The document summarizes the polymetallic mineralization in the Wadi Al-Masilah Sedimentary Basin in Yemen. Key minerals include Pb, Zn, V, Mn, Fe and barite. Mineralization formed through hydrothermal processes and weathering, and is structurally controlled along fault zones. Weathering led to supergene enrichment of Zn, Pb, V, Mn and Fe. Notable minerals include hemimorphite, willemite, smithsonite, galena, cerussite, anglesite, wulfenite, descloizite and vanadinite. The basin hosts large Pb-Zn ore bodies exceeding 100,000 tons. Geochemical data indicates high

1. Polymetallic-barite mineralizations at Wadi Al-Masilah Sedimentary Basin, Mahrah province, Yemen
Kamal Mohamed Ali Mattash
University of Aden-Yemen, kamalmattash@yahoo.com
Abstract
The Wadi Al-Masilah property totals 300 km2
and hosts high grade ore minerals, including Pb, Zn, V, Mn, Fe and barite.
Economic minerals occur entirely in the residual sedimentary plateau, which overlies late Proterozoic basement. They
formed by hydrothermal processes and by the action of slightly acidic groundwater. The prospecting area is structurally
controlled and located in a NW-SE major fault zone and its parallel lineaments. Tectonic uplift and intensive fracturing
systems, along with favorable climatic conditions gave rise for the development of supergene Zn, Pb, V, Mn and Fe
mineralizations.
Barite crops out as vein fillings, as stratiform type, or as an association with the silicified dolostones. Deposition of barite
mineralization based on fossil records is believed to have occurred during the Cretaceous and continued up to the
Quaternary. Geochemical results indicate high quality for the vein material, in which barite contents averaging 96 wt.%,
and its specific gravity beats the API values, and may reach 4.37 g/cm3
.
Hypogene mineralizations consist of sphalerite, galena and pyrite, and the ore bodies follow the NW extensional fault
system. Available data indicate several lead-zinc ore bodies at variable depths, and some of which are considerably large
with dimensions attaining 60x30x50m, and the tonnages of individual large ore bodies exceed one hundred thousands
tons of Pb and Zn combined.
Nonsulfide minerals occur in a variety of structures, such as: shear zones, fractures, vugs, cavities, brecciated veins, and
open spaces of the vein barite sidewalls, and connected mainly to the dolostone sequence. Variety of zinc contents are
reported, ranging from low (1-8 wt.%), passing through medium (13-23 wt.%), and reaching higher grades (30 to >50 wt.
%), contained in: hemimorphite, willemite, smithsonite, hydrozincite and zincite. However, lead grade ranges between 1
and 20 wt.%, included in galena, cerussite, hydrocerussite, along with lesser contents of wulfenite, anglesite, plattnerite,
and plumbonacrite.
Vanadium ores occur in association with lead-zinc as vanadate, and mineralization is characterized by the following
types: i) dissemination, ii) stockwork, and iii) massive descloizite developed on the veins sidewall or in cavities.
Descloizite has crystals with different sizes (up to 3 mm) and shapes, including tabular, pyramidal and prismatic, mostly
having strong metallic luster. Color is varying from metallic black to olive-green, to pale-brown and pale-red, and rarely
golden. Results of geochemical analyses indicate that V content may locally grade up to 12.30 wt.%. Anomalous V
contents, grading up to 0.1%, 0.2%, and 0.7% are reported from Mn ore minerals, silicified dolostones and siliclastic
materials, respectively. However, vanadinite crystals are small six-sided prisms ranging in size up to 1 mm, having bright
orange to brown color and characterized by hollow structure. Vanadinite occurs in vugs and fissures of the limestone and
is reported from deep oxidation zone in association with nonsulfide lead-zinc minerals. These prospects contain one of
the extra amazing occurrences of descloizite known, and descloizite is probably the mineral resource for which Wadi Al-
Masilah would be most promising.
Mn and Fe ore minerals, include several discordant irregular veins striking NW-SE, near horizontal or in some cases
steep dipping ore bodies. Mineralogy consists of hollandite, romanechite, cryptomelane, pyrolusite, hematite, goethite
and limonite. Mn ore occurs either as massive veins, stockwork breccia-cementing material, or as porous filling types,
characterized by botryoidal of oolitic structure, indicating shallow marine depositional environment, whereas Fe ore
tends to form massive veins. Results of geochemical analyses indicate high quality Mn with an average value of 64%
MnO, however Fe contents exceed 55%.
Anomalous Ag, Cd, Co, Cr, Cu, Mo, Ni, Sb, and less commonly Ga and Se are reported from all the mineralized
prospects.
Introduction
Barite ore mineralization, occurring singly or in various combinations throughout the sedimentary sequence of Wadi Al-
Masilah, connected mainly to the Cretaceous carbonates (Saa’r Formation). Barite crops out both as vein fillings and as
bedded type (stratiform), it is also found as an association with the brecciated and silicified dolostones. Vein barite ore
material is formed by the effect of hydrothermal and/or by weathering activity and was carried by hot fluids and generally
deposited in WNW-ESE trending faults, fractures, joints, as well as in planes of bedded limestone and dolostones. Veins
are characterized by extreme variation in length and can be traced through a distance of several kilometers.

2. Supergene weathering processes of the hypogene ore materials had played an important role through long periods and in
several intervals of the sedimentary strata from the late Jurassic time till the recent. Occurrence of hematite and gossans
indicate the oxidation of Fe, Pb, and Zn sulfides reported from the Wadi Al-Masilah prospects, and penetration of
hematite and gossan to greater depths suggests that nonsulfide ores occur from the surface (Saa’r Formation) to deeper
fracture zones and cavities throughout the older carbonates of the Jurassic. Oxidation of sulfides generally extends from
the surface to an average of 80-100 m in depth. This is also indicated by the occurrence of vanadinite at 97-100 m depth.
The nonsulfide Zn minerals reported from Wadi Al-Masilah prospects are mixture of minerals such as Zn silicates
[hemimorphite, Zn4(Si2O7)(OH)2·H2O]. and willemite (Zn2SiO4) and Zn carbonate (smithsonite, ZnCO3). Cerrucite
(PbCO3) is frequently found in association with the zinc mineral assemblage. Nonsulfide ore minerals are white, brown,
and reddish in color. They mostly occur as massive vein material, and in some cases as fine-grained crystals associated
with the contact of vein barite material. Sometimes they are connected to vuggy and brecciated structures.
The form of the nonsulfide mineralizations varies from massive replacement veins, to the drusy and cavity structures
infilling of euhedral transparent fine-grained hemimorphite crystals at different depths of the oxidation zone. All these
prospects are located in a NW-SE major fault zone and its parallel lineaments of Wadi Al-Masilah, which are composed
of limestone, dolostones, and silicified limestones, siliclastic material and shale. Significant economic nonsulfide
mineralization overlaid the dolomitic limestone, as well as the silicified limestone. The middle part of these prospects has
a shape of a subsident infill, and as a whole they extend for a length amounts 25 kilometers, and a width exceeding 10
kilometers. Our results are based on bulk chemical as well as X-ray diffraction analyses of the mineralized veins and
other altered units. All specimens analyzed are relatively Fe-poor varities, where Fe content does not exceed 3.4 wt.%.
Mg contents range from 0.14 to 0.29 wt.%, whereas Ca values range from 0.6 to 13.6 wt.%. However, Ag, Cd, Co, Ga,
and Mo are moderately to highly anomalous. Generally, the most interesting issue is that Cr in these prospects is very
highly anomalous and its values may reach 6800 ppm.
The oxidized zinc ore is a structurally controlled wall side fillings, connected to vein barite materials, in the Saa’r
dolomitic limestone of Cretaceous age.
No other known deposit consists of diversity of oxidized minerals like those found in Wadi Almasilah deposit, i.e., barite,
hemimorphite, willemite, smithsonite, zincite, hydrozincite, cerussite, anglesite, wulfenite, descloizite, vanadinite,
hollandite, romanechite, cryptomelane, pyrolusite, hematite, limonite, and goethite. Although we think that this deposit
is similar to several deposits throughout the world, we did not experience that such mineral assemblages do occur in any
type of deposits connected to limestones or dolomites. Moreover, the iron sulfide of our original ore body was oxidized to
hematite and limonite.
Within the nonsulfide assemblage, willemite, is the first mineral to be deposited, in which willemite crystals has replaced
sphalerite directly. Smithsonite partially replaces willemite and locally replaces hemimorphite, whereas hemimorphite
replaces coarse-grained willemite crystals, therefore, smithsonite and hemimorphite in particular signify the final
generations of the nonsulfide mineral assemblage. Mostly, nonsulfide minerals are associated with Fe-Mn oxides and/or
hydroxides. However, sauconite is mainly combined with hemimorphite.
In some cases, significant content of galena is found in association with some portions of vein barite material and varies
between 1 and 25% of the bulk.
The mineral assemblage at Wadi Al-Masilah prospects extends from overbearing hemimorphite, smithsonite, and
cerussite on/or near the surface, to a mixture of smithsonite, hemimorphite, willemite, and zinc clays in deeper zones (up
to 50-70 m, below the surface, or deeper). Massive willemite ore bodies are assumed to be available at the lower part of
the oxidized supergene product’s deposit, hinted with variable amounts of hemimorphite and smithsonite. Significant
amount of cerussite was reported as an associated ore connected to the zinc mineral assemblage, in which Pb may reach
up to 8%. The probable source of SiO2 that entered the hemimorphite and willemite is the underlying weathered igneous
rocks and/or the silicified limestones and dolostones, as well as the shale materials.
The mineralogy of the oxidized ore material, in the order of abundance, is given as follows: Hemimorphite, smithsonite,
willemite, hyrozincite, zincite, calcite, maghemite and qurtz.
Conclusions
Economic minerals of Wadi Al-Masilah prospects occur almost entirely in the residual sedimentary plateau, which
overlies late Proterozoic basement rocks. They formed by hydrothermal processes caused by the circulation of fluids
produced by Quaternary volcanic activity, and/or by the action of slightly acidic groundwater which dissolved the soluble
carbonates through a long period of weathering and erosion.
The prospecting area is located in a NW-SE major fault zone and its parallel lineaments. Tectonic uplift and intensive
vertical and horizontal fracturing systems, along with favorable climatic conditions gave rise for the development of
supergene zinc-lead mineralizations. Nonsulfide minerals usually occur in a variety of structures, such as: shear zones,
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3. fractures, vugs, cavities, brecciated veins, and open spaces between the vein barite sidewalls, all connected mainly to the
upper portion of the Cretaceous dolostone sequence. They are characterized by a variety of zinc contents ranging from
low (1-8 wt.%), passing through medium (13-23 wt.%), and reaching higher grades (30 to >50 wt.%), contained mainly
in: hemimorphite, willemite, smithsonite, hydrozincite and zincite minerals. The first two occur either as massive
constituents of the matrix of supergene breccias having fine to very fine-grained transparent, white, pale yellow, pale
brown to pale red euhedral crystals, or as reddish-brownish botryoidal concretions. Smithsonite and hemimorphite,
mostly developed at the expense of willemite, signifying the final generations of the nonsulfide mineral assemblage.
Willemite-rich materials characterized by bright-green iridescence. However, the Pb grade ranges between 1 and 20 wt.
%, included mainly in neoformed galena, anglesite, cerussite, plattnerite, hydrocerussite, plumbonacrite, pyromorphite
and wulfenite.
The ore minerals are usually disseminated in a variety of structures such as solution breccias and pipes, shear zones,
fractures and cavities. The mineralized zones usually occur in the upper portion of the sedimentary sequence (Saa'r
Formation), in particular.
Barite is formed as a result of hydrothermal process, and/or as the result of weathering processes, where Ba mainly
reached the solution in the form of BaCl2, but when reacts with SO4-root it forms barite immediately. Barite can be easily
solved, but because of increasing in redox-potential (Eh), again it precipitates as barite. Individual ore bearing structures
are largely sedimentary bedded planes, usually associated with near horizontal dolostone high. Mineralization, based on
fossil records is believed to have occurred during the Cretaceous time and continued up to the post-rift phase of the Gulf
of Aden. Some ore appears to be pre-faulting, some is prepared by faulting and in association with silicified zones by the
affect of hydrothermal activity, and some is present as patches in faults, cavities and openings.
All the characteristics of the nonsulfide Zn–Pb ores in Wadi Al-Masilah prospects indicate alteration and oxidation of
sphalerite and galena that resulted in the formation of the supergene mineral assemblage, yielding hemimorphite,
willemite, smithsonite, hydrozincite, and zincite as well as cerussite, hydrocerussite, anglesite, wulfenite, pyromorphite,
plattnerite and plumbonacrite by precipitating them in fractures, cavities, brecciated veins (sometimes stockwork), barite
side walls, silicified host rock and partially as replacements. Tectonic uplift and intensive fracturing system, along with
favorable climatic conditions gave rise for the development of supergene deposits, including Fe, Mn, Pb, V, and Zn.
Evidence of the formation of Pb-Zn nonsulfide deposits can be expressed by the effect of oxidation intensity that had
substantially reached considerable depths ranging up to 100 m and probably to greater than 150 m below the surface. In
addition, formation of willemite can be ascribed to the hydrothermal processes that caused by the influence of the
volcanic activity.
The preponderance of hemimorphite, indicates the persistent replacement of willemite, in the upper levels of the Wadi
Al-Masilah nonsulfide deposits, and this is in accordance with the typical mineral assemblage of hemimorphite and/or
smithsonite found in most European supergene nonsulfide ores (e. g., Calamine deposits of Belgium). However,
willemite is considered as a product of hydrothermal activity (e.g., Berg Aukas, of the Otavi Mountains, Namibia) at
temperatures exceeding 100 °C, for that willemite becomes more significant in the lower levels of the deposits, and due
to reaching of oxidizing fluids, it is intensively replacing sphalerite. This is because willemite is more stable at higher
temperature as well as at lower water activities. In addition, willemite overcomes smithsonite at lower CO2 fugacity,
along with higher silica activity, which was provided by the hydrothermal alteration of igneous rocks of the underlying
metamorphic basement.
Geochemically, vanadium has lithophile character, but at the surface and within shallow depth, vanadium indicates
oxyphile affinity. The relative slight siderophile character of vanadium, indicates that the concentration of vanadium may
locally exceed hundreds times the value given by Clarck and Goldshmidt. In such oxidation zone vanadium concentration
shows similar mechanism to the formation of iron "laterite infiltration". One of these varieties is the oxidation zone of the
polymetallic lead-zinc deposits, where descloizite [Pb(Zn,Cu)(OH)VO4], and vanadinite [Pb5VO4)3Cl] are concentrated as
independent heavy metal minerals and vanadium is found as V+5
. Descloizite represents late stage of formation in which,
mineralization took place when metavanadate solutions were introduced into the deposit along vertical and near vertical
fractures. Therefore the ore types are either breccia type occurrences or metal-enriched silicified dolostones as well as
siliclastic layers, that are found near the surface, and characterize semiarid regions.
Manganese ores are practically related to the contact of the Cretaceous sandstone and the shallow marine carbonate of
Umm Er Radhuma Formation. They are probably the product of the chemical decomposition of igneous rocks, where
released manganese primarily solves and migrates as hydrocarbonate, and rarely as sulfate complex forms. Manganese
has similar characteristic feature to that of iron, where it is generally solving near lower Eh values, and precipitates when
these values are higher. Diluted and weak acidic environment affects the strongly basic Mn+2
to remain in solution. This
can be applied for the occurrence of manganese mineralization, which was precipitated in such an environment.
Based on our intensive geological and geochemical investigations carried out on Wadi Al-Masilah mineralized prospects,
we found it very promising, meanwhile we strongly recommend to follow up detailed geological, geochemical, and in
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4. particular geophysical surveys and drilling works in order to enhance an ultimate economic potential.
Nowadays, there is an increasing interest in the supergene polymetallic deposits, particularly after successfully
developing a number of procedures to facilitate processing and concentrations. In this regard, some authors have paid an
attention to the significance of the formation of zinc oxide (nonsulfide zinc deposits), for instance Sangster, 2003.
Based on our available geochemical results the Wadi Masilah Property is highly prospective for ‘Medium scale’ barite-
nonsulfide, as well as strata bound lead, zinc, iron, manganese and vanadium deposits.
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