TEXTURAL AND CHEMICAL EVIDENCE FOR MELTING IN SPHALERITE-RICH ORE FROM BROKEN HILL

Textural and microprobe studies of sphalerite-rich ore from the Western A lode at Broken Hill provide more evidence that the Broken Hill ore body melted. The sample studied contains 74% sphalerite, 16% gangue (quartz, spessertine, and rhodonite), 8% galena, 2% pyrrhotite and 1% chalcopyrite. Galena has cuspate margins against sphalerite whereas chalcopyrite and pyrrhotite form isolated lensoid grains decorating the sphalerite-sphalerite grain boundaries. Grain boundary angles between sphalerite-sphalerite pairs and galena, pyrrhotite, and chalcopyrite range from 10° to 140°. For each of these phases the measured angle distribution extends well below the population of grain boundary angles from ores that have recrystallized in the solid state. Furthermore 48% of the galena-sphalerite-sphalerite angles, 53% of the pyrrhotite-sphalerite-sphalerite angles, and 73% of the chalcopyrite-sphalerite-sphalerite angles are lower than 60°, the angle below which one phase wets the grain boundaries of the other. This strongly suggest that chalcopyrite, pyrrhotite and galena crystallized out of a sulfide melt that wetted the sphalerite-sphalerite grain boundaries. The wide distribution of angles suggests that reequilibration ceased before the minimum grain boundary energy was attained.

X-ray maps of the sphalerite, the most refractory of the sulfides show subtle depletion of Fe toward the margins, even in grains completely encased in galena. These relations are consistent with melting relations in the PbS-FeS-ZnS system. At the temperature of peak metamorphism, the Broken Hill orebody should have consisted of a sphalerite-rich restite and a Pb -rich melt that contained most of the Fe, and Cu in the ore body. Crystallization of pyrrhotite+galena in this system will enrich the melt (and pyrrhotite) in Fe, thus explaining the weak Fe zoning in the sphalerite.