- Email: [email protected]
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region, Kingdom of Saudi Arabia
Journal of African Earth Sciences. V o l .
4. pp. 1 8 3 - 1 8 8 . 1 9 8 6
0 7 3 1 - 7 2 4 7 / 8 6 $3.0t) + 0 . 0 0 Pergamon Press Ltd.
P r i n t e d in G r e a t B r i t a i n
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region, Kingdom of Saudi Arabia JAFFAR AL TAYYAR, NORMAN J. JACKSON* and SAEED AL-YAZID! Directorate General of Mineral Resources, P.O. Box 345, Jiddah, Kingdom of Saudi Arabia Abstract--The Jabalat post-tectonic granite pluton is composed of albite- and oligoclase-bearing, low-calcium, F-, Sn- and Rb-rich subsolvus granites. These granites display evidence of late-magmatic, granitophile- and metallic-element specialization, resulting ultimately in the development of post-magmatic, metalliferous hydrothermal systems characterized by a Mo--Sn-Cu-Pb-Zn-Bi-Ag-F signature. Two main types of mineralization are present within the pluton and its environs: ( 1) weakly mineralized felsic and aplitic dikes and veins enhanced in Mo, Bi, Ag, Pb and Cu; and (2) pyrite-molybdenite-chalcopyrite-bearing quartz and quartz-feldspar veins rich in Mo, Sn, Bi, Cu, Zn and Ag. A satellite stock, 3 km north of the main intrusion, is composed of fine-grained, miarolitic, muscovite-albite-microcline (microperthite) granite. The flanksof this intrusion and adjacent dioritic rocks are greisenized and highly enriched in Sn. Bi and Ag. Quartz veins which transect the satellite stock contain molybdenite and stannite.
INTRODUCTION
outcrop within 500 m to 3 km of the northern, southern and eastern margins of the granite.
THE JABALAT granite pluton (No. 3017 in the Saudi Arabian Mineral Occurrences D o c u m e n t a t i o n System, M O D S ) forms part of a N-trending, 150 kin-long array of post-tectonic granitic intrusions in the Asir region in southern Saudi Arabia, referred to as the Bahah granite belt by Jackson et al. (1985). The granite pluton is situated close to the At T a i f - A I Bahah highway, about 40 km SSE of Ai Jibub and 90 km N o f A l Bahah, at 20°42'N, 41°13'E (Fig. 1). Reconnaissance field work and sampling indicated that the pluton was radioactively anomalous and enhanced in Mo, Bi, Pb, Ag and Sn. Subsequent ground prospecting of the pluton and its environs led to the discovery of a n u m b e r of minor Mo and Sn mineral occurrences.
GEOLOGY
Main features The Jabalat pluton is a crudely rectangular body about 4 km long and 2.5-3 km wide (Fig. 2). Erosion of the core has produced a central low-lying area surrounded by rounded, steep-sided hills. Intrusive contacts with the metasedimentary host-rocks are sharp and vary in 35°
]fl°
41°
a4"
47 °
GEOLOGIC SETTING The granite was emplaced discordantly into d e f o r m e d , greenschist-facies metasediments of the Bahah group (the m3 unit of G r e e n e and Gonzalez 1980). The commonest rocks in the vicinity of the intrusion are finegrained slates and phyllites with a strong slaty cleavage, c o m p o s e d of quartz, sericite, chlorite, biotite and opaque minerals (iron oxide and graphite). Quartzite, granofels (equigranular siliceous metasediments) and l i m e s t o n e - p e b b l e conglomerate are also present. The metasediments have undergone polyphase deformation resulting in isoclinal folding and a regional N-trending fabric (bedding, cleavage, fractures, faults and fold axes). The D h u r a h diorite-tonalite complex lies 1 km west of the Jabalat intrusion; other diorite-tonalite intrusions
*Present address: 6 Fulmere Court, Haughton Lane, Swinton, Manchester, England. 183 4:SI-H
Fig. 1. Map showing location of the Jabalat granite.
J. AL TAYYAR, N. J. JACKSON and S. AL-YAzID!
184 +
+ ++
t+t+'l'+t+ + + + + <
ToAtTa'if~--:,/.-_
20"45" N
(a)
'
reisenized .LEGEND N
/ ++++ +
+
+
+ ~
+
+
Sn, Mo, Cu, stannlte, molybdenita0 chalcopyrlta
4
+++4
++++ ~ + + + +~. + +
Porphyritic biotite granite N o n - p o r p h y r i t i c biotite granite Quartz diorite and tonalite Undifferentiated hornfelsed metasediments ..... Felsic dike _ _ _ Mafic dike / / • :_':--.- Wadi Contact / i/ • t Road • i I MINERALIZATION TYPE i i • Quartz vein // • Disseminated .~_s /I • Pegmatite / • Replacement METAL Ag Silver Bi Bismuth Cu Copper Fe Iron Mo Molybdenum Pb Lead \ Sn Tin ,,. Zn Zinc
///
+
ite
Cu, MO';':';':';'?;';';';';';';';';';';';, bmalachite, molybdenlte,':.: pyrlte,:.:-:,:.:.:.:-:.:-:,:.:-:.:.:.:.:,:.:.
, m8
+
+ + ++~+++ + +
'
Pegmatite Muscovite microgranite
+++/l + + + ,~r',?
+~+
~
;'
i~'::;i:i:i:i:!:i:i:i:i:i:i:i:i:: "
i/
~:i:i:::~:~...-,~-¢.,.,
ff ,"
.
......
,
i
.\_
•
Wadl ShUqU .'~"
"-..,~ . . . . . •.
,." To
/
t
"%,.--.
'SSE -Porphyritic mlcrogranlte roof/facies Original roof of intrusion ~ .
--1560
/'~ / /
1440
2kms I
(b)
Metres --1680
1 I
I
Pegmatlte-apllte capping cupola
~ J~
~'\
Mo-Sn-BI-Pb-Ag veins ---.:-----_.J~':'.'l / "IT.'.%
/
Satellite mlcrogranlte
Bahah
| ~ ~ ~
NNW
metasedlments
I . . . . . I MO-m-I-'b-Ag/ Z n - C u veins
N %
^ uuartz
G.relsen MO--:~n - veins
/cap f
Dlorltq
\N
Fig. 2. (a) Simplifiedgeological map showing location of mineralizedsites in the vicinity of the Jabalat granite. (b) Schematiccross-sectionshowingthe dispositionof mineralizationin the roof of the Jabalatintrusion. attitude from shallow dipping in the NW to, more typically, subvertical elsewhere. The contact zones of the granite are generally finer grained, indicative of chilling. Wall-rocks are cut by numerous granite veins as much as 50 m from the contact. The granite is well jointed, relatively free from xenolithic inclusions, and cut by numerous aplitic and microgranite dikes.
Lithologies The rocks comprising the Jabalat pluton are mediumgrained (3-8 mm) and white to reddish in color, with a
color index of less than five. In the field, two main phases can be distinguished: (a) non-porphyritic, pink biotite granite which forms most of the southern part of the pluton; and (b) grey to white, porphyritic, biotite granite (containing muscovite) which forms most of the northern part of the pluton (Fig. 2a). The equigranular variety is generally fresh and is cut by few veins or dikes, whereas the porphyritic variety is more weathered and cut by numerous aplite, pegmatite and quartz veins. The two highest peaks within the area of the intrusion are composed of porphyritic granite, capped by porphyritic alkali-feldspar granite. Xenoliths of a third type, fine-
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region GEOCHEMISTRY
O •
185
Monzogranlte
O Subsolvua alkali-feldspar
9renlte •
A .
Satellite mlcrogranlle
.
.
.
.
.
P
Fig. 3. Modal composition of the Jabalat granite and microgranite (classification according to Streckeisen 1976, Ramsay et al. 1986).
grained porphyritic biotite granite, which occur within the medium-grained porphyritic granite, might have been derived from a chilled roof facies. The main rock-forming minerals are quartz (25-44%), plagioclase of albite or oligoclase composition (2546%), microcline (22-45%) and 2-5% biotite and muscovite. The non-porphyritic biotite granite also has up to 1% magnetite, zircon, fluorite and apatite. The modes (Fig. 3) indicate that the Jabalat pluton is composed of two distinct types of subsolvus granite, alkali-feldspar granite containing microcline and albite (An6_+3) and monzogranite containing microcline and oligoclase (An29_+3). These cannot, however, be distinguished in the field. An interesting feature of the alkali-feldspar mineralogy is the presence of orthoclase or cryptoperthite, microcline-perthite and microcline in both major rock-types. This feature probably indicates that alkali-feldspar crystallization began above the solidus curve but subsequently continued below it. The development of secondary sericite and chloritization of biotite are common features. Other rock-types include: (1) pink, albite-microclineperthite microgranite capping porphyritic granite; (2) pegmatite-aplite capping a small cupola 100 m west of the pluton and present as dikes and veins in the metasedimentary wall rocks; and (3) pink, felsic or aplitic dikes which transect the pluton. Satellite microgranite
A small, 200 m x 100 m elliptical intrusion of microgranite (Fig. 2) crops out 3 km N of the main Jabalat pluton (MODS 3018). It forms a prominent, white, quartz-capped hill about 60 m high. The main rock type is a pink, muscovitic, microcline-perthite-albite (An4) microgranite (Fig. 3) containing accessory magnetite, zircon, fluorite, apatite and allanite. It is miarolitic, with cavities 2-4 cm across containing quartz and muscovite. Both rock-types have been hydrothermally altered to greisen for distances of 1-2 m on either side of the steeply dipping southern contact.
Granites of the main pluton are highly siliceous (>74% SIO2), with low to moderate contents of MgO and CaO and high contents of NazO, K20, Be, F, Li, Rb and Sn compared to the average Hijaz alkali-feldspar granite (Odell et al. in press). The granites are plumasitic, specialized granites (Ramsay 1986, this volume). The microgranite (Table 1, D, E) has more SIO2, Rb and Sn and less A!203, iron, CaO, Ba, Ce, F, La, Sr, Y and Zr than granites of the main pluton. The very low F content is interesting, and the complementary high F content of greisenized microgranite indicates that depletion was due to removal in a volatile fluid phase. Hydrothermal alteration resulted in a significant introduction of SiO 2, iron, MgO, CaO, MnO, OH, F, Sn, Zn'and Zr and leaching of NazO, K20 and A1203 (Table I, F). The modal and chemical composition of the Jabalat granite and microgranite, together with that of the Ibrahim, Nis, Shadah and AI Asdah granites of the Bahah area (Jackson et al. 1985) are shown in Fig. 4. Except on a normative Q - A B - O R diagram, the Jabalat granites are obviously dissimilar to the Bahah granites, though the most calcic Jabalat granite lies at the edge of the Bahah granite fields. The most noteworthy chemical feature of the main granite is the high content of F, Rb and Sn, elements which are commonly enhanced in metalliferous and mineralized granites (Tischendorf 1977, Stemprok 1979). Compared with the other granites in the Bahah area, those of Jabalat are strongly depleted in Ba and Sr and enriched in F, Li, Nb, Rb, Sn, Y and Zn.
METALLOGENESIS Types of mineralization
Mineral occurrences and rocks with anomalous metalliferous contents are shown in Fig. 2, and selected assay data are given in Table 2. The main types of mineralization are: (1) felsic to aplitic dikes containing minor disseminated pyrite or gossanous hematitic spots, with anomalous Ag, Bi, Cu, Pb, Mo and Sn; (2) pegmatitic quartz veins or quartz-feldspar-mica veins containing disseminated pyrite and malachite with anomalous Bi, Cu, Pb, Mo and Sn; (3) quartz veins and vein swarms with muscovite or sericitized selvedges containing disseminated pyrite, chalcopyrite, molybdenite, powellite and stannite; and mineralized in Ag, Bi, Cu, Pb, Zn, Mo, Sn and minor W; (4) greisen at the contact of the satellite microgranite and Dhurah diorite, anomalous in Ag, Bi, Pb, Zn, Mo and Sn; and (5) magnetite replacements in a thin bed of limestone conglomerate. No mineralization so far identified is of economic significance.
186
J. A L TAYYAR, N . J. JACKSON a n d S. AL-YAZIDI Table 1. Major-oxide and trace-element composition of the Jabalat granite and satellite microgranite Jabalatgranite B 348655
C 348689
Major oxides (wt. percent) SiO2 74.10 AI203 13.30 FezO3 0.74 FeO 0.94 MgO 0.21 CaO 1.02 Na20 4.55 K20 4.78 TiO 2 0.16 P205 0.03 MnO 0.05 F 0.33 Total 99.88 O~F 0.13
74.90 13.38 0.56 0.98 0.14 0.67 4.50 4.63 0.12 0.03 0.04 0.22 99.95 0.09
75.80 13.36 0.50 0.60 0.09 0.60 4.35 4.48 0.06 0.02 0.03 0.29 99.89 0.08
76.40 13.20 0.31 0.38 0.12 0.18 4.75 4.21 0.02 0.02 0.03 0.03 99.46 --
78.00 12.12 0.41 0.30 0.08 0.09 4.45 4.41 0.03 0.00 0.03 0.03 99.92 --
83.20 8.62 0.94 0.78 0.32 0.28 0.99 2.63 0.05 0.00 0.07 0.16 97.88 0.06
Total
99.86
99.81
99.46
99.92
97.82
A 348688
99.75
Trace elements (ppm) Ba 233 Be 7 Ce 53 Co 3 Cu 5 F 3290 La 29 Li 112 Mo N Nb 38 Ni 11 Pb 30 Rb 198 Sr 89 Sn 20 V 11 Y 43 Zn 63 Zr 140 NKC/A K/Rb
136 5 38 3 5 2190 19 91 N 58 8 50 205 53 20 9 62 74 120
1.09 190
Satellite microgranite D E F 348663 348664 348622
99 5 19 2 5 2900 10 40 20 32 8 100 195 36 10 10 34 29 64
1.02 178
0.985 181
29 5 27 2 5 253 10 36 N 46 8 70 309 27 100 7 28 47 44
60 2 0 I 5 328 0 52 N 38 5 15 259 29 30 5 24 29 73
0.965 107
1.00 134
35 3 27 2 5 1600 10 36 N 46 8 N 291 27 150 7 28 76 93 ---
A, oligoclase monzogranite; B, albite monzogranite; C, porphyritic albite-microcline microgranite, roof facies; D, E, albite-microcline/microperthite microgranite; F, greisenized microgranite. NKC/A, molecular proportions of (Na20 + K20 + CaO)/AI203; N, below detection limits; - - , not calculated. Major oxides and minor elements by Inductively Coupled Plasma Spectrometry, using the method of Walsh (1980) (King's College, London; analyst N. Jackson). FeO by volumetric method and F by selective ion electrode; Be, Mo, Pb and Sn, semi-quantitative, by emission spectrography (Directorate General of Mineral Resources/U.S. Geological Survey Laboratory, Jiddah).
Table 2. Semi-quantitative assay data for mineralized and geol:hemically anomalous rock samples Type of mineralization Felsic dikes Quartz-pegmatite veins Quartz-muscovite-molybdenite veins Greisen
No. of samples
Ag
Bi
Cu
30 17 12 16
N-7 N-2 N-20 N-10
N-50 N-200 N-200 N-500
L-200 L-500 100-500 L-50
Pb Zn (range of values, in ppm) 10-150 N-100 L-700 L-500
N N-L N-1500 N-300
N, not detected (detection limits: Cu 3, Mo 3, W 25, Zn 100, Pb 5). L, detected but not quantifiable.
Mo
Sn
W
L-100 N-100 70-2000 N-200
N-30 N-70 N-700 20-700
N N N-30 N-L
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region Q
Q
AN
(a)
(c)
A
P
AB
FeOt
Na20+K20
187
OR AB K20
MgO
Na20
OR
FeOt
CaO CaO÷MgO
AI203
Fig. 4. Modal and chemical composition of the Jabalat granite and satellite microgranite (solid shading, contains 5 data points) and other granites of the Bahah area (stippled area. contains 13 samples from the Ibrahaim. Nis. Shadah and AI Asdah intrusions) (Jackson et al. 1985). (a) QAP diagram (Streckeisen 1976, Ramsay et al. 1986). (b) Normative An-Ab-Or diagram (after Barker 1979), (c) Normative Q-Ab-Or diagram (cotectic surfaces and ternary minima for 1 and 5 kb PH,o are from Tuttle and Bowen 1958). (d) AFM diagram (weight percent), (e) K.,O-Na20-CaO diagram (weight percent). (f) FeO-(CaO + MgO)-AI20~diagram (weight percent).
Structural control
The emplacement of dikes and veins was controlled by joints and fractures in the pluton and its hornfelsed envelope. Dikes within the pluton trend either N-S or E - W , but mineralized veins and vein swarms are emplaced in a prominent set of closely spaced joints which trend NW, at 310 °. Veins are narrow ( 5 - 5 0 cm), have strike-lengths of several tens of meters, and are internally structureless or contain rugs with no signs of tectonic disturbance. These features suggest that the veins have a dilational-extension origin, and that they were possibly formed as a result of vertically directed pressure within a N E - S W - o r i e n t e d regional, compressive stress-field.
Evolution
The likely spatial relationships of the various mineralization p h e n o m e n a and an intepretation of the original geometry of the intrusion are shown in Fig. 2b. Mineralization is apparently confined to the roof zone (perhaps the upper 200 m) of a partially exposed granite body about 8 km in diameter. It is predominantly fracturecontrolled and post-dates the crystallization and consolidation of the granite. However, the weakly mineralized
felsic-aplitic dikes and the development of pegmatite, massive quartz cap-rock and miarolitic facies in apical zones indicates that mineralization began during the late-magmatic evolutionary stage. Extensive post-magmatic hydrothermal activity is indicated by (1) pervasive sericitization of feldspar and chloritization of biotite in the main pluton, (2) the local development of greisen and fracture-controlled sericitic alteration, and (3) extensive quartz veining in the pluton and its wall rocks. In mineralized systems of this type, hydrothermal fluids were generally only moderately saline and meteoric or mixed meteoric-magmatic in origin (Sheppard 1977). Petrographic examination of fluid inclusions in vein and greisen quartz indicates that the Jabalat mineralizing fluids were, indeed, only moderately saline (daughter salt crystals were not observed), but were rich in CO,. These fluids probably penetrated the roof and flanks of the intrusion along fractures and joints, and interacted with a volatile phase rich in metals. Metals were transported into the roof zone and precipitated in open fractures. Small-scale hydrothermal cells were probably focused on areas of elevated relief, such as the satellite intrusion. The restricted nature of the mineralization and the consistency of its metalliferous signature are probably indications that the hydrothermal system was of the 'single-pass-type', a short-lived event with limited convective circulation (Henley 1973).
188
J. A L TAYYAR, N. J. JACKSON a n d S. A L - Y A Z I D I
Acknowledgment--We would like to thank Colin Ramsay, Christopher Legg and Alan Drysdall for their constructive reviews and Mr Javed Akhtar for providing modal mineralogical data.
REFERENCES Barker, F. 1979. Trondhjemite----definition, environment and hypothesis of origin. In: Trondhjemites, Dacites and Related Rocks (Edited by Barker, F.), pp. 1-12. Elsevier, Amsterdam. Greene, R. C. and Gonzalez, L. 1980. Reconnaissance geology of the Wadi Shuqub Quadrangle, sheet 20/4 I A, Kingdom of Saudi Arabia. Saudi Arabian Dir. Gen. Miner. Resour. Geologic Map GM-54, scale 1 : 100,000. Henley, R. W. 1973. Some fluid dynamics and ore genesis. Trans. Inst. Min. Metall. Sect. B. 82, B1-B8. Jackson, N. J., Alyazidi, S. and AI Tayyar, J. 1985. Mineral potential of post-tectonic felsic plutons in the AI Bahah area. Saudi Arabian Deputy Ministry for Mineral Resources Open-File Report DGMROF-05-43. Odell, J., Douch, C. J., Drysdall, A. R., Jackson, N. J. and Ramsay, C. R. (in press). Mean chemical compositions of some granitic rocks in the Hijaz Region. Saudi Arabian Deputy Ministry for Mbwral
Resources Prof. Pap. PP-2. (Issued as DGMR-OF-03-10, 1983, 24 pp.). Ramsay, C. R. 1986. Specialized felsic plutonic rocks of the Arabian Shield and their precursors. J. Afr. Earth Sci. 4, 153--168. Ramsay, C. R., Stoeser, D. B. and Drysdall, A. R. 1986. Guidelines to classification and nomenclature of Arabian felsic plutonic rocks. J. Afr. Earth Sci. 4, 13--20. Sheppard, S. M. F. 1972. Identification of the origin of ore-forming sOlutions by the use of stable isotopes. In: Volcanic Processes in Ore Genesis, pp. 25-41. Geol. Soc. Lond. Sp. Pub. 7. Stemprok, M. 1979. Mineralized granites and their origin. Episodes3. 20--24. Streckeisen, A. 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12, 1-33. Tischendorf, G. 1977. Geochemical and petrographic characteristics of silicic magmatic rocks associated with rare-element mineralisation. In: Metallization Associated with Acid Magmatism, Vol. 2, pp. 41-96. Czechoslovakia Geological Survey, Prague. Turtle, O. F. and Bowen, N. L. 1958. Origin of granite in the light of experimental studies in the system (NaAI)Si~Os-(KAI)Si~O~-SiOjHzO. Mere. geol. Soc. Am. 74, 1-53. Walsh, J. N. 1980. The simultaneous determination of major and trace constituents of silicate rocks using inductively coupled plasma spectrometry. Spectrochem. Acta 35B, 107-111.
4. pp. 1 8 3 - 1 8 8 . 1 9 8 6
0 7 3 1 - 7 2 4 7 / 8 6 $3.0t) + 0 . 0 0 Pergamon Press Ltd.
P r i n t e d in G r e a t B r i t a i n
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region, Kingdom of Saudi Arabia JAFFAR AL TAYYAR, NORMAN J. JACKSON* and SAEED AL-YAZID! Directorate General of Mineral Resources, P.O. Box 345, Jiddah, Kingdom of Saudi Arabia Abstract--The Jabalat post-tectonic granite pluton is composed of albite- and oligoclase-bearing, low-calcium, F-, Sn- and Rb-rich subsolvus granites. These granites display evidence of late-magmatic, granitophile- and metallic-element specialization, resulting ultimately in the development of post-magmatic, metalliferous hydrothermal systems characterized by a Mo--Sn-Cu-Pb-Zn-Bi-Ag-F signature. Two main types of mineralization are present within the pluton and its environs: ( 1) weakly mineralized felsic and aplitic dikes and veins enhanced in Mo, Bi, Ag, Pb and Cu; and (2) pyrite-molybdenite-chalcopyrite-bearing quartz and quartz-feldspar veins rich in Mo, Sn, Bi, Cu, Zn and Ag. A satellite stock, 3 km north of the main intrusion, is composed of fine-grained, miarolitic, muscovite-albite-microcline (microperthite) granite. The flanksof this intrusion and adjacent dioritic rocks are greisenized and highly enriched in Sn. Bi and Ag. Quartz veins which transect the satellite stock contain molybdenite and stannite.
INTRODUCTION
outcrop within 500 m to 3 km of the northern, southern and eastern margins of the granite.
THE JABALAT granite pluton (No. 3017 in the Saudi Arabian Mineral Occurrences D o c u m e n t a t i o n System, M O D S ) forms part of a N-trending, 150 kin-long array of post-tectonic granitic intrusions in the Asir region in southern Saudi Arabia, referred to as the Bahah granite belt by Jackson et al. (1985). The granite pluton is situated close to the At T a i f - A I Bahah highway, about 40 km SSE of Ai Jibub and 90 km N o f A l Bahah, at 20°42'N, 41°13'E (Fig. 1). Reconnaissance field work and sampling indicated that the pluton was radioactively anomalous and enhanced in Mo, Bi, Pb, Ag and Sn. Subsequent ground prospecting of the pluton and its environs led to the discovery of a n u m b e r of minor Mo and Sn mineral occurrences.
GEOLOGY
Main features The Jabalat pluton is a crudely rectangular body about 4 km long and 2.5-3 km wide (Fig. 2). Erosion of the core has produced a central low-lying area surrounded by rounded, steep-sided hills. Intrusive contacts with the metasedimentary host-rocks are sharp and vary in 35°
]fl°
41°
a4"
47 °
GEOLOGIC SETTING The granite was emplaced discordantly into d e f o r m e d , greenschist-facies metasediments of the Bahah group (the m3 unit of G r e e n e and Gonzalez 1980). The commonest rocks in the vicinity of the intrusion are finegrained slates and phyllites with a strong slaty cleavage, c o m p o s e d of quartz, sericite, chlorite, biotite and opaque minerals (iron oxide and graphite). Quartzite, granofels (equigranular siliceous metasediments) and l i m e s t o n e - p e b b l e conglomerate are also present. The metasediments have undergone polyphase deformation resulting in isoclinal folding and a regional N-trending fabric (bedding, cleavage, fractures, faults and fold axes). The D h u r a h diorite-tonalite complex lies 1 km west of the Jabalat intrusion; other diorite-tonalite intrusions
*Present address: 6 Fulmere Court, Haughton Lane, Swinton, Manchester, England. 183 4:SI-H
Fig. 1. Map showing location of the Jabalat granite.
J. AL TAYYAR, N. J. JACKSON and S. AL-YAzID!
184 +
+ ++
t+t+'l'+t+ + + + + <
ToAtTa'if~--:,/.-_
20"45" N
(a)
'
reisenized .LEGEND N
/ ++++ +
+
+
+ ~
+
+
Sn, Mo, Cu, stannlte, molybdenita0 chalcopyrlta
4
+++4
++++ ~ + + + +~. + +
Porphyritic biotite granite N o n - p o r p h y r i t i c biotite granite Quartz diorite and tonalite Undifferentiated hornfelsed metasediments ..... Felsic dike _ _ _ Mafic dike / / • :_':--.- Wadi Contact / i/ • t Road • i I MINERALIZATION TYPE i i • Quartz vein // • Disseminated .~_s /I • Pegmatite / • Replacement METAL Ag Silver Bi Bismuth Cu Copper Fe Iron Mo Molybdenum Pb Lead \ Sn Tin ,,. Zn Zinc
///
+
ite
Cu, MO';':';':';'?;';';';';';';';';';';';, bmalachite, molybdenlte,':.: pyrlte,:.:-:,:.:.:.:-:.:-:,:.:-:.:.:.:.:,:.:.
, m8
+
+ + ++~+++ + +
'
Pegmatite Muscovite microgranite
+++/l + + + ,~r',?
+~+
~
;'
i~'::;i:i:i:i:!:i:i:i:i:i:i:i:i:: "
i/
~:i:i:::~:~...-,~-¢.,.,
ff ,"
.
......
,
i
.\_
•
Wadl ShUqU .'~"
"-..,~ . . . . . •.
,." To
/
t
"%,.--.
'SSE -Porphyritic mlcrogranlte roof/facies Original roof of intrusion ~ .
--1560
/'~ / /
1440
2kms I
(b)
Metres --1680
1 I
I
Pegmatlte-apllte capping cupola
~ J~
~'\
Mo-Sn-BI-Pb-Ag veins ---.:-----_.J~':'.'l / "IT.'.%
/
Satellite mlcrogranlte
Bahah
| ~ ~ ~
NNW
metasedlments
I . . . . . I MO-m-I-'b-Ag/ Z n - C u veins
N %
^ uuartz
G.relsen MO--:~n - veins
/cap f
Dlorltq
\N
Fig. 2. (a) Simplifiedgeological map showing location of mineralizedsites in the vicinity of the Jabalat granite. (b) Schematiccross-sectionshowingthe dispositionof mineralizationin the roof of the Jabalatintrusion. attitude from shallow dipping in the NW to, more typically, subvertical elsewhere. The contact zones of the granite are generally finer grained, indicative of chilling. Wall-rocks are cut by numerous granite veins as much as 50 m from the contact. The granite is well jointed, relatively free from xenolithic inclusions, and cut by numerous aplitic and microgranite dikes.
Lithologies The rocks comprising the Jabalat pluton are mediumgrained (3-8 mm) and white to reddish in color, with a
color index of less than five. In the field, two main phases can be distinguished: (a) non-porphyritic, pink biotite granite which forms most of the southern part of the pluton; and (b) grey to white, porphyritic, biotite granite (containing muscovite) which forms most of the northern part of the pluton (Fig. 2a). The equigranular variety is generally fresh and is cut by few veins or dikes, whereas the porphyritic variety is more weathered and cut by numerous aplite, pegmatite and quartz veins. The two highest peaks within the area of the intrusion are composed of porphyritic granite, capped by porphyritic alkali-feldspar granite. Xenoliths of a third type, fine-
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region GEOCHEMISTRY
O •
185
Monzogranlte
O Subsolvua alkali-feldspar
9renlte •
A .
Satellite mlcrogranlle
.
.
.
.
.
P
Fig. 3. Modal composition of the Jabalat granite and microgranite (classification according to Streckeisen 1976, Ramsay et al. 1986).
grained porphyritic biotite granite, which occur within the medium-grained porphyritic granite, might have been derived from a chilled roof facies. The main rock-forming minerals are quartz (25-44%), plagioclase of albite or oligoclase composition (2546%), microcline (22-45%) and 2-5% biotite and muscovite. The non-porphyritic biotite granite also has up to 1% magnetite, zircon, fluorite and apatite. The modes (Fig. 3) indicate that the Jabalat pluton is composed of two distinct types of subsolvus granite, alkali-feldspar granite containing microcline and albite (An6_+3) and monzogranite containing microcline and oligoclase (An29_+3). These cannot, however, be distinguished in the field. An interesting feature of the alkali-feldspar mineralogy is the presence of orthoclase or cryptoperthite, microcline-perthite and microcline in both major rock-types. This feature probably indicates that alkali-feldspar crystallization began above the solidus curve but subsequently continued below it. The development of secondary sericite and chloritization of biotite are common features. Other rock-types include: (1) pink, albite-microclineperthite microgranite capping porphyritic granite; (2) pegmatite-aplite capping a small cupola 100 m west of the pluton and present as dikes and veins in the metasedimentary wall rocks; and (3) pink, felsic or aplitic dikes which transect the pluton. Satellite microgranite
A small, 200 m x 100 m elliptical intrusion of microgranite (Fig. 2) crops out 3 km N of the main Jabalat pluton (MODS 3018). It forms a prominent, white, quartz-capped hill about 60 m high. The main rock type is a pink, muscovitic, microcline-perthite-albite (An4) microgranite (Fig. 3) containing accessory magnetite, zircon, fluorite, apatite and allanite. It is miarolitic, with cavities 2-4 cm across containing quartz and muscovite. Both rock-types have been hydrothermally altered to greisen for distances of 1-2 m on either side of the steeply dipping southern contact.
Granites of the main pluton are highly siliceous (>74% SIO2), with low to moderate contents of MgO and CaO and high contents of NazO, K20, Be, F, Li, Rb and Sn compared to the average Hijaz alkali-feldspar granite (Odell et al. in press). The granites are plumasitic, specialized granites (Ramsay 1986, this volume). The microgranite (Table 1, D, E) has more SIO2, Rb and Sn and less A!203, iron, CaO, Ba, Ce, F, La, Sr, Y and Zr than granites of the main pluton. The very low F content is interesting, and the complementary high F content of greisenized microgranite indicates that depletion was due to removal in a volatile fluid phase. Hydrothermal alteration resulted in a significant introduction of SiO 2, iron, MgO, CaO, MnO, OH, F, Sn, Zn'and Zr and leaching of NazO, K20 and A1203 (Table I, F). The modal and chemical composition of the Jabalat granite and microgranite, together with that of the Ibrahim, Nis, Shadah and AI Asdah granites of the Bahah area (Jackson et al. 1985) are shown in Fig. 4. Except on a normative Q - A B - O R diagram, the Jabalat granites are obviously dissimilar to the Bahah granites, though the most calcic Jabalat granite lies at the edge of the Bahah granite fields. The most noteworthy chemical feature of the main granite is the high content of F, Rb and Sn, elements which are commonly enhanced in metalliferous and mineralized granites (Tischendorf 1977, Stemprok 1979). Compared with the other granites in the Bahah area, those of Jabalat are strongly depleted in Ba and Sr and enriched in F, Li, Nb, Rb, Sn, Y and Zn.
METALLOGENESIS Types of mineralization
Mineral occurrences and rocks with anomalous metalliferous contents are shown in Fig. 2, and selected assay data are given in Table 2. The main types of mineralization are: (1) felsic to aplitic dikes containing minor disseminated pyrite or gossanous hematitic spots, with anomalous Ag, Bi, Cu, Pb, Mo and Sn; (2) pegmatitic quartz veins or quartz-feldspar-mica veins containing disseminated pyrite and malachite with anomalous Bi, Cu, Pb, Mo and Sn; (3) quartz veins and vein swarms with muscovite or sericitized selvedges containing disseminated pyrite, chalcopyrite, molybdenite, powellite and stannite; and mineralized in Ag, Bi, Cu, Pb, Zn, Mo, Sn and minor W; (4) greisen at the contact of the satellite microgranite and Dhurah diorite, anomalous in Ag, Bi, Pb, Zn, Mo and Sn; and (5) magnetite replacements in a thin bed of limestone conglomerate. No mineralization so far identified is of economic significance.
186
J. A L TAYYAR, N . J. JACKSON a n d S. AL-YAZIDI Table 1. Major-oxide and trace-element composition of the Jabalat granite and satellite microgranite Jabalatgranite B 348655
C 348689
Major oxides (wt. percent) SiO2 74.10 AI203 13.30 FezO3 0.74 FeO 0.94 MgO 0.21 CaO 1.02 Na20 4.55 K20 4.78 TiO 2 0.16 P205 0.03 MnO 0.05 F 0.33 Total 99.88 O~F 0.13
74.90 13.38 0.56 0.98 0.14 0.67 4.50 4.63 0.12 0.03 0.04 0.22 99.95 0.09
75.80 13.36 0.50 0.60 0.09 0.60 4.35 4.48 0.06 0.02 0.03 0.29 99.89 0.08
76.40 13.20 0.31 0.38 0.12 0.18 4.75 4.21 0.02 0.02 0.03 0.03 99.46 --
78.00 12.12 0.41 0.30 0.08 0.09 4.45 4.41 0.03 0.00 0.03 0.03 99.92 --
83.20 8.62 0.94 0.78 0.32 0.28 0.99 2.63 0.05 0.00 0.07 0.16 97.88 0.06
Total
99.86
99.81
99.46
99.92
97.82
A 348688
99.75
Trace elements (ppm) Ba 233 Be 7 Ce 53 Co 3 Cu 5 F 3290 La 29 Li 112 Mo N Nb 38 Ni 11 Pb 30 Rb 198 Sr 89 Sn 20 V 11 Y 43 Zn 63 Zr 140 NKC/A K/Rb
136 5 38 3 5 2190 19 91 N 58 8 50 205 53 20 9 62 74 120
1.09 190
Satellite microgranite D E F 348663 348664 348622
99 5 19 2 5 2900 10 40 20 32 8 100 195 36 10 10 34 29 64
1.02 178
0.985 181
29 5 27 2 5 253 10 36 N 46 8 70 309 27 100 7 28 47 44
60 2 0 I 5 328 0 52 N 38 5 15 259 29 30 5 24 29 73
0.965 107
1.00 134
35 3 27 2 5 1600 10 36 N 46 8 N 291 27 150 7 28 76 93 ---
A, oligoclase monzogranite; B, albite monzogranite; C, porphyritic albite-microcline microgranite, roof facies; D, E, albite-microcline/microperthite microgranite; F, greisenized microgranite. NKC/A, molecular proportions of (Na20 + K20 + CaO)/AI203; N, below detection limits; - - , not calculated. Major oxides and minor elements by Inductively Coupled Plasma Spectrometry, using the method of Walsh (1980) (King's College, London; analyst N. Jackson). FeO by volumetric method and F by selective ion electrode; Be, Mo, Pb and Sn, semi-quantitative, by emission spectrography (Directorate General of Mineral Resources/U.S. Geological Survey Laboratory, Jiddah).
Table 2. Semi-quantitative assay data for mineralized and geol:hemically anomalous rock samples Type of mineralization Felsic dikes Quartz-pegmatite veins Quartz-muscovite-molybdenite veins Greisen
No. of samples
Ag
Bi
Cu
30 17 12 16
N-7 N-2 N-20 N-10
N-50 N-200 N-200 N-500
L-200 L-500 100-500 L-50
Pb Zn (range of values, in ppm) 10-150 N-100 L-700 L-500
N N-L N-1500 N-300
N, not detected (detection limits: Cu 3, Mo 3, W 25, Zn 100, Pb 5). L, detected but not quantifiable.
Mo
Sn
W
L-100 N-100 70-2000 N-200
N-30 N-70 N-700 20-700
N N N-30 N-L
Geology and mineralization of the Jabalat alkali-feldspar granite, northern Asir region Q
Q
AN
(a)
(c)
A
P
AB
FeOt
Na20+K20
187
OR AB K20
MgO
Na20
OR
FeOt
CaO CaO÷MgO
AI203
Fig. 4. Modal and chemical composition of the Jabalat granite and satellite microgranite (solid shading, contains 5 data points) and other granites of the Bahah area (stippled area. contains 13 samples from the Ibrahaim. Nis. Shadah and AI Asdah intrusions) (Jackson et al. 1985). (a) QAP diagram (Streckeisen 1976, Ramsay et al. 1986). (b) Normative An-Ab-Or diagram (after Barker 1979), (c) Normative Q-Ab-Or diagram (cotectic surfaces and ternary minima for 1 and 5 kb PH,o are from Tuttle and Bowen 1958). (d) AFM diagram (weight percent), (e) K.,O-Na20-CaO diagram (weight percent). (f) FeO-(CaO + MgO)-AI20~diagram (weight percent).
Structural control
The emplacement of dikes and veins was controlled by joints and fractures in the pluton and its hornfelsed envelope. Dikes within the pluton trend either N-S or E - W , but mineralized veins and vein swarms are emplaced in a prominent set of closely spaced joints which trend NW, at 310 °. Veins are narrow ( 5 - 5 0 cm), have strike-lengths of several tens of meters, and are internally structureless or contain rugs with no signs of tectonic disturbance. These features suggest that the veins have a dilational-extension origin, and that they were possibly formed as a result of vertically directed pressure within a N E - S W - o r i e n t e d regional, compressive stress-field.
Evolution
The likely spatial relationships of the various mineralization p h e n o m e n a and an intepretation of the original geometry of the intrusion are shown in Fig. 2b. Mineralization is apparently confined to the roof zone (perhaps the upper 200 m) of a partially exposed granite body about 8 km in diameter. It is predominantly fracturecontrolled and post-dates the crystallization and consolidation of the granite. However, the weakly mineralized
felsic-aplitic dikes and the development of pegmatite, massive quartz cap-rock and miarolitic facies in apical zones indicates that mineralization began during the late-magmatic evolutionary stage. Extensive post-magmatic hydrothermal activity is indicated by (1) pervasive sericitization of feldspar and chloritization of biotite in the main pluton, (2) the local development of greisen and fracture-controlled sericitic alteration, and (3) extensive quartz veining in the pluton and its wall rocks. In mineralized systems of this type, hydrothermal fluids were generally only moderately saline and meteoric or mixed meteoric-magmatic in origin (Sheppard 1977). Petrographic examination of fluid inclusions in vein and greisen quartz indicates that the Jabalat mineralizing fluids were, indeed, only moderately saline (daughter salt crystals were not observed), but were rich in CO,. These fluids probably penetrated the roof and flanks of the intrusion along fractures and joints, and interacted with a volatile phase rich in metals. Metals were transported into the roof zone and precipitated in open fractures. Small-scale hydrothermal cells were probably focused on areas of elevated relief, such as the satellite intrusion. The restricted nature of the mineralization and the consistency of its metalliferous signature are probably indications that the hydrothermal system was of the 'single-pass-type', a short-lived event with limited convective circulation (Henley 1973).
188
J. A L TAYYAR, N. J. JACKSON a n d S. A L - Y A Z I D I
Acknowledgment--We would like to thank Colin Ramsay, Christopher Legg and Alan Drysdall for their constructive reviews and Mr Javed Akhtar for providing modal mineralogical data.
REFERENCES Barker, F. 1979. Trondhjemite----definition, environment and hypothesis of origin. In: Trondhjemites, Dacites and Related Rocks (Edited by Barker, F.), pp. 1-12. Elsevier, Amsterdam. Greene, R. C. and Gonzalez, L. 1980. Reconnaissance geology of the Wadi Shuqub Quadrangle, sheet 20/4 I A, Kingdom of Saudi Arabia. Saudi Arabian Dir. Gen. Miner. Resour. Geologic Map GM-54, scale 1 : 100,000. Henley, R. W. 1973. Some fluid dynamics and ore genesis. Trans. Inst. Min. Metall. Sect. B. 82, B1-B8. Jackson, N. J., Alyazidi, S. and AI Tayyar, J. 1985. Mineral potential of post-tectonic felsic plutons in the AI Bahah area. Saudi Arabian Deputy Ministry for Mineral Resources Open-File Report DGMROF-05-43. Odell, J., Douch, C. J., Drysdall, A. R., Jackson, N. J. and Ramsay, C. R. (in press). Mean chemical compositions of some granitic rocks in the Hijaz Region. Saudi Arabian Deputy Ministry for Mbwral
Resources Prof. Pap. PP-2. (Issued as DGMR-OF-03-10, 1983, 24 pp.). Ramsay, C. R. 1986. Specialized felsic plutonic rocks of the Arabian Shield and their precursors. J. Afr. Earth Sci. 4, 153--168. Ramsay, C. R., Stoeser, D. B. and Drysdall, A. R. 1986. Guidelines to classification and nomenclature of Arabian felsic plutonic rocks. J. Afr. Earth Sci. 4, 13--20. Sheppard, S. M. F. 1972. Identification of the origin of ore-forming sOlutions by the use of stable isotopes. In: Volcanic Processes in Ore Genesis, pp. 25-41. Geol. Soc. Lond. Sp. Pub. 7. Stemprok, M. 1979. Mineralized granites and their origin. Episodes3. 20--24. Streckeisen, A. 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12, 1-33. Tischendorf, G. 1977. Geochemical and petrographic characteristics of silicic magmatic rocks associated with rare-element mineralisation. In: Metallization Associated with Acid Magmatism, Vol. 2, pp. 41-96. Czechoslovakia Geological Survey, Prague. Turtle, O. F. and Bowen, N. L. 1958. Origin of granite in the light of experimental studies in the system (NaAI)Si~Os-(KAI)Si~O~-SiOjHzO. Mere. geol. Soc. Am. 74, 1-53. Walsh, J. N. 1980. The simultaneous determination of major and trace constituents of silicate rocks using inductively coupled plasma spectrometry. Spectrochem. Acta 35B, 107-111.