Late Paleaozoic Granitic Rocks And Mineralization Of The Southern Margin Of The Altun Mountains

Located at the intersection between Altun and Southern Qilian tectonic belts, Huashigou Copper ore region contains late Paleozoic granitic magmatism and the related Cu-Au ore and W-Sn mineralized spots. The origins and mineralization of the granitic rocks in the studied area have not been studied systematically for a long time, so that the great development in ore-prospecting has not been made. Based on advance in the research of granites and mineralization, this paper is in the point of the scientific thoughts of tectonic plate theory, and by use of comprehensively studied methods of petrology, petro-geochenmistry, isotopic geochemistry and tectonics, and by the means of the modern test technology, to research tectono-magma-mineralization and tectonic geodynamic processes of the formation and ore-forming of the granites, and then to establish the granites and their metallogenic model. The creative results on the below:1) Based on the survey of field geology, together with the studied results, this paper takes Huashigou ore region to the southern margin of west end of the Southern Qilian tectonic belt, closely adjacent to the mid-Altun and northern Altun tectonic belts. Therefore, the tectono-magma-mineralization in the studied area was strictly controlled by the above two tectonic belts.2) Based on the studies of mineralogy and petrology of the granitoids, the granitoids in the studied area are classified by use of petro-chemistry, and they are granodiorite (Ⅰ-type), quartz-diorite (Ⅰ-type), toellite/porphyry (Ⅰ-and S-type), adamellite (Ⅰ-and S-type) and alterated sericite-quartz rock.3) The highly precise isotopic chronology of the zircon LA-ICP-MS U-Pb ages of the granites is first studied, and the results show that the weighted average age of granodiorite (sample No:YQ-57) distributing in the west of fault F1 in the region is 368.7±3.5 Ma, and that granodiorite (YQ-1), adamellite (YQ-25), quartz-diorite (YQ-81), toellite/porphyry (YQ-91) between faults Fl and F3, are ages of 261.1±3.8 Ma,252.0±2.1 Ma,261.8±1.5 Ma, and 262.2±1.4 Ma, respectively. Meanwhile, these indicate that two very important extensional tectono-magmatic events took place in Paleozoic period in the studied area, they are also important chronological evidence for building the regional geological framework.4) The studied results of major-, trace-, rare-earth-elements and Sr-Nd-Hf-O isotopic geochemistry show:(1) late Devonian granodiorite (368.7±3.5Ma) is high-K calc-alkaline series, has high-Sr and low-Y (Yb) contents, obviously enriched with Rb, K, Th, Zr-Hf and LREE, relatively depleted in Ba, Nb, Ta, Sr, P, Ti, andδEu= 0.64-0.89; greatly high (87Sr/86Sr), (0.712261-0.712614) and low (143Nd/144Nd)I (0.511838-0.511825), (176Hf/177Hf)I= 0.282487, negativeδNd(t) values (-6.748~-6.989) andδHf(t) (-2.334), characteristic of the highly enriched source rocks with residual minerals of plagioclase+ homblende+pyroxene, and lower differentiation of the magma; (2) Mid-Permian granodiorite (261.1±3.8Ma) dues to calc-alkalineⅠ-type granite, has high-Sr and low-Y (Yb) contents, rich in Rb, Th, U, K, Sr, Zr-Hf and LREE, relatively depleted in Ba, Nb, Ta, P, Ti, and averageδEu value equal to 0.83; (87Sr/86Sr) I= 0.706963-0.706571, (143Nd/144Nd)I=0.512246-0.512266,δNd(t)=-1.23847 and -0.84907, (176Hf/177Hf)1=0.282766, and highδ18OV-smow values (12.1-12.5‰), indicating that the rocks originated from the partial melting of the underplating basaltic rocks, and relic minerals of hornblende+garnet, and that have good Cu-Au metallogenic potentiality; (3) Mid-Permian quartz-diorite (261.8±1.5Ma) is low-K to calc-alkalineⅠ-type, rich in Na, Ba, Rb, Th, U, K, La, Sr, Ce, Sm and Zr-Hf, depleted in Nb, Ta, P, Ti, and averageδEu value equal to 0.87, showing the granitic magma originated from lower crust, has no Cu-Au and W-Sn metallogenic potentiality; (4) late Permian adamellite (252.0±2.1Ma) is calc-alkaline to high-K calc-alkaline series with high-Na and-Y and low-K and-Sr, enriched relatively with Rb, Th, U, K, Zr-Hf and LREE, depleted in Ba,Nb,Ta,P and Ti, and averageδEu value equal to 0.62; (87Sr/86Sr)1,= 0.705968-0.706571, (143Nd/144Nd)I=0.512393-0.512404,εNd(t)=+1.89479 and +1.69254, having characteristic of the source rocks of juvenile crust at relatively shallow level with residual minerals of hornblende+plagioclase, and W-Sn metallogenic potentiality; (5) Mid-Permian toellite/porphyry (262.2±1.4Ma) dues to high-K calc-alkaline series, has two groups of A/CNK>1.1 and A/CNK<1.1, is S- and I-types, respectively, some due to A-type with (Ga/Al)×104>2.6; has low-Sr and -Yb, characteristic of the Miocene leucogranites in higher Himalaya; and enriched relatively with Rb, Th, U, K, Zr-Hf and LREE, depleted in Ba,Nb,Ta,Sr,P,Ti, andδJEu=0.42-0.89; Sr-Nd isotopic composition may be divided into two groups:one is slightly enriched Sr-Nd isotopic composition, (87Sr/86Sr)I=0.706211-0.706309, (143Nd/144Nd)I= 0.512342-0.512344, and the other highly enriched, (87Sr/86Sr)I= 0.729009-0.732882, (143Nd/144Nd)I=0.511861-0.511899, andδNd(t)<0(-8.55910 and -7.80714), showing that the rocks originated from the partial melting of high pressure granulite of the heterogeneous lower crust with minerals of plagioclase+hornblende+garnet+pyroxene, having Cu-Au mineralization.5) Compared studies of geochemistry andδ18O of gsericite-quartz rock with toellite/porphyry, this paper points out that the fluids in the alteration-were from the hydrothmermal solution of the late toellitic magmas, not from the other fluids.6) Two important granitic magmatic events took place in the region responding to post-orogenic compression-extensional transform periods during Paleozoic:the magmatic source of late Devonian granodiorites (YQ-57-YQ-63,369Ma) with low-Sr and high-Y(Yb) contents located at relatively shallow levels (ca.< 35Km) of continental crust, so they have little metallogenic potentiality; mid-late Permian granitic magmatasm may be divided into two epochs of intrusion:the granodiorites with high-Sr and low-Y contents, and toellite/porphyry with low-Sr and-Y (Yb) formed at 262Ma, and resulted from partial melting of the underplating basaltic rocks of thickened lower crust (>45Km), have good Cu-Au metallogenic potentiality; late Permian adamellite (252Ma) with W-Sn metallogenic capacity originated from partial melting of thinned lower crust due to the long extension of the crust during the late Permian. According to the results, the diagenetic and metalogenic models of the granitoids in the region are scientifically established, and the direction of ore-prospecting is also pointed out.