Intrasedimentary gypsum deposits of El Siasgo Creek, Monte and General Paz districts, Buenos Aires Provinc

Abstract

The El Siasgo Creek is a tributary stream of the Salado River system, located in the north-east part of Buenos Aires province. In its lower basin, the river cuts a great depression, a remnant of an old infilled deflation basin, in which mainly aquatic sedimentary environments have operated from the Upper Pleistocene. At present, this depression represents a 2,750 ha lentic temporary environment, divided in two parts by a 2 km long, 0.7 km wide narrowing of the water basin, to the north, the minor water body named "La Cañada" and to the south the bigger one, "laguna del Siasgo". Important accumulation of intrasedimentary mainly sand-side gypsum are shown in the clastic host sediments of this palaeolentic environment The gypsum crystallized at different stratigraphic levels and with a diversity of habits, textures, fabrics and structures, The host sediment is essentially mud, but the original texture has been modified by this epigenic mineral into sandy muds, sandy silts and muddy sands. The gypsum deposit covers an area of 25,6 km², with 2.15 m mean thickness in "laguna del Siasgo" and 0.91 m in "La Cañada". The total volume of gypsum sediments reaches 48 hm³ with a minimum grade of 53,5% and 53,623,000 tons of impure gypsum , in the bigger water body, whereas in the minor one the mean grade is 20,1% which represents only 1,361,000 tons of impure gypsum. The low gypsum content in "La Cañada" body precludes the posibilities of economic exploitation. However, the "laguna del Siasgo" body has high ecomonic potential, even at its lower mineral content. The genesis of these gypsums are linked to the interaction between surficial and ground waters, during the greater aridity periods of regional eolian episodes (early and middle Holocene). Isotopic determinations of d³⁴S in these gypsums indicate that the sulphates were precipitated in a closed basin (continental sabkha).