International Journal of Geosciences, 2011, 2, 657-668
doi:10.4236/ijg.2011.24067 Published Online November 2011 (http://www.SciRP.org/journal/ijg)
Copyright © 2011 SciRes. IJG
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Mineralogy of Metacarbonate Rocks and Garnet Deposits
at Two Selected Areas at Asir Region, Southwestern KSA
Asaad Mohammed Bakor Moufti
Department of Mineral Resources and Rocks, Faculty of Earth Sciences,
King Abdul Aziz University, Jeddah, Saudi Arabia.
E-mail: ambmoufti@hotmail.com
Received June 21, 2011; revised August 14, 2011; accepted September 27, 2011
Abstract
The mineralogical data materialized in the present work suggest that the previously described skarns at Ad
Darb in the literature are actually marble deposits intercalated with schists and phyllites of different compo-
sitions. The marble and associated metasediments lie to the west of striking ridges of marbles that are nearly
aligned in the NNW-SSE direction. Garnet at Al Madhiq occurrence often occurs in the form of bands con-
formable with rock foliation (gneissosity and schistosity). It is suggested that the paragenesis “quartz-gar-
net-epidote” is developed due to the percolation of some Al-rich solutions along rock foliation of the horn-
blende gneiss, i.e. metasomatic garnet. Careful field investigation collaborated with petrographic and SEM
studies, suggest the occurrence of another garnetiferous paragenesis associating quartz, mica and feldspar in
pegmatites, aplites and quartz veins, i.e. exclusively igneous garnet. Metasomatic garnet in the calc-silicates
of Al Madhiq is of grossular composition. It is commonly unzoned but some distinctly to slightly zoned
crystals are observed where the core is andradite-rich and the rim is grossular. Metasomatic events response-
ble for growth of garnet in the calc-silicates led also to formation of epidote post-dating grossular. Hand
specimens, microscopic investigation and BSE images prove that this epidote post-dates and replaces gros-
sular, and even rims it in some instances. Igneous garnet at Al Madhiq (almandine-spessartine) is found only
in pegmatites and aplites that are genetically related to alkali granitoids. Sulphides (dominated by pyrite)
occur in intemate association with domains rich in grossular and hence these sulphides are more likely
hydrothermal indicating reducing conditions for formation of grossular.
Keywords: Asir, Metacarbonate, Calc-Silicates, Grossular, Almandine-Spessartine, Metasomatic, Igneous.
1. Introduction
The present paper documents the main geological and
mineralogical aspects of some meta-carbonate rocks from
Asir terrane, particularly from Ad Darb and Al Madhiq
areas. Here, the present investigators aim to define any
possible skarns and calc-silicates accurately, particularly
those containing garnet in economic quantities.Marble
and skarn deposits were documented previously in Saudi
Arabia by some authors, e.g. [1-3].
According to the geological map of Fairer [4] shown
in Figure 1, there is a considerable mass of skarn deposit
cropping close to the town of Ad Darb in Asir terrain,
near the Red Sea coast. Careful survey by the present in-
vestigators revealed that nothing documents such nomen-
clature in literature and now detailed petrologic studies
have been carried out in order to decide whether they are
actually skarns or not. The nomenclature given by Fairer
[4] did not rely on sampling and petrographic rock iden-
tification. It was possibly suggested because of stereo-
scopic investigation using topographic sheets and aerial
photographs.
The paper aims to characterize and distinguish meta-
carbonate rocks at both Ad Darb and Al Madhiq areas at
Asir terrain in order to fit their proper lithostratigraphic
position, in addition to synopsis on their mineralogical
composition and possible economic potentialities. In this
respect, it is proposed to distinguish metacarbonate rocks
(either in the field or microscopically). Also, the paper
furnishes a trial to testify the presence of possible skarn
deposita at Al Madhiq area which were previously sug-
gested by some authors [5-7].
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2. Procedures and Techniques
For the sake of a detailed petrographic study, several
representative samples were collected from the two stu-
died areas at the Asir terrain, namely Ad Darb and Al-
Madhiq. Thin- and polished-sections of representative
samples were selected for the petrographic study, both in
transmitted and reflected lights.
X-ray diffractograms were obtained using a PW 1840
Philips diffractometer with Ni-filtered Cu Kα radiation at
30 Kv and 20 mA and normal scanning rate of 12θ/1
cm/minute, housed at the research laboratories of Faculty
of Earth Sciences, King Abdulaziz University. For addi-
tional credibility, some representative ground samples
were also prepared and sent to the XRD laboratory of the
Saudi Geological Survey (SGS) in Jeddah in order to
verify the mineralogical identification of ore minerals
and accessory silicate minerals of low modal abundance.
For the identification of both silicate and ore minerals in
the investigated samples, selected Powder Diffraction
Data for minerals documented in the ASTM electronic
cards were used.
Scanning electron microscopy with energy dispersive
X-ray attachment (SEM-EDX) will be conducted on a
Philips machine Model XL 30 workable at 30 kV acce-
lerating voltage housed at the Central Laboratories of the
Geological Survey of Egypt. The obtained data are in the
form of spot semi-quantitative microanalyses of some
ore minerals, calc-silicate minerals (e.g. garnet, feldspars,
epidote, zircon, amphibole & sulphides) and finally some
accessories such as apatite and zircon. Such SEM survey
can help to detect any metallic mineralization in the stud-
ied skarn, and accordingly can be considered here as a
powerful tool for metal exploration.
3. Geologic Set Up and Field Observations
3.1. Ad Darb Marble Deposits
Current field observations by the present author proved
that there are no skarns there at all, and the whole mass is
actually marble deposits intercalated with schists and
phyllites of variable compositions. The marble and asso-
ciated metasediments lie to the west of striking ridges of
marbles that nearly aligned in the NNW-SSE direction
(Figure 1).
Figure 1. Different types of Quaternary deposits to the southwest of Abha area and possible Precambrian skarn outc rops, sky
blue colour at center of the map (Fairer, 1985).
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The schists associating the marbles are mostly chlo-
ritic and graphitic with occasional occurrence of biotite
and garnet indicating regional metamorphism in green-
schist to lower amphibolite facies conditions. The schist
itself shows remarkable mineral lineation and grain size
variation turning the rock to phyllite in parts. The schists
and phyllites are dissected by barren quartz veins and
andesite dykes where the latter display distinct spheroidal
weathering. In some instances, the marbles are invaded
by conformable basaltic to andesitic sill. In many sites, the
volume of marble seems much greater than that of the me-
tasediment intercalations.
At some other sites, the marble is intercalated with
mafic schists, mostly actinolite schist and in parts the
marble becomes black and highly silicified. The contact
between mafic schists and marble can be easily distin-
guished in the field where the contact is sometimes in-
terrupted with mafic dykes. In other cases, the contact
between mafic schists and marble facilitates the invasion
of mafic sills in between. Deformation in marble is in-
tense near major faults where shows fine grain size and
becomes sliced and/or crenulated.
Some pinkish varieties of marbles were also identified
at Ad Darb area where the pinkish variety is more de-
formed than the whitish and yellowish varieties. The
pinkish variety sometimes associates purple schists and
kinked graphite phyllite. From the stratigraphic point of
view, these metasediments belong to the Bahah group
which is underlain by dark agglomerates of the Jeddah
group with coarse volcanic clasts. These are similar to
metacarbonate of similar setting elsewhere in the world,
e.g. [8].
The pelitic schists associating marble become mafic in
some other sites and to amphibolite as well where the
contact between marble and the mafic lithologies can be
easily defined due to distinct colour variation. Stratigra-
phycally, both the schists and marble belongs to the Ba-
hah group along major fault contacts with the underlying
dark mafic agglomerates of the Jeddah group.
3.2. Al Madhiq Calc-Silicate Rocks
Ahmed [5,6] presented the first description of calc-sili-
cate rocks at Al Madhiq area in Asir terrain (Figure 2)
south to Abha and Khamis Mushayt cities, particularly
about 20 km to the south of the latter city. Ahmed (op
cit.) identified these calc-silicate rocks (rich in garnet) as
“exoskarn” confined to amphibolites that were formed
near upper margin of intrusive granitoids.
The area of Al Madhiq is occupied by a series of Pre-
cambrian rocks that are capped by Cambro-Ordovician
sandstone of the Wajid Formation. The Precambrian
basement rocks consist mainly of two groups, the first is
a metamorphosed one (high-grade gneisses and schists)
as a part of thick volcano-sedimentary succession, and
second intrusive rocks that are dominated by granitoids.
The granitoids comprise different types of gneissose
granite and granodiorite, in a ddition to frequent pegma-
tites (up to 2.5 m thick) and aplite dykes. The gneisses
themselves are represented mainly by hornblende gneiss
that grades to amphibolites in part.
In the sense of skarn definition and mineralogy [9-12],
Al Madhiq garnetiferous samples are not proper skarn
but garnet is common like several calc-silicate minerals
within metasedimentary terrains [13,14].
Garnet at Al Madhiq occurrence often occurs in the
form of bands conformable with rock foliation (gneissos-
ity and schistosity). It occurs in bands of variable thick-
ness in the range of 5 - 90 cm and associates dense
quartz and epidote peripheral zones. The quartz-garnet
bands are sometimes dislocated along normal faults de-
fined by narrow aplite dykes.
Some specimen display clear rimming of garnet by
epidote, which was supported microscopically as will be
given in some back-scattered electron images (BSE). The
paragenesis quartz-garnet-epidote is developed due to
percolation of some Al-rich solutions along the rock fo-
liation of hornblende gneiss, i.e. metasomatic garnet.
Careful field investigation collaborated with petro-
graphic and SEM studies, suggest the occurrence of an-
other garnetiferous paragenesis associating quartz, mica
and feldspar in pegmatites, aplites and quartz veins, i.e.
exclusively igneous garnet.
4. XRD Runs of Al Madhiq Samples
Some representative samples were selected for XRD runs
of some samples from Al Madhiq area in order to define
accurately the mineral paragenesis in the calc-silicates
and igneous dykes (pegmatite & aplite).
Two samples of garnetiferous calc-silicates were ana-
lyzed using the XRD technique after mineral purification
in order to minimize the effect of co-existing minerals. It
was obtained that the garnet is grossular (Figure 3) and
this will be supported by the mineral chemistry in the
next section.
On the other hand, igneous garnet in both pegmatite
and aplite was hardly detected because of its low modal
abundance (~5%). The resulted patterns were for major
silicate minerals such as feldspars (microcline & albite)
and quartz. It was possible to identify igneous garnets in
these igneous dykes in samples investigated using the
SEM technique. High magnification enables us to iden-
tify such igneous garnet and do spot chemical analysis
for it.
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Figure 2. Detailed geological map of Al-Madhiq area at Asir terrain (Ahmed, 2002).
5. Mineral Chemistry of Al Madhiq
Garnetiferous Calc-Silicate Rocks
The present section focuses on the chemical composition of
mineral paragenesis of the studied calc-silicate rocks that
are characterized by common occurrence of garnet, i.e. gar-
netiferous. On field, microscopic and mineral chemistry
bases, it was possible to distinguish between two different
types of garnet, 1) Metasomatic garnet (mainly grossular) in
calc-silicate rocks, and 2) Igneous garnet (almandine-spes-
sartine) dispersed in pegmatite and aplite dykes. Identifica-
tion of different garnet species was possible based on XRD
runs and optical discrimination following the mineralogical-
chemical schemes of [15,16].
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Figure 3. XRD patterns of garnetiferous calc-silicate samples from Al Madhiq area.
5.1. Metasomatic Garnet and Epidote
The metasomatic garnet is usually abundant (up to 70
modal %), massive and occurs in the form of thick ag-
glomerated garnet crystals in the form of bands. Such
garnet bands can be traced megascopically and micro-
scopically as meso- and micro-bands, respectively. Seve-
ral garnetiferous calc-silicate samples were analyzed
using the SEM-EDX technique in order to have a broad
idea about the chemistry of existing garnet and other
minerals in the paragenesis.
Spot chemical analyses given in Table 1, supported by
SEM back-scattered electron imaging (BSE), prove that
the garnet in Al Madhiq calc-silicate rocks is either
zoned or unzoned. Zoned crystals are much less common
that the unzoned ones. The unzoned garnet is commonly
coarse, locally sub-idioblastic and appears interstitial to
other silicate phases (Figure 4). The last figure shows
also abundant coarse quartz inclusions in this unzoned
garnet of grossular composition. Considerable amounts
of Mg and Na in parts of unzoned grossular are attributed
to the presence of fine amphibole and albite inclusions
with preferential concentration at the peripheral zone. Dis-
tribution of both Ca and Fe along rim-core-rim profile in
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Figure 4. Very slight zoning in nearly unzoned gr ossular wi th coarse quartz inclusions.
Table 1. Spot microanalyses of metasomatic garnet profiles from Al Madhiq calc-silicates.
Sample No. MQ 2 MQ 2
Analysis No. 3 4 5 6 7 8 9 10 11 12
Notes R IC C IC R R IC C IC R
SiO2 35.03 34.91 34.71 33.09 33.82 34.93 35.91 35.01 35.90 34.43
Al2O3 18.77 18.58 18.35 17.48 18.32 18.30 20.57 19.60 21.66 19.13
CaO 31.15 31.54 31.97 30.31 28.06 31.62 34.94 35.29 35.47 30.33
Fe2O3 11.99 11.51 12.67 12.02 14.23 12.17 6.04 6.82 5.31 12.99
MgO - - - - 3.69 - - - - -
MnO 3.05 3.46 2.30 2.22 1.89 2.99 2.54 2.55 1.67 3.11
Na2O - - - 4.88 - - - 0.74 - -
Total Hypothetically equals to 100 wt% for all
- Not detected
unzoned grossular is almost homogeneous where CaO
ranges from 28.06 wt% to 31.97 wt% while Fe2O3 ranges
from 11.51 wt% to 14.23 wt% (Table 1). These Ca and
Fe contents may suggest that some rims of grossular
would bear little andradite component in solid solution if
it is compared with the core.
There are few zoned garnet crystals that can be ob-
served both optically (Figure 5) and chemically (Table
1), where there core is represented by andradite while the
rim is grossular. The andradite core is characterized by
remarkable Fe-depletion where Fe2O3 lies in the range of
5.31 wt% - 6.82 wt% whereas the amount is almost dou-
bled at the rim (12.17 wt% - 12.99 wt%). The variation
from andradite at core to grossular at rim is controlled by
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remarkable ionic substitution of Fe2+ for Ca2+ at the rim.
The given spot analyses of metasomatic garnets (Table 1)
either the zoned or unzoned show constant and similar
ranges of MnO (1.67 wt% - 3.11 wt% and 1.89 wt% -
3.46 wt%, respectively). This indicates no possible ionic
substitution of Mn by other divalent cations regardless
the metasomatic garnet is zoned or not, i.e. constant
spessartine component in soild solution from core to rim.
Ahmed and Hariri [7] oppose such conclusion as they
conclded that both spessartine and almandine in solid
solution of Al Madahiq garnet from the same calc-sili-
cate assemblage increase from core to rim. The present
garnet analyses show agreement with Ahmed and Hariri
[7] that grossular component decreases towards rim be-
cause this is applicable only to garnet crystals with dis-
tinct optical and chemical zoning.
Spot analyses of independent garnet crystals from Al
Madhiq calc-silicates (Table 2) indicate that they all for
grossular from three selected samples, namely MQ 4,
MQ 6 & MQ 7. BSE images suggest that most of these
crystals are unzoned and usually traversed and rimmed
by epidote as a metasomatic mineral too that can be at-
tributed to abrubt increase in alumina due to injection of
felsic bodies, mostly alkali granitoids and related pegma-
tites and dykes. This hypothesis is supported by the ap-
pearance of few sporadic igneous garnet crystals (alman-
dine, analysis# 25, sample# MQ 4) co-existing with me-
tasomatic grossular (Table 2). An aluminous alkali-rich
igneous fluid that injected the garnetiferous calc-silicates
is also responsible for 2.00 wt% K2O in this occasional
igneous almandine in the calc-silicates (Table 2).
Figure 5. BSE image and chemical rim-core-rim profile of zoned grossular garnet crystal with andradite core.
Table 2. Microanalyses of independent metasomatic garnet crystals from Al Madhiq calc-silicates.
Sample No. MQ 4 MQ 6 MQ 7
Analysis No. 19 21 22 23 25 37 40 41 44 45
Notes* Light greyLight grey Light greyLight greyLight greyLight greyLight to
dark greyLight grey Light to
dark greyLight grey
SiO2 32.71 34.28 33.19 31.93 37.52 35.00 34.56 34.71 34.22 34.90
Al2O3 9.28 10.33 8.26 9.77 12.26 16.51 16.98 16.30 17.09 18.08
CaO 28.73 30.67 26.79 26.68 11.60 34.73 34.30 33.85 30.60 30.29
Fe2O3 24.48 21.12 27.02 22.70 30.92 13.67 12.46 12.30 14.65 14.50
MgO - - - 4.57
4.31 - - - 1.42 -
MnO 3.93 3.60 4.74 3.41 1.40 - 1.70 2.84 2.04 2.23
TiO2 0.87 - - 0.97 - - - - - -
K2O - - 2.00 - - - - -
Total Hypothetically equals to 100 wt% for all
- Not detected. * Tone of grey shades as observed on the back-scattered electron (BSE) images.
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BSE images that are shown in Figure 6 witness inva-
sion of metasomatic grossular garnet with metasomatic
epidote. Epidotization of grossular is either regular along
sub-parallel cubic cleavage or irregular depending on the
intense of replacement (Figure 6(a)). Someimes, epidote
appears invading and rimming grossular (Figure 6(b)).
In some few instances, epidote severely replacing gros-
sular (Figure 6(c)) represents immature epidote compo-
sition with garnet fingerprints (analysis# 47, Table 3).
Such analysis shows considerable alkali content (6.40 wt%
Na2O) that again supports metasomatic origin of epidote
contemporaneous with the injection of felsic bodies.
(a) (b)
(c)
Figure 6. BSE images of analyzed epidote showing its textural relationship with grossular. (a) Epidote invading grossular
garnet. (b) Epidote rimming and invading garnet. (c) Epidote (with verimiclar quartz inclusions, dark grey) replacing gros-
sular (light grey).
Table 3. Spot analyses of epidote associating garnet in Al Madhiq calc-silicates.
Sample No. MQ 4 MQ 6 MQ 7
Analysis No. 18 20 24 36 39 47*
SiO2 36.64 35.17 35.49 35.52 37.00 33.32
Al2O3 22.74 21.48 21.20 24.75 24.41 22.94
CaO 23.63 25.09 24.36 26.95 26.24 15.05
Fe2O3 16.98 18.26 17.33 12.78 12.35 11.91
MgO - - 1.62 - - -
Total Hypothetically equals to 100 wt% for all
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5.2. Igneous Garnet
Modal percentage of igneous garnet in pegmatites and
aplites is remarkably low (up to 5% only). It occurs as
dispersed crystals in these felsic rocks. Sometimes, non-
equant garnet can be observed adjacent to mica flakes
(Figure 7(a)) where it is cracked and xenomorphic. Cra-
cking in other garnet crystals is rarely found (Figure
7(b)). Equant sub-idiomorphic garnet are common oc-
curring interstitially to quartz and sodic feldspar (Fig-
ures 7(c) and (d)). Figure c documents the presence of
fine zircon inclusions in both garnet and other silicates.
Geochemically, the igneous garnet is transitional in
composition between almandine and spessartine (Table
4) and hence it can be safely considered as Fe-Mn garnet
that characterizes aluminous and peraluminous granitoids.
Garnet crystal with maximum spessartine component has
the highest MnO and Fe2O3 contents (analysis# 48 with
25.59 wt% and 16.79 wt% respectively) indicating ionic
substitution between Mn2+ and Fe2+ (Table 4). Also, some
almandine-spessartine crystals witness limited cationic
exchange between Mn2+ and Ca2+ where CaO attains the
value of 7.32 wt% (analysis# 32, Table 4).
(a) (b)
(c) (d)
Figure 7. BSE of Fe-M n garnet in igneous bodies. (a) Cracked gar net associating mica in pegmatite. (b) Uncracked garnet in
between albite (ab) and quar tz (qz) in pegmatite. (c) Cracked equant garnet in betw een albite (ab) and quartz (qz) in pegma-
tite. Notice fine zircon inclusions (bri ght spots). (d) Subhe dral garnet in betw een albite (ab) and quartz (qz) in aplite.
Table 4. Microanalyses of igneous Fe-Mn garnet (fine, cracked & uncracked).
Sample No. MQ 5 MQ 8
Analysis No. 31 32 33 47 48
SiO2 32.43 33.63 33.15 34.39 32.43
Al2O3 19.00 19.57 20.97 20.01 20.29
CaO 4.02 7.32 2.84 4.30 4.90
Fe2O3 26.14 24.61 19.62 19.42 16.79
MnO 18.40 14.88 23.42 21.88 25.59
Total Hypothetically equals to 100 wt% for all
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5.3. Zircon
Survey both igneous and calc-silicate samples using the
scanning electron microscope (SEM) revealed the pres-
ence of abundant zircon in both types. Zircon in the
calc-silicates is most probably of igneous protolith being
connected to the metasomatized ortho-gneisses (mostly
hornblende gneiss). It is euhedral and some of it occurs
as hollowed crystals or cracked zoned ones.
Spot analyses of zircon from either pegmatites (sam-
ple# 5) or clac-silicates (samples# 2 & 6) lack any ra-
dioactive elements such as U or Th and hence all the
tabulated analyses of zircon is for non-mentamict variety
(Table 5). Traces of Ca, Fe and Al are attributed to fine
silicate inclusions in relatively coarser zircon crystals.
Zircon analysis# 30 with 21.23 wt% Fe2O3 (Table 5)
owes such very high Fe-content to hematite impurities.
Both igneous and metasomatic zircons are Hf-free.
5.4. Feldspar and Amphibole
The presented analyses of feldspars are from pegmatite
whereas amphibole is from calc-silicate (Table 6 ). It is evi-
dent that the there are two varieties of feldspar in the peg-
matite, one as microcline perthite and the other is homoge-
neous albite. The analyzed microcline spot in the perthite is
exclusively potassic with neither traces of Na nor Ca
(analysis# 26, Table 6). Plagioclase is almost sodic with
very little anorthite component in solid solution where CaO
amounts 1.24 only. Mineral chemistry of the different varie-
ties of feldspars in the pegmatites and aplites is supported
by the microscopic investigation as well as the XRD runs.
The presented amphibole from the calc-silicate is horn-
blende, which occurs as relics in growing epidote cluster
and the hornblende itself contains euhedral apatite.
Analysis# 42 of this apatite is not tabulated because it is
solely formed of Ca and P oxides only.
Table 5. Spot analyses of zircon from Al Madhiq area.
Sample No.* MQ 2 MQ 5 MQ 6
Analysis No. 2 30 34 35 38
SiO2 36.43 29.55 35.35 31.30 36.73
ZrO2 63.57 49.22 53.50 67.85 60.06
Fe2O3 - 21.23 7.32 - -
CaO - - 3.83 0.85 -
Al2O3 - - - - 3.22
Total Hypothetically equals to 100 wt% for all
- Not detected. * MQ 2 & MQ 6 are calc-silicates, whereas MQ 5 is pegmatite.
Table 6. Spot analyses of feldspars* and amphibole** from al madhiq area.
Mineral K-feldspar in perthite
(microcline) Na-plagioclase (albite) Amphibole
Sample No. MQ 5 MQ 5 MQ 7
Analysis No. 26 29 43
SiO2 64.22 50.93 40.85
Al2O3 20.18 22.53 -
TiO2 - - -
CaO - 1.24 20.27
Fe2O3 - - 18.45
MgO - - -
MnO - - -
Na2O - 25.30 15.31
K2O 15.59 - -
Total Hypothetically equals to 100 wt% for all
- Not detected. * from pegmatite (igneous rock member). ** from calc-silicate (metasomatic rock member)
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5.5. Sulphides
Ore microscopic investigation collaborated with the BSE
images revealed that sulphides are absent in the studied
pegmatites and aplites but some are encountered in the
calc-silicates, e.g. samples# 2 & 4. Sulphides in the calc-
silicates are dominated by pyrite that displays distinct
size and abundance variations.
Also, mode of occurrence of pyrite varies as it in cases
occurs as fresh pentagonal pyritohedron as inclusions in
grossular. This is a probable clue for reducing conditions
for development of host metasomatic grossular garnet.
This is also supported by the common existence of pyrite
with a metasomatic mineral assemblage rich in grossular
and epidote. Coarse pyrite crystals are often oxidized to
colloform bands of goethite and semi-mineraloid aggre-
gate in the form of "limonite". Chalcopyrite, when pre-
sent, occurs as fine inclusion in much coarser pyrite.
Chemical analyses of pyrite and chalcopyrite are pre-
sented in Table 7 that shows few traces of Cu of 2.51
wt% in the structure of pyrite hosting chalcopyrite.
6. Conclusions
1) Current field observations by the present author pro-
ved that there are no skarns at Ad darb area, and the
whole mass of metacarbonates is actually marble deposits
intercalated with schists and phyllites of different composi-
tions. The marble and associated metasediments lie to the
west of striking ridges of marbles that are aligned nearly
in the NNW-SSE direction.
2) Garnet at Al Madhiq occurrence often occurs in the
form of bands conformable with rock foliation (gneissosity
and schistosity). It occurs in bands of variable thickness
in the range of 5 - 90 cm and associates dense quartz and
epidote peripheral zones. The quartz- garnet bands are
sometimes dislocated along normal faults defined by
narrow aplite dykes.
Table 7. Spot analyses of sulphides from Al Madhiq calc-
silicates.
Mineral Pyrite Chalcopyrite
Sample No. MQ 2 MQ 4 MQ 4
Analysis No. 1 17 16
Notes Fresh
Relics in collformed
goethite
Relics in collformed
goethite
S 39.38 25.91 19.89
Fe 60.62 71.59 16.68
Cu - 2.51 59.07
Si - - 4.27
Total Hypothetically equals to 100 wt% for all
- Not detected.
3) The present project argues that Al Madhiq garnet
deposits can not be classified as garnetiferous skarn be-
cause of the followings: a] absence of any carbonate
minerals and marbles in the paragenesis, b] confinement
of the garnet bands to main structural trends and rock
schistosity and gneissosity, c] common occurrence of
silica and even quartzite bands associateing garnet bands,
and finally d] zoned garnet crystals at the garnet-country
rocks contact are uncommon indicating weak fluid cir-
culation in contrast to thermal aureoles icharacterizing
skarn deposits.
4) It is suggested that the paragenesis quartz-garnet-
epidote is developed due to percolation of some Al- rich
solutions along the rock foliation of hornblende gneiss,
i.e. metasomatic garnet. Careful field investigation col-
laborated with petrographic and SEM studies, suggest
the occurrence of another garnetiferous paragenesis as-
sociating quartz, mica and feldspar in pegmatites, aplites
and quartz veins, i. e. exclusively igneous garnet.
5) Metasomatic garnet in clac-silicates of Al Madhiq
is of grossular composition. It is commonly unzoned but
some distinctly to slightly zoned crystals are observed
where the core is andradite-rich and the rim is grossular.
6) Metasomatic events responsible for growth of gar-
net in the calc-silicates led also to formation of epidote
post-dating grossular. Handspecimen, microscopic inves-
tigation and BSE images prove that this epidote post-
dates and replaces grossular, and even rims it in some
instances.
7) Igneous garnet at Al Madhiq is observed in pegma-
tites and aplites connected to alkali granitoids. In this
case, the garnet is intermediate in composition between
almandine and spessartine. The present study is the first
record of igneous garnet in these felsic dykes because
previous studies in the same locality failed to record ig-
neous garnet in the pegmatites cutting the granitoids,
gneisses and amphibolites, e.g. [7].
8) Zircon in igneous dykes (pegmatites & aplites) and
metasomatic calc-silicates is non-metamict and of or-
tho-origin in both cases.
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