Molybdenittmineraliseringer i Kleivangranitten, Lyngdal
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Mineralization of molybdenite were discovered in the Kleivan granite, Lyngdal and document that the ore-forming potential of this rare example of a differentiated, A-type, granite. The cryptic zonation up through the igneous stratigraphy was confirmed and is the result of fractional crystallization. A decrease in compatible elements (Sc, Zr, Ba, Ti, Sr, Fe, Zn and Cu) and an increase in incompatible elements (mostly Rb) towards the most evolved parts of the intrusion are advantageous for the genesis of an H2O phase enriched in Mo. Field observations of molybdenite in aplite and pegmatite in the cupola of the Kleivan granite, together with veins and veinlets of quartz imply that magmatic-hydrothermal ore forming processes were active. A high abundance of primary fluid inclusions in magmatic quartz in the upper part of the intrusion show that igneous aqueous fluids in deed were present during solidification of this part of the intrusion. However, regular hydrothermal stockwork mineralizations are not exposed. They may be present somewhere in the roof section of the hostrock that is covered by quarternary sediments or, simply they never formed. Mineralization of molybdenite in aplites and pegmatite`s formed during terminal of the Kleivan intrusion supports that the conditions for molybdenite formation were present (fO2, S and decreasing T). Calculations of the size of a possible Mo deposit with different initial Mo (1, 15, 29 ppm) in the parent melt at different thicknesses (1.5, 3, 6, 12 km) of the intrusion gives reasonable sizes of a deposit. Mo will apparently not be a limiting factor in the ore forming process. The amount of exsolved H2O in the Kleivan granite is unknown and is crucial in the control of the ore forming process. The discovery of spinel ([Zn, Fe]Al2O4) and chrysoberyll (BeAl2O4) in the apex of the intrusion, together with the occurrence of sillimanite and garnet makes a complex petrography and the Al rich conditions here. The contents of Mo in rocks from the early crystallization of the intrusion are surprisingly low (<1 ppm). In aplite and pegmatite in the upper part of the intrusion, the content of Mo is considerably higher (from 10 to 600 ppm). The theory of a hydrothermal fluid scavenging the melt for Mo is supported by these findings. Finding a magmatic hydrothermal mineralization of molybdenite associated with the Kleivangranite is highly possible. The potential for further exploration in the Kleivan granite in the search for an interesting molybdenite deposit is present.