Geochemistry of the Permian Kalatongke Mafic Intrusions, Northern Xinjiang, Northwest China: Implications for the Genesis of Magmatic Ni-Cu Sulfide Deposits
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©2009 Society of Economic Geologists, Inc. Economic Geology, v. 104, pp. 185–203 Geochemistry of the Permian Kalatongke Mafic Intrusions, Northern Xinjiang, Northwest China: Implications for the Genesis of Magmatic Ni-Cu Sulfide Deposits ZHAOCHONG ZHANG,† State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, P. R. China JINGWEN MAO, FENGMEI CHAI, SHENGHAO YAN, BAILIN CHEN, MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, 100037, P. R. China AND FRANCO PIRAJNO Geological Survey of Western Australia, 100 Plain Street, East Perth WA 6004, Australia School of Earth and Geographical Sciences, The University of Western Australia, Crawley 6009, Australia Abstract The Kalatongke Cu-Ni sulfide deposit in northern Xinjiang, northwest China, is located on the southern side of the regional Irtysh fault zone, which is the boundary between the early Paleozoic Altai orogenic belt to the north and the late Paleozoic Junggar terrane to the south. In the Kalatongke region there are 11 mafic intru- sions which were emplaced in Lower Carboniferous strata. Economic Ni-Cu sulfide ores are found within three of these intrusions, which are well differentiated, and compositionally zoned. Four rock types are recog- nized which, from top to base of the three mineralized intrusions, include biotite diorite, biotite-hornblende norite, biotite-hornblende-olivine norite, and biotite hornblende-dolerite. The biotite-hornblende olivine norites and biotite-hornblende norites are the most favorable host rocks for Ni-Cu mineralization. The initial 87 86 ( Sr/ Sr)t (t = 280 Ma) ratios of the intrusions vary from 0.70375 to 0.70504, and εNd(t) from 6.3 to 8.2, im- plying that the magmas originated from depleted asthenospheric mantle. However, the strong enrichment of lithophile elements such as K, Rb, Th, U, and LREE, the negative Nb and Ta anomalies, and the high δ18O values of whole rocks (5.4–10.6‰) suggest significant crustal contamination. These patterns are present in al- most all samples from the intrusions, indicating that the contamination took place before the magma was em- placed at its present level in the crust. Crustal contamination is interpreted to have driven the magma to S-sat- uration and brought orthopyroxene onto the liquidus. Convection, or perhaps flow differentiation of the crystal-bearing magma during ascent, caused the dense sulfide melt with entrained olivine and orthopyroxene crystals to become concentrated in the center of the intrusions, whereas the fractionated magmas formed other, less mafic intrusions, which intrude the nearby strata. Introduction ppb) and 3.4 t of Pd (0.20 ppb). A mine was subsequently de- IN MANY DEPOSITS, crustal contamination has been identified veloped in 1985 and started production in 1989. In 2005, a as having a major role in triggering S saturation in the mafic new Ni-Cu orebody consisting of massive sulfide ores with magmas (Brugmann et al., 1993; Naldrett, 2004; Lightfoot about 200,000 t of Cu and Ni (avg grades of 4.09 and 3.73 wt and Keays, 2005; Wang and Zhou, 2006), although some de- %, respectively) was discovered at the boundary between Y2 posits (e.g., Jinchuan and Nebo-Babel in Western Australia) and Y1. The Kalatongke deposit is now the second largest Ni- are thought to have originated as continuous magma chono- Cu deposit in China, after Jinchuan (Lehmann et al., 2007). liths with multiple and related magma pulses (Seat et al., The ore-bearing intrusions contain a large amount of or- 2007; Tang et al., 2007). The main issues are the following: thopyroxene, probably suggesting that the parental magmas what were the contaminants (e.g., pyrite-rich chert at Kam- were siliceous high Mg basalts (SHMB) that were formed by balda, anhydrite at Noril’sk, and S-rich metasediments at crustal contamination of komatiites (Arndt and Jenner, 1986; Voisey’s Bay), and at what stage in the evolution of the mag- Barley, 1986; Cattell, 1987; Skulski et al., 1988; Barnes, 1989; mas did the contamination take place? Sun et al. 1989; Skulski and Percival, 1996). This feature pro- The Kalatongke copper-nickel sulfide deposit is located in vides us with an opportunity to evaluate the role of crustal Fuyun County, about 380 km north of Urumqi, the capital of contamination in the generation of the Kalatongke intrusion Xinjiang, northwest China. It was discovered in 1978 by the and ore formation. No. 4 Party of the Xinjiang Bureau of Geology and Mineral Although the Kalatongke intrusions and hosted ore de- Resources. The exploration program for the No. 1 and No. 2 posits have been previously studied, most publications are in mafic intrusions (named Y1 and Y2, respectively) established Chinese (e.g., Wang and Zhao, 1991; Wang et al., 1992; Zhang a reserve of 419,000 tons (t) of Cu (avg grade of 2.46 wt %), et al., 2003). In this paper, we present the results of an addi- 240,000 t of Ni (avg grade of 1.42 wt %), 8,355 t of Co (avg tional field and petrologic-geochemical study of the intrusions grade of 0.049 wt %), as well as 2.5 t of Pt (avg grade of 0.15 that host the Kalatongke deposit. On the basis of our obser- vations and results we propose a new model for the formation † Corresponding author: e-mail, [email protected] of the deposit that involves flow differentiation. 0361-0128/09/3807/185-19 185 186 ZHANG ET AL. Geologic Setting movement. The intersections of northwest-striking faults with The Kalatongke region is located in the East Junggar ter- west-northwest–striking faults were favorable sites for the rane, which is bounded by the Siberian craton, the Kaza- emplacement of mafic complexes. khstan block, and the Tianshan orogenic belt in the central The Kalatongke area is underlain by Devonian to Carbonif- portion of Central Asian orogenic belt (Fig. 1a, b). The Irtysh erous volcanic and sedimentary rocks. The Devonian succes- fault is a boundary between the early Paleozoic Altai orogenic sion consists of marine sedimentary clastic rocks intercalated belt to the north and the Late Paleozoic Junggar terrane to with carbonates and tuffaceous rocks and overlain by inter- the south (Fig. 1c; Coleman, 1989; Huang et al., 1990; Xiao et mediate-mafic flows and pyroclastic rocks intercalated with chert (Beitashan Formation). These are followed by interme- al., 1990). The Junggar terrane is traditionally divided into the diate-mafic volcanic rocks and minor interbedded sedimen- East and West Junggar terranes and the Junggar basin. The tary rocks (Yundukala Formation). The Carboniferous is rep- Junggar basin contains thick continental deposits and is resented by the Heishantou and Nanmingshui Formations, bounded to the south by a south-dipping foreland thrust which comprise a succession of intermediate-mafic and felsic zone, which is composed mainly of an east-west–trending vol- volcanic and pyroclastic rocks, chert, sedimentary and vol- canic arc complex. The East Junggar terrane comprises sev- canic breccias, carbonate and tuffaceous beds. These rocks eral northwest-trending, highly deformed metasedimentary were metamorphosed to lower greenschist facies during the and ophiolite assemblages which were accreted to the south- Triassic. ern margin of the Siberian plate along the Irtysh fault. The West Junggar terrane was accreted to the Kazakhstan block to The Kalatongke Intrusions and the west. Associated Cu-Ni Mineralization The East Junggar terrane comprises several accretionary Eleven northwest-trending mafic intrusions are present in complexes that were generated by subduction-accretion the 1.7 km2 Kalatongke region (Fig. 2a). All intrude the sedi- processes in Paleozoic times (Coleman, 1989; Feng et al., mentary and volcanic rocks of the Lower Carboniferous Nan- 1989). Two highly deformed and dismembered belts of ophi- mingshui Formation. The three largest intrusions (Y1, Y2, olites, the Wulunguhe and Kalamaili ophiolites, occur in the and Y3) are well differentiated, zoned, and strongly mineral- East Junggar terrane; the Wulunguhe ophiolite was dated to ized, but mostly not exposed. In contrast, Y4, Y5, Y6, Y7, Y8, 481 ± 5 and 489 ± 4 Ma by Jian et al. (2003) using SHRIMP Y9, Y10, and Y11 intrusions are weakly differentiated and U-Pb zircon methods, whereas the Kalamaili ophiolite was contain only uneconomic copper mineralization. Rb-Sr and determined to be 373 ± 10 Ma (Tang et al., 2007). These Sm-Nd isochron ages of the Y1, Y2, and Y3 intrusions, yield ophiolites reflect the formation of oceanic crust in the early to ages of 285 ± 16.7, 297 ± 23, 301 ± 28, 290 ± 33.5, and 297.7 middle Paleozoic. Subduction of the oceanic lithosphere be- ± 11 Ma, respectively (Wang and Zhao, 1991; Li et al., 1998). neath the Altai orogen and the Kazakhstan block is mani- Moreover, recent U-Pb dating of zircons from the Y1 intru- fested by the presence of thick marine volcanic rocks interca- sion yielded an age of 287 ± 3 Ma (Han et al., 2004), and Re- lated with sedimentary rocks of Devonian to Carboniferous Os dating of chalcopyrite and pyrrhotite in the ores from Y2 ages. This was followed by accretion and imbrication of arc gives an age of 285 ± 17 Ma (Zhang et al., 2008a). series and back-arc basins toward the Kazakhstan block, as The Y1 intrusion exhibits an irregular lensoid shape that is the three blocks (Tarim, Kazakhstan, and Siberian craton) 695 m long and 39 to 289 m wide with an outcrop area of converged. Final closure of the Paleo-Asian ocean with colli- about 0.1 km2. Its long axis strikes northwest, and dips at 60° sion between the Kazakhstan and Siberian plates occurred by to 85° to the northeast. In cross section, the Y1 intrusion the Late Carboniferous.