American Mineralogist, Volume 70, pages 1075-1086, 1985

Geology of the Silsilah ring complex, and associated tin mineralization,Kingdom of Saudi Arabia-a synopsis

Eow.cno A. ou Bnly

U.S. Geological Suruey-MS 905Box 25046, Denuer Federal Center

Denuer, Colorado 80225

Abstract

A significant tin deposit has been identified in the northeast part of the Late ProterozoicArabian Shield. Cassiterite-topaz-quartz greisens are distributed over 16 km2 in flat-toppedcupolas of a highly evolved, zinnwaldite-bearing alkali-feldspar granite which composes partof the silsilah ring complex (lat 26"06'N, long 4240'E). The 587 m.y. old alkali-feldspargranite is overlain by a carapace of aplite and pegmatite. The carapace acted as an imperme-able boundary, beneath which fluids accumulated and caused greisenization of the alkali-feldspar granite and deposition of disseminated cassiterite. Subsequently emplaced quaftz-wolframite veins cut the alkali-feldspar granite and the aplite-pegmatite carapace.

The rest of the Silsilah ring complex is composed of peralkaline granite, its hypabyssalequivalent, and alkaline dacite. The occurrence of alkaline dacite, peralkaline granite, andalkali-feldspar granite (oldest to youngest) in a single ring fracture suggests that these rocksform a single differentiation series. Major, trace, and rare-earth element data support thishypothesis. The alkaline dacite evolved to the peralkaline granite by fractionation of sodicplagioclase and Fe-Ti oxides. The peralkaline granite continued differentiation by frac-tionation of anorthoclase, Na-pyriboles, and zircon. This process yielded the peraluminouscomposition, incompatible trace element enrichment, and flat REE patterns with large nega-tive europium anomalies that are characteristic of the alkali-feldspar granite. This unusualdifferentiation trend may be typical of other, highly evolved intrusive suites and associated,yet undiscovered, tin-tungsten deposits in the Arabian Shield.

Introduction

Deposits of tin, tungsten, and other rare metals are as-sociated with a group of petrologically similar granitoidrocks known variously as S-type (Chappell and White,1974), ilmenite series (Ishihara, 1977), and metallogenicallyspecialized (Tischendorf, 1977). These rocks are charac-terized by highly evolved major element compositions, in-compatible trace element enrichment, are peraluminous,and often contain an aluminum-rich silicate such as musco-vite. Mineral exploration programs have demonstratedthat rocks of this type also occur within the PrecambrianArabian Shield (Elliott, 1980; Stoeser and Elliott, 1980; duBray and others, 1982; du Bray, 1983a) and that the poten-tial for discovery of deposits of tin, tungsten, and other raremetals associated with these highy evolved granitoid rocksis significant.

Results of a systematic geochemical survey in the north-east Arabian Shield (Allen et al., 1983) indicate that wadisthat drain the Silsilah ring complex (Fig. 1) contain highlyanomalous concentrations of Be, Nb, Pb, and Sn. Du Bray(1983b) identified the Fawwarah alkali-granite, a compo-nent of the complex, as a metallogenically specialized gran-ite and subsequently identified two intensely mineralized,cassiterite-bearing greisens. A large body of milky whiteqtrartz, numerous weakly mineralized greisen pods, quartz

veins in greisen envelopes, and quartz-wolframite veins, allfound in the southwest part of the complex indicate that anextensive hydrothermal system was once active here. Per-cussion drill samples of the cassiterite-bearing greisens con-tain an average tin content ofless than 1000 ppm, althoughsome I m intervals contain as much as several percent tin.The economic viability of the deposit could be predicatedby the location ofa very large tonnage oflow-grade ore.

General characteristics of the Silsilah ring complex

The Silsilah ring complex is an igneous suite, mostlycomposed of highly evolved rocks (du Bray, 1983b), thatcrops out in a ring fracture centered at lat 26'O7' N. andlong 42"42' E. (Fig. 1). A U-Pb zircon age for the Faw-warah alkali-feldspar granite, the youngest component ofthe ring complex, is 587+8 m.y. (J. S. Stacey, writtencomm., 1984). Rocks of the ring complex intrude gray-wacke of the Murdama group. The sandstone, locallyknown as the Maraghan lithic graywacke, crops out withinthe ring complex and occurs in a very large area outsidethe ring. The rocks exposed in the ring complex form aprominent toroidal topographic feature, hereafter referredto as the ring, 12 km in diameter that rises between 10 and300 m above the surrounding pediment surface; theaverage relief along the ring is about 100 m. The cross-

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DTJ BRAY: SILSILAH RING COMP.LEX TIN MINERALIZATION

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Fig. 1. Geologic sketch map of the Silsilah ring complex.

sectional dimension of the toroidal feature is between 30 min the northeast and about 3 km in the southeast.

Three principal and several minor rock types comprisethe ring complex (Fig. 1). The northwest and southeastquadrants of the complex are composed of Silsilah alkaligranite, the southwest quadrant of Fawwarah alkali-feldspar granite, and the Hadhir aplite forms a sill-likemass that crops out on top of the alkali-feldspar granite.These rocks account for about 40,20, and 10 percent of thering complex, respectively. Two other lithologies crop outin the northeast and southeast qu4drants of the complexand together account for about 30 percent of its area. Oneis very fine grained alkaline rock composed of mugearite,benmoreite, trachyte, and tristanite (Irvine and Baragar,1971). These are the alkaline analogs of dacite in thesystem of subalkaline volcanic rocks, and are called alka-line dacites hereafter for simplicity. The other is composedof fine-grained, porphyritic comendite (Noble, 1968).

Quartz-potassium feldspar pegmatite crops out betweenthe Fawwarah alkali-feldspar granite and the Hadhiraplite. Quartz veins penetrate each of the plutonic units,and a pegmatitic plug is located 1 km inside the southwestmargin of the ring. Two strongly mineralized greisens arelocated near the plug. Weakly mineralized greisen pods

occur at several places inside the southwest part of the ring

and several more occur within the Fawwarah alkali-

feldspar granite.The Maraghan lithic graywacke is intruded by, and

therefore is older than, rocks of the Silsilah ring complex.

These layered rocks occur everywhere at the outer periph-

ery of the ring complex and in a circular area of about 40

km2 within the ring. It is a massively bedded immature

sandstone with minor interbedded siltstone. Stoping does

not seem to have been operative during emplacement of

the ring complex rocks because blocks of the host metase-

dimentary rocks found within the intrusive units are rare

and of limited size. The Maraghan lithic graywacke was

regionally metamorphosed in lower greenschist facies con-

ditions. Locally, the grade was very slightly enhanced by

emplacement of the ring complex.The alkaline dacite is locally tuffaceous and vesicular,

suggesting that some of the magma vented and that the

remainder solidified at a shallow depth. Intrusive relations

indicate that the alkaline dacite magma was emplac€dprior to the other components of the complex. This

grayish-black to blackish-red rock weathers recessively and

forms dike-like outcrops.The comendite is characterized by petrographic at-

tributes similar to those of the Silsilah alkali granite, whichsuggests that it represents part of the alkali granite magmathat was quenched at a shallow level in the ring fracture.This unit intrudes, and therefore is younger than, the alka-line dacite. This pale-reddish brown comendite formsprominent dike-like outcrops.

The Silsilah alkali granite is the volumetrically predomi-nant component of the ring complex. Intrusive relations atthe southeastern edge of the ring complex indicate that theSilsilah alkali granite intrudes and therefore is youngerthan the alkaline dacite and the comendite.

The Hadhir aplite is a volumetrically minor componentof the complex. It crops out in a flat-lying sheet less than50 m thick on top of the Fawwarah alkali-feldspar granitein the southwest part of the complex. The aplite also cropsout as recessive-weathering slabs located inside the south-western limit of the ring complex where again it appears torest on top of the Fawwarah alkali-feldspar granite. About4 km west of Jabal al Hadhir at the southern limit of thecomplex a sill-like mass of aplite approximately 15 m thickunderlies the Silsilah alkali granite and indicates that theaplite intrudes and therefore is younger than the alkaligranrte.

Coarse grained quartz and potassium feldspar in graphicintergrowth form a flat-lying pegmatite sheet that pinchesand swells along strike between the Hadhir aplite and theunderlying Fawwarah alkali-feldspar granite. It consists ofa series of coalescing pegmatite pockets and layered aplite.It is between 0.1 and about l0 m thick, and locally con-tains large, bladed interstitial crystals of oxidized fayalite.Contacts between the pegmatite and both the alkali-feldspar granite and the aplite are gradational.

The Fawwarah alkali-feldspar granite crops out promi-nently throughout the southwestern quadrant. Intrusiverelations between the Hadhir aplite and the alkali-feldspargranite indicate that the latter is younger. It crystallizedbelow the structurally coherent, impermeable, aplitic cara-pace.

The large size and circular shape of the Silsilah ringcomplex and the fine grain size characteristic of much ofthe rock that composes it suggest that it represents magmathat was emplaced in the near surface part of a major,closely-spaced ring fracture system. Elliptical pegmatiticpockets found throughout the Hadhir aplite and the Faw-warah alkali-feldspar granite and the compositional simi-larity between the intrusives and experimentally deter-mined low pressure minimum melt compositions indicatethat they crystallized at very shallow depths.

Evidence in favor of piston-like subsidence of a centralcore, like that demonstrated for large calderas (Smith andBailey, 1968), is scarce. The concentric fault patterns thatare found in areas where caldera core subsidence has oc-curred are not apparent. A shallow trench excavated radi-ally inward from the alkali-feldspar granite-alluvial inter-face demonstrates that the host lithic graywacke is not infault contact with the rocks of the complex. The presenceand structural coherence of the Maraghan lithic graywackeinside the ring also argues against a major episode of caul-

r077

dron subsidence. These rocks would, in most cases, be sig-nificantly more disrupted and dismembered, had they ex-perienced significant pistonJike subsidence. There is littleevidence of copious dike injection that would have oc-curred in a structurally dismembered, foundered block lo-cated above an active magma chamber.

Several lines of evidence indicate that rock, presumablythe central pluton from which the currently exposed rocksof the ring complex evolved, is present at a shallow depthbelow the graywacke. Audiomagnetotelluric profiles acrossthe ring indicate that the depth to intrusive rock at thecenter of the ring, that is the thickness of the graywacke,does not exceed several hundred meters (Flanigan and Za-blocki, 1983). The presence ofthe two cupolalike bodies ofFawwarah alkali-feldspar granite inside the ring (Fig. 1), ofaplite dikes that penetrate the graywacke, and of hy-drothermally altered rock throughout the southwesternquadrant of the complex also suggest that intrusive rock ispresent close to the surface. At the present level of ex-posure, all extrusive products that may have been part ofthe svstem have been removed bv erosion.

Petrography

Host rockThe Maraghan lithic graywacke is principally composed of

brownish-green to olive-gray fine sandstone and siltstone. Thematrix is a silt-size intergrowth of turbid, fine-grained clay rnin-

erals. Volcanic lithic clasts (50%) outnumber monocrystallinequartz clasts (30%,), which in turn are more abundant than sub-angular clasts of plagioclase (20Vo). The lithic fragments are felsite,

turbid grains of argillite, and distinctive, fine-grained volcanic

clasts that contain small phenocrysts ofplagioclase.

Intrusiue rocksThe alkaline dacite is hypidiomorphic inequigranular and is

composed of an intergranular to weakly trachytic intergrowth ofsodic plagioclase and iron-titanium oxides. Most samples arecomposed of between 50 and 80%, unzoned, subhedral plagioclase

laths 0.2 mm long. Anhedral to subhedral grains of iron-titaniumoxides 0.05 mm in diameter compose from 20 to 30% of the rock.Apatite, which occurs in subhedral grains as much as 0.01 mmlong, is the only accessory mineral.

The comendite is allotriomorphic to hypidiomorphic granular

and distinctly porphyritic. Quartz forms rounded to bipyramidal,anhedral to subhedral phenocrysts as much as 4 rnm in diameterthat compose about 5%, of the rock. Anorthoclase forms subhe-dral to euhedral laths as much 4 mm long that compose as muchas 60'2, of the rock. Ragged interstitial iron-titanium oxide grains

are 0.2 to 4 mm in diameter and compose about 1%. Spindleshaped laths of arfvedsonite as much as 0.5 mm long composeanother 2 to 3Vo. Anhedral, interstitial albite forms as much as10%,. The matrix is characterized by irregularly-shaped, interstitialpatches of quartz and feldspar in micrographic intergrowth andby spherulitic overgrowths of alkali feldspar on quartz and anor-thoclase phenocrysts.

The grayish-red Silsilah alkali granite is hypidiomorphic granu-

lar and is locally characterized by a micrographic groundmass.

The felsii constituents are plagioclase, qu'artz, and anorthoclase(Fig. 2). Average contents of these minerals are 10,21, and 63'h,respectively; the color index iS 6. Phenocrysts of quartz and anor-

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

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Fig. 2. Quartz-potassium feldspar-plagioclase (QKP) ternary dia-

gram showing the modal composition of the Silsilah ring com-plex components. Each plotted point represents a modal analysis(between 400 and 700 points counted on a stained slab measuringat least 50 cm2) for a single sample. Albite is counted as plagio-

clase so that the ratio of albite to potassium feldspar is discernible.Properly plotted for modal classification (Streckeisen, 1976) thesepoints plot on the quartz-alkali feldspar sideline of the ternary at

the indicated modal quartz content. Plot symbols: open circles-

Silsilah alkali granite, triangles-Hadhir aplite, solid circles-

Fawwarah alkali-feldspar granite.

thoclase give the rock a distinctly porphyritic character. Anhedral,

rounded quartz phenocrysts are as much as 2.5 mm in diameter.

Blocky laths of perthitic anorthoclase form subhedral to euhedral

phenocrysts as much as 4 mm long that are Carlsbad-twinned.

Albite forms subhedral phenocrysts as much as 1 cm long in a few

samples but is principally found in the groundmass. Soda pyri-

boles such as katophorite, minor arfvedsonite, and aegirine-augite

are the mafic silicates in the alkali granite. Fayalite was identified

in one sample. Abundant zircon, iron-titanium oxides, and allanite

are the accessory minerals. The groundmass is a fine-grained, allo-

triomorphic granular intergrowth composed of quartz and perthi-

tic alkali feldspar.

The Hadhir aptite is fine grained and allotriomorphic inequi-

granular. The felsic constituents are albite, quartz, and potassium

feldspar (Fig. 2). Average contents of these minerals are 33, 9, and

56%,, respectively; the color index is 2. The apparently low quartz

content of this rock may be spurious and could result from the

diffrculties encountered in performing modal analyses of fine-

grained rocks such as this. Quartz, albite, and potassium feldspar

form scarce, anhedral phenocrysts as much as 2 mm in diameter.

Potassium feldspar phenocrysts are perthitic and partly altered to

clay minerals. Biotite and white mica form scarce anhedral to

subhedral crystals as much as I mm long. The white mica is

weakly pleochroic from colorless to very light brown and is prob-

ably zinnwaldite. Fluorite is the principal accessory mineral and

lorms anhedral grains as much as 0.8 mm in diameter. Thegroundmass is an allotriomorphic, locally micrographic, inter-

growth composed of quartz and alkali feldspar. Irregularly

shaped, quartz-filled miarolitic cavities as much as 2 mm in diam-

eter are present-The Fawwarah alkali-feldspar granite is very light gray, medium

grained, and allotriomorphic equigranular The felsic constituents

are albite, quartz, and microcline (Fig. 2). Average contents of

these minerals are 25, ]3, and 38%r, respectively; the color index is

4. Quartz forms subround anhedral grains 2 to 3 mm in diameter.

Microcline is perthitic and forms anhedral to subhedral grains

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

that are 2 to 3 mm long. Some grains poikilitically enclose grains

of albite and quartz. The plagioclase is unzoned albite that forms

anhedral to subhedral laths as much as 3 mm long, interstitial to

quartz and microcline. White mica and rare biotite are present.

The white mica is zinnwaldite and forms subhedral interstitial

grains 0.5 to 1.5 mm long that are pleochroic from nearly colorless

to either very light brown or very pale bluish green. A trace

amount of zircon is present as very small subhedral crystals in the

groundmass. Other accessory minerals present include fluorite and

topaz. As much as 1 modal% fluorite forms subhedral' purple-

tinged grains as much as 1 mm in diameter and wormy, interstitial

grains that are commonly associated with the micas. As much as

0.5 modal%, topaz forms colorless wormy crystals in the ground-

mass although it too forms scarce subhedral grains as much as 2

mm in diameter.

Geochemistry of the igneous rocks

Analytical methods are described and individual analy-ses from which the averages of Table 1 were computed arein du Bray (1984).

Major elements

The major element chemistry of the ring complex com-ponents displays systematic variation as a function of age.The alkaline dacite is characterized by low contents ofsilica and low differentiation indices whereas the youngercomponents, especially the Fawwarah alkali-feldspar gran-ite, are characterized by high silica contents and high dif-ferentiation indices (Table l). Total iron, AlrOr, MgO'CaO, TiOr, PrOr, and MnO contents decrease from theolder to the younger rocks, that is, from the low silica tothe high silica rocks. NarO contents are nearly constant.KrO contents increase with increasing silica contents'reach a maximum in samples of the alkali granite, and thendecrease slightly with increasing silica. Fluorine contentsincrease with increasing silica contents. The NarO/K2Oratio in samples of the complex varies discontinuously.

The alkaline dacite and comendite were treated as vol-canic rocks and classified on the basis of their chemistry.Samples of the alkaline dacite are metaluminous andfollow an iron-enrichment trend (Fig. 3) similar to thatdisplayed by the rocks of the Skaergaard intrusion (Wagerand Deer. 1939). The alkaline dacite is characterized byhigh alkali element content, high total iron content, andlow silica content. Samples of this rock type are alkaline, asdefined by Irvine and Baragar (1971); members of both thesodic and potassic suites of alkaline rocks are representedin the complex. The composition of the alkaline daciteranges from benmoreite and mugearite to tristanite andtrachyte. The tristanite and trachyte are compositionallytransitional to samples of comendite, The chemistry ofcomendite samples (Figs. 3-5) support the petrographic in-ferrence that these rocks are quenched equivalents of theSilsilah alkali granite. The samples are peralkaline andacmite normative.

Major element analyses for the three principal constit-uents of the ring complex indicate that these are highlyevolved rocks (Table 1). The Silsilah alkali granite isweakly peralkaline and acmite normative. Differentiation

indices indicate that the Silsilah alkali granite is slightlymore evolved than the low-calcium granite (Table 1). Inparticular, the alkali granite is characterized by greatercontents of NarO, KrO, CaO, TiO, and total iron andlower contents of AlrOr, MgO, and PrOr. The Hadhiraplite is metaluminous whereas the Fawwarah alkali-feldspar granite is weakly peraluminous and corundumnormative. The differentiation indices for samples of thesetwo plutons are greater than that of the low-calcium gran-ite; the aplite is a little less evolved than the alkali-feldspargranite. These two plutons are characterized by very highcontents of SiOr; high contents of NarO, KrO, and F; andvery low contents of total iron, MgO, CaO, TiOr, PrOr,and MnO.

Certain similarities exist between the maior element

Table 1. Average composition of Silsilah ring complex compo-nents. N, indicates not detected at the indicated value: L. indicatesless than the indicated value; hyphen indicates not present; ND,

indicates not determined.

LO79

Fig. 3. Alkali-iron-magnesium (AFM) ternary diagram, inweight percent, for samples of the Silsilah ring complex. A :(NarO + KrO), F :(FeO +0.8998 x FerOr), M : Mgp. Trendlines from Irvine and Baragar (1971). Plot symbols: squares-alkaline dacite, plusses----comendite, open circles-Silsilah alkaligranite, triangles-Hadhir aplite, solid circles-Fawwarah alkali-feldspar granite.

composition of some components of the Silsilah ring com-plex and highly evolved igneous rocks identified elsewherein the World. The aplite and especially the alkali-feldspargranite have compositions similar to those presented byTischendorf (1977) for metallogenically specialized granites.

Similarities also exist between the composition of thealkali-feldspar granite and the aplite and S-type granites(White and Chappell, 1977), ilmenite-series granites (Ishi-

hara, 1977), and topaz rhyolites (Burt et al., 1982\. Each ofthese highly evolved rock types, like the metallogenicallyspecialized granites, is associated with deposits of tin, tung-sten, and other rare metals. In contrast, the composition ofthe Silsilah alkali granite is not as evolved as compositionsofthese highly evolved rock types.

Trace elements

Each of the three principal ring complex componentscan be distinguished on the basis of trac€ element compo-sition. Relative to the average low-calcium granite (Ture-kian and Wedepohl, 1961) the Silsilah alkali granite is de-pleted in Li, F, Rb, Sr, Ba, and Th and is enriched inY,Zr,Nb, Sn, U, and Be; its Rb/Sr ratio is 3.1 and is slightlygreater than that of the low-calcium granite. The trace ele-ment chemistry of the comendite is similar to that of theSilsilah alkali granite (Fig. 5), which confirms that it is thequenched equivalent of the alkali granite. The Hadhiraplite is enriched in Li, F, Rb, Y, Nb, Sn, Th, U, and Beand depleted tn Sr,Zr, Ba, and Cu; its Rb/Sr ratio is 17.5and is much greater than that of the low-calcium granite.The Fawwarah alkali-feldspar granite is enriched in Li, F,

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

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Fig. 4. Ternary diagram showing the normative quartz, albite,and orthoclase contents (Q-ab--or) of components of the Silsilahring complex. Circled points from top right to bottom left, repre-sent the minimum melting compositions in the experimentalsystem SiOr-KAlSi.Or-NaAlSi3Os-H2O for P(HrO) : P(Total): 2, 4, 5, l0 kbat (Winkler and others, 1975). Circled plusesindicate minimum melting compositions at I kbar with excessHrO and 1,2, and,47o fluorine (Manning, l98l); other plot sym-bols as in Fig. 3.

Rb, Y, Nb, Sn, Th, U, and Be and depleted in Sr, Zr, Ba,and Cu; its Rb/Sr is 28.1 and is about fifteen times that ofthe low-calcium granite. Trace element enrichments anddepletions identified in the Hadhir aplite are better devel-oped in the Fawwarah alkali-feldspar granite. The degreeto which the latter is enriched in some elements and deple-ted in others is extreme.

The degree of incompatible trace element enrichment ob-served in each of these rocks demonstrates that the Hadhiraplite and the Fawwarah alkali-feldspar granite are signifi-cantly more evolved than the Silsilah alkali granite (Table1). Tischendorf (1977) indicates that incompatible elementenrichment, including combinations of F, Rb, Li, Sn, Be,W, and Mo and high Rb/Sr ratios are diagnostic character-istics of metallogenically specialized granites. Topaz rhyo-lites described by Burt et al. (1982) and S-type granitesdescribed by Chappell and White (1974) are similarly en-riched in these same elements.

The trace element composition of the alkaline dacite isdistinct. In particular, these rocks are not enriched, but aredepleted instead, in most incompatible trace elements andthey contain significantly more barium than the Silsilahalkali granite. The high zirconium content of these rocks is

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

significant in light of the zirconium content of the Silsilahalkali granite.

Rare-earth elements

The REE chondrite-normalized patterns (Table I, Fig. 6)for the alkaline dacites have a gentle and negative slopeand lack a europium anomaly. The comendites have REEpatterns identical to those ofthe Silsilah alkali granite. Thetotal REE content increases from almost 200 ppm in thealkaline dacites to about 450 ppm in the comendites. Thetotal REE content and chondrite-normalized REE patternsfor samples of the Silsilah alkali granite are similar to thosedetermined for other peralkaline rocks in the northeasternArabian Shield (Stuckless et al., 1982). Of the rocks thatcompose the Silsilah ring complex, the Silsilah alkali gran-ite has the highest average REE content, 469 ppm. TheREE patterns are negatively sloping, parallel to the averagepattern for the alkaline dacites and are characterized by amoderate, negative europium anomaly. The average totalREE content in the Hadhir aplite is about 145 ppm, lessthan a third of that in the alkali granite. Chondrite-normalized REE patterns for samples of the Hadhir apliteare nearly flat but slightly negatively sloping and arecharacterized by a negative europium anomaly slightlygreater than that observpd in the alkali granite. Theaverage total REE content of the Fawwarah alkali-feldspargranite is about 164 ppm, whereas the low-calcium granitecontains about 210 ppm of the same nine REE (Turekianand Wedepohl, 196l). Chondrite-normalized REE patternsfor the Fawwarah alkali-feldspar granite are nearly flat andcharacterized by a large negative europium anomaly. FlatREE patterns and large negative europium anomalies arecharacteristic of highly evolved granitoid rocks (Miller andMittlefehldt, 1982).

Fig. 5. Trace element variation diagram showing the relativeproportions of strontium, potassium, and rubidium in compo-nents of the Silsilah ring complex. Plot symbols as in Fig. 3.

SrRb

K/ 100

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

uJEEozoI(J

UJJo-

o

La Ce Nd Tb Dy Yb Lu

Fig. 6. Average chondrite-normalized rare-earth element pat-terns for components of the Silsilah ring complex. Silsilah alkaligranite average includes data for two comendite samples. plot

symbols as in Fig. 3.

Economic geology

GreisensTwo types of greisenized rock, which represent complete

and incomplete greisenization of the Fawwarah alkali-feldspar granite, are associated with rocks of the ring com-plex. These greisen lithologies are spatially unrelated toquartz veins. Primary igneous textures characteristic of theFawwarah alkali-feldspar granite are distinctly preserved inthe incompletely greisenized rock and in rock associatedwith the completely greisenized rock that has undergoneargillic alteration. Primary igneous textures have been ten-tatively identified in the completely greisenized rock.

Strongly mineralized, completely greisenized alkali-feldspar granite is present immediately beneath the Hadhiraplite and pegmatitic rock found at the base of the aplite.The aplite itself crops out subhorizontally beneath theMaraghan lithic graywacke. The intensity of greisenizationdecreases downward and within a few meters incompletelygreisenized alkali-feldspar granite is present. Incompletelygreisenized rock grades downward into progressively lessaltered Fawwarah alkali-feldspar granite. Completely greis-enized rock is resistant in the weathering environment, andforms low hills, whereas incompletely greisenized rock hasbeen more readily eroded, and crops out peripherally onthe flanks of hills formed by completely greisenized rock.Greisen is emergent beneath aplite throughout the south-west part of the complex. These geologic relations suggestthat additional sheet-form or cupolalike bodies of greise-nized rock may exist beneath thin veneers of aplite and inas yet unexposed positions below the Maraghan lithicgraywacke.

The completely greisenized rock contains qvartz, topaz,and cassiterite, and trace amounts of iron-titanium oxidesand white mica. This lithology has been identified in two

small hills located less than I km inside the southwesternpart of the ring (Fig. 1). Completely greisenized rock has anirregular outcrop pattern and grades into reddened, argilli-cally altered alkali-feldspar granite and to incompletelygreisenized rock. Disseminated grains of cassiterite occur instructureless qvartz-topaz greisen that constitute from 0 toabout 10% of the rock. Cassiterite also occurs in ellipticalpods 0.05 to 5 m in diameter that are 50 to 90% cassiteritein a matrix of quartz and topaz. Contacts between almostbarren quartz-topaz rock and the cassiterite pods are ex-tremely sharp.

Incompletely greisenized rock contains quartz, whitemica, topaz, and trace amounts of albite and cassiterite.This lithology is not as abundant as completely greisenizedrock. Within the complex incompletely greisenized alkali-feldspar granite is encountered in several different settings.In addition to occurrences peripheral to completely greise-nized rock, it occurs as pods within the Fawwarah alkali-feldspar granite. Several of these pods were identified inisolated positions within the southeastern part of thealkali-feldspar granite pluton. Isolated pods are also associ-ated with the two small masses of alkali-feldspar granitelocated inside the ring. These pods of greisenized alkali-feldspar granite are surrounded by Maraghan lithic gray-wacke. The pods are approximately elliptical in shape andare between 10 and 200 m in diameter.

The mean and standard deviation of concentration foreach element in a set of 64 samples of the two stronglymineralized greisens is given in Table 2. Each sample is

Table 2. Compositions of samples collected from the two in-tensely mineralized greisens in the southwest part of the Silsilahring complex. Samples composited from rock chips collected in 10m intervals. All values in parts per million, except iron, mag-

nesium, calcium, and titanium which are in percent.

E l . e m e n t M e a n S t a n d a r d N u n b e rd e v i a t i o n s a n p l e s

F e 2 . 7 6 1 . 1 5 6 8M g 0 . 0 q 0 . 0 3 6 8C a , 8 4 . 6 1 6 8T 1 . 0 1 . 0 3 1 r 3Mn 429 52\ 684 s 9 5 6 3B 1 3 U 2 2Ba 61 33 688 e 2 2 2 \ts i 53 5U 66cr 295 93 68C u 1 0 6 6 8 6 8M o 1 2 8 5 7N b 6 4 1 9 6 8N i 7 2 5 9Pb 661 \77 58S c 6 2 7S r 1 3 0 \ 2 2 3v 1 6 5 5 t' 1 1 2 3 2 27n 32 \ 1 37 21zr 83 2C 68A s 1 3 6 7 1 9 1 3 6 \Sn 9527 21 3a7 6ct i 175 2 \1 6 ) )Rb 351 393 649 r t 8 9 i 3 ? 8 5 r r

Sm Eu

N u a b e r o f s a m p l e s w i t h u n q u a i i f i e d

1082 DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

composited from rock chips collected semicontinuouslywithin 10 m intervals. Of particular interest are the meanconcentrations of Ag, B, Bi, Mo, Nb, Pb, Zn, As, Sn, W,and F which are present in anomalous to highly anoma-lous concentrations relative to the low-calcium granite ofTurekian and Wedepohl (1961). The elements Sn, W, As,Bi, Pb, and Zn are all present in highly anomalous con-centrations and have wide ranges of abundances. However,only tin and tungsten occur at economically interestinggrades. Few samples contain less than 1000 ppm tin (duBray, 1983c).

Quartz ueins

Samples of quartz veins that cut the Fawwarah alkali-feldspar granite contain interesting concentrations of sometrace elements. As much as 300 ppm silver was determinedand most contain 0.5 ppm. As much as 500 ppm bismuthwas determined and many samples contain 10 ppm. Severalsamples contain 700 ppm molybdenum and most contain10 ppm. Most of the quartz vein samples contain 100 to

high but variable concentrations of tungsten.Most of the quartz veins that cut the Silsilah alkali gran-

ite don't contain anomalous concentrations of the tracemetals. A quartz vein system at the south end of the ringcomplex 5 km west of Jabal al Hadhir contains very highthough erratic concentrations of tungsten. Samples fromthese veins contain as much as 50 ppm silver, 300 to 2@0ppm As, 15 to 300 ppm bismuth, 10 to 700 ppm molyb-denum, 15 to 7000 ppm lead, 200 to 1000 ppm tin, up to900 ppm lithium, and as much as 5000 ppm fluorine.

Petrogenesis of the igneous rocks

The allotriomorphic and micrographic textures thatcharacterize the Hadhir aplite and the Fawwarah alkali-feldspar granite suggest that the melts represented by theserocks solidified by cotectic crystallization. The normativecompositions of the complex components are compared tominimum melting compositions in the experimental systemSiO2-KAlSi3Or-NaAlSi.Or-HrO (Fig. 4). Normativecompositions for samples of the Silsilah alkali granite plotwell below the minima trend; they contain more of the aband or components and less of the 0 component than ex-perimentally determined minimum melt compositions.These samples are characterizedby a nearly constant abforratio while Q displays significant variation. The Hadhircomposition plots slightly below the I kilobar (kbar) mini-mum composition. The Fawwarah alkali-feldspar graniteshows considerable compositional variation parallel to anddisplaced slightly below the minima trend, towards slightlyor-enriched, Q-depleted compositions that are appropriateto crystallization in the P(HrO) range between about 0.5and 4 kbar. By interpolation, the average composition ofthe Fawwarah alkali-feldspar samples plots slightly below

the minimum melting composition that corresponds toP(total) : P(HrO) : about 1.5 kbar which is equal to 4km of overburden.

The effect of varying amounts of fluorine on the mini-mum melting composition is also displayed because theaplite and the alkali-feldspar granite contain large amountsof fluorine. Fluorine dramatically affects the phase equilib-ria in the granite system. It shifts the minimum melt com-positions towards ab-enriched compositions (Manning,1981). Samples of the alkali-feldspar granite whose compo-sitions suggest the greatest crystallization depths generallycontain the highest fluorine concentrations. The averagenormative composition of the alkali-feldspar granite is co-incident with the minimum melt composition that wouldbe obtained for about 0.25% fluorine and P(Total) : 1kbar or about 3 km of overburden. This is a maximumdepth estimate because the fluorine-dominated greiseniza-tion indicates that some fluorine initially contained in thealkali-feldspar granite melt exsolved and was lost duringlate magmatic processes. The fluorine content of samples istherefore a minimum value for the melt.

The presence of the white mica zinnwaldite does notrequire that the alkali-feldspar granite crystallized at highpressure, that is, at great depth. Experimental work (Miller

et al., 1981) has demonstrated the stability of non-idealwhite micas extends to the much lower pressures than thatproposed for muscovite (Yoder and Eugster, 1954).

The Hadhir aplite is considered to be the quenchedequivalent of the Fawwarah alkali-feldspar granite becauseof the similarity of their chemical compositions and be-cause of their close spatial association. The texture charac-teristic of the Hadhir aplite may be a function of its havingexperienced pressure quenching. Soon after the magmarepresented by these two lithologies was emplaced in thering fracture, it may have begun to exsolve a fluid phase.The volume expansion and increased pressure created bythe exsolved fluid could have caused extensive fracturing ofthe wallrock which in turn would have led to dramaticreduction of confining pressure. With confining pressuregreatly reduced, the conditions necessary for exsolution ofa fluid phase would have been enhanced and rapid fluidloss would greatly increase the magma's liquidus and sol-idus temperatures. Rapid solidus depression would haveresulted in rapid, disorderly crystallization of the magma(Tuttle and Bowen, 1958). The process would have oc-curred rapidly and the fractures would have been sealed byaplite dikes. Once resealed, more orderly solidification ofthe remaining magma, represented by the alkali-feldspargranite, which was located below some finite thickness ofpressure-quench aplite, could proceed. Small chemical dif-ferences between the aplite and the alkali-feldspar graniteindicate that some differentiation continued following thequenching event.

The Silsilah alkali granite is slightly porphyritic and soits solidification history involves at least some crys-tallization under noncotectic conditions. The trend of in-creasing Q displayed by samples of the alkali granite (Fig.4) may indicate that noncotectic crystallization of anortho-

clase is responsible for the compositional variation dis-played by this pluton. Crystallization and removal of anor-thoclase with a composition like that of the perthite in thealkali granite, namely abruorno, would have the effect ofchanging the composition of the remaining liquid along atrend coincident with that depicted by samples of the alkaligranite and comendite (Fig. 4) and toward the compositionof the Hadhir aplite. The most Q-enriched sample of com-endite can be considered to approximate the cotectic com-position towards which the liquid represented by the alkaligranite was evolving. Consequently, the alkali granite mustalso have crystallized less than 4 km below the surface.

Since feldspar contains more molar AlrO. than molarNarO plus KrO its crystallization and removal from themagma represented by the alkaline dacite caused the com-position of the remaining liquid to become enriched inNarO and KrO relative to AlrO. and the liquid evolved toa peralkaline composition. Peralkalinity was achieved andpermitted crystallization of soda pyriboles includingaegirine-augite, katophorite, and arfvedsonite. The com-endite is the most peralkaline component, a fact which isemphasized by the presence of arfvedsonite, end-membersoda amphibole. The amphiboles are less sodic in the lessperalkaline Silsilah alkali granite. The crystallization andremoval of the soda pyriboles, with agpaitic ratios muchgreater than 1, caused the composition of the remainingmelt to evolve toward a peraluminous composition such asthat characteristic of the Hadhir aplite and the Fawwarahalkali-feldspar granite.

Igneous processes that yield a peralkaline liquid favorpartitioning of zirconium into the liquid phase becausealkali-zircono-silicate complexes are stabilized (Watson,1979). Zirconium is concentrated in peralkaline magmasduring the differentiation process but is buffered at 100 to200 ppm in nonperalkaline magmas by crystallization ofzircon. The extreme enrichment of zirconium in the Silsilahalkali granite is a function of this complexing phenomenonand is characteristic of the peralkaline granites of thenortheastern Arabian Shield (Stuckless et al., 1982). Thehigh zirconium content of the alkaline dacite may alsoresult from this process.

The dramatic reduction in zirconium content betweenthe alkali granite and both the aplite and alkali-feldspargranite is probably related to fractionation of zircon whichoccurred in response to soda amphibole fractionation. Am-phibole fractionation can cause a peralkaline magma toevolve to one with a peraluminous composition and in sodoing will greatly reduce the ability of the magma to stabil-ize zircono-alkali-silicate complexes (Watson, 1979). Withthe destabilization of these complexes, the magma becamezirconium-saturated and zircon began to crystallize, as in-dicated by the abundance ofaccessory zircon in the Silsilahalkali granite, causing dramatic depletion of zirconium inthe remaining magma. In high-silica liquids amphibolesthemselves have mineral-melt distribution coeflicients forzirconium that are greater than one. Thus, fractionation ofsoda amphibole may cause additional reduction of themagma's zirconium content.

1083

The observation that the chemical evolution of the ringcomplex is time dependent and systematic is confirmed bythe fact that the alkaline dacite, the oldest component ofthe complex, is strontium-enriched, the Silsilah alkali gran-ite, of intermediate age, is potassium-enriched, and theHadhir aplite and the Fawwarah alkali-feldspar granite,the youngest components of the complex, are rubidium-enriched (Fig. 5). The contents of these elements are con-trolled by their mineral-melt distribution coeflicients andthe prevailing phase equilibria. Primitive, basaltic magmasare characterized by high concentrations of strontium rela-tive to potassium (Turekian and Wedepohl, 1961). Compo-sitions such as this become increasingly potassium enrichedas plagioclase crystallizes and removes strontium from theliquid (Hanson, 1978). Rubidium, which acts incompatibly,remains in the melt so that rocks of intermediate compo-sition contain little more rubidium than the primitivemagma from which they are derived. When potassium feld-spar nucleates, late in magmatic evolution, potassiumbegins to be effectively removed fiom the liquid (Hanson,1978) and the relative concentration of rubidium begins toincrease. If this trend continues, a small volume of highlyevolved magma with a very high rubidium content mayresult. Rubidium is finally and effectively removed from theliquid by crystallization of micas (Hanson, 1978). This dif-ferentiation process is depicted by a smooth variationcurve on a ternary strontium-potassium-rubidium diagramthat begins near the strontium apex, trends toward thepotassium apex, and then turns dramatically toward therubidium apex. Geochemical data for the components ofthe Silsilah ring complex follow this trend and suggest thatthe components are cogenetic and evolved by processes ofigneous differentiation.

The barium content of the complex's components furthersubstantiates the proposed cogenesis of the complex's com-ponents and suggests that the alkali-feldspar granite andthe aplite represent a small part of the primitive, parentalmagma. The high barium content of the alkaline dacite andthe affrnity of biotite and potassium feldspar for barium(Hanson, 1978) suggest that one or both of these mineralswas part of the partial melt assemblage. The dramatic and

monotonic decrease of barium, from the oldest component

to the youngest, demonstrates the evolution of the magmatoward barium-depleted compositions by crystallizationand removal of potassium-bearing sodic plagioclase fromthe alkaline dacite magma, by crystallization and removalof anorthoclase from the peralkaline magma, and finally bycrystallization and removal of mica and potassium feldsparfrom the peraluminous magma. The extremely low bariumcontent of the complex's youngest components, relative toits content in the oldest component, indicates that theformer represent a small proportion of the latter.

Relative REE content and chondrite-normalized pat-terns also provide some insight into the genesis of the ringcomplex components. The lack of a europium anomaly insamples of the alkaline dacite suggests that these rocksrepresent primary liquids which have undergone little, ifany, differentiation. Plagioclase and iron-titanium oxides

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATI}N

1084

are major constituents of the alkaline dacites but arealmost absent in the other components. Their near absenceindicates that they were removed from the melt, perhaps byfilter pressing or crystal settling. Except for europium inplagioclase, the REE have mineral-melt distribution coef-ficients that are less than I in both plagioclase and iron-titanium oxides (Hanson, 1978). Consequently, crys-tallization and removal of iron-titanium oxides and plagio-clase from the alkaline dacite liquid did not cause REEfractionation, but their removal led to the overall REEenrichment observed in the alkali granite. The removal ofplagioclase did cause development of a negative europiumanomaly.

Derivation of the chondrite-normalized REE patternsthat typify the alkali-feldspar granite and the aplite fromthat characteristic of the peralkaline rocks was alsoachieved by crystal fractionation. As indicated by the totalREE content of these rocks (Table 1), the tendency for theREE to act incompatibly and to be enriched in residualmelt was more than balanced by their removal in the min-erals crystallizing from the magma represented by thealkali granite. Relative to the alkali granite, REE patternsfor the aplite and alkali-feldspar granite are flattened andcharacterized by much larger negative europium anoma-lies.

The REE distribution coefficients in anorthoclase(Hanson, 1978), such as that which is the major mineralphase in the weakly peralkaline Silsilah alkali granite, aresuch that its crystallization and removal caused the mag-nitude of the negative europium anomaly to increase andcaused the overall REE content of the remaining liquid toincrease. However, the tendency for REE abundances toincrease was counteracted by crystallization of zircon andallanite from the alkali granite magma. The REE distri-bution coefficient patterns for these two minerals are com-plimentary. Their simultaneous crystallization in equalamounts leads to overall reduction of the magma's REEcontent and to slightly greater light REE (LREE) depletionthan heavy REE (HREE) depletion because distributioncoeffrcients for LREE in allanite are greater than distri-bution coeffrcients for HREE in zircon (Mahood and Hil-dreth, 1983).

Absolute enrichment of the HREE as well as enrichmentrelative to the LREE was achieved by several processes.Some flattening of the REE patterns and absolute enrich-ment of the HREE may have been achieved by anortho-clase fractionation. Distribution coeflicients for the LREEin anorthoclase are greater than for the HREE, and sinceabundant anorthoclase crystallized and was removed fromthe magma represented by the alkali granite the HREEbecame enriched in the melt relative to the LREE. Mahoodand Hildreth (1983) have demonstrated that in high silicamagmas mineral-melt distribution coefficients for REE iniron-titanium oxides are greater than one and are signifi-cantly greater for LREE than HREE. Crystallization andfractionation of iron-titanium oxides, abundant in thealkali granite and extremely scarce in the aplite and alkali-feldspar granite, may have caused further LREE depletion.Finally, Candela (1984) has demonstrated that LREE are

DU BRAY: SILSILAH RING COMPLEX TIN MINERALIZATION

more strongly partitioned into halogen-enriched hy-drothermal fluids than HREE. Thus, magmas that evolve ahalogen-enriched fluid phase, such as the fluorine-richfluids that escaped from both the aplite and alkali-feldspargranite, will be depleted of LREE relative to the HREE.

The temporal, spatial, and tectonic association of theSilsilah ring complex components suggest that these rocksare cogenetic. The components of the complex may repre-sent samples that depict the successive chemical evolutionof a large magma. The chemical variation portrayed by thecomponents depicts a discontinuous process that involvedmagma evolution via separation of melt from earlierformed crystals, and emplacement of batches of magmawhose compositions represent stages of the parent magma'sevolution. Petrographic observations and chemical datasuggest that the early chemical evolution of primitivemagma represented by the alkaline dacite to a compositionlike that of the alkali granite was controlled by the frac-tionation of iron-titanium oxides and sodic plagioclase.Chemical evolution by crystallization and removal of anor-thoclase, soda pyriboles, zircon, and iron-titanium oxidesand evolution of a LREE-enriched fluorine-rich hy-drothermal fluid caused the composition of the remainingmagma to become like those of the aplite and the alkali-feldspar granite.

Petrogenesis of the mineralized rocks

The genesis of the mineralized rocks, in particular, thequartz-cassiterite-topaz greisens and the wolframite-bearing quartz veins are only partly understood. Thenearly complete separation of high-grade accumulations ofcassiterite in greisens from high-grade but erratic accumu-lations of wolframite in quartz veins suggests that a large,strongly zoned hydrothermal system once existed in thesouthwest part of the ring complex.

The Fawwarah alkali-feldspar granite contains a highlyanomalous quantity of tin (Table 1) so that it, and not thegraywacke, is the likely source of the tin. Tin acted as anincompatible element during crystallization and its con-centration in the melt increased progressively. Tin was ulti-mately partitioned into a fluorine-rich fluid phase (Bailey,1977) that exsolved as the Fawwarah alkali-feldspar granitecrystallized, although a small proportion of tin was parti-tioned into zinnwaldite. This fluid phase seems to havebeen trapped below the previously crystallized, competentand impermeable Hadhir aplite and caused recrystalliza-tion of the alkali-feldspar granite in its roof zone. Mineral-ization may have been localized by faults or in fracturesthat developed as the alkali-feldspar granite cooled andcontracted. Similarly, weakly mineralized greisen patcheswithin the alkali-feldspar may be structurally controlled.

The results of experiments in the granite-NaF-SnOtsystem (Stemprok, 1982) indicate that two immiscible liq-uids, one silicate-dominated and the other fluoride-dominated, evolve under conditions appropriate to mag-matic and hydrothermal processes. Tin is almost exclu-sively partitioned into the silicate liquid. The pod-like ac-cumulations of cassiterite in the greisens may represent theimmiscible unmixing of silicate and fluoride liquids during

the greisenization process, and the concommitant par-titioning of tin into the former.

Summary

A tin greisen deposit is associated with an alkali-feldspargranite that forms part of a ring complex at Jabal as Silsil-ah. The petrologic and geochemical characteristics of theFawwarah alkali-feldspar granite are like those of graniteslocated elsewhere in the World that also are associatedwith deposits of tin, tungsten, and rare metals. The alkali-feldspar granite is peraluminous, incompatible traceelement-enriched, and is characterized by a flat chondrite-normalized REE pattern that includes a very large, nega-tive europium anomaly. The mineralogy of the alkali-feldspar granite is also distinctive because it includes zinn-waldite and topaz.

Differentiation and mineral fractionation controlledmagma evolution as components of the ring complex weresequentially emplaced. The Fawwarah alkali-feldspar gran-ite and its quenched equivalent, the Hadhir aplite, werederived from the Silsilah alkali granite, which in turn wasderived from liquid represented by alkaline dacite. Separa-tion of iron-titanium oxides and plagioclase from the geo-chemically primitive alkaline dacite magma caused the re-maining magma to evolve toward a composition repre-sented by the Silsilah alkali granite and its quenched com-enditic equivalent. The crystallization and separation ofsoda pyriboles, anorthoclase, and zircon then caused theremaining magma to evolve to a composition representedby the Fawwarah alkali-feldspar granite and the Hadhiraplite.

Evolution of the ring complex and its associated tin de-posit concluded with local, intense alteration of the Faw-warah alkali-feldspar granite to a cassiterite-bearing greis-en. The alteration was achieved by a late-stage, hy-drothermal fluid that was probably fluorine dominated.The fluid was locally trapped in cupolas beneath the imper-meable carapace formed by the Hadhir aplite. The effect ofthe fluid was to convert the alkali-feldspar granite to anassemblage of quartz, topaz, and, cassiterite. Ellipticallyshaped, high-grade accumulations of cassiterite may owetheir existence to the evolution of a second, immisciblefluid phase. Samples of greisens suggest that the locus ofeconomically viable deposits of tin is not restricted to thetwo known, strongly mineralized greisens.

,A,cknowledgments

The work on which this study is based was done in accordancewith the work agreement b€tween the U,S. Geological Survey andthe Saudi Arabian Ministry of Petroleum and Mineral Resources.Early drafts were reviewed and improved by C. R. Ramsay, N. J.Jackson, R. J. Kamilli, and D. H. Lindsley.

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Manuscript receiued:, October 25, 1984;accepted fior publication, July I 5, 1985.