| As a comprehensive and cutting-edge survey of regional geology, three-dimensional(3D) geological survey or 3D geological mapping is to investigate the spatial distributions of geological objects and their relationships on 3D scales, and to reveal the deep geological structures and geological processes and their possible effects on mineral resource distributions and geological environments based on technologies and methods integrated by surface geological mapping, geophysical survey, drilling, and 3D visualization, through which some evidences are provided for solving major geological issues, predicting the deep mineralizing potentials, warning destructive geological disaster, and assessing the base stability of underground engineering. 3D geologic modeling as a basic and key issue in regional geological survey has become a hot research topic in the geology and mining sectors at domestic and abroad.The study area, located in the north area of Dandong city, Liaoning Province, is suited at the Liao-Ji Paleoproterozoic tectonic belt,namely the Liao-Ji rift, at the northern part of the Eastern blocks in North China craton. The area is not only the typical one in the study of the precambrian geology in China, but also the important mineralized one within East Liaoning-South Jilin metallogenic belts. In Liao-Ji rift zone, Paleoproterozoic Liaohe groups, with a thickness of ten thousand meters, and magmatic rocks formed at different evolved stages of the zone are widely distributed. The Liao-Ji rift zone enriched abundant mineral resource such as gold, silver, lead, zinc, magnesite, talc, boron, cobalt, copper, iron, etc. The distribution of these different types of the mineral deposits existed in Liaohe Group showed some strata-bound characteristics, in which Boron deposit mainly occured in Lieryu formation, Graphite deposits in Gaojiayu formation, and lead-zinc, magnesite, gold and silver deposits in Dashiqiao formation.Some researchers, aimed at the formation and evolution of North China craton, the tectonic features of Jiao-Liao-Ji Paleoproterozoic tectonic belt, the division and comparison of Paleoproterozoic sedimentary formation, the forming mechanisms and the deep exploration potentials of main type deposit in Liaohe group, and the Mesozoic tectonic-magmatic activity and its mineralization and so forth, have carried out some extensive studies using the methods of geology, geochemistry, isotope chronology, geophysics, and they have gained some valuable data and research results about basic geology and mineral exploration. However, those research results are mainly derived from surface geological survey, and the issues such as the tectonic properties of the Liao-Ji rift and its evolution, the origin of the Paleoproterozoic granite, formation and evolution of Liaohe group, the origin of zinc, gold, silver and lead ores in Qingchengzi ore-concentrated area and boron deposits in Wenquangou area in Fengcheng city are debated among scholars. Meanwhile, the studies about the deep structure of Liao-Ji rift belts, the basement characteristics and its composition have less been conncerned with the comprehensive approaches of geological and geophysical on 3D scales, which to certain extent limits the understanding of some major geological and metallogenic issues in the study area.The objectives of this paper are supported by the program ‘Deep geological survey of Benxi –Linjiang area’ grafted by China Geological Survey,( 1) to characterize the deep geological structures about the main geological interface, fault structures, Mesozoic basins, intrusions in the study area and highlight the understanding of deep geological structures employing the methods of potential field continuation of gravity and magnetics, integrated backward inversions of gravity and magnetics data, and 3D density inversions, through processing interpreting geophysical data from regional geological survey data of 1:250000 scales in Danong area and regional geophysical data of 1:200000 scales;(2)to construct the 3D geological models of major geological units with GoCAD software using the methods of multistage profile controls and block devisions in order to show 3D distributions of Liaohe group and others, and basement features and its compositions of Liao-Ji rift, and to reveal the coupled relations of regional geological tectonic settings and deep geological processes to mineralization, and to provide the base for solving major geological problems of Liao-Ji belt and predicting the deep mineral resources potential in 3D scales. The research results of the paper are as followings:1. The tectonic boundary between Longgang block and Liao-Ji rift has been determined, which is along the lines from Shuangtaling,through Sandaoling, to Lianshanguan in northern part of study area.The boundary actually is a normal fault between Longgang block and Liao-Ji rift, which plays an important role in the control of magmatic activity, sedimentation and distribution of mineral resources in study area.This further confirms that Liao-Ji tectonic belt is of a rift.2. Liao-Ji granite is characterized by low density and magnetic parameters. Liao-Ji granite and Liaohe groups have a similar distribution pattern of trace elements and rare earth elements to late Archean granites and Anshan groups, along with the record of multiple episodes of Archean geological processes by the inherited zircons in Liao-Ji granite and Liaohe groups, all which suggested that both Liao-Ji granite and Liaohe groups were derived from Archean crystalline basement.3. The Liao-Ji rift is formed on the base of Archean metamorphosed basement, and the basement composition varied with tectonic position. The northern part of the study area is mainly composed by the late Archean plutonic intrusions and small scale supracrustal rocks, while the center part of the study area mainly the Liao-Ji granite.4. Small Mesozoic volcanic sedimentary basins are developed along the fault striking northeast. The faults controlling the basin boundary are of normal ones, which are formed by strike-slip stretching. The basin boundary is reworked by compression stress.5. The deep geological structure and 3D geometry of Mesozoic intrusions have been reconstructed. It has found that some concealed intrusions were seated beneath Qingchengzi-Tongyuanpu and Xiuyan area, which takes much effects on the mineralization potentials within the area. The potential area for mineral exploration is the one where the concealed intrusions may be exited beneath Liaohe group, which includes the hidden areas beneath the Hanjialing intrusions in Xiuyan and Shuangdinggou intrusions in Qingchengzi area.6. The 3D geological models of Longchang-Caohekou and Pianling-Suzigou area have been constructed, which make the geological structure of Liao-Ji rift in Dandong area be transparent above the depth of 5000 m, and characterize the geometry of the main geological interface, main faults, Mesozoic intrusions, and Mesozoic basin in three-dimensional ways.The research results can provide new evidences for solving major geological problems such as tectonic characteristics and evolution of Liao-Ji rift and deep mineral prospecting. At the same time, the flowchart of 3D geological survey, the approaches for researching deep geological structure and 3D geological modeling proposed in the paper can be beneficial to other researchers. |
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