Several differentiated intrusions
are enriched in F, Li, W, Sn, Sb, e.g., Bir Tawilah tungsten (265,000 t at
1.2% WO3, Jebel Abal, and Wadi Hazarh (MODS 04573). These intersect
the Ash Shiam series and seem to postdate the main gold-mineralization.
Polymetallic mineralization, Pyrite
occurrences, Oxidized gossans
Three exhalative sedimentary
occurrences in black shale were studied in detail. They are the Shaib Lamisah
prospect, the Ar Rjum Zinc prospect, and the Al Garith prospect (mostly
argentiferous), but their resources are likely to be too small to be economic.
About 20 gossans and pyrite
occurrences are described from black shale in the Ash Shiam and Ad Dafinah
areas. Several of these have never been tested for gold.
Al Himar is an epithermal-type
occurrence. It contains a stockwork with chalcedony and manganese oxide, and
was discovered during a geochemical survey. This mineralization may be related
to the volcanism that accompanied the Jibalah-basin faulting, around 550 Ma,
i.e. post Naj faulting.
Exploration perspectives
Several old gold workings areas (As Suq-Al
Haffirah, Azzeraib, Bir Tawilah, Jabal Mugherah, Mangar An Nam) are partially
covered by wind-blown sand or thin alluvial deposits. They strongly merit
additional exploration, such as bedrock geochemistry and geophysics, to locate
extensions to known mineralization, a technique that was very successful at
Zalim. The Bir Tawilah – Ghadarah thrust fault also has a high potential for
mineralization associated with syn-tectonic intrusions and listwaenite. The gold
occurrences associated with differentiated F-W intrusions also should be
re-investigated from a porphyry or perigranitic viewpoint (case of the tungsten
intrusions mentioned above). In a volcano-sedimentary context, the polymetallic
occurrences of Shaib Lamisah and Ar Rjum Zinc should be retested for gold,
something that was not done either for some of the other base-metal targets. At
Shaib Lamisah, gold anomalies were identified, but not checked by drilling.
Mineralized district N° 6: Samran – Mahd adh Dhahab-Jabal Sayid
Mineralization
Gold
Only a dozen gold occurrences are
known from this district, which nonetheless contains Mahd adh Dhahab, the most
important gold occurrence of Saudi Arabia. This is an epithermal, low-sulfidation,
adularia-type, polymetallic deposit, the total production of which, including
that by the Ancients, has been estimated at over 100 tons of gold (Doebrich
et al., 1999). The reserves before the last stage of exploitation were
around 1.2 Mt at 24 g/t Au, 92 g/t Ag, 0.65% Cu and 3.11% Zn.
Systematic rock-geochemistry over a
50 m x 50 m grid discovered new reserves (Roberts et al., 1978) in the
metavolcanic rocks hosting the old workings in rich quartz veins. The
polymetallic mineralization of chalcopyrite, sphalerite, galena, gold and
silver tellurides, and rare electrum occurs in a dense vein network. The veins
were filled in successive stages, are ribboned, and contain cockade breccias
that cut felsic agglomerate and crystal tuffites. A subsequent porphyritic
rhyolite intrusion may have been the driving engine for fluid circulation. The
mineralization is older than 709 Ma, the age of the intersecting dikes (Huckerby
et al., 1983). Zinc grades increase to the north and at depth, whereas the
gold grades decrease commensurately. In November 1999, the remaining 0.65 Mt
were estimated to contain 12 g/t Au and 3.11% Zn. Four successive alteration
stages are observed: early alteration with quartz, sericite, pyrite; an
intermediate stage with chlorite, sericite, microcline, sphalerite, pyrite;
the gold-bearing stage with quartz, chlorite, sphalerite, galena, pyrite,
gold, tellurides, and accessory siderite-calcite-hematite; and a final stage
with quartz, calcite, and barite.
Minor (apparently) gold indications
are known from near Mahd adh Dhahab, as Lahuf, or an older (769 Ma) epithermal
silver occurrence associated with the Ram Ram caldera (Saeed Al Yazidi, 1997).
Volcano-sedimentary massive sulfides
Four occurrences
of this type are known in the district: Umm ad Damar, Samran, Shayban, and
Jabal Sayid. The last is a deposit with substantial reserves.
Jabal Sayid lies 40 km east of
Mahd adh Dhahab, in acid metavolcanic rocks that may be younger than the Mahd
succession. A major Cu gossan indicated the presence of ore in the north limb
of an anticlinorium in the roof of rhyolitic paleo-domes. The ore-bearing
level is marked by a temporary halt in volcanic activity, and chemical
sedimentation of chert and chloritite (Milesi, 1984; Pittre, 1985). The tree
main ore types are: massive breccia copper ore (orebody 1), finely ribboned
ore that is rich in zinc and depleted in copper (orebodies 1 and 4), and
stockwork ore with dominant chalcopyrite (orebodies 2 and 4). Total resources
of the copper-rich ore are 20 Mt at 2.68% Cu and 5 Mt of zinc-dominant ore at
4% Zn and 0.5% Cu. Lead content is low (120-900 ppm) and gold has not been
systematically assayed for, even though high spot values were found (12 g/t in
the gossan, 2.3 g/t in a Zn stockwork in orebody 4, and 18 g/t in orebody 1,
which grades are systematically associated with good silver values.
Umm ad Dammar comprises several
gossans with rich copper that were intensively mined by the Ancients. Located
in the structural continuation of the Sayid formation, Umm ad Dammar may be an
extension of the Jabal Sayid mineralized system, if one restores the hostrock
to its position before sinistral NNW-trending strike-slip faulting offset the
Jabal Sayid unit from the Mahd adh Dhahab one.
The Samran–Shayban–Baydan belt
contains over 14 occurrences of the volcano-sedimentary massive-sulfide or
disseminated-polymetallic-sulfide type. The copper stockwork of Samran
contains a resource of 0.9 Mt at 2% Cu. The Baydan mineralization (Bellivier
et al., 1997) is a concealed polymetallic VMS (0.6 Mt at 20% Zn equiv.)
associated with black shale intercalated in acid volcanic rocks; and laterally
within the black shale, barite and jasper are found outcropping. Three classic
types of mineralization for this type of deposit were found: Au-Ag (Cu-Pb-Zn)
remobilized mineralization in black shale, massive polymetallic, but
dominantly zinc mineralization in ribboned sulfides, and Cu-Zn stringer and
disseminated mineralization in the chloritized breccia stockwork of the
footwall. The deposit is partly affected by faulting and folding, but moreover
an adverse topography/mineralization-dipping combination is a strong handicap
for the search of additional resources.
The Bari prospect includes a network
of east-west quartz veins, more than 4 km wide, intersecting Hufayriyah
tonalite on the edge of calc-alkaline granite near Harrat Khisb. Within this
area, showing a relatively high density of old workings for gold, a
polymetallic Pb-Zn-Cu-Au-Ag-As mineralization (Cassard
et al., 1987) was found over 39 m in a drill-hole as vein sulfides with
sphalerite (1.2% Zn), pyrite, and arsenopyrite with a high gold content (7
g/t), surrounded by a Sb halo. No extension to this rich mineralization was
found at that time. However, it might be profitable to resume exploration as
many positive indicators exist for the existence of polyphase intrusions or a
porphyry: biotite microgranite in sills; subvolcanic dikes; pegmatite with
magnetite and hornblende; a Cu-Pb-Zn-Sn-Sb geochemical zonation; the existence
of disseminated pyrite and magnetite in the boreholes testing the IP anomaly
north of the prospect; and potassic and propylitic alteration in the veins.
The Bari prospect evokes Hollister’s (1978) "Diorite model", i.e. dominant
albitic and propylitic alteration (carbonate, zeolite, epidote) and only minor
potassic alteration, low sulfur activity leading to the coexistence of pyrite
and magnetite (as at Bari), and a late gold-pyrite-base metal paragenesis.
Rare Earths, thorium, uranium
mineralization
South of Jabal Sayid, the
alkaline Nb-Y-Zr Jabal Hadb / Ash Sharar granite contains reserves of 23 Mt at
0.13% Nb; 0.13% Ce, >1.7% Zr, and 134 ppm U (Elliott, 1997). REE are found in
bastnaesite, doverite, monazite, and synchisite; Nb is associated with
pyrochlore, Zr with zircon, and thorium with thorite and thorianite. The
monzogranitic complex hosting these rocks is dated 573 Ma (Rb/Sr: Calvez and
Kemp, 1982).
Several copper occurrences of
marginal interest are found in the Bir Umq serpentinites north of Jabal Sayid.
Exploration perspectives
This district has a high potential for
various types of mineralization. Epithermal, volcano-sedimentary, and prophyry
occurrences merit a systematic geochemical coverage with the aim of finding
other mineralized structures, in particular near interruptions in the volcanic
activity. The last are indicated by the presence of breccia, epiclastic rock,
and black shale. Other areas of potential interest are around rhyolite domes and
subvolcanic microgranites.
At Bari, some deep drill-holes on the
IP anomalies north of the prospect should test for the presence of Zn- and
Au-bearing diorite porphyry.
At Jabal Sayid, it is recommended to
make an accurate assessment of the gold potential in the gossan, in the poorly
tested sulfidic zones, and in particular in the polymetallic baritic
mineralization found in a lateral position to the stockwork.
The Bir Umq sub-district warrant
additional investigations for base- (mainly copper) and precious- metal (4 m at
20 g/t in one drill-hole) mineralization.
Mineralized district N°7: As Safra-Musayna’ah
Mineralization
Four types of mineralization are found in the As Safra
- Al Maham - Jabal Hamick - Nuqrah belt : massive-sulfide sedex,
epithermal, and porphyry,
and vein-type installed on shear-zones.
The belt, which in addition to the
volcano-sedimentary component contains black shale and turbidites, hosts over 18
gold occurrences, one of which, Bulgah, is potentially economic (50 Mt at 1 g/t
Au), as well as 29 gossans or stratiform pyritic occurrences and 8 copper
occurrences of which 2 are associated with serpentinite slivers.
Massive-sulfide sedex
The Nuqrah massive-sulfide sedex
mineralization is disseminated and polymetallic, within graphite schist and
carbonates near the top of the pyroclastic Halaban formation (Delfour, 1971).
The mineralization occurs in north and south zones, 4 km apart and each marked
by a gossan. That of the (largest) south gossan represents about 1 Mt at
0.8% Cu, 1.8% Pb, 5.6% Zn, 220 g/t Ag, and 4-33 g/t Au. The disseminated and
massive sulfides with sphalerite, galena, chalcopyrite, and pyrite also contain
Pb-Bi sulfo-antimonides and Pb, Bi, Ag, and Ni tellurides. Molybdenite and
linneite are known from the paragenesis. The mineralization of the north gossan
is smaller: 400,000 t at 0.75% Cu, 1.22% Pb, 6% Zn, 332 g/t Ag, and 2.5 g/t Au,
and strong talc alteration affects the carbonates. In addition, Mo grades of
118-275 ppm were intersected in drill-holes, as were mercury values of 2,300 to
13,300 ppb) and pyritized rhyolites dikes.
The Jabal Hamick sedex prospect, in the
center of the Hulayfah volcano-sedimentary belt, has ancient workings that
stretch north-south over more than 1.5 km. The hostrock is chlorite-schist
(meta-rhyolitic tuffs) with a subvertical carbonate bed, in dacitic and
andesitic tuff. The gold (4 g/t) occurs in ferruginized chlorite schist with an
anomalous zinc content of >1%, but depleted in Cu and Pb. A silver-bearing level
is found nearby. The prospect merits further work: an EM anomaly that can be
followed over 3 km to the north has not been tested.
At Jabal Mardah, several gossans
overlie basic volcaniclastic rock to the northwest of the central belt, which is
part of the Darb Zubaydah ophiolite succession. Disseminated sulfides (pyrite,
polybasite, vaesite, polydymite, sphalerite, chalcopyrite) occur in
volcani-clastic rock and sandstone, overlying basalt affected by propylitic
alteration. A rough resource estimate is 1.5 Mt at 0.8%Ni, but no data are
available on precious-metal contents.
Epithermal mineralization
occurs in a carbonate context at As Safra, and in younger volcanic
rocks at Asfar al Hadawi.
The As Safra prospect has extensive old
workings in a shear-zone developped in rhyolitic tuff associated with
carbonates, intruded by subvertical rhyolitic dikes. Two types of mineralization
are found: copper with variably high (nul to 40 g/t Au) gold in schistose
metavolcanic rock, and base metals plus locally very high (> 200 g/t Au) gold in
carbonate breccia and metavolcanic rock. Skarn-type alteration (epidote, garnet)
affects the rocks and could indicate thermal metamorphism related to the late
dike injection. The old workings were mainly focussed upon vein mineralization,
but the hypothesis of epithermal mineralization trapped at the intersection of
carbonate beds and rhyolite dikes merits further testing.
The Asfar al Adawi occurrence lies
northwest of Hulayfah, in an undeformed (Shammar age?) ignimbritic caldera. It
consists of fractures filled with barite, and silver and uranium minerals,
corresponding to a gold-depleted epithermal acid-sulfate type. Other fluorite
and Mn-oxide occurrences are associated with rhyolitic Shammar breccia in the
east of the district.
Porphyry type mineralization
The Musayna’ah prospect, over 6 km
long, is one of the largest concentrations of old workings for copper in the
Shield. Delfour (1971) studied the area for the possibility of massive-sulfides
associated with andesite. Work carried out as part of this project has shown the
Cu-Au potential of the area to be very large and of four types:
1) Disseminated Cu sulfides
(chalcopyrite mainly) in andesitic breccia, with minor Au and Cu in veinlets
cutting andesite and rhyolite dikes. K-feldspar alteration, biotite,
magnetite, and hematite accompany the Cu sulfides (cuprite, tenorite).
Laterally, other parageneses with chalcedony, chlorite, carbonate (propylitic
alteration), and auriferous pyrite (lateral epithermal type) are more
indications for a large porphyry system centered on syntectonic grano-dioritic
intrusion. The last also contains pegmatites with magnetite, tourmaline, and
Cu oxides.
2) Vein-type rich copper-irregular
gold (up to 14 g/t in gossan) sub-massive sulfides (main ore of the ancient
miners)
3) Disseminated Cu and Au (up to 13
g/t) in breccias displaying cockade structure;
4) Other Cu occurrences associated
with magnetite are located south of the intrusion (Hamra) and skarn type
parageneses are known farther north.
Vein-type mineralization
Bulgah, some 75 km south of Nuqrah,
occurs in diorite of the same age as that of Sukhaybarat, and is polyphase as
well. Quartzitic tonalite and mafic and felsic dikes cut the diorite intrusion,
which is affected by a shear zone that caused micro-fractures with arsenopyrite,
pyrite, and pyrrhotite. The main geochemical signature is Au-As, with in the
south of the prospect a Sb-Cu-Zn-Ag signature resembling the one found at Bari.
Mineable reserves (heap leaching) are presently (Portacio, pers. comm., 11/99)
estimated at 33 Mt at 1.1 g/t Au
Maham is another vein-type gold
prospect, in a favorable intersection of acid metavolcanic rocks and black
shale. The mineralization is epigenetic with Ni (pentlandite), Cu
(chalcopyrite), Zn (sphalerite), molybdenite, and gold-bearing arsenopyrite in
quartz, as well as a stratiform mineralization with a similar composition and
additional magnetite, marcasite, and graphite in black shale. A drill-hole
intersection gave 29 g/t Au over 9 m.
Tayma shows old workings over several
hundreds of meters in granodiorite. At the contact with quartz veins, sericite,
carbonate, pyrite alteration occurs. A drill-hole intersected 3 m at 8 g/t Au.
Exploration perspectives
The Cu-Au Musayna’ah prospect has to be
re-studied with a Cu-Au porphyry perspective. Mapping of the surface alteration
coupled with a rock-geochemistry survey, and an adapted ground geophysic
campaign are recommended for delineation of drilling targets.
The gossan prospects with anomalous
Ni-Mo-Ag-As grades of the central area merit study, either from the viewpoint of
low-grade gold associated with young polyphase intrusions of the Bulgah type, or
from that of Au, PGE, Mo, Cu mineralization in black shale.
Mineralized district N° 8: Sukhaybarat-Silsilah
Mineralization
The Sukhaybarat-Silsilah district
contains over 40 gold occurrences where silver is subordinate. These are
generally installed on shear-zones and hosted , either by Murdama
volcano-sediments, or by post-Murdama intrusives of the Idah suite. Some Cu-Mo
porphyries also occur in this context, as does the Mibari Cu-Au prospect (E.
Jaques, A. Al Jehani 1998) or the zoned Silsilah greisen intrusive with tin
mineralization.
Gold mineralization in shear zones
Gold mineralization in shear zones is
found in extensional fractures related to shear-faulting. The hostrock is either
an intrusive whose emplacement was related to the structural movement, or a
metasedimentary rock that was affected by the thermal halo around the intrusion.
Biotite hornfels occurs up to 100 m distant from the intrusion and is marked by
a negative ground-magnetic anomaly. The post-Murdama intrusives were dated at
622 Ma (Walker et al., 1994).
Gold mineralization in intrusive rocks
Sukhaybarat mine was discovered as the result of a
systematic re-evaluation of old workings. The mine, which is now almost worked
out, will have produced 21 tons of gold from ore grading on average 2.5 g/t Au
The gold occurs in en echelon
fractures filled with quartz with arsenopyrite and pyrite, and traces of
chalcopyrite and galena. The fractures intersect hornblende-biotite diorite
cut by a tonalite and late porphyritic microgranite dikes. Alteration with
actinolite, pyrite, sericite, chlorite, arsenopyrite, and carbonates affects
the walls over 30-40 cm. Gold grades in the walls are significant at
0.5-1 g/t. Geochemically, the alteration stands out through a clear increase
in K2O, CaO, MgO, and CO2, and a decrease in Na2O.
The An Najadi prospect attracted attention
because of its very large size (4 x 0.7 km) and the fact that its alteration
halo is very similar to that of Sukhaybarat. Resources of 250,000 tons at
1.76 g/t Au were found in ancient dumps (Walker et al., 1994), but
subsurface grades were found to be erratic and no economic resource could be
sized. The fracturing that affects the small near-surface diorite stock might be
related to the rise of a larger and deeper batholith beneath an anticlinal axis
in the Murdama, and more exploration appears warranted for this large and
unexplained old-working area.
The Meshaheed prospect is composed of several
vein clusters, associated to the shear faults northeast of An Najadi and,
locally, to hematitic alteration. Recent re-evaluation of this prospect as a
large-tonnage/low-grade target was unsuccessful (Lewis et al., 1993), but
more exploration should be carried out.
Several gold occurrences occur in rather flat-lying
shear zones with northeasterly dips, in the thermal halo of the Silsilah
granite. The Raha and Shaila prospects are very close to a major thrust fault
with a northeast vergence that affects Murdama rocks and serpentinite schuppen,
i.e. a favorable host environment for the trapping of peri-intrusive
gold-bearing fluids. Wadi Shaba is another prospect of this type that was
drilled in 1993, with disappointing results of 260,000 tons at 2.4 g/t Au for
Wadi Shaba East. (Schull, 1993). Hematite alteration and traces of stibnite were
found in the dumps.
Porphyry-type mineralization with Cu-Mo
Several occurrences with gold and
silver around intrusions, or with copper and molybdenum within them, were
studied in detail as porphyry-type or epithermal mineralization targets.
-
Al Habla, east of Sukhaybarat, has a great potential
for this type of mineralization: old workings extend over 3.7 km and the
average grade in the dumps is around 7.8 g/t Au.
-
The Mibari Cu-Mo-Au prospect (devoided of old
workings) was explored in 1998 (E.Jaques and A. Al Jehani, 1997-1998),
searching for the potassic core of a porphyry system with Cu and Mo. But
trenches and relatively shallow drilling remained in the propylytic zone, and
if the volume of Cu-Au mineralized rocks is significant (around 80 Mt), the
average grades were disappointingly too low (0.18 g/t Au, 0.12% Cu).
Nevertheless this exploration suggests that Cu-Au (Mo) low-grade/high tonnage
targets, not attractive for the ancient miners, might as well be underexplored
and missed by exploration carried out along the last decades.
-
The Hibshi prospect is located near a major shear
fault. It is known to contain a Cu-Mo mineralization and should be re-visited
with new work hypothesis and in particular tested for gold.
Greisen-type mineralization with
cassiterite related to the Silsilah intrusions
The Silsilah intrusions form a huge
ring structure of 12 km diameter, dated at 587 Ma. Its core is alkali granite,
with a rim of aplite and peralkaline commendites. This intrusive complex has
been interpreted as a subsidence structure of the "Simple Bell Jar" type (Roobol
and White, 1986). Within the alkali granite, a greisen-type alteration with
muscovite and topaz contains disseminated cassiterite mineralization. The sized
resources were about 1.5 Mt at 0.19% Sn, but the full potential may be not fully
assessed.
Exploration perspectives
The district has a good potential for
shear-zone-type mineralization related to young intrusions as well as for
porphyry/epithermal deposits and perigranitic mineralization. Systematic
regional geochemical sampling over a regular grid as well as ground geophysics (gravimetry,
magnetism) should provide a clearer idea of the potential of this district,
which is marked by a high density of ancient workings.
Mineralized district N° 9: Baid Ad Jimalah – Ad Dawadimi
Mineralization
Four types of mineralization are known
from the district:
1) Polymetallic sedex mineralization
at Ar Ridanyah, on the edge of the Abt Schist.
2) Perigranitic Sn-W mineralization
at Baid Ad Jimalah, Minyah, and Umm Hadhir.
3) Au-(Cu) mineralization related to
the Ruwah-Halaban shear zone in an ultrabasic context, or Au-(Pb-Zn-Ag)
mineralization in Abt schist near rhyolite dikes (Hajlan).
4) Silver vein mineralization that
might be epithermal in a context of polyphase intrusions.
Several small Cu, Mo, or Ag occurrences
related to subvolcanic or young peralkaline intrusions are potentially of the
porphyry type (Ar Ruwaydah complex, etc.), or the topaz-rhyolite type enriched
in F-Sn-Nb-Zr-Th (Hajlan prospect; Ferrand, 1985).
Two major targets, Ar Ridanyah and
Arjah (Ag) have been evaluated in more detail:
Several gossans occur where
microgranite dikes intersect marble and metavolcanic chlorite-biotite schist.
The schist, associated with basalt, might be part of the Humayan ophiolites
below the Abt Schist to the east, where they are thrust over the Dawadimi
batholith to the west. A resource of 1.5 Mt at 5% Zn was indicated by drilling.
Several IP anomalies were found, one of which ("B") is associated with graphite
schist with disseminated pyrrhotite depleted in base metals. In addition to the
stratiform mineralization, it is possible that porphyry type mineralization
could be found: a Sn-Mo-Cu-Ag anomaly ("G") related to a breccia intersecting a
quartz porphyry was noted northeast of the prospect (possible analogy with
Hajlan).
Samrah ancient mine and the Arjah–Dawadimi
silver district
More than eleven silver occurrences,
including the ancient mine of Samrah, are located on the Dawadimi batholith and
its surrounding Abt Schist.
Samrah ancient mine lies in the
Dawadimi batholith, an andesine granodiorite to hornblende-oligoclase granite.
It encloses xenoliths of the old mafic Rharaba complex that comprises gabbro and
norite. In its center, the batholith is folded into a N-S antiform with potassic-feldspar
alteration, cut by aplite (muscovite granite) and pegmatite dikes. Along the old
workings, a shear foliation (N.070°E. strike and southeast dip) extends over
more than 1.2 km. Along the edge, the granodiorite is hydraulically brecciated
and re-cemented by quartz, chlorite, and carbonates. A pink microgranite dike
was injected parallel to the shear. All indications point at an epithermal
mineralization related to this late subvolcanic episode. The resources at Samrah
were evaluated at 278,000 t at 653 g/t Ag.
Most silver prospects, whether in
intrusives (Sydriah) or Abt schist (Arjah) have a paragenesis of galena,
sphalerite, Ag, Pb and Cu sulfo-antimonides, and carbonates, and are related to
horsetail extension fractures that may be caused by movement along the Halaban (Najd)
fault. The paragenesis and the propylitic alteration indicate an emplacement
through low-temperature (250°) and low salinity fluids (L. Bailly, pers. comm.).
Exploration perspectives
Paupy et al. (1985) argued that
the probability of finding an economic silver target in this setting was low.
However, it might be worthwile to (1) re-appraise the Ad Dawadimi silver
district with the hypothesis of porphyry/epithermal models (such as Mexico Ag-Pb-Zn
districts) and (2) re-investigate certain gold occurrences in the thermal halo
of W-Sn leucogranites (Baid Ad Jimalah, Umm Hadhir, Minyah, etc.). For this, an
airborne radiometric survey will be of precious help.
Mineralized District N°10: Al Amar
Mineralization
Several occurrences and a Pb-Zn deposit
are of the volcano-sedimentary type; other precious- and base-metal deposits,
such as Al Amar, are epithermal. About ten gold occurrences are related to
shears associated with the major Al Amar Fault Zone. A major magnetite deposit (Jabal
Idsass) is related to andesite, and several copper occurrences are part of the
basic/ultrabasic Jabal Rugaan complex. Skarn-type and peri-granitic occurrences
are known as well.
Polymetallic volcano-sedimentary
mineralization
The Knaiguiyah Zn-Cu deposit
The Khnaiguyiah deposit consists of
four stratiform ore lenses, intercalated at the top of a felsic metavolcanic
succession and accompanied by hydrothermal carbonate breccia. The whole was
deformed in a shear zone. To the northeast, Phanerozoic rocks cover the deposit.
The mineralization consists of
sphalerite, chalcopyrite, pyrite, magnetite, and hematite, and the manganese
minerals rhodonite, rhodochrosite, and Mn-garnet. The gangue is rich in epidote,
chlorite, and carbonates. As in other massive-sulfide models, copper enrichment
is noted at the base of certain bodies. An asymmetry exists between the
K-Cu-Fe-B alteration haloes in the footwall, and those with Mn-Zn-As-Pb in the
hanging wall and to the sides of the orebodies (Prévot and Barbier, 1984). The
demonstrated resource is about 16 Mt at 5.8% Zn and 0.8%Cu.
Other, similar-type occurrences in the
district are Wadi ar Rufia and Marjan.
Epithermal mineralization
The Al Amar deposit was found beneath
extensive old workings in metavolcanic rocks. Initially, it was interpreted as
Au-Cu-Pb-Zn vein mineralization; then it was seen as an exhalative
massive-sulfide deposit with base metals, before being recognized as epithermal
gold mineralization accompagned by Ag, Cu, and Zn. Five basic volcanic cycles,
intermediate to acid, were found at Al Amar. An early mineralization developed
as a stockwork with quartz, pyrite, chlorite, sericite, barite, and sphalerite
in "Unit 2", and sulfide lenses with barite and talc occur at the top of felsic
"Unit 3". Most of the gold mineralization is found in two zones: the "North
vein", striking N.100°E. with a 70°SW dip, and the subparallel "Breccia vein".
The North vein shows polyphase filling
with cockade breccia and ribbons of sphalerite, pyrite, and chlorite. There is a
correlation between zinc and gold grades. Gold and silver tellurides are found,
as at Mahd adh Dhahab.
The estimated resource is 1 Mt at
33 g/t Au, 31 g/t Ag, 7.8% Zn, and 0.87% Cu for the "North vein", and 1.1 Mt at
9.4 g/t Au, 26 g/t Ag and 5.75% Zn and 0.68% Cu for the southern "Breccia vein".
Several occurrences of the same type,
but with few old workings and much more discrete hydrothermal alteration,
yielded disappointing results, such At Taybi (south of Al Amar) where Zn-Cu-Ag
sulfides were intersected, Umm Ad Dabah (270,000 t at 2.5% Cu), Umm ash Shalahib
(epithermal with Cu-Zn-Au), and Marjan (Au-Ag-Cu-Zn).
Gold prospects related to the Al Amar
fault or its satellites
Several vein areas are found along the
mylonitic N-S or NE-SW zones that are superimposed to the Al Amar Suture Zone,
and may locally correspond to the later Najd deformation episode (Selib, Wadi
Merjan and Fawarah). The mineralized quartz veins generally contain pyrite
(abundant in the wall-rocks, as at Selib) and carbonate alteration (particularly
developed at Fawarah) that can be sericite-rich in the wall-rocks. Such veins
commonly are late extension features that cut across mylonitic schistosity,
which itself can be marked by early and barren quartz boudins, such as at Wadi
Merjan.
Some veins occur in flat faults with
sub-horizontal striations that can be slightly wavy because of late compression,
as at Selib, related to transpression along Najd faults.
Other gold prospects are associated
with vein zones in the wall-rocks of structurally late microgranitic or
rhyolitic dikes, e.g., Umm Shaddon and Wadi Khyam. At Umm Shaddon, the
geochemical signature of the mineralized areas is clearly polymetallic (Au-Pb-Zn-Cu-Ag-As-Sb).
At Umm Ash Shara, the gold is associated with a fracture network that intersects
the Al Amar fault, marked by listwaenite and cut by small plagiogranite and
diorite intrusives.
Porphyry type mineralization
At Wadi Garrah, a circular peralkaline
granite cupola of about 100 m diameter occurs in non-outcropping reg conditions,
within a regional setting of gneissic tonalite. The intrusion has an altered
(F-K) core with a diffuse quartz stockwork, and a ring of flat-lying quartz.
Orientation rock-sampling (BRGM Mission, 1997) yielded significant grades of Mo
(up to 0.6%), Au (up to 0.8 g/t), and Cu (up to 0.6%). Given these values, the
real extension of this porphyry-type mineralization assessed, as well as other
similar deposits should be searched.
Exploration perspectives
Re-investigation of the At Taybi and
Umm Ad Dabah occurrences appears warranted, on the base of the Al Amar
deposit-model.
The magnetite lenses at Jabal Idsas,
whose economic interest as iron ore is limited might indicate the presence of
blind gold-copper porphyry mineralization according to the Hollister (1978)
model.
The copper occurrences of Jabal Rugaan
merit a check from the viewpoint of possible gold and PGE mineralization
associated with ultrabasic cumulates.
Exploration of selected (with the help
of compiled previous geochemical samplings) areas of the Al Amar Belt for
porphyry deposits is necessary, as well as a regional re-appraisal of the Al
Amar Fault Zone for mainly gold and PGM.
Mineralized district N°11: Ash Shumta- Al Khushaymyah
Mineralization
Three groups of occurrences are known.
In the northwest, the As Shumta area is mostly auriferous; in the center, the
Aklyah (gold) Afif area contains base metals associated with metavolcanic rocks
and black shale; in the south, the Kirsh-Al Kushaymyah-Hammat Um Musnifah area
hosts porphyry indications with Cu, Mo and base metals, and some silver-gold
occurrences.
The Ash Shumta area contains a dozen
gold occurrences agenced like a crown around the contact between granodiorite of
the Idah suite and metavolcanic Hulayfah rocks. The Ash Shumta prospect, which
resembles Sukhaybarat, has been the subject of detailed work. It is a bundle of
flat veins at the roof of a granodiorite cupola in contact with Murdama rocks. A
greisen-type alteration developed in the vein walls (Bounny et al.,
1987). A halo with pyrite, sericite, and K enrichment is seen in the wall. Gold
is associated with pyrite. The paragenesis also includes base-metal sulfides and
trace sulfosalts and oxides (magnetite, hematite, ilmenite, rutile). The main
difference with Sukhaybarat is that arsenopyrite is practically absent. Despite
the apparent large size of the prospect, results from reconnaissance percussion-
and core-drilling campaigns at Ash Shumpta were disappointing.
The Simfan umm Shieh prospect, 5 km
southwest of Ash Shumta, contains several old workings related to silicified
fractures in granodiorite and a set of perpendicular rhyolite dikes. Here, the
average values from dump samples were high, 5 to 50 g/t Au, but the volumes of
mineralized rocks appear limited.
The Al Qoom intrusion located to the
west is potentially interesting because of the large number of gold occurrences
within and around this cupola in a structurally disturbed area with abrupt
changes in the directions of late dikes, a setting comparable with the one of
Sukhaybahrat or Zalim, but with significant additional copper content that may
suggest a porphyry signature.
The Aklyah prospect, farther south,
comprises pyritiferous quartz veinlets that intersect a syenite intrusion. The
resources identified by drilling are modest.
The Afif area hosts medium-grade
metamorphic metavolcanic rocks, including marble, gneiss, amphibole micaschist,
and pyritiferous quartzite and chert, in a graben surrounded by granitic and
granodioritic intrusives. Surface geochemistry and a geophysic investigations (EM-INPUT)
revealed several Pb-Zn anomalies. But two reconnaissance drilling campaigns on
the "North" and "South" prospects only found disseminated mineralization (6 m at
3.2% Pb, 3.1% Zn, and 0.8% Cu) of apparently limited extension.
The Kirsh/Wadi Salamah area
The Kirsh/Wadi Salamah area, which lies
in the center of the district, contains several porphyry-type Cu-Mo indications
associated with young alkaline intrusions that cut Murdama rocks, with gold
occurrences at the contact or within the thermal haloes. These occurrences are
suggested to have a strong potential, either for epithermal-, or for skarn-type
mineralization.
The Khushaymiyah-Zaen area
The Khushaymiyah-Zaen area is centered
on the Uwayjah-Al Khushaymiyah ring structure, composed of alkaline granite
intruded by a young calc-alkaline granite in the Murdama basin in the south of
the district. Cu-Mo occurrences were found in the core of the intrusion, and
silver/base-metal occurrences associated with quartz and siderite intersect the
metasedimentary hostrock. Despite this favourable organization, the disposition
seems to be of marginal interest because of the scarcity of sulfides.
In the south, the Umm Hadid prospect
contains epithermal-type mineralization associated with riedel-shear veins
related to Najd fracturing with late rhyolite and microgranite, which intersect
granodiorite that intruded metavolcanic rocks. In addition to Fe-Zn-Pb sulfides
and Pb-Ag sulfosalts, the presence of fluorite and the tungsten minerals
huebnerite and scheelite was noted. Probable resources of 500,000 t at 340 g/t
Ag were indicated by previous drilling, but no assays for gold were done.
Exploration perspectives
For gold, the Ash Shumta prospect and
the Al Qoom intrusions should be tested with deeper boreholes. It is also
recommended to re-analyze the Afif and Umm Hadid drill cores.
The gold and base-metal potential of
the district is considerable: a string of gossans has been described from the
vitric tuff and volcano-sedimentary rocks in the upper Afif group southwest and
west of Ash Shumta. Some of these gossans are associated with quartz porphyry
intrusives that cut the Afif group near its contact with Murdama rocks. These
intrusions have a calc-alkaline composition with biotite and hornblende, and
locally the plagioclase is albitic. It is recommended to systematically check
these gossans, some of which yielded significant Mo values (e.g. at Jabal Al
Yanufi by Letalenet in 1976), for precious and base metals, either in a porphyry
or an epithermal setting.
Mineralized district N° 12: Al Wajh- Umm Lajj
Mineralization
Two major mineralized sectors are
recorded in the Northwestern Shield: the Al Wajh area that hosts a dozen of vein
structures related to a Najd shear zone and two tin-tungsten occurrences
occuring in leucogranite; and the Umm Lajj area containing eight gold
occurrences related to north-south faults.
In addition to these, two lead-zinc
prospects of interest occur within the sedimentary rocks of the Coastal Plain :
Jabal Dhaylan, recently re-studied by the DMMR/USGS Mission, and Wadi Azlam to
the north of Al Wajh.
Gold mineralization
The Al Wajh area (Leanderson et al.,
1995) hosts mesothermal veins with a low sulfide content, which are related to
the main Najd deformation. The basement here comprises conglomerate, sandstone,
and basalt of the Miyah formation, and sandstone, pyritic black shale, and
carbonates of the Kibrah formation. The district was subjected to four
deformation phases: gold is associated with phases 2 and 3 that were accompanied
by a sericite-carbonate-pyrite alteration. Notwithstanding the extensive ancient
workings, and/or the presence of attractive prospects such as at Al Qubbah, the
resources found by the last exploration work were found to be limited, such as
500 kg of gold at Umm al Quayrat. Also was noted the fact that basalt was
regionally found to have an abnormally high background value of 20 ppb Au, even
away from any hydrothermal features. Scheelite occurrences were found by
alluvial prospecting, in particular around the ancient workings of Wadi Arjah (Jacquin
et al., 1983).
The Umm Lajj area also hosts
mesothermal gold mineralization related to the conjugate opening of Najd faults,
as around Al Wajh. This area as well merits a more systematic re-assessment.
The tin-tungsten leucogranites of Jabal
Liban and Wadi Unaybick have apparently a limited interest, as placer samples
below these occurrences did not show economic grades. However, at Wadi Unaybick,
a 1-km-long vein yielded good gold assays (Jaquin and Quinet, 1984).
A nickel-copper occurrence is related
to the ultramafic intrusion of Jabal Garhabah. This area, that contains gossans
in a setting of ultrabasic cumulates, merits further investigation with indirect
methods such as adapted geochemical sampling.
The Jabal Dhaylan prospect in
sedimentary setting
Several lead-zinc occurrences are
located in Miocene reef limestone overlying a paleorelief in basement rocks.
Such mineralization seems to be related to rejuvenated basement faults along the
Red Sea. Copper occurrences exist in Oligocene red sandstone. Claystone and
anhydrite, as well as brines moving along the faults, have played a direct role
in the deposition of this mineralization. At Calamine Hill (Jabal Dhaylan
prospect), 21 boreholes showed a resource of 1.2 Mt at 5.6% Zn and 1.4% Pb.
(Carlson, 1996). The deposits are under re-assessment by DMMR/USGS mission that
interpreted them as salt-dome related systems (T. Hayes, A. Siddiqui 1998) in a
complex setting, but having a real potential that warrants more investigations.
Exploration perspectives
The gold potential of Umm Lajj, Wadi
Unaybick, and Al Wajh clearly merit more regional approach as well as detailed
studies from a viewpoint of mesothermal gold related to quartz veins near or
within dioritic intrusions and leucogranite.
For lead and zinc, the potential of the
coastal plain warrants to be fully assessed, using in particular indirect
markers for Pb-Zn mineralization, i.e. geophysics for locating structural and
lithologic traps, as well as gas and fluid geochemistry.
Mineralized district N°13: Ash Shizm
Mineralization
Four types of occurrence are
found in the district:
1) Polymetallic
volcano-sedimentary mineralization of Ash Shizm and Qalat Zummurud
2) Porphyry-type
mineralization with Mo and Ag
3) REE, Nb, and Zr
mineralization related to differentiated alkalines intrusions
4) Titaniferous magnetite
occurrences in the differentiated basic/ultrabasic Qabqab intrusion.
5) Listwaenite-related gold occurrences.
The Ash Shizm prospect is a
150-m-thick stockwork that intersects keratophyric quartz lava and pumice-like
breccia, between two pillow-lava layers. Magnesian chlorite alteration is seen
laterally and in the roof, which is covered by carbonates and jasper breccia.
The mineral paragenesis includes chalcopyrite, sphalerite, magnetite, and
bornite, with accessory pyrite, tellurides, and selenides. The Fe content in
sphalerite increases in the stockwork at depth, and the appearance of cobaltite
shows an upwards-decreasing temperature gradient. Drilling (Donzeau et al.,
1980) resulted in a resource of 1 Mt at 2.93% Cu, 0.7% Zn, and 18 ppm Ag. Gold
was not systematically analyzed, notwithstanding the favorable character of such
stockworks for trapping precious metals (Leistel et al., 1994), but was
evidenced at significant level (8 to 30 g/t; Angel, 1974) in neighbouring
chlorite schists and gossan.
The Qalat Zummurud prospect
is a stratiform occurrence with Cu-Pb-Zn depleted in gold, in basic metavolcanic
rock intruded by a diorite and deformed by conjugate faults. The prospect merits
a second look from the porphyry viewpoint, as several Cu, Mo, and Ag occurrences
occur at the intersections of the late faults near the intrusions.
Gold was encountered in
alluvial panned samples within listwaenitic faulted corridors nearby the Al
Quayran occurrence.
The quartz-syenite intrusion
of Hamra, located northeast of Qalat Zummurud, is one of the richest REE
intrusions known in Saudi Arabia. The resource estimated in silexite with Fe
oxides, bastnaesite, and monazite is around 18 Mt at 0.17% Nb, 0.26% La, 0.34%
Ce, and 1.3% Zr (Elliott, 1994).
Exploration perspectives
A re-assessment of the Ash
Shizm district is necessary, including (1) a re-evaluation of the Ash Shizm VMS
deposit for which, in addition to potential discoveries of extensions of the
stockwerk and/or desolidarized massive sulfide bodies, an added content of
precious-metal might well be identified through re-analysis of core-samples and
newly collected samples, and (2) a regional re-appraisal of other VMS and sedex
potentialities.
Exploration is also warranted
for listwaenite-hosted gold deposits, as well as for PGM in mafic/ultramafic
outliers.
Mineralized district N° 14: Wadi Sawawin
Mineralization
Mineralization is found in
four areas:
- In the south, the Wadi
Azlam area hosts seven volcano-sedimentary-type Cu-Zn occurrences
- Around Wadi Sawawin,
japillite and banded iron formations (BIF)
- In the north, several
occurrences are associated with young intrusives, either of the Cu-Mo type
(porphyries), or REE-Nb associated with peralkaline intrusions
- A coastal area with Jabal
Dhaylan type mineralization with Zn-Pb, or Cu red beds.
The stratiform Cu-Zn
occurrences of Wadi Azlam can be divided into disseminated Cu-Zn mineralization
associated with volcano-sedimentary chert overlying basalt, and Cu occurrences
in fractures near late plutons (Wadi Dama, Wadi Matelian). At Muzubia,
remobilization of Cu and Zn (1-2%) (and Pb) is seen in the metavolcanic hornfels
in contact with gabbro, but the extensions of the mineralization appears very
limited (I. Salpeteur, M. Sahl, 1999).
The jaspilitic iron deposit
of Wadi Sawawin is of significant size (> 245 Mt at 42% Feox), but of limited
economic interest today. It classically signals a favourable setting for gold,
but previously carried-out sampling brought negative results.
Among the young intrusions,
the Ghurrayah microgranite with abundant quartz, microcline, and sodic
amphiboles is a potentially economic target with 440 Mt of ore at 0.22% Nb,
0.85% Zr, 0.13% Y, 400 ppm Th, and 117 ppm U. Some drilling was carried out in
2000 as an attempt to locate richer zones. Another intrusive of this type, the
sickle-shaped Jabal Tawlah is much smaller (6 Mt), but with higher Zr, Nb, and Y
grades.
The mineralization of the
Tertiary Maqnah basin in the north includes phosphates in the black pyritic
claystone of Miocene age, and barite-Pb-Zn occurrences at the base of Late
Miocene evaporites.
Exploration perspectives
From what is known, the
occurrences of the coastal plain merit to be investigated for base metals, using
paleogeography and microgravimetry studies. Additional regional gold exploration
is recommended in the iron-ore area of Wadi Sawawin and around the copper
occurrences of Wadi Dama.
- Nevertheless, in reason of
its location far from Jeddah with access difficulties, the Northwestern part of
the Shield obviously received less attention than other parts, a fact that must
be kept into mind.
Mineralized district N° 15: Wadi Kamal – Al Ays
Mineralization
The district has a high
potential for PGE-Ni-Cu mineralization related to ultrabasic cumulates. Two
areas are very promising: the basic-ultrabasic Wadi Kamal complex and the
chromiferous Al Ays ophiolite complex. Within and to the east of Al Ays, gold
occurrences related to shear zones affecting late diorite intrusions and
associated to listwaenites are potentially interesting.
PGE-Ni-Cu mineralization
related to basic/ultrabasic rocks
The Wadi Kamal complex is
55 km long and has an asymmetric synclinorium shape. From bottom to top, it
contains dunite (serpentinized), lherzolite, wehrlite, websterite, leuconorite,
anorthositic gabbro, and late trondjhemite (Chevremont and Johan, 1980). The
ultrabasic base, exposed in the south, contains a Cu-Ni gossan-occurrence that
was initially drilled for these two substances. The gossan was later (Cassard
et al., 1986) found to be rich in Pt-Pd (7 g/t Pt) and Au (6.2 g/t). The
re-analysis of the KMS1 drill-hole, undertaken in 1996 (DMMR/BRGM Mission)
confirmed the copper (0.3-0.4% Cu and nickel (0.6-1% Ni), and enhenced
significantly mineralized intervals for PGE (twice 2 m at 1495 ppb Pt+Pd) within
dunitic cumulates. Strongly contrasting Pt anomalies were found in the wadi
sediments draining ferrogabbro and melanogabbro with titaniferous magnetite of
the upper (northern) part of the complex, where Cu-Ni indications were noted in
the footwall of the main anorthosite (Salpeteur and Ziab, 1996).
The Al Ays complex is located
100 km north of Wadi Kamal. It was interpreted as part of an ophiolite complex
that was dismembered during the accretion of island arcs (Prichard et al.,
(1996). The complex comprises sepentinized harzburgite with chromite lenses,
dunite cumulates, wehrlite, and gabbro. The chromite lenses are too small to be
economic. Os-Ir-Ru alloys were described from these chromites, which are common
features in this context (Augé and Legendre, 1984), and Prichard et al.,
(1996) found Pt+Pd grades over 2 g/t in a chromite sample enriched in Ni.
Gold mineralization at
Murayib and Billiwy
This gold mineralization was
investigated for placer deposits by Riofinex, but without success. At Billiwy,
however, the possibility exists for large-tonnage/low-grade gold mineralization
related to a shear zone in diorite and its alteration halo (Sukhaybarat type).
Trenching found 32 m at 1.7 g/t Au and a 700-m-long Au-As anomaly remains to be
checked.
Exploration perspectives
The PGE potential of the Wadi
Kamal complex is high and should be systematically investigated, starting with
stream-sediment and heavy-mineral-concentrate geochemistry. The PGE potential of
Al Ays might be low, but still significantly attrative. The gold potential of Al
Ays and its eastern and southeastern surroundings, of probable listwaenite/epithermal/porphyry
type (remind the Hg-cinnabar alluvial occurrence of Wadi Itan) and resembling
Jabal Ghadarah-Bir Tawilah or Hamdah, could be of first interest and remains to
be assessed.
The Murrayjib-Billiwy area
remains interesting, with a potential for disseminated gold in a shear-zone in
diorite at Billiwy.
Mineralized district n° 16: Madinah North
Mineralization
The district includes five
gold occurrences in NW-SE shear zones with sinistral movement that intersect
altered andesitic and rhyolitic metavolcanic rocks of the Al Ays group, as well
as a copper occurrence. Gold is associated with silver and at Mutaheel with lead
and zinc.
Pyritization is intense and
strong arsenic anomalies reinforce the potential of certain structures, such as
at An Numraniyah.
As many of the other
vein-gold districts, this one merits a re-evaluation, but with lower priority
according to the very limited extensions found by previous exploration.
Mineralized district N°17: Al Lith-Taif
Mineralization
Several gossans and
polymetallic mineralization were found through regional geochemical surveys on
these slopes of the northern Asir. Re-examination of priority anomalies, in
particular upstream from gold-bearing boulders in wadis, appear necessary for a
better understanding and consequently a more accurate definition of the real
potential of this district.
Mineralized district N° 18: Ghurayrah-Khamis Mushayt
The Ghurayrah-Khamis Mushayt
district is located in the heart of the Asir. It comprises metasedimentary and
metavolcanic rocks, affected by medium-grade metamorphism and intruded by
gabbros.
The district contains a dozen
scheelite occurrences discovered through alluvial prospecting (Gaukroger, 1984).
The main occurrence at Jabal Marya consists of disseminated scheelite lenses in
leucogabbro enriched in hornblende. The best drilling intersection was 0.81 m at
0.9% WO3, but the extension of the mineralization appeared very
limited.
The scheelite mineralization
hints at a gold potential. In amphibolitic greenstones, scheelite is a good
indicator (Mueller and Grooves, 1991) for gold mineralization, or at least a
significant neighbour-substance as for example in the giant (>800 t Au) Kolar
deposit in India (GSI/BRGM report, 1993).
