Metallogenic Tectonic Setting, Metallogenic And Prospecting Models For Precambrian Iron-formation In The Anshan-benxi Area

The Anshan–Benxi greenstone belt is situated in the east segment of the northern margin of the North China Craton (NCC), concerned by domestic and foreign scholars due to the Widely distributed Iron‐formation(IF). In order to further reveal tectonic setting, metallogenic mechanism and establish metallogenic, exploration models for IF Mine, We carry out the study for the regional geology, deposit geology, diagenesis and metallogenic geochronology, ore deposit geochemistry and geophysical characteristics of the various types IF iron ore, on the basis of previous research. The main advance achievements from this study are as followings.1. According to the research on g regional geology, deposit geology and geophysical characteristics, we further divide the study area into Anshan, Gongchangling, Waitoushan‐Beitai and Nanfen‐Dataigou ore concentration area.The main IF iron ores in Anshan ore concentration area are Xianshan, Qidashan, Dagushan, Hujiamiaozi, Yanqianshan, Xiaofangshen, and so on, hosted in the Yingtaoyuan Group of the Anshan complex. The ore‐controlling structure is the asymmetric domed structure system, as Tiejiashan Archean granitic complex as the core. The south ore zone is east‐west trend and the north ore zone is south‐north trend, which main orebody are single layer and thickness>100m. The upper part of the iron ore is consist by the martite quartzite and magnetite quartzite, the lower ore is composed by magnetite quartzite and chlorite magnetite quartzite. Geophysics is characterized by the Tiejiashan negative magnetic anomaly center, with a ring of high magnetic anomaly, north‐south trend and north‐west distribution aeromagnetic anomaly belt. It is noteworthy that Xiaofangshen hematite quartzites type is hosted in the purple shale of the Nanfen Group, and the Containing siderite ore type in the north side of the Dagushan main ore body is hosted in the early Proterozoic phyllite.The main IF iron ores in Gongchangling ore concentration area are First mine, Second mine, Third mine and Dumu mine, hosted in the Cigou Group of the Anshan complex. The ore‐controlling structure is Gongchangling anticline, with the mixed granite as the core and the Cigou Group and Liaohe Group stratum as the wings. Orebody with plagioclase amphibolite were interbedded output, multilayer thin‐bedded, single‐layer thickness of <50m, The main types of ore is magnetite quartzite. The Senond mine is known by lage scale rich iron ore, with the wall rock alteration of chlorite, garnet, muscovite, tourmaline, carbonate and pyrite. Geophysics is characterized by large magnetic anomaly on the south side, thin strip positive anomaly on the north side and the overall aeromagnetic NWW trend.The main IF iron ores in Waitoushan‐Beitai ore concentration area are Waitoushan, Beitai, Mianhuapuzi, Daheyan, hosted in the Cigou and Dayugou Group of the Anshan complex. The ore‐controlling structure is lying east and north‐south trend duplex anticlinal folds. The deposits Generally have westward tilt and NNE trending three‐layers orebodies, with magnetite quartzite and magnetite quartzite containing amphibole as the main ore types. Geophysics characterized by having a plurality of oval‐shaped aeromagnetic anomaly zone, with high residual magnetic anomaly.The main IF iron ores in Nanfen‐Dataigou ore concentration area are Nanfen, Dataigou, Sishanling, Huanxiling, hosted in the Cigou, Yingtaoyuan and Dayugou Group of the Anshan complex. The ore‐controlling structure is Pianling left–lateral displacement Fault，due to the slightly west‐dipping strata. The orebody is thickness single‐layer and multilayer thin‐bedded, with martite quartzite, magnetite quartzite and hematite quartzite as the main ore types. Geophysics have the performance of aeromagnetic anomalies towards NNW, north eastern side of the gradient zone over the southwest side of the dense, showing occurrence west‐dipping seam characteristics.2. Studies on diagenetic and metamorphic epoch show that the coarse‐grained amphibolites in the Gongchangling second mine and Waitoushan mine are occurred in2548±12Ma and2540±13Ma, with the metamorphic age of2475±15Ma and2476±8Ma. The massive fine‐grained amphibolites, intruded to those coarse‐grained, have a diagenetic age of2527±15Ma, and a metamorphic age of2476±19Ma. Those zircon U‐Pb ages indicate that Anshan‐Benxi areas exist large‐scale mafic volcanic eruption in the Late Archean, intrusive mafic dikes subsequently, and experienced strong tectonic metamorphism in the Early Paleoproterozoic. The xenocrystic core of zircon grain from the massive fine‐grained amphibolites exhibits a207Pb/206Pb age of4174±48Ma, corresponds to206Pb/238U and207Pb/235U ages within analytical uncertainty. The leptite in the Gongchangling second mine and Waitoushan mine have lots of Middle‐Early Archean zircons, with207Pb/206Pb ages of~3.1Ga,~3.3Ga, worthy noting that there is a initial Early Archean detrital zircon, with207Pb/206Pb age of3763±14Ma. Those ages indicate that the Middle‐Early Archean ancient earth materials have a wider distribution in the Anshan–Benxi greenstone belt.3. The martite quartzite form the Xianshan deposit and magnetite quartzites form Dagushan, Qidashan, Gongchangling and Waitoushan have magmatic detrital zircons U‐Pb weighted average age of2530±16Ma，2548±7.9Ma，2541±7.4Ma，2535.7±8.1Ma and2542±2.0Ma, with a metamorphic age of2484±9Ma, other Mid‐Late Archean detrital zircons age of2764±27Ma,2757±40Ma、2635±16Ma and Early Archean detrital zircons age of3563±14Ma，3498±14Ma. In addition, the Containing siderite ore type in the north side of the Dagushan main ore body has the detrital zircons U‐Pb207Pb/206Pb weighted average age of1872±18Ma and2507±9.6Ma. The hematite quartzites from Xiaofangshen deposit has the detrital zircons U‐Pb207Pb/206Pb weighted average age of820±13Ma, with a detrital zircon207Pb/206Pb age of1564±66Ma. The ages of detrital zircons indicate that there is final Late Archean, Late Paleoproterozoic and Middle Neoproterozoic metallogenic epoch for IF iron ore,, and middle Early Archean, middle‐early Late Archean and early Mesoproterozoic magmatic tectonic events.4. The main ore types of IF iron deposits are magnetite quartzite, martite quartzite, hematite quartzite and rich magnetite ore. The major elements show that the sum of SiO2, Fe2O3and FeO is more than95%, low content of TiO2, with the inverse relationship between SiO2and TFe2O3, demonstrated little terrigenous material in the ore. In the primitive mantle‐normalized trace element distribution patterns comparison chart, different types of iron ores are characterized by Sr, Ta, Nb, Ti, Zr relatively negative anomaly, Rb, U, P, Y relatively positive anomalies, loss on of high field strength elements. Magnetite quartzite shows low REE+Y average contents with∑REE+Y of21.8×10‐6. Post Archean Australian Shale (PAAS) normalized REE patterns for Magnetite quartzite show that HREE are strongly enriched with La/YbPAAS of0.1～1.2. Magnetite quartzite displays significantly La, Eu and Y positive anomalies with La/La*PAAS, Eu/Eu*PAAS and Y/Y*PAAS of1.0～4.1,1.7～7.8and1.1～2.5, slight Ce negative anomalies or no anomalies with Ce/Ce*PAAS of0.60～1.5. Final Late Archean martite quartzite shows low REE+Y average contents with∑REE+Y of15.3×10‐6. PAAS normalized REE patterns for martite quartzite show that HREE are strongly enriched with La/YbPAAS of0.19～0.84. Martite quartzite displays significantly La, Eu and Y positive anomalies with La/La*PAAS, Eu/Eu*PAAS and Y/Y*PAAS of1.26～4.19,1.97～4.27and1.42～2.37, slight Ce negative anomalies or no anomalies with Ce/Ce*PAAS of0.81～1.32. Late Paleoproterozoic martite quartzite shows low REE+Y average contents with∑REE+Y of20.19×10‐6. PAAS normalized REE patterns for Late Paleoproterozoic martite quartzite show that HREE are strongly enriched with La/YbPAAS of0.31～0.54. Late Paleoproterozoic martite quartzite displays relatively lower La, Eu and Y positive anomalies with La/La*PAAS, Eu/Eu*PAAS and Y/Y*PAAS of1.3～2.54,1.52～1.84and0.89～2.13, slight Ce negative anomalies or no anomalies with Ce/Ce*PAAS of0.78～1.19. Middle Neoproterozoic hematite quartzite shows obviously higher than other iron ores REE+Y average contents with∑REE+Y of117.8×10‐6. PAAS normalized REE patterns for hematite quartzite show that HREE are very strongly enriched with La/YbPAAS of0.12～0.22. Hematite quartzite displays slight La and Eu positive anomalies with La/La*PAAS and Eu/Eu*PAAS of1.24～1.38and1.27～1.41, slight Ce positive anomalies and no Y anomalies with Ce/Ce*PAAS and Y/Y*PAAS of1.07～1.24and0.77～1.07. Rich magnetite ore shows similar to magnetite quartzite REE+Y average contents with∑REE+Y of20.7×10‐6. Post Archean Australian Shale (PAAS) normalized REE patterns for rich magnetite ore show that HREE are very strongly enriched with La/YbPAAS of0.02～0.68. Rich magnetite ore displays significantly La, Eu and Y positive anomalies with La/La*PAAS, Eu/Eu*PAAS and Y/Y*PAAS of0.73～5.42,1.2～5.3and0.7～1.9, obviously Ce positive anomalies or no anomalies with Ce/Ce*PAAS of0.66～3.75.5. Elements geochemistry indicate that the protolith of final Late Archean amphibolites is tholeiite, and divided into TH1tholeiite with depleted LREE and TH2tholeiite with enriched LREE. TH1tholeiite relatively enrich MgO, Al2O3, Ni, and deplete SiO2, TiO2, P2O5, incompatible elements and LILE, without fractionation of REE ((La/Yb)N=0.85~1.10,(La/Sm)N=0.91~1.16,(Gd/Yb)N=0.72~1.05), and high the ratio of Zr/Hf、Zr/Sm and Nb/Th. TH2tholeiite is characterized by the obviously enrichment of LREE ((La/Yb)N=4.44～4.93,(La/Sm)N=2.28–2.41), without Eu anomaly, Nb, Ta, P, Ti high field strength elements with adjacent REE systematic loss, and hight the ratio of Th/Nb and La/Nb. Sr‐Nd isotope show that: TH1tholeiite have low initial87Sr/86Sr ratio (ISr=0.70066~0.70287), high147Sm/144Nd ratio (147Sm/144Nd=0.1978~0.1988), εNd(t)=+4.5~+4.6, fSm/Nd=+0.01, the single‐stage model ages of2512~2523Ma，the two‐stage model ages of2540~2542Ma；TH2tholeiite is characterized by high initial87Sr/86Sr ratio (ISr=0.70849~0.70977), low147Sm/144Nd ratio (147Sm/144Nd=0.1387~0.1412), εNd(t)=+1.8~+2.4, fSm/Nd=‐0.28~‐0.29, the single‐stage model ages of2754~2822Ma Ma，the two‐stage model ages of2706~2754Ma；The protolith magma of TH1tholeiite was derived from a long–term depleted mantle that evolved as LREE–depleted sources, without crustally contaminated. Positive anomalies at Nb (Nb/Th>8) have been interpreted to be the recycling of ocean slab into a mantle plume. The protolith magma of TH2tholeiite were formed by melts from a depleted mantle source that were contaminated with15%–20%of upper crustal materials before eruption.6. Regional tectonic evolution Show: final Late Archean IF iron deposits were formed in the plume‐arc tectonic setting, where strongly mafic magma erupted and volcanic hydrothermal activity brought lots of Si and Fe; Late Paleoproterozoic IF iron deposit was formed in the back‐arc extensional basin tectonic environment, where strongly mafic magma erupted and volcanic hydrothermal activity brought lots of Si and Fe for the Containing siderite ore type in the north side of the Dagushan main ore body; Middle Neoproterozoic IF iron deposit was formed in the marine sedimentary basins in extensional the tectonic environment in the “Snowball” event.7. According to the deposit geology, metallogenic chronology and geochemical characteristics, we divide the IF iron deposit in the Anshan‐Benxi area into Algoma‐type IF、Algoma transition to Superior‐type IF、Superior‐type IF and Rapitan‐type IF. The Algoma‐type IF are mainly distributed in Gongchangling ore concentration area, Waitoushan‐Beitai ore concentration district and Nanfen iron ore deposit etc. Algoma‐type IF deposited in the environment of the deep anoxic basin, with closely related to submarine volcanic hydrothermal activity, and formed in the hydrothermal mixed with seawater. Algoma transition to Superior‐type IF are mainly distributed in Anshan ore concentration area, relative to Algoma‐type IF, deposited closer to the shallow shelf, with sulfur isotope Mass independent fractionation effects, and derived from high‐temperature hydrothermal minerals mixed with seawater, But contains a higher component of ancient seawater. Superior‐type IF is the containing siderite ore body on the north side of the Dagushan main ore body,1.87Ga age limit for the deposition. Superior‐type IF mineralization is significantly affected by submarine volcanic hydrothermal activity, has the causes of hydrothermal and deep anoxic seawater mixed, but accept more terrigenous clastic sediments, and decrease the hydrothermal component, relative to Late Archean IF. Rapitan type IF is hosted in purple shale of Nanfen group in the Xiaofangshen hematite quartzite deposits. Due to the impact of Mid‐Neoproterozoic “snowball” event, ice hinder marine microbial photosynthesis and the exchange between atmosphere and ocean with oxygen and other substances, so that the gradual depletion of oxygen in the water make the ocean anoxia. Because of the massive presence of the late Archean IF in the ancient weathering environment, the formation of iron‐rich clastic sediments and moraines result in the large‐scale dissolution of Fe2+in the seawater. The degree of oxidation of the oceans rise as the ice melts, the Fe2+deposit in the near. Rich magnetite ore are produced with lean ore body, and has the similar PAAS normalized REE distribution patterns to magnetite quartzite, with close relation to altered rock. High temperature metamorphic hydrothermal leach the magnetite quartzite, and make it desilication and the recrystallization to form rich magnetite ore.8. Aeromagnetic anomalies in the Anshan‐Benxi area are summarized as tall magnetic anomalies, complex magnetic anomaly, low and gentle magnetic anomaly, deep and enormous magnetic anomalies and residual magnetic anomaly. The long axis of magnetic anomalies reflect the orebody trend, the ore bodis have greater extension as the longer of the long axis. Short axis direction reflects the thickness of the ore body, the greater the thickness of the ore bodies as the wider short axis direction. Gradient of the magnetic anomalies reflects the depth of burial ore body, the steeper the gradient, the more shallow buried ore bodies, whereas the deeper. Field strength of the magnetic anomalies reflects the size of the ore body, the larger size of field strength with more remarkable ore body.