Zebra pattern in rocks as a function of grain growth affected by second-phase particles

Abstract

Alternating fine grained dark and coarse grained light layers in rocks are often termed zebra patterns and are found worldwide. The crystals in the different bands have an almost identical chemical composition, however second-phase particles (e.g., fluid filled pores or a second mineral phase) are concentrated in the dark layers. Even though this pattern is very common and has been studied widely, the initial stage of the pattern formation remains controversial. In this communication we present a simple microdynamic model which can explain the beginning of the zebra pattern formation. The two dimensional model consists of two main processes, mineral replacement along a reaction front, and grain boundary migration affected by impurities. In the numerical model we assume that an initial distribution of second-phase particles is present due to sedimentary layering. The reaction front percolates the model and redistributes second-phase particles by shifting them until the front is saturated and drops the particles again. This produces and enhances initial layering. Grain growth is hindered in layers with high second-phase particle concentrations whereas layers with low concentrations coarsen. Due to the grain growth activity in layers with low second-phase particle concentrations these impurities are collected at grain boundaries and the crystals become very clean. Therefore, the white layers in the pattern contain large grains with low concentration of second-phase particles, whereas the dark layers contain small grains with a large second-phase particle concentration. The presence of the zebra pattern is characteristic for regions containing Pb-Zn mineralization. Therefore, the origin of the structure is presumably related to the mineralization process and might be used as a marker for ore exploration. A complete understanding of the formation of this pattern will contribute to a more accurate understanding of hydrothermal systems that build up economic mineralization.

A review of numerical modelling of the dynamics of microstructural development in rocks and ice: Past, present and future

“…Pinning particles can also be moved around so that growing grains become clean. Such a process can lead to layered rocks, for example, zebra dolomites (Kelka et al, 2015).…”

Reaction-induced porosity fingering: Replacement dynamic and porosity evolution in the KBr-KCl system

“…The formation of pore space in this system is similar to dolomitization, where the created porosity ranges from 13% to 25% (Merino and Canals, 2011;Kondratiuk et al, 2016). Dolomitization is an important process in sedimentary basins, reservoir rocks (Al-Helal et al, 2012;Ferket et al, 2003;Gomez-Rivas et al, 2014;Machel and Buschkuehle, 2008;Machel and Mountjoy, 1986;Maliva et al, 2011;Montaron, 2008;Whitaker et al, 2004) and ore bodies (Bons et al, 2014;Kelka et al, 2015;Kelka et al, 2017;Merino, 2006;Merino and Canals, 2011;Nielsen et al, 1998). Questions about the mechanisms that govern dolomitization of large geobodies are debated, and focus on how the huge amount of magnesium required to complete the replacement could enter the system, without involving billions of cubic meters of fluid.…”

Micro‐scale chemical and physical patterns in an interface of hydrothermal dolomitization reveals the governing transport mechanisms in nature: Case of the Layens anticline, Pyrenees, France

“…Indeed, an alternation between lighter and darker stripes in thin section shown in Fig. 2D is similar to the zebra texture that is typical of base metal ore deposits (Zn‐Pb) linked to hydrothermal dolomite formations (Leach & Viets, 1992; Wallace et al ., 1994; Krug et al ., 1996; Paradis et al ., 2007; Palinkaš et al ., 2009; Diehl et al ., 2010; Kelka et al ., 2015, 2017; Kelka, 2017). The zebra pattern indicates that the original calcite experienced recrystallization and coarsening prior to or during the early dolomitization stage.…”

“…The zebra pattern indicates that the original calcite experienced recrystallization and coarsening prior to or during the early dolomitization stage. Such a recrystallization process can be related to either fluid pressure or stress waves linked to a first step of fluid infiltration, and/or to stress around fracture zones (Kelka et al, 2015(Kelka et al, , 2017Centrella et al, 2018). Trace element maps (Fig.…”

“…The second category relates to late dolomitization, occurring in relation with an influx of ascending hydrothermal, Mg‐heavy evolved fluids that flow and interact with carbonates during tectonism (Swennen et al ., 2012; Martín‐Martín et al ., 2015; Mozafari et al ., 2019). The latter is usually referred to as hydrothermal dolomite regardless of the fluid origin (Machel & Lonnee, 2002), and is of important economic interest (Wallace et al ., 1994; Diehl et al ., 2010), namely because of a genetic link with base metal deposits (Gomez‐Rivas et al ., 2014; Kelka et al ., 2015; Martín‐Martín et al ., 2015; Kelka et al ., 2017).…”