Zircon U–Pb Age and Deformation Characteristics of the...

Original Article

Zircon U–Pb Age and Deformation Characteristics of theJiama Porphyry Copper Deposit, Tibet: Implicationsfor Relationships between Mineralization, Structureand Alteration

Jilin Duan,1,2 Juxing Tang,2 Russell Mason,3 Wenbao Zheng2 and Lijuan Ying

2

1State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 2MLR Key Laboratory ofMetallogeny and Mineral Resource Assessment, Institute of Mineral Resources, CAGS and 3China Gold International ResourcesCorp. Ltd., Beijing, China

Abstract

The Jiama copper deposit is one of the largest deposits recently found in Tibet and is composed ofthree types of mineralization including skarn, hornfels and porphyry. To investigate the relationshipbetween mineralization, structure and alteration, we report new zircon U–Pb age and present field obser-vations on the deformation characteritics associated with the copper mineralization in Jiama. Two mainperiods of deformation were identified, represented by D1 and D2 in Jiama. The first deformation (D1)occurred around 50 Ma, whereas the second deformation (D2) that was closely related to mineralizationoccurred later. Previous zircon U–Pb and molybnite Re–Os dating results indicate that the mineralizatoinoccurred at ∼15 Ma and thus the D1 regional deformation significantly occurred before the mineralizationtime, although the D1 deformation probably provided important space for the development of significantcopper deposition. Our new mapping and observations on the D2 deformation demonstrate that the min-eralization was closely coeval with or slightly later than the time of D2 deformation. The new U–Pb zirconage further indicates that the aplite formed in ∼17.0 Ma and thus the D2 deformation happened later thanthis time because the D2 deformation cut across the aplite, which is proposed to be the key control forcopper mineralization. Altered laminated hornfels including three types of alteration (A-, K- and S-type)were found spatially associated with the D2 deformation. The type-A is mainly silicification, with fine sericiteor chlorite, as well as abundant disseminated sulphides on fracture surfaces; the type-S is mainly fine-grained silicification with patches of chlorite, epidote and common sulphides; the type-K (potassic altera-tion) appears to be fine-grained biotite. Such types of alteration indicate the presence of skarns at depthwhere ore shoots are located. Taken together, the multiple structural-magmatic-mineralization events con-tributed to the formation of the supergiant Jiama porphyry copper deposit in Tibet. The results have generalimplication for regional exploration.

Keywords: deformation, Jiama, mineralization, structure, Tibet.

Received 8 March 2014. Accepted for publication 18 July 2014.Corresponding author: J. Tang, MLR Key Laboratory of Metallogeny and Mineral Resource Assessment, Institute of Mineral Resources,CAGS, Beijing 100037, China. Email: [email protected]

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doi: 10.1111/rge.12043 Resource Geology Vol. 64, No. 4: 316–331

© 2014 The Society of Resource Geology316

1. Introduction

Studies on porphyry copper mineralization potentiallyinclude aspects such as mineralization age, petrologi-cal and geochemical features of ore-host rocks,mineralization-related alteration, tectonic settings ofmineralizaiton as well as structrual control on miner-alization. Each of them plays an important role inexploration at mine scale or regionally. The giant por-phyry copper mineralization that formed during theLate Cenozoic in Tibet has been a focus in recentdecades (Schärer et al., 1984; Aitchison et al., 2000;Meng et al., 2003; Qu et al., 2004b, 2007, 2009; He et al.,2007; Yang et al., 2009; Gao et al., 2010; Hou et al., 2011,2012a; Searle et al., 2011). To date, a lot of studies havefocused on the age, geochemistry and tectonic settingsof large porphyry copper deposits such as Jiama inTibet (Qu et al., 2004a; Tang et al., 2010, 2011; Zhenget al., 2010; Qin et al., 2011, 2012; Hou et al., 2013; Zhonget al., 2013; Ying et al., 2014). These studies have shownthat the copper deposits in Tibet mainly formed duringthe Late Cenozoic and their origin seems to be relatedto the input of mantle-derived basaltic magmas intothe thick crust at post-collisional setting after the Asia–India collision at the Cenozoic (Hou et al., 2011). Thehost rocks associated with copper mineralization inTibet are mainly intermediate-felsic rocks derived frompartial melting of the newly formed, thick lowercontinental crust (Hou et al., 2004, 2011). The source ofcopper was therefore probably located in the newlyformed, water-rich arc-like lower crust. Although thegeneral age, petrogenesis and tectonic settings forcopper deposits in Tibet have been well studied, littlework has concentrated on figuring out the relationshipbetween structure, alteration and mineralization on arelative small scale, e.g., in the scope of a deposit (Burg& Chen, 1984; Yin et al., 1999).

The Jiama porphyry copper deposit in Tibet is one ofthe largest deposits recently found in Tibet, which con-tains approx. 750 Mt Cu and approx. 170 t Au as well asother metals (e.g., Pb, Zn, Ag, etc.). The ore-bearingwere emplaced at approx. 14 Ma and are mainly com-posed of dioritic porphyry and granitic porphyry con-taining mafic enclaves (Tang et al., 2010; Qin et al.,2012). The rocks are high-K and calc-alkaline with lowMgO content. The genesis of the ore-host rocks in theJiama copper deposit and the tectonic settings underwhich the magmas formed have attracted extensiveattention in recent studies (Tang et al., 2010, 2011;Zheng et al., 2010; Qin et al., 2011). Similar to most por-phyry copper deposits in South Tibet, previous studies

have shown that the Jiama copper deposit was formedin a post-collisional setting, and was derived fromnewly formed lower crust (Hou et al., 2009, 2012b; Tanget al., 2010; Zheng et al., 2010; Qin et al., 2011). The keystructural controls on the mineralization process andore shoots after the magma formation, however,remain unclear.

Two main structural events have affected rocks thathost the Jiama giant copper deposit, including the firstdeformation (D1) and the second deformation (D2). Thefirst deformation is correlated with a regional struc-tural event timed at around 50 Ma and has been wellstudied in previous work (Kapp et al., 2007; Tang et al.,2010; Zhong et al., 2012, 2013). This structural eventoccurred significantly prior to the copper mineraliza-tion and it probably provided important space for thedevelopment of copper deposition (Zhong et al., 2012).The D1 deformation was named as the Jiama-kajunguothrust-gliding nappe tectonic system and it might havebeen tectonically caused by the continental–continentalcollision between the Indo-Asia plates during theCenozoic (Zhong et al., 2012, 2013). The D2 deformationis interpreted as the most important control for thefinal development of mineralization because it wastemporally associated with the main time of mineral-ization in Jiama and is spacially associated with altera-tion. The detailed nature and characteristics of the D2

deformation have not been studied yet.In this study, we present district structural studies

for the Jiama copper deposit in order to investigatethe major structural control (e.g., the D2 deformation)on the copper mineralization. Given that the D1 defor-mation occurred significantly prior to the main timeof mineralization, we have focused mainly on the D2

deformation that is interpreted to be closely associ-ated with copper mineralization. In addition, to pre-cisely confirm the age of the D2 deformation in theJiama deposit, we report zircon U–Pb age for aplitefrom the Jiama deposit. We propose that D2 deforma-tion in the Jiama porphyry copper deposit is synchro-nous or slightly post-dates the mineralization. Theresults have crucial implications for further regionalprospecting.

2. Geological background2.1 Region and district geology

The Jiama copper deposit is a porphyry-skarn typecopper polymetallic deposit (Fig. 1). It is located inthe south part of the Gangdese-Nyenchen Tanglhaterrain. The strata that host Jiama are mostly composed

Jiama porphyry copper deposit, Tibet


Fig. 1 The regional tectonics of the Jiama porphyry copper deposits modified from (Qin et al., 2011). Inset denotes thelocation of the Jiama area in Tibet. 1. Quaternary alluvium and diluvium. 2. Chumulong Group, quartz sandstone withdark gray slate. 3. Linbuzong Group, sand slate and hornfels. 4. Duodigou Group, limestone and marble. 5. Quesangspring Group, calcareous siltstone with quartz sandstone. 6. Yeba Group third section, rhyolitic crystal tuff. 7. Yeba Groupsecond section, dacitic and andesitic crystal tuff with rhyolitic crystal tuff. 8. Granite porphyry. 9. Silicified rocks at thesurface. 10. Skarn. 11. I: skarn orebody. 12. Stratigraphic boundary. 13. Hornfels alteration boundary. 14. Normal fault. 15.Reverse fault. 16. Overturned/Normal syncline. 17. Overturned anticline. 18. Plate boundary zone and the subductiondirection. 19. Ocean crust subduction thrust front. 20. The main boundary thrust fault.

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of island arc volcanic rocks and passive marginsediments. The strata hosting mineralization aremainly composed of the rocks belonging to theDuodigou Group (J3d) (pale white marble, crystallinelimestone, marl, gray black calcirudite and debrismicrite) and the Linbuzong Group (K1l). The upperpart of the Linbuzong Group is interbedded with lithicsandstone, quartz sandstone, lithic quartz siltstone andargillite; the lower part consists of argillite, carbona-ceous shales with fine-siltstone and fine sandstonewith bioclastic micrite.

Many small dykes are exposed at Jiama, whereas themineralized porphyry stocks were emplaced at depth(not exposed on the surface). The sequence of emplace-ment of mineralized intrusive rocks from oldest toyoungest is: quartz dioritic porphyry, granitic por-phyry, monzogranitic porphyry and granodioritic por-phyry. The formation age of these magmatic rocks ismainly at 16.3 to 14.8 Ma with a peak at 15 Ma based onzircon U–Pb dating (Qin et al., 2011). The Re–Os iso-chron ages of molybdenites in mineralized porphyry,skarn and hornfels range from 15.5 to 14.0 Ma (Yinget al., 2010, 2014), which are consistent with the forma-tion time of the host magmatic rocks.

The structural trend at Jiama is due to the collisionbetween the Indian and Eurasian plates at the southernedge of the Gangdese-Nyenchen Tanglha terrane. As aresult of long-term regional strike-slip, sub-structures

mostly trend to WNW and thus result in developmentof many WNW directed nappes. The Jiama deposit ismainly controlled by nappe structures that are vergentfrom north to south and have a transform directionfrom south to north (Zhong et al., 2012). Nappe struc-tures in the mining area consist of a series of over-turned folds, including the Hongta and Niumatanganticlines and the Xiagongpu syncline. A detachednappe-gliding body extends from the Copper Moun-tain throughout Bulang ditch to the Mogulang ditch.This slide-nappe is divided into three parts from southto north: the front, central and rear (Fig. 2) sections(Zhong et al., 2012) and the outcrop of the gliding areais approximately 4 km2 (Fig. 1).

2.2 Geological features of skarn-typecopper-rich orebody

At Jiama, there are three types of copper orebody:skarn, porphyry and hornfels. The skarn copperorebody is shaped like a thick layer parallel plate. TheLinbuzong Group overlying the skarn consists of sand-stone slate and hornfels, whereas the Duodigou Grouplimestone and marble underlie the skarn orebody. Theskarn orebody appears to occupy a structural zonebetween the two groups. The orebody strikes NW–SE(300°) extending 2.85 km and dips NE (30°) extendingabout 2.5 km, although the margin is not well defined.

Fig. 2 The front, central and rear section of Jiama-Kajunguo sliding nappe tectonic system. 1. Sand slate. 2. Limestone. 3.Feldspar quartz sandstone. 4. Carbonaceous slate. 5. Porphyry. 6. Silicon cap. 7. Faults: ① reverse thrust nappe in earlyHimalaya period, ② Paleogene-Neogene reverse/growth normal fault. 8. Oriented sample number and occurrence.



The tendency of the orebody clearly varies from steepto slow, and then changes to steep again as a result ofthe nappe structures (Fig. 2). Steep dips of the upperpart, which are located in Qianshan region, rangefrom 50 to 70°. The central zone is the main orebodyand dips generally less than 20°, becoming slightlysteeper around the marginal area of Niumatang. Thismarginal area in Niumatang is exactly located betweenXiagongpu and Zegulang and covers nearly 5 km2. Thesteeper orebody at depth is located in the north ofZegulang typically dipping 30 to 40°. On the basis ofthe latest drillhole data, multiple high-grade (copper)blocks occur in this skarn-type orebody. The geologicalcharacteristics of the high-grade blocks are presentedin the following sections (Fig. 3).

The typically horizontal skarn orebody is shown inFigure 3. The main trend of high-grade mineralization

is 120°. However, apparent ore shoots trending 30°might be an artificial effect due to drilling layout andmay not be representative of the alteration of the origi-nal ore body. The 030° direction represents a regionalstructure from Jiama to Qulong. The 120° orientationparallels to the axial plane of D2. The 120° faults aresmall displacement faults that are mapped in the pitand are clearly part of D2. The 030° structures (and anapparent jog in that structure that locates the intrusiveporphyry) appear to be reactivation of an inferred base-ment fault and many different intrusive centres lie onthis trend (Qulong, Xiangbaishan, etc). The main anti-cline that hosts the highest grade skarn terminatesagainst this zone, implying differential movement onthe 030° structure during D2 deformation. From thecross-section seven in Figure 3a, the distribution ofmultiple high-grade orebody blocks appears to be inclose spatial relationship with folds or faults. It can beclearly seen in the cross-section (Fig. 3) that high-gradecopper-rich orebodies have been intersected by drillholes of ZK702, ZK710, ZK714, and ZK722 (Fig. 3a).Some smaller orebodies around drill hole ZK701 andZK707 have unknown structural relationships.

3. Field observations3.1 Lithologies, alteration and structures inNiumatang Pit

Several lithologies are found in the Niumatang pit.The upper part of the pit comprises two mainmetasedimentary units: a well-bedded, mostly fine-grained carbonaceous unit with interbedded mud-stone and thin fine-grained sandstone, and a massivecoarse-grained unit comprising fine to mediumgrained sandstone with uncommon interbedded mud-stone. Towards the bottom of the pit, closer to the min-eralized zone, a number of skarn lithologies areexposed. Strong skarnification of the metasedimentaryrocks has resulted in a massive to banded, generallypale green rock unit composed of green garnet anddiopside with common small patches and zones ofcarbonate and gypsum. A less altered sedimentary pre-cursor is indicated by the scale, thicknesses and mor-phology of banding in altered areas and are similar tothose observed in the hornfels sediments. The strongdevelopment of skarn resulted in the formation ofmassive rocks composed of green garnet, ±browngarnet, ±diopside, and ±wollastonite. The transitionzone from skarn to hornfels is composed of skarn withgradational variation and is in irregular contact withmarble. It is difficult to determine the precursor of this

(a)

(b)

Fig. 3 (a) The characteristics of Jiama skarn-type copper-rich ore body. Cross-section 7. (b) The skarn copper-rich ore body distribution characteristics in Section 7.

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type of skarn in the Niumatang pit and therefore, alltypes of skarn lithologies have been mapped as a singleunit.

Three types of alteration have been mapped in theNiumatang pit in this study. The most common type ofalteration occurs on higher berms and is developed inhornfels sediments that are associated with quartz-sulphide veins and appears most obviously as a lightgrey or light green-grey colour and overprinting thesediments and contrasts with the usual dark greycolour. This is defined as type-A alteration and ismainly silicification, particularly proximal to veins

with fine sericite and/or chlorite, as well as abundantdisseminated sulphides together with sulphidesoccurring on fracture surfaces (see Fig. 4a). The secondalteration type (type-S) occurs on lower berms of theNiumatang pit and is readily distinguished based onthe white to pale green colour of the rocks. In detail,this alteration type is mainly fine-grained silicificationwith patches of chlorite, epidote and commonsulphides. This alteration (type-S) appears to cross-cutbedding in the sediments (Fig. 4b). A third type,mapped in only a tiny area of the pit proximal to skarnmineralization is characterised by red-brown colour of

(a) (b)

(c)

Fig. 4 (a) Alteration associated withquartz-sulphide veins compris-ing silicification with sericite and/or chlorite with sulphides. Leadmountain. (b) Alteration type-A.Alteration of hornfels sediments.This alteration comprises mostlystrong, fine-grained silicificationwith chlorite, epidote andsulphides. Copper Mountain. (c)Alteration type-S. Dark patchesare remnant hornfels andthe cross-cutting nature ofbedding planes and foliation inthe Niumatang pit. Whereobserved the same angular rela-tionships prevailed. A regionalanticline occurs to the SSW of theNiumatang pit.



the hornfels sediments. It appears to be the fine-grained biotite and therefore interpreted as potassicalteration (type-K).

A number of structures have been mapped in theNiumatang pit. Foliation is intermittently developed inthe fine-grained hornfels sediment lithologies. Thisappears to be a regionally developed foliation and maybe associated with thrusting and folding in this area.Bedding and foliation are commonly oriented such thatfold vergence towards the anticline is always directedto the north or northeast (Fig. 4c), indicating that theNiumatang pit is located on the northeast limb of aregional anticline. Although a number of clay gougefilled faults have been mapped and measured, only onefault contains some kinematic indicators that suggest anormal fault. Some faults have quartz-sulphide veinssuggesting that these faults significantly post-date min-eralization. Other faults persist across several bermsand high walls, although it is anticipated that theirdisplacement is small.

Three types of veins have been discriminated inthe Niumatang pit, but only two of them have beenmapped because the unmapped type is steeplydipping, commonly parallel with each other, and com-prises a thin (several micrometre) calcite infill and areconsidered as late-stage structures commonly paralleljoints in the area. Quartz veins within the skarn com-monly contain open space which is filled by textureswith sulphides or secondary copper minerals. Theseveins are commonly steeply dipping in an approxi-mately north–south direction and are only observedwithin skarn. The most striking veins in the Niumatangpit are those that the structures therein contain alteredzones with many quartz-sulphide veins that are vari-ably deformed. These zones are discrete and containveins that display a range of deformation geometries.These include boudinaged, folded, and strongly flat-tened veins with intrafolial folds. No lineation orrodding type structure was observed. The wall rocksare strongly silicified and fine-grained and all veinscontain sulphides. These structures are shown inFigure 5.

3.2 Lead Mountain/Copper Mountain’slithologies, alteration and structures

Marble is more widely exposed in Lead Mountain andCopper Mountain pits than in the Niumatang pitarea, but their lithologies are generally similar. Thetransition from unaltered marble, having a prevailingfabric developed in coarse-grained carbonate, to

well-developed skarn is observed. Skarn-type altera-tion of the limestone is well exposed in the Lead Moun-tain area. The rocks in this transition zone are pale todark grey and contain carbonate which partially origi-nated as carbonate veining with fine-grained greengarnets. In some areas substantial carbonates gradeinto well-developed coarse-grained skarn.

In the Lead Mountain area, folds are well developedin the marble and in the hornfels sediments (Fig. 6a).The folded layering comprises bedding, S1 foliationand sulphide-bearing quartz veins. Crenulation cleav-age is observed especially in the hinge regions of F2

folds (Fig. 6b). An aplitic dyke in the Lead Mountainpit is folded (see Fig. 6c). A recumbent fold within themarble is exposed in the southern part of the LeadMountain pit. The ubiquitous fabric developed inmarble is interpreted as S1 which is axial planar to thisfold. The fabric in the marble (Fig. 6d) has been clearlydeformed, but in part it resembles bedding and showssome variations in grain size between layers. In themarble exposed in the Lead Mountain pit, S1 andbedding (S0) are mostly parallel to each other. Foldingof bedding (decimetric scale layering) is exposed incliffs above the Lead Mountain pit and is consistentwith the F2 folding. This supports the interpretationthat S1 and S0 are sub-parallel in this area and that bothare folded by F2 folds. Measurements of bedding, S1

and Sc (composite foliation comprising S1, veins and S0)have been combined because they are sub-parallel inboth the Copper Mountain and Lead Mountain pitareas. Together, these folded fabrics help to specify theF2 fold orientation and define D2 deformation zones.

4. Methods

Geological mapping was carried out in Niumatang,Lead Mountain, and Copper Mountain pits of theJiama deposit. Field observation formed the basis formap and cross-section interpretation. The processincluded: construction of geological maps, structuraldata recording, map interpretation, cross-sectionconstruction, and an interpreted deformation/mineralization/alteration history.

Zircons were separated from the studied aplite inLead mountain (Fig. 6c) using magnetic and heavyliquid separation methods, and then hand-pickedunder a binocular microscope. Approximately 100–200grains for the sample were mounted into an epoxyresin disc. Prior to isotope analysis, all grains werephotographed under both transmitted and reflectedlight, and subsequently examined using the

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cathodoluminescence (CL) image technique. Isotopicratios were analyzed using a Laser Ablation Induc-tively Coupled Plasma Mass Spectrometer (LA-ICP-MS) at the University of Science and Technology of

China (USTC). During the analyses, the standardzircon 91500 was used as the external standard forU–Pb isotopic analysis. Generally, 32-micrometerdiameter spots were used, with 44- or 60-micrometer

(a)

(c) (d)

(e) (f)

(b)

Fig. 5 (a-b) Folded quartz-sulphideveins with apparent foldedfoliation S1 such that an incipientcrenulation foliation S2 isdeveloped. Lead mountain. (c)Boudinaged quartz-sulphideveins. (d) Strongly foldedquartz-sulphide veins. Leadmountain. (e) Fold limbsare commonly “sheared out”.Niumatang pit. (f) Folded quartz-sulphide veins. Strongly flattenedquartz veins indicated by thepresence of highly lenticularveins, intrafolial folds and strongwall-rock fabric development.Lead mountain.



diameters sometimes, depending on the size of theanalyzed zircons. Final isotopic ratios and ages of thezircons were processed using the CommPbCorr#program (Andersen, 2002). The weighted mean 206Pb/238U age calculated using the ISOPLOT program(Ludwig, 2001) was used to represent the age of theaplite.

5. Zircon U–Pb dating results

The zircon U–Pb dating results are reported in Table 1.Zircons from the aplite sample (A3467) collected fromthe Lead mountain area are generally prismatic, color-less, transparent, and euhedral. Most zircon grainsdisplay oscillatory zoning as shown in the CL images(Fig. 7), which are typical of magmatic zircons. Analy-ses of thirteen spots yielded generally concordantU–Pb ages, with a weighted mean 206Pb/238U age of16.93 ± 0.4 Ma (Fig. 7), which is considered as the crys-tallization time of aplite intrusion in the Lead moun-tain area.

6. Discussion6.1 Characteristics of deformation D2

Structures attributed to the second deformation (D2)have been interpreted on cross-section and in 3Dmodels of Figure 8a. D2 folds trend 120° sub-parallel toD2 high strain zones. The model in Figure 8a representsthe top of skarn orebody (green) whereas yellow partdenotes the base. Porphyry rocks are shown as purplein this figure. A cross-section which cuts across theNiumatang pit shows high-grade skarn orebodiesand D2 deformation zones interpreted to depth. Themapping results and observations from drilling coresare used to mark bedding, foliation and D2 deforma-tion zone in the cross-section 23 (Fig. 8c). The deforma-tion region of D2 parallels to the fold axis in 120°direction. The drilling line is designed nearly 30° whichis vertical to the direction of 120° such that D2 occurs asa narrow area in the cross-section.

The mapping of the Niumatang pit is shown inFigure 9c. The Niumatang pit is mostly occupied byhornfels siliciclastic sediments with various skarn

(a)(a)

(d)(d)

(b)(b) (c)(c)(a)

(d)

(b) (c)

Fig. 6 (a) Folded quartz-sulphidevein in hornfels sediments. Awell-developed crenulationcleavage S2 is axial planar to thefold and an earlier S1 foliation canbe seen. Niumatang pit. (b)Folded composite layering inhornfels sediments. The folds areinterpreted as F2 folds since theydeformed an earlier S1 foliation.Lead mountain. (c) Folded aplitedyke contact. Folds in the dykecontact are consistent with foldedveins in the country rock and isinterpreted to reflect that foldingis post-dyke intrusion. The age ofthis intrusion is the maximumage of the D2. Lead mountain. (d)Folds in carbonate veins withinmarble. Lead mountain.

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bodies in the lower, western part of the pit. The contactbetween the skarn and siliciclastic sediments is irregu-lar and transitional with strongly altered hornfels.Several types of alteration have been mapped, whichvary with proximity to the skarn. For example, themost distal alteration zones are the A-type alteration(silicification, sericite, chlorite), whereas more intenseand widespread S- and K-type alteration occurredcloser to the skarn (Fig. 10). A-type alteration is spa-tially associated with many deformed veins, and S2

foliation is locally developed in this alteration zone.The alteration type changes from A-type into S- andK-type with distance closer to the skarn.

The geological mapping for the berms in the LeadMountain pit is shown in Figure 9a. Hornfels siliclasticsediments generally occur in the northern part,

whereas marbles occur in the south. A garnet-bearingskarn is well-developed along the contact. Alterationbecomes stronger closer to the skarn with variousalteration types in both the marble and hornfels sedi-ments. Skarn-type alteration zones containing garnetswithin marble occur further to the south. Three intru-sive dykes have been mapped in this area. Two of themare typical of the porphyry dykes containing abundantquartz and feldspar phenocrysts with rare maficminerals (hornblende). The third dyke is mainly fine-grained aplite and is in temporal association withthe porphyry dykes and D2 deformation. The mappingresults demonstrate that a porphyry dyke cuts across(and therefore post-dates) both skarn and the fine-grained aplitic dyke (see Fig. 9). The aplite dyke wasdeformed by D2 deformation whereas the porphyrydyke was not. The age of this porphyry dyke is dated atapprox. 15 Ma (Qin et al., 2011; Ying et al., 2014). Theaplite dyke therefore pre-dated the D2 deformation,and the age of approx. 17 Ma we obtained should rep-resent a maximum age for the D2 deformation.

The hornfels sediments in the Copper mountain pithave been strongly altered and generally lack internalstructure such as bedding and foliation. The massivenature of these sediments indicates that they belong toa massive fine-grained sandstone unit similar to thatmapped in the upper part of the Niumatang pit. Thecontact zone between the hornfels sediments and themarble is composed of skarn and altered rocks sur-rounding skarn. Like that encountered in the LeadMountain pit, the layering in skarn and in the adjacentmarble and hornfels sediments is steep to vertical and,in some cases, possibly overturned. Large porphyrydykes cut the sequence in the Copper Mountainpit, are generally steeply dipping and strike in an

Table 1 LA-ICP-MS zircon U–Pb isotopic data of aplite sample from the Jiama area, Tibet

Spot No. 207Pb/235U ± 1σ 206Pb/238U ± 1σ 208Pb/232Th ±1σ 207/235 age ±1σ 206/238 age ±1σ

No. 01 0.01317 0.00278 0.00236 0.00014 0.00193 0.0001 13 3 15.2 0.9No. 02 0.01551 0.00387 0.00261 0.00019 0.00204 0.0002 16 4 17 1.0No. 03 0.01606 0.00168 0.00266 0.00009 0.00185 0.0001 16 2 17.1 0.6No. 04 0.01541 0.00216 0.00252 0.0001 0.00194 0.0001 16 2 16.2 0.6No. 05 0.01233 0.00314 0.00269 0.00015 0.00191 0.0002 12 3 17.3 0.9No. 06 0.01914 0.00275 0.00251 0.00012 0.00204 0.0002 19 3 16.2 0.8No. 07 0.01683 0.00182 0.00272 0.0001 0.00205 0.0001 17 2 17.5 0.7No. 08 0.01667 0.00317 0.00261 0.00013 0.00222 0.0002 17 3 16.8 0.8No. 09 0.02338 0.00248 0.00266 0.00011 0.00195 0.0001 23 2 17.1 0.7No. 10 0.01876 0.00262 0.0026 0.00011 0.0018 0.0002 19 3 16.8 0.7No. 11 0.01783 0.00319 0.00289 0.00013 0.0019 0.0002 18 3 18.6 0.8No. 12 0.01819 0.00209 0.00264 0.00011 0.00178 0.0001 18 2 17 0.7No. 13 0.01474 0.00267 0.00267 0.00013 0.00208 0.0002 15 3 17.2 0.8

Fig. 7 Zircon U–Pb concordia diagram and weightedmean age of the aplite from Jiama, Tibet.



(a)

(b)

(c)

Fig. 8 (a) Interpreted folds in theskarn are also likely to be D2

deformation zones that can bemodelled as surfaces. (b) Spatialdistribution characteristics ofskarn D2 deformation zone. (c)The observation of D2 phenom-enon in drill cores of line 23.

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(a)

(b)

(d)

(c)



approximately NNE direction. These dykes in theCopper Mountain area contain more quartz phe-nocrysts and particularly abundant xenoliths (usuallyless than 100 mm) than those in the Lead Mountainarea. Clay gouge filled in fault zones, cut the hornfelssediments and indicate a late stage deformation withlittle significant displacement.

An interpretation of the D2 deformation zones andalteration is shown in Figure 10. It can be seen that thealteration becomes more intense closer to skarn. Thelowest berm includes large areas of altered hornfelssediment outcrop suggesting that significant areas ofskarn may have developed on hornfels sediments.West of the mapped area, skarn outcrops in contact

Fig. 9 (a) The mapping of Lead Mountain. (b) The mapping of Copper Mountain. (c) The mapping of Niumatang. (d)Legends. SLH, Well-bedded, dark grey to black mudstone, siltstone and minor fine sandstone. Carbonaceous metamor-phosed fine-grained hornfels. SAH, Dark grey, fine to medium-grained massive, carbonaceous sandstone with minorinterbedded politic hornfels. SCM, Grey coarse-grained marble with common carbonate (calcite) veins. AKG, Pale green,mostly massive rock comprising green garnet, ± brown garnet, ±diopside, ±wollastonite and minor carbonate (±gypsum)patches. Common sulphides disseminated include chalcopyrite, molybnem and other secondary copper minerals.Common sulphides and some banding are observed. IFQ, Pale to white, very fine-grained quartz, feldspar aplite dyke.IPF, Pale grey to brown, porphyritic dyke with abundant quartz and feldspar phenocrysts (1–7 mm), minor hornblende.It is diorite porphyry dyke. IPQ, Brown to pale grey porphyry dyke with abundant quartz and feldspar phenocrysts.Abundant xenoliths of slightly more mafic composition (also porphyry) with common reaction rims/haloes in granodi-orite porphyry. A, Green to light grey alteration of siliciclastic metasediments. Common disseminated suphides (pyrite,chalcopyrite and pyrrhotite). Silicification plus sericite and chlorite alteration. S, White to pale green, fine-grained stronglysilicified rock (with patches of chlorite and patches of epidote), disseminated sulphides common. Remnant hornfels in lessaltered parts. K, Red-brown colouring of hornfels sediments. Very fine-grained biotite-potassic alteration. Commonlyoverlaps with A and S type alteration. G, White to pale green alteration of marble. Comprise fine-grained green garnet andcarbonate. Transitional to coarse garnet skarn in marble.

◀

(a) (b)

Fig. 10 (a) Interpretation of the relationship between alteration, structure and mineralization. (b) Stain developing progres-sion in D2 zone which indicates mineralization process.

J. Duan et al.


with marble occur, so that at least some of the skarnwas developed on marble as well. The location of theoriginal contact between hornfels sediments andmarble is uncertain because it has been replaced byskarn. A possible interpretation is that the skarn iswell developed in the Niumatang pit area due tothe presence of altered D2 deformation zones. Inother words, D2 deformation zones which developedduring alteration and mineralization (as evidenced bymineralised Vsb veins in D2 zones) provided additionalfluid access to the contact between marble and hornfelssediment, and ultimately enhanced the fluid trap.

6.2 The forming mechanism of high-gradeorebody—the effect of D2

The formation of D2 deformation, which controlsskarn orebody development, is divided into four stages(Fig. 10): Stage 1, σ stress caused conjugate veinsshowing feature of an acute bisector; Stage 2, veinsbegun to fold when the initial foliation started todevelop; Stage 3, veins strongly deformed into foldswith foliation developed; and Stage 4, veins were com-pletely flattened and only early fold hinges remain andfoliation developed extremely strongly during thisstage.

Based on the observations from pit mapping (Figs 5,9, 10), we infer that the strong deformation zone ofquartz sulfide veins occur within the high strain zone.The D2 deformation typically deformed S1 whichregional foliation D2 was formed later than the regionalfolds and overthrust (Zhong et al., 2012). In addition,the occurrence of sulfides indicates that this deforma-tion may be formed simultaneously or slightly laterthan with mineralization. No linear fabric wasobserved in the D2 deformation zone, indicating theirformation was due to flattening. Structural phenomenarelated to this deformation zone display differentextents of stress change within the entire region. Asobserved in Figure 10, we find that the deformation inthe center (high stress) is stronger than that at the edge.However, systematic stress gradient has not beenwidely observed in the deformation zone. In the D2

deformation zone, the main deformation characteris-tics are like those commonly observed in the third andfourth stages, although the deformation features of thesecond stage have also been observed.

As shown in Figure 6, an interpreted F1 fold has beenfolded by F2 (D2 deformation) in the south part of theLead Mountain area. This suggests that a D1 deforma-tion structure was previously developed in this region.

The gliding-nappe attributed to D1 deformation andnamed the Jiama-Kajunguo thrust system, formed atapprox. 50 Ma (Zhong et al., 2013). The D1 is actually apassive far field event and the D2 is formed during themain collisional orogeny (including the formation ofnappes). D2 mainly provided the space for the develop-ment of deposits, and thus contributed significantly tothe metallogensis of the deposit. It might representthe main regional deformation event response to theIndia–Asia collision during the Cenozoic (Kapp et al.,2007; Hou et al., 2009; Zhong et al., 2012, 2013). Apliteveins in Lead Mountain were formed earlier than theD2 deformation because the veins were subject to D2

deformation. Therefore, the D2 deformation shouldoccur slightly after 17 Ma. Mineralization was later orsynchronized with D2 deformation. Mineralized veins(quartz sulfide veins) were prior to the development ofskarn alteration. The porphyry emplacement was laterthan D2 deformation around approx. 15 Ma. Then thereare some other small-scale structure activities resultingin micro-structures such as fault gouge, which are typi-cally even later.

6.3 The relationship between alteration,structure and mineralization

Three types of alteration (A-, S- and K-type) have beenmapped in different locations in the Jiama area. Thetype-A alteration occurs in the distal to the Jiamadeposit. Type-S occurs intermittently and is recognizedby the appearance of white to pale green colour ofthe rocks. In detail, this alteration type is mainly fine-grained silicification with patches of chlorite, epidoteand common sulphides (pyrite, chalcopyrite, borniteand tetrahedrite). Type-K alteration was mapped inonly a tiny area and is recognised by red-brown colourof the hornfels sediments. The deformation in the D2

deformed zones initially resulted in vein development,thus improving fluid access to those zones, as evi-denced by development of alteration in wall rocks.Skarn was located below the D2 deformed zones, andD2 developed a series of altered rocks which weregradually transformed into skarn, the major depositorebody.

6.4 Exploration implications

The results from the regional structural mappingand the observation of rock alteration in this studymay have important significance for exploration.First, although there were at least two periods of



deformation in the Jiama district and perhaps more inother regions, only the second deformation (i.e., D2)was found to be associated with alteration and coppermineralization. The D2 deformation can be identified,dated and described in detail. The zircon U–Pb ageobtained from the aplite in the Jiama area has con-strained the time of the D2 deformation. In addition,high-grade ore shoots appear frequently in the D2

deformation zones. Secondly, the K, S and A alterationtypes are also associated with in the D2 deformation.Further exploration of the deeper areas or thoseperipheral to the Jiama deposit should fully take intoaccount the key control of the D2 deformation whichwas generally closely related to copper mineralizationas demonstrated in this present study. In addition, therock and alteration types closely related to the D2 defor-mation might be useful pathfinders to mineralization.

7. Conclusion

In this paper, we presented detailed deformation char-acteristics associated with copper mineralization in theJiama deposit district, Tibet, and reported new zirconU–Pb age for the aplite from this area to confirm theprecise time of the structural deformation that is asso-ciated with and may have controlled the coppermineralization. Two periods of deformation were dis-cerned in this area. Both deformation events are impor-tant for the development of copper mineralization, andin the following we summarize several key points:

1 Based on structural mapping and observations ofrock alteration in the Niumatang, Copper Mountainand Lead Mountain areas of Jiama, Tibet, we dem-onstrate that there was a close relationship betweenstructure, mineralization and alteration. The relation-ship is gradually variable. Mineralization developedwidely in the alteration area accompanying deforma-tion, with more veins being related to strongerdeformation.

2 The progressive deformation characteristicsobserved in the field at the Lead Mountain andNiumatang pits indicate the similar strain intensity.The results therefore indicate that the intensity ofmineralization is also variable at different areas. Thishas general implications for regional exploration.

3 Field-based observations indicate that mineralizationwas coeval or slightly later than D2 deformation inthe Jiama area. Our new zircon U–Pb dating indi-cates that aplite in the Lead mountain area formed in

16.9 Ma and therefore the D2 deformation occurredslightly later than this time. Together with previousmolybdenite Re–Os dating which revealed that themajor copper mineralization occurred at approx. 14Ma, we propose that the copper mineralization inJiama was temporally related to the D2 deformation.

4 Our observations suggest that the copper mineraliza-tion was closely related to the D2 deformation whichcan be efficiently confirmed with thorough detailedmapping and observation of alteration features. Forexample, the observations that the D2 deformationcontrolled ore shoots can be interpreted on severalcross-sections through the Jiama deposit. This resultcould provide a good indication for future explora-tion of high-grade copper orebodies.

Acknowledgment

We thank Tibet Huatailong Mining Development Co.,Ltd. for the support of the field geological studies inthe Jiama deposit. We thank Dr. Qiufeng Leng andDr. Bin Lin for hard field work. Dr. Zhenhui Houis thanked for zircon U–Pb dating. We thank GregHall for comments and the editor for handlingwhich largely improved this manuscript. Thiswork was funded by the third subject of NationalBasic Research Program of China (973) grant(2011CB403103), National Natural Science Foundationof China (41302060) and Geological Survey Project(12120113093700).

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