| The Southeastern Hubei Ore-concentrated District(SHOD)is an important part of the Middle and Lower Yangtze Cu-Fe-Au metallogenic belt,which located in the western end of this belt.It hosts a large number of copper,iron,gold,molybdenum,tungsten,lead and zinc deposits related to the medium-acid intrusive rocks and is one of the most important iron and copper production bases in China.Since the Mesozoic,this district has undergone Multi-stage structural superposition,and related magmatic activities and hydrothermal mineralization have occurred in the district.Therefore,it is worthwhile to study how the structures control the formation of ore deposits in the SHOD.Previous studies on the metellogentic mechanism of ore deposits in this district since the Mesozoic suggest that the predominant ore-control structures are the contact zone structure(or layer-controlled structure)and the node structure,which has being used as a useful guide for mineral prospecting.However,the relationship between tectonic and metallogenic evolution is still not fully understood,and it is still unclear what is the geodynamic control for regional mineralization.In this study,several typical ore deposits including Tonglushan,Jiguanzui,Yangxin,Chengchao,Daye,Jinshandian,and Lingxiang in the SHOD are selected,and their pre-,syn-and post-mineralization structures and related deformation characteristics are distinguished based on the field observation,and the main features of multi-layered detachment system in these deposits are summarized.Combined with previous researches,this paper further concludes tectonic types,tectonic combinations,and the relationship between tectonic types and mineralization in the SHOD,and consequently establishes a tectonic-controlled ore-forming sequence.Magmatic and hydrothermal zircon and apatite grains were selected for fission track thermochronologic studies which reveal the temperature-time evolution history of ore deposits(rock mass).By quantitatively calculation of the uplifting-erosion rates of ore bodies we figure out their variation and preservation process.This study provides another angle to theoretically understand how the multi-layered detachment delamination system controls mineralization and can be used to assess mineral resources potential and exploration strategies in this area.The main achievements and understanding of this paper are as follows:1.The main characteristics of multi-layered detachment in the SHOD are summarized.Based on the field geological survey,drillhole profile data,and previous researches,the main characteristics of the multi-layered detachment faults in the SHOA include the following two aspects.Firstly,significant structural erosion occurs during the detachment process,and the strata have generally being thinned or even missing.Due to the extension of the mantle plume,multi-layered detachment faults are formed between the uplifted base and the strata that have a large difference in competence.In the study area,multi-layered detachment faults mainly occur between Devonian quartz sandstone,siliceous rock and Silurian silty mudstone,and between Jurassic quartz sandstone,siliceous rock,and Triassic limestone intercalated with argillaceous siltstone.Among them,the Middle and Lower Devonian and Upper Jurassic strata were missing in the entire SHOD,where Permian and Carboniferous strata were missing locally.Secondly,the ductile and brittle transition occurs between the hanging wall and footwall of the multi-layered detachment faults.The ductile shear deformation accompanied by a series of interlayered shear fold occurs at the footwall,whereas the normal fault can be seen in the hanging wall.2.It is demonstrated systematically how the multi-layered detachment faults control the ore bodies in the SHOD.Combined with regional geological data and previous research results,the multi-layered detachment faults in this area can be divided into upper,middle and lower detachment fault in terms of their position,detachment degree,and faults combination and rock deformation characteristics.The lower detachment faults consist of the denudational fault in the basement and two denudational faults occurs between Ordovician and Silurian,Silurian and Devonian strata in the hanging wall;the central detachment faults formed by the main denudational fault between the Middle Carboniferous Dapu Formation and Huanglong Formation,and the secondary denudational fault between the Upper Permian and the Lower Triassic strata;the upper detachment faults consist of brittle fault in the hanging wall and the denudational fault between the Middle Triassic Puzhao Formation and the Jialingjiang Formation.The rocks and ore bodies controlled by the multi-layered detachment faults have the following characteristics.In terms of geometry,the rocks and the orebodies are distributed along denudational fault,with an irregular strip shape on the plane and a plate shape on the section.There occur no flow and deformation structures inside the rocks.The multilayered detachment faults intersect the surrounding strata.In the plane view,the rocks expand or shrink,and are unevenly distributed.In the outcrop,fault breccia produced by multi-layered detachment faults can be seen locally.3.A tectono-metallogenic sequence in the SHOD has been established.Based on the geological surveys of outcrops and mine roadways in the field,combined with regional tectonic and evolutionary processes in the Middle and Lower Yangtze metallogenic belt,this study deciphered the tectonic deformation characteristics in this region,and summarized different tectonic types,tectonic combinations and their relationships with mineralization.This work also identified the pre-,syn-,and post-mineralization structures.The intrinsic relations among the pre-Indosinian period,Yanshanian cycle and Himalayan cycle and their corresponding tectonic systems,deformation sequences,tectonic patterns and metallogenic sequences were analyzed and elaborated.Consequently,the tectono-metallogenic sequence in the SHOA was established.4.Apatite and zircon samples have been selected and their fission track age and length parameters are obtained.Seven mining areas were sampled in the SHOD and the analytical results are summarized as followings: The apatite fission track ages of the Tonglushan are concentrated between 40-65 Ma with average track lengths of 11-13?m.The zircon fission track ages are concentrated at 60-90 Ma;The apatite fission track ages of Jiguanzui are concentrated between 53-65 Ma with mean track lengths of 11-13 ?m,and the zircon fission track ages are concentrated at 60-83 Ma;The apatite fission track ages of Yangxin are concentrated at 39-65 Ma with average track lengths of 12-14 ?m.The zircon fission track ages are concentrated at 63-119 Ma;The apatite fission track ages of Chengchao mining area are concentrated at 27-61 Ma with average track lengths of 11.5 to 13.4 ?m.The zircon fission track ages are concentrated at 56-81 Ma;The apatite fission track ages of Daye are concentrated at 38-54 Ma with mean track lengths of 13.1 to 13.8 ?m.The zircon fission track ages are concentrated at 39-63 Ma;The apatite fission track ages of Jinshandian are concentrated at 50-69 Ma with average track lengths of 13.1 to 14.2?m.The zircon fission track ages are concentrated at 64-81 Ma;The apatite fission track ages of Lingxiang are concentrated at 43-72 Ma with average track lengths of 12.9 to 13.3?m.The zircon fission track ages are concentrated at 83-95 Ma.5.The distribution characteristics of apatite fission track lengths are summarized for seven mining areas in the SHOD.The distribution characteristics of fission track lengths of Tonglushan,Jiguanzui,Yangxin and Chengchao are bimodal and the mixed type,indicating that the mining areas have experienced complicated thermal history processes.The distribution characteristics of fission track lengths of Daye,Jinshandian and Lingxiang mining areas are inconsistent.The distribution pattern of fission track lengths of one group sample are undisturbed basement,and the other group samples show bimodal and hybrid,indicating these three mining areas have experienced at least one tectonic heat event.6.Thermal history simulation of apatite fission track was performed on the samples from seven mining areas.The simulation results show that at Late Cretaceous(about 65 Ma)and Eocene(about 50 Ma),the t-T curves of Tonglushan,Jiguanzui,Yangxin,and Chengchao have two structural thermal events characterized by rapid cooling transit to quick warming,reflecting that the area experienced two significant tectonic events during this period.The t-T curves of Daye,Jinshandian and Lingxiang mining areas are divided into two groups.Only one thermal event occurs in one group at either Late Cretaceous(about 65 Ma)or Eocene(about 50 Ma).The other group is a simple cooling process at same period.These data are generally consistent with the data obtained from the neighbouring area of the SHOD.7.According to the thermal history of the seven mining areas,the total erosion depths are calculated for each mining area.The eroded depths of the seven mining areas since the late Yanshanian period(about 70 Ma)are roughly between 5.2 km and 2.2 km,which are broadly consistent with that of the neighboring area of the SHOD(4.8-6.0 km obtained by fission track thermogeodological method).In comparison,the total erosion depths of Daye,Jinshan,and Lingxiang mining areas are relatively smaller.The deep prospecting potential of each mining area needs further calculation and evaluation based on the specific metallogenic depth and regional geological conditions.In summary,there are two main aspects of innovation results and understanding achieved in this study.In theory,through field geological surveys and typical exploration section analysis,the pre-,syn-,and post-mineralization structures are identified and their corresponding structural deformation characteristics are summarized.The features of multi-layered detachment faults in the study area are analyzed and their relationship with ore bodies has also been studied,and the regional structure and oreforming sequence is established.The fission track thermochronological method can be used to reveal the thermal evolution history of the ore concentrated district,to illustrate the uplift and erosion process of the orebodies,and to quantitatively calculate the erosion depths since the Yanshanian period in the SHOD.This study will help to guide the future deep mineral exploration and potential assessment. |
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