| The Huoqiu iron mine of Anhui is located in the southern margin of the NorthChina Craton, and situated in the west of Huoqiu County, extending about40km fromsouth to north and with width about2.7km from east to west. The ore-body is hostedin Archean huoqiu group, with few magmatic rocks developed in this area. The totalthickness of Huoqiu group is greater than2070m, which is a set of metamorphic rocksof low-amphibolite facies, and can be divided into Huanyuan group, Wuji group andZhouji group in ascending order. The ore-body, whose dip and strike is consistent withthe attitude of the wall rocks, is mainly near bedded, conformable contact with thewall rocks, and has clear boundaries, all of which are evident for strata-bound oredeposits. The ore-body mainly occurred in the upper section of Wuji group, upper partof lower section of Zhouji group, and lower part of the upper section of Zhouji group.Based on field observations and laboratory identifications, this thesis dividesrocks of Huoqiu group into amphibolite, schist, gneiss-leptynite, marble,magnetite-quartzite and migmatite. Among them, the total rare earth content ofamphibolite is lower,∑REE=50.1010-6~101.08×10-6, the average is69.87×10-6,and the fractionation between HREE and HREE is not obvious, the (La/Yb)N is0.94～3.87, with average is2.07; the rare earth content in schist and gneiss-leptyniteis relatively high, with average of ΣREE is larger than10010-6, significantlyenriched in light rare earth; the ΣREE of marble is1.5910-6~23.17×10-6, withaverage is10.96×10-6, and the (La/Yb)N is2.23~23.88, with average is14.20,enriched in light rare earth elements. Through restoring of the original rock, theoriginal rock of amphibolite is volcanic basalt, schist is clay rock and shaly sand,gneiss-leptynite is graywacke or intermediate-acidic volcanic rocks (tuff), and marbleis dolomitic limestone. Hence, the original rock of Huoqiu group is a set of volcanicrock-clay rock-carbonate rocks, sandwiched with ferrosilicon formations. In theviewpoint of stratigraphic section, the early stage is mainly intermediate-maficvolcanic eruption, followed by intermediate-acid volcanic activities that sandwichedwith mafic volcanic activities and occasional deposition, and volcanism in the late stage weakened that became normal marine sediments, which represents a completevolcano-sedimentary cycles, with much developed rhythmic layers. Hence, theHuoqiu group may be equivalent to a part of the upper part of the standard section ofthe worldwide recognized greenstone belt. The tectonic environment that formedoriginal rock of Huoqiu group possibly is back-arc basin–Huoqiu basin which isbetween Dangshan volcanic island arc and Yinxianji volcanic island arc. It can bediscriminated by the major elements of all kinds of rock that the main source ofsediments in the wall rocks of the mine is terrigenous clastic materials andintermediate-acid pyroclastic.The formation age of Huoqiu iron ore is confined to the early period of LateArchean (2754～2773Ma) by using zircon Cameca U-Pb method on the sandwichedplagioclase amphibolite and granulite of the ore-body.The iron bearing formation of Huoqiu can be divided into three types, oxidephases, silicate phases and carbonate phases. Oxide phases including hematite faciesand magnetite facies, with hematite facies distributed in the north and south ends ofthe mine, and magnetite facies widely developed in the region; silicate phases oftenoutput as magnetite oxide facies with small scale; and carbonate phases only appear inLilaozhuang deposits within the mining area, suggesting the specificity ofdepositional environment in Lilaozhuang deposit. Judged by phase transitioncharacteristic and bearing strata yielded by each phases, the basin that yielded Huoqiuiron ore should not be an open and flat basin, but is likely to be open or semi-enclosedbay. And the appearance of carbonate phases at Lilaozhuang illustrates the water bodyat Lilaozhuang is a lagoon basin which is relatively deep and closed.Huoqiu basin is abay with a major axis near north-south and slightly westwards, in the eastern part ofwhose east-west axis is the Lilaozhuang lagoon basin.Geochemical analysis on quartz magnetite, amphibole-quartz magnetite, quartzspecularite, carbonate-magnetite and a few bonanza show that all types of ore has lowTiO2and P2O5, and Al2O3content of most ores are less than0.5%, except for a fewores likely to be contaminated by terrigenous clastic materials that induced a slightlyhigher Al2O3(>1%), quartz magnetite and quartz specularite are mainly composed ofSiO2and Fe oxides, and the remaining oxides content were very low; amphibole-quartz magnetite is mainly composed of SiO2and Fe oxides, and it also has a highcontent of MgO and CaO (MgO+CaO>2%); Carbonate-magnetite has a low contentof SiO2and a high content of MgO and CaO (MgO>> CaO), in addition to Fe oxides;and bonanza is mainly composed of Fe-rich oxides, with total iron content TFe2O3 greater than90%. Except for a few ores contaminated by terrigenous clastic materialsthat have relatively flat REE form (show a high Al2O3content in major elements), thenormalized PAAS REE diagram of most ores show a leftist form of light rare loss. Inaddition, the ores has an obvious enrichment of La, Eu, Y, without abnormal on Ce,These are typical characteristics of BIF iron ore. The lack of abnormal on Ceillustrates the ore is formed in reducing environment with low oxygen fugacity; andabnormal on Eu illustrates the formation of Huoqiu iron ore is closely related withhigh-temperature hydrothermal, while the high Y/Ho ratio implies metallogenicmaterial has an affinity with seawater. It can be judged preliminary from the Y/Ho,Eu/Sm, Sm/Yb ratio that sources of metallogenic material of Huoqiu iron ore are0.1%mixed solution of submarine volcanoes hydrothermal and seawater, which is subjectsto varying degrees of contamination of terrigenous clastic materials.Although the formation of Huoqiu iron ore is closely related to volcanic activity,the most favorable ore-forming position is not near the center of volcanic activity, butis at sedimentation developmental areas of continental margin. The occurrence layersalso have significant characteristics of shallow-marine deposits of continental margin.Hence the thesis considers Huoqiu BIF iron ore belongs to type of Lake Superior BIFThe formation of Huoqiu iron ore has undergone two stages. The first phase is avolcano-sedimentary stage (2.7～2.2Ga), which has deposited mafic volcanic ofHuayuan group, intermediate-acidic volcanic rocks and ferrosilicon deposition ofWuji group, and clastic sedimentation, ferrosilicon deposition and carbonatedeposition of Zhouji group. The second stage is a tectonic deformation andmetamorphism, and uplift stage (2.2～1.0Ga). The tectonic deformation has formedthe north-south trending Zhouji syncline and east-west trending Fanqiao andLilaozhuang syncline, which controls the distribution of iron ore in this region. Theuplift causes the absence of previous strata of Miocene.The Huoqiu basin is not stableuntil Miocene and stops deposition, which forms the iron pattern that we see today inthis region. |
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