GISdevelopment.net ---> AARS ---> ACRS 1992 ---> Poster Session P The arcuate – structures on space borne remotely sensed imagery : their genesis and domination in gold mineralization Lin Qizhong, Guo Huadong, Shao Yun Institute of Remote Sensing Applications, Chinese Academy of Sciences, P.O. Box 775, Beijing 100101, P.R.China Abstract It is discovered that the arcuate image anomalies are well-developed in Xinjiang, northwestern China with the interpretation of spaceborne remotely sensed images. They have’ regular dimention, orientation and distribution. The conclusion of the study is : they are specific geological bodies formed under special conditions during the geological evolution course. They study area is located at the connection of Siberia plate and Junggar plate, where is sedimentary formation of volcanic and pyroclastic rocks. A series of grand abyssal faults, developed in early stage and strike in NWW direction, are the major dominating structures during the tectonic evolution in the study area. Owing to the strike slip and shear movement, stress concentrates in some sections of the faults, where rocks are fractured and low-medien hydrothermal alteration are well developed. The non-uniform of physical property of rocks and stress distribution took shape the arcuate structures in various of genetic mechanism. The arcuate structures have typical brittle shear zone character. Their evolution synchronized low –medien hydrothermal alteration. If they developed in “Mineral source” strata, they would be favourable for gold and copper enrichment and macro indicators for mineral exploration in use of Remote Sensing. This paper presents the image characteristics of arcuate structures and their emergence and developing, discuss their genesis and dominating in mineralization. Introduction The most important contribution of spaceborne Remote Sensing to geology is its ability to show the pattern and distribution of structures and indirectly express the mineralization process. The Landsat MSS, TM, SPOT HRV image how a number of arcuate imagery features (numbered as 1 1 – 5 in fig. 1) within Ertix structural zone in northwester China. They are assemblage of geological bodies in arcuate shape including structures, metamorphic rocks and a few relic blocks with typical tectonic characters and consistent evolution process. More-over they are strongly related to the formation of low-medien hydrothermal mineral deposits. They its why they are named as acrcuate structures. This paper discuss their genesis and domination in mineralization. Figure 1. Distribution Map of Arcuate Structures in Ertix Region The Characteristcs of Arcuate Structures 1) Obvious image tone anomalies in arcuate shape The tone anomalies are the most distinctive and recognizable character of arcuate structure. Each of arcuate structure is in sharp contrast to the image background: dark tone anomalies against the background in light tone such as No. 1 arcuate structure in fig. 1. An image anomalies in dark tone come out of light background such as No. 5. 2) Regular extension and distribution It is easily delineate the arcuate structures on the space borne image. They extend tens kilometers long and number kilometers, even more than ten kilometers wide, and occupy hundred square kilometers area. The No. 3 arcuate structures, one of them, extends eastwards into Mongolia. The arcuate structures mainly protrude to south which means the compressive stress comes from north. 3) Similar lithological combination in Hercynian formation. All of late Palaeozoic strata expose in Ertix region. The arcuate structure are strongly related to the middle Devonian and early Carboniferous rocks which are volcanic formation consists of felsic or femic volcanic rocks, pyroclastic rocks, clastic rocks interbeded with carbonatite. 4) Related to composite folds The arcuate structures developed ins strongly folded strata, especially in linear tight composite synclines. 5) Develop along large abyssal faults and controlled by large compressive and torsional faults. The arcuate structure developed at lateral part of large abyssal faults and strike in parallel with them (fig. 1). These faults play an important domination role in tectonic evolution of Ertix region. They are typical compress ional faults or torsional faults with compressive fracture zone in tens or hundreds meters wide. Schistosity, mylonitization, breccia and compressive lends are well developed within the fracture zone. In some section of the faults are the Devonian rock thrust over the Mesozoic rocks. Some of the arcuate structures are surrounded by faults, such as No. 1, no. 3 and No. 4, they have faults boundaries, which make the arcuate structures be a relatively independent “micro tectonic units”. 6) Relatively independent evolution process and character. The arcuate structures are relatively independent “microtectonic units” so they have internal consistent in their components, extension, distribution and combination of lithological composition. Firstly, they harmonize with regional geology environment, eg. the faults at two sides of No. 3 arcuate structure are the boundaries of “micro tectonic unit” as well as a part of No. 3 arcuate structure are the boundaries of “micro tectonic unit” as well as part of No. 3 arcuate structure. The block thickness in between of two faults are also the exposure width or arucate structure. The strike of strata bended as the fault strike changed. Secondly, each component within arcuate structure coordinate each other in their composition shape. Thath means the arcuate structures as an organic whole genesised and developed during geological evolution. Each component were formed under the internal stress re-distribution. The Genesis of Arcute Strcture Ertix region where the arcuate structures developed actually is a intense compressive tectonic zone composited of compressional or torsional, large abyssal faults and folds in NW direction (fig. 1). The major part of Ertix tectonic zone are linear tight composite folds, the arcuate structures occurs at such a complex environment. It is inferred from the Bouguer gravity anomaly map that the Ertix zone lies in the southern side of first – class gravity anomaly gradient belt. The gravity value varied from -125gm to -200mg within tens kilometers range. The depth of Moho discontinuity increase from north to south, shown in fig. 2 (Yong Zhongren, 1983). The arcuate structure mainly developed at the south side of gravity gradient belt close to the edge of Junggar plate where the crust movement is extremely active. Two positive magnetic anomalies in Figure 2. Moho Depth Map of Northern Xinijiang (Yang Zhongren, 1983) NW direction are the products of magmatism controlled by faults, which proves that the Ertix region is an active tectonic zone, more over, proves that the arcuate structure is a product of strong crustal movement. In palaeozoic era, the Siberia continental paleo – plate moved southwards and collided with the Junggar oceanal plate, where formed a collision belt in Ertix region. The gravity anomaly gradient belt, elongated magnetic anomaly belt, a number of large compressive, torsional abyssal faults, the uplift of Altai mountains and many levels of structural rock benches I he piedmont of Altai mountains, in NW direction, present the NNE nappe of Siberia plate to south. The nappe of Siberia plate is the dynamics of tectonic stress field in Ertix region. Owing to eh understhrust of plate, thick deposits were compressed, deformed and napped at the underthrust belt where the imbricate thrust occurs. The protruding direction of deposits is from the ocean (fig. 3 CopoxTHH, 1979) Figure 3. Thick deposits on the underthrust are compressed and deformed (COpoxTHH, 1979) It is infered consequently that Hercynian volcanic deposits were strongly compressed and deformed when the Siberia plate moved southwards Junggar plate overthrusted and napped, then formed a number of imbricate structures in parallel with the underthrust belt. Finally, the imbricate structures turn to curreat compressive, torsional abyssal faults. Inside of rocks are quite nonhomogeneous, so each component with block is under nonuniform pressure, stress condition, their strain and deformation are different. This is the genesis of various arcuate structures. General speaking, they have following types : The rigidity bodies of magmatic intrusion as a withstand column bend the block to be an arcuate structure. The block limited by former structure plane turn to a limited arcuate structure. The later stress field inherits the former conjugated fault system then generates the joint arcuate structure. The slip movement of large duatls form traction arcuate structure. The arcuate structures in Ertix region have region have intact shape and distinctive boundaries which means they genesised at late Hercynian and developed after the Collision of Siberia and Junggar plate. b>Ore-Controlling Arcate Structure The arcuate structures have two characters : their lithologic formation include oceanic volcanic rocks, continental clastic sediments and chemical deposits. They all have flexibility. The shear stress always exists during the genesis and development of arcuate structure. These two characters of arcuate structure is favourable for enriching the gold and copper element. The arcuate structures provide tunnel and space for hydrothermal liquid to transport and deposit. Furthermore, their existence causes pressure difference between the country rocks and the ore bodies, which is the drive dynamics of hydrothermal liquid. The fieldwock show that most of arcuate structure in Ertix region have gold enrichment sites. The arcuates structure could be the macro indicator for low medien hydrothermal mineral exploration in use of Remote Sensing. The top of the arc, the reflect are and the cross point with the linear structure are the favourable places for mineral deposits. Conclusion The arcuate structure is a product of plate collision, a special tectonic phenomenon due to Siberia plate napped southwards to collide with Junggar plate. They took place at those places where deposits muddy, tufferous rocks. They have stronger shear stress field than any other structures. The genesis of arcuate structure is valuable for the movement of low-medien hydrothermal mineral liquid, especially when the strata are “mineral-source” strata. The arcuate structure could be macro indicator for mineral exploration in use of Remote Sensing. The stress concentration position, the cross point with linear structure are the favourable place for mineralization.