergranular gold. A geochemical study indicates that the Xifengshan gold deposit is characterized by the magmatic hydrothermal quartz vein type. The Rb-Sr isochron age of the fluid inclusions in quartz is 272 ±3 Ma, with an initial ⁸⁷Sr/⁸⁶Sr value of 0.7061 (Zhang, 1999).

Fig.4.

Local geology map of the Xifengshan gold deposit, eastern Tianshan

Jinwozi gold deposit

The Jinwozi gold deposit is located in the eastern portion of East Tianshan near the Gansu province (Fig.1). This deposit is one of vein-type mineralization hosted in a small Permian granodiorite body. The granodiorite body is about 4500 m in length and 600–900 m in width. An auriferous quartz veins were emplaced into extensional fractures developed in the granodiorite body. Individual orebodies, consisting of Au-bearing quartz veins, vary from 20 to 500 m in length and 0.5 to 5.5 m in width. The ore grade of the gold is 8–25 g/t. Ore minerals include native gold, pyrite, and galena with minor amounts of sphalerite and chalcopyrite. The ore minerals occur as banded and/or nodular aggregates in auriferous quartz veins. Quartz, calcite, sericite and chlorite are the dominant gangue minerals. Gold shows a high fineness, generally >850.

The δ³⁴S compositions of sulfide in the ore range from 5.0‰ to 6.8‰. Fluid inclusions are dominated by liquid inclusions, with secondary gaseous inclusions and CO₂ inclusions. Homogenization temperatures of the primary fluid inclusions vary between 220°C and 330°C. The δ¹⁸O values of the mineralizing fluid vary between 2.71‰ and 6.94‰, and the δD values between -87.6‰ and -101.7‰. The δ¹⁸O and δD compositions of fluid inclusions indicate that the ore fluids were mainly derived from magmatic water (Chen et al. 1999). The Jinwozi deposit is typical of magmatic hydrothermal gold deposits.

Porphyry copper deposits

Tuwu porphyry copper deposit

No significant breakthrough was achieved in the search for porphyry copper deposits until late 1990’s, when the Tuwu-Yandong superlarge porphyry Cu deposits were discovered in east Tianshan, NW-China (Wang et al. 2001). The Tuwu porphyry Cu deposits are located in the west-south section of the Dananhu island arc, about 80 km southwest of Hami city, east Xinjiang (Fig.1). Initial reports by the No. 1 Geological Team indicate that the Tuwu porphyry deposit contains about 7 million tons of copper at a cutoff grade of 0.72% Cu. In addition, the deposit is estimated to contain 10 tons of Au at an average grade of 0.16 g/t, and also significant quantities of Ag (Wang et al. 2001).

The porphyry copper deposit is located about 1–3 km north of the Kanggurtag fault and is hosted in plagiogranite porphyry and diorite porphyry that intruded into early Devonian intermediate to mafic volcanic rocks (dated 416 Ma ago using Sm-Nd dating, and 390 Ma by zircon U-Pb dating, Rui, 2001, 2002a). The Tuwu deposit comprises two larger orebodies (Fig.5A). No. I is 1400 m long, 10–130 m thick, dipping 180 ^o at an angle of 65^o–80^o. The explored orebodies extend over 600 m below the surface (Fig.5B) in the dip direction. Stockwork-hosted and disseminated sulfide minerals in the porphyritic and granodioritic host intrusion, are mainly pyrite and chalcopyrite, with lesser content of bornite. Biotite is closely associated with the main ore zones, but K-feldspar is absent (Rui et al. 2002b). Surrounding alteration zones are dominated by massive silicification outward to a quartz-sericite zone, then sericite-kaolinite, and finally an epidote-rich outer halo (Fig.5B). Mineralization may be classified into four stages. Stage 1 is characterized by the occurrence of a chalcopyrite + bornite + pyrite assemblage and disseminated mineralization; Stage 2 is a quartz + pyrite + chalcopyrite assemblage, occurring as veins; Stage 3 is a quartz + molybdenite assemblage, occurring as veins and disseminations; Stage 4 is characterized by the occurrence of an assemblage of carbonates (calcite) + laumontite + minor sulfides.

Fig.5.

Local geology of the Tuwu copper deposit, eastern Tianshan, in A. plan view and B. cross-section

According to the study of Han et al. (2002), the δ³⁴S compositions of sulfide in the ore range from -0.2 to +8.0‰, indicating that the sulfur came from a deep source. Fluid inclusions in the quartz show characteristics of middle temperature (150–350^oC), middle salinity (9.0–12 wt% NaCl equ.) with a few high-salinity examples (25–40 wt% NaCl equ.). The δ¹⁸O values of the mineralizing fluid vary between 0.3‰ and 8.0‰ and the δD values between -69‰ and -44‰. These data indicate that ore-forming fluids were derived mainly from magmatic water mixed with meteoric water.

Magmatic deposit

Huangshan Cu-Ni deposit

Ultrabasic-basic complexes, consisting of gabbro, diorite, gabbro-norite and websterite, are a part of the melange accumulation in the Wutongwuozi Group (Fig.6A). The basic volcanics belong mainly to an intra-oceanic volcanic arc and belong partly to an oceanic volcanic rock.

Fig.6

Local geology of the Huangshan Cu-Ni deposit, eastern Tianshan, in A. plan view and B. cross-section

The Huangshan Cu-Ni deposit is a super-large deposit. However, as the bulk of the ore bodies are lean and deep-seated (Fig.6B), the deposit is still mined. The mineralization has a special relationship with gabbro-norite. The rock body occurs along EW- or NEE-trending fault. Most orebodies are 300–600 m in length and 50–100 m in width. Metal minerals include pyrrhotite, pentlandite, chalcopyrite, and pyrite. Nonmetal minerals include olivine, pyroxene, amphibole, plagioclase and phlogopite with the alteration minerals chlorite, serpentine and talc. The ores are characterized by xenomorphic-granular and sideronitic textures, disseminated, vein, breccia and banded structures. The ore bears 0.2–0.65% Cu, averaging 0.31%, 0.3–0.5% Ni, averaging 0.4%.

The δ³⁴S and initial ⁸⁷Sr/⁸⁶Sr values of the sulfides are +0.5‰ to -0.2‰ and 0.70475–0.70585, respectively. They suggest that the ore-forming material came from a deep magma and the deposit type is magmatogenic (Mu, 1998).

Volcanic sedimentary deposit

Xiaorequanzi Cu-Zn deposit

The Xiaorequanzi copper-zinc deposit, a newly discovered, promising copper-rich deposit, lies in the west section of the Kanggurtag Au-Cu ore belt (Fig.1). The deposit occurs in tuffaceous sedimentary rocks of the Lower Carboniferous Xiaorequanzi Formation in the Dananhu island arc. The ore deposit is closely related to volcanic edifices and volcanic depressions and the distribution of the orebodies is controlled by NW-SE trending folds and faults (Sun and Shang, 1998). The Xiaorequanzi deposit contains 22 ore bodies, which are mainly distributed in the north of the mining area, and of which the largest one is 520 m long and 1–30 m wide, with an average width of 12.46 m (Fig.7A). Major ore bodies occur in stratiform and stratoid form (Fig.7B). The grade of the ore is 0.5–19.2% of copper and 0.5–2.6% of zinc. Ore minerals are mainly chalcopyrite, sphalerite, bornite, pyrite, galena, chalcocite and pyrrhotite; gangue minerals include chlorite, quartz, calcite and sericite. The ore structures include disseminated, veinlet, nodular and breccia types.

Fig.7

Geological section of the Xiaorequanzi ore area, in (A) plan view and (B) cross-section

The δ³⁴S values of sulfides are 1.5–11.1‰, averaging 6.15‰. The δ¹⁸O value of quartz are 7.5–11.7‰, averaging 9.23‰. The δD values of fluid inclusion in quartz are -66 to -121‰, averaging -93‰ (Chen, 1999). The data discussed above show that the mineralizing materials of the copper-zinc deposit are characterized by crust-mantle mixing sources, and the formation of the deposit is characteristic of syngenesis and superimposed mineralization.

Skarn-type copper deposit

Weiquan copper deposit

Situated 30 km southeast of the Kanggur gold mine, the Weiquan Cu deposit is a new type of copper deposit discovered in 1999 (Fig.1). It is presently being drilled and studied by the No.1 Geological Team of the Xinjiang Bureau of Geology and Mineral Resources. It has been indicated that this area has great promise of geological exploration.

The country-rocks are the Tubulake Formation of late Carboniferous age, and are composed of acid-intermediate-basic pyroclastic rocks interbedded with limestone. Skarn minerals are well-developed around a Late Carboniferous-Permian biotite-granite body. The skarn consists of andradite, diopside, idocrase and epidote. Altered minerals include actinolite, chlorite and fluorite. An oxidized ore body near the surface contains malachite, azurite and jarosite. Its length is 250 m and its width is 0.4–24 m (average 9.28 m). It strikes EW and dips north at 54° - 75°. The mineralization shows a clear vertical zonation with Ag, Pb and Zn enrichment in the upper orebody, and Cu enrichment in the lower orebody. The Cu grades of oxide ore vary from 0.5% to 36.7%. The primary ore contains 0.2%–4.1% Cu, accompanying 466 g/t Ag, and 2.7% Pb and Zn. The ore minerals include chalcopyrite, bornite, pyrite, magnetite, sphalerite, galena, argentite and molybdenite. The gangue minerals include skarn minerals, altered minerals, quartz and calcite. The ores are characterized by granoblastic and replacement-remnant textures, and disseminated and nodular structures. The δ³⁴S compositions of sulfide in the ore range from -2.7‰ to -0.6‰. Homogenization temperatures of fluid inclusions vary between 150 ^oC and 297 ^oC. The salinity of fluid inclusions are on average 10.1 wt % NaCl equ. (Han et al. 2002).

Discussion

Metallogeny and ore-controlling factors

The eastern Tianshan is a typical Paleozoic accretionary arc system. It is quite similar to the circum-west Pacific volcanic Au-Cu belt, ranging from Japan, Taiwan, Philippines, Indonesia, and Papua New Guinea to New Zealand, where most large deposits formed during the late Miocene, Pliocene and Quaternary. All the Au-Cu deposits in the eastern Tianshan formed in the late Paleozoic (340–240 Ma) and represent an ancient accretionary arc setting and subduction activity. The volcanic-intrusive relationships, metal deposit types, and their association in eastern Tianshan show some similarity with the metallogenesis of the southeast Asia Cenozoic archipelago (Qin et al. 1999).

(1):

Control of intrusions and subvolcanic bodies on gold-copper mineralization. Gold-copper mineralization in the eastern Tianshan area is related without exception to intrusive rocks and subvolcanic porphyries. Mineralization occurs within intrusions, in contact zones, or in near wall rocks. Intrusive and subvolcanic bodies offer heat and ore sources for mineralization. The combination of intermediate-basic volcanic rocks with intermediate-acid intrusions favors especially Au and Cu mineralization. The Tuwu porphyry copper deposit is hosted by a plagiogranite porphyry and diorite porphyry. The Xitan gold deposit is intimately associated with a granite-rhyolite porphyry; the orebodies are distributed on the northwestern margin of a caldera. Pirajno et al. (1997) suggested that the ore-controlling structure of the Xitan gold deposit is a breccia pipe of Permian age. The Kanggur and the Matutan gold deposits are only about 4 km south of a tonalite stock. The Mazhuangshan gold deposit occurs within a subvolcanic body. Thus the intrusions and orebodies are temporally and spatially associated.

(2):

Control of ductile shear zones on gold-copper deposits. Some of the gold-copper deposits, such as the Kanggur gold deposit, the Kangxi gold deposit and the Huangshan Cu-Ni deposit, are controlled by the Qiugemingtashi-Huangshan ductile shear zones, which provided conduits for the transport of ore fluids and structural sites for the deposition of ore-forming materials. Strata with more intense ductile shearing have high contents of gold and copper. Structural sites that have late-stage brittle faults superimposed on earlier ductile fabrics are favorable locations for the formation of gold-copper deposits. The Kanggur, Kangxi, Qishitan and numerous other ore occurrences are distributed on the southern margin of the ductile shear zone.

(3) :

Control of strata and lithology to Au-Cu mineralization. The strata hosting Au and Cu deposits are complex in lithology and contain mainly volcanic rocks and sedimentary clastic rocks. For example, the Xiaorequanzi Cu-Zn deposit occurs in tuffaceous sedimentary rocks of the Xiaorequanzi Formation of Lower Carboniferous age, and the Kanggur Au deposit is hosted in the Aqishan Formation intermediate-acid volcanic rocks. Geochemical surveys have revealed that volcanic rocks have a higher content of Au and Cu, whereas clastic rocks have a lower metal content. This indicates that volcanism provides a primary material basis for the formation of Au and Cu deposits.

Spatial and temporal distribution of gold and copper deposits

The different types of the main gold and copper deposits of along the Kanggurtag metallogenic belt are shown in Fig.1, with most being clustered in the Devonian and Carboniferous island arc rocks. It is evident that the distribution of these gold and copper deposits are, to some extent, related to their tectonic setting (Table2). Type-1 deposits (e.g. Xitan and Mazhuangshan) occur mainly in volcanic basins that are superimposed on the late Paleozoic island arc, in the eastern and western part of the Kanggurtag metallogenic belt. The ore-forming process is likely related to the volcanic activity. Isotopic dating shows that the age of the host rocks is 300–260 Ma and the age of ores is 290–240 Ma (Table4). Type-2 deposits (e.g. Kanggur) are hosted mainly in mylonitic volcanic rocks, and are largely distributed in the middle part of the Kanggurtag metallogenic belt. These deposits also show evidence of multiple mineralization periods. The main mineralization that reaches economic grade consists of Au-bearing altered rock that is attributed to late Paleozoic syn-metamorphism at 300–280 Ma (Zhang et al. 2002). In the interval of 275–254 Ma, the intrusion of the tonalite and extension within brittle-ductile shear zones allowed the ore-forming solution to be injected into open fractures. This is thought to have formed the primary gold orebody and the superimposed gold-bearing pyrite-quartz veins, gold-bearing polymetallic sulfide-quartz veins and sulfide poor-calcite-quartz vein in the middle-low orebody (Table4). The Type-3 deposits (e.g. Xifengshan) are located in the southern section of the Kangurtag metallogenic belt, where the age of ore formation isapproximately 270 Ma.

Table 4 The age data of ores and hosting rocks in Kanggurtag Au-Cu metallogenic belt

Type-4 Cu-Mo deposits are located in the middle section of the Dananhu island arc. Systematic isotopic dating of the Tuwu-Yandong porphyry Cu deposits, applying several methods have been done by Qin et al. (2001; 2002). A single-grain zircon U-Pb age for plagiogranitic porphyry is 356 ±8 Ma, an ⁴⁰Ar-³⁹Ar age of an ore-bearing quartz vein is 347 ±2.3 Ma, and a K-Ar age of the altered sericite is 341 ±4.90 Ma. The most reliable age estimate is probably a new Re-Os isochron date of 322.7 ±2.5 Ma for seven molybdenite samples from the deposit (Rui et al. 2002b). These ages represent the first crystallization from magma, hydrothermal alteration, and mineralization, respectively. Therefore the intrusive and metallogenic epoch of the Tuwu-Yandong Cu deposit is late Devonian to early Carboniferous in age rather than late Carboniferous or Permian as suggested by Liu et al. (2001). The Type-5 Cu-Ni deposit was formed in ultrabasic-basic complexes in the eastern part of the Kanggurtag Au-Cu metallogenic belt, with an ore-forming age of 305–285 Ma. Type-6 Cu-Zn deposits are located in the northwestern section of the region. The age values of sulfides in type-6 deposits are between 339.1 Ma and 230.2 Ma (Chen, 1999; Li et al. 1998). Type-7 skarn-copper deposits are located in the central part of the Kanggurtag ore belt. Amphiboles from the skarn ores of the Weiquan Cu deposit were selected for ⁴⁰Ar-³⁹Ar dating, and indicate a step-heating age of 275.8±2.87 Ma, interpreted as the age of an extensional event of the Tianshan orogen (Mao et al. 2002).

As a whole, mineralization of the Kanggurtag metallogenic belt has the following characteristics: the Cu-Mo deposits are distributed in the Devonian-Carboniferous island arc located to the north of the Kanggurtag fault; the Cu-Ni deposits are located to the south of the fault; and the Au deposits are distributed in the Carboniferous island arc, south of the Kanggurtag fault system. From north to south, the ore-forming ages become younger.

Gold- and copper-deposit ore-forming processes

Located between the Siberian and Tarim blocks, the Kanggurtag metallogenic belt went through a complex tectonic evolution and mineralization history since the Paleozoic (Fig.8):

Fig.8

Matellogenic geodynamics and model of the Kanggurtag Au-Cu ore belt, east Tianshan

(1) :

During the early-late Devonian, the Tianshan ocean was subducted beneath the old Dananhu (Ordovician-Silurian?) island arc, forming the Devonian arc volcanic rocks, and the Kanggurtag accretionary wedge. At the same time, a S-dipping subduction zone was active beneath the northern Tarim block (Fig.8A).

(2):

In the early to mid-Carboniferous, N-dipping subduction beneath the Dananhu arc generated magma intrusion, leading to the formation of the porphyry Cu deposits. The magmatic front migrated southward forming the Yamansu arc which was contructed upon the Kanggurtag accretionary wedge (Xiao et al. 2004; Fig.8B).

(3) :

In the latest Carboniferous to early Permian, during the closure of the ancient Tianshan ocean, a large volume of mafic-ultramafic complex rocks (310–285 Ma) were emplaced along the eastern section of the Huangshan-Kanggurtag structural zone, resulting in several magmatic copper-nickel deposits of large or medium size. At the same time, along the Qiugemingtashi-Huangshan ductile shear zone and in its vicinity, the shear zone-type gold deposits were formed (Fig.8C). The Kanggur deposit represents a typical example of a shear-zone-type gold deposit. The main mineralization stage of the deposit was formed in the period of 290–270 Ma, simultaneously with collisional orogenesis in Tianshan (C₂-P₁). These results indicate that the Kanggur gold deposit is of orogenic gold (Groves et al. 1998; Kerrich et al. 2000; Goldfarb et al. 2001).

(4) :

Volcanic- or subvolcanic-hosted epithermal gold deposits, such as the Xitan and Mazhuangshan gold deposits, are associated with extensional tectonics and volcanic activity in the post-collision period. Following this event, magmatic hydrothermal and skarn deposits were formed. The magmatic hydrothermal gold deposits are related to intrusions of intermediate-basic diorite, granodiorite, and plagiogranite. The ore deposits are located not only in intrusions but also in contact zones and even wall rocks (e.g. Jinwouzi Au deposit). On the other hand, the skarn zones were developed between a Late Carboniferous-Permian granite body and the Carboniferous carbonate (e.g. Weiquan Cu-Ag deposit; Fig.8C).

Conclusions

The gold and copper deposits along the Kanggurtag metallogenic belt of Eastern Tianshan can be divided into seven types according to their ore geneses: (1) epithermal gold-bearing quartz veins hosted by late Paleozoic volcanics and sub-volcanics; (2) ductile-shear-zone-type gold deposits including (I) those hosted by carboniferous volcanics and (II) those hosted by carboniferous turbidites; (3) magmatic hydrothermal gold-bearing quartz veins; (4) porphyry Cu-Mo deposits; (5) magmatic Cu-Ni deposits; (6) volcanic sedimentary Cu-Zn deposits hosted by tuffaceous sedimentary rocks of the Lower Carboniferous; and (7) skarn-rock-type Cu deposits.

The plate margin and plate convergent zones are the most important gold-copper mineralization regions. Various styles of mineralization were performed in different tectonic settings. The porphyry copper deposits are products of mature island-arc volcano-plutonic activity. Orogenic gold deposits were formed during compressional and transpressional deformation processes in accretionary and collisional orogens. Epithermal gold deposits and porphyry copper deposits were formed in the shallow parts of both island arc and continental arcs, although they were produced at different times and locations. The ore-forming processes of the main gold and copper deposits correlate to processes active now in the SW Pacific subduction system.

In the early to mid-Carboniferous, N-dipping subduction beneath the Dananhu arc may have generated magma intrusion, leading to formation of the porphyry Cu deposits. From late Carboniferous to early Permian, several magmatic copper-nickel deposits are related to emplacement of mafic-ultramafic complexes. The formation of shear-belt-type gold deposits are associated with collisional events. The volcanic- or subvolcanic-hosted epithermal gold deposits, such as the Xitan and Mazhuangshan gold deposits, are associated with extensional tectonics and volcanic activity in post-tectonic period, followed by the formation of magmatic hydrothermal and skarn deposits.