Mineralogical and geochemical characterisation of the Kipawa syenite complex, Quebec: implications for rare-earth element deposits

The Kipawa rare-earth element (REE) deposit is located in the Parautochton zone of the Grenville Province 55 km south of the boundary with the Superior Province. The deposit is part of the Kipawa syenite complex of peralkaline syenites, gneisses, and amphibolites that are intercalated with calc-silicate rocks and marbles overlain by a peralkaline gneissic granite. The REE deposit is principally composed of eudialyte, mosandrite and britholite, and less abundant minerals such as xenotime, monazite or euxenite. The Kipawa Complex outcrops as a series of thin, folded sheet imbricates located between regional metasediments, suggesting a regional tectonic control. Several hypotheses for the origin of the complex have been suggested: crustal contamination of mantle-derived magmas, crustal melting, fluid alteration, metamorphism, and hydrothermal activity. Our objective is to characterize the mineralogical, geochemical, and isotopic composition of the Kipawa complex in order to improve our understanding of the formation and the post-formation processes, and the age of the complex. The complex has been deformed and metamorphosed with evidence of melting-recrystallization textures among REE and Zr rich magmatic and post magmatic minerals. Major and trace element geochemistry obtained by ICP-MS suggest that syenites, granites and monzonite of the complex have within-plate A2 type anorogenic signatures, and our analyses indicate a strong crustal signature based on TIMS whole rock Nd isotopes. We have analyzed zircon grains by SEM, EPMA, ICP-MS and MC-ICP-MS coupled with laser ablation (Lu-Hf). Initial isotopic results also support a strong crustal signature. Taken together, these results suggest that alkaline magmas of the Kipawa complex/deposit could have formed by partial melting of the mantle followed by strong crustal contamination or by melting of metasomatized continental crust. These processes and origins strongly differ compare to most alkaline complexes in the world. Additional TIMS and LA-MC-ICP-MS analyses are planned to investigate whether all lithologies share the same strong crustal signature.

Zircon U–Pb geochronology and Hf isotope analyses of the Wulian complex in the Sulu orogenic belt, eastern China: tectonic affinity and implications for early Precambrian crustal growth and recycling in the South China Craton

The Wulian complex is located on the northern margin of the Sulu orogenic belt, and was formed by collision between the North China Craton (NCC) to the north and South China Craton (SCC) to the south. It consists of the metasedimentary Wulian Group, gneissic granite and meta-diorite. The U–Pb analyses for the detrital zircons from the Wulian Group exhibit one predominant age population of 2600–2400 Ma with a peak at c. 2.5 Ga and several secondary age populations of > 3000, 3000–2800, 2800–2600, 2200–2000, 1900–1800, 1500–1300 and 1250–950 Ma; some metamorphic zircons have metamorphic ages of c. 2.7, 2.55–2.45, 2.1–2.0 and 1.95–1.80 Ga, which are consistent with magmatic-metamorphic events in the SCC. Additionally, the Wulian Group was intruded by the gneissic granite and meta-diorite at c. 0.76 Ga, attributed to Neoproterozoic syn-rifting bimodal magmatic activity in the SCC and derived from partial melting of Archaean continental crust and depleted mantle, respectively. The Wulian Group therefore has tectonic affinity to the SCC and was mainly sourced from the SCC. The detrital zircons have positive and negative ϵHf(t) values, indicating that their source rocks were derived from reworking of both ancient and juvenile crustal rocks. The major early Precambrian crustal growth took place during c. 3.4–2.5 Ga with a dominant peak at 2.96 Ga and several secondary peaks at 3.27, 2.74 and 2.52 Ga. The two oldest zircons with ages of 3307 and 3347 Ma record the recycling of ancient continental crust (> 3.35 Ga) and crustal growth prior to c. 3.95 Ga in the SCC.

Discussion about Gold Ores Mineralization of Collision-Type Orogeny in the East of Shandong

The mineralization of collision orogeny is an important part of continental dynamics. For the process of continental dynamics of Shandong, adoption of tectonic facies mapping is main carrier and specific expression form to these researches such as divergence of continental mass, convergence, collision and orogeny. Shandong tectonic facies mapping of 1:500000 scale worked out by author shows that there are two very important events of collision orogeny in Mesozoic this areaIndochina and Yanshan collision orogeny. The Indochina orogeny is mainly characterized as subduction from Yangtze to North China Plates, based on which Sulu high-ultra high pressure zone of metamorphism, syn-orogenic granite and post-orogenic high alkali sinaite are formed. Continental dynamics environment of the Yanshan orogeny derives from transformation from Central Asia-Tethys tectonic domain to marginal-Pacific tectonic domain and subduction of Pacific plates, and it appears as three orogenys and three stretching in the east of Shandong. Magmatic rocks of orogeny related with gold ores can be divided into four combinations as follows: Linglong gneissic granite of the early orogenic period (J3), Guojialing granodiorite-granite of the middle orogenic period (K1), Weideshan diorite-granodiorite-granite of the late orogenic period (K1) and A-type Laoshan geode parlkaline alkali granitesyenogranite of the post orogenic period. For combination of Guojialing granodiorite-granite of the middle orogenic period, SHRIMP U-Pb ages concentrate in 130~126Ma, which are closely related with emplacement of gold ores, and formed ages of gold ores this area concentrate in 115~120Ma, which basically stand for the age of main mineralization period. Polymetallic ores are related with combination of Weideshan diorite-granodiorite-granite of the late orogenic period, and it was also the superimposed mineralization period in the east of Shandong. Tectonics-magma activities and gold ores mineralization are controlled by interaction of three tectonic domains that are tethys, Paleo-Asian Ocean and Pacific. Dynamics background of gold ores this area is transition of tectonic system and lithospheric thinning in Mesozoic, which is related with collision of North China and Yangtze Plates and subduction of Pacific Plates.