Geochemistry of Calcium Sulphate Rocks of Fat`ha Formation atFour Localities within Nineveh District, Northern Iraq(with emphasis on strontium distribution)

Abstract

Strontium distribution within (90) calcium sulphate samples (20 anhydrite, 25 massive gypsum, 25 selenite and 20 fibrous gypsum) from Fat'ha Formation (Middle Miocene) at four selected localities in northern Iraq were investigated. The study revealed that there was a loss of (182 ppm, 15.50%) strontium during the hydration of anhydrite to gypsum. This loss is attributed to the incapability of monoclinic structure of gypsum to hold all the released strontium from anhydrite. Massive gypsum contains almost twice and (3.46) times strontium than selenite and fibrous gypsum respectively. These differences are related to the different mode of formation of each type. There is a negative correlation between crystal size of gypsum and strontium content. The relationship is linear on a logarithmic scale. Estimated partition coefficients of strontium (KSr G) for the three typs of gypsum are all equal to (0.20) at an estimated temperature and brine concentration of (32ºC) and (X3.4) respectively. Estimated partition coefficient for anhydrite is (0.4) at an estimated temperature and brine concentration of (35ºC) and (X7.5) respectively. Multivariate discriminant analyses proved that strontium is the most discriminating element between (anhydrite and gypsum) and between (massive gypsum, selenite and fibrous gypsum).