1988 Volume 38 Issue 209 Pages 203-214
Pyrite, chalcopyrite and sphalerite are the main constituent sulfides of the ordinary ores of the Besshi-type deposits in the Sambagawa belt. Pyrrhotite and bornite occur in significant amounts in some deposits. The amounts of other sulfides and sulfosalts are extremely small. Minerals uncommon in ordinary massive ores, such as galena, tetrahedritess, etc., are found more frequently in trace amounts in copper-rich ores occurring as offshoots, tongues and veins in and around the main stratified orebodies. These minerals may have been formed during the metamorphic deformation of the deposits and the recrvstallization of constituent common sulfides.
The notable geochemical characteristics of the Besshi-type ores are their high cobalt, low lead and low barium contents. Although most of the available sulfur isotope data of sulfides are only from the Besshi deposit, the spread of sulfur isotope ratios of the Besshi-type deposits is very narrow, ranging in δ34S from + 0.4 to +3.9‰ with a mean of +2.1‰. The Se/S ratios of pyrite also display a narrow distribution around its mean of 1.32×10-4 in six examined deposits.
These mineral and geochemical characteristics of the Besshi-type ores can possibly be ascribed to the chemistry of related basic volcanics. Modern copper-rich sea-floor sulfide deposits are quite similar to the Besshi-type deposits in their chemistry. These two deposits is somewhat different from each other in mineralogy, but sea-floor alteration and hydrothermal replacement in the mound interiors should change the primary mineral assemblages of the sea-floor sulfide deposits toward those of the Besshi-type deposits.
