Mineral paragenesis, a key in determination of physico-chemical conditions of skarnification, a case study, Mohammad-Abad district, SW Yazd Province

Mineralogical and geochemical investigations indicate two general stages of skarnification i.e, prograde and retrograde stages in the Mohammad-Abad district, south west of Yazd province. Metasomatic skarn (Prograde stage) occurred with precipitation of anhydrous calc-silicate mineral assemblages (andradite and diopside- hedenbergite) within the temperature ranges of 470 to 550 ºC. During the retrograde stage (<470 ºC) considerable amounts of anhydrous calc-silicates were altered and replaced by a series of hydrous calc-silicates (epidote, tremolite–actinolite). Finally, both anhydrous and hydrous calc-silicates were altered to fine aggregates of chlorite, calcite, quartz and clay minerals at temperatures lower than 300 ºC. The absence of wollastonite among the calc-silicate assemblages may suggest that andradite and hedenbergite were crystallized from metasomatizing fluids at lower than 550 ºC in the Mohammad-Abad skarn. The presence of intergrowth texture and non-replacive crystal boundaries between garnets and pyroxenes suggest that they were formed contemporaneously within the temperature and fO2 ranges of 430–550 ºC and 10-26–10-23, respectively. Andradite converted to quartz, calcite and magnetite below 470 ºC and fO2 = 10-24–10-21. Since, both magnetite and pyrite along with quartz and calcite are present in mineral assemblages of sub-stage III, it can be reasoned that the metasomatizing fluids probably had an approximate fS2 10-6.5 and a temperature of about 430 ºC.