Mineralogical and Geochemical Study of the Subsurface Acidification Products Ninivite, Alunite and Jarosite, Northern Iraq

Abstract

Petrographic, mineralogical and chemical analyses of marl were investigated to verify the formation processes of many alteration products under low temperature -pressure conditions, where an ample source of sulphate exists. Marl (calcareous claystone) comprising about one third of the lithological constituents of the Middle Miocene Fat'ha Formation (previously named Lower Fars Formation) in northern Iraq in addition to evaporites and limestones. The extensive reduction of its sulphate members produces economically significant deposits of sulphur as well as sulphate and salt. Oxidation of H 2S produces sulphuric acid due to the effect of Thiobacillus bacteria. The interaction of H2SO4 with marl, leaching of K, Na, Al, Fe and Ca leaves behind a porous silica -rich (ninivite) center of the zoning pattern due to the alteration- precipitation processes, followed outwards by alunite, which is, in turn, surrounded by a narrow zone of jarosite and finally iron oxides in the rim. Secondary gypsum (selenite) is precipitated in shrinkage cracks as veins or as.floating crystals in the ninivite.