Metallogenic Process Of Important Iron Deposits In Ningwu Basin,Eastern China

Ningwu ore district is the most important concentration of iron ore deposits in the Middle and Lower Yangtze metallogenic belt,and is the birthplace of the"Porphyrite Iron Ore Deposits" metallogenic model,which includes a variety of iron ore deposit forms.The porphyrite iron ore deposits metallogenic model was proposed by Chinese scholars,however the concept is not widely accepted elsewhere.Recently,some researchers have compared the genesis of iron ore deposits and concluded that some porphyrite iron ore deposits(Meishan,Washan,Gaocun,Gushan)in the Ningwu basin could be classified as IOA type deposits.The porphyrite iron ore deposits in the Ningwu basin show close similarities to typical IOA type deposits elsewhere,but differ significantly in that they also develop strong skarn alteration.Thus,the relationship between porphyrite iron ore deposits and typical IOA type deposits elsewhere needs further study.It has been widely accepted that evaporites play an important role in the metallogenesis of porphyrite iron ore deposits,but did they influence the formation of magnetite-apatite-actinolite IOA type ore?The genetic role of evaporites needs further investigation.Porphyrite iron ore deposits show distinctive hydrothermal alteration and mineral zoning.Despite extensive studies of their mineralogy,whole rock geochemistry and isotopic compositions,how variations in metallogenic conditions controlled the diversity of mineralization and alteration remains unclear.This has prevented development of a deep understanding of porphyrite ore mineralization processes and their fluid evolution.Detailed exploration drilling and opencast mining of several ore deposits in the basin has provided detailed access allowing systematic research on spatial variations in mineral characteristics.This promotes study of the metallogenesis and helps to advance understanding of porphyrite iron ore deposits.Previous detailed research established the classic model of "three categories and eight types" for the Ningwu porphyrite iron ore deposits."Gaocun type" deposits,typically with intrusion-hosted disseminated magnetite mineralization,are the most common deposit type.Their intimate temporal and spatial relationship to intrusions provides important information about porphyrite ore mineralization.In this thesis two typical and important "Gaocun type" deposits have been chosen for study:Gaocun and Heshangqiao.Grid-based systematic sampling has been done in the open pits and on drill core,and systematic studies on timing and mineralization processes have been carried out based on petrography and in situ mineral trace element analysis.The following major advances in understanding have been achieved.The ages of the ore-related diorite porphyrites are 131.6±2.6 Ma.in Gaocun deposit,and 128.8±1.1 Ma.In Heshanqiao deposit.The age of the two deposits at about 130Ma is consistent with the second phase of ore formation in the Middle and Lower Yangtze metallogenic belt.The Gaocun and Heshanqiao iron deposits are mainly composed of disseminated and veinlet magnetite ore,and locally develop magnetite-apatite-actinolite veins.The veins formed late in the hydrothermal paragenesis,and cut early disseminated ore.The trace element chemistry of magnetite in the veins shows high V and Ti content(Gaocun vein magnetite averages 24148ppm Ti?and 3299ppm V;Heshanqiao vein magnetite averages 10919ppm Ti and 2042ppm V).This is significantly higher than the Ti and V in disseminated magnetite(Gaocun disseminated magnetite averages 11149ppm Ti and 1437ppm V;Heshanqiao disseminated magnetite averages 10919ppm Ti and 2042ppm V).This indicates that the formation temperature of the magnetite-apatite-actinolite veins was higher than that of the disseminated magnetite that deposited in the initial high temperature hydrothermal stage.Thus the deposit had at least two stages of magmatic hydrothermal activity.The content of Mg,Mn and Al of magnetite in Gaocun and Heshanqiao deposits is consistent with the content of Ti and V,which shows that these elements are less influenced by sedimentary rocks during magnetite crystallization.Compared with typical IOA deposits,Mg,Mn and Al in magnetite show little variation,which indicates that the components of deep marine sedimentary strata are continuously replenishing.The contents of Ti and V in the disseminated,veinlet and brecciated magnetite in the two deposits have roughly the same range,which shows that the disseminated,veinlet and brecciated magnetite in the two deposits all formed in the stage of early magmatic hydrothermal activity.The content of Ti+V of magnetite in the magnetite-apatite-actinolite veins is very high,all of the data are plotted in Fe-Ti,V area in the Al+Mn vs.Ti+V diagram,however,the geological and geochemical characteristics of this kind of magnetite are significantly different from that of magmatic liquated magnetite,it was formed in very high temperature hydrothermal environment.Disseminated and veinlet magnetite are mainly plotted in Fe-Ti,V and Kiruna areas,some are plotted into porphyry and skarn area,they were formed by high temperature hydrothermal fluid.Magnetite from quartz-magnetite-pyrite veins in Gaocun deposit mainly plot in the Skarn and IOCG fields,consistent with formation in a medium to low temperature hydrothermal environment.The distribution and variations of V,Ti,Al and Mn in magnetite are directly related to the mineralization temperature,so they can be used to detect the deposit centre.Grid sampling and analysis of disseminated magnetite in Gaocun deposit shows the content of V,Ti,Al and Mn is higher in the deposit center,corresponding to the location of magnetite-apatite-actinolite veins;it may also correspond to the high temperature hydrothermal channel for the second stage of hydrothermal mineralization.The high V,Ti,Al and Mn value region in Heshangqiao deposit is located in the east part of the open pit,and corresponds to the centre of the deposit.Trace element compositions of apatite may be controlled by various geochemical conditions during the formation process,which makes apatite an ideal mineral for study of hydrothermal processes.This thesis reports the results of a mineral geochemistry study on magmatic and hydrothermal apatites from Gaocun,Heshangqiao,Meishan and Gushan deposits in Ningwu volcanic basin.Generally,these iron deposits can be divided into three types:(1)deposits in the upper parts of diorite porphyry,such as Gaoun and Heshangqiao;(2)deposits in the contact between diorite porphyry and Cretaceous andesites,such as Meishan;and(3)deposits in the contact between diorite porphyry and Triassic sedimentary rocks,such as Gushan.V content in the magmatic apatite indicates the oxygen fugacity of Heshangqiao intrusion was higher than that of Gaocun,Gushan and Meishan.High Mn and Cl content in the Heshangqiao magmatic apatite indicates Heshangqiao intrusion had higher salinity than Gaocun intrusion.Comparison of the content of F-Cl in magmatic apatite from the four deposits shows that the Cl content of magmatic apatite in Gaocun,Heshanshang and Meishan deposits is higher than that of hydrothermal apatite.Gushan shows the opposite,which suggests that evaporites were mainly involved in hydrothermal processes in deposits that formed at the contact between diorite and sedimentary rocks(Gushan).Deposits that formed in the upper part of the diorite(Gaocun and Heshangqaio)or at the contact between diorite and volcanic rock(Meishan)had more involvement of evaporites during the magmatic stage.The magnetite-apatite deposits in Ningwu basin have similarities to magnetite-apatite deposits in other parts of the world,especially those in the Andean iron belt in Chile.In both China and Chile the metallogenic epoch was mainly early Cretaceous.During the mineralization process,from early to late,and from deep to shallow,the alteration deposited albite,diopside,epidote,chlorite,carbonate,pyrite and kaolinite.The source of associated magmatic rocks was,in all cases,metasomatic enriched mantle.The trace element characteristics of magnetites indicate that the deposits all formed from high temperature magmatic hydrothermal fluids.The isotopic characteristics,and major and trace element contents of magnetite and apatite indicate that the magnetite-apatite deposits in Ningwu basin are significantly affected by involvement of evaporites in the hydrothermal fluids.By contrast,the magnetite-apatite deposits in the Andean iron belt are not apparently affected by evaporites.The results of this study further clarified the relationship between Gaocun and Washan type deposits.Where there was strong interaction between the ore-forming fluid and Triassic evaporites,a large amount of skarn minerals formed,and the main mineralization type was disseminated magnetite formed by metasomatic replacement of wallrock(Heshangqiao deposit).Where a new phase of ore-forming fluid rose,participation of Triassic evaporites was weaker,and mineralization was controlled by magmatic-hydrothermal processes;there more apatite and less skarn minerals formed,and the main mineralization type was magnetite-apatite-actinolite veins.Where fluid pressure and shallow tectonic activity was weak,only fractures in the intrusion were filled with magnetite-apatite-actinolite mineralization(Gaocun deposit).Where fluid pressure and shallow tectonic activity was strong,a large amount of massive ore and coarse magnetite-apatite-actinolite veins formed in incompetent parts of the intrusion(Washan deposit).