The Touissit-Bou Beker Mississippi Valley-Type District of Northeastern Morocco: Relationships to the Messinian Salinity Crisis, Late Neogene-Quaternary Alkaline Magmatism, and Buoyancy-Driven Fluid Convection

Abstract

The Touissit-Bou Beker Mississippi Valley-type (MVT) district of northeastern Morocco includes five major strata-bound Pb-Zn-Ag deposits that produced, over a century of mining, more than 70 Mt of ore at an average grade of 4.0 wt % Pb, 3.5 wt % Zn, ~1 wt % Cu, and 120 g/t Ag. Economic orebodies are hosted by a 25-m-thick sequence of unmetamorphosed, flat-lying, diagenetically and hydrothermally dolomitized carbonate platform rocks of Aalenian-Bajocian (174-168 Ma) age. The sulfide mineralization consists principally of galena and sphalerite, and occurs as open-space fillings and partial to massive replacements of medium-to coarse-grained dolostones.Throughout the district and surrounding prospects, sulfur isotope compositions of sulfide minerals range from -8.6 to 12.9‰ CDT (n = 194) and display spatial and temporal variations characterized by an overall progressive decrease of d 34 S values: (1) with advancing paragenetic sequence, and upward stratigraphic position; (2) toward distal prospects; and (3) within single grains from core to rim. Similarly, the distribution of 3 He/ 4 He ratios shows an overall lowering from 0.5 RA to <0.01 RA with advancing paragenetic stage. Conversely, lead isotope compositions of galena display fairly uniform 206 Pb/ 204 Pb (18.31-18.37), 207 Pb/ 204 Pb (15.61-15.66), and 208 Pb/ 204 Pb (38.45-38.62) ratios, which contrast significantly with the highly heterogeneous isotopic compositions ( 206 Pb/ 204 Pb = 17.76-18.49, 207 Pb/ 204 Pb = 15.61-15.70, 208 Pb/ 204 Pb = 37.76-39.55) shown by potential metal source rocks.The large variations in d 34 S values are explained in terms of inorganic and/or bacteriogenic reduction, at different rates, of seawater sulfate in seawater or pore fluids, in reservoirs both open and closed with respect to sulfate. The highly variable 3 He/ 4 He values reflect different degrees of mixing between mantle and crustal He components along the fluid flow path. Estimated contributions of the mantle-derived ( 4 He) component involved in the mineralizing fluids of the paragenetically early Zn-rich ore-stage I, and in the later Pb-rich main ore-stage II, are in the range of ~3 to 8%. With time, the mantle-derived input to the mineralizing fluids decreased, and contemporaneously the involvement of surface waters increased, consistent with the lowering of d 34 S with advancing paragenetic stage. This geochemical evolution toward lower isotopic values indicates that the Touissit-Bou Beker hydrothermal system became exclusively dominated by crustally derived fluids, and probably incorporated at shallower levels the influx of progressively larger volumes of cooler, dilute, and oxidized meteoric waters.Whole-rock and galena lead isotope compositions are consistent with a model in which metals were extracted from the local country rocks, particularly the basement Visean rhyodacites and associated siliciclastic rocks, and from the overlying Triassic to Upper Bajocian sediments. The extraction of lead likely occurred during the late Neogene, coincident with the Messin...

Geologic, Hydrothermal, and Geochemical Relationships Between Bimodal Magmatism and Massive Sulphide Mineralization in the Central Jebilet-Guemassa Province (Western Moroccan Hercynides)

“…It is noteworthy that these Pb isotope compositions exclude any major component from a juvenile mantle-derived lead reservoir. Moreover, the plot of isotopic compositions of the Zn-Pb sulphide separates within the field of isotopic compositions defined for Visean rhyodacite and associated schistose pelite (Bouabdellah et al 2015) indicates that a quantitatively significant proportion of the Pb, and by inference other metals, could have been derived from leaching from these local host rocks.…”

“…Yellow field defines isotopic compositions for VHMS deposits of the Iberian Pyrite Belt (IPB). Grey field shows range of whole-rock lead isotopic compositions for Visean rhyodacite and schist from the eastern Meseta (Bouabdellah et al 2015) Another possibility is that the radiogenic end-member represents externally derived Pb that was incorporated into the sulphides during regional metamorphic recrystallization and fluid-rock reactions.…”

Geology, Geochemistry, and Current Genetic Models for Major Mississippi Valley-Type Pb–Zn Deposits of Morocco

“…The summaries presented hereafter draw extensively from the recently published papers of Bouabdellah et al (2012Bouabdellah et al ( , 2015 as these constitute the most exhaustive and comprehensive descriptions published on this district.…”

“…Indeed, more than 90 % of the exploited deposits and prospects occur within Lower and Middle Jurassic carbonates and, to a lesser extent, Triassic dolostones. Major deposits are concentrated in six separate mining camps, including in Morocco the word-class and historically important Touissit-Bou Beker district (>100 Mt at 4 wt% Pb, 3.5 wt% Zn, <1 wt% Cu, 120 g/t Ag; Bouabdellah et al 2012Bouabdellah et al , 2015, the Upper Moulouya district (>30 Mt at 4 wt% Pb), and the Jbel Bou Dahar district (>30 Mt at 4 wt% Pb, 4 wt% Zn), the El Abed district in Algeria that constitutes the eastern termination of the Moroccan Touissit-Bou Beker district, and the Bou-Grine district and related deposits of the diapir zone in Tunisia (Fig. 1).…”

“…However, in spite of abundant available geological, mineralogical, and geochemical data (Rajlich 1983;Dupuy and Touray 1986;Makhoukhi 1993;Bouabdellah 1994;Bouabdellah et al 2012Bouabdellah et al , 2015, the basic questions of genesis remain controversial with proposed models ranging from syngenetic to epigenetic. The lack of minerals suitable for radiometric age dating, together with the omnipresence of post-ore oxidizing events, have left the age of Pb-Zn mineralization as the subject of ongoing debate, and therefore any robust genetic model open to controversy.…”