1977 Volume 27 Issue 146 Pages 355-365
The contact metamorphism was superimposed on the regional glaucophanitic one in the deeper levels of the Besshi deposit. The mineral assemblages change progressively with increasing depth by this event. Sulfide mineral assemblages were studied on regional and contact metamorphosed ores of the deposit.
Pyrite, chalcopyrite and sphalerite assemblage is the majority of ores suffered the regional metamorphism. Pyrite-chalcopyrite-bornite and pyrite-hematite-magnetite assemblages, which constitute univariant equilibria in Cu-Fe-S and Fe-S-O systems, are sometimes observed in these ores. T and fS 2 of the metamorphism were discussed from the mutual relation of these univariant equilibria. T and fS 2 were also obtained at high pressures from FeS contents of sphalerite (0.48-0.90 mole %) associated with pyrite-chalcopyrite-bornite assemblage. They are 280°-350°C and 10 -8.7 -10 -6.4 atm, supposing 5 Kb lithostatic pressure.
By contact metamorphism, pyrrhotite is formed from pyrite in the levels deeper than 18 L. Fe contents of pyrrhotite increase with increasing depth, from monoclinic modification containing 46.7 atomic % Fe and hexagonal one with 47.3 atomic % Fe in 18-25 L to 47.5-48.0 atomic % Fe in 26-33 L. FeS contents of sphalerite associated with pyrrhotite are in the range of 16 to 20 mole %, which does not correspond to pyrrhotite with 47.5-48.0 atomic % Fe. Hexagonal pyrrhotite coexists commonly with pyrite, which shows sometimes euhedral shape. This association is incompatible in the light of the low temperature phase relation of Fe-S system. hitherto established. Pyrrhotite alone may have changed its composition during the retrogressive metamorphism, reflecting the T-fS 2 environments. Sphalerite and pyrite, on the contrary, may preserve the high temperature states. It is therefore difficult to obtain metamorphic T and fS 2 from the compositions of coexisting sphalerite and pyrrhotite, unless detailed study is made on the compositional change of these minerals in the retrogressive metamorphism. The association of hexagonal pyrrhotite and euhedral pyrite may be showing that pyrrhotite of high temperature type and pyrite were once in equilibrium at the climax of the metamorphism.
Sphalerite has no exsolution dots in the contact metamorphosed ores. The absence of exsolution dots will be due to the low fS 2 environments in the retrogressive metamorphism, corresponding to pyrrhotite with 47.5-48.0 atomic % Fe.
