Invited research articleZebra textures in carbonate rocks: Fractures produced by the force of crystallization during mineral replacement
Introduction
Zebra textures are visually striking and enigmatic rock fabrics that consist of millimetre- to centimetre-scale regularly alternating light and dark bands within sedimentary strata. The textures have variously been referred to as “diagenetic crystallization rhythmites” (Fontboté and Amstutz, 1980, Fontboté and Amstutz, 1982; Fontboté, 1981), rhythmites (Sass-Gustkiewicz and Mochnacka, 1994), banded or ribbon ores (e.g. Sass-Gustkiewicz et al., 1982; Tompkins et al., 1994), expansion structures (Beales and Hardy, 1980) or most commonly as zebra rock/texture/fabric/dolomite (e.g. Beales and Hardy, 1980; Wallace et al., 1994; Nielsen et al., 1998; Diehl et al., 2010; Morrow, 2014). Zebra textures have frequently been documented as consisting of dolomite, but precisely analogous textures are also found in ankerite, siderite, sphalerite, marcasite, barite, fluorite, magnesite and other minerals (Arne and Kissin, 1989; Fontboté, 1993; Wallace et al., 1994; Tompkins et al., 1994; Sass-Gustkiewicz and Mochnacka, 1994).
These banded textures have considerable economic significance because they commonly occur in association with carbonate-hosted zinc-lead mineralization (e.g. Arne and Kissin, 1989; Sass-Gustkiewicz and Mochnacka, 1994), and can form the main ore textures within such deposits. Zebra textures have also been found in carbonate reservoir rocks, particularly in “hydrothermal dolomite reservoirs” (Davies and Smith Jr, 2006; Morrow, 2014; Hiemstra and Goldstein, 2015). Earlier researchers (Fontboté and Amstutz, 1980; Fontboté, 1981; Fontboté and Amstutz, 1982) favoured a synsedimentary or early diagenetic origin for the structures, but more recent research has favoured a late diagenetic origin (Table 1). The banding has been variously suggested as being caused by replacement of sedimentary structures such as evaporite laminae (Beales and Hardy, 1980; Tompkins et al., 1994), replacement of dolomite (Sass-Gustkiewicz et al., 1982; Arne and Kissin, 1989), fracturing (Wallace et al., 1994; Nielsen et al., 1998; López-Horgue et al., 2009), dissolution (Morrow, 2014), hydraulic overpressuring (e.g. Boni et al., 2000; Swennen et al., 2003; Vandeginste et al., 2005; Swennen et al., 2012) and grain growth during recrystallization (Kelka et al., 2015, Kelka et al., 2017). Thus, despite their economic significance, after more than 30 years of research, there is no agreement on the origin of the structures (Table 1).
Zebra textures have been the subject of a large number of detailed petrological, fluid inclusion and geochemical investigations that help constrain the timing, types of fluids, and conditions under which these structures and associated ore deposits form (Table 1). While there is no clear consensus on the origin of zebra textures, this large body of literature exhaustively documents the characteristic features of zebra textures from a huge variety of settings. In this report, we therefore concentrate not on describing new occurrences or descriptions of zebra textures, but on synthesizing and interpreting the features that are ubiquitous to zebra textures globally. We present a concise review of the petrologically relevant features of zebra fabrics across a range of mineralogies that constrain the origin of these textures. This is based on features described from the literature, coupled with petrographic analysis. From this evidence, we suggest a new and simple model for the origin of zebra textures that can explain the characteristics and occurrence of these structures.
Section snippets
Methods
Polished thin sections were examined in plane light and using cathodoluminescence microscopy. Cathodoluminescence microscopy was carried out using a Nuclide ELM-2B cold cathodoluminoscope, attached to a Wild M400 photomacroscope (operating conditions of 8 kV voltage and around 0.6 milliamps beam current) with a SPOT Flex 4 megapixel digital camera attached. Plane light photography was carried out on the same instrument. In this report, we have documented zebra textures from the Cadjebut
Results
Zebra textures occur in a variety of different geological settings and are associated with both carbonate-hosted ore deposits (Mississippi Valley type mineralization, intrusive-related lead-zinc deposits and Carlin-style gold deposits) and hydrothermal dolomite reservoirs, (Emmons et al., 1927; Fontboté, 1993; Davies and Smith Jr, 2006; Diehl et al., 2010; Morrow, 2014; Hiemstra and Goldstein, 2015). Zebra textures commonly consist of coarsely crystalline dolomites, but can also consist of a
Historical models for zebra textures
The majority of previously suggested origins for zebra textures can be categorised as being related to either fracturing, dissolution, recrystallization/replacement or combinations thereof (Table 1). From the petrographic constraints discussed above, it is difficult to interpret the light bands as anything other than elongate cement-filled cavities, which limits the possible origins for the cavity systems. This also explains why the majority of previous researchers have favoured either
Conclusions
We suggest that zebra textures are caused by two processes occurring in close proximity: 1. Mineral replacement; and 2. Dissolution and open space generation. The pressure produced by growing replacement crystals within carbonates adjacent to fluid-filled open space (produced by dissolution) causes fracture systems to develop. The fractures develop because, in the absence of a lithostatic stress field (due to adjacent, fluid-filled open space), the force of crystallization can exceed the
Acknowledgements
This research was partially funded from Australian Research Council Discovery Grant DP130102240. AvSH acknowledges funding from a NASA Astrobiology Institute Postdoctoral Fellowship and a University of Melbourne Puzey Fellowship. We thank Elizabeth Turner, whose comments substantially improved an earlier version of the manuscript. We also thank Hilary Corlett and Jeff Lonnee for their helpful reviews and Brian Jones for his editorial handling, all of which improved the manuscript.
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