KINETIC ASPECTS OF SECTOR ZONING IN ARSENOPYRITE - A CASE-STUDY

A simple quartz-arsenopyrite vein assemblage formed at temperatures of 280-200-degrees-C occurs in the Madan lead-zinc ore district, Bulgaria. Spear-shaped arsenopyrite is bounded by {101}, {010} and {210}; compositionally it is pure FeAs1-xS1+x of variable x. The BSE image of an oriented (001) section reveals a pronounced sector zoning defined solely by differences in the S/As ratio. The boundary between the (210) and (010) growth sectors reflects the changes of their growth rates V010 and V210 in perfect correlation with zonal S/As oscillations. The V010/V210 ratios determined in 20 zones range from 0.1 to 2.4, and correspond to S variations from 39 to 34 at.% derived from 7 microprobe point analyses. The S content in the (010) sector is on average 1.5 at.% lower than in the (210) one, the difference decreasing from about 2.5 to less than 0.33 at.% with the decreasing V010/V210. The rate/composition relationship obeys the following rules: the higher the S/As the lower the V010/V210, and the slower the (010) growth. The phenomenon is interpreted in terms of crystal growth theory of a solid solution analyzing the arsenopyrite growth history on hand specimen, microscopic and structural scales. Sector zoning in this particular case is explained by processes taking place very close to the growing crystal surface which are superimposed on those produced by larger-scale variations in the hydrothermal environment.