The mineralization of an ore-formation province generally developed a specific mineralization type during its geological evolution with several or one tectono-magmatic cycle. The enrichments of metallogenic elements probably are controlled by continental material heterogeneously, whereas the type of ore deposit is also controlled by regional geological structures. An ore-formation province is usually made up of several huge ore-forming belts, each huge ore-forming belt contains a number of large ore-college areas, and each large ore-college area contains one large or super-large ore deposit at least. A huge ore-forming belt can produce one or several unique ore-college areas at a certain crustal evolutional stage. The ore-forming processes are generally controlled by one tectono-magmatic zone, regional structure, or metamorphism. A large ore-college area refers to ore field distribution zones that are characterized by similar regional mineralization.
Numerous terranes with different properties and evolutional histories in the CAMR have distinct metallogenic natures and host various types of ore deposits. The core part of the CAMR consists of four ore-formation provinces: Altay, Balkhash-Junggar, Chu-Yili-Tianshan, and Southwest Tianshan. Due to the independence of geological evolution of these ore-formation provinces, their evolution histories and mineralization-types are highly variable. This paper focuses on the Balkhash-Junggar and Chu-Yili-Tianshan ore-formation provinces. Each ore-formation province contains several huge ore-forming belts. The classification scheme is listed in Table 1 and shown in Fig. 2.
Altay ore-formation province Ⅰ: Irtysh-Zaisan ore-forming belt Balkhash-Junggar ore-formation province Ⅱ: Zharma-Saur ore-forming belt (1) Saimusek Au-W-REE
(2) Kensay Cu-Mo
(3) Kuorzhenguora Au-Cu
Ⅲ: Tarbahtay-Xiemistay ore-forming belt (4) Ayagus Cu-Pb-Zn
(5) Baiyanghe-Xiemistay Cu-Au-Be- U
Ⅳ: Aktogay-Baerluke ore-forming belt (6) Aktogay Cu-Mo-Au
(7) Baerluke (Suyunhe) W-Sn-Mo- Cu-Fe-Au
Ⅴ: Balkhash-western Junggar ore-forming belt (8) Kunrund Cu-Mo
(9) Sayak Cu-Au-Mo
(10) Hatu-Baobei-Sartohay Au-Cr
(11) Baogutu Cu-Au
Chu-Yili-Tianshan ore-formation province Ⅵ: Alatau-Sairimu ore-forming belt (12) Wurazar-Karagaire W-Mo-Pb-Zn
(13) Burutashi-Kailongur Pb-Zn-W- Fe-Sn
(14) Kekesay-Aerkeley Cu-Mo-Au
(15) Jiekeli-Keguqin Pb-Zn-Cu
(16) Alatau W-Sn
Ⅶ: Chu-Yili-Bolehuole ore-forming belt (17) Akbakai Au
(18) Boguty W
(19) Dabate-Lailisigaoer Cu-Mo
Ⅷ: Issyk-Awulale ore-forming belt (20) Axi-Yermande Au
(21) Bala Chichkan-Jerooy Au-Cu
(22) Akejuz Pb-REE-Cu-Mo-Fe
(23) Taldybulak-Kensu-Oktorkoy Au- Cu belt
(24) Awulale Cu-Ag-Fe-Pb
(25) Katebasu Au
(26) Changannuor-Shikebutai Fe-Cu
(27) Tianger-Wangfeng Au belt
(28) Motuosala Fe-Mn
Ⅸ: Kazharman-Nalaty ore-forming belt (29) Kazharman Au-Cu-Fe-W-Sn
(30) Jietmu Fe-W
(31) Karaker Cu-Mo-Au
(32) Kumtor Au
(33) Sawayardun-Dashankou Au
Southwest Tianshan ore-formation province X: Southwest Tianshan ore-forming belt (34) Sairajiaz Sn
(35) Chanhansara Au-Sb-Hg
Table 1. Ore-formation province, huge metal-ore belt, and large ore-college area in the core part of Central Asian metallogenic region (CAMR)
The Balkhash-Junggar ore-formation province, consisting of 4 huge ore-forming belts (Zharma-Saur, Tarbahtay- Xiemistay, Aktogay-Baerluke, and Balkhash-western Junggar ore-forming belts, which were numbered with Ⅱ, Ⅲ, Ⅳ, and Ⅴ, respectively, in Fig. 2), is adjacent to the Altay province bounded with the Irtysh-Zaisan ore-forming belt in northeastern and the Alatau-Sairimu ore-forming belt in southwestern, which contacts with the Chu-Yili-Tianshan province. The Balkhash-Junggar province hosts numerous large metal deposits, including hydrothermal Au-Ag-Wu-Sn, skarn-porphyry, porphyry Cu-Mo-Au, and epithermal deposits of rare metals. All these porphyry deposits are controlled by Late Carboniferous intrusions and related hydrothermal systems.
There is a good coupling between the evolution and mineralization in the Balkhash-Junggar ore-forming province. Although the proportion of the crust-mantle component varies greatly in a single magmatic system, most rocks have similar +εNd values. Positive εNd value of the Late Paleozoic magmatic rocks that spread widely is another important characteristic of the CAMR (Zhou T F et al., 2015, 2008; Yang et al., 2014; Zhu et al., 2009; Kovalenko et al., 2004; Han et al., 1997). The ore-forming ages of porphyry Cu-Au deposits in the Balkhash area vary from Ordovician to Permian, and mostly at Late Carboniferous (An et al., 2015; Chen X H et al., 2015; Chen Y F et al., 2013; Yang et al., 2012). The skarn Cu-Mo-Sn deposits, epithermal Au-Ag deposits, and porphyry Cu deposits, constitute a unique ore-forming system in the Balkhash-Junggar ore-formation province.
For example, the Balkhash-western Junggar ore-forming belt (the belt V, see Fig. 2) contains at least 4 large ore-college areas: Kunrund Cu-Mo, Sayak Cu-Au-Mo, Hatu-Baobei- Sartohay Au-Cr, and Baogutu Cu-Au. The Hatu-Baobei- Sartohay Au-Cr and the Baogutu Cu-Au ore-college areas are the most important regions for Au-Cu-Cr mining activities in northwestern China due to recent progresses in mineral exploration. Locating on the central part of the CAMR, the western Junggar is characterized by distributions of Early Paleozoic ophiolites, Late Paleozoic volcano-sedimentary rocks and intermediate to granitic intrusions (see Fig. 3). The Tangbale, Baijiantan-Baikouquan, Darbut-Sartohay, Kujibai-Honguleleng ophiolite belts have been studied for interpreting the Paleozoic subduction-accretion processes. The Tangbale ophiolite mélange consists of radiolarian chert, pillow lava, metagabbro, serpentinite, harzburgite and lherzolite. Consistent with stratigraphical records, radiometric dating suggested that the Tangbale ophiolite mélange was formed at Cambrian (Ren et al., 2014). The Baijiantan-Baikouquan ophiolitic mélanges are mainly composed of serpentinite with lherzolite lenses, blocks of metagabbro and amphibolite. Consisting with recently reported isotopic age data (Zhu et al., 2015), Middle Ordovician radiolarians (He et al., 2007; Buckman and Aitchison, 2001) found in West Junggar suggested that the evolution of the paleo-ocean lasted until Late Ordovician.
Figure 3. Geology map of the western Junggar in the core part of the CAMR (modified from Zhu et al., 2013) labeled with ore deposits including gold, copper and chromitite.
The low-grade metamorphic gabbro samples in the Darbut- Sartohay region were dated to be 426.0±5.8 Ma, implying that the paleo-ocean floor spreading happened until Middle Silurian (Zhu et al., 2013). Serpentinite with chromitite lenses occasionally occurs at the stratigraphic bottom of the Sartohay ophiolitic mélange (Zhu et al., 2016). The Devonian sandstone, containing plant fossils (Ma et al., 2015, 2011; Zong et al., 2015), and Devonian to Early Carboniferous volcanoclastics cover ophiolitic mélange and flysch. These newly uncovered geological data are conflicting with the previously proposed tectonic models. For example, the widespread Early Carboniferous volcano- sedimentary rocks in western Junggar, filling in the post-orogenic basin and occurring as molasses in most cases, were misunderstood as island arcs in geological literatures. Increase of geological data could illustrate a completely new version for geology evolution in the western Junggar.
For the Hatu-Baobei-Sartohay Au-Cr large ore-college area, zircons separated from andesite in the Hatu region give an average age of 335.2±2.9 Ma, and zircons separated from tuff in the Baobei region give an average age of 328.1±1.8 Ma (Zhu et al., 2013). Large granite bodies in West Junggar were dated to be ~300 Ma (Han et al., 2006), while small granitic to diorite plutons and various dykes intruded into the Devonian to Early Carboniferous volcano-sedimentary units were dated to be 312–277 Ma. For example, two small intrusive bodies intruded into the Baikouquan ophiolitic mélange were dated to be 310.7±3.7 and 312±3 Ma (Zhu et al., 2013).
For the Baogutu Cu-Au large ore-college area (Fig. 4), the widespread Early Carboniferous volcano-sedimentary rocks were dated to be 310–316 Ma (Wei et al., 2011; Liu et al., 2009), small intrusive bodies consisting of granodiorite, quartz diorite, and diorite were dated to be 310–325 Ma, felsic dykes were dated to be 280–310 Ma, and diorite dykes were dated to be ~280 Ma (Zhu et al., 2014). The Baogutu porphyry Cu-Au deposit was formed at ~310 Ma, while gold deposits formed probably 10 Ma later (at ~300 Ma). Studies show that felsic magma intruded volcanic-sedimentary sequence was rich in water and Cu-Au with high oxygen fugacity, which finally formed porphyry Cu-Au and hydrothermal gold deposits.
Figure 4. Genetic model showing gold and copper mineralization in the Baogutu region of the western Junggar. There are three stages of magmatic intrusions: granite-diorite pluton and granitic porphyry hosting porphyry Cu-Au deposit formed during 310–316 Ma, felsic dykes formed at 280–310 Ma, which was followed by diorite/gabbro dykes intruded at ~280 Ma. The Au-As-Sb-Bi mineralization occurred after the intrusion of granitic magma but before mafic dykes.
In general, tectonic evolution and ore-formation in the Balkhash-western Junggar ore-forming belt could be illustrated in a model as shown in Fig. 5. The subduction of oceanic crust continued until the end of Silurian, and the Paleo-Junggar Ocean closed at Early Devonian. Afterwards, volcano eruption formed widespread volcanic-sedimentary rocks during extensional stage at Early Carboniferous period, which was probably caused by delamination of the thick lower crust formed during previous accretionary processes. Felsic magma intrusion caused formation of porphyry Cu-Au deposit at ~310 Ma and related hydrothermal gold deposits at ~300 Ma.
Figure 5. Tectonic model showing the evolution of western Junggar and related ore-formation. The subduction process continued until the end of Silurian (a, b). The Paleo-Junggar Ocean closed probably at Early Devonian (c). The following extensional stage formed volcanic-sedimentary basin at Early Carboniferous. Felsic magma intrusion caused formation of porphyry Cu-Au deposit at ~310 Ma (d) and related hydrothermal gold deposits 10 Ma later (e).
The Chu-Yili-Tianshan ore-formation province consists of four huge ore-forming belts: Alatau-Sairimu (the Ⅵ belt in Fig. 2), Chu-Yili-Bolehuole (the Ⅶ belt in Fig. 2), Issyk-Awulale (the Ⅷ belt in Fig. 2), and Kazharman-Nalaty (the Ⅸ belt in Fig. 2). The Chu-Yili-Tianshan magmatic zone formed within three stages: (a) with the subduction of the oceanic crust, andesite-rhyolite and diorite-granite assemblages formed during the Silurian to Carboniferous periods; (b) large-scale felsic magma eruption and following gabbro-granite intrusions at Late Carboniferous; (c) bimodal volcanic rocks and diorite- syenite-alkaline assemblages developed in post-collisional setting at Early Permian (Gou et al., 2012; Seltmann et al., 2011; Konopelko et al., 2008).
The Northwest Tianshan in Xinjiang, belonging to the eastward extension part of the Chu-Yili-Tianshan ore-formation province, developed the Late Paleozoic Alatau-Tulasu-Keguqin volcanic arc, accompanied with intensive volcanic activities during Devonian to Carboniferous. Zircons separated from the andesite collected from the Axi and Jingxi-Yelmand gold mines were dated to be > 360 Ma (An et al., 2013; An and Zhu, 2008). In addition to volcanism-related gold deposits, some large gold deposits in Chu-Yili-Tianshan were mainly controlled by shear zone. For example, the Bingdaban-Tianger shear zone, located along the northern margin of central Tianshan, controls the Wangfeng, Tianger, and Saridala gold deposits. Studies on these gold deposits showed that the ductile shear zone activity lasted over 50 Ma, and the final shear deformation at Triassic period controlled gold deposition (Zhu, 2011).
The Alatau-Sairimu ore-forming belt is characterized by the formation of Pb-Zn deposits in Early Paleozoic and porphyry, hydrothermal Au, and Wo-Mo deposits in Late Paleozoic. Five large ore-college areas could be identified: Wurazar- Karagaire W-Mo-Pb-Zn, Burutashi-Kailonger Pb-Zn-W-Fe-Sn, Kekesay-Aerkeley Cu-Mo-Au, Jiekeli-Keguqin Pb-Zn-Cu, and Alatau W-Sn ore-college areas. Generally, the Jiekeli-type deposit is pyritic polymetallic deposit hosted in carbonate-clastic rocks. Their ore-bodies are distributed along a narrow band of the Lower to Middle Ordovician carbonate-clastic rocks that underwent metamorphism. The outline of the ore-body is coordinated with fractures. The host country rocks are composed of lensoid mudstone-marlstone-limestone-dolomite and pyrite- bearing siliceous layers, which underwent metamorphism at greenschist facies. Two ore-forming stages have been identified: (a) the SEDEX with ore beds alternated with layers of chert-mudstone-calcareous rocks at Caledonian extensional stage; (b) metamorphic hydrothermal fluids, migrating along fault system, extracted ore-forming elements from country rocks including the pre-existing pyrite-sphalerite ore-bodies, and finally formed echelon ore-bodies at the Hercynian orogenic stage. The Jiekeli large ore-college area extends eastward into Junggar-Alatau in Xinjiang. The Sairimu Cu-Pb-Zn large ore-college district may be the eastward extension part of the Jiekeli-Keguqin ore-forming belt. The Proterozoic Kusongmuqieke Group, consisting mainly of neritic carbonate with clastic rocks and distributing along the margin of the Sairimu terrane, was covered by the Middle Devonian conglomerate-sandstone intercalated with bioclastic limestone.
The Chu-Yili-Bolehuole ore-forming belt corresponds to the Early Paleozoic Bolehuole orogenic belt. Several porphyry Cu-Au polymetallic and hydrothermal vein-type Au-Cu deposits, including the Lamasu Cu-Zn deposit, Lailisigaoer Cu-Mo deposit, Dabate Cu deposit, Kekesay Cu deposit were found during the last decade. The first two deposits are related to the Late Carboniferous to Early Permian felsic intrusions, and the latter two deposits are related to Early Carboniferous granite.
In the Issyk-Awulale ore-forming belt, Neo-Proterozoic to Early Cambrian andesite-rhyolite overlies the Early to Middle Proterozoic amphibolite-gneiss. The Early Caledonian structural layer is composed of Late Cambrian sandstone and Early Ordovician altered basalt-diabase, while the Late Caledonian layer consists of Ordovician and Devonian sequences. The Devonian terrestrial volcanic-sedimentary rocks cover on the Ordovician flysch and molasses. The Issyk-Awulale ore-forming belt is characterized by multi-stage with multiphase superposition of various types of mineralization. The epithermal, exhalative sedimentary Pb-Zn, and VMS Fe-Mn deposits were formed in the Naman-Jialayier suture zone. The porphyry copper deposits are related to the Ordovician diorite- granodiorite (Jenchuraeva, 1997). For example, the Taldybulak porphyry copper deposit was hosted in the quartz diorite porphyry with explosive breccias. The alterations outward from the intrusive center are potassic alteration and argillzation, along with the prophylitic alteration in wall rocks. This huge porphyry Cu-Au ore-forming system (for example, the Taldybulak-Karakol-Oktorkoy-Zharkulak porphyry Cu-Au ore-forming belt) was recognized in the Ordovician arc on the northern edge of the Issyk terrane and the ore-forming age of the Taldybulak porphyry Cu-Au deposit was dated to be 464 Ma by Re-Os method (Yakubchuk et al., 2011). The Andash porphyry copper deposit formed in two explosive breccia pipes of the Ordovician dioritic-granodioritic intrusions. Potassic alteration and silicification developed in the breccia-pipes, while argillic-propylitic alteration mainly developed along margins. The discovery and exploration of the Ordovician Kendeketas-Talas porphyry copper ore belt provide new ideas for searching porphyry deposits in Xinjiang. The Carboniferous granite in this ore-forming belt is also an important exploration target. Significant breakthroughs have been achieved recently in the eastern part of this belt. Dozens of large ore deposits, including the Dunde Fe-Au-Zn and Cartebasu Au deposits were found. For example, the Cartebasu gold deposit hosted in granite-diorite intruded into Early Carboniferous volcanic- sedimentary rocks, and this pluton is very likely to host a large porphyry Cu-Au deposit in the deep.
The Kazharman-Nalaty ore-forming belt is separated from the Southwest Tianshan ore-formation province by the Nikulaev Line-Nalaty fault system. The Early Paleozoic Caledonian structural layers, overlain by the Devonian volcanic- sedimentary basin, principally consist of the Early–Middle Devonain volcanic and volcanic-sedimentary rocks in the lower part, the Late Devonian to Early Carboniferous carbonate rocks-littoral facies and red continental clastic deposit in the middle part, the Late Carboniferous to Permian sandstone- mudstone and felsic volcanic rocks in the upper part. Barite-Pb-, polymetallic vein-, pyrite- and skarn-typed deposits formed during the Early Caledonain period, while the rare metal- bearing greisen-quartz vein-, skarn-carbonate-greisen-, and granite intrusion-related W-Mo-Sn-typed mineralization formed at late stage of Caledonian period. In most cases, gold deposits were hosted in carbonaceous shale (the so called black-shale) controlled by shear zones.
Substantial gold endowment of Permian age (Yakubchuk et al., 2005; Mao et al., 2004; Cole et al., 2000) occurs in a number of giant deposits. For example, the Proterozoic to Early Paleozoic marine clasics, mafic volcanic rocks, carbonaceous shale, and dolomite are exposed in the Muruntau area in the form of structural window frequently. Two important shear zones formed at Early Permian period, which finally produced the Muruntau gold deposit. The direct Re-Os estimate for arsenopyrite from the Muruntau deposit of 287±1.7 Ma (Morelli et al., 2007). Bierlein and Wilde (2010) provided new constraints on the polychronous and possibly polygenetic nature of the Muruntau District. In addition to the prominent gold deposits, a large group of mercury and mercury-antimony deposits is known in the Alay segment. Further east, coeval, relatively small gold deposits occur in the eastern part of the South Tianshan. Several stages of mineralization within individual deposits between 290 and 220 Ma could be identified, which are probably in association with tectono-thermal fluid pulses (Mao J W et al., 2014; De Jong et al., 2009).
Ore-formation in Southwest Tianshan is characterized by porphyry Cu-Au and related hydrothermal Au-Ag-Sb-Hg. The geological structures are complicated and shear zones that control the large gold deposits are well developed. Only small portion of this ore-formation province has been shown in Fig. 2 and the main part exists in the west outside of this map. The key scientific issues of Southwest Tianshan ore-formation province include: (a) the constraint of fluid-magma process for the ore-forming processes during the subduction of the south Tianshan oceanic crust; (b) the ore-forming mechanism of epithermal Au-Ag-Sb-Te-Hg deposits in volcanic rocks distributed along the Nikulaev-Nalaty fault system; (c) the environment for the formation of huge porphyry Cu-Au belt and the reconstruction of the Late Paleozoic geotectonic framework; (d) the characteristics of the Central-South Tianshan ductile shear zone, and its significance for gold mineralization. The Caledonian and Hercynian magmatism developed in the Southwest Tianshan along with two oceanic crust subdution-arc systems, respectively. For example, the Ordovician strata in the central- western Tajikistan is overlain by the Silurian to Devonian thick carbonate rocks and thin terrestrial clastic rocks. The granitic magma intruded into these carbonate rocks resulting in skarn and related mineralization. The mineralization is closely linked with the granodioritic intrusions. Intensely folded and the northward thrust napped structures were developed during Hercynian period. The granodiorite, granite, and tonalite intruded the Silurian to Devonian terrestrial carbonate-clastic rocks, the magmatic fluid finally produced the Jilau W-Au deposit. Nearly vertical quartz veins and lensoids of ore shoots exist in strongly altered granodiorite. The native gold, associated with arsenopyrite, scheelite, and bismuth minerals precipitated through the second boiling of H2O-CO2-CH4-N2 fluids with low salinity. The gold grade is in response to the high CH4 concentration in fluid. This suggests that a portion of mineralization fluids derived from the reducing carbonaceous strata interacted with skarn tungsten ore-bodies and formed the Au-W-rich ore-forming fluids (Cole et al., 2000). The formation of many hydrothermal gold deposits is closely associated with CH4-rich fluids and the ophiolites in the Central Asian are rich in this kind of fluids. These facts and a good correlation between giant deposits in Southwest Tianshan and the deep structures indicate that the long-term activities and effective accumulation in specific locations of deep ore-forming fluids are necessary for the formation of huge deposits.