Fluid inclusion studies in fluorite from the Askrigg area of north-west Yorkshire.

A study of fluid inclusions in fluorite from the epigenetic mineralization in lower Carboniferous strata of the Askrigg block in the Northern Pennine Orefield has been made. The purpose of the study was to ascertain the nature, origin and significance of these inclusions and to obtain information concerning the temperature, salinity and chemistry of the environment of crystallization of these ore deposits. The information subsequently obtained oan be used to place constraints on postulated theories of genesis of these deposits. In Chaper 1 a short introduction to the study of fluid inclusions in minerals is given. The suitability for inclusion study of minerals from the Askrigg area is discussed in Chapter 2. A method for the preparation of polished samples is also described together with criteria for the recognition of inclusion types. The importance of the recognition of post-formational changes in inclusions such as leakage and necking-down is also stressed. Chapter 3 contains a detailed geological outline of the ore deposits and a brief review of ideas regarding their genesis. Detailed homogenization studios have been performed on fluorite from numerous localities. The data obtained is then used to determine the minimum temperature of formation of the fluorites. The statistical evaluation of the significance of these results is also described together with details of the positive pressure correction applied to the results. The distribution of temperature in the orefield is described. A detailed account of a freezing stage is presented in Chapter 4. The behaviour of fluid inclusions in fluorite on freezing is outlined together with detailed salinity measurements from various samples. The results obtained are then discussed in the light of other inclusion studies from similar deposits. Chapter 5 contains a detailed appraisal of the analytical methods used to study the chemistry of fluid inclusions. The methods used to analyze leached portions of this inclusion fluid for sodium, potassium, lithium, magnesium, calcium, chloride, and sulphur are described. The chemistry of the ore-forming fluid in the light of these results plus other electronmicroprobe and atomic absorption studies is then discussed. The theories regarding the mode of transport of metals in the inclusion are reviewed in the light of fluid inclusion evidence. The genesis of the ore deposits of the Mississippi Valley type in the light of fluid inclusion studies is discussed in Chapter 6. Comparison is made between the Askrigg and other deposits of similar type and a possible genetic model is postulated for the Askrigg block mineralization.