Permissive tracts for sediment-hosted lead-zinc-silver deposits in the Islamic Republic of Mauritania (phase V, deliverable 73)
<p>Although Mississippi Valley-type (MVT) deposits have not been recognized in Mauritania there are permissive tracts for these deposits in the regionally…
Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.
Prepared in cooperation with the Ministry of Petroleum, Energy and Mines, Islamic Republic of Mauritania Second Projet de Renforcement Institutionnel du Secteur Minier de la République Islamique de Mauritanie (PRISM-II) Permissive Tracts for Sediment-Hosted LeadZinc-Silver Deposits in the Islamic Republic of Mauritania: Phase V, Deliverable 73 By Jeffrey L. Mauk
Open-File Report 2013‒1280 Chapter J U.S. Department of the Interior U.S. Geological Survey
U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director U.S. Geological Survey, Reston, Virginia: 2015 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit ://www.usgs.gov or call 1–888–ASK–USGS For an overview of USGS information products, including maps, imagery, and publications, visit ://www.usgs.gov/pubprod To order this and other USGS information products, visit ://store.usgs.gov This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Suggested citation: Mauk, J.L., 2015, Permissive tracts for sediment-hosted lead-zinc-silver deposits in the Islamic Republic of Mauritania (phase V, deliverable 73), chap. J of Taylor, C.D., ed., Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II): U.S. Geological Survey Open-File Report 2013‒1280-J, 13 p., ://dx.doi.org/10.3133/ofr20131280. [In English and French.] Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Multiple spellings are used in various literatures, and may be reflected in the text. This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or for stratigraphic nomenclature. The report is being released in both English and French. In both versions, we use the Frenchlanguage names for formal stratigraphic units. ISSN 2331-1258 (online)
Contents Summary 1 Introduction 1 MVT deposits 2 Global attributes 2 Regional attributes 2 SEDEX Deposits—Global Attributes 3 Mineral Deposits and Occurrences in Mauritania 3 Mississippi Valley-Type (MVT) Pb-Zn Deposits 3 SEDEX Pb-Zn Deposits 4 Potential Mineral Tracts 6 Mississippi Valley-Type Pb-Zn (Ag) Deposits 6 4.2 SEDEX Pb-Zn (Ag) Deposits 10 Conclusions 11 Acknowledgments 11 References 12
Figures Figure 1. Permissive geology for Pb, Zn, and Ag deposits in sedimentary rocks 5 Figure 2. Generalized stratigraphy of part of the Hodh Supergroup. Redrawn from Pitfield and others (2004) 6 Figure 3. Generalized stratigraphic column for the Northern subbasin and the southern part of the Mauritania subbasins (modified from Brownfield and Charpentier, 2003) 9 Figure 4. Schematic cross section showing the distribution of sedimentary facies in the Mauritania offshore, northern Senegal Basin, northwest Africa (From Brownfield and Charpentier, 2003) 10
Conversion Factors SI to Inch/Pound Multiply By To obtain Length centimeter (cm) inch (in.) millimeter (mm) inch (in.) decimeter (dm) foot (ft) meter (m) foot (ft) kilometer (km) mile (mi) Area hectare (ha) acre square meter (m2) 0.0002471 acre square kilometer (km2) square mile (mi2) Volume cubic kilometer (km3) cubic mile (mi3) Mass gram (g) ounce, avoirdupois (oz) kilogram (kg) pound avoirdupois (lb) megagram (Mg) ton, short (2,000 lb) megagram (Mg) ton, long (2,240 lb) metric ton per day ton per day (ton/d) megagram per day (Mg/d) ton per day (ton/d) metric ton per year ton per year (ton/yr) Pressure kilopascal (kPa) atmosphere, standard (atm) kilopascal (kPa) bar Energy joule (J) 0.0000002 kilowatt hour (kWh) ppm, parts per million; ppb, parts per billion; Ma, millions of years before present; m.y., millions of years; Ga, billions of years before present; 1 micron or micrometer (µor µm) 1 × 10-6 meters; Tesla (T) the field intensity generating 1 Newton of force per ampere (A) of current per meter of conductor Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8 Coordinate information is referenced to the World Geodetic System (WGS 84)
Acronyms AMT
Audio-magnetotelluric ASTER
Advanced Spaceborne Thermal Emission and Reflection Radiometer AVIRIS
Airborne Visible/Infrared Imaging Spectrometer BIF
Banded iron formation BLEG
Bulk leach extractable gold BGS
British Geological Survey BRGM
Bureau de Recherches Géologiques et Minières (Mauritania) BUMIFOM The Bureau Minier de la France d’Outre-Mer CAMP
Central Atlantic Magmatic Province CGIAR-CSI Consultative Group on International Agricultural Research-Consortium for Spatial Information DEM
Digital Elevation Model DMG
Direction des Mines et de la Géologie EC
Electrical conductivity EMPA
Electron Microprobe Analysis EM
Electromagnetic (geophysical survey) EOS
Earth Observing System eU
Equivalent uranium GGISA
General Gold International GIF
Granular iron formation GIFOV
Ground instantaneous field of view GIS
Geographic Information System HIF
High grade hematitic iron ores IHS
Intensity/Hue/Saturation IAEA
International Atomic Energy Agency IOCG
Iron oxide copper-gold deposit IP
Induced polarization (geophysical survey) IRM
Islamic Republic of Mauritania JICA
Japan International Cooperation Agency JORC
Joint Ore Reserves Committee (Australasian) LIP
Large Igneous Province LOR
Lower limit of reporting LREE
Light rare-earth element METI
Ministry of Economy, Trade and Industry (Japan) MICUMA Société des Mines de Cuivre de Mauritanie MORB
Mid-ocean ridge basalt E-MORB Enriched mid-ocean ridge basalt N-MORB Slightly enriched mid-ocean ridge basalt T-MORB Transitional mid-ocean ridge basalt Moz
Million ounces MVT
Mississippi Valley-type deposits NASA
United States National Aeronautics and Space Administration
Nlaps
National Landsat Archive Processing System OMRG
Mauritanian Office for Geological Research ONUDI
(UNIDO) United Nations Industrial Development Organization PRISM
Projet de Renforcement Institutionnel du Secteur Minier PGE
Platinum-group elements RC
Reverse circulation drilling REE
Rare earth element RGB
Red-green-blue color schema RTP
Reduced-to-pole SARL
Société à responsabilité limitée SEDEX
Sedimentary exhalative deposits SIMS
Secondary Ionization Mass Spectrometry SNIM
Société National Industrielle et Minière (Mauritania) SP
Self potential (geophysical survey) SRTM
Shuttle Radar Topography Mission SWIR
Shortwave infrared TDS
Total dissolved solids TIMS
Thermal Ionization Mass Spectrometry TISZ
Tacarat-Inemmaudene Shear Zone TM
Landsat Thematic Mapper UN
United Nations UNDP
United Nations Development Program US
United States USA
United States of America USGS
United States Geological Survey UTM
Universal Transverse Mercator projection VHMS
Volcanic-hosted massive sulfide VisNIR
Visible near-infrared spectroscopy VLF
Very low frequency (geophysical survey) VMS
Volcanogenic massive sulfide deposit WDS
Wavelength-dispersive spectroscopy WGS
World Geodetic System
1U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 U.S.A.
Second Projet de Renforcement Institutionnel du Secteur Minier de la République Islamique de Mauritanie (PRISM-II) Permissive Tracts for Sediment-Hosted LeadZinc-Silver Deposits in the Islamic Republic of Mauritania: Phase V, Deliverables 73 By Jeffrey L. Mauk1 Summary Although Mississippi Valley-type (MVT) deposits have not been recognized in Mauritania there are permissive tracts for these deposits in the regionally extensive Proterozoic carbonate rocks of the Taoudeni Basin. Permissive tracts for undiscovered MVT Pb-Zn-Ag deposits in the Proterozoic carbonate units are supported by the occurrences of MVT mineral and alteration assemblages, presence of evaporites, proximity to major orogenic events that have produced MVT ores elsewhere, red bed sequences and basal aquifers that may have been potential brine migration pathways for large MVT hydrothermal systems. Permissive tracts for SEDEX (sedimentary exhalative) deposits coincide with those for MVT deposits. However, the geodynamic setting of the Taoudeni Basin is unlike that of SEDEX ores elsewhere on Earth, and therefore the potential for this class of deposits must be rather low. SEDEX deposits occur along tectonically active, shale dominated passive margins or in intracontinental rift basins. Introduction Sediment-hosted Pb-Zn deposits are divided into two broad sub-types: MVT (Mississippi Valley-type) and SEDEX (sedimentary exhalative). Deposits of both subtypes display a broad range of relationships to enclosing host rocks that includes stratiform, stratabound, and discordant ores. Mississippi Valley-type and SEDEX deposits have similar mineral assemblages consisting of sphalerite and galena, and iron sulfide minerals can be abundant in some deposits. Minor amounts of copper minerals and barite may be present. Common alteration minerals include siderite, ankerite, calcite or dolomite.
Mississippi Valley-type deposits dominantly formed in platform carbonate sequences, mostly in extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intra-continental or failed rifts, and in passive continental margins. Sedimentary packages that contain SEDEX and MVT deposits have similar stratigraphic elements, consisting of a basal clastic and (or) volcanic dominated succession, which is overlain by shales and (or) carbonates. SEDEX deposits are hosted mainly by reduced, fine-grained siltstones-shales-mudstones and (or) carbonate units within clastic sediments, whereas MVT deposits are located in carbonate-dominated sequences. Although most SEDEX and many MVT deposits were localized by extensional faults, SEDEX deposits are mainly related to growth faults active at the time of mineralization. Most MVT ores are tens of millions of years younger than their host rocks; however, a few are close m.y.) to the age of their host rocks. In the absence of direct dates for SEDEX deposits, their age of formation is generally constrained by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of most SEDEX ores was coeval with sedimentation or early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation. MVT deposits are mainly a Phanerozoic phenomenon; the ages of their host rocks as well as ages of ore deposition are mainly Phanerozoic. Nevertheless, important MVT deposits do occur in Archean and Proterozoic rocks. The oldest MVT deposit in the world is the Pering deposit in South Africa that is hosted by Archean stromatolitic limestone; however, ore formation occurred during the Mesoproterozoic (Schaefer, 2002). The ages of SEDEX deposits are grouped into two major groups: one in the Proterozoic and another in the Phanerozoic. Exploration and Mineral Assessment for MVT and SEDEX deposits: Leach and others (2005) summarized the important ore deposit attributes that can be used to assess the favorability for undiscovered MVT and SEDEX deposits. These attributes form the basis for identifying permissive mineral tracts for MVT and SEDEX deposits in Mauritania. MVT deposits Global attributes 1. Ore is hosted in platform carbonate sequences. Phanerozoic carbonate sequences have greater potential for hosting MVT ores relative to Archean and Proterozoic carbonates. 2. Districts and deposits are regionally associated with evaporites or evaporative facies. Location within evaporative latitudes during ore formation is considered highly favorable. 3. The most important ore controls are extensional faults that were active during major contractional tectonic events. Favorability is related to the density of faulted terrain and proximity 500 km) to major orogenic belt. Regional attributes Locally important assessment criteria include extensive dissolution collapse breccias, paleokarst, and erosional unconformities.
Higher potential is given to carbonate sequences containing regional aquifers, basal sandstones, and red bed sequences. Regional permeability contrasts expressed by shale edges, limestone to dolostone transitions, and basement topography. Presence of geochemical anomalies and mineral occurrences are highly suggestive indicators of the effects of large hydrothermal systems required for MVT deposits. Geochemical indicator elements include Zn, Pb, As, Tl, and Cd. Mineral indicators are barite and sparry dolomite cement that may form widespread halos about mineralized regions. SEDEX Deposits—Global Attributes The presence of mixed clastic and carbonate sedimentary sequences in Atlantic style passive margins and intracontinental rift basins. SEDEX deposits are absent from rocks older than 2.2 Ga and are notably absent from rocks about 1.4 to 0.6 Ga in age. Therefore, sedimentary sequences older than 2.2 and between 1.4 to 0.6 Ga are considered unfavorable for SEDEX deposits. The presence of evaporites or evaporative facies in proximal sedimentary environments, or indications that the tectonic environment was active in evaporative latitudes are highly favorable. The presence of sedimentary facies that are enriched in organic matter that occur in passive margins and intracontinental rift basins are favorable. The presence of active growth and (or) transtensional faults in the above mentioned sedimentary sequences are favorable. Higher potential is given to tectonic environments that contain regional aquifers, basal sandstones and red bed sequences. Presence of geochemical anomalies and (or) mineral occurrences with stratiform or tabular forms that extend for tens of kilometers. Important geochemical anomalies are Mn, Fe, Zn, and Tl. Mineral Deposits and Occurrences in Mauritania Mississippi Valley-Type (MVT) Pb-Zn Deposits Mississippi Valley-type deposits have not been recognized in Mauritania despite the presence of regionally extensive Proterozoic carbonate rocks in the stratigraphic sequence of the vast Taoudeni Basin. The remarkable absence of MVT deposits is likely due in part to the lack of modern exploration efforts for these deposits, limited geochemical and geological data that are useful for assessments, and the generally low global potential for carbonate rocks older than about 900 Ma to host MVT deposits. In the report prepared as part of Phase 5 of the USGS program for PRISM-II (Marsh and Anderson, 2015) there are geochemical data from samples of stratiform mineralization that are suggestive of being indicators for “syngenetic” mineralization. These occurrences are: a. Gouirat El Khatt—Three samples of stromatolitic limestone, collected by J.P. Mroz in the Phase I PRISM report, contain disseminated galena with lesser
amounts of sphalerite and copper minerals in a stratiform zone extending more than 25 kilometers (km) (fig. 1). The samples are from stromatolitic limestone in the Mesoproterozic Khatt Formation in the El Mreïti Group. This zone of mineralization is interpreted to be syngenetic in origin but with an aspect of epigenetic mineralization. Gangue calcite and dolomite are associated with the mineralized zone. The report notes that fractured zones are mineralized. b. Timzac—One sample of mineralization is reported to be in the upper part of the Char Group (fig. 1). The notes in PRISM-I state that the mineralization extends for more than 16 km. This occurrence is described as stratiform in nature and contains dolomite, sphalerite, and geochemical anomalies of lead.
It is important to note that the presence of MVT-style mineralization, even in minor to trace amounts as in the above occurrences, is highly suggestive of a larger hydrothermal system that may have produced undiscovered ore deposits in the region. Typical MVT ore forming processes generally are regional in scale, affecting thousands of square kilometers and may produce widespread traces of mineralization. Furthermore, carbonate-hosted mineralization with a stratiform nature has commonly been misinterpreted as syngenetic in origin. As Leach and others (2005) point out, this is not necessarily correct, and many MVT deposits with stratiform mineralization have commonly been misclassified as SEDEX. Considerably more examination is needed to arrive at more rigorous classification for the Mauritania occurrences. SEDEX Pb-Zn Deposits No confirmed SEDEX lead-zinc deposits have been described in Mauritania. However, the potential for SEDEX mineralization in the southern Mauritanides was suggested in the PRISM-I reports and include the Ba-Mn-(Au) stratiform deposits (Bou Zrabie, Mbout, Eli Ajar, Ouechkech, Vararate Sud, and Oued d’Amour) hosted by the Groupe d'El Mseigguem. These deposits have been compared to deposits in the Carolina Slate Belt of the U.S. However, USGS review of these deposits (Taylor and Giles, 2014) concluded that this classification is unjustified. A more likely classification is that these occurrences are similar to volcanic hosted massive sulfide deposits (veins and tabular ore bodies).
Figure 1. Permissive geology for Pb, Zn, and Ag deposits in sedimentary rocks. Potential for SEDEX lead-zinc mineralization is recognized in the Taoudeni Basin. The presence of geochemical and mineralogical data that are favorable for MVT mineralization must also be considered permissive for SEDEX in the Taoudeni Basin. These occurrences are described in section 4.1 and include: c. Gouirat El Khatt—Three samples of stromatolitic limestone, collected by J.P. Mroz in the Phase I PRISM report, contain disseminated galena with lesser amounts of sphalerite and copper minerals in a stratiform zone extending more than 25 kilometers (km) (fig. 1). The samples are from stromatolitic limestone in the Mesoproterozic Khatt Formation in the El Mreïti Group. This zone of mineralization is interpreted to be syngenetic in origin but with an aspect of epigenetic mineralization. Gangue calcite and dolomite are associated with the mineralized zone. The report notes that fractured zones are mineralized.
d. Timzac—One sample of mineralization is reported to be in the upper part of the Char Group (fig. 1). The notes in PRISM-I state that the mineralization extends for more than 16 km. This occurrence is described as stratiform in nature and contains dolomite, sphalerite, and geochemical anomalies of lead. Potential Mineral Tracts Mississippi Valley-Type Pb-Zn (Ag) Deposits The vast Proterozoic and Paleozoic Taoudeni Basin represents a major epicratonic platform sedimentary sequence of the West African craton. The sedimentary cover of the Taoudeni Basin includes regionally extensive carbonate rocks, dominantly stromatolitic limestone and dolomitized limestones, which are potential hosts for MVT ores. Favorable units include the El Mreïti Group, the Char Group and the Atar Group, which comprise a 400 meter (m) thick Proterozoic section of dominantly siliciclastic rocks with alternating sections of stromatolite-bearing carbonates (fig. 2). Figure 1 shows the distribution of sedimentary rocks of the Taoudeni Basin that are classified as dominantly carbonate (Bradley and others, 2015). The El Mreïti Group, the Char Group and the Atar Group are highlighted because they contain, or are spatially related to MVT style mineralization, and they contain geological attributes that are favorable environments for MVT mineralization.
Figure 2. Generalized stratigraphy of part of the Hodh Supergroup. Redrawn from Pitfield and others. (2004).
The attributes of the Proterozoic rocks in Mauritania that favor MVT mineralization include: Carbonate rocks of the El Mreïti Group, the Atar Group and the Char Group comprise a vast Proterozoic carbonate platform of the Taoudeni Basin. There are occurrences of MVT style mineralization in these rocks. Mauritania and the Taoudeni Basin experienced major orogenic events that are essential for producing regional MVT hydrothermal systems and the fracture/fault controlled permeability required for large scale brine migration. These include the important continental-scale ore-forming events of the Pan African and the Hercynian orogenies. The presence of evaporites and evaporative environments, which are essential for generating the sedimentary brines that are the MVT ore fluids. The presence of extensive unconformities that provide a network of karst dissolution features that focus ore fluid migration. The presence of basal siliciclastic aquifers in the basin that can provide long distance fluid flow. The presence of red bed sequences in the basin can provide important sources for the metals.
Despite these favorable attributes for the presence of undiscovered MVT resources in the Taoudeni Basin, on a global perspective, there are few economic MVT deposits hosted by Proterozoic carbonates (see discussion in Leach and others, 2005). Globally, abundant stromatolites characterize the Archean and Proterozoic carbonate platforms. The appearance of abundant MVT deposits in the latest Neoproterozoic and early Phanerozoic marks the transition from carbonate platforms consisting of relatively impermeable stromatolitic limestone to platforms dominated by bioclastic carbonates. This observation supports the contention that carbonates older than the latest Neoproterozoic in the Taoudeni Basin have significantly lower potential for MVT deposits. Nevertheless, the Pering and Bushy Park MVT deposits in South Africa are hosted in highly fractured and karsted Archean stromatolitic limestone. Therefore, consideration of fault density and distribution of unconformities in the carbonates of the Taoudeni Basin will yield a higher confidence in the delineation of favorable tracts. An additional permissive favorable track includes the carbonate-bearing siliciclastic rocks of Cambrian age. These rocks must be considered permissive tracts for MVT deposits based on the presence of carbonate rocks of this age that are favored host rocks for MVT deposits on a global scale. The other area of Mauritania that could be considered permissive for MVT deposits is the Coastal Basin, which includes Mesozoic to Cenozoic sedimentary rocks that are up to 9 km thick in the depocenter to the west of Noukchott (Pitfield and others, 2004). The Coastal Basin is part of the Senegal Basin, which is the largest Atlantic margin basin in northwest Africa (Wissmann, 1982). The Cenozoic rocks in the Coastal Basin are not permissive for MVT deposits, because Cenozoic rocks do not host MVT deposits elsewhere (Leach and others, 2005). The Mesozoic rocks are permissive because Jurassic and Cretaceous rocks elsewhere host MVT deposits, including: the El Abed deposit, Algeria (38 Mt @ 2.3 percent Pb, 3.5 percent Zn); the Mehdiabad deposit, Iran (217 Mt @ 2.3 percent Pb, 7.2 percent Zn); and the Reocin deposit, Spain (62 Mt @ 1.4
percent Pb, 11.0 percent Zn) (Leach and others, 2005; Bouabdellah and others, 2012; and references therein). The Coastal Basin formed as a result of rifting that led to the breakup of Gondwana, which began along the present northwestern margin of Africa in the Middle Triassic. Crustal separation that created the Central Atlantic began in the Early Jurassic (Janssen and others, 1995). The early rifting produced a sedimentary environment conducive to the accumulation of thick Triassic salt deposits, whereas the marine conditions in the Jurassic and Cretaceous allowed development of carbonate rocks in the offshore region of the Mauritanian segment of the Coastal Basin (fig. 3). Mesozoic sedimentary rocks of the Coastal Basin thin onshore, and then pinch out to the east; at the outcrops that delineate the eastern margin of the Coastal Basin, Cenozoic sedimentary rocks sit unconformably on Paleozoic to Precambrian basement (Pitfield and others, 2004). This onshore thinning of the Coastal Basin means that if carbonate rocks occur in the onshore Mesozoic stratigraphy, they may occur at a shallow enough depth to form a permissive tract for MVT deposits. This hypothetical permissive tract is entirely concealed, because the Mesozoic carbonate rocks do not outcrop onshore in the Coastal Basin. Delineation of such a permissive tract would require lithofacies mapping of the concealed Mesozoic sediments in the onshore region of the Coastal Basin. Published data do not provide sufficient detail to allow this, but future research that delineates the distribution of lithofacies in the subsurface would be very worthwhile for mineral and petroleum prospectivity studies. Nevertheless, available offshore data indicate that there is a very low probability that substantial thicknesses of Mesozoic carbonate rocks occur in the onshore portion of the Coastal Basin (Brownfield and Charpentier, 2003; fig. 4), so at this time the onshore portion of the Coastal Basin cannot be considered permissive for MVT deposits because it does not appear to contain favorable host rocks.
Figure 3. Generalized stratigraphic column for the Northern subbasin and the southern part of the Mauritania subbasins (modified from Brownfield and Charpentier, 2003).
Figure 4. Schematic cross section showing the distribution of sedimentary facies in the Mauritania offshore, northern Senegal Basin, northwest Africa (From Brownfield and Charpentier, 2003). 4.2 SEDEX Pb-Zn (Ag) Deposits The same tracts in the Taoudeni Basin that are considered permissive for MVT Pb-Zn-Ag deposits must also be considered permissive for SEDEX ores. The presence of stratiform Pb-Zn mineralized samples over a large area is characteristic of SEDEX ores. Furthermore, 22 percent of all SEDEX metals known on the Earth are hosted by carbonate units in clastic sedimentary sequences (Leach and others, 2005). However, the most critical attributes of SEDEX ores are their locations in tectonically active passive margin sequences dominated by shales or in continental rift basins. Neither of these attributes can be applied to the Taoudeni Basin. The Coastal Basin could also be considered permissive for SEDEX deposits. As with MVT deposits, there are no recognized SEDEX deposits of Cenozoic age, so only the Mesozoic portion of the Coastal Basin could be considered permissive. The basin formed on a passive margin, and has recognized evaporite and shale sequences. However, these favorable units occur in the offshore portion of the basin, and there is little to no evidence to suggest the occurrence of subbasins that contain thick shale sequences in the onshore portion of the Coastal Basin (Brownfield and Charpentier, 2003; fig. 4). There is moderate potential for SEDEX mineralization to have recently formed or to presently be forming offshore of Mauritania in the Coastal Basin area. This area
contains 4 of the 7 global attributes for the occurrence of SEDEX ores given in section 3. Of particular importance is the presence of active seawater evaporation sites that generate the fluids capable of transporting lead and zinc and the presence of west facing passive margin sequences containing organic matter as reductants for seawater sulfate (Leach et al., 2005). SEDEX deposits form in subbasins where sedimentation rates are commonly very low, and where anoxic bottom water allows sulfide accumulation (Leach and others, 2005). However, the eastern Atlantic continental margin of northwest Africa, particularly between 15°N and 26°N, is characterized by to infrequent but large-scale mass movements that produce debris flows and turbidity currents (Weaver and others, 2000; Antobreh and Krastel, 2006, 2007). Therefore, even though the modern offshore coastal basin may have the appropriate chemical conditions to allow recent to present deposition of SEDEX mineralization, it appears to lack the appropriate sedimentary environment for this mineralization to accumulate and be preserved as significant deposits. Future work could evaluate whether the offshore region contains stable subbasins with low sedimentation rates where accumulation of SEDEX deposits may be favorable. Conclusions The sedimentary cover of the Taoudeni Basin with regionally extensive stromatolitic limestone and dolomitized limestones are permissive tracts for MVT deposits. Analogs to similar MVT deposits include the Pering and Bushy Park deposits of South Africa. Favorable indications that these carbonates are permissive include the presence of evaporites, unconformities, proximal major orogenic belts, and mineral and geochemical occurrences that appear to be similar to MVT assemblages. Late Neoproterozoic carbonate platforms that mark the first significant host for MVT deposits (located in South Africa) may be an analog for the Proterozoic carbonate platforms of Mauritania. If this is correct, a focus on faults and karst dissolution areas may be prospective. Permissive tracts for SEDEX deposits coincide with the tracts indicated for MVT resources. However, the potential for SEDEX deposits is much less because the geodynamic model for the Taoudeni Basin differs from the geological settings that host world SEDEX deposits. Available data suggest that the onshore Coastal Basin is unlikely to contain appropriate host rocks for MVT or SEDEX deposits, so based on current knowledge, this area cannot be considered permissive. Acknowledgments I thank David L. Leach, who prepared the preliminary version of this report, and John Horton, who prepared figure 1 and the plate that accompany this report. Al Hofstra and Andy M. Killick provided helpful and insightful reviews.
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