Noel C. White

Genesis of late carboniferous granitoid intrusions in the Dayinsu area, West Junggar, Northwest China: evidence of an arc setting for the western CAOB

ABSTRACT The Dayinsu area is located in the northern part of the West Junggar district near the border between China and Kazakhstan and is an important component of the Central Asian Orogenic Belt (CAOB). The Dayinsu area hosts numerous granitoid plutons in Devonian–Carboniferous volcano–sedimentary strata. The older Laodayinsu and Kubei (345–330 Ma) plutons are located in the west with the younger Bayimuzha and Qianfeng (330–325 Ma) plutons in the east. The whole-rock SiO2 contents of the four granitoid plutons range from 52.22 to 68.42 wt.% and total alkaline contents (K2O + Na2O) range from 4.94 to 9.16 wt.%. The granites are enriched in large ion lithophile elements and light rare earth elements with depletions in Nb, Ta, Ce, Pr, P, and Ti. The plutons are metaluminous with I-type signatures. The geochemistry of the intrusions suggests that they formed in a subduction zone setting, and subsequently underwent fractional crystallization during emplacement, with higher degrees of fractionation in the eastern sector than in the west. Similarities in the geochronology and geochemical characteristics of the granitoid plutons in Dayinsu to those in the Tabei district (west to Dayinsu area) suggest that both districts are part of the Carboniferous Tarbagatay Mountain intrusive event. The early Carboniferous (345–324 Ma) granitoid intrusions in the Tarbagatay Mountain likely formed in an island arc subduction setting during the evolution of the CAOB.

Characteristics of the Intracontinental Porphyry Deposits in the Middle‐Lower Yangtze River Valley Metallogenic Belt, Eastern China

world’s copper (Cu) and one fifteenth of the world’s gold (Au) (Sillitoe, 2010), with 97% of the giant-large porphyry Cu (Mo-Au) deposits being generated in magmatic arc setting (Richards; 2003; Cooke et al., 2005; Sillitoe, 2010). Nevertheless, recent studies indicate that important porphyry deposits may also be formed in subduction-unrelated environments, e.g., along continental collision-related orogeny and intracontinental setting (Hou et al., 2009; Chen, 2013; Zhou et al., 2011). The Middle-Lower Yangtze Metallogenic Belt (MLYB) is one of the most important Cu-Au-polymetallic metallogenic belts in eastern China, and has been studied extensively in the past (Zhou et al., 2008). Various models have been proposed concerning the regional metallogeny, with major ones including the “porphyrite iron deposit” (NW Group, 1978), “stratabound skarn deposits” (Chang et al., 1991) and the “superimposed metallogenic system” (Zhai et al., 1992; Chang et al., 2012). With the discoveries of large porphyry Cu-Au deposits at Shaxi and Shujiadian in the recent years addition to the previous discovered porphyry deposits such as Chengmenshan and Fengshandong and other deposits, porphyry deposits are becoming more important exploration targets in the MLYB. Their nature and origin are still controversial, and are attributed variably to be the products of intracontinental magmatism (Hou et al., 2009; Chen, 2013; Zhou et al., 2011) or of the subduction of the Paleo-Pacific Plate (Ling et al., 2009, 2011; Liu et al., 2010; Sun et al., 2010; Xie et al., 2012), and recent study are more and more tend to the former point (Chen et al., 2014; Lv et al., 2014; Wang et al., 2014). To further illustrate the differences between the porphyry deposits formed in intracontinental setting and magmatic (continental or island) arc setting, we have chosen typical continental margin arc porphyry deposits, i.e., Bingham Canyon (US) and Bajo de la Alumbrera (Central Andes); island arc porphyry deposits, i.e., Panguna (PNG) and Batu Hijau (Indonesia) to compare with the MLYB porphyry deposits. It is summarized that: 1. Distribution of magmatic (continental or island) arcgenerated porphyry deposits is commonly linear, and is parallel to the orogeny and perpendicular to the subduction zone (Sillitoe, 2010). The MLYB porphyry deposits are distributed along the Yangtze Fault, oblique to the PaleoPacific subduction zone. 2. Magmatic arc-generated porphyry metallogenic systems are commonly preceded by calc-alkaline or alkaline felsic volcanism (Sillitoe, 1973) that occurs ca. 0.5 ‒ 3 Ma prior to the porphyry emplacement, as evidenced at Bingham (Waite et al., 1997), Farallón Negro (Argentina; Sasso and Clark, 1998; Halter et al., 2004), Yerington (Dilles and Wright, 1988; Dilles and Proffett, 1995), Tampakan, Philippines (Rohrlach and Loucks, 2005) and Yanacocha (Longo and Teal, 2005). No coeval (or similar age) volcanism with the porphyry deposits has been documented in the MLYB. 3. Types of wall rocks vary in different deposits, suggesting porphyry Cu deposits are indiscriminative towards their wall rocks. Wall rocks for magmatic arcgenerated porphyry deposits are commonly volcaniclastic rocks, whereas wall rocks for the intracontinental MLYB porphyry deposits contain sandstone (e.g., Shaxi) and carbonates (e.g., Tongshankou). 4. Major metal sulfides in the MLYB include chalcopyrite, pyrrhotite, pyrite,and bornite. Pyrrhotite is closely associated with skarn, and may suggest the reducing nature of the wall rocks (Kósaka and Wakita, 1978; Perelló et al., 2003), in accordance with the formation of magmatic arc-generated porphyry deposits (Sillitoe, 2010). The MLYB porphyry deposits contain the same vein types as typical magmatic arc-generated porphyry deposits, but with different vein type proportion: ZHOU Taofa, WANG Shiwei, FAN Yu, YUAN Feng, ZHANG Dayu and Noel White, 2014. Characteristics of the Intracontinental Porphyry Deposits in the Middle-Lower Yangtze River Valley Metallogenic Belt, Eastern China. Acta Geologica Sinica (English Edition), 88(supp. 2): 667-669.