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Xiarihamu |
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Qinghai, China |
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Ni Co Cu
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Super Porphyry Cu and Au
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The Xiarihamu magmatic sulphide nickel-cobalt-copper deposit is located SW of Wutumeirenxiang, which is ~150 km WNW of Golmud City in northwestern Qinghai Province, on the Tibetan Plateau. It is ~850 km WSW of the Jinchuan Ni deposit and ~820 km NNE of Lhasa in Tibet.
Regional Setting
The Xiarihamu deposit is located on the northern margin of the east-west trending East Kunlun Orogenic Belt in the northern part of the Qinghai-Tibet Plateau (Jiang et al., 2015; Li et al., 2015). The Kunlun Orogenic Belt constitutes the western section of the greater Central China Orogenic Belt, which extends across China over a length of >3000 km, from the the major, north south Tan-Lu Fault in the east, to the ENE-WSW trending Altyn-Tagh Fault Complex to the west. The eastern section of of the Central Orogenic Belt is the Qinling-Dabie Orogen that separates the North China and Yangtze cratons. The Kunlun Orogen separates the Qaidam Block to the north from the Bayan-Har Terrane of the Songpan-Ganzi Complex/Terrane to the south. The Qaidam Block is a fragment of the Tarim Micro-continent that has been displaced by the sinistral Altyn-Tagh Fault Complex and is concealed by up to 15 km of Cainozoic continental sediments of the Qaidam Basin. To the west, the East Kunlun Orogen, whilst terminated by the Altyn-Tagh Fault Complex, continues as the offset, progressively east-west to NW-SE curving West Kunlun Orogen which marks the southern edge of the Tarim Micro-continent in far western China and into Central Asia.
The East Kunlun Orogenic Belt belt is split into the Northern Qimantagh, Middle East Kunlun and the Southern East Kunlun domains, separated respectively by the Qimantagh-Xiangride ophiolitic mélange zone (or Northern Kunklun Fault), and the Aqikekulehu-Kunzhong ophiolitic mélange zone (or Middle Kunlun Fault). The Southern East Kunlun Domain is separated from the Bayan-Har Terrane by the Muztagh-Buqingshan-Anemaqen ophiolitic mélange zone (or Southern Kunlun Fault; Jiang et al., 1992, 2000; Dong et al., 2018).
The Northern Qimantagh Domain and Qimantagh-Xiangride mélange on its southern margin, are interpreted to represent the Qimantagh back-arc basin behind the Early Palaeozoic northward subduction of the Proto-Tethys Ocean below the Middle East Kunlun Domain and Qaidam Block. The Northern Qimantagh Domain is predominantly represented by erosional remnant outliers of back arc basin sequences overlying the southern margin of the Qaidam Block. These remnants decrease in frequency and extent from almost complete cover in the north, to total removal in the SE. The Middle East Kunlun Domain is taken to represent an Early Palaeozoic island-arc and Late Palaeozoic-Triassic active continental marginal arc, related to two main pulses of subduction. The Palaeozoic Aqikekulehu-Kunzhong ophiolite fragments and associated high-pressure eclogites that occur along the Middle East Kunlun fault between the Middle and Southern East Kunlun domains are consistent with that regional fault representing a fossil suture zone between two micro-continental blocks (Jiang et al., 1992; Yang et al., 1999, 2000; Chen et al., 2002). The Aqikekulehu-Kunzhong ophiolitic mélange zone and Southern East Kunlun Domain to its south, and then the Muztagh-Buqingshan-Anemaqen ophiolitic mélange zones and Bayan-Har Terrane respectively, represent the early-Palaeozoic and late-Palaeozoic to Triassic subduction zones and associated accretionary complex as subduction migrated to the south. The latter pair are related to the closure of the Palaeo-Tethyan Ocean (Dong et al., 2018).
The three domains of the East Kunlun and intervening ophiolitic mélange zones reflect the evolution of the Proterozoic to Early Palaeozoic Proto-Tethys Ocean and the Late Palaeozoic to Mesozoic Palaeo-Tethys Ocean, with voluminous ophiolites dated at ~537 to 436 Ma and ~345 to 243 Ma (Yang et al., 1996; Chen et al., 2001; Dong et al., 2018). The Bayan-Har Terrane of the Songpan-Ganzi Complex comprises a Triassic turbidite sequence, now composed of sandstones and slates (Rao et al., 1987, 1991; Nie et al., 1997) that unconformably overlies older sequences of the Southern East Kunlun Domain and the Carboniferous to Middle Permian Muztagh-Buqingshan-Anemaqen ophiolitic mélange zone. The sequence is interpreted to have been deposited in a marine shelf environment (Wang et al., 2009). Recognition of a series of fault-bound blocks of Permian limestone in the Triassic turbidites suggests the latter were deposited on a wide Permian carbonate platform that formed part of the Tethyan ocean (Yin and Zhang 1998). The basins containing the turbidite sequences closed during the late Middle Triassic to early Late Triassic, followed by unconformably overlying successions of terrestrial volcanic rocks and coal-bearing clastic rocks during the Late Triassic to Early Jurassic, synchronous with voluminous granite intrusions.
The basement to the East Kunlun Orogenic Belt in all three domains is predominantly Proterozoic granitic gneiss, amphibolite and migmatites, with minor granitoids. This sequence is more prevalent in the Middle East Kunlun Domain, but is largely only found in the northern part of the Southern East Kunlun Domain. The granitic gneiss of this sequence has a peak magmatic age of ~920 Ma and a metamorphic age within a 500 to 400 Ma window (Chen et al., 2007). The Proterozoic basement is overlain by weakly metamorphosed Palaeozoic sedimentary and volcanic rocks which include clastic and volcanic rocks, and limestone (Jiang et al., 1992; Yang et al., 1996; Bian et al., 2004; Chen et al., 2013; Meng et al., 2013; Huang et al., 2014). Similar to the Proterozoic sequences, exposure of middle Palaeozoic strata in the Southern East Kunlun Domain is restricted to its northern part. Mesozoic to Cenozoic sedimentary and volcanic rocks are also known, though largely restricted to the Southern East Kunlun Domain and the Bayan Har Terrane. Extensive Early Palaeozoic to Mesozoic magmatic rocks occur throughout the East Kunlun Orogenic Belt, mainly as granitoids emplaced in two episodes, from 466 to 390 Ma in the Ordovician to Lower Devonian; and the Permo-Triassic from 257 to 200 Ma (Mo et al., 2007; Huang et al., 2014; Shao et al., 2017; Dong et al., 2018; Zhao et al., 2020). Granitoids of both episodes evolved from early calc-alkaline granodiorites to late monzogranites and syenogranites (Zhang et al., 2014; Chen et al., 2020), and are interpreted to be related to the subduction of the Proto- and Paleo-Tethys Oceanic plates respectively (Yin and Harrison, 2000; Mo et al., 2007; Chen et al., 2017). However, Palaeozoic and Mesozoic granitic plutons in the Southern East Kunlun Domain are much smaller than those in the Middle East Kunlun Domain.
Zones of Ultra-High Pressure (UHP) metamorphism have been mapped within the greater Central China and adjacent related orogens. These include occurrence within the i). Qinling-Dabie Orogen in the east, ii). North Qaidam Belt along the northern margin of the Qaidam Block; iii). Qilian Orogen across the Qilian Block to the north; iv). South Altyn, within to the Altyn-Tagh Fault Complex; and v). East Kunlun Orogen, where UHP eclogite lenses are mapped at Kehete and Wenquan near the eastern extremity of the orogen, and in the Xiarihamu district, ~400 km to the WNW, where HP (but not UHP) metamorphic rocks are encountered. These occurrences are characterised by the presence of diagnostic UHP indicators within the metamorphic assemblage, which include varying amounts of coesite, and/or diamond bearing eclogite or lherzolite. Coesite is a high-pressure modification of SiO2 generated at depths of >100 to 120 km at PT conditions of 29 to 30 kbar and 610 to 675°C; (Song et al., 2018). These UHP metamorphic assemblages are interpreted to have formed during deep subduction, and where present in continental crust, occur where such crust trailing an oceanic slab, follows that slab beneath the opposing plate during subduction (Zheng et al., 2012). UHP metamorphic rocks exposed in orogenic belts are taken as evidence that those rocks were tectonically transported to depths of >100 km, and in some cases exceeding ~150 km (Erdman and Lee, 2014), and have subsequently been exhumed. A proposed mechanism for such exhumation involves the 'break-off' of the attached heavier leading oceanic slab, and the consequent tectonic rebound of the lighter, more bouyant continental crust along what becomes a reversed subduction zone. This rebound creates an extensional regime (e.g., Davies and Blanckenburg, 1995; Chemenda et al., 1996; Zhang et al., 2014). Slab 'break-off' is interpreted to occur when the bouyancy of the trailing continental crust being subducted exceeds the strength of the leading oceanic slab. The UHP and HP occurrences in the East Kunlun Orogen are restricted to the Middle East Kunlun Domain, and are found in lensoid occurrences within the Aqikekulehu-Kunzhong ophiolitic mélange zone at Kehete-Wenquan and within Proterozoic marbles and gneisses at Xiarihamu, distal to the mélange zone and the Middle Kunlun Fault. At Kehete, the UHP eclogites are exposed as lens-shaped blocks of varying sizes from 5 to 100 m in length within metasedimentary hosts, mainly metapelitic gneisses and marble. They are composed of 20 to 30% anhedral garnet crystals, 30 to 40% omphacite, 5 to 15% amphibole, 5% quartz and 5% phengite, with accessory rutile, ilmenite and zircon. Coesite inclusions and its pseudomorph have been recognised in garnet, omphacite and zircon (Song et al., 2018; Bi et al., 2018). Serpentinite blocks have also been discovered in the same area, varying in size from 5 to 10 m in length. These locations are part of a 500 km long eclogite belt that experienced UHP metamorphism in Siluro-Devonian times (Song et al., 2018). The same authors describe this eclogite belt as consisting of EMORB and OIB affinity eclogite blocks, metapelites, marble and minor serpentinite blocks, the rock assemblage of which, together with the presence of ophiolite and arc-volcanic rocks, suggest a tectonic mélange and accretionary complex. Bi et al. (2002) indicated the Medium Temperature UHP eclogites to the east in the Kehete-Wenquan eclogite belt experienced peak prograde ~30 kbar and 650 to 720°C, followed by retrograde conditions of 10 kbar at ~650 to 700°C, and then further retrogression at 3 to 6 kbar and 500 to 580°C. In the Xiarihamu area to the west, low temperature-HP eclogite indicates peak conditions of ~27 kbar at 570°C, followed by retrograde conditions at 10 ±2 kbar and 470 to 560°C. They also suggest from their investigations that two types of eclogite are represented from the two districts. The eclogites are interpreted to be related to metamorphism of oceanic slab material to the west, and continental crust to the east. This they explain as the result of an advancing oblique oceanic to continental crust transition entering the subduction zone first in the east, and progressively later to the west, and being at a greater depth in the east when the oceanic slab was detached (Bi et al., 2002). UHP assemblages are more obvious in mafic protoliths, and their presence within the surrounding country rock assumes the latter were also subjected to the same pressures and temperatures.
Xiarihamu Intrusive Complex and Setting
The Xiarihamu Ni deposit is located within the northern half of the Middle East Kunlun Domain, which is characterised by a widespread Proterozoic metamorphic basement. The sequence in the Xiarihamu deposit area comprises:
Palaeoproterozoic
• Jinshuikou Group, Baishahe Formation - biotite granite gneiss, quartz schist, plagioclase amphibolite and marble;
Neoproterozoic
• Granitic gneiss, which are discriminated from Palaeoproterozoic gneisses by zircon U-Pb dating of ~920 Ma (Chen et al., 2006);
Unconformity
Upper Ordovician to Lower Devonian
• Qimantage Group, siliceous rocks, basalt and dolostone, sub-divided into:
- Lower Formation, composed of sandstone and siliceous rocks, representing bathyal to shallow marine environments;
- Middle Formation, which mainly consists of basalt with a zircon U-Pb age of 428 ±3 Ma, where the basalt has been interpreted to be tholeiitic, deposited in an extensional arc-back basin environment (China University of Geosciences, Wuhan, 2011; Qinghai Geological Survey, 2012). The age of this basalt is close to that of the older gabbro of the Xiarihamu intrusive complex which is 431 Ma (Li et al., 2015);
- Upper Formation, comprising dolostone and dolomitic limestone, representing a continental shelf environment).
Both the Proterozoic metamorphic rocks and the late Ordovician and Silurian strata were intruded by voluminous 420 to 390 Ma (U-Pb zircon) Siluro-Devonian granitoid plutons that occupy ~50% of the eastern section of the Middle East Kunlun Domain (Mo et al., 2007; Xu et al., 2007; Cui et al., 2011, Gao and Li, 2011; Liu et al., 2012).
No Early and Middle Devonian sedimentary strata have been encountered in the deposit area.
Upper Devonian
• Maoniushan Formation, (after Qinghai Geological Survey, 2012)
- Lower clastic member, composed of fuchsia-colored medium-grained quartz sandstone and coarse-grained feldspar-quartz conglomerate, representing lake-phase molasse deposition;
- Upper volcanic member, comprising brecciated tuff and lava, suggesting a continental volcanic environment.
The Qimantage Group is interpreted to represent back-arc basin strata (Ren, 2010; Wang et al., 2010; Gao et al., 2011), whilst the Maoniushan Formation is regarded as marking the end of an orogenic cycle and the beginning of the late Palaeozoic basin phase (Mo et al., 2007; Li et al., 2008).
Lower Carboniferous
• Dagangou Formation, sandstone and dolomitic limestone;
Cenozoic sedimentary cover.
Four mafic-ultramafic complexes, denoted Xiarihamu I to IV, have been intruded into the Xiarihamu area, distributed over an area of ~10 x 4 km. The Xiarihamu magmatic sulphide deposit is hosted within the Xiarihamu I intrusion, although additional resources may occur in the other three that have not been as intensely investigated. These are part of a cluster of intrusions in the surrounding district, all intruding Jinshuikou Group country rock. They include i). Neoproterozoic granitic gneiss, one occurrence of which is ~3 x 2 km and surrounds the Xiarihamu I intrusion in three sides, whilst a second abuts the Xiarihamu II intrusion; ii). two small eclogite bodies, one in contact with the Xiarihamu III intrusion, the other within 1 km to the SE of Xiarihamu II which have peak metamorphic ages of ~436 Ma and retrograde metamorphic ages of ~406 Ma, although as detailed above, the core of constituent zircons have been dated at 934 Ma, suggesting a Tonian protolith; iii). the four Xiarihamu intrusions, as discussed below, dated at between 431 and 405 Ma (see below); iii). an Upper Silurian to Middle Devonian, 411.7 to 391.1 Ma, >3 km wide, WNW-ESE belt of A2-type granites within a kilometre to the north of the Xiarihamu I intrusion (Chen et al., 2013; Liu et al., 2013; Wang et al., 2013; Gan, 2014; Yan et al., 2016); iv). Large masses of Permian and Upper Triassic to Lower Jurassic granite are mapped within 5 km to the south of the Xiarihamu I intrusion (Song et al., 2016). Although not represented in the immediate vicinity of Xiarihamu, elongated batholiths of Ordovician to Silurian granitoids intrude the Proterozoic metamorphic sequences of the Middle East Kunlun Domain to within 85 km to the east (Song et al., 2016).
The Xiarihamu I mafic-ultramafic complex was emplaced into both the Palaeoproterozoic Jinshuikou Group granitic gneiss, schist and marble (Song et al., 2016), and Neoproterozoic granitic gneiss (Gan, 2014; Wang et al., 2016). It occurs as an irregular, gently WSW dipping lensoid lopolith, with a plan extent of ~1400 x 900 m by <50 to 600 m in thickness, covering an area of 1.33 km2. In cross-section, it is overall thinnest to the west, expanding to a thicker mass in the east. The western section of the complex dips below the host sequence, while the southern part is masked by Quaternary cover. The upper margin intrudes Neoproterozoic granitic gneiss, whilst the base is in contact with marble in its western section and Neoproterozoic gneiss to the east. The country-rock marble and granitic gneiss contains minor amounts of sulphide, generally totalling <0.1% S, occurring as 10 to 100 µm pyrite crystals within the granite gneiss. Overall, the intrusion is semi-concordant with the country-rock.
The bulk of the exposed intrusion is composed of gabbronorite and gabbro, dated at 405.5 ±2.7 Ma (zircon U-Pb SHRIMP; Song et al., 2016) near surface towards the centre of the exposed intrusion, while another determination at 340 m depth, 400 m to the WSW, returned an age of 431.3 ±2.7 Ma (LA-ICP-MS) U-Pb zircon; Li et al., 2015). These dates are taken to imply the gabbro most likely formed in two stages. The gabbroic portion is principally composed of medium-grained gabbronorite which contains 50 to 60 modal.% plagioclase, 25 to 35% orthopyroxene, 10 to 20% clinopyroxene, interstitial hornblende and <5% biotite, with minor magnetite and sulphide. The ultramafic portion of the intrusion principally comprises medium and fine-grained rocks, which includes olivine orthopyroxenite, overlain by orthopyroxenite in the west and orthopyroxenite overlain by websterite in the east. The olivine orthopyroxenite is composed of 10 to 40% olivine, 50 to 80% orthopyroxene, <10% clinopyroxene and plagioclase, minor hornblende, and biotite. Orthopyroxenite/websterite are exposed to the ENE and sporadically across the centre of the intrusion, and is generally over- and underlain, and flanked by the gabbronorite and gabbro, particularly to the NNW, where the intrusion is almost entirely a thick section of gabbronorite and gabbro. Websterite from this lithofacies has been dated at 411.6 ±2.4 (Li et al., 2015) and 406.1 ±2.7 Ma (Song et al., 2016) towards the centre of the intrusion. Harzburgite forms a large mass near the upper contact to the WSW, as recognised by Liu et al. (2018), although Song et al. (2016) have not differentiated it from the orthopyroxenite.
The lithofacies described above are as recognised by Song et al. (2016) and shown on the accompanying sections. However, Liu et al. (2018) have further subdivided the lithofacies of the intrusion to include dunite, harzburgite and wehrlite, as well as the orthopyroxene/websterite and gabbro/gabbronorite. These same authors studied drill holes across the intrusion and have divided it into a western and an eastern part. In the western part, the sequence passes upward from a basal, relatively thin, 15 to 50 m thick pyroxenite that varies from an orthopyroxenite to websterite. This is overlain by 35 to 155 m of dunite, which is, in turn, followed by wehrlite that varies from 5 to 80 m thick in the drill holes studied by Liu et al. (2018). In the eastern part of the intrusion, the drill holes studied contain a sequence that comprises a thin basal gabbro, which is overlain by pyroxenite that varies in thickness from 50 to 265 m. This is followed by dunite that varies from a maximum of 87 m decreasing to as little as 3 m, and is overlain by a similar thickness of harzburgite that is bounded above by Proterozoic granitic gneiss (Liu et al., 2018).
Liu et al. (2018) recognised three mineralised positions within their magmatic stratigraphy of the Xiarihamu I intrusion, both within the western and eastern sections, namely: i). the No. 1 orebody zone within the dunite and intervening orthopyroxenite, as defined by the 0.6 wt.% Ni contour, which, in the eastern section of the deposit, encloses two narrower intervals of >1.5 wt.% Ni in the upper dunite intervals of the intersection, which migrates across the deposit to the lower part of the dunite interval; ii). the No. 2 orebody zone within the underlying orthopyroxenite-websterite, migrating from the central portion of the lithofacies in some ares, to the lower portion in others; iii). the No. 3 orebody zone, hosted by the basal gabbro. This latter zone only accounts for a minor proportion of the deposit, with >95% of the ore grade mineralisation within the ultramafic portion of the intrusion. Most of the individual bodies of ore are generally from 100 to 700 m long and 1.5 to 80 m thick, occurring as thick layers/lenses, while a few are funnel-shaped and/or irregular shapes (Han et al., 2020).
According to Song et al. (2016), more than 95% of the Ni reserves of the Xiarihamu I intrusion are hosted in the largest composite orebody, known as M1, composed of multiple ore lenses/layers. The main mass of the orebody is continuous and hosted in both olivine orthopyroxenite and orthopyroxenite, although laterally it occurs as two thick ore pods, a larger eastern and smaller western, separated by a narrow 'pinchout'. This pinching and swelling of the ore pods in three dimensions is a direct result of pinching and swelling of the host ultramafic lithofacies. On the eastern margin of the main orebody (and eastern pod), mineralisation splits into several thinner sublayers which are only hosted by orthopyroxenite. A few lens-shaped blocks of Jinshuikou Group metamorphic rock and gabbronorite are enclosed in the western ore pod bifurcating the orebody. To the south, the orebody thins to <50 m and usually contains <15% sulphides, whilst an up to 100 m thick sulphide-barren orthopyroxenite layer occurs beneath the sulphide ore layer. Intercalated layers of weakly mineralised or sulphide-barren orthopyroxenite within the orebody to the south and east divide it into separate sublayers, whilst the sub-layers are further divided into multiple layers by thinner sulphide barren orthopyroxenite bands that are <1 m thick. The eastern ore pod has a maximum thickness of up to 300 m and is 600 m wide locally where the roof of the ultramafic portion of the intrusion rises. The two main pods are dominantly composed of disseminated mineralisation, containing 5 to 20 modal.% sulphides with lesser net-textured or 'blotchy' textured ores with 20 to 70 modal.% and massive >75 modal.% sulphides. The sulphide contents of the M1 orebody tend to decrease from NW to SE in both pods (Song et al., 2016).
NOTE: The differences in the descriptions of the distribution of mineralisation by Liu et al. (2018) and Song et al. (2016) in the last two paragraphs are based on their separate subdivisions of the magmatic sequence. The M1 orebody of Song et al. (2016) would appear to combine both the No.s 1 and 2 orebody zones of Liu et al. (2018) as a composite orebody that includes the individual bodies of ore the latter describe, and intervening waste.
The sulphide content of the lithofacies varies according to the degree of mineralisation. Well mineralised dunite contains 10 to 15% disseminated sulphide, whereas in poorly mineralised intervals there is almost no sulphides, or they are only sparsely disseminated. Dunite can contain as much as 90% olivine with up to 7% interstitial pyroxene crystals varying from ortho- to clinopyroxene. Harzburgite typically contains 2% disseminated sulphide, mainly pentlandite and pyrrhotite, accompanied by 45% olivine, 45% orthopyroxene, 2 to 3% clinopyroxene and 1 to 2% spinel. The sulphides occur at the contact between olivine and orthopyroxene, or within the orthopyroxene. With increasing depth in the harzburgite, the grain sizes of the olivine and orthopyroxene increase, and the proportion of orthopyroxene gradually increases. Websterite contains either sparse disseminated, or massive sulphide, with two distinct orthopyroxene crystal sizes; 1 to 1.5, and 0.2 to 0.5 mm. Sulphides are mainly located interstitial to the orthopyroxene crystals, although minor sulphides are also enclosed within orthopyroxene. They constitute 0.5 to 1.5% of the lithofacies, and are mainly pyrrhotite and pentlandite, with minor chalcopyrite. The upper portion of the websterite has more chalcopyrite in the upper portions of some drill holes, decreasing downward. Gabbro in the lower mineralised interval in the eastern section of the intrusion contains both 5 to 6% disseminated, and massive sulphides. Angular fragments of gabbro are found at the contact with the massive sulphide which are mainly composed of pyrrhotite with minor pentlandite (Liu et al., 2018). Gabbronorite contains 50 to 60 modal % plagioclase, 25 to 35% orthopyroxene, 10 to 20% clinopyroxene, interstitial hornblende and biotite (<5%) plus minor magnetite and sulphide (Song et al., 2016). The Xiarihamu sulphide ores are characterised by high Ni tenor with Ni100% to 18.11%), but extremely low PGE tenors where PGE100% may be in the range 46 to 235 ppb (Liu et al., 2018).
There are no crosscutting relationships between dunite, harzburgite and wehrlite, implying sequential deposition during a single stage of magmatic evolution. In contrast, websterite veins cut through the dunite and harzburgite, indicating that the websterite formed later than the other two lithofacies. Similarly, gabbro has intruded websterite suggesting one phase of the gabbro formed slightly later than the websterite. Zircon dating indicate one of the gabbros formed 20 m.y. before the websterite (Li et al., 2015). Based on these observations, Liu et al. (2018) conclude the intrusive sequence is as follows: gabbro at 431 Ma → dunite-harzburgite-wehrlite → websterite or orthopyroxenite at 411 to 405 Ma → the younger basal gabbro.
Trends are evident in the distribution of mafic-ultramafic rocks and mineralisation. The upper part of the intrusion is composed of peridotitic facies, including dunite, harzburgite and wehrlite, whilst the lower part is composed of pyroxenite facies including orthopyroxenite and websterite. The mineralisation style changes gradually from top to bottom. At higher levels, mineralisation is mainly 'spotted', veined and disseminated, whilst massive and densely interstitial mineralisation occurs at ~100 m greater depths. There is a gradual transition between the different rock types of a single lithofacies, with structural fracture zones or veining often marking lithofacies changes.
The major elements of chromite, olivine and pyroxene, together with the rare earth elements; trace elements and bulk rock isotope composition of the Xiarihamu Intrusive Complex have been interpreted to indicate that the parental magma originated from partial melting of the asthenospheric mantle that was modified by melts derived from the subduction slab, followed by multi-stage silicate fractional crystallisation and liquation of sulphides and silicates (Wang et al., 2014; Feng et al., 2016; Zhang Z.B. et al., 2016; Du, 2018; Song et al., 2020; Han et al., 2020).
The geotectonic history of the East Kunlun Orogen can be summarised as follows. Subduction in the East Kunlun Orogen commenced after 508 Ma (Mo et al., 2007) or 520 Ma (Song et al., 2018) in the Cambrian. As detailed previously, this initially involved the northward subduction below the Middle Easy Kunlun Domain of oceanic crust of the Proto-Tethys Ocean which preceded the approaching Southern East Kunlun Domain microcontinent. This led to the establishment of a back-arc basin over the Northern Qimantagh and northern section of the Middle East Kunlun domains, straddling the North Kunlun Fault; and a magmatic arc centred on the Middle East Kunlun Domain that involved intrusion of voluminous Cambrian to Ordovician granitoids. Deposition of the Upper Ordovician Qimantage Group Lower Formation took place in the back-arc basin. The transition from oceanic subduction to continent-continent collision between the Southern and Central East Kunlun domains began at ~438 Ma (Liu et al., 2013). The UHP metamorphism in the East Kunklun Orogen has been dated at 430 to 410 Ma (U-Pb, Zircon; Song et al., 2018). This took place as the Palaeo- and Neoproterozoic continental crust of the Southern East Kunlun Domain was subducted below the Central East Kunlun Domain. In the immediate Xiarihamu area, the Wenquan and Xiarihamu eclogites have peak eclogite facies metamorphic ages of ~428 Ma and ~436 Ma respectively (Meng et al., 2013), interpreted to have formed during this stage. This is also reflected by mean SHRIMP U-Pb ages of the core and rim of zircon crystals from these coarse-grained eclogites, of 934 and 428 Ma, respectively, implying an early Neoproterozoic (Tonian) mafic protolith was overprinted by Palaeozoic HP metamorphism (Meng et al., 2013). The Xiarihamu eclogite also has a retrograde metamorphism age of 409 Ma, taken to reflect exhumation of the Middle and Southern East Kunlun domains following the progressive 'break-off' of the oceanic slab between 428 and 409 Ma (Zhang et al., 2017). Slab 'break-off' resulted in the tectonic rebound of the deeply subducted bouyant continental crust, reversal of subduction and rapid exhumation of HP and UHP metamorphosed Proterozoic rocks. This uplift also resulted in erosion of the bulk of the Lower Palaeozoic accretionary wedge overlying the subduction zone between the Middle- and Southern East Kunluk domains. As a result of this reversal, an extensional regime was created forming an arc-rift in the Middle East Kunlun Domain, and further expansion of the earlier formed back-arc basin to its north, as well as the development of an ophiolitic mélange centred on the suture zone reflected by the Middle Kunlun Fault. This slab 'break-off' and 'subduction reversal' was accompanied by upwelling of depleted asthenospheric mantle formed during the subduction stage. The upwelling, in turn, was accompanied by decompression melting and formation of deep hydrous mafic partial melts. These partial melts presumably ponded at the Moho where both fractionation and crustal assimilation occurred before the magma chamber was tapped and magma rose in the extensional regime into the arc-rift. This magma was the source of the Xiarihamu mafic-ultramafic intrusions between 431 and 405 Ma, and the Qimantage Group Middle Formation basalts in the back-arc basin and rift. The ponding chamber at the Moho was also responsible for lower crustal anatexis that produced the 411.7 to 391.1 Ma, Upper Silurian to Middle Devonian, granitoids. Geochemical criteria support this scenario, as they suggest the Xiarihamu ultramafic rocks were mainly influenced by deep hydrous melts that form at high temperatures and ultra-high pressures of >20 kbar, rather than shallower crustal sources (Liu et al., 2018). However, Liu et al. (2018) calculate crustal sulphur most likely accounts for 40 to 60% of the total sulphur content on average in the Xiarihamu deposit. This contamination presumably occurred during one or more of: i). asthenospheric contamination by partial melting of the subducting oceanic and continental crustal slab prior to 'break-off'; ii). ponding and crustal assimilation at the Moho, or iii). during subsequent transit to the Xiarihamu Intrusive Complex. These observations led Liu et al. (2018) to the conclusion that, if the 'slab break-off' interpretation is accepted, there is a direct connection between the simultaneous appearance of the Xiarihamu magmatic Ni sulphide deposit and the HP-UHP exhumation in the East Kunlun Orogen.
Renewed subduction related to the closure of the Palaeo-Tethys ocean between the Southern East Kunlun Domain and the Bayan-Har Terrane is reflected by extensive Permo-Triassic intrusive and extrusive magmatism and accretionary wedge deposition to the south.
Resources
The deposit contains ~157 Mt of sulphide ores @ 0.65 wt.% Ni, 0.013 wt.% Co and 0.14 wt.% Cu (No. 5 Geological and Mineral Survey Institute of Qinghai
Province, 2014, as quoted by Liu et al., 2018).
The most recent source geological information used to prepare this decription was dated: 2020.
This description is a summary from published sources, the chief of which are listed below.
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Selected References:
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Bi, H., Whitney, D.L., Song, S. and Zhou, X., 2022 - HP-UHP eclogites in the East Kunlun Orogen, China: P-T evidence for asymmetric suturing of the Proto-Tethys Ocean: in Gondwana Research v.104, pp. 199-214.
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Han, X., Liu, Y. and Li, W., 2020 - Mineralogy of Nickel and Cobalt Minerals in Xiarihamu Nickel-Cobalt Deposit, East Kunlun Orogen, China: in Frontiers in Earth Science v.8, 14p. doi:10.3389/feart.2020.597469.
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He, H.-L., Chen, L.-M., Song, X.Y., Fu, B., Yi, J.-N., Yu, S.-Y. and Deng, Y.-F., 2022 - Genesis of the Xiarihamu Magmatic Ni-Co Sulfide Deposit in the East Kunlun Orogen, Northern Tibetan Plateau: In Situ Oxygen Isotope and Geochemical Perspectives: in Econ. Geol. v.117, pp. 1827-1844. https://doi.org/10.5382/econgeo.4949.
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Liu, Y.-G., Li, W.-Y., Jia, Q.-Z., Zhang, Z.-W., Wang, Z.-A., Zhang, Y.-B., Zhang, J.-W. and Qian, B., 2018 - The Dynamic Sulfide Saturation Process and a Possible Slab Break-off Model for the Giant Xiarihamu Magmatic Nickel Ore Deposit in the East Kunlun Orogenic Belt, Northern Qinghai-Tibet Plateau, China: in Econ. Geol. v.113, pp. 1383-1417.
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Song, S., Bi, H., Qi, S., Yang, L., Allen, M.B., Niu, Y., Su, L. and Li, W., 2018 - HP-UHP Metamorphic Belt in the East Kunlun Orogen: Final Closure of the Proto-Tethys Ocean and Formation of the Pan-North-China Continent: in J. of Petrology v.59, pp. 2043-2060. doi: 10.1093/petrology/egy089.
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Song, X.-Y., Yi, J.-N., Chen, L.-M., She, Y.-W., Liu, C.-Z.,Dang, X.-Y., Yang, Q.-A. and Wu, S.-K., 2016 - The Giant Xiarihamu Ni-Co Sulfide Deposit in the East Kunlun Orogenic Belt, Northern Tibet Plateau, China: in Econ. Geol. v.111, pp. 29-55.
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Wang, Q., Zhao, J., Zhang, C., Yu, S., Ye, X. and Liu, X., 2022 - Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt, NW China: in Geoscience Frontiers v.13, 18p. doi.org/10.1016/j.gsf.2021.101271.
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