Bjerkreim- Sokndal
About this presentation
Rogaland
County
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| Henrik Schiellerup | Christian Tegner |
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| Gurli Meyer | Are Korneliussen |
Henrik Schiellerup
Christian Tegner
Gurli B. Meyer
Are Korneliussen
Brian Robins
Agnes Raaness
The rationale of the work was to located, characterise and estimate the volumes of the combined recourses of apatite, ilmenite and magnetite. The reason for not dealing with the Sokndal and Mydland lobes (see map below) was that these areas were partly locked by claims during the course of this work. These claims expired in 2005. Vegetation cover is also more extensive in these two areas making hand specimen sampling difficult.
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| The Bjerkreim, Mydland and Sokndal lobes. |
The occurrences of geological units containing apatite, low Mg-ilmenite and vanadium rich magnetite were well established as a result of earlier work in the Bjerkreim-Sokndal layered intrusion. However, determination of precise mineral compositions and their concentrations needed more detailed work in order to delimit zones of economic interest. Our work focused on sampling and describing profiles in relatively well-exposed outcrops along roads and the shores of lakes. Samples were collected using a hammer on natural outcrops or fresh road cuts. A few profiles were located in fields and hilly wood terrain where the outcrops were restricted and the rocks influenced by surface weathering. These areas were included to provide a better general picture of the mineral distribution.
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Sampling on natural outcrop in the fields at Åsen farm. Stratigraphic up is to the right.
Photo: H. Schiellerup, NGU |
A question that arises when working on layered intrusions is whether the sampling is representative of the average rock composition. It is important to bear in mind when reading this presentation that hand specimens from natural outcrops will never give an adequate picture of the average rock composition. Shallow core drilling was performed at two localities in order to determine how well the hand specimen sampling represents the “real” picture. We refer to the section dealing with future perspectives for a comparison of the two methods.
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Modally graded layering in gabbronorite at Teksestemmen. Stratigraphic up is to the left.
Note pencil at lower right corner for scale. The thin light-coloured lens-shaped body just above the pencil is a xenolith of country-rock gneissic rocks. There are several other smaller lenses of this type visible in the outcrop. Photo: G. Meyer, 2002, NGU |
In the zones of interest modal layering is generally on a scale of 5-10 centimetres, but locally less or more (up to 0.5-1 meter or non layered sequences of several metres). Both modally graded layering, isomodal 'average' layers and alternating isomodal felsic and mafic layers occur. An attempt has been made to sample pairs of mafic and felsic layers adjacent to each other. In outcrops where this was not possible it was attempted to sample 'average' layers, in other words, layers that visually seemed most representative of the local sampling point. We also attempted to maintain a uniform spacing between individual samples of 10-15 m. However this strategy only turned out to be realistic only along lakeshores and road cuts. In areas covered by Quaternary sediments and/or dense vegetation the spacing between samples is irregular.
Alteration skin was removed from samples either in situ or later by sawing. Petrographic studies and electron probe micro-analyses were performed on polished thin sections. Whole rock geochemical analyses were performed by XRF analyses. The normative composition was calculated from the XRF analyses.