MINERALOGY, SULFUR ISOTOPE GEOCHEMISTRY AND THE DEVELOPMENT OF SULFIDE STRUCTURES AT THE BROKEN-SPUR HYDROTHERMAL VENT SITE, 29-DEGREES-10'N, MID-ATLANTIC RIDGE

A large collection of hydrothermal sulphides from the Broken Spur hydr othermal vent site, including representative samples of mound sulphide materials, has been characterized using optical mineralogy and sulphu r isotope analysis. Young mound sulphides from Broken Spur have a pyrr hotite-dominated mineralogy unusual for bare ridge vent systems. Howev er, pyrrhotite is metastable and is ultimately converted to iron disul phides. Mature sulphides are indurated, recrystallized and contain abu ndant quartz. Sulphide mound materials are developed by three major pr ocesses: (i) coalescing of chimney structures; (ii) accumulation of ta lus from mass wasting and (iii) precipitation and growth in response t o hydrothermal flow. Progressive maturation of mound materials is by m odification of primary textures, development of mineralogical zoning a nd replacement of metastable phases. Sulphur isotope analysis of 35 mi neral separates returned delta(34)S values of -0.5 to +3.2 parts per t housand. These values are similar to those previously measured for Bro ken Spur and Snakepit, but are distinctly S-32 enriched compared to th e TAG active mound and some Pacific sites. Seawater entrainment and su lphate reduction within the subsurface feeder zone below Broken Spur m ounds do not appear to be important processes at Broken Spur, in contr ast to the TAG active mound.