Ore-Forming Regulations And Prospecting Evaluation Of Banded Iron Formations From Macheng Deposit,Eastern Hebei

The Macheng BIFs iron ore deposit is located in the eastern part of the Qinglonghe fault zone on the southwest edge of the North China platform,Yanshan platform fold belt and Shanhaiguan uplift.Its geographical location is in Luannan County,Eastern Hebei Province.In this paper,the metallogenic environment of the Macheng iron deposit has been studied in detail,including fluid inclusions,major element,rare-earth element(REE),trace element geochemical analyses and H-O isotope compositions.The metallogenic model of the deposit is finally established by regional tectonic analysis.On this basis,through the research on the exploration data of Macheng Iron Mine,the three-dimensional geological model of the deposit was established,and the prospect evaluation area was delineated and qualitatively evaluated.The results provide basic geological data and support for further exploration of the Macheng iron deposit.Zircon SHRIMP U-Pb dating and related age data shows that the zircon age of the biotite plagioclase gneiss and biotite granulite on the roof and floor of the orebody and the zircon age of the potassium migmatite in orebody is2529±7Ma?2539±8Ma,which represents the formation age of BIF and the volcanic sedimentary age of Luanxian Group.Temperature measurements of fluid inclusions show that the homogeneous main body temperature at the peak stage of metamorphism in Macheng Iron Mine is 250-300?,the low temperature alteration main body temperature at the later stage is 150-200?.The range of homogenization temperature and salinity varies greatly in each period,which may represent the multi-stage characteristics of late alteration.Major elements in Macheng iron deposit show the high average total content of SiO2+TFe2O3+MgO accounting for more than 90%.However,other oxide components are opposite and the content of Al2O3+TiO2 is very low.These results indicate that Macheng iron deposit formed in a pure environment of chemical deposition without any terrigenous clastic.The ratio of MnO/TFe2O3 in ore is extremely low with an average of 0.002,and that of K20O/Na2O is less than 1,which demonstrates environment of hot water sediment.The ratios Co/Zn?Ni/Zn and other trace element characteristics of Macheng iron deposit indicate that the ore-forming materials were derived from hydrothermal fluid induced by volcanic activities within sedimentary basins.Light rare earth elements(LREE)are depleted and,by contrast,heavy rare earth elements(HREE)are enriched;In addition,positive anomalies in La and Y,and Y/Ho ratio are similar to those of modern seawater indicate Macheng iron deposit are formed in seawater environment by the characteristics of ratio ?REE+Y;Strong positive anomalies of Eu show ore-foming fluids were derived from hydrothermal water,and moreover,mixed with meteoric water by the ratio(La/Yb)N is less than 1;The range of Ce/Ce*varies form 0.82 to 0.95 less than 1.The results show there is no obvious abnormality suggesting the iron deposit formed in an oxygen deficient environment.The H-O isotope value and ?D-?180 fluid result indicates that quartz formed in fluid similar to that of metamorphic water in Macheng iron deposit.Based on the relationship between metallogenic age and volcanic activity record,we considered that the ore-forming material source of the Macheng BIFs iron deposit is related to a large number of hydrothermal fluids induced by submarine volcanic activity and intermittence period 2.5 billion years ago during the late Archaean instead of terrestrial material.Volcanism and high-temperature heating of fluids raised the seawater temperature in an anoxic environment,and then formed a mixture of seawater and hydrothermal fluids.In addition,the main temperature of the mixed fluid is ranging form 250 to 300 ?.The mixed fluid leaches out and extracts some elements including Fe and Si from original volcanic sediment forming BIF iron deposit,accompanied by metamorphism of epidote-amphibolite facies.The comparative analysis of iron-bearing rock series and geochemical characteristics shows that the areas between Macheng Iron Mine and the Sijiaying Iron Mine is not large-scale syncline structure and there is little possibility of turning in the deep part.The prospect of prospecting in this area is not great.Comparing the section with the three-dimensional solid model,it is confirmed that the No.? orebody of Macheng Iron Mine is enclosed in 0 line-580m,showing the characteristics of tight inverted fold syncline structure.The No.? orebody and No.? orebody are enclosed by the 23 line-1140m elevation and show the characteristics of tight inverted fold syncline structure.Geophysical prospecting and drilling speculate that V orebody has tended to the turning point of "orientation" in depth.The above findings prove that there are synclinal structural features in Macheng Iron Mine.Combined with geological section,three-dimensional geological model and geophysical data,the prospects of the Macheng iron deposit mining area are evaluated,and threer extremely promising prospects are finally determined,such as the deep part of the No.? ore body.