Holocene ochreous lacustrine sediments within the Famatina belt, NW Argentina: A natural case for fossil damming of an acid drainage system.

MAZA, SANTIAGO; COLLO, GILDA; ASITNI, RICARDO A.; NIETO, FERNANDO; NIETO, JOSÉ MIGUEL

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 p. 149 - 165

0895-9811

A 44 m-thick lacustrine succession of silty-clay banded ochres and subordinated sandstones, and conglomerates(known as the Corral Amarillo Formation) is superbly exposed within the Famatina Belt(Central Andes of Argentina) after deep entrenchment by the present-day Amarillo river due to strongrecent uplifting and consequent relative drop in base level. The unusual ochreous-rich succession wasproduced by natural damming (3.48e3.54 14C kyr BP) of an acid drainage system linked to the alterationcap of polymetallic deposits. Facies of silty-clay ochre (wet season) and banded ochre (dry season) fromthe paleolacustrine setting are composed of jarosite þ goethite and goethite respectively. Geochemically,these layers record high concentrations of Fe2O3 (25e55 wt. %) and trace elements (Cu, Zn, Co, As, andMo with mean concentrations of 2759; 2467; 109; 375 and 116 ppm, respectively). Their origin is inferredfrom a comparative analysis with the present-day Amarillo river, which has a pH of w3, (SO4)2 concentrationsof w5000 mg/l, and jarosite as the dominant phase, in the upper catchments. Watersdownstream have pH values of 3e4.5, (SO4)2 concentrations of w3000e480 mg/l, and schwertmanniteas the dominant phase. Thus goethite in the paleolake facies is likely related to schwertmannite transformationby an aging process, whereas jarosite is probably transported from the river but could also beassociated with post-depositional formation regulated by variations in grain size and the pore fluidchemistry. The Corral Amarillo Formation offers a Natural model, which may be employed to infer theeffect on nature of acid drainage of mineralized areas.