Zebra and boxwork fabrics in hydrothermal dolomites of northern Canada: Indicators for dilational fracturing, dissolution or in situ replacement?

Abstract

AbstractZebra and boxwork fabrics in Manetoe and Presqu'ile hydrothermal dolomites provide critical data for discrimination between hypotheses for their origin. In a unique occurrence, sets of weakly imbricated, decimetre‐sized curvilinear lenticles of white dolospar are developed within sub‐metre sized tongues of greyish‐white dolomitized crinoidal packstone and grainstone. This nascent zebra fabric has developed along wavy to nodular intra‐bed sedimentary partings. Primary bedding can be traced through dolomitized masses containing these fabrics, into the surrounding undolomitized limestone. All zebra and boxwork fabrics are confined within single beds, consistent with an early burial time of origin. No change in bed thickness occurs across zebra or boxwork fabrics within beds; this indicates a dissolutional, rather than a dilational, origin for the creation of pore space partly cemented with white dolospar. The dolomitized groundmass displays an abruptly gradational or sharp transition to centripetal saddle dolomite cement, which partially to nearly completely, occludes vugs. Circulation of geothermally heated hypersaline Devonian Elk Point basinal sea water brines led to dissolution and replacive dolomitization of limestone adjacent to vugs, and near‐contemporaneous precipitation of white dolospar within dissolutional vugs, consistent with geochemical simulations. The presence of downward‐extending galleries of white dolospar‐cemented solution‐collapse breccia provides further support for an interpretation of regional thermally driven convection of hydrothermal evaporative sea water brines across a broad area of northern Canada. The absence of gases under pressure in vacuoles within fluid inclusions from Manetoe and Presqu'ile dolospars is also more consistent with a dissolutional and contemporaneous dolomitization origin for these fabrics, rather than an origin involving dilational fracturing for space creation and dolospar precipitation. The ubiquitous presence of zebra and boxwork fabrics in hydrothermal dolomite reservoirs indicates that they are not confined fault zones and instead occur wherever precursor limestones had relatively greater porosity and permeability prior to dolomitization.