Metallogenesis Of The Xiaobaishitou W-(Mo) Deposit In East Tianshan,Xinjiang

The Xinjiang East Tianshan Orogenic Belt,which is located on the southwestern part of the Centern Asian Orogenic Belt and adjacent to the Beishan Orogenic Belt,is characterized by the Late Paleozoic mineralization.In recnet years,many W and Mo deposits have been discovered in East Tianshan Orogenic Belt and Beishan Orogenic Belt,forming a huge Triassic tungsten–molybdenum metallogenic belt.The Xiaobaishitou deposit?36,264 tonnes@0.78 wt.%WO₃?in the Eastern Tianshan Orogenic Belt is a medium-sizded skarn deposit characterized by W–?Mo?ore bodies that developed along contacts between Triassic biotite granite and Mesoproterozoic carbonaceous rocks.As a unique tungsten-molybdenum deposit,the study of its tungsten-molybdenum symbiosis mechanism not only enriche the theory of mineralization in the East Tianshan-Beishan,but also lead to futher prospecting work.In this contribution,we report a comprehensive study of the Xiaobaishitou W–?Mo?deposit basing on the detailed field investigation,intrusive rocks,mineralogy,geochromology,fluid inclusions,and isotopic analyses?C,H,O,S,He,and Ar?to determine the metallogenic tectonic setting,ages of intrusion and mineralization,fluid and metal sources,and the physicochemical conditions of scheelite and molybdenite deposition,thereby forming the basis for a genetic model of the deposit.Based on the comparison with the other skarn W deposits in the Chinese,the tungsten metallogenic regularity of the East Tianshan Orogenic Belt was summarized.The following achievements have been withdrawn in this study:Intrusive rocks are developed in the Xiaobaishitou district,such as the Neoproterozoic biotite monzonite granite?zircon U–Pb age of 908.1 Ma?,the Devonian granodiorite?zircon U–Pb age of406.8–412.3 Ma?,the Carboniferous gabbro-diorite?zircon U–Pb age of 324.7 Ma?,and Triassic biotite granite?zircon U–Pb age of 246.4–252.2 Ma?related to mineralization.Molybdenite Re–Os model ages ranging from 45.0±1.7 Ma?2?;MSWD=0.101?to 251.1±1.6 Ma?2?;MSWD=0.70?are interpreted as the time of the hydrothermal activity mineralization?245–251 Ma?.The Xiaobaishitou Trassic biotite granite are characterized by high SiO₂ and K₂O+Na₂O content,medium Al₂O₃ content and Mg^#values,and low CaO content,belonging to the high-K calc-alkaline and magnesian granites.They show weak negative Eu anomaly,enrichment LREEs,and I-type granite features.Mineralogy,whole-rock geochemistry,Sr–Nd and Lu–Hf isotope studies have shown that they are derived from a mixture of mantle-derived and crust-derived materials,and more crustal materials were added during the later assimilation and contamination process,which were formed in an intra-continenetal setting in the East Tianshan.The skarns and W/Mo orebodies are distributed along the contact zone between biotite granites and carbonaceous rocks of Kawabulag Group.Four types of W and Mo orebodies are distinguished,including skarn-type,quartz vein-type,altered granite-type,and marble-type.Mineralization of the Xiaobaishitou deposit are divided into prograde stage?I?,retrograde stage?II?,quartz–sulfides stage?III?,and calcite stage?IV?.Scheelite is the dominant product of the stages II and III,while molybdenite is the dominant product of the stage III.Mineral studies have shown that the oxygen fugacity of the ore-forming fluids in stage I gradually increased and evolved towards weak alkalinity,with pyrite and other sulfides formed in a moderate-low temperature environment.The early stage II ore-forming fluids have high oxygen fugacity and scheelite began to precipitate,mainly from magmatic hydrothermal fluid.The reducibility of ore-forming fluids in the late stage II and stage III is increasing.In the stage III,metamorphic fluids are added and directly participate in the formation of scheelite.Based on the fluid inclusion study,ore-forming fluids in the four mineralization stages are characterized by moderate to low temperatures?peaks at 310°C,300°C,290°C and 170°C,150°C?,moderate to low salinities?peaks at 6.5 NaCl equiv.,4.5 NaCl equiv.,4.5.NaCl equiv.,2.5 NaCl equiv.?,shallow metallogenic depth?2.6–3.3 km,0.7–0.9 km,0.8–1.0 km,>0.2 km?.The addition of meteoric water and abrupt trapping pressure decrease during stages II and III led to the fluid boiling and formation of inhomogeneous fluids,which resulted in a large amount precipitaton of scheelite and molybdenite.Stable isotope?C,H,O,S,He,and Ar?studies have shown that the stage I fluid are mainly derived from magma,and experienced strong fractionation in the late stage of magma-hydrothermal activity;the stage I fluid are mainly from magma,and mixed with minor meteoric water;the stage III fluid are mainly comes from the mixing of magma and significant meteoric water;stage IV fluid are mainly composed of meteoric water.The ore-forming materials are mainly derived from crust-derived granite,mixed with deep-source materials.The key to the symbiosis of tungsten and molybdenum is the participation of crust and mantle materials,full magmatic evolution and an open metallogenic environment.It is revealed that the geological tectonic background of tungsten and molybdenum deposits in the East Tianshan-Beishan Triassic metallogenic belt provides similar material sources.The participation of old and young crustal components in ore-forming magmatic rocks is the fundamental reason for the formation of different deposit types.The tungsten deposit formed earlier than the molybdenum deposit,and the East Tianshan deposits formed earlier than Beishan.Porphyry molybdenum deposits are more closely related to regional structures.Compared with typical tungsten deposits in South China,the East Tianshan Triassic tungsten deposit are very similar to them in magmatic rock,structure,surrounding rock,fluid evolution,and material source,and has great prospecting potential.