Zebra dolomitization as a result of focused fluid flow in the Rocky Mountains Fold and Thrust Belt, Canada

Discordant zebra dolomite bodies occur locally in the Middle Cambrian Cathedral and Eldon Formations of the Main Ranges of the Canadian Rocky Mountains Fold and Thrust Belt. They are characterized by alternating dark grey (a) and white (b) bands, forming an ‘abba’ diagenetic cyclicity. These bands developed parallel to both bedding and cleavage. Dark grey (a) bands consist of fine (< 300 μm) non‐planar crystalline impure dolomite. The white (b) bands are composed of coarse (up to several millimetres) milky‐white pure saddle dolomites (b1) which are often covered by pore‐lining zoned dolomite (b2). Thebphases often possess a saddle‐shaped morphology. In contrast to the replacement origin of theadolomite, the zonedb2dolomite rims are interpreted as a cement formed in open cavities. Theb1dolomite is interpreted as the result of recrystallization with diagenetic leaching of non‐carbonate components. All the zebra dolomites studied are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ depending on the sample locality. Fluid inclusion data indicate that the dolomites formed from relatively hot (TH = 130–200 °C), saline (20–23 wt% CaCl2eq.) fluids. A diagenetic high temperature origin is also supported by depletedδ18O values (−20 to −14‰ VPDB). A contribution of87Sr‐enriched fluids is reflected in the87Sr/86Sr values (0·7091–0·7123). Zebra dolomite development is explained by focused fluid flow, which exploited areas of structural weaknesses (e.g. basin‐platform, rim areas, faults, etc.). Expulsion of hot basinal brines in a tectonically active regime generated overpressures, which explains the development of secondary porosity during zebra dolomitization as well as the intra‐zebra fracturing at decimetre to micrometre scale.