Multi-data integration of exploration criteria and selection of prospecting targets: An example in Shengyuan Uranium producing basin

Multi-data integration of exploration criteria and selection of prospecting targets: An example in Shengyuan Uranium producing basin

Liu Dechang, Zou Jingke, Sun Maorong, Wang Guojuan
Beijing Research Institute of Uranium Geology China

Abstract
Based on the analysis of the exploration criteria for ShengYuan Basin a uranium ore field, the multi-data integration and information extraction of exploration criteria are carried out on computer and image processing system so that the areas with best combinations of exploration criteria are directly displayed on the screen. Six prospecting targets are selected through the field examination.

Shengyuan basin in Jiangxi province is a uranium-producing, Jurassic-Cretaceous volcanic sedimentary basin with an area of about 400 sq. km . Its basement consists of Sinian-Cambrian rocks with Caledonian granites intruded. Several uranium deposits, occurrences and anomalies have been discovered over the basin region which, therefore, becomes a very important uranium ore field China.

Geologic analysis of exploraiton criteria for Uranium deposit
Through the comprehensive research of remote sensing, geologic, geophysical and geochemical data in Shengyuan basin, eight basic criteria were proposed: 1. favourability of lithology, 2. crustal fracture intensity. 3. metallogenic fault, 4. airborne radiometric anomaly of uranium, 5. Th/U radio, 6. hydrogeochemical anomaly of uranium, 7. absolute elevation of relief, and 8. topographic low. Furthermore, these eight criteria were converted into three combined criteria: 1. enrichment conditions, 2. enrichment indicators ,and 3. ore preservation conditions (Table 1) When the uranium source exists, it is very important to consider the presence of enrichment conditions which are referred to the concentration environments or places of ore-forming solution. However, the places with enrichment conditions will not always show the uranium concentration therefore, enrichment indicators must be the ore preservation conditions which prevent uranium deposits from being eroded. So, the most prospective areas for exploration should be those with both favorable metallogenic conditions and good ore preservation conditions.

The above-mentioned enrichment conditions, enrichment indicators and ore preservation conditions belong to geologic, geophysical and morphological criteria, respectively. Their combinations reflect the multi-data integration of exploration criteria.

Table 1 Classfication of Exploration Criteria for Shengyuan Basin

Classification of Criteria	Criteria
Enrichment Conditions	Favorability of Lithology : J3d, J3e, Z, G, J2s, K1, r, K2 Crustal fracture intensity : Total number of faults per unit area Metallogenic fault : SN, EW, NE-trending faults
Enrichment Indicators	Airborne radiometric anomalies of uranium, Th/U ratio <3, Hydrogeochemical anomalies of uranium
Ore preservation conditions	Absolute elevation of relief : 0-100m, 100-200m, 200-300 m Topographic low

Multi-source integration and information extraction of exploration criteria
Multi-source data integration of exploration criteria is based on the geologic concepts: the more highly the exploration criteria concentrate , the better the metallogenic conditions; the more disperse the exploration criteria, the worse the metallogenic conditions. The areas with similar combinations of exploration criteria should have similar metallogenic conditions to the areas with known ore deposits. Hence, the areas like this are most prospective for exploration of similar deposits.

The main integration procedure of exploration criteria is as follows (Fig.1):

Fig. 1 Flow chart showing exploration criteria integration

It must be emphasized that the exploration criteria need, at first, to be graphed . In other words, the criteria need to be converted into the plain figures. In order to realize the multi-source data integration, some of plain figures must be preprocessed into maps which are easy for imaging. For example, linear and circular structures interpreted by remote sensing should be easy for imaging . For example, linear and circular structures interpreted by remote sensing should be preprocessed into density or frequency maps. Then, the plain figures are input into computer by digitizer or TV camera. Vector data files are formed in digitizing plain figures. After being coded and interpolated, the vector data files are converted into the grid data files. Coding is the process of conversion of components on maps into grey-scale values ranging from 0 to 255. The grid data files saved in computer are for processing in image system. Finally, the grid data files of exploration criteria are registered, which is necessary for multi-data integration and information extraction. Exploration criteria integration can be achieved by direct integration of basic criteria. Also, the basic criteria can be classified according to their properties, and those basic criteria with same property are integrated to form the combined criteria. Thereafter, the final image of multi-data integration can be obtained by integration of combined criteria (Fig.2)

Fig. 2 Schematic diagram showing multi-source data integration
and superposition procedure of exploration criteria

Both geologically weighted superposition and multiple statistical analysis were employed in the integration of exploration criteria to produce weighted superposition integration image and principal component analysis integration image, respectively. Accordingly, two extraction images were got by the supervised extraction, which is based on the knowledge of existing deposits and grey-scale value histogram of image. The extracted anomalous areas have similar combinations of exploration criteria to the areas with existing deposits. Therefore these extracted areas are thought to be the most promising for finding new deposits in the region.

After the classification by using the threshold values, different colors were assigned to the different classification. Thus, as color composite image, reflects the uranium resources evaluation in Shengyuan basin, was finally obtained (Fig.3).

1. Most favorable areas for uranium mineralization
2. favorable areas for Uranium mineralization
3. Relatively favorable areas for uraniam minealization
4. Unfavorable areas for uranium mineralization

Fig. 3 Sketch map of uranium resources evaluation in Shengyuan basin

Metallogenic prognosis based on the multi-data integration of exploration criteria
To carry out the metallogenic prognosis, first of all, the exploration criteria multi-source data integration and information were completed. Then the results were analyzed with other geoscience information according to the specific situation in order to preliminarily determine the favorable areas of mineralization. Furthermore, the prospecting targets for explorations were selected through the field examination.

Analysis to the Images of Exploration Criteria Integration --- information Extraction
Through the integration processing of 8 exploration criteria by using two different integration methods: geologically weighted superposition and principal component analysis, the supervised extraction was carried out based on the knowledge of areas with existing deposits. So the areas which have similar metallogenic conditions to the areas with existing deposits were determined. Two results extracted from the two-way-integrated images were compared. The favorable areas resulted from the weighted superposition and principal component analysis are approximately accordant with each other. This illustrates that the results of two different integrations are quite similar although there are some differences. The most obvious difference is that one favorable are in the extraction image with principal component analysis has no correspondency in the image with weighted superposition. But during the field examination from both integration images should be fully considered to scientifically select the prospecting targets.

Aimed at the spatial distribution analysis of the determined favorable areas, the Landsat images of the region were interpreted in detail and were superimposed by the exploration criteria integration-inforamtion extraction images. The analysis shows there are two cases for the favorable areas and the interpreted circular structures: some of favorable areas are inside the circular structures or on their rims, some others are apart from the circular structes. According to the dating analysis of uranium mineralization in the region, the mineralization process is devided into two stages : in the early stage, there was U-P mineralization with age of 127-136 ma which corresponds to the end of Late Jurassic and the beginning of early cretaceous . In late Jurassic, the volcanic activity gradually weakened and finally expired. The faulted -blocks in the ore field began to form. According to the research on the typical deposits in the region, the mineralization of the early stage is believed to be related to the volcanic apparatuses. In the late stage, there was U-Mo mineralization with age 90-100 ma which ranges from Early to late Cretaceous. At his time, no volcanic activity happened. Therefore, internal parts and rims of circular structures are the favorable areas for superimposed uranium mineralization (two stages of mineralization may occur). And the areas with out circular structures often show hydrothermal-solution -type uranium mineralization (late stage mineralization). According to the integration analysis of the extraction image with weighted superposition and interpreted linear structures, it is concluded that most favorable areas of exploration criteria integration are controlled by the SN-EW - adn NE- trending fault intersections. In addition, based on the integration of the extracted favorable areas and the ground gamma data, it is known that the ground anomalies in deposit areas and the favorable areas from the exploration criteria integration are spatially overlapped . Some of the ground anomalies are adjacent to the extracted favorable areas. Some other ground gamma anomaslies, however, are far away from the determined favorable areas. Obviously, those gamma anomalies which are adjacent to or at the favorable areas are of importance in exploration. Certainly , the integration can also be analyzed with other metallogenic factors to provide some useful information.

Also, the extracted favorable areas can be superimposed on the relief maps or Land sat images to serve as basic information for field examination.
Field geological examination
According to the exploration criteria integration-information extraction images and the analysis, the field geological examination was undertaken . There are two situations : 1 The information from the integration analysis is accordant with the field examination results. 2. The information from the integration analysis is not accordant very well with the field examination results. For example, the airborne-surveyed anomaly in the determined favorable area in Tongjia is caused by the ore piles which is underlain by late Jurassic volcaniclastic rocks. And there are some faults of different directions across this place. So a false favorable place occurred on the exploration criteria integration image.

Based on the analysis of the exploration criteria integration-information extraction images and the field geological examination, the prospecting targets were selected in Shengyuan basin: Three targets of grade -1, two targets of grade -2 and one target of grade -3

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