Petrogenesis Of The Late Mesozoic Uranium-Bearing Volcanic-Intrusive Complex In The Gan-Hang Tectonic Belt And Their Implications For Uranium Mineralization

The Gan-Hang volcanic rock-hosted uranium mineralization belt is one of important uranium mineralization belts(region)in China,where uranium deposits are mainly distributed in several volcanic basins,such as Xiangshan,Shengyuan,Daqiaowu,Dazhou and Furongshan.For these volcanic-intrusive complexes in these basins,there are debates on their ages,petrogenesis geodynamic settings and potential as uranium source rocks.Especially for the geochemical compositions of volcaniclastic rocks,no reliable and valuable data are available.In this study,we comprehensively use petrography,geochronology,elemental geochemistry,Sr-Nd-Pb-Hf isotopes and melt inclusion to explore the petrogenesis of volcanic-intrusive rocks in Xiangshan,Daqiaowu and Furongshan,and to evaluate their contributions to uranium mineralization.The zircon U-Pb.dating reveals that volcaniclastic rocks of the Huangjian formation in Daqiaowu were generated at 128?135 Ma,and the crystallization ages of the granite porphyry and diabase dikes are 125?128 Ma.These results are consistent with the intrusive contact between the volcanic and intrusive rocks.The zircon U-Pb dating yields crystallization ages of 127?129 Ma for the Furongshan granite porphyry and its mafic microgranular enclaves.By combining these ages and previous dating for other volcanic-intrusive complexes,we propose that volcanic-intrusive rocks distributed along the Gan-Hang Tectonic Belt(GHTB)were mainly generated during the period of 137?120 Ma and exhibit younger eastward.Primary geochemical compositions of volcaniclastic rocks in the Daqiaowu and felsic lava in the Xiangshan by analyzing their melt inclusions,together with the whole rock geochemical data of intrusive rocks in the GHTB,indicate that volcanic-intrusive rocks in the GHTB show A-type granitic affinities:they are enriched in alkalis,REE and HFSE,and show high FeOT/MgO and Ga/Al ratios,high Zr + Nb + Ce + Y contents but low CaO,MgO,TiO2 contents.They are strongly depleted in Sr?Ba?P?Ti and Eu.Zircon saturation thermometry indicates relatively high magmatic temperature(750 to 860?).The decreases of Zr + Nb + Ce + Y with increasing SiO2 contents observed in Daqiaowu granite porphyry,Jiuhuashan and Tongshan granites as well as Xiangshan porphyritic lava,suggest fractionated A-type granites.The DQW granite porphyry(?125 Ma)samples have low initial 87Sr/86Sr ratios(0.7093 to 0.7117),relatively high ?Nd(t)(-6.6 to-4.4)and a large range of zircon ?Hf(t)values(-9.9 to-0.5)with double peaks,suggesting a petrogenetic model related to magma mixing.Hf isotopic compositions of the Daqiaowu volcaniclastic rocks and granite porphyry exhibit a gradual rise from-13.0(133-135 Ma)to-3.0(125 Ma).Meanwhile,zircon saturation thermometry shows that magmatic temperature increases from?750? to?830?.These observations suggest that the petrogenesis of volcaniclastic rocks of the upper Huangjian Formation and granite porphyry in the Daqiaowu is related to mixing of magmas derived from mantle and crust.Most zircons from the Furongshan mafic microgranular enclaves(MMEs)have newly grown rims.Their rounded cores have the same ?Hf(t)values and U-Pb ages as zircons from the granite porphyry.So zircon cores in the MMEs were most likely to be captured from the felsic magma as xenocrysts.In addition,MMEs generally show fine-grained.igneous microstructures and have needle-like apatites and feldspar xenocrysts with sieve-like resorption holes.These observations also support that the Furongshan MMEs and granite porphyry were produced by magma mixing rather than restite or xenolith.The similarity of trace element contents between them is evident(except Eu,Sr,Ba and P associated with fractional crystallization):enrichments in LILE(K,Rb),REE,HFSE(Th,U,Hf and Zr);depletions in Nb,Ta;identical Sr-Nd isotopic compositions(average(87Sr/86Sr)i= 0.7075,?Nd(t)=-3).Only a few major elements and zircon ?Hf(t)values are different.These facts indicate that a high level of element diffusion and chemical equilibration should have occurred between the Furongshan MMEs and host rocks.In comparison with other MMEs and host granitoids in the Xiangshan and Muchen,chemical equilibration of commingled magmas in the eastern GHTB exhibit a much higher degree than that in the Xiangshan of the western GHTB.During?140-125 Ma,Nd and Hf isotopic compositions of A-type volcanic-intrusive complexes in the GHTB show a significant increase from-9.0 to-1.0 and from-10.0 to 4.0,respectively.This tread is consistent with the fact that magmatic temperatures increase from 750 ? to 830? and 860 ? for volcanic-intrusive rocks in the Daqiaowu and Xiangshan,respectively,reflecting an increasing addition of mantle-derived high temperature magma.uranium-bearing volcanic-intrusive rocks in several typical uranium deposits tend to plot along the same Sr-Nd mixing curve reflecting a similar mixing mechanism.The percentage of mantle components is 0%-15%for the Xiangshan granite porphyry,10%-30%for the Daqiaowu granite porphyry and 50%for the Furongshan granite porphyry,respectively.The highest percentage of mantle components(?60%)was recorded in the Muchen monzonite with no uranium mineralization.The Daqiaowu diabase dikes(128 Ma)have relatively high initial 87Sr/86Sr ratios(0.7066 to 0.7085),negative ?Nd(t)(-3.3 to-1.1)and zircon ?Hf(t)values(-6.3 to-1.6),and display enrichment in LILE and stronger depletion in Nb,Ta.They also exhibit low Nb/U(1.3-11.9)and Ce/Pb(3.9-9.9)ratios,indicating arc-like magmatic characteristics.Based on the ages and geochemical features,mafic rocks emplaced along the GHTB could be divided into two groups,the Early Cretaceous arc-like and Late Cretaceous OIB-like rocks.In contrast to the latter,the former is characterized by markedly high Ba/Nb and La/Nb ratios,considerably low Ce/Pb and Nb/U ratios,low Nb and Ta contents,and enriched Nd isotopic compositions.The Early Cretaceous arc-like rocks,including the Daqiaowu diabase dikes,were derived from the mantle wedge hybridized by 1%?6%subducted sediments(or silicic melts derived from the subducted sediments),while the Late Cretaceous OIB-like rocks were derived from relatively depleted asthenospheric mantle.These differences imply a plausible geodynamic change.Combined data from mafic and felsic rocks,the GHTB may have been in a period of transition from continental arc to back-arc extension during 140-100 Ma,but the geodynamic setting has transferred to back-arc completely during 100-80 Ma.Melt inclusions and whole rock geochemistry of volcanic-intrusive rocks along the GHTB suggest that their U and Th contents decrease with the intensified magma mixing through time(140-125 Ma)and from west to east,affecting their metallogenetic potential.This may be an important reason for the fact that the eastern parts of the GHTB have relatively less and smaller uranium deposits than the western part.However,the intensified magma mixing has also resulted in the rise of promising mineralizers such as F and Cl,which would greatly benefit uranium mineralization if extra uranium sources exist.These mineralizers may be an important source for strong fluoritization commonly observed in many uranium deposits.Thus,such areas with combined extra uranium source and the overlying Early Cretaceous volcanic rocks may be promising prospecting targets in the eastern GHTB,for example,the Maoyangtou uranium deposit(an indosinian granite as basement and the overlying volcanic strata).