Table of Content

20 December 1996, Volume 2 Issue 4

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Article

A THOROUGH INVESTIGATION OF GEO-ANOMALY: A BASIC OF METALLOGENIC PROGNOSIS

Zhao Pengda, Chi Shundu, Chen Youngqing

1996, 2(4):  361-373. 

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The factors related to mineralization are all regarded as geological anomalous events in geologically evolving processes. All geo-anomalies related to mineralization can be considered as “possible regions of exploring mineral resources”, the geo-anomalies (or anomalous zones) where expected mineral deposits may be found are called “practicable zones for prospecting”, among them those abounded with more direct and indirect ore-prospecting information may be defined as “favorable segments for prospecting”. Geo-anomaly is of self-similarity on a different scale. Spatially, it is of complicated genetic and chaotic dynamic mechanism. In form of expression, it contains two categories: hidden and obvious geological bodies (or associations). There is an intrinsic connection between geo-anomalies and mineral deposits both in spatio-tmporal relation and in genesis. Study on geo-anomalies must be based on the division of geological background fields. Geo-anomalies can be quantitatively delineated by certain methods and rules. This paper points out the different features of delineating geo-anomalies on different scales. The sophisticated techniques such as GIS, GPS RS etc. are powerful tools of studying and delineating geo-anomaly. Fractals, self-organizations and chaos facilitate, from sides of morphology (geometry), process and mechanism, to explore the complexity in existence and development of geo-anomaly. There are two kinds of relationships, i.e. telescoping and coupling, between geo-anomaly and geophysico-geochemical anomaly. Eventually the paper gives the compilation of geological anomalous map and prognostic map for favorable area of ore-prospecting.

MODELING OF WATER-MAGNESIAN CALCITE SYSTEM AND ITS APPLICATION

Mei Hongming

1996, 2(4):  374-381. 

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This paper shows an elementary attempt to model the water-magnesian calcite system by making use of the up-to-date achievements in the researches on magnesian calcite and water-calcite system. Then, on the basis of this system, a general lake model was developed which includes climatic, hydrologic and hydrochemical conditions. The lake model is applied to the quantitative study on the phenomenon of the alternating carbonated and clastic rich rhythmic deposition in lakes controlled by seasonal climatic change. The evolution process of the lacustrine hydrochemistry producing rhythmic deposition of the above style is demonstrated in this paper. It is verified that the carbonate deposits in the rhythm might be formed in summer of each year.

EXCESS ARGON IN METAMORPHIC AMPHIBOLE BY TECTONIC SHEARING: EVIDENCE FROM Sm-Nd, Rb-Sr AND 40Ar/39Ar DATING ON PLAGIOCLASE AMPHIBOLITES OF THE DANFENG GROUP, NORTH QINLING AREA

Sun Weidong1, Li Shuguang1 2, Chen Wen3, Sun Yong4, Zhang Guowei4,

1996, 2(4):  382-389. 

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The evolution history of the famous E-W-striking Qinling-Dabie orogenic belt which separates the North China Block and Yangtse Block, has been a matter of concern for many years. There are still many disagreements on the formation time of this orogenic belt. The continental subduciton of the Yangtse Block occurred in Triassic time, as shown by the ages of ultrahigh-pressure metamorphic rocks from the Dabie Mountains, while the calc-alkaline magmaticsm terminated in Devonian in North Qinling as shown by U-Pb analyses of zircons from I-type granites which is consistent with the Sm-Nd mineral isochron age of mafic pluton in Heihe, North Qinling belt. One important way to solve this problem is to figure out the uplifting history of the North Qinling Mountains. Since the closure temperature for argon in amphibole has been accurately determined, its 40Ar/39Ar method is widely used in studying the uplifting history of orogenic metamorphic rocks. It is very important to know how and in what kind of condition the excess argon occurred in metamorphic amphibole because that excess argon in amphibole has often been reported. Two metamporphic amphibolte samples of the Danfeng Group in north Qinling area were determined using Sm-Nd, Rb-Sr and 40Ar/39Ar methods. Sm-Nd, Rb-Sr mineral isochron ages of sample QSP-10 and QH-3 are consistent with each other. They are also consistent with other dating results on metamorphic time of the Danfeng Group. In fact 400 Maagrees well as a collision time in Shangdan fault. The 40Ar/39Ar spectrum age (488Ma) of amphibole in QSP-10 sampled from a sheared zone in Puyu is much higher than its Sm-Nd and Rb-Sr mineral isochron ages (414Ma, 410Ma). While the 40Ar/39Ar age of amphibole from QH-3, which was not sheared, is similar to its SM-Nd and Rb-Sr mineral isochron ages. In general, amphibole is an ideal mineral for K-Ar or 40Ar/39Ar dating because of good retention properties for radiogenic argon. The result of their study suggests that amphibole from sheared zone is different from others. One possible reason for this is that shearing leads to fluid activity and hence mobiled argon is easy tobe transferred to metamorphic amphibole from country rocks by fluid. It could be concluded that shearing might play an important role on the formation of excess argon in metamorphic amphibole from tectonic zone. To avoid sampling in shear zone is essential to reduce the possibility of excess argon and to improve the reliability of results from studies on orogenic isochronology with 40Ar/39Ar methods.

INFRARED-STIMULATED LUMINESCENCE DATING OF SEDIMENTS FROM QUATERNARY TECTONIC TERRACES IN YUMEN, GANSU PROVINCE

Chen Jie, Lu Yanchou, Yin Gongming, Liu Aiguo, Hu Biru

1996, 2(4):  390-399. 

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The optically stimulated luminescence (OSL) dating method, a technique closely related to TL, which is developing extensively in recent years, determines the time since the sediment was last exposed to sunlight. A monochromatic light is used in OSL dating, instead of heat, to excite electrons (or holes) from thermally stable light sensitive traps on a geological time scale. The luminescence obtained from feldspar grains when they are stimulated by infrared photons, is termed infrared stimulated luminescence (IRSL). The OSL dating technique offers several major advantages over conventional TL dating. Not only is the residual signal following deposition lower for OSL, but also much improved signal to noise ratios, greater precision and speed of sample measurement, and some degree of mineral selectivity. IRSL dating of sediments from Quaternary tectonic terraces in Yumen, Gansu Province, demonstrates that the formation periods of terraces of Shiyouhe, Baiyanghe and Dahuanggou are at about 90, 70, 44, 29, 20, 13 and 5.6ka B.P, respectively, which shows agreement with field observation.

THE EARLY SILURIAN DELTAIC SEDIMENTS IN YONGSHUN-LONGSHAN AREA, NORTHWESTERN HUAN PROVINCE

Wang Zezhong

1996, 2(4):  400-408. 

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The Yongshun-Longshan area in northwestern Hunan Province was an area of deltaic sediments from Longmaxi Formation of Lower Silurian. The deltaic sedimentary system can be divided into predelta, deltaic front and deltaic plain facies. The predelta was composed of the Longmaxi Formation, in which three microfacies were identified: in-situ shale and mud, turbidites and bottom-current deposits. The later two facies mainly developed in the lower part of the Longmaxi Formation. The deltaic front was represented by the Xiaoheba Formation, in which four facies were recognized: front and sheet, reef, oolite (bioclastic shoal), inter-reef (shoal) mud. The Rongxi Formation was deltaic plain which includes bracnched channels and marsh. The river for building the delta was characterized by low velocity, high discharge, high concentration of fine clay and sand, and associated with reef (shoal) in delta front.

A DISCUSSION ON THE CONTROL OF UNDERLYING SAND BODY OVER SULPHUR CONTENT IN COAL SEAM

Huang Caoming, Zhou Anchao, Wang Liqing

1996, 2(4):  409-420. 

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The paper mainly discusses the relationship between sulphur content in coal seam and the thickness of underlying sand body of 20 coal seams in Huoxi, Xishan, Pingshuo, Dtong, Qinshui coalfields of Henan province. By detailed analysis of superimposed position of isogram of sulphur content in coal seam and isopach of the thickness of underlying sand body, it is revealed that the peak value area of sulphur content is often situated on the areas where underlying sand bodies are thinner or pinch out. In other words, they are arranged in a staggered position. Among the 188 peak value areas of sulphur content from 20 pairs of isogram maps, 8 are located in superimposed position with the peak value areas of underlying sand body in most part (4.26%); 10 are partly located in superimposed position (5.32%); whereas 121 are located on the are a of thinner underlying sand body (64.36%); and 49 are located on the pinch out area of the underlying sand body(26.06). It shows that the numbers of peak value areas of sulphur content, which are located on the thinner and pinch out parts of the underlying sand body make up 90.42%, so the staggered position between these two peak value areas is a general character with a reliability of 90%. Mean while, it is revealed that the isogram lines of the lines of peak value areas of sulphur content and underlying sand body often have concordant outlines, they are often in mosaic or parallel patterns. The control factor of these kinds of phenomena is the difference in the base relief of coal-forming peat swamp determined by underlying sand body. Where the underlying sand body is thicker, the base relief is higher. In the lower area, in comparison with the higher area, commonly existed more underground or surface streams, which carried more fine suspending clays absorbing iron ions and calcium sulface that would be reduced later and transformed to more pyrite. As a result of this process, the coal seam formed in the lower area contains much more slphur than that formed in the higher area. This relationship can be used for prediction and searching for the coal area with lower sculphur content, and predicting the alternate tendency of coal seam’s sulphur content in the process of coalfield exploration,

CUMULATIVE POLARITY BIAS AND ITS APPLICATION TO EVALUTION OF DYNAMO MODELS

Lung Sang CHAN

1996, 2(4):  421-430. 

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The magnetic polarity reversal polarity reversal record is often illustrated by means of a column with black and white intervals depicting the two polarity. The cumulative polarity bias curve is introduced here as an alternative method to demonstrate the record of geomagnetic polarity reversals. This new methods, which is obtained by compounding the length of magnetic polarity bias of the magnetic field over any period of time, as well as the hysteretic properties of the field. It also serves as an efficient means for the evaluation of geodynamo models. An example is presented in this paper to demonstrate how the cumulative polarity bias can be used to evaluate the effects of variations in both the dimensionless parameters and the signal/noise ratio on the polarity bias, length and frequency of polarity zones.

A STUDY ON THE BALANCED CROSS SECTION OF WU WEI BAISN, ANHUI PROVINCE

Hu Dezhao, Xu Mingjie, Huang Zhongjin

1996, 2(4):  431-436. 

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In this paper, based on the representative seismic profile 28, a balanced cross section of Wu Wei basin is made by computer. So the basic style of structure and its evolution model of Wu Wei basin is established. The calculated results show that: the Lun-tou anticline is a multi-bend folding made up of superposed Silurian strata, the shortening of superposed Silurian system is 7.9km, the rate of shortening is 17.3%, the denudation of the fold in late evolutions reaches 2.5km. The tension distance by normal faults situated at the basin boundaries is 5.4km, the subsidence of the basin reaches 3.4km. The study in this paper is in accord with the geological data from neighbouring and deep structures.

FINITE-DIFFERENCE CALCULATIONS OF TRANSIENT TIME DOMAIN ELECTROMAGNETIC FIELD OF A TWO-DIMENSIONAL

Ruan Baiyao1, Xu Shizhe2

1996, 2(4):  437-447. 

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According to the works of Kuo and Cho(1980) and Oriistaglio and Hohmann(1984), a modified finite-difference approach technique was presented in this paper for solving time-stepping Maxwell’s equations of an arbitrary two dimension was reduced to a series of one dimension by the methodology of transferring direction was reduced to a series of one dimension by the methodology of transferring direction, and the previous technique was retained in which the boundary condition of an upward continuation at the earth air interface was applied to avoid finding the solutions of the air layer in the grid. Thus the computer’ memory was saved, calculating method was simplified and calculation can speed up. Solutions of various models showed that this method has great calculation precision.

THE FINITE ELEMENT METHOD FOR MODELING 2-D MT FIELD ON A GEOELECTRICAL MODEL WITH CONTINUOUS VARIATION OF COUDUCTIVITY WITHIN EACH BLOCK

Li Yuguo, Xu Shizhe, Liu Bin, Yu Tao

1996, 2(4):  448-452. 

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The finite element method is used for modeling 2-D MT field on a geoelectrical model with continuous variation of conductivity within each block. In the first part of this paper, the calculating formulae of coefficient matrixes in FEM were given. This second part gives the results of calculation for four models. In the FEM, the bi-linear and bi-guadratic interpolations are used respectively. The results of calculation by the FEM are compared with those by the analytic method or the finite difference method, which coincide well each other. The calculation examples show that this method has merits as high accuracy, simple net division and precise simulation.

THE EXPORATION OF GOB AREA BY USING HIGH-RESOLUTION SEISMIC TECHNOLOGY----- Exemplified by Jiawang mining district

Liu Jiahua1, Pong Xiangfeng1, Sun Zhifeng2

1996, 2(4):  453-457. 

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High-resolution seismic technology can be widely used in engineering geophysical prospecting. The location and size of gob area can be prospected by using high-resolution seismic prospecting, and the reliable data which evaluates the stability of the ground work of the construction can be provided. This paper also discusses the acquisition, processing and interpretation of data from high-resolution seismic technology in both theory analysis and field experiment, and introduces briefly the practical effect in Jiawang mining district. Experimental results show that high-resolution seismic technique can prospect the mining gob in the ground-work of construction.

STUDY OF PRESSURE AND PHASE TRANSITION OF GAS (OIL)-BEARING FLUIDS SYSTEM

Hu Wenxuan1, Fu Qi, Lu Xiancai1, Zhenhao Duan2

1996, 2(4):  458-465. 

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Based on principles of physicochemistry, this paper studies fluid pressure and phase states of gas (oil) bearing fluid systems. The fluid pressure consists of hydrostatic pressure and system pressure. The later is a function of fluid properties (composition, volume and temperature) of the fluid system, and it can be quantitatively calculated by relevant components. According to components and saturation degree of gas and oil, the phase states are classified into eight types. Then, major factors (temperature, pressure and composition) influencing the phase transition are discussed by using equation of state for a typical fluid system. This provides a new approach to quantitative study on properties and states of gas (oil) bearing fluid systems.

CONODONTS OF CAMBRIAN-ORDOVICIAN BOUNDARY IN TANGSHAN-LULONG AREA, HEBEI AND PHYLOGENY OF SOME IMPORTANT SPECIES

Wang Zhihao1, Fang Yiting2

1996, 2(4):  466-474. 

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The present paper restudies the conodonts of Cambrian-Ordovician boundary from Tangshan-Lulong area, Hebei province. An evolutionary sequence of Eoconodontus notchpeakensis through Cordylodus primitivus, C.proavus, C. caboti, C. intermedius to C. angulatus is recognized here. Several proposals for the Cambrian- Ordovician boundary based on the evolution of Cordylodus are also discussed in this paper.