Gold in intrusive-rock biotite.

Au distribution in biotite from intrusive and metamorphic rocks is discussed. Biotites have high Au contents relative to other rock-forming minerals: maximal mean contents occur for biotite in gabbroic rocks (14.5 X 10^-7%), while minimal levels occur in granitic rocks (5.8 X 10^-7%) in Palaeozoic folded regions. In all folded regions there are two distinct groups of granitic rocks: with low and elevated levels in the biotite, (0.7-1.9) X 10^{- 7}% and (12.5-84.4) X 10^-7%, respectively. The biotite of autometasomatically altered granitic rocks has Au content reduced by a factor of 1.5-6, while biotite from altered granites around ores has Au raised by a factor of 2-3 relative to the biotite in the initial rocks. Estimates are made of the mean Au contents in biotites from various types of intrusive rocks.-T.R.