©2015 Society of Economic Geologists, Inc. Economic Geology, v. 110, pp. 1455–1484 The Touissit-Bou Beker Mississippi Valley-Type District of Northeastern Morocco: Relationships to the Messinian Salinity Crisis, Late Neogene-Quaternary Alkaline Magmatism,

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 d34S 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 3He/4He ratios shows an overall lowering from 0.5 RA to <0.01 RA with advancing paragenetic stage. Conversely, lead isotope com-positions of galena display fairly uniform 206Pb/204Pb (18.31–18.37), 207Pb/204Pb (15.61–15.66), and 208Pb/204Pb (38.45–38.62) ratios, which contrast significantly with the highly heterogeneous isotopic compositions (206Pb/204Pb = 17.76–18.49, 207Pb/204Pb = 15.61–15.70, 208Pb/204Pb = 37.76–39.55) shown by potential metal source rocks