Results of Gold Exploration in the Al Maham Prospect

SGS-TR-2001-2 J.M. Leistel, Ghazi Kattu, Ibrahim Khalil, J.M. Eberlé, Ayman Naderh, Zaben Al Ghinady, and Alim A. Sidduqui
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ABSTRACT

 

Al Maham prospect (lat. 24°25.835¢N, long. 41°57.655¢E., GPS coordinates) is located about 480 km northeast of Jiddah.
It was explored by BRGM, first in 1968 (695.50 m of trenches, sampling, SP test profiles over gossans), then in 1986-87 (mapping, geophysics [surface IP, gravimetry, magnetism, IP logging, and SP], drilling by ten percussion drill-holes totaling 697 m, and a systematic geochemical survey).
Dump sampling yield grades up to 15.8 g/t Au, and some of the drill-holes intersected mineralized structures grading up to 29.7 g/t Au over 9 m.
Surface geochemistry outlined several mineralized zones showing values between 1 and 17 g/t.
Motivated by these intersections and by the potentially large extension of the mineralization, a reassessment of this prospect by BRGM-SGS led to the drilling from February to June 2000 of eleven core drill-holes (1,450.35 m), six reverse-circulation drill-holes (400 m), and fifteen trenches (781 m).
A detailed structural study of this prospect was also performed.
The Al Maham prospect is underlain by volcano-sedimentary rocks of the Nuqrah Formation and by later intrusive dikes.
Some of the volcanic rocks clearly present the geochemical characteristics of komatiitic succession: typical Ni, Co, Cr enrichment, and Al2O3, TiO2, and V decreasing correlated with the increasing of MgO.
From the structural viewpoint, the prospect lies in a folded shear-zone underlined by a gneiss layer and characterized by N.030ºE-20ºSW lineation.
This ductile deformation is linked to the Nabitah orogeny that took place around 650 Ma.
Late brittle deformation is marked by NW-SE faults, and corresponds mostly to the formation of the Najd fault system.
Two main types of mineralization are present: 1) stratiform sulfides hosted by the volcano-sedimentary succession, locally with a Ni signature, which form easily recognizable surface gossan; 2) gold mineralization linked to quartz veins and disseminated sulfides, which is outlined at the surface by old workings scattered over a 0.5 x 2 km area.
Stratiform sulfides are mostly composed of pyrite and pyrrhotite.
They are locally enriched in nickel (13.95 m at 0.28% Ni in hole C005) with traces of cobalt, copper, chromium, and molybdenum, but generally without Au and base metals.
For the regional point of view, it is interesting to emphasize the link between this nickel indication and the komatiitic signature of the host rock.
Elsewhere in the world, important nickel sulfide deposits are associated with komatiite flows and associated intrusions.
The gold concentration may have resulted from structural control during the ductile deformation phase.  Gold appears to be associated with arsenopyrite, the highest grades being confined to meter-wide structures bearing quartz veins, whilst disseminated sulfides in the host rock give large (up to 40 m) gold bearing zones but with low grades (about 0.5 g/t).
The signature of the mineralization is dominated by Au and As, with locally traces of Zn.
In the northern part of the prospect, a continuous mineralized envelope can be drawn, including quartz veins and low-grade dissemination.  Its average gold grade is low, about 0.85 g/t Au, and the tonnage (rough estimation of 2 Mt) is too low to be of economic range.
The southern part of the prospect is marked by a N-S trending structure, with discontinuous gold bearing quartz veins.
Gold grades are a little bit higher (1.9 to 5.6 g/t Au), but the boudinaging of the veins along the structure precludes any estimation of the gold potential.
The low-grade mineralization could be an interesting point only if a high tonnage could be found.
This cannot be assumed at this stage, and the extension and continuity of “gold-rich” quartz veins is severely hampered by their frequent splitting during ductile deformation.
In conclusion, the gold potential of the Al Maham prospect does not appear very high.
Nevertheless, all the possible extensions of the mineralization were not fully tested.
Taking into account all this features, this prospect appears to be a lower priority target for gold exploration when compared with the other promising prospects.
From the regional point of view, the presence of a komatiitic signature in the volcano-sedimentary rocks of the Nuqrah Formation draws the attention on the nickel potential of this area.
The Jabal Mardah nickel deposit, located in the same formation about 80 km to the northwest, is also a good indication that strengthens this possibility.

 

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Leistel, J.M., Kattu, G., Khalil, I., Eberlé, J.M., Naderh, A., Al Ghinady, Z., and Sidduqui, A.A., 2001, Results of gold exploration in the Al Maham prospect, Kingdom of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2001-2, 39 p., 23 figs, 10 tables, 5 apps,