Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar'a caldera, Anti‐Atlas, Morocco

Abstract

This paper presents a case study of the sublacustrine precipitation of hydrothermal silica ± TiO2 in the Ediacaran Mançour Group of the Saghro inlier, Anti‐Atlas, Morocco. Lacustrine carbonates containing stromatolitic mats and bioherms occur in ephemeral ponds developed within the Oued Da'ra caldera. Its syn‐eruptive infill consists of pyroclastites, ashflow tuffs, and subsidiary lava flows and sills, whereas inter‐eruptive deposition is mainly represented by slope‐related debris‐flow breccias and landslides, alluvial fans and fluvial channels. Carbonate production took place in a mosaic of differentially subsiding, fault‐bounded intra‐caldera blocks controlled by episodic collapse‐induced drowning, pyroclastic blanketing and migration of alluvial/fluvial environments. After microbial carbonate production, the carbonates recorded several early‐diagenetic processes, punctuated by polyphase fissuring (controlling secondary permeability) locally linked to hydrothermal influx. Three generations of carbonate cements are recognisable: (i) fibrous, botryoidal and blocky/drusy mosaics of calcite; (ii) idiotopic mosaics of dolomite caused by flushing of hypersaline Mg‐rich brines; and (iii) euhedral to drusy calcite via dedolomitization. The δ13C and δ18O values from carbonate cements broadly become successively isotopically lighter, as a result of meteoric and hydrothermal influence, and were probably overprinted by the Panafrican‐3 phase that affected the top of the Mançour Group. Two mechanisms of silicification are involved: (i) early‐diagenetic occlusion of interparticle pores at the sediment/water interface of pyroclastic substrates and reefal core and flanks; and (ii) hydrothermal precipitation of silica ± TiO2 lining fissures and vuggy porosity encased in the host rock. Silica conduits cross‐cutting lacustrine mats and bioherms exhibit high potential of preservation in collapsed volcanic calderas. Primary fluid inclusions of hydrothermal silica contain brine relics with NaCl/CaCl2 ratios of 2·1 to 4·4, representing minimum entrapment temperatures of about 142 to 204°C, and abiotic hydrocarbons (heavy alkanes) related to serpentinization of the volcanic and volcanosedimentary basement of the Oued Dar'a caldera.

A 571 million‐year‐old alkaline volcanic lake photosynthesizing microbial community, the Anti‐atlas, Morocco

“…Álvaro and González‐Acebrón (2019) have characterized a number of diagenetic aspects in the Amane Tazgart microbialites similar to other sections of the Anti‐atlas. They described several early‐diagenetic processes, punctuated by polyphasic fissuring related to hydrothermal activity, with the generation of fibrous, botryoidal and blocky/drusy mosaic calcites, idiotopic mosaics of dolomite, and euhedral to drusy calcite produced by dedolomitization.…”

“…The northeast margin is intruded by pink granite interpreted as parent magma of the volcanic rocks (Figure 1b). The southwestern margin (Choubert & Faure‐Muret, 1970) is marked by a 200 m coarse‐grained, quartz‐rich, and granulated volcaniclastic deposit resting on the caldera's infill ash‐flow tuff that is intercalated with lacustrine beds containing purplish‐green microbialites (Álvaro et al., 2010; Álvaro & González‐Acebrón, 2019; Walsh et al., 2012). The age of the caldera infill has been estimated to the last major volcanic eruption from the caldera ~571 ± 5 Ma according to Walsh et al., 2012 (Figure 1b).…”

“…Choubert and Faure‐Muret (1970) were the first to propose a lacustrine depositional setting for these meter‐thick, carbonate‐bearing volcanosedimentary units, in view of their restricted geographic expansion and the obvious nearness of domal and conical stromatolites. The development of the Amane Tazgart microbialites was controlled primarily by the caldera's volcanic and geothermal activity, combined with silicate hydrolysis under subtropical conditions (Álvaro et al., 2010; Álvaro & González‐Acebrón, 2019).…”

“…Choubert and Faure-Muret (1970) were the first to propose a lacustrine depositional setting for these meter-thick, carbonate-bearing volcanosedimentary units, in view of their restricted geographic expansion and the obvious nearness of domal and conical stromatolites. The development of the Amane Tazgart microbialites was controlled primarily by the caldera's volcanic and geothermal activity, combined with silicate hydrolysis under subtropical conditions (Álvaro et al, 2010;Álvaro & González-Acebrón, 2019). Powdered samples were analyzed at the 2-65° 2θ angular range, using 0.017° 2θ step sizes per 0,7 s. After air drying, ethylene-glycol solvation, and heating at 350 and 550°C, the clay fraction was analyzed over an angular range of 2-35° 2θ at 0.017° 2θ using 0.7 s step sizes.…”

“…Chlorite (Chl), Illite (Il), mixed-layer minerals (MLM) and chlorite/smectite (C/S), represent the Clay mineralogical assemblages of the various microbialitic facies gray aphanitic andesitic magmas and overlain by a volcanosedimentary succession including andesitic lava flows, peperites and epiclastic micro-conglomerates and sandstones rich in volcanic clasts. The surrounding depositional setting dominated by subaerial volcanic and sedimentary setting of the Oued Dar'a Caldera, as well as the overall facies features of the Amane Tazgart, suggest short periods of attenuated volcanic activity that are accompanied by lacustrine sediment deposition in ephemeral ponds (e.g., Álvaro et al, 2010;Álvaro & González-Acebrón, 2019).…”