Geology, mineralogy, and geochemistry of the Zarloukh Bentonite -Tuff deposit, Hemrin South Mountain, northern Iraq: implications for genesis and geotectonics

Authors: YAWOOZ KETTANAH

Abstract: The Quaternary Zarloukh Bentonite -Tuff (ZBT) deposit occurs within the Hemrin South Mountain, northern Iraq. The ZBT deposit occurs as depression-filling exposed on the erosional surface of the siliciclastic Pliocene Muqdadiya Formation and covered by an overburden of recent sediments. The thickness of the studied industrial bentonite bed is ~80-100 cm, occurring at the bottom of these depressions, covered by ~3-4 m thick bedded volcanic tuff, which also contains many 10-12 cm thick bentonite layers along its bedding planes. The volcanic ash at the bottom of lakes/swamps with shallow water content acted as basins for the deposition of falling volcanic ash, which immersed in water and devitrified to bentonite at a later stage by hydration and chemical interactions, meanwhile the continued fallen ash consolidated as tuff beds protecting the bentonite formed at the bottom of the depressions. The bentonite bed shows mini-scale trough crossbedding as a sign for its formation within a low energy, shallow agitated water in lakes/swamps. The bentonite and its precursor tuff show some differences in the concentration of Ca, Mg, Na, and K representing the exchangeable elements in smectite (montmorillonite), which is the predominant clay mineral in bentonite because of the probable gain of bentonite for these elements during the process of bentonitization of volcanic ash, which also formed the tuff. The ZBT has most probably the same origin as the Hemrin Basalt located NW of ZBT deposit. The chondrite-normalized REEs distribution pattern of ZBT and the Hemrin Basalt is similar, both showing enrichment and negative slope for the LREEs relative to the flat-lying HREEs. The Th vs. Co and the Th/ Yb vs. Ta/Yb diagrams indicated that the ZBT and the Hemrin Basalt fall within the field of high-K calc-alkaline basalt and shoshonite and the andesitic basalt-andesite rocks reflecting their common origin.

Keywords: Zarloukh deposit, bentonite, tuff, Hemrin basalt, montmorillonite, Iraq

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