Polymetallic Mineralisation And Associated Magmatic And Volcanic Activity In The Luzong Basin, Anhui Province, Eastern China

The Luzong (Lujiang~Zongyang) volcanic basin is located in the central part of theMiddle-Lower Yangtze (Changjiang) River Valley metallogenic belt, the Northern margin of theYangtze block in Anhui Procvince, which is one of the most important polymetallic mineralizationbelt in east-central China. As a representative volcanic basin in the Middle-Lower Yangtze(Changjiang) River Valley metallogenic belt, the Luzong basin shows noticeable feature ofmineralization and magmatism and occured multiplicity deposits, which contains four shoshoniticvolcanic units and many intrusive rocks of cretaceous age. With the Nihe iron deposit wasdiscovered in2006, the evaluation of the mineralization ability of the Luzong basin has beenimproved. In order to clarify the diagenetic and metallogenic epoch, the ore genesis, metallogenicregularity and the relationship between the magmatism and the mineralization many comprehensiveresearch work needs to do. This paper taking the Luzong basin as the subject, based on thesummarise of previous studies, by the means of detailed geological investigation, sampling andmicroscope observation, combine with geochemistry, geochronology, fluid inclusion geochemistryand in-situ laser ablation trace elements analysis methods, aims to achieve a comprehensiveunderstanding of magmatism and mineralization.The intrusive rocks in the Luzong basin can be divided into two stages and three types. Theearly stage monzonite-diorite intrusions mainy distributed in the north part of the basin, ages from134Ma to130Ma, which have close relationship with Longmenyuan and Zhuanqiao cycle volcanicactivity. The late stage syenite and A-type granite intrusions distributed in the south part of the basin,ages from129Ma to123Ma, which have close relationship with Shuangmiao and Fushan cyclevolcanic activity. New LA-ICP-MS zircon U-Pb dating results of the intrusive and subvolcanic rocksfrom the the main mineral deposits shows the age range from134Ma to129Ma. The magmatic rocksin the Nihe district (include diorite porphyry, monzonite porphyry, trachyandesite porphyry, syeniteand aplite), the diorite porphyry in the Daling deposits, the trachyandesite porphyry in the Xiaolingdeposits, diorite porphyry in the Dabaozhuang deposit, andesitic porphyry in the Jingbian deposit,diorite porphyry in Yangshan deposits and trachyandesite porphyry in the Yueshan deposit arebelong to the early stage magmatism, the syenite porphyry in Nihe deposit and the quzrtz syeniteporphyry in Makou deposit belongs to the late stage magmatism. Compare with tthe magmaticactivity in the Middle-Lower Yangtze River Valley metallogenic belt, the intrusions in the mainmineral deposit in the Luzong basin were the product of the second(135Ma~127Ma) and thirdperiod(126Ma~123Ma) magmatic activity and obviously later than high-k calc-alkaline intrusionsrelated to skarn-porphyry Cu-Au deposits in uplift areas in the Middle-Lower Yangtze River Valleymetallogenic belt.The geochemical features of the four groups volcanic rocks and two stage intrusive rockssuggest that the volcanic rocks and intrusions might be derived from a same source, themetasomatic mantle closing to the enriched mantle I (EMI). The form of the source might berelated to the subduction of the paleo-plate. The evolution of the magma was controlled by thecrystal fractionation mainly and influenced by the crust materials partially in the high-level magmachamber. The Shuangmiao group and Fushan group volcanic rocks and the late stage intrusive rockshows a higher degree crystal fractionation and much more influenced by the crust contaminationthan the Longmenyuan and Zhuanqiao group volcanic rocks and the early stage intrusive rocks. TheThe tectonic setting of Luzong basin existed a change from the transition of compressional to extensional regime to the extensional regime in Early Cretaceous, and the time was about130Ma. The magmatic rocks of Longmenyuan and Zhuanqiao stage formed in the transitional setting fromcompressional to extensional regime, and those of the Shuangmiao and Fushan groups formed inthe extensional regime.The40Ar-39Ar dating of phlogopite from Nihe, Longqiao, Mabianshan, Yangshan, Makou andfluid inclusions in quartz from Jingbian deposit shows the mineralization age of these deposit rangefrom133Ma to127Ma. Combine with the age of the magmatic rocks which closely related to themineralization, which can indirect constrain the mineralization ages, of other mineral deposits wecan summarize the time sequence of the mineralization in the Luzong basin as following: thePanshiling iron deposit formed earliest, the age consistent with the Zhuanqiao formation volcanicrocks(~134Ma); the vein-type Cu-Au deposits and the Yueshan Pb-Zn deposit formed in133Ma~132Ma, which consistent with the ages of the subvolcanic rocks or monzonite intrusions ofZhuanqiao formation; the porphyry iron deposits(include Luohe, Nihe, Yangshan, Longqiao, Daling,Xiaoling and Dabaozhuang) are "booming" formed in a short interval, around130Ma; the age of theFanshan alunite deposit might be consistent with or slightly younger than the porphyry iron deposits;the last mineralization event is the Fe-Cu-Au-U mineralization occurs in the late stage syenit and theA-type granite, the age approximately≤127Ma.We present the geologic characteristics and subdivided the alteration and the mineralizationstages of the main deposits in the Luzong basin. Detailed mineral chemistry studies on the main oreminerals and gangue minerals (pyrite, magnetite, pyroxene, garnet, apatite and anhydrite) have beendone by means of EPMA and LA-ICP-MSanalyses. These data are used as an indicator of the natureof the hydrothermal fluids involved in the formation of deposits. For instance, the trace elementsLA-ICP-MS map of the pyrite core from magnetite stage of Nihe deposit enriched in Co, Ni, As, andSe. While pyrite rim from sulphide stage is enriched in Cu, Pb, Zn, Ag, Au, Tl and so on. Thegeochemical composition of apatite from the Nihe, Makou and Taocun iron deposit are much similarwhereas shows some differents from the Kiruna type deposit and IOCG deposits, which mightindicate that the iron deposits in the Luzong basin is not belongs to the Kiruna type deposits and theIOCG type deposit. The anhydrite fromdifferent stage of the Nihe iron deposit display different color.The Ba, Na, Y and REE are drasticly decrease from pink color anhydrite(magnetite stage), redcolor anhydrite(sulphide stage) to white color anhydrite(quartz-calcite-sulphide vein stage).The alteration in the Nihe iron deposit can be divided into upper leucocratic alteration zone(late stage), middle Melanocratic alteration zone(middle stage), lower leucocratic alterationzone(early stage). The alteration geochemistry study results shows that the early alteration stageresult in the Fe,Ca,Mg add to the solution and the Na add to the wall rock, the skarn alteration ofmiddle stage addition Fe,Ca,Mg and minor Si, with a volume of volatile (F,P and CO2). There isno obvious component addition or loss in the middle alteration stage. However, the volatile (H2Oand CO2) making the anhydrous silicate minerals (pyroxene and garnet) into the hydrous silicateminerals (chlorite and epidote) and calcite, siderite. A large number of Ca,Fe,S and Si enrichment inthe late stage, and formed the anhydrite ore bodies and pyrite ore bodies.Fluid inclusion study shows the temperature and the pressure of the mineralization stageore-forming fluids decrease from magnetite stage of the Nihe deposits, magnetite stage of theLongqiao deposit, to the sulfide stage of the Nihe deposits. The vein-type Cu-Au deposits showsmiddle level temperature ore-forming fluids, the ore-forming fluids of the alunite deposit belongs tothe epithermal system. The hydrogen and oxygen isotope composition analyses results indicate thatthe ore-forming fluids of the Nihe iron deposit were predominated by magmatic thermal fluid, theascending ore-forming fluids reacted with the wall rock, and the crust materials were entered ore-forming fluids. The ore-forming fluids of Longqiao iron deposit derived from the magma and mixed with the stratum and meteoric water. The vein-type Cu-Au deposits shows the meteoric waterdominated ore forming fluids. The C isotope study of Longqiao deposit shows the carbon in thefluids of the later mineralization stages is a mixed source from both of the magmatic andsedimentary rocks (the Dongma'anshan Formation). Sulfur isotopic compositions show that thesulfur in the deposit was derived from a mixture of magmatic hydrothermal fluids andcarbonate-evaporite host rocks. The diorite porphyry is an important engine of the ore-forming fluidsin the Luzong basin, with flowing of the ore-forming fluids in different environment, interact withwall rock and mixed with the meteoric water, the changes of principal physicochemical conditions ofore-forming fluids cause different types mineralization occurred in different place. The ore-formingfluids was corresponding to the porphyry iron mineralization system of the Middle and LowerYangtze River Polymetallogenic mineralization system.Based on the above comprehensive study results, and compareing with the mineralizationsystem around the world systemic, we discussed the genesis of the main deposits, and propose ametallogenic model of the Luzong basin. This model shows the polymetallic mineralization andmagmatic activity in the Luzong basin is a continuous process, different magmatic rocks anddifferent mineral deposits are the products of the magmatic-hydrothermal system in differentevolution stages. The petrogenetic and metallogenetic of Luzong basin is one part of the Mesozoictectonic-magmatic-metallogenic event controlled by the geodynamic setting of eastern China in theYanshanian period. Widespread volcanic basins and Fe, Cu polymetallic mineralization formedduring135Ma to123Ma in early Cretaceous, in the extensional tectonic setting, whichcorresponding to the subduction of the pacific plate, lithospheric de delamination and asthenosphericupwelling in eastern China.