Jing-Jing Zhu

Geology, Sulfur Isotopes and Scheelite Sm‐Nd Age of the Kukaazi Pb‐Zn‐(Cu‐W) Polymetallic Deposit, Yecheng County, Xinjiang, China

The Kukaazi Pb-Zn-(Cu-W) polymetallic deposit, a newly discovered massive sulfide polymetallic deposit, is located in Xihexiu Twonship, Yecheng County, Xinjiang, China. Tectonically, it lies in the Western Kunlun central terrain and Phanerozoic magmatic arc (Sun et al., 2003), bordered by the Kegang Fracture zone in north and Kangxiwa Fracture zone in south, in the Western Kunlun Oregenic belt which is believed to be a Proto-Tethyan Orogenic belt (Xu et al., 2011). The Pb-Zn-(Cu-W) mineralization of the Kukaazi deposit occurred in Precambrian strata. Through primary studies on ore deposit geology, sulfur isotopes of sulfides and sulfate, Sm-Nd geochronology of scheelite, the origin of oreforming materials and the metallogenesis of the deposit have been probed and discussed in this paper.

Titanite major and trace element compositions as petrogenetic and metallogenic indicators of Mo ore deposits: Examples from four granite plutons in the southern Yidun arc, SW China

Abstract Major, minor, and trace element abundances in titanite crystals from four granitic plutons in southern Yidun arc, SW China, have been determined using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. The selected plutons are the Cretaceous Xiuwacu (CXWC) pluton, with quartz vein-type Mo mineralization (economic-Mo), the Tongchanggou (TCG) pluton, with porphyry-type Mo mineralization (economic-Mo), the Triassic Pulang (PL) pluton, with porphyry-type Cu mineralization (subeconomic-Mo), and the Triassic Xiuwacu (TXWC) pluton, without any Mo mineralization (Mo-barren). Our study reveals that the chemical compositions of titanite crystals from these plutons such as REE, Sr, Ga, δEu, δCe, Fe2O3/Al2O3, halogens, and Mo can be used to track magma compositions, oxidation states, metal fertility, and crystallization history. The data from this study also show that titanite crystals from these plutons with different potential of Mo mineralization have similar Mo contents and exhibit an irregular variation between Mo and Sr abundances (indicating non-Mo enrichment in the residual melt during the progressive crystallization) for some Mo-mineralized plutons. Our new observations support the recent hypothesis that high initial Mo contents in magma and the enrichment of Mo in residual melts formed by fractional crystallization are not the only requirements to form a granite-related Mo ore deposit. Efficient extraction of the residual melts, possibly facilitated by high concentrations of magmatic F is also critical to the ore formation. Evidence for high-F concentration in felsic magma, which facilitates melt and fluid separation and economic Mo mineralization during magma evolution, may be traced by the presence of F-rich titanite crystals in the two Mo-mineralized granite plutons (CXWC and TCG). These new findings from this study confirm that titanite is indeed a good petrogenetic and metallogenic indicator. However, in light of the limited contribution of metal fertility to Mo mineralization, we suggest that titanite Mo concentrations should be used along with other crucial proxies, such as titanite F contents and Fe2O3/Al2O3 ratios to better evaluate the Mo-mineralized potential of granites.

Contribution of Previously Arc Cumulates to the Post-Subduction Yangla Skarn Cu Deposit, SW China

The Yangla skarn Cu deposit (150 Mt at 1.03% Cu) is located in the central segment of the Jinshajiang Metallogenic Belt within the Sanjiang (three rivers) region, SW China. The ore associated granodiorite was emplaced at 233.1 ± 1.4 Ma and 231.0 ± 1.6 Ma at 2σ by zircon U-Pb dating (Zhu et al., 2011), coeval to the Cu mineralization (232.0±1.5 Ma; Re-Os dating by molybdenite). Both significantly postdated the Early Triassic collision (246– 247 Ma; Zi et al., 2012) between the Qamdo–Simao terrane and the Zhongza terrane along the Jinshajiang Suture following the closure of the Jinshajiang Paleo-Tethys Ocean. Based on their geochemical compositions, two groups of granodiorite have been defined. Group 1 is characterized by relatively low eNd(t) values (-5.1– -6.7) and high initial 87 Sr/ 86 Sr ratios (0.7078–0.7148), with ancient two stages Nd isotope model ages (TDM2 = 1420– 1551 Ma). In contrast, Group 2 has mantle-like eNd(t) and initial 87 Sr/ 86 Sr ratios (eNd(t) = 1.5–2.9; ( 87 Sr/ 86 Sr)i = 0.7042 –0.7047), with obvious younger TDM2 ages (769–882 Ma). Both of them are enriched in high ion lithophile elements and depleted in high field strength elements, typical subduction signatures. In addition, Group 2 shows higher Sr/Y ratios (45–81) than Group 1 (Sr/Y = 11–37), indicative of a hydrous magma source which would suppress plagioclase fractionation and be in favor of hornblende crystallization (Naney, 1983; Richards and Kerrich, 2007). In combination, we propose Group 2 was probably derived from the melting of previously subduction-modified lithosphere, especially hydrous arc cumulates in the lower crust. However, rocks from Group 1 were derived from a mixing magma from the melting of both residual metasomatic lithosphere and ancient lower crust. The arc cumulates associated with the subduction of the Paleotethys might provide Cu, S, and water for the fertile magmas. For the contribution of the Neoproterozoic slab subduction, further work should be done.