Applying Radar Imagery to Gold Exploration in Altai Area, Northwest China

Applying Radar Imagery to Gold Exploration in Altai Area, Northwest China

Li Lin,Lin Shudao,Guo Huadong,Lin Qizhong
Institute of Remote Sensing Application Chinese Academy of Sciences
P.O. Box 775, Beijing 100101, P.R.

Abstract
Radar imagery is advantageous in the analysis of geological structure and plays an important role in geological mapping and mineral exploration. In this paper, the gold-bearing structures in Altai area are identified using radar imagery. The relationship between the imagery features and the geological information of gold mineralization are discussed and the paper suggests that radar imagery is an efficient tool for the analysis and enhancements of lineaments and the discrimination of geological structures controlling gold – bearing bodies.

Introduction
Radar imagery has been used to not only the map-making region geology and petroleum exploration, but also mineral prospecting. Meanwhile, it has been widely used for gold exploration. Through the interpretation of radar imagery in Altai area, the paper evaluated the capability of radar imagery for the discrimination of lithologic units and the detection of lineaments, discussed the relationship between gold – bearing units and their features of radar imagery. Also, the paper suggested that radar imagery can detect the arcuate structure and gold-bearing fracture zone, which are not easily distinguished using multispectral SPOT imagery.

Study Area Description
The study area lies on the east side of Zasang depression and the south side of Altai fold belt, the major structure trends are in NW, NEE direction, the secondary NNW, EW direction. Arxile fault is one of several important structures in the area, striking in 320 degree direction and cutting Habahe basic complex body. The hypothermal alteration minerals in Arxile fault include kaolinite, illite and monmorillinite. The minor faults and joins well develop and cut one another, they are in parallel with Arxile major fault or meet it with small angle.

In the area, Magmatic rocks which belongs to the first and third time instrusive mass in intermedient period of Variscian movement include hornblende gabbro and plagioclase – granite. Vein rocks in the area maily include intrusive is site and quarts. The former mostly strikes the same direction as a result of its being controlled by Arxile fault, the latter chiefly distributes among basic complex and plagioclase – granite bodys.

There are two types of gold deposit in the area : gold – bearing fracture zone and quartz. The former exists among Habahe basic complex body composed of hornblende gabbro and diorite, the fracture zone made of kaolinite, quarts and pyrite develops gold mineralization and strikes in accordance with Arxile fault, a lot of crevasses and joints can be found in the middle and periphery of Habahe basic complex body and they provide gold-bearing body with mineralization space. Many gold-bearing quarts have been discovered among Habahe basic complex and plagioclase – granite bodies and both sides of their adjoining rocks develop the alternation such as chloritization, epidotization, sericitization, silicification and kaolification Most quarts veins striking in correspondence with the major structure trend or meeting the major faults with minor angles grow a lot sulfide mineralization such as chalcopyritization and pyritization. That a long term cultivation results and Quarts vein separating out ferrous cations and becoming red and pink, makes quarts veins be defined as possible gold – bearing geological bodies.

Geological Interpretation of Radar Imagery

1 Lineament
The interpretation of radar imagery reveals that major structural trend have two directions of NW and NE-NEE. The faults stretching in NW direction include Saresuke (F1), Tasibai (F2), Sikubastao (F3), Talerder(F4) and in NE-NEE direction there are three fracture compression zones as follows: Hujieter (F5), Suku (F6) and Artar (F7). The faults stretching in NW direction can be distinguished basing their lineal characteristics on radar imagery, these lineaments composed of separately bright and dim lines result from the different relief of both sides of the faults in general, the lineaments stretch continuously very long and cutting NE-NEE fracture compression zones. The fracture compression zones are composed of a series of small crevasses and joints, they can be distinguished through the textural features resulted from small shearing crevasses and joints, stretch continuously and shorter than the faults stretching in NW direction, only means of careful interpretation can they be distinguished.

In study area, many quarts veins striking in NW direction and crossing Arxile faults with the 20-60 degree angle contain higher gold tenor than those stretching in other direc- tion and they mostly occur at the cross joints of NW and NE-NEE structures, the sites which NW faults cut NE-NEE fracture compression zone often be defined as favourable place of gold-bearing body exist.

2 Arcuate Structure
Arcuate structure can be distinguished by virtue of radar imagery, Arxile (F9) arcuate fault has a few kilometers length 500 meters width, its disjunction sticks out to the northeast, it is a gental relief zone made up of clay and many small crevasses. The rocks of both sides of arcuate zone have the phenomenon of mylonitization, it can be defined as a dull and width strip on the radar imagery as a result of clay smooth surface and high complex dielectric constants. Hujiete (F8) arcuate fault presents an arcuate belt and topographic pattern and narrow and long depression having ten kilometers length and one kilometer width. Its two limbs manifest brighter strips resulted from the strong backward scattering of the forest growing on the surface of solid rocks as a corner reflector, the figure 1 presented the distribution of lineaments and arcuate structures in study area. The arxile arcuate fault has been examined by means of geochemical measurement. The analytic result of soil samples show that most samples gold contents are 10 times higher than gold Clark value, the highest 900 ppb, suggest that the Arxile altered fracture zone is a strongly anomalous zone of a gold diffusion halo (Guo Guandong et al. 1990). As a counter part of Arxile arcuate zone, Hujiete fracture zone has the similar imagery features to Arxile arcuate and the former scale is very larger than the latters, it has been defined as an other prospective target of gold mineralization, it is worth while to carry out further exploration.

Figure 1. Lineament interpretation map made on the SPOT imagery gold mine site
Evaluation of Radar Imagery for Gold Exploration
Because of radar imagery high space resolution, side looking effects and big coarseness and complex dielectric constant of fracture zone and vegetation emergence as a corner reflector, radar imagery can be used to identify the more lineaments than SPOT, especially in NE-NEE direction and it can enhance ten arcuate belt features, not apparent on the SPOT imagery. Another element influencing the effects of enhancing lineament and arcuate belt depends on the relationship between radar side-looking direction and major structural trend. Because that in the area, the first rank valley (lineament) is long enough to be identified on the radar imagery and topographic slope degree smaller than 25 degree, these satisfy the condition of illuminated geometric model (figure 2) from Yasushi Yamaguchi, 1985), so it has :

Figure 2. Geometric model for maximum enhancement the first order vally (from Yasushi Yamasushi, 1985)

a : azimuth angle between valley and detective direction of radar wave beam
b : depression angle
g : valley floor inclination
P : illumination wall
G : shadow wall
Q : valley wall inclination
AB : a line in parallel with the first order valley

Figure 3. Relative length of the valley AB in figure 3
(a)g=20 (b)g=20-40 (c)g=15-30

Sin a tan g + cos a tan q = tan b ------------------(1)

When depression angle changing from b₁ to b₂, formula (1) suggests that the most optimum condition of enhancing lineament is :

X / H = (cos b₁ – cos b₂) / cos a

X : lineament length:
h : sensor altitude:
x/h : corresponding value with a angle

The curve of the most optimum lineament enhancement (figure 3) can be obtained basing formula (1) and (2). In the area, some lineaments in NE-NEE direction have been enhanced because of the satisfaction of the conditions, this is the reason for NE 60 – 65 degree lineament to occupy eminent frequency in the rose map of lineaments (Guo Huadong et. al, 1990) The two limbs of two arcuates structures have been NW and NEE strike, so they are enhanced, this also is the reason for two arcuate structures to have apparent imagery features on radar imagery in addition to high coarseness of fracture zone, altered clay big complex dielectric constant and corner reflectance of vegetation in the inner of fracture zone. SPOT imagery lower space resolution than radar imagery and big depression angle constrain the enhancement of lineament.

Conclusion
Radar imagery has the most optimum ability of enhancing lineament among Remote Sensing data, especially in the moderate and general relief area one can make the most optimum detection through crossing 20 – 30 degree angle of radar side – looking direction with major structural trend. It can also be used to distinguish the arcuate belt. Along with the emergence of multiband, mulitpolarization radar imagery, it will provide the more suitable information for gold exploration.

References

Guo Huadong, et al., The use of SPOT and Airborne Radar Imagery for Gold Exploration in Qiaben, Northern Xinjiang, China Proceedings of Remote Sensing and Operational Technology of the Mining and Petroleum Industries, pp 205 – 212, 1990.
Yasushi Yamasushi, Image-Scale and Look-Direction Effects on the Detectablity of lineament in Radar Images, Remote Sensing of Environment, Vol. 17, pp. 117 – 127, 1985.