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PALEOSUBMARINE VOLCANISM AND MINERALIZATION FROM QILIAN

MOUNTAINS AND ADJACENT AREAS

 

XIA Linqi, XU Xueyi and XIA Zuchun

Xi'an Institute of Geology and Mineral Resources, Xi'an 710054, China

I.  PREFACE

Qilian Mountains, a big famous compound organic belt, strides across Gansu and Qinghai Provinces and situates in the suture between north and south plates in China, it undergoes three kinds of tectonic systems since Proterozoic,, namely continental rift, plate tectonic and intracotinental orogeny respectively. From Proterozoic to Cambrian, continental riftism with different intensity occurred on the basement of Archean-Proterozoic crust and Qilian Mountains underwent different stages of continental breaking-up, ocean basin opening, expanding, subducting, closing and plates colliding, forming three secondary orogenic belts, called North Qilian Mountains with large scale and Laji Mountains and north margin of Qaidam with small scale. Volcanic rocks from Proterozoic to Silurian are very developed in North Qilian orogenic belt; middle Cambrian to late Ordovician volcanic rocks occurred in Laji orogenic belt and volcanic rocks with small range of late Ordovician occurred disjointedly in north margin of Qaidam orogenic belt (Xia Linqi et al., 1998, 2001).

 

II.  PALEOSUBMARINE VOLCANISM AND MINERALIZATION FROM NORTH QILIAN

MOUNTAINS

1. The Proterozoic-Cambrian (2349514 Ma ) Continental Rift Volcanism and Mineralization

Since the middle period of Paleo-Proterozoic, with the beginning of the asthenosphere mantle plume ascending, the processes from the riftism developing on the Archean-Proterozoic basement to the continental crust thinning, splitting and finally ocean crust forming occurred. At present, the Beidahe Group (1980 Ma) which distributes as a fault block in west section of North Qilian orogeny, the Dunhuang Group and Milan Group (24602789 Ma) in Dunhuang Massif in the northeast margin of Altin tectonic belt, the paleo-Proterozoic Huangyuan Group, Hualong Group in Middle Qilian Massif and Alashan Group, Longshoushan Group in Alashan Massif in the south margin of North China Plate may be the remnants of Archean-Paleoproterozoic basement before the Proterozoic riftism in North Qilian Mountains. With the deep mantle plume material ascending, thermal erosion occurred at the bottom of lithosphere, leading to the lithosphere thinning and finally splitting, producing quantities of melt and forming CFB, the follower represents the co-active products of mantle plume and lithosphere. The Proterozoic continental flood basaltic volcanic rocks (CFB) in Qilian Mountains are determined in west section of North Qilian Mountains today. They are composed of tholeiitic lavas and pieces of dolomite in lower part (23491780 Ma), low-grade metamorphic microclastic rocks with Fe-bearing layer (1309 Ma) in middle part, dolomite and alkali basalt suite (1032604 Ma) in upper part, these volcanic rock series which is preserved in North Qilian Mountains as residual fault blocks with the area over 1000 km2 today are similar to those of CFB from Deccan, India.

The syngenetic submarine sprouting sedimentary iron deposit, represented by big Jingtieshan-type iron deposit, is formed in low metamorphic iron-bearing microclastic series in middle part of Proterozoic continental flood basaltic volcanic rocks. The high copper content in iron-bearing rock series is suggested that Cu had undergone primary pre-enrichment during the Middle Proterozoic iron-forming period. In middle-Ordovician, the crushing altered rock-type copper deposit (also named Jingqieshan-type copper deposit) controlled by crushing hydrothermal alteration zone is formed by activated re-enrichment of tectonic-magma-hydrothermal events at the stages of subduction and orogeny of Caledonian ocean basin in North Qilian Mountains. Moreover, the Dadonggou type Pb-Zn deposit is also formed in phyllite  and  carbonate rocks in the middle part of Proterozoic volcano-sedimentary rocks and undergoes the same processes of the Proterozoic primary pre-enrichment stage and the Caledonian magma-hydrothermal activated re-enrichment stage.

With the increasing degree of continental riftism, the Neoproterozoic-Cambrian continental rift volcanism occurred. The volcanic rocks formed in this period belong to high volcanic rift type volcanic rocks, which can be divided into two groups based on their occurrence: One is the large bi-model volcanic domes formed by central-type exploding-effusing -empacing-invading and the other is layered flowing volcanic rocks formed by fissure-type exploding and effusing. The former are represented by Baiyinchang orefield, Heishishan dome in Gansu Province and Qingshuigou-Bailiugou, Shitougou-Xiangzigou domes in Qinghai Province. In the domes, the basic volcanic rocks are dominated by alkali basalts, which is derived by low degree partial melting in upper mantle at the condition of relative low expanding speed of continental rift; the acid calc-alkali volcanic rocks are very developed, which are the products of anatectism occurred at lower part of sialic crust after the basic magma ascending to the bottom or invading into the continental crust. The acid volcanic rocks developed in central and lower parts of the domes and basic volcanic rocks, small amount intermediate rocks developed in margin and upper parts of the domes. The  layered  flowing  volcanic rocks, which is formed in high intensity and at rapid rate of rift expanding, are dominated by basic and intermediate volcanic rocks of tholeiitic series with no or less acidic volcanic rocks. In the region of Neoproterozoic-Cambrian volcanic rocks, the Baiyinchang copper multimetal deposit, the Gadaban copper multimetal deposit with great prospective reserve and Xiangzigou pyrite deposit with middle-large scale occur in acid volcanic rocks developed in large-scale domes with two-layer structure, the ore-bearing volcanic rocks have sharp REE chondrite-normalized patterns with LREE slightly enriched flatten patterns ((La/Yb))N=3.38) and middle degree of Eu negative anomaly (dEu=0.60.9).

2.  Late Cambrian-Early Ordovician (522~495Ma) Mid-ocean Ridge-ocean Island Volcanism and Mineralization

By Late Cambrian, with the increasing degree of continental splitting, the continent breaking-up and the transformation from continental fault depression to ocean basin formation had finished. The mid-ocean ridge-ocean island volcanic rock belt preserved as obducted slices in north Qilian Mountains is the mark of North Qilian Early Paleozoic ocean basin, which situated from Yushigou (522 Ma), Chuancigou (495 Ma) areas in north slope of Telaishan to Diaodaban in northwest direction, Yongan old city, northwest Menyuan County at southeast direction, and disjointedly to Dakecha, Tianzhu County. The characteristics of soure magma of the mid-ocean ridge-ocean island type volcanic rocks with well-developed ophiolitic suites (overturned today) is low-K tholeiitic series and ocean island alkali and tholeiitic series with E-MORB and T-MORB features (Xia Linqi et al., 1996, 1998, 2001).

Based on the lithogeochemistry method, we estimate that the expanding rate of North Qilian Paleozoic ocean ridge is 1.22.38 cm/a, similar to that of Atlantic ridge (0.63 cm/a, Hekinian, 1982) and belongs to slow expanding ridge. According to the ridge expanding rate with the time span of the ocean basin opening, the width of North Paleozoic ocean basin before subduction is approximately estimated as 10802142 km, indicating the ocean basin is a limited one. In theory, only when the high level magma chamber beneath the ridge and hot spot which drives the floor thermal brine circulation system is at large scale, the steady thermal circulation system can be formed and it is advantageous to the formation of large steady industrial massive sulfide deposits, which is indeed the features of rapid expanding ocean floors; at the slow expanding ocean floor, the high-level magma chamber under ocean ridge or hot spot is at small scale and scatter, the steady hydrothermal circulation system with large scale can not be formed and it is not advantageous to the formation of large scale massive sulfide deposites.

3. Ordovician (486445 Ma) Trench-Arc-Basin Volcanism and Mineralization

With the ocean basin extending, the oceanic lithospheric plate becomes cold and dense, which subducted beneath North China continental plate from southeast to northwest and formed a subduction complex belt along the site corresponding to today's south slope of Zoulangnanshan. Because of the oceanic lithospheric plate's subduction ,  basalts which consist of the oceanic crust and sediments that are not scraped off at the site of the trench are heated gradually as transport to deep mantle and undergo complex process of dehydration. Aqueous fluids formed by dehydration reaction release into the mantle wedge overlying the subdution slab, induce partial melting and produce island-arc volcanism, further the island-are extending in the rear side and cause secondary seafloor spreading, leading to the development of back-arc basin or marginal basin and volcanic activity related to back-arc spreading center which are not evenly distributed.

The subduction complex belt which marks the site of Paleozoic paleo-trench subdution in North Qilian Mountains situates in northeast side of oceanic ridge-oceanic isand volcanic rock belt in north slope of South Tuolai mountains and extends over 400 km from Yingzuishan in west to Changma, Shiyougou of south Yumen twon in southeast, further to the beginning of Bianmagou-Qinshuigou-Baijingshi in more southeast and until to Jingyangling. It is wedge subdution complex accretionary in the forearc region formed by oeanic lithospheric plate subdution and scraped off, including blueschist belts (two belts), basic-ultrabasic blocks, volcanic rock microlithons, mlange accumulations, remnants of radiolarian siliceous rocks and accretionary wedge consisting in slump accumulation, turbidity sediments and flysches (Xu Zhiqin et al., 1994). The greatest and lowest K-Ar and 39Ar/40Ar isotopic ages reported by other authors from muscovite, lepidomorphite and glaucophane in blueschist are 459 Ma and 388 Ma respectively, most are among 450420 Ma. Another subduction complex belt with small scale extending from west Baiquanmen to Jiugequan is the product of secondary subduction during the process associated with back-arc forming.

At present, the mineral resource types are relatively single in subduction complex belts discussed above, for example, the gold deposit in ultrabasic block in Yingzuishan subduction belt in west section of North Qilian Mountains comes up to middle scale, whose formation is related to the tectonic-thermal events occurred at the initial stage of collision orogeny in North Qilian Mountains (Late Silurian-413 Ma).

The Late Cambrian-Ordovician (523455 Ma) continental marginal island arc volcanic rock belt develops in northeast side of paleo-trench subduction complex belt discussed above, distributing along south Zoulang Mountains and the typical site is Shihuigou area, Yongdeng County, Gansu Province. The arc volcanic rocks appear to evolve forward associated with different stages of island-arc evolution from early to late and with the near to far distances away from the subduction belt: the lower part consists in tholeiitic basalts, which are the eruption of volcanism in the early stage of the arc development; following this, the eruption of dominantly calc-alkaline marine volcanic activities developed and produce island arc calc-alkaline volcanic rocks dominated by intermediate volcanic rocks in middle part; to Late Ordovician (457445 Ma), on the layer over the island-arc volcanic rocks, a series of alkali volcanic rocks with the source magma characteristics of shoshonitic series occurred, which mark the mature stage of island-arc system in Late Ordovician in North Qilian Mountains.

Study of the authors (Xia Linqi et al., 1996, 1998, 2000) indicates that the Ordovician island-arc volcanic rock series extend steadily, toward east it can reach Baiyinchang and the eastern part areas, where it overlap the Late Proterozoic-Cambrian volcanic rocks, toward west it can reach Xidaoliu area, Minle County, and Bianmagou area, Qilian County, through the Nanbei river area in Jiayuguan city, it can reach Dishuishan-Zhujiadashan area in the west of Yumen Town, finally being cut off by Altin fault (Xu Zhiqin, 1999). According to this, We suggest that the island arc volcanic rocks belt extends more than 1000km form northwest to southeast.

All the tectonic altered rock type (represented by big Hanshan gold deposit, metallogenetic epoch: 395303 Ma) and quartz-vein-type (represented by small Chelugou gold deposit, metallogenetic epoch is older than 427 Ma and younger than 253 Ma) gold deposits in west section of North Qilian Mountains occurs in island-arc fine pyroclastic rocks, the island-arc volcanic rocks are the source bed of the two types gold deposits, the formation of the gold deposits is related to the tectonic-thermal events during or after the colliding period in North Qilian Mountains.

Back-arc basins are formed by riftism of active volcanic island arcs (Karig, 1971). The island-arc riftism can split island arc or occur in a side of island arcs (Taylor and Karner, 1983). Whose geodynamics may be related to the additional convection induced by the viscous dragging in asthenosphere mantle produced by slabs' subduction and sinking occurring in mantle wedge behind island arcs, which leads to mantle material upwelling and induces back-arc spreading. Although it is accepted that these back-arc basins are extensional features formed by seafloor spreading processes similar to those occurring at mid-ocean ridges. However, the feature of seafloor spreading in back-arc basins are different from that in normal mid-ocean ridges. In a single word, back-arc basin seafloor spreading processes accompany with subduction. As a whole, back arc basin development always marks the steep angle subduction of the slab and induces back-arc extending, the rules (Cross and Pilger, 1982) deduced from west Pacific subducion belt can be used to study the steep angle in Paleozoic subduction belt in North Qilian Mountains. The diapir upwelling asthenosphere mantle beneath back-arc basins cause partial melting under double effects of adiabatic decompressing and aqueous fluids derived from the subduciton slabs, leading to eruption of the back-arc basin volcanic magmas, so the volcanic rocks of back-arc basin show both MORB-like and arc-like lithogeochemistic characteristics.

The Middle-Late Ordovician (469-544 Ma) back-arc basin volcanic rock belt develops in north slope of Zhulangnanshan. Present's studies indicate that the volcanic belt stretchs from east of Laohushan area in Jingtai County, Gansu Province, through Biandukou in Minle County, Suyouhe in Zhangye County, to Baiquanmen-Huoshixia in Sunan County with increasing width in the west, and the west end of the belt possibly reaches Yushugoushan in northwest Jiayuguan City. The ophiolite suites and back-arc spreading ridge marine volcanic rocks with overturned layers are well developed in Laohushan region in the east and Dacha-Daban, Huoshixia-Baiquanmen region in the west, indicating they are the obducted seafloor slices of back-arc basins. The volcanic rocks in the belt show clearly both MORB-like and island-arc-like lithogeochemistic features.

The Middle-Late Ordovician back-arc basin induced by back-arc spreading is at certain scale, the east and west parts spread violently with opening rate varying from 1.0-1.31 cm/a (Xia Linqi et al., 1996) and the back-arc spreading ridge ophiolite suites developed and deposits which emerge in basic lava in the upper parts of ophiolite suites and mainly consist in the metals of Cu and Zn are found (for example, small scale Zhuzuiyaba deposit, Laohushan in the east, small scale Jiugequan and middle scale Shijuligou deopists in the west). The metallogenic mechanism of these deposits is similar to that of Cu-Zn deposits occurring in mid-ocean ridge ophiolite suites. The regions of back-arc  with seafloor spreading violently and lager scale ophiolite suites is advantageous to finding these kind of deposits.

 

4.  Valcanism and Mineralization of Late Ordovician-Silurian (445428 Ma) Oceanic Basin Closing Period

 

The ocean basin closing is caused by ocean crust successive subduction, leading to Ordovician island arcs distributing in Zhulangnanshan colliding with Middle Qilian Massif in Late Ordovician, and accumulating initial molasses formation dominated by terrigenous clastics and some intercalated pyroclastics with more than 3000 m thickness in residual ocean basin (Mayinggou Group). Because partial metallogenetic material come from Paleozoic Cu-bearing volcanic rocks, some Cu-rich layers emerge in Silurian sandstone layers and sandstone-type Cu deposits ( i.e. Tianlu sandstone-type Cu deposits in Sunan County ) have been found in local areas.

Accompanying with the initial molasses formation, some Late Ordovician passive continental rift volcanic rocks (445 Ma, Xia Linqi et al., 1996) occur in some areas of Binggou in Qilian County, Honggou in Menyuan County and Huluhe in Jingning, Zhuanlang Counties, which are the products of small-scale passive continental rift caused by lithosphere spreading induced by local elastic resistance after arc-continental plate collision. Although at small scale, the passive continental riftism can also form the same local high-thermal circulation environment as active riftism, and has the same condition of mainly consisting in acid volcanic rocks. The basic volcanic rocks are rich in LREE ((La/Yb)N=3.55-23.4) and dominated by alkali basalts; the acid volcanic rocks have the similar REE chondrite-normalized patterns to Late Proterozoic-Cambrian ore-bearing acid volcanic rocks with LREE slightly riched ((La/Yb)N=4.47) and moderate negative Eu anomaly (dEu=0.66) (Xia Linqi et al., 1996, 1998) . At present, the middle-scale Honggou Cu-rich deposit in Qinghai Province and Jiaolongzhang Pb-Zn sulfide deposit in Gansu Province have been found.

In Devonian, Middle Qilian Massif in south collided with Alashan Massif (north part of China Plate) in deep of North Qilian orogenic belt, the huge overthrust masses thrust in fan shape towards north and south sides respectively, forming colliding-typekepa tectonic (Xu Zhiqin and Cui Junwen, 1996).  At this moment, in the intermountain and foreland basins of North Qilian Caledonian upwelling area cumulated pediment facies, intermountain river facies and lake facies sandstone, conglomerate formations with the thickness of 3000 m or so, including Laojunshan Group in the west and Xueshan Group in the east, which mainly stretch along the line of Sunan-Lenglongling-Jingtai. Accompanying with the Devonian collision orogeny, intrusive rocks of plagiogranite develop in Qingshixia area and the north side of Yingzuishan and intrusive rocks of adamellite occurring in colliding period develop in main peak of Qilian Mountains, Shiyouhenao and Jinfeshi areas ( Rb-Sr isochron dating of Jinfeshi rock body: 419-403 Ma, Zhangdequan et al., 1995).

North Qilian area entered post-orogeny stretching stage after Late Devonian and the larger-scale stretching leaded to crust thinning, ductile shear zones developing, which impelled the upwelling of deep metamorphic blocks in interior of the whole orogeny and invasion of post-orogeny granitic magma activities (Xu Zhiqin et al., 1997). the alkali granites in Baishuiquan and syenites in Renganggou, Gangou are the products of post-orgeny magma activities. Moreover, diorite-granodiorite-granite-alaskite belt of Late Permian distributing along the south side of Altin faults in northwest margin of this area indicate that magma events induced by the heat produced in the processes of strike-shear faults activity had existed in deep crust since Late Permian. The formation of continental stretching at post-orogeny stage accompanied with that of the suprabasins in Carboniferous, Permian, Triassic, Jurassic and Cretaceous at shallow crust. The genesis of these founded dominated gold deposits in northwest section of North Qilian Mountains are mainly related to the tectonic-magma-hydrothermal events occurred at the colliding orogenic stage and post-orogeny continental stretching stage.

North Qilian orogenic belt is neither similar to the Alpine-type orogeny represented by a continental plate superimposed over another in shallow tectonic layer without magma arcs, nor to the Himalaya-type orogeny represented by directly coactive collision between two continental plates (Sengor, 1990, 1991), which isaccretionary arc-typeorcontinent-accretionary arc-typecolliding orogeny (also calledTurkey-typeorogeny, Sengor, 1990, 1992) and underwent the processes of continental plate riftism at pre-orogeny stage and the formation of paleo-ocean basin®subdution of paleo-Qilian ocean basin to north direction at mainly orogenic stage®subduction, accretion, ocean basin closing, collision of arcs and continent , formation of Caledonian orogenic wedge®post-orogenic stretching and crust thinning.

As the backland of Tethys-Himalaya orogeny in Himalaya period, North Qilian Mountains had undergone the re-orogeny since Tertiary, formed thrust-napple faults and made the pre-Tertiary faults reactive, accompanying with the formation of piedmont and intermountain molasses basins, the mountains uplift again and formed the present's North Qilian Mountains.

 

III.  PALEOSUBMARINE VOLCANISM AND MINREALIZATION FROM SOUTH QILIAN

MOUNTAINS

Following the continental riftism occurred in Laji Mountains and north margin area of Qaidam basin in South Qilian Mountains on the metamorphic basement of Proterozoic in Cambrian-Ordovician, the Laji orogeny and North Margin of Qaidam orogeny are formed after the rift closure (Xia Linqi et al., 1998).

Based on the study from Qiu Jiaxiang et al. (1997), continental rift occurred in Middle-Late Cambrian in Laji Mountains and evolved to intracontinental rift, finally formed intracontinental rift-type small oceanic basin with residual ophiolitic suites, the width of the small oceanic basin is only 130 km and the pulling rate is 0.25 cm/a. Meantime, the dominated alkali basalts and tholeiitic basalts erupted with some high-MgO andesitic volcanic rocks appeared. The small oceanic basin closed in Ordovician and intracontinental rift belt formed by the processes of suduction and impelling and the volcanic rocks formed in this time was marked by island-arc-type calc-alkaline series and shoshonite series, in addition the orogenic O-type and post-orogenic A-type granites developed. The Paleozoic mantle characteristics in the area were cryptic metasomatic enriched mantle.

North margin of Qaidam orogenic belt only emerge Late-Ordovician volcanic rocks and continental rift occurred in early period, the opening width is about 64 km and the pulling rate is 0.4 cm/a, the bi-modal volcanic rock suites mainly consisting in basic and acid volcanic rocks are developed; Continental rift closed in Middle-Late period and accompanied with the eruption of basic-intermediate volcanic rocks with dominated intermediate volcanic rocks, moreover, granitic magma activities occurred at orogenic stage. The Paleozoic mantle characteristics in the area were metasomatic enriched mantle.

The genesis of copper, gold, multimetal mineral resources in Laji area and Pb-Zn deposits in North margin of Qaidam basin is closely related to Paleozoic volcano-magma activity in time and space, which mainly develop in Paleozoic volcanic and sedimentary rocks. The occurrences of the deposits in Laji area are spatially controlled by NWW faults and that of Pb-Zn deposits in North margin of Qaidam basin are closely spatially related to NW bedding tensional faults, the larger shear faults are very important to the mobilization migration of metallogenic elements.

IV.  SILURIAN MARINE VOLCANISM AND MINERALIZATION IN GONGPOQUAN AREA

IN BEISHAN MOUNTAINS, GANSU PROVINCE

Gongpoquan area is situated in Ordovician-Silurian active continental margin in northeast margin of  Tarim Plate and develops completely Silurian trench-arc-basin system (Xia Linqi et al., 1998). The north Yaodongnuru-Gongpoquan island-arc belt is a subaquatic island-arc belt developing on continental crust basement and volcanic rocks are dominated by calc-alkaline andesites and dacites with basalts and rhyolites in small volumes, the lavas and pyroclastics occur in alternating beds; the south Yemajing-Tongchangkou back-arc basin is situated in middle section of Hongliuhe-Niujuanzi-Jianquanzi back-arc basin and volcanic rocks are dominated by basalts with pillow structure developing. Compared to island-arc volcanic rocks, the proportion of andesites, dacites and pyroclastics reduce greatly. The residual ophiolite microliths develop in margin of the back-arc basin.

The Gongpoquan Cu deposit situates in the volcanic dome in east of Yaodongnuru-Gongpequan island arc and its genesis is related to Late Silurian calc-alkalic dicitic porphyry, granodioritic porphyry, which can attribute to island-arc-type porphyry copper deposit. The Cu-bearing porphyry and volcanic rocks are products of eruption, invasion of the same source magma according to temporal evolution. The volcanic eruptive sediments-magma hydrothermal fluids superimposing remaking copper deposits occur inside Upper Silurian andesites and andesitic tuffs developing in Yemaling-Tongchangkou back-arc basin.

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