Magma Evolution And Its Controls On The Cu And Fe Skarn Mineralization In The E’dong District,with Implications For Mineral Exploration

The E’dong ore district is located in the westernmost of Middle-Lower Yangtze River Valley Metallogenic Belt,eastern China.In this ore district,five Early Cretaceous granitoid plutons（Echeng,Tieshan,Jinshandian,Lingxiang,and Yangxin）host numerous Fe and Cu-polymetallic skarn deposits,whereas the Yinzu pluton is barren with no associated mineralization of economic significance having been documented yet.There are two major types of skarn deposits（Fe skarn vs.Cu skarn）in the E’dong district,both being genetically related to late Mesozoic high-K calc-alkaline granitoids.The origins of these ore-related granitoids have been intensively investigated,but the reasons why they are characterized by two distinct mineralization styles remain not well understood.Constraints on the derivation and evolution of magmatic rocks that have formed contrasting skarn mineralization is not only of significance in better understanding the petrogenesis of the ore-related magma,but also have implications for future ore exploration.In this dissertation,I present a comprehensive study of the intrusions associated with two Fe skarn（Chengchao and Lingxiang）and three Cu skarn deposits（Tongshankou,Tonglushan and Ruanjiawan）in the Edong ore district.Based on detailed field petrological and mineralogical observations,I further use geochemistry of the whole-rock and constituent minerals（amphibole,biotite,plagioclase,titanite,apatite and zircon）to constrain the magma source,oxidation state,volatile compositions and evolution of the ore-related intrusions.The objective of this study was to reveal the possible major factors that control magmatic processes and,as a consequence,the contrasting Cu and Fe skarn mineralization styles in the E’dong ore district.Lastly,I analyze compositions of zircon,apatite,amphibole and biotite combined with whole-rock geochemistry of the Yinzu pluton to its mineralization fertility.Apatite from granitoids related to Cu skarns and Lingxiang Fe skarn haveε_Nd（t）values of-8 to-4,whereas zircon from these granitoids haveε_Hf（t）values ranging from-11.3 to-1.1.These values are similar to those of the contemporaneous mafic rocks in the E’dong ore district,which were dominantly derived from an enriched lithospheric mantle source.Apatite and zircon grains from granitoids associated with the Chengchao Fe skarn have lowerε_Nd（t）values of-15 to-9 andε_Hf（t）values of-11.6 to-25.3,suggesting larger degrees of contaminations from ancient lower crust materials.The apatite Nd and Hf isotope data indicate that the Cu and Fe-related intrusions were generated by partial melting of enriched lithospheric mantle following variable degree of lower crust contamination.Contents of multivalent elements in zircon and apatite suggest that Cu-related magmas have higher oxygen fugacity than Fe-related varieties.Interpretation on geochemical and geophyscial data suggests that such a difference in oxygen fugacity most likely resulted from variation in crustal thickness under which the lithospheric mantle were partially molten and subsequent contrasting magma evolution.The Cu-related magmas were associated with thickened（>50 km）crust and experienced high-oressure garnet fractionation,which explain their higher oxygen fugacity.Apatite from Cu skarn-related intrusions displays much higher Sr/Y and Eu/Eu*ratios of 1.16to 5.71 and 0.40 to 0.77,respectively,compared to values of the Fe skarn-related varieties（Sr/Y=0.11-1.12 and Eu/Eu*=0.16-0.38）.Fractionation modelling based on apatite Sr/Y and Eu/Eu*ratios suggests that magmas related with the Cu skarns have experienced amphibole-dominated fractionation under high pressure and hydrous conditions,whereas those associated with Fe skarns have undergone plagioclase-dominated fractionation at relatively low pressure and dry conditions.Zircons from the Cu-related intrusions have Eu/Eu*ratios ranging from 0.41 to 0.89,whereas the variety from Fe skarn-related intrusions displays lower Eu/Eu*（0.25～0.43）ratios.These contrasting zircon Eu/Eu*ratios indicate the prevalence of early amphibole fractionation and the suppression of plagioclase crystallization,and thus more hydrous property for the Cu-related magmas.The apatite H₂O concentrations determined via attenuated total reflection Fourier transform infrared spectroscopy（ATR-FTIR）suggest apatite from Cu-related intrusions have H₂O concentrations of0.229 to 0.660 wt.%（0.476 wt.%on average）,much higher than the values of the Fe-related intrusions（0.009～0.419 wt.%,0.233 wt.%on average）.Based on compositions of apatite and apatite-melt partitioning data,the estimated magma Cl contents for the Fe skarn range from 3260 ppm to 13940 ppm（8396 ppm on average）,which are significantly higher than those for the Cu skarn（Cl=430-5990 ppm,2711ppm on average）.The apatite and plagioclase from Fe-related granitoids have(⁸⁷Sr/⁸⁶Sr)_tranging from 0.7073 to 0.7082 and 0.7066 to 0.7100,respectively,whereas the variety from Cu skarn-related intrusions displays lower(⁸⁷Sr/⁸⁶Sr)_tof 0.7054 to0.7061 and 0.7030 to 0.7062,respectively.These Sr isotope data,combined with whole-rock S isotopes of this study and previous investigations(δ³⁴S values of+8.3 to+16.3‰for Fe skarn-related intrusions and+2.8 to+7.4‰for Cu skarn-related intrusions)suggest that the Fe skarn related intrusions have assimilated larger amount of evaporite-bearing carbonate during magma ascent and emplacement.The assimilation process may have not only promoted magmatic water exsolution but also provided sufficient amount of Cl^-and SO₄^2-,which facilitated effective complexation and transportation of Fe²⁺and subsequent oxidization of ferrous Fe to precipitate magnetite,respectively.We suggest that evaporite assimilation into the granitoid magmas has played a crucial role in the large-scaled Fe skarn mineralization in the E’dong ore district.This study highlights that oxidation state,volatile composition and fractionation are likely the key factors controlling the Cu and Fe skarn mineralization.The Yinzu pluton mainly consists of quartz diorite and diorite phases that have zircon U-Pb ages of 148.4±1 Ma and 146.6±1.6 Ma,respectively.Whole-rock Sr-Nd isotopic compositions((⁸⁷Sr/⁸⁶Sr)_t=0.70635 to 0.70741;ε_Nd（t）=-8.74 to-3.73)and zircon hafnium isotopes(ε_Hf（t）=-9.1 to-4.8)overlap with values of the Cu-fertile Tongshankou,Tonglushan and Ruanjiawan granitoids,implying a common enriched lithospheric mantle source.The whole-rock geochemical data suggest that both the diorite and quartz diorite have experienced variable degrees of fractional crystallization of olivine,amphibole,titanite and magnetite.The Ce⁴⁺/Ce³⁺ratios（16-445）and oxygen fugacity（logf O₂）calculated using trace element concentrations of zircon（ΔFMQ=+0.2to+2.4）suggest that both intrusive phases crystalized from oxidized melts similar to Cu-fertile magmas elsewhere,with the quartz diorite being well comparable to the Tongshankou,Tonglushan and Ruanjiawan granitoids.Based on available melt-apatite partitioning models,the estimated S and Cl contents for the magmas parental to the Yinzu pluton are akin to those of the Cu-fertile granitoids in the Daye district.Rocks of the Yinzu pluton display high apatite Sr/Y（0.38-3.01）and Eu/Eu*（0.35-0.71）ratios as well as high zircon Eu/Eu*（0.51-0.86）and 10000*（Eu/Eu*）/Y（2-27）ratios,indicating significant fractionation of amphibole with limited plagioclase fractionation and high magmatic water contents（>4 wt.%H₂O）.This view is consistent with the calculated melt H₂O content of 4.3-6.1 wt.%based on the amphibole compositions.Collectively,results presented here indicate that magmas generating the Yinzu pluton are hydrous,oxidized and S-rich,characters similar to the Cu-fertile granitoids in E’dong and elsewhere.Thus,the Yinzu pluton is considered to be Cu-（Au）fertile.This consideration is partially supported by occurrences of several vein-type Cu prospects within and disseminated Au ores in Silurian clastic sedimentary rocks immediately to the southeast of this pluton.Based on the regional metallogeny and results of this study,we suggest that the Yinzu pluton is a promising target of future Cu-（Au）exploration.Collectively,our results suggest that the Cu and Fe-related intrusions were both derived from an enriched lithospheric mantle source,but experienced distinctively different magmatic evolution.A synthesis of geological,and geochemical data demonstrates that oxidation state,volatile composition and fractionation are likely the key factors controlling the Cu and Fe skarn mineralization.Magamas responsible for the Yinzu pluton are hydrous,oxidized and S-rich,similar to the Cu-fertile granitoids in Daye and elsewhere.Results presented in this dissertation highlight the combination of whole-rock geochemistry and minerals（apatite,zircon,amphibole,titanite,plagioclase and biotite）chemistry in better understanding the metallogeny of granitoids and as an indicator for mineral exploration.