Genesis Of Metamorphic Mafic Volcanic Rocks In The Huachanggou Gold Orefield, Western Qinling Orogen And Its Implication For Gold Mineralization

The Huachanggou orogenic gold deposit is located in Mianlue suture, western Qinling Orogen. The metamorphic mafic volcanic rocks in the Huachanggou gold orefield include spilites and metamorphic basalts. The spiltes distribute in phyllite in the Sanhekou Group and the metamorphic basalts in tuff and tuffaceous phyllite in the Bikou Group as banded or lenticular bodies. Based on detailed field geological survey and systematic laboratory studies including microscope observation, major and trace element analysis and isotope analysis, the author investigated the genesis of the metamorphic mafic volcanic rocks and their relationship with gold mineralization.The result shows that the spilites in the Sanhekou Group have two typical altered mineral assemblages, i.e., albite+chlorite+epidote±actinolite and albite+ Fe-dolomite+sericite and they belong to tholeiite series. The spilites were generated by a moderate-degree (5-20%) partial melting of spinel-lherzolite, and were composed of 10?20% lower crust component and 80?90% mantle component, which resulted from lower crustal contamination during upward migration of the magma. The spilites erupted in Neoproterozoic at 786.4±4.5 Ma (MSWD=2.9, n=33) and the protolith of the phyllite in the Sanhekou Group formed in Devonian later than 397 Ma, the sediment sources of which were the Proterozoic basement and Neoproterozoic volcanic materials from the Bikou Goup Volcanic Rocks (BVT) in the south of the Sanhekou Group. The Sanhekou Group in the Huachanggou gold orefield is a tectonic melange, which consists of rocks that formed in Neoproterozoic and Devonian. The spilites were assumed to be the product of Neoproterozoic volcanic eruption in the Bikou Group, and later mixed into the sedimentary rocks in the Sanhekou Group resulting from the collision between the Yangtze plate and the Qinling plate during the late Permian to early Triassic.The Bikou Group metamorphic basalts in the Huachanggou area mainly consist of plagioclase, chlorite and epidote and they belong to tholeiite series. Zircons from the metamorphic basalts are characterized by ages in two intervals, i.e.,306?325Ma and 843?867 Ma, of which the former gives a weighted mean age of 316.3±6.0Ma (MSWD=0.78; n=7). This age is interpreted as the crystallization age of the basalts. Isotope modeling suggests the basaltic magma was most likely generated from a mixture of 15?20% lower crustal component (i.e., Neoproterozoic Bikou Group volcanic basement and Paleoproterozoic basement) and 80?85% mantle component, resulting from delamination of these lower crustal materials. It is concluded that BVT formed in two separate volcanic stages. The Neoproterozoic volcanism and subsequent metamorphism formed the Neoproterozoic basement, which was overprinted by the late Paleozoic volcanism represented by the Huangchanggou metamorphic basalts.The H, O, C, S and Pb isotopic compositions reveal that both mantle and crustal components provide ore-forming material for gold mineralization, and the mantle component came from the metamorphic mafic volcanic rocks in the Huachanggou orefield. Therefore, the ore forming material sources came from these metamorphic mafic volcanic rocks. In addition, the study of primary halo shows that the ore body of main ore zone between line No.36?70 is extending to the deep part below the 990 middle section; and that the Yangjiadaqu area in the western Huachanggou gold orefield has good prospecting potential.