Iron terraces in acid mine drainage systems: A discussion about the organic and inorganic factors involved in their formation through observations from the Tintillo acidic river (Riotinto mine, Huelva, Spain) Javier Sánchez España* Esther Santofi mia Pastor Enrique López Pamo Unidad de Recursos Minerales y Geoambiente, Instituto Geológico y Minero de España (IGME), Rios Rosas, 23, 28003, Madrid, Spain ABSTRACT of TIFs, whereas abiotic parameters, such as and internal structure of such iron terraces, via water composition, fl ow rate and velocity, or extracellular and intracellular assimilation of Iron terraces that form in acidic mine stream channel geometry, also appear to be iron and other metals. Further, TIFs have been drainage settings are unique and extreme essential variables. proposed as modern analogs for ancient (Pre- geomicrobiological systems that can provide cambrian) banded iron formations (BIF) of the highly relevant information about the inter- Keywords: acid mine waters, acidophilic mi- geological record (Hasiotis et al., 2001; Brake action between microbes and their surround- crobes, iron, terraced formations, Tintillo River, et al., 2002), as well as for the Proterozoic and ing aqueous environments. These singular Riotinto. present-day stromatolite-building colonies of systems can represent, additionally, poten- cyanobacteria (Leblanc et al., 1996; Brake et al., tial models for the study of ancient geologi- INTRODUCTION AND SCOPES 2002, 2004), and they have also been considered cal formations (e.g., banded iron formations, by other authors as terrestrial equivalents of the stromatolites) and/or for the cycling of iron The presence of ferruginous terraces of milli- iron oxide deposits recently discovered on Mars on Mars. This work describes geochemical, metric to metric scale is probably the most strik- (Fernández-Remolar et al., 2004). However, mineralogical, morphological, and micro- ing feature observed in acid mine drainage set- the relative importance of the microbial activ- biological evidence obtained in the highly tings worldwide. These terraced iron formations ity with respect to inorganic processes, such as acidic and Fe-rich Tintillo River (Riotinto (TIFs) are usually developed during the oxida- water composition, Fe(III) precipitation rate, mines, Huelva, SW Spain), which can be tion and hydrolysis/precipitation of dissolved stream fl ow velocity, or channel geometry, has used to speculate about the origin and nature iron in the acidic solutions after they emerge not yet been evaluated. A further and critical of the terraced iron formations (TIFs) that from waste piles, tailings, or mine portals, and question arises about whether the formation of are being currently formed in acid mine they display a morphological pattern similar to these ocherous terraces is determined by param- drainage environments. The size (up to 36 m that observed in calcareous travertines formed eters such as stream channel geometry or fl ow 2+ – long and 1 m thick) and continuity (strong in Ca -HCO3 –rich spring waters. TIFs differ velocity, or if, on the other hand, they control the development over 3.5 km) of the iron ter- from calcareous travertines, however, in their evolution of such parameters. races offer a unique opportunity to study mineralogical composition, which is character- The Tintillo River (Huelva, SW Spain) is the different organic (mainly microbial) and ized by hydrous iron (oxy)hydroxides and/or probably a world-class example of an acid mine inorganic processes involved in the construc- hydroxysulfates (Sánchez-España et al., 2005a, drainage–impacted stream in the sense that it tion of these characteristic, travertine-like, 2005b, 2005c), in agreement with the typical is almost entirely formed by highly acidic and 2– sedimentary structures. Evidence presented Fe(II)/Fe(III)-SO4 chemical composition of metal-rich acid mine drainage solutions ema- in this study suggests that both types of pro- most acid mine drainage solutions (Nordstrom nating from waste piles located near the Corta cesses appear to be controlling factors in the and Alpers, 1999). Atalaya open pit (Riotinto Mines; Fig. 1). This formation and internal arrangement of the TIFs have been, until now, the subject of stream course shows chemical, physical, and TIFs, although no defi nitive evidence has little scientifi c attention. The most remarkable microbiological features that seem to favor the been found to support the prevalence of any studies available in the literature have been development of these travertine-like, ferrugi- of these mechanisms with respect to another. focused on the acid mine drainage systems of nous deposits (Sánchez-España et al., 2005a, The photosynthetic production of dissolved the Carnoulés Pb-Zn mine, France (Leblanc et 2005c). For example, it has: oxygen by eukaryotic microorganisms (green al., 1996; Casiot et al., 2004) and the Green Val- (1) initially near-anoxic water with a high algae, euglenophytes, and diatoms) and the ley coal mine, Indiana, USA (Brake et al., 2001, concentration of dissolved ferrous iron (on the Fe-oxidizing metabolism of acidophilic pro- 2002, 2004; Hasiotis et al., 2001). From this order of 2 g/L Fe[II]) at the source point; karyotes are critical factors for the formation research, it is apparent that microbes (including (2) an initially turbulent fl ow near the source acidophilic bacteria and eukaryotic microor- point (downslope of the waste pile) with succes- *[email protected]. ganisms) play a critical role in the construction sive, centimeter-scale water falls that promote Geosphere; June 2007; v. 3; no. 3; p. 133–151; doi: 10.1130/GES00069.1; 13 fi gures; 2 tables. For permission to copy, contact [email protected] 133 © 2007 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/3/133/865180/i1553-040X-3-3-133.pdf by guest on 28 September 2021 Sánchez España et al. Figure 1. Confi guration of the Tintillo acidic river (in red) and its main tributaries (in blue). For simplifi cation, only spring a is shown in the source area. The locations of water and terraced iron formation (TIF) samples are also indicated. water oxygenation and enhance the oxidation work emphasizes the organic versus inorganic Huelva province (Fig. 1). This river is mainly of Fe(II); origin of these structures and presents results fed by leachates of acid mine drainage ema- (3) abundant mat-forming, benthic communi- that include: (1) the chemical composition of nating from the base of large, sulfi de-bearing, ties of acidophilic, Fe-oxidizing bacteria, which the stream waters and its downstream evolution, waste-rock piles and tailings impoundments have colonized the stream substrate and appear (2) the major morphological, mineralogical, and situated in the surroundings of Corta Atalaya, a to enhance the oxidation of Fe(II); chemical characteristics of the TIFs, and (3) the vast open pit exploited from 1907 to 1991 by (4) a considerable initial fl ow rate of between spatial (downward) evolution of living benthic the company Minas de Riotinto. At the head- 15 and 30 L/s; and microbes (algal and bacterial colonies) and their waters of the river, there are a number of small (5) an initial pH around 2.6–2.8, which permits relation to the aqueous chemistry and TIF devel- acid mine drainage springs (named as spring a, the hydrolysis/precipitation of aqueous Fe(III). opment. Finally, we hypothesize about the origin spring b, etc.), which have variable fl ow rates on As a result of such favorable geochemical, of TIFs by comparisons with similar acid mine the order of a few liters per second and which hydrodynamic, and microbiological condi- drainage systems, as well as calcareous traver- fi nally converge in the T-1 sampling point (Sán- tions, spectacular terraces of decametric scale tines formed in carbonate-rich environments. chez-España et al., 2005c). Subsequently, the have formed along the fi rst 3.5 km of the river Tintillo River meets some tributaries (the Gan- (Fig. 2). ENVIRONMENTAL SETTING gosa and Escorial creeks; Fig. 1), which are also The present study is aimed at providing fur- acidic and show comparable fl ow rates but lesser ther insight into the development of TIFs in acid Location and Hydrological Confi guration sulfate and metallic content. Finally, the Tintillo mine drainage systems through preliminary of the Tintillo Acidic River River converges with the Odiel River, causing hydrogeochemical, morphological, mineral- a strong environmental impact on the latter and ogical, and microbiological fi ndings from the The Tintillo River is 10 km in length and a sharp decrease of its water quality (Sánchez- exceptional Tintillo acidic river. Particularly, this drains an area of 57 km2 in the northern part of España et al., 2005c, 2006a). 134 Geosphere, June 2007 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/3/133/865180/i1553-040X-3-3-133.pdf by guest on 28 September 2021 on 28 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/3/133/865180/i1553-040X-3-3-133.pdf Geosphere, June2007 135 A B C Iron terraces in acid mine drainage: Organic vs. inorganic factors in the Tintillo River Tintillo inthe factors vs.inorganic Organic inacidminedrainage: terraces Iron 3 m D E F 3 m 0.5 m 2 m Figure 2. Terraced iron formations (TIFs) of the Tintillo acidic river. (A–B) Panoramic views show- G H ing the general aspect of the Tintillo stream course and its iron terraces, as seen from sampling points T-3 (A) and T-4 (B). (C–D) Fan-shaped deposit of ocherous iron oxides, which form terraces made of consecutive pools and rims. These pictures cor- respond to the same site (stream reach between T-2 and T-3) but were taken at different times: C is from 2004, when the downstream fl ow of water along the terraces provoked the development of continuous centimeter-scale waterfalls (the water fl ow is from left to right of the photograph), while D is from 0.5 m 2 m 2005, when the fl ow had been diverted, after several heavy rainfall events, to the left margin of the river, thus causing the abandonment of the terraces.