Franco Pirajno
Geological Survey of Western Australia
September 9, 2004
Crustal uplifts, continental rifting, the emplacement of anorogenic magmas and large igneous provinces (some basalt-dominated, others dominated by silicic lavas; see Bryan et al., 2000) can be linked to a great variety of ore deposits, both orthomagmatic and hydrothermal (Pirajno, 2004).
The last 16 months have seen important developments in mantle plume geology and its link with ore deposits. Several papers have been published dealing with large scale tectonics, continental reconstructions and associated metallogenesis, in which mantle plumes are thought to have been implicated. In the following I briefly review the salient points (as far as I am aware; not easy these days to keep up with large volumes of literature) of recent developments in central Asia, a region full of promise for new challenges in the earth sciences.
I begin by referring to a special issue of the Journal of Asian Earth Sciences (Jahn et al., 2004), which provides some powerful insights into the Phanerozoic geodynamic evolution of the Earth’s largest land mass. In this special issue, Yakubchuk (2004) details the mineral deposits settings of the Altaid orogenic belt (also known as Central Asian Orogenic Belt, CAOB; a complex collage of accreted magmatic arcs, ophiolites sensu lato, back-arc basins and fragments of continents). In this context I draw attention to the fact that the CAOB is well-endowed with mafic and ultramafic rocks. Most, if not all, are interpreted as ophiolites in the classic sense, that is ancient oceanic crust that was obducted or accreted during collision events onto continents (see Sengor and Natal’in, 1996, for an excellent overview of the tectonics of Asia). In the special issue referred to above, Yakubchuk (2004) mentions the Siberian superplume as the principal cause of mafic-ultramafic magmatism and related Cu-Ni-PGE deposits. Important is the inference by this author that a number of plume-related “oceanic plateaux” as well as areas of continental flood basalts could have been present in the Paleotethys Ocean. The mafic-ultramafic rocks in these plateaux and continental flood basalts were subsequently accreted during the final amalgamation of the CAOB (figure 7 in Yakubchuk, 2004). In other cases, post-orogenic extension and mantle plume activity (shallow?) occurred following collision (Zhou et al., 2004). The bottom line here is that many of the “old” ophiolites may in fact be either fragments of LIPs (oceanic plateaux and/or continental flood basalts), or post-orogenic intrusions in rift settings. This is a very important point because it has major economic implications. Wu et al. (2004) report on the geochronology of Cu-Ni-bearing ultramafic complexes at the eastern end of the CAOB, in NE China, where more than a thousand of these complexes have been recognised. The authors clearly show that the timing of emplacement of the causative intrusions is post-orogenic. Zhou et al. (2004) found the same for the Huangshan Ni-Cu-PGE deposits in NW China. In addition, it is well established that these mafic-ultramafic complexes are associated with widespread A-type granitoids in the region (Jahn et al., 2000). Several studies now suggest that these A-type granitoids developed in rift settings and are the result of crustal melting due to underplating of mantle melts inferred to be related to the impingement of mantle plumes (e. g. Ling et al., 2003; Wu et al., 2004).
In terms of ore deposits, and apart from Cu-Ni sulphides in mafic-ultramafic complexes related to plume magmatism (including the world-class Jinchuan deposit; Naldrett, 1999), several hydrothermal deposits, such as the Au epithermal Axi and Shyingtan, and even Cu-Zn VMS deposits like Ashele (Mao et al., 2003), all appear to be associated with Carboniferous-Permian rift settings or, as in the case of Ashele, with ancient oceanic plateaux (see Wang et al., 1998). A spin off is the exciting notion that low-sulphidation epithermal deposits may also form in LIP provinces and are not exclusive of magmatic arc settings. I am currently working on one of these in the Djibouti area in the horn of Africa, where Au-bearing hot spring deposits are associated with Ethiopian-Yemen LIPs.
Space limitations preclude further details, but I would also like to mention Zorin et al.’s paper (2003) on the Baikal rift zone and the emplacement of, what in my opinion amounts to a large igneous province, namely the abundant Oligocene-Quaternary volcanics that extend from Tibet, to NE China, Mongolia and the Siberian platform. The possible role of mantle plume(s) in this vast region is perhaps not well-constrained (the possibility of lithospheric delamination due to the subduction front of the Pacific plate needs to be evaluated and considered). The economic resources of this region are considerable, including the fact that many of the associated rift basins contain hydrocarbons.
References
Bryan, S. E., Ewart, A., Stephens, C. J., Parianos, J., Downes, P. J., 2000. The Whitsunday volcanic province, central Queensland, Australia: lithological and stratigraphic investigations of a silicic-dominated large igneous province. Journal of Volcanology and Geothermal Research, 99, 55-78.
Jahn, B-m., Wu, F-y., Chen, B., 2000. Granitoids of the Central Asian Orogenic Belt and continental growth in the Phanerozoic. Transaction of the Royal Society of Edinburgh: Earth Sciences, 91, 181-193.
Jahn, B-m., Windley, B., Natal’in, B., Dobretsov, N., (eds), 2004. Phanerozoic crustal growth in central Asia. Journal of Earth Sciences, v. 23.
Ling, W., Gao, S., Zhang, B., Li, H., Liu, Y., Cheng, J., 2003. Neoproterozoic tectonic evolution of the northwestern Yangtze Craton, South China: implications for amalgamation and break-up of the Rodinia Supercontinent. Precambrian Research, 122, 111-140.
Mao, J-w., Goldfarb. R. J., Seltmann, R., Wang, D-h., Xiao, W-j., Hart, C. (eds), 2003. Tectonic evolution and metallogeny of the Chinese Tianshan. Centre for Russian and Central Asian Mineral Studies and Natural History Museum of London. IAGOD Guidebook Series, v. 10.
Naldrett, A. J., 1999. World-class Ni-Cu-PGE deposits: key factors in their genesis. Mineralium Deposita, 34, 227-240.
Pirajno, F., 2004. Hotspots and mantle plumes: global intraplate tectonics, magmatism and ore deposits. Mineralogy and Petrology (in press; DOI 10.1007/s00710-004-0046-4).
Sengor, A.M. C., Natal’in B. A.,1996. Paleotectonics of Asia: fragments of a synthesis. In Yin, A. and Harrison, T. M. (eds) The tectonic evolution of Asia, Cambridge University Press, pp. 486-640.
Wang, D-h., Chen, Y-c., Mao, J-w., 1998. The Ashele deposit: a recently discovered volcanogenic massive sulphide Cu-Zn deposit in Xinjiang, China. Resource Geology, 48, 31-42.
Wu, F-y., Wilde, S. A., Zhang, G-l., Sun, D-y., 2004. Geochronology and petrogenesis of the post-orogenic Cu-Ni sulphide-bearing mafic-ultramafic complexes in Jilin Province, NE China. Journal of Asian Earth Sciences, 23, 781-797.
Yakubchuk, A., 2004. Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model. Journal of Asian Earth Sciences, 23, 761-779.
Zhou, M-f., Lesher, C. M., Yang, Z-x., Li, J-w., Sun, M., 2004. Geochemistry and petrogenesis of 270 Ma Ni-Cu-(PGE) sulphide-bearing mafic intrusions in the Huangshan district, eastern Xinjiang, northwest China: implications for the tectonic evolution of the Central Asian Orogenic Belt. Chemical Geology, 209, 233-257.
Zorin, Yu. A., Turutanov, E. Kh., Mordvinova, V. V., Kozhevnikov, V. M., Yanovskaya, T. B., Treussove, A. V., 2003. The Baikal rift zone: the effect of mantle plumes on older structures. Tectonophysics, 371, 153-173.