Occurrence of zinc in granitic biotites

37 granitic plutons in Great Britain and the United States were sampled and biotite separates prepared. These biotites were analysed for zinc and iron and a modal (point-count) analysis was conducted on the granitic samples. The results of these analyses were examined for differences that would characterise mineralised and non-mineralised granites, and for differences between biotites coexisting with muscovite and those coexisting with hornblende. The possibility of differentiating between mineralised and non-mineralised granites on the basis of the zinc content of biotites is rejected, but significant differences in composition are found between biotites from muscovite-biotite-granites and those from hornblende-biotite-granites. Muscovite-bearing granites have low biotite contents, these biotites having high iron and low zinc concentrations; hornblende-bearing granites have high biotite contents but these biotites, in contrast, have comparitively low iron and high zinc concentrations.

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Zusammenfassung Biotitkonzentrate aus 37 granitischen Plutonen aus Großbritannien und den U.S.A. wurden auf ihre Zn- und Fe-Gehalte analysiert. Granite, mit denen Zn-Vererzungen zusammenhängen, können nach diesen Untersuchungen nicht aufgrund der Zn-Gehalte ihrer Biotite von Graniten ohne Zn-Vererzungen unterschieden werden. Die Biotite aus muskovit- und aus hornblendeführenden Graniten unterscheiden sich hingegen: Erstere haben hohe Fe- und niedrige Zn-Konzentrationen bei geringem Biotitanteil am Gestein; bei letzteren ist es umgekehrt.