Statystyczne badania cech jakościowych okruszcowania skał rejonu izerskiego

Andrzej J. Krawczyk, Ksenia Mochnacka


Statistical studies of qualities of mineralizes rocks of the Izera Region (Sudety Mts)

The presented paper aims at using statistical methods to define the dependence between mineralogical and petrographical characteristic of rocks and occurrence of radiometric anomalies. The area of occurrence of the uranium mineralization, on which the statistical studies were carried out, lies in the Izera Highlands. The zone, described above, is situated in its central part, to the south of the schist belt, surrounded by the Izera gneisses. In the area under examination there occur: gneises, Izera granites (granite-gneisses), mica schists, amphibolites and dyke-rocks (K. Smulikowski , 1958, M. Kozłowska-Ko ch, 1965, J. Pawłowska, 1967, 1969, J. Oberc, 1967, J. M. Szałamacha, 1968). These series occur accordingly and they dip N at an angle of about 70—80° (the strike of the foliation area is, approximately, E-W). The rock series are intersected by a fault of a NW-SE direction; its face dips abruptly towards NE. Two dislocations, the northern and the southern one, run parallel to the main fault. The presence of many secondary displacements, important for the uranium ore-emplacement, has also been found there. An area of about 1,5 km2, with 41 bore-holes in it, was under precise examination. In the described rocks the following uranium minerals have been found: autunite, metaautunite, metatorbenite, uranophane and gummite (M. Banaś 1969). The following rocks have been distinguished macroscopically: dark-grey Izera granites (granite-gneisses), of a medium-or coarse-crystalline texture and non-directional structure with considerable amount of biotite; light granites which resemble the former, yet differ from them in having a considerable smaller amount of biotite containing pink feldspars; white leucogranites of a coarse-or medium-crystalline texture; extremely fine-crystalline rocks of mineral structure, resembling the formerly described ones, which are macroscopically similar to aplites; gneisses which resemble the granites but have a distinct gneissose structure; pegmatites of an extremely coarse-crystalline texture; silver-grey mica schists of a lepidoblastic texture and schistose structure, often rusty-coloured; amphibolites; breccias which are usually firmly cemented rock fragments, often with an indistinct outline. In the descriptions of cores, which had been made by the geological staff, rocks described as lamprophyres were also found. The described rocks often have dark-violet inclusions of fluorite and tourmaline. In certain zones the rocks are strongly fissured and ferruginized. In these areas and sometimes elsewhere the occurrences of pale-green or yellow uranium micas have been found. Accumulations of uranium minerals have often been found in the areas of considerable radiometric anomalies and also independently of them. The examination of cores has not given an explicit answer to the problem of the occurrence of anomalies in rock areas, as well as to the problem of existence of a more privileged pétrographie variety which is connnected with the anomalies. It also concerns the problem of accumulations of uranium minerals. Even a rough estimation of geological structure of the area and of the character of mineralization allows an assumption that tectonics in the main agent which is necessary for the ore emplacement. It can also be assumed that certain petrographie varieties may be reducing agents of uranium, as they constitute the environment permitting ore accumulation. It is not always clear what part the ferruginized zones play in the process of accumulation of uranium minerals and in occurrence of anomalies. Studies of the following types of dependences have been carried out: the petrographie type — the radiometric anomaly; the petrographie type — the fissuring; the pétrographie type — ferruginization; the pétrographie type — uranium mineralization; the anomaly — fissuring; the anomaly — ferruginization; the anomaly — uranium mineralization; the fissuring — ferruginization; the fissuring — uranium mineralization; the ferruginization — uranium mineralization. The material from 41 exploratory borings has been analysed statistically. The authors reduced the descriptions of cores made by the geological staff to 6 basic lithological subdivisions: granite, leucogranite, amphibolite (together with amphibole schists and amphibole-chlorite schists), mica schist, lamprophyre and breccia. In this way all the cores have been divided into sections which have the characteristics of a definite lithological formation. Each section bears the following characteristics: the maximum value of radiometric anomaly occurring within this section, and the presence (or absence) o fissuring, ferruginization and uranium mineralization. While describing the value of radiometric anomaly, the authors accepted a value of 100[mikro]R as a reference level; all anomaly values smaller than this one were regarded as absence of anomaly (i.e. zero anomaly). On the whole, 504 samples were obtained from the examined borings: 217 granite samples, 197 — leucogranites, 39 — amphibolites, 21 — mica schists, 16 — lamprophyres and 14 — breccias; at the same time each sample was characterized by the six parameters, mentioned above. In the examined of characteristics there is only one quantity characteristic (the value of radiometric anomaly); the remaining ones are quality characteristics. Three of them (fissuring, ferruginization and uranium mineralization) have been regarded as dichotomic characteristics. In this situation the statistical studies of the dependence of characteristics resolve themselves into the analysis of contingency tables. In this case the x2 independence test is a basic method of testing the He hypothesis on the lack of correlation between characteristics, where the value of x% was estimated according to the formula (1), in which e are the expected frequencies of individual cells of the contingency table (where the characteristics are independent). If any of the expected frequencies e is smaller than 5, the test should noit, be applied. To extend the range of its applicability in case of the 2x2 table, the so-called Yates’s correction has been used (F. Yates , 1934). The Fisher’s test was also used for contingency tables 2x2 and 2x3. The following dependences between particular characteristics of rocks have been obtained: 1. A strong relation between the petrographie type of rocks and the values of radiometric anomalies has been found. Lamprophyres are the rocks which distinctly influence the origin of this dependence. 2. An essential correlation between the type of rocks and the fissuring has been displayed. Micas are especially poorly fissured and amphibolites still less than the former, while granite-gneisses display an outstanding fissuring. 3. Granites and breccias are most strongly ferruginized, while leucogranites and mica schists display poor ferruginization. 4. There is a relation between the occurence of uranium mineralization and the petrographie development of rocks. Especially in granites, the intensity of abundance of uranium minerals has been greater than expected. 5. A relation between the occurrence of radiometric anomalies and the fissuring has been found. This dependence is clearly visible in granites and leucogranites. 6. A dependence between the occurrence of radiometric anomalies and ferruginization has been found. A distinct dependence occurs in leucogranites. 7. There is no correlation between the occurrence of uranium mineralization and radiometric anomalies. 8. There is a distinct dependence between the fissuring and ferruginization within granites and leucogranites; it has not been found in amphibolites and lamprophyres. 9. A relation between the occurrence of rock fissurings and their uranium ferruginization has been discovered. This dependence is quite distinct in granites, while there is no sign of it in breccias. 10. The strongest relation between the occurrence of ferruginized zones and the accumulations of uranium minerals has been found in leucogranites, a weaker one occurs in granites, and there is nome in breccias. The statistical studies of the material from bore-holes seem to confirm a number of regularities which have been found in regions where mineralization of rocks is richer. Besides, certain seemingly unnoticable dependences, which may become prospecting indices, have been discovered.

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