1. Introduction
From north to south, the Tibetan Plateau comprises the Songpan–Ganze flysch complex, the eastern Qiangtang Terrane, the western Qiangtang Terrane, the Lhasa Terrane, and the Himalaya (
Figure 1a, [
1]) separated by the Jinsha, Longmu Tso–Shuanghu, Bangong–Nujiang, and Indus–Yarlung Zangbo suture zones, respectively. These blocks and terranes represent relicts of Tethyan oceanic material of various ages. The Sanjiang Metallogenic Belt is located between the Jinshajiang and Bangonghu–Nujiang sutures along the eastern and northern margins of the Tibetan Plateau and extends for nearly 1500 km [
2]. This belt is an important Pb-Zn-producing region within the Himalayan–Tibetan metallogenic domain. The Sanjiang Metallogenic Belt evolved as part of a Paleozoic–Mesozoic Tethys archipelagic arc basin over which was sequentially superimposed a Tertiary foreland basin, a strike–slip pull-apart basin, and a thrust-nappe structure that formed during Himalayan orogenesis [
3].
The conditions of metallogenesis in the Sanjiang Metallogenic Belt were favorable for the formation of large-scale deposits [
6,
7]. A series of Pb-Zn deposits developed during the Cenozoic, mainly along the margins of Mesozoic–Cenozoic continental basins. The main deposits are distributed from southeast to northwest and include the Jinding and Baiyangping super-large Pb-Zn deposits in the Lanping basin [
8,
9], the Zhaofayong Pb-Zn deposit in the Changdu Basin [
10], and the Dongmozhazhua and Mohailaheng Pb-Zn deposits in the Yushu basin (
Figure 1a) [
11,
12]. The Tuotuohe area, located in the northern part of the Sanjiang Metallogenic Belt, hosts several medium- to low-temperature hydrothermal-vein-type, porphyry-type, MVT and VMS-type deposits [
13,
14,
15]. Many Pb-Zn deposits and other sites of mineralization have been discovered in the Tuotuohe area, including the Chaqupacha super-large Pb-Zn deposit, the Chuduoqu large Pb-Zn-Cu deposit, and the Basihu medium-sized Pb-Zn deposit, as well as the Nariniya, Nabaozhalong, and Zhalaxiageyong Pb-Zn deposits (
Figure 2).
Exploration of the Chuduoqu Pb-Zn-Cu deposit started in 2007. From 2007 to 2011, the Qinghai Fifth Institute of Geological and Mineral Survey carried out work in the northern part of the deposit, where geochemical data indicate favorable conditions for mineralization, and a set of ~N–S-oriented ore-bearing zones was discovered. These zones show good surface and shallow-subsurface mineralization, weak deep mineralization, and generally unsatisfactory indications for ore-prospecting. In 2011, fracture zone SBIII was discovered, which is the major ore-controlling structural zone formed by the NWW-oriented fault, indicating that ore-prospecting be undertaken on this fracture zone. The Pb-Zn-rich ore body M9 in fracture zone SBIII was the first such body to be discovered in the hanging wall of the main fault, followed by Cu-Ag ore body M10 and Pb-Ag ore body M11 (
Figure 3a) [
15]. The Chuduoqu Pb-Zn deposit has estimated metal reserves of 402,547 t Pb, 112,672 t Zn, 9197 t Cu, and 593 t Ag [
15]
. Various studies have reported the geological features, mineralization, fluid inclusions, ore-controlling structures, and results of exploration of the Chuduoqu deposit [
13,
15]. Most of these studies have shown that this deposit formed in relation to Cenozoic magmatic hydrothermal activity. The deposit is a mesothermal hydrothermal type that is most likely associated with the intrusion of Cenozoic syenite porphyry dykes. However, the detailed characteristics and mechanisms of the mineralization of the Chuduoqu deposit are poorly constrained, especially when compared with the detailed information available for other Cenozoic Pb-Zn deposits in the Tuotuohe area.
The origin, properties, and evolution of the ore-forming fluids, as well as the genesis of the Chuduoqu deposit, are still not fully understood, which limits our overall understanding of the genesis of hydrothermal vein-type Pb-Zn mineralization in the region. Using field observations and petrographic studies, we investigated the ore-controlling structures, the composition and characteristics of fluid inclusions, and the stable (C–H–O–S) and radiogenic (Pb) isotope systematics of the Chuduoqu Pb-Zn-Cu deposit. In this paper, we report the results of our study, discuss the characteristics of the mineralizing fluids and metal sources as well as the mechanisms of mineralization, and constrain the genesis of the deposit. In doing so, we provide an important basis for understanding the Chuduoqu Pb-Zn-Cu deposit and similar deposits in the Tuotuohe area. Our findings should prove valuable for prospecting in the Tuotuohe area and in the Sanjiang Metallogenic Belt.
2. Geological Background
The Chuduoqu Pb-Zn-Cu deposit is located in the Tuotuohe area in the northern part of the Sanjiang Metallogenic Belt, central Tibet (
Figure 1, [
17]). The Tuotuohe area is positioned on the margin of the northern Qiangtang Terrane between the Jinsha River suture zone and the Longmucuo Shuanghu suture zone [
6,
15,
16]. The oldest rocks in the Tuotuohe area are Carboniferous clastic and carbonate sediments that are thought to have formed in a passive continental margin setting (
Figure 2, [
18]); the Permian to Triassic units consist mainly of marine carbonate, clastic, and volcanic rocks. Recent studies of Permian magmatic rocks in the Yushu area have suggested that these units were deposited in a continental-margin-arc setting associated with northward subduction of the Shuanghu oceanic plate between ca. 275 and 248 Ma [
19]. Lower and Middle Triassic rocks are absent from the area, meaning that the Upper Triassic rocks unconformably overlie the underlying units. During the Late Triassic, the Tuotuohe area was in a subduction setting involving the southward subduction of the Jinsha oceanic plate [
20].
Lower Jurassic rocks are absent from the study area (
Figure 2, [
16,
21]; Middle to Upper Jurassic units in the area consist of clastic and carbonate rocks of (from bottom to top) the Qumocuo, Buqu, Xiali, Suowa, and Xueshan Formations [
22]. During the Cretaceous, the Tuotuohe area entered a continental sedimentary stage, when thick successions of clastic deposits were laid down. The Cenozoic units comprise terrigenous clastic and carbonate rocks of the Eocene Tuotuohe Formation, the Eocene–Oligocene Yaxicuo Formation, and the Miocene Wudaoliang Formation [
18,
23,
24], which are exposed mainly in the northern part of the Tuotuohe area (
Figure 2).
The Tuotuohe area contains a large-scale thrustnappe structure and strike-slip system as a result of India–Eurasia collisional orogenesis during the Cenozoic [
2,
9]. The thrustnappe structural belt comprises a series of NWW-trending thrust faults and folds, most of which dip to the southwest [
25]. The large-scale thrustnappe in the Tuotuohe area underwent two main episodes of thrusting, one at around 52–42 Ma and the other at around 24 Ma [
25]. Between these two episodes, strike-slip activity developed with the formation of a series of strike-slip fault systems [
26].
Magmatic activity in the Tuotuohe area started during the late Paleozoic and ended during the Cenozoic. The magmatism was characterized by relatively weak intrusive and strong volcanic activity. Volcanic rocks are widespread at a regional scale. These volcanic rocks comprise Permian basaltic andesite interlayered with basalt; Late Triassic andesite, basalt, and pyroclastic rocks; and Cenozoic trachyte. The Cenozoic volcanic rocks are distributed mostly around the locality of Nariniya and dated at 45–22 Ma [
27,
28,
29].
Magmatic intrusions are widely dispersed, with numerous igneous rock outcrops, but the total area covered by these rocks is quite small. Magmatic rocks include those formed during the Indosinian, Yanshanian, and Himalayan periods. Late Permian–early Triassic syenites and diorite bodies are found in the Chaqupacha deposit [
30], and Middle Triassic diorite is present in the Basihu mine [
31]. Late Cretaceous granites occur in the Longyala and Munai areas of the Tanggula mountains [
32]. Paleogene olivine gabbro–diabase has been discovered in the Quemocuo mining area (
Figure 2, [
33]), and Cenozoic porphyry bodies have been discovered in the Zhamuqu, Nariniya, Zhalaxiageyong, and Saiduopugangri areas [
13,
32,
34]. The Cenozoic volcanic rocks and granites mentioned above consist predominantly of shoshonitic to high-K calc-alkaline rocks and were formed in a geodynamic setting of crustal shortening, thickening and melting [
27,
28,
29,
32,
35]. Their occurrence and ages are supporting evidence for Cenozoic crustal shortening and uplift of the plateau in the study area.
3. Ore Deposit Geology
Rocks exposed in the studied mining area include those of the Middle Jurassic Buqu and Xiali formations, the Upper Jurassic Suowa Formation, and Quaternary deposits (
Figure 3a). The Middle Jurassic Buqu Formation (J
2b) is composed predominantly of light-gray to dark-gray limestone, with purple-red and gray argillaceous siltstone and quartz–feldspar sandstone. This formation is rich in marine fossils and is distributed primarily in the southwestern part of the mining area. The Buqu Formation rocks strike at 110–130°, dip at 30–70° to the NNE, and conformably overlie the Upper Xiali Formation (J
2x). The Middle Jurassic Xiali Group (J
2x) is an important ore-bearing group in the study area and consists of the following three lithological sections: (a) the lower section comprises blue-gray crystalline limestone, purple-red muddy siltstone intercalated with purple-red feldspar debris sandstone, and gray feldspathic quartzitic sandstone; (b) the middle section has a lithological association of purple-red feldspathic lithic quartz sandstone interbedded with bioclastic crystalline limestone; and (c) the upper section comprises purple-red feldspar arkose intercalated with gray-green feldspathic quartzitic sandstone. Sedimentary rocks of the Xiali Formation (J
2x) are distributed primarily in the central part of the mining area. In the northern-central part of the mining area, they strike at 170–200° and dip at 30–70° to the east, and in the southern-central part, the beds strike at 130–160° and dip at 30–70° to the NE. The Xiali Formation conformably underlies the Upper Suowa Formation (J
3s). The Upper Jurassic Suowa Group (J
3s) is distributed mainly in the eastern part of the mining area. The lower part of this group consists of gray-green calcareous siltstone and mudstone intercalated with biological calcareous siltstone, interbedded with thick layers of muddy crystal limestone, whereas the upper part comprises thick layers of dark-gray muddy crystalline limestone interbedded with thin layers of muddy crystalline limestone. The Suowa Group rocks strike at 110° and dip at 30–50° to the east.
The studied mining area is characterized by NWW- and N-trending faults. The NWW-trending faults have four associated fracture zones: SBIII, SBIV, SBV, and SBVI. Of these, fracture zone SBIII (
Figure 3b) is the main ore-controlling fracture zone in the area and measures 1000 m in length and 200–300 m in width, strikes at 120–135°, and dips at 65–75° to the SSW. A series of parallel secondary faults are developed in the hanging wall of the main fault and are associated with fracture zones SBIV, SBV, and SBVI. A second group of faults trends N–S and occurs in the footwall of fracture zone SBIII. These N-trending faults have formed several fracture zone structures, including SBI and SBII.
The intrusive rocks in the Chuduoqu mining area include syenite porphyry veins, diabase veins, and fine-grained granite dykes, syenite porphyry veins are oriented NE to E and NW, diabase veins are oriented E, fine-grained granite dykes are oriented NE (
Figure 3a). Syenite porphyry veins have been identified in boreholes. These veins occur in the rocks of the Xiali Formation (J
2x) and show strong alteration, mainly baritization and limonitization, as well as carbonation and silicification. The contact zone of the syenite porphyry with the host rocks of the Xiali Formation (J
2x) is highly mineralized, with the development of massive, vein-like, and disseminated pyrite, chalcopyrite, and galena (
Figure 3b). Locally, crystal lithic tuff overlies the Xiali Group (J
2x) across an unconformity (
Figure 3b). The U-Pb age of the crystal lithic tuff is 68.3 ± 0.7 Ma (unpublished data) and was erupted prior to the onset of mineralization.
The mineralization in the ore bodies is most enriched in and around the intersections of secondary faults (including fracture zones SBIV, SBV and SBVI) with the main fracture zone SBIII. There are six main mineralized alteration zones in the mining area. Eleven polymetallic ore bodies have been identified, including four Pb-Zn-Ag ore bodies (M1, M5, M7, and M9), four Pb-Ag ore bodies (M2, M3, M4, and M11), one Pb-Cu-Ag ore body (M8), one Cu-Ag ore body (M10), and one Pb ore body (M6) (
Figure 3a). The main ore-body-hosting rocks of the Chuduoqu deposit are those of the Xiali Formation (J
2x), chiefly cataclastic micritic silty limestone and cataclastic quartz–feldspar sandstone. Of the eleven identified ore bodies, six are oriented ~N–S and five are oriented NWW. The six approx N–S-trending ore bodies (M1–M6) occur in N-oriented fracture zones as layers and veins. These ore bodies have lengths of 150–1350 m and thicknesses of 4–16 m, dip at 42–62° to the SE, and host good quality surface mineralization but poor discontinuous mineralization at depth. The five NWW-trending ore bodies are distributed in NWW-oriented fractured zones.
In the Chuduoqu Pb-Zn-Cu deposit, the geology and metal resources of ore bodies M1, M2, M8, M9, and M10 have been investigated (
Table S1), whereas the resources of the remaining six ore bodies remain unknown. The main ore body, M9, occurs in the altered fracture zone SBIII, which is the main ore-controlling structure. Ore body M9 is layered, extends for more than 500 m, varies in thickness from 3.0 to 24.7 m, and dips at 20° to the south. This body contains an average Pb grade of 2.22% (locally reaching 21.13%), an average Zn grade of 1.41% (locally up to 8.69%), and an average Ag grade of 49.5 g/t (locally up to 220 g/t) (
Table S1), indicating very good prospecting potential. The degree of host-rock fragmentation in the main fracture zone of ore body M9 varies greatly, with the alteration and mineralization being strongest in regions of highly fractured limestone and sandstone, and weakest in regions of weak host-rock fragmentation.
The ore minerals of the Chuduoqu Pb-Zn-Cu deposit include specularite, magnetite, pyrite, chalcopyrite, bornite, tetrahedrite, pearceite, galena, sphalerite, limonite, malachite, and azurite and the gangue minerals include quartz, calcite, dolomite, barite, sericite, chlorite, and epidote. The ores show mainly xenomorphic granular texture, with subordinate idiomorphic–hypidiomorphic granular texture. In addition, the ores exhibit cataclastic and metasomatic characteristics. The ores show mainly block and vein structures, as well as local disseminated structures. Hydrothermal alteration is widespread in the Chuduoqu Pb-Zn-Cu deposit, with the most intensive alteration occurring in and around the mineralized Pb-Zn-Cu veins. The key components of alteration assemblages include silicification, chloritization, epidotization, sericitization, carbonation, and baritization. Distinct episodes of hydrothermal alteration are recognized: an early episode of silicification, three intermediate episodes (baritization, phyllic and propylitic), and a late carbonatization. Silicification is the most widespread alteration type in the Chuduoqu Pb-Zn-Cu deposit, which coexists with minor early precipitated specularite (
Figure 4a). Silicification was overprinted by baritization and phyllic alteration, which consists of barite, quartz and sericite. Baritization and phyllic alteration appear closely related to Cu metal sulfides deposition (
Figure 4c,d). Phyllic alteration was overprinted by propylitic alteration, characterized by an assemblage of chlorite, epidote, and quartz. Propylitic alteration appears closely related to base metal sulfides deposition (
Figure 4d,e). The final stage of hydrothermal alteration is carbonatization, which overprinted all the previous alteration types coexisting with minor pyrite. In addition, there is no obvious spatial zonation of various hydrothermal alteration types, in most cases, the alteration assemblages are superimposed upon one another.
Based on field observations, mineral assemblages, and crosscutting relationships (
Figure 4), we divided the mineralization history of the deposit into a hydrothermal mineralization phase (which is subdivided into four mineralization stages) and a supergene phase. The characteristics of the mineral associations in the four hydrothermal stages are as follows (
Figure 5).
3.1. Quartz–Specularite Ore (Stage I)
In this stage, specularite ore is the main metallic mineral and occurs as needle-like crystals with idiomorphic–hypidiomorphic texture. The mineral assemblage of this stage is quartz + specularite, cut by late-stage quartz–barite–chalcopyrite veins (
Figure 4a).
3.2. Quartz–Barite–Chalcopyrite (Stage II)
In this stage, the gangue minerals are mainly quartz and barite, and the ore minerals are mainly pyrite and chalcopyrite (
Figure 4c). Small amounts of bornite and tetrahedrite are found, mostly with idiomorphic–hypidiomorphic texture. The mineral assemblage is quartz + barite + pyrite + chalcopyrite + bornite + tetrahedrite (
Figure 4g,h). This stage is the main metallogenic stage for Cu.
3.3. Quartz–Polymetallic Sulfide Stage (Stage III)
This stage is the main stage of deposit formation. The gangue minerals are dominated by quartz, and the metallic minerals are chiefly galena, sphalerite, pyrite, and minor pearceite and chalcopyrite (
Figure 4d), mostly showing idiomorphic and hypidiomorphic textures. The mineral assemblage is quartz + galena + sphalerite + pyrite + chalcopyrite+ pearceite (
Figure 4e,i). This stage is the main metallogenic stage of Pb and Zn.
3.4. Quartz–Carbonate Stage (Stage IV)
This stage is characterized by quartz–calcite veins with fewer metal sulfides compared with stage III. Some fine-veined disseminated pyrite is found. Numerous calcite veins are present (
Figure 4f), with minor quartz veins. The mineral assemblage is calcite + quartz + pyrite.
The supergene phase involved the formation of cerussite, malachite, azurite and limonite.
8. Conclusions
(1) The ores of the Chuduoqu Pb-Zn-Cu deposit in central Tibet are hosted in limestone and sandstone of the Middle Jurassic Xiali Formation (J2x) and are structurally controlled by NWW-trending faults cutting the host sediments. The mineralization of the Chuduoqu Pb-Zn-Cu deposit can be divided into four stages: quartz–specularite (stage I), quartz–barite–chalcopyrite (stage II), quartz–polymetallic sulfide (stage III), and quartz–carbonate (stage IV).
(2) H, O, C, S, and Pb isotope data of samples from the Chuduoqu deposit reveal that the ore-forming fluids had a dominantly magmatic signature but were mixed with meteoric water. The most likely source of metallogenic material was a regional-scale potassic magmatic hydrothermal fluid system, and the mineralization occurred between 40 and 24 Ma. Specifically for the Chuduoqu Pb-Zn-Cu deposit, the magmatic activity of a syenite porphyry intrusion most probably provided the heat source and main metallogenic material for the mineralization.
(3) Fluid mixing and cooling mainly contributed to the ore precipitation. In addition, small scale fluid boiling did take place in some quartz from stage III.
(4) The Chuduoqu Pb-Zn-Cu deposit is a mesothermal hydrothermal vein deposit and shares many similar features with those of Cordilleran-type vein deposits worldwide, and it was formed in an extensional environment related to late intracontinental orogenesis caused by India–Asia collision.