Hypogene Cave Patterns in Iron Ore Caves: Convergence of Forms Or Processes?
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A. AULER, L. PILÓ, C. PARKER, J. SENKO, I. SASOWSKY, H. Barton HYPOGENE CAVE PATTERNS IN IRON ORE CAVES: CONVERGENCE OF FORMS OR PROCESSES? Augusto S. Auler1, Luís B. Piló1, Ceth W. Parker2, John M. Senko2,3, Ira D. Sasowsky3, and Hazel A. Barton2,3 peleogenesis in iron ore caves may involve generation of porosity at depth with a later surficial phase associated Swith slope hydrological processes. The earlier phreatic phase results in morphological features similar to but much more irregular at wall and ceiling scale than what is observed in hypo- gene caves. Processes responsible for the generation of caves do not seem to follow normal karst geochemical paths, but instead occur through bacterially mediated redox reactions. INTRODUCTION Caves and small voids in iron-rich rocks have been reported in the Brazilian geological literature since the 19th Century, but only recently, due to the expansion of iron mines, have they been subject to detailed studies. Initial research was performed by American geologists from the United States Geological Sur- vey in the Iron Quadrangle region of southeastern Brazil, with George C. Simmons providing pioneering insights on cave gen- esis and mineralogy (Simmons, 1963; 1964). Since 2005, with the increase in iron ore prices and the regulatory mandate to as- sess the significance of any void over 5 m in length, intensive research has resulted in the identification of approximately 3,000 caves, the majority of them being located in the two major iron ore provinces, Carajás ridge in northern Brazil (Amazonia) and the Iron Quadrangle area (Fig. 1). Cave mapping and geospele- ological studies have provided new insights on the morphology and genesis of these enigmatic and little known caves. Figure 1. Location of iron ore outcrops and caves. Besides the GEOLOGY OF IRON ORE CAVES better known areas of Carajás and the Iron Quadrangle, iron ore caves are known from the western area of Corumbá, and the Iron ore is a generic umbrella term that denotes iron-rich rocks south central Espinhaço ridge (Caetité and Conceição do Mato with economic value. These rocks include, besides the original Dentro areas). BIF (Banded Iron Formation), a series of heterogeneous altera- tion products with varying iron content. Although BIF repre- known as canga (Fig 2). Alteration of BIF can occur at great sents the original rock, due to its long term tectonic and weath- depths, as frequently shown by the occurrence of friable high- ering history, unaltered BIF is seldom found at or close to the grade ore deep in open pit mines. Iron compounds are by far surface. Chemical alteration of BIF is neither a continuous nor the more resistant constituent of BIF, silica together with other a synchronous process, resulting in a complex array of highly elements (carbonates, etc) being more easily removed, resulting heterogeneous rocks with distinct levels of alteration, includ- in a more porous and friable rock horizons referred to as “pale ing a surficial iron-rich breccia cemented by ferruginous matrix zones” (McFarlane and Twidale, 1987). 1Instituto do Carste, Rua Brasópolis 139, Belo Horizonte, Minas Gerais, 30360-260, Brazil, [email protected] 2Department of Biology, The University of Akron, Akron, OH 44325-3908, USA, [email protected] (Parker), [email protected] (Senko), [email protected] (Barton) 3Department of Geosciences, The University of Akron, Akron, OH 44325-4101, USA, [email protected] (Sasowsky) Hypogene Cave Morphologies 15 KARST WATERS INSTITUTE SPECIAL PUBLICATION 18 A. AULER, L. PILÓ, C. PARKER, J. SENKO, I. SASOWSKY, H. Barton In the Iron Quadrangle, iron-rich rocks comprise BIF and al- in both Carajás and the Iron Quadrangle, silica undergoes chemi- teration products of the 2.5 Ga Cauê Formation of the Minas cal weathering, yielding a more porous rock and a higher-grade Supergroup (Babinsky et al., 1995). These rocks outcrop at the ore. In the Iron Quadrangle some BIFs also contain carbonates, top of ridges, forming a narrow strip of iron topped by canga, which are also prone to be dissolved away. Irregular, non-con- corresponding to the highest elevations in this mountainous area. nected voids would be created at this early stage. In the Carajás area, the iron ore belongs to the 2.7 Ga Carajás Formation, Grão Pará Group (Trendall et al., 1998). The local The chemical processes related to silica and carbonate dissolu- geomorphology comprises a series of irregularly linked flat- tion are well established (Ford and Williams, 2007; Wray, 2013). topped plateaus capped by canga. As a general rule, the very low The mobilization of iron, however, is a more complex process rates of denudation in iron outcrops (Spier et al., 2006; Shuster that has recently been shown to involve bioreduction of Fe (III) et al., 2012) result in the iron formations occupying the highest by iron reducing bacteria that convert insoluble solid Fe(III) into elevation terrains. aqueous Fe(II), allowing for the mobilization of iron and genera- tion of voids (Parker et al., 2013a, b). The majority of caves develop in the irregular contact between altered BIF and canga. Caves entirely in altered BIF or entirely As denudation progresses, isostatic rebound will slowly move in canga also tend to be common. Caves within BIF, on the other the caves above the water table towards the land surface. Iron hand, are relatively rare. It must be stressed that the higher fre- ore caves are located in high elevation areas are relict dry fea- quency of caves in the canga/altered BIF contact may reflect a tures, hydrologically active in only a limited way; once they sampling bias towards erosional exposure of more superficial get close to the surface, the presence of the very resistant canga caves. There is some indication of a significant number of en- cover will further protect the caves from weathering and unroof- trance-less voids existing at depth. Table 1 provides brief general ing. However, these caves will eventually become integrated to data on cave features relative to the lithology. the near-surface hydrological processes that occur at the slopes, particularly subsurface flow at the canga/iron ore interface. The original hypogene morphology will tend to be partially obliterat- REMARKS ON THE GENESIS OF IRON ed by the connection between chambers, masking or sometimes ORE CAVES AND THE RESULTING even completely overprinting the original hypogene features. A MORPHOLOGY schematic model of cave evolution is shown in Figure 3. The existence of substantial voids at great depths in iron areas Slope hydrological processes will link isolated chambers and is well established by numerous proprietary borehole logs from will result in a more linear pattern parallel to the slope. In this iron exploration studies. Porosity generation and thus speleo- analysis we will focus in what is interpreted as early hypogene- genesis may start at an unknown depth, on the order of several like features of these caves. hundred meters below the present surface. As already discussed, Figure 2. Examples of iron ore rock types. A. Unaltered Banded Iron For- mation (BIF) showing silica and iron bands; B. Altered BIF. Silica has been leached. C. Canga showing whitish bacterial colonies. D. Sharp contact between canga (top) and altered BIF (bottom). 16 Hypogene Cave Morphologies KARST WATERS INSTITUTE SPECIAL PUBLICATION 18 A. AULER, L. PILÓ, C. PARKER, J. SENKO, I. SASOWSKY, H. Barton Table 1. General morphological features relative to rock type. Morphology Rock Type Cave Frequency Macro Micro BIF Rare Rounded chambers Polished walls, less irregular at ceiling and wall levels Altered BIF Common Rounded chambers tend to be Irregular at ceiling and wall levels, evident sharp rock projections are common. Canga Common highly irregular, rounded cham- Presence of pendants and pillars bers Contact Canga/Altered BIF Very Common highly irregular, rounded cham- Display features common to both rock bers types Differentiating Between Hypogene and Later Modification Features In the “hypogene” category we include morphological features that are interpreted as being generated deep below the water table. As previously mentioned, later slope processes result in a more elongated pattern that considerably masks the original morphology. Among these later vadose processes one can in- clude the alignment of the cave with the slope gradient, its close proximity to the surface and the existence of smaller passages linking rounded irregular chambers (Fig. 4). Furthermore, small channels mostly at the contact between floor and walls are quite abundant, but probably represent later generated inlets related to the expansion of the cave along the slope allowing the input of fine grained altered iron sediment. Ubiquitous breakdown is Figure 3. Schematic evolution of iron ore caves. A. Isolated vugs attributed to the very surficial nature of the caves, in which un- at depth. Denudation and isostatic rebound positions the caves above the water table and closer to the surface. The contact loading (release) joints favor ceiling collapse. between canga (darker red) and altered ore is a favorable zone for subsurface flow parallel to the slope (blue arrows). B. Caves Hypogene Morphology at Plan View reach the contact and become integrated to the slope hydro- logical system. C. Connection between once isolated vugs. D. The morphology at plan view indicates many features that re- Scarp retreat intersects a former entranceless cave. Connection semble hypogene morphology: between chambers and headward expansion of caves along the contact. At these later stages, the original “hypogene” morphol- • Absence of an entrance. Iron ore caves usually display ogy may become obliterated. an entrance that is much smaller than the remaining inner passages. These entrances are associated with clastic sediment sequences are entirely composed of material the evolution of scarps, fortuitously intersecting once generated within the cave, being highly homogeneous in terms isolated chambers (Fig. 5).