The Salpeterkop ring structure, Cape Province, South Africa

Tectonophysics, 171 (1990) 275-285 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

275

The Salpeterkop ring structure, Cape Province, South Africa

W.J. VERWOERD

Department of Geology, University of Stellenbosch, Cape Province (South Africa)

(Revised version accepted November 15,1988)

AbStlWt

Venvoerd, W.J., 1990. The Salpeterkop ring structure, Cape Province, South Africa. In: L.O. Nicolaysen and W.U. Reimold (Editors), Cryptoexplosions and Catastrophes in the Geological Record, with a Special Focus on the Vredefort Structure. Tectonophysics, 171: 275-285.

Salpeterkop (32O29’S, 21” 51’E) near Sutherland forms part of a dome with radial fractures extending to a distance of at least 16 km. Four concentric zones of decreasing megascopic deformation are distinguished, including a central crater 1 km in diameter, but no unequivocal shock features could be found. Intense brecciation is indicative of explosive eruptions. A radial and concentric fracture pattern has been exploited by trachytic and carbonatitic dykes. The regular outward dip of the surrounding strata is ascribed to uplift by an intrusive plug represented by numerous clast-rich trachytic apophyses at surface. The Salpeterkop structure is interpreted as a dome overlying an alkaline- carbonatite ring complex, obscured by a volcanic superstructure that has been only partly eroded.

Intruduction

The spectacular ring structure, 5 km iu diameter, centred on Salpeterkop peak in the Sutherland district, Cape Province (Fig. 1) has escaped the attention of researchers studying meteorite impact structures, probably because an association with intrusive and extrusive rocks obviously pointed to a volcanic origin. In addition, the crater has been strongly eroded to such an extent that it is not easily recognizable. Nevertheless, several features (almost perfectly circular outline, isolated posi- tion, updoming and prominent brecciation) tempt one to include the Salpeterkop ring among cryptoexplosion structures of Class IV a/2, i.e. “complex structures with central uplift but without enclosing ring depression or evidence of shock metamorphism” (McCall, 1979). Consideration of the origin of this structure can contribute signifi- cantly to the knowledge on updoming, associated

oo40-1951/90/$03.50 Q 1990 Elsevier Science Publishers B.V.

alkali rocks and intense brecciation in cryptoexplosion structures.

The Rich& Dome in Mauritania is in many respects similar to Salpeterkop, although much larger. The concentric outcrops of updomed strata, 38 km in diameter, have been described as “probably the most striking earth structure so far photo- graphed from near space” (Dietz et al., 1969). It has been shown conclusively (Dietz et al., 1969; Woolley et al., 1984) that Rich& is not an astro- bleme as originally supposed. The central area is occupied by flat-lying meta-arkose; by analogy with Salpeterkop this may well prove to be a crater deposit.

Previous work

The existence of the Salpeterkop structure has been known since the early part of the century (Rogers and Du Toit, 1903). It was described as a

216

volcanic neck accompanied by at least twenty

subsidiary necks that pierce the surrounding up-

arched strata. Rogers and Du Toit drew attention

to a possible genetic connection between kimber-

lite, “melilite basalt” and at least some of the

pipe-like bodies near Salpeterkop (e.g. the Silver

Dam locality just beyond the western boundary of

Fig. 4). De Wet (1975) presented the first detailed

geological map of Salpeterkop and provided pet-

rographic descriptions of the breccias as well as

associated K-trachyte, carbonatite and olivine

melilitite dykes and plugs. Subsequently, McIver

and Ferguson (1979) postulated that this assem-

blage originated by fractional crystallisation, liquid

immiscibility and crystal-liquid reaction phenom-

W.J. VERWOERD

ena operating in an ascending CO&h kimber-

litic magma. The doming of the country rock was

ascribed to pressure of volatiles.

Until recently, structural aspects of the dome

were neglected in favour of petrological studies of

the associated intrusive and so-called pyroclastic

rocks. De Wet (1975) measured 445 joints and 124

carbonatite dyke orientations, revealing two major

trends. The fracture pattern was studied in more

detail on aerial photographs and Landsat images

by Newton (1987) who concluded that radial frac-

tures were superimposed on two pre-existing sets

-a regional “regmatic” pattern and two major

lineaments with northeasterly and N-S trends.

The purpose of the present investigation is to

Fig. 1. Vertical aerial photograph of the Salpeterkop ring structure.

SALPETERKOP RlNG STRUCTURE

contribute more information on the orientation of bedding, joints, dykes, sills and other structural features that may provide an insight into the cause

of the deformation.

Regional setting

Salpeterkop (32”29’S, 21°51’E) is situated 20 km southeast of Sutherland within the Palaeozoic Karoo basin of South Africa. The country rocks are sandstone, siltstone and shale with a shallow (2”-3”) northward dip and belong to the Permian Teekloof Formation of the Beaufort Group. Sedi- mentary rocks are intruded by Early Jurassic dolerite sills. Although small diatremes and minor intrusions of olivine melilitite, carbonatite and related rocks post-dating the dolerite are widely scattered t~ou~out the Su~erland district, the Salpeterkop disturbance is unique in the area. The closest analogy in terms of structure, age and associated rock types is the 84 m.y. old Gross Brukkaros crater 780 km to the northwest, but there is no colon lineament between Salpeter- kop and Brukkaros or the other two circular structures listed in Table 1 (cf. Fig. 2). On the basis of a fracture analysis Newton (1987) concluded that the location of Salpeterkop may be constrained by the intersection of several large fractures with a major NE-trending lineament (Fig. 5A). However, in view of the large number of fractures present, this argument cannot be considered decisive. In a wider context, Salpeterkop lies within a 400 km wide zone of alkaline magmatism straddling the boundary of the 1000 m.y. old Kalahari Craton and parallel to the western margin of the African continent (Fig. 2). Craton-margin magmatism of this type is a well-known phenomenon (Moore and Verwoerd, 1985). The Latest Cretaceous to Tertiary age of these volcanics (and probably also of Salpeterkop) can be correlated with the later stages in the opening of the Atlantic Ocean. The South America-Africa rift commenced at 130-120 Ma and was punctuated by changes in the pole of rotation at 80 Ma, 56.8 Ma and at about 40 Ma, each time coincident with magmatic activity on both continents (Garson, 1984). A broad geo- tectonic control for most of the circular structures listed in Table 1 thus seems plausible, even if they

271

TABLE 1

Circular structures of the southwestern Cape Province and South-West Africa/Namibia (localities in Fig. 2)

Structure Diameter Diameter Reference of crater of updomed

(km) area (km)

Brukkaros 3.0 10.0 Janse (1969) Hatzium Unknown 2.4 Heath and

Toerien (1962) Roter Kamm 2.5 If present, Miller and

unknown Reimold (1986); Reimold et al. (1988)

Salpeterkop 1.0 5.5 This paper

4 i: !j I1

L Fig. 2. Tectonic setting of Salpeterkop. Map shows edge of continental shelf, present coastline, boundary of Kalahari Cra- ton (thick dashed line) and approximate boundary of postulated zone of a&a&e magmatism parallel to the west coast of Southern Africa. Dots are alkaline plugs, non-di~ondifer- ous kimberlite pipes and intrusive complexes of Late Creta- ceous and Tertiary age (35-70 Ma). Cretaceous diamondifer- ous as well as barren kimberlites occur further east and are older (SO-90 Ma) (Mitchell, 1986). Location of ring structures

are indicated by circles.

278

do not fall on lineaments; Roter Kamm, however,

is considered to be a meteorite crater by, among

others, Miller and Reimold (1986) and Reimold et

al. (1988).

Geology of the Salpeterkop structure

Salpeterkop itself is a conical hill of silicified

oligomictic breccia rising about 250 m above the

surrounding topography and is 1767 m above sea

level (Fig. 3). Tuffaceous deposits and carbonatite

occur immediately south of the hill. The surround-

ing normally almost flat-lying shale and sandstone

dip away in all directions, becoming more shallow

away from the centre. Numerous plugs and dykes

of contaminated K-trachyte and a few small intru-

sions of olivine melilitite penetrate these beds.

Thin carbonatite dykes and sills are present up to

at least 3 km from Salpeterkop. Any possible

fall-out deposits have been completely eroded.

The concentric ridges of sandstone, the subsidiary

plugs and the main peak all owe their topographic

prominence to differential resistance to weather-

ing.

The ring structure can be conveniently subdi-

vided into four concentric zones, each with char-

W.J. VERWOERD

acteristic megascopic deformation features de-

creasing in intensity outwards (Fig. 4).

Central Zone

This is a zone of total disruption. It is now

clearly recognized for the first time that the

Salpeterkop breccia is only part of an almost

circular crater rim around a deposit of bedded tuff

that accumulated in a crater lake or summit

caldera. The rim measures about 1.5 km across

and the remnant of the crater that is preserved has

a diameter of 800 m. Together these rocks con-

stitute the hub of the ring structure.

On the southern slope of Salpeterkop the crater

lake deposit is to a large extent obscured by scree

and calcrete. The khaki-coloured tuff is exposed in

two gullies and shows 40”-60” inward dip. The

rather crude bedding is brought out by lenses of

coarser material. The southern contact appears to

be near-vertical, indicating the likelihood of

caldera collapse by faulting. The “tuff” varies

from clay to grit in grainsize but it was not

possible to determine whether it consists of

volcanic ash completely altered to clay minerals,

or of finely cornminuted country rock. It contains

Fig. 3. Salpeterkop as seen from the north.

SALPETERKOP RING STRUCTURE 279

CARBONATITE

DOLERITE SILL

Fig. 4. Geological map of Salpeterkop and inferred cross section along A-B. Arrows indicate width of Central Zone (CZ),

Intermediate Zone 1 (IZl), and Intermediate Zone 2 (IZZ). Dashed lines show bedding traced from aerial photograph (Fig. 1). Note

sills of carbonatite and their postulated conical feeders.

both rounded pieces of shale and unaltered breccias and includes outcrops in the northwestern hornblende crystal fragments. No evidence of the sector that were formerly mapped as tuff (De Wet, presence of molten or glassy materials has been 1975). The breccias were undoubtedly formed in a

found. volcanic vent. The clasts are predominantly of The crater rim consists (in the north) of various local Karoo sandstone and shale, but dolerite,

280 W.J. VERWOERD

white quartzite, gneiss and schist in addition to

biotite, hornblende and ilmenite xenocrysts are

also present, obviously derived in part from great

depths. The average diameter of the clasts is l-2

cm, but there are also blocks of Karoo sandstone

more than a metre across. All variations in shape,

from rounded to angular, are observed. The sili-

ceous breccia is cemented and replaced by a quartz

mosaic. Barite occurs occasionally in the siliceous

breccia as vein fillings and centimetre-sized drusy

crystals. Ferruginous patches containing red and

black haematite may perhaps represent the sites of

ancient fumaroles. West of the crater lake deposit

De Wet (1975) recognized an irregular intrusion of

carbonatite (flow-banded siivite). This is con-

sidered to be part of the crater rim. The carbonate

becomes more extensive to the southeast than has

previously been mapped. The carbonatite, in this

area is more ferruginous and is contaminated with

breccia fragments. It intruded and broke up a

slightly older carbonatized variety of breccia and

is therefore not always readily identifiable as

carbonatite. The rim is breached by erosion in the

southeast and then continues northward, joined

by a strip of green breccia containing small coun-

try rock clasts, biotite xenocrysts and calcite and

celadonite in a tuffaceous matrix. It is clear that

the breccia body was not formed by a single event

because there are breccia-in-breccia fragments and

cross-cutting relationships between the various

phases. The green breccia in particular sends

apophyses into the main breccia. The sequence of

emplacement of mappable units’ is apparently as

follows: (1) breccia, (2) green breccia, (3) carbona-

tized breccia, (4) silicified breccia. The carbonati-

zation and silicification events probably post-date

the brecciation.

Intermediate Zone I

This zone is about 700 m wide. It consists of

indurated and intensely jointed country rock in

which bedding has been obliterated. The joints are

predominantly vertical and oriented in many dif-

ferent directions. Although in-situ deformation

seems to reach a maximum here, no brecciation,

pseudotachylite or shatter cones were seen. Two

other features characterize this zone:

(1) Thin ( < 1 m) carbonatite dykes (“alvikite”),

which weather with a light to dark brown colour,

are plentiful. They are often filled with country

rock inclusions and can seldom be traced for more

than a few metres. From 69 dyke orientations that

were measured in the two intermediate zones it is

concluded that most of them are either radial or

tangential (concentric) with respect to the central

zone, and that these groups are about equally

represented. The tangential dykes may be assumed

to occupy concentric cylindrical or conical frac-

tures by analogy with carbonatite complexes

worldwide, but it must be admitted that they

could never be followed sufficiently far to prove

an arcuate shape. The best exposure found so far

is in a radial gully at the northern foot of Salpe-

terkop where no less than thirteen such dykes are

closely spaced over a distance of 75 m perpendicu-

lar to their strike direction. They are either vertical

or dip steeply inward, as would be expected of a

locality close to the centre of the structure. By

extrapolating the dips at surface, an approximate

depth of convergence of between 2 and 4 km may

be estimated for these dykes.

(2) Two large and several small intrusive bodies

that were previously interpreted as “adventive

necks of pyroclastic breccia” (Truter, 1949; Du

Toit, 1954) penetrate this zone. De Wet (1975) has

shown that they are not breccias but actually

consist of a feldspathic rock heavily contaminated

with country rock fragments; the uncontaminated

equivalent occurs in the form of thin dykes with a

trachytoid texture consisting mainly of potash

feldspar with nepheline, biotite, apatite and fluo-

rite as minor constituents. Petrographic study and

chemical analysis (De Wet, 1975) indicated that

the rock is not a trachyte sensu strict0 but that the

term “potassium-trachyte” as defined by Suther-

land (1965) for a rheomorphosed potassic fenite

appears to be applicable. The emplacement of

these bodies was obviously controlled by radial,

concentric and NE-trending fractures. The irregu-

lar shapes of the two larger intrusions can be

explained by enlargement at dyke intersections.

Many of the smaller plugs are aligned in radial

fashion.

Intermediate Zone 2

The collar of outward dipping, tilted Beaufort

beds surrounding the previously discussed zones is


about 2 km wide. From a distance of about 3.5 km, a gradual steepening of dip takes place to- ward the centre on all sides, reaching a maximum of 30”-40’ at the inner margin of Intermediate Zone 2. Steeper angles have been recorded in the vicinity of intrusions, but these are regarded as local effects. Since to regular bedding can be recognized further inward, it is impossible to say whether the updoming took the form of an arch or whether further steepening took place adjacent to the breccia ring. It is also noteworthy that the overturning, folding and contortion generally associated with a diapir are absent, and only very limited faulting with displacements of a few metres has been observed.

Radial and concentric carbonatite dykes as well as trachytic dykes and plugs of contaminated K- trachyte continue into this zone. The plugs in both intermediate zones become smaller outwards and less plentiful to the east. In all there are about 27 plugs, including some that are designated as “carbonatized breccia” (Fig. 4). The K-trachyte also displays varying degrees of replacement by calcite. Barite occurs along joints in sandstone and in small vugs in carbonatite (De Wet, 1975). Another feature of this zone is the occurrence of several carbonatite sills exploiting bedding planes in shale and sandstone. One of these carbonatite sills, where the inward dipping feeder is exposed and clearly cuts across a dolerite sill, has been described long ago by Rogers and Du Toit (1903). It occurs in the Outer Zone, 4 km northwest of Salpeterkop beyond the boundaries of Fig. 4.

Outer Zone

This is a zone of indeterminate width because it is unknown how far the effects of the Salpeterkop structure extend beyond the two intermediate zones. From an interpretation of aerial photographs Newton (1987) found that the pattern of radial fractures around Salpeterkop is still recognizable at a distance of 16 km.(Fig. 5B). This is sufficient to indicate that the structure is the result of a major disturbance.

Several small breccia pipes, dykes and olivine melilitite plugs are known within a radius of 7 km, including the deep-seated Silver Dam-type pyro-

elastics which have been linked to kimberlite (Mc- Iver and Ferguson, 1979). Similar features also

occur much further afield (e.g. in the Roggeveld Mountains 40-60 km west of Sutherland (South African Geological Survey Sheet 3220, 1983). Olivine melilitite occurs as solitary pipes and small pipe swarms in the Swellendam, Robertson, Gamoep, Garies and Bitterfontein districts of the Cape Province (Moore and Verwoerd, 1985). All these alkaline intrusives constitute the distinctive volcanoplutonism of the Kalahari Craton edge. While the precise interrelationships between Salpeterkop, the breccia pipes and olivine melili- tites are still undefined, there is evidence from elsewhere in the world for a linkage between melilite-bearing rocks (such as alniiite) and carbonatite complexes.

Dyke and fracture pattern

The orientation of dykes in the intermediate zones of the Salpeterkop structure confirms the conclusion of Newton (1987) that, over a much larger area, a radial pattern has been superimposed on and somewhat modified by a set of pre-existing fractures, among which a NE-trending swarm is pre-eminent (Figs. SA and B). Some of the dykes follow this trend. Newton (1987) also found that “there is a complete lack of any fractures tangential or concentric to the dome,” yet carbonatite dykes following such fractures are undoubtedly present between 0.8 km and 2.5 km from the centre. This discrepancy is ascribed to different scales of observation. It also means that the carbonatite-filled fractures are much less con- spicuous than the concentric rings etched out by the differential erosion of uptilted strata.

Extrapolation of the near-vertical radial dykes indicates that they may perhaps converge to more than one geometrical centre within or near the boundaries of the Central Zone breccia (Figs. 5C

and D). The trachytic dykes appear to have axes of origin which are different from the carbonatite dykes; this accords with field evidence which indicates that the former are older (De Wet, 1975). One of the three supposed trachytic centres coin- cides with a plug at the present level of exposure, whereas the others either predate the breccia of

282 W.J. VERWOERD

1

25 Km

)Km 1Krn

big. 5. Fracture pattern m the vicnnty of Salpeterkop at various scales of observatton. A. Traced from Landsat image (Newton, 1987). B. Interpreted from aerial photographs (Newton, 1987). C. Extrapolated from orientation of radial trachytic dykes (dotted) and trends of feldspatbic plugs (crosses). D. Extrapolated from orientation of carbonatite dykes: radial (continuous lines) and concentric

(dashed lines). Note exploitation of both local and regional fractures by dykes.

the Central Zone, or they never reached the present land surface. If the various carbonatite centres have any real significance, in view of the fact that they depend on large extrapolations from limited data, they could confirm that the Central Zone was emplaced as a breccia pipe in several pulses.

Except in the case of the closely spaced concentric carbonatite dykes mentioned during the discussion of Intermediate Zone 1, no attempt has been made to extrapolate the tangential dykes in their direction of dip because insufficient mea-

surements are available. It seems possible that they too may emanate from different foci.

Discussion

The following possibilities could be considered in an attempt to explain the Salpeterkop structure:

(1) Tectonic deformation. Structural domes occur at the intersection of anticlinal fold axes or in association with salt diapirs. Neither of these is


present in the Karoo basin at Sutherland, and such features do not occur in isolation, as is the case with Salpeterkop. The tilting of the beds is more like a cusp than a dome. A purely tectonic explanation will fail to account for the bedded tuff, the breccia rim, the intrusive rocks and the barite veins.

(2) Meteorite impact. This hypothesis can also be quickly discounted. Although impact has been favoured for isolated circular structures, even where no evidence of shock deformation is present, the relative size of the crater and the amount of uplift at Salpeterkop entirely contradict an impact origin. At Salpeterkop a small crater occurs on top of a large uplift, whereas large meteorite craters are characterized by compara- tively small central uplifts due to rebound. In addition, the association between brecciation, crater formation and intrusion of alkaline rocks at Salpeterkop is much too close to be fortuitous. The structure has had a history of multiple episodes impossible to reconcile with meteorite impact.

(3) Catastrophic eruption. Explosive volcanism is typical of alkaline, especially CO,-rich magmas erupting in a cratonic environment. Chivas et al. (1987) have discussed geochemical and other evidence for the existence of liquid CO, or high-den- sity CO,-fluid of magmatic origin at a depth of 2.5-3.4 km in South Australia, and suggested that this fluid would expand explosively from shallow levels in the Earth’s crust and be responsible for large phreatic eruptions. Von Eckermann (1966), in the final version of his classical model for the emplacement of the Alnii alkaline complex, postulated the existence of a series of explosion foci at depths between 3 and 10 km below the surface, from which carbonatite cone sheets as well as radial dykes intruded very rapidly. Cone sheets are characteristic of many other subvolcanic carbonatite complexes such as Chilwa (Garson and Smith, 1985), Tundulu (Garson, 1962) and Homa Mountain (Le Bas, 1977) where similar foci have been construed by extrapolating the dip of the cone sheets. At Homa Mountain at least nine foci appear to lie on a spiral path rising from a depth of 2 km to 150 m below the present surface. The data from Salpeterkop are still incomplete,

but the concentric and radial carbonatite dykes may well define a similar pattern. However, seri- ous doubts have been expressed whether these foci, if they exist, really imply dramatic explosive events. Bahat (1979) interpreted the Homa Moun- tain spiral in terms of Hertzian fractures produced by diapiric rise of a viscous liquid column. Kres- ten (1980) proposed that the dykes and cone sheets at Alnii were emplaced quite passively in a stress field caused by updoming as a result of a central intrusive body, followed by subsidence. Cone sheets in a carbonatite complex cannot be regarded as clear evidence of explosive phenomena.

Although violent eruptions provide the most likely mechanism for the formation of the Salpe- terkop Central Zone breccias, the updoming of the country rock remains unexplained.

(4) Volcanoplutonic intrusion. Rogers and Du Toit (1903) pointed out that a quaquaversal dip of the extent observed at Salpeterkop is highly unusual around a volcanic neck; to them it was reminiscent of the long-discarded “elevation crater” theory of the formation of volcanic mountains. Accordingly, Du Toit (1954) postulated the existence of a buried laccolithic mass, probably of alnbite, “which arched up the overlying beds be-

fore relief was afforded by the drilling of holes through the stretching cover and the violent dis- charging of heated gases through the perforations thus produced”. De Wet (1975) contended that gas pressure would cause deformation of greater regularity than a viscous magma, if the medium being deformed is sufficiently plastic.

A better explanation presents itself if the K- trachyte plugs (formerly described as satellite vents) are considered as apophyses of a major, carbonatite-related intrusive body responsible for the updoming. This model for Salpeterkop would accord with that of many alkaline ring complexes, notably the six famous Upper Cretaceous carbonatite occurrences in Minas Gerais and Goias, Brazil (Tapira- Ma, Serra Negra-82 Ma, Salitre-81 Ma, Araxa-91 Ma and Catalao I and II-83 Ma) where the surrounding Pre- cambrian schists and quartzites have been strongly domed (Ulbrich et al., 1981). A similar conclusion has been reached about the genesis of the Rich& Dome in Mauritania (Woolley et al., 1984). There

284 W.J. VERWOERD

seems to be a strong possibility that Salpeterkop

not only shares a similar mode of emplacement

with these dome complexes, but that they are all

coeval and genetically related, despite their wide

geographical separation both before and after con-

tinental drift.

Comparison with Gross Brukkaros

Salpeterkop and Gross Brukkaros have the fol-

lowing features in common: (1) craton margin

tectonic setting, (2) doming of country rock, (3)

crater lake deposits, (4) radial carbonatite dykes,

(5) satellite breccia bodies, (6) minor associated

alkaline intrusives and (7) probable correspon-

dence in age.

Janse (1969) compared the Brukkaros-Hatzium

association on the one hand and the Wells Creek

Basin-Hicks Dome (U.S.A.) on the other, dis-

counting a meteorite impact origin for these fea-

tures. He considered the Brukkaros structure to be

an incipient carbonatite volcano that was char-

acterized by phreatic eruptions. The updomed area

and crater remnant of Brukkaros is at least twice

the size of Salpeterkop (Table 1); this could ex-

plain why sagging of the dome occurred at Bruk-

karos but not at Salpeterkop.

Conclusion

The Salpeterkop structure is interpreted as being

the eroded surface manifestation of a typical

volcanoplutonic alkaline ring complex. In this

context, outward tilting of adjacent strata in cir-

cular fashion is not unusual. The updoming at

Salpeterkop was probably caused by the forceful

emplacement of a pipe-like intrusion 3-4 km in

diameter, as demonstrated by offshoots that form

a broad arc coinciding with the western half of the

two intermediate zones. The larger volume of plugs

in this area compared to the east may merely

reflect closer proximity of a buried syenitic ring.

At great depths this intrusion may consist of bio-

tite pyroxenite, ijolite, nepheline syenite, syenite

and carbonatite, possibly with economic con-

centrations of pyrochlore, perovskite, monazite

and apatite. In addition, clasts of fenitized granite

and quartzite have been brought up from below

(De Wet, 1975). At the present volcanic level, the

offshoots from the main collar consist of a rock

composed mainly of K-feldspar and crammed with

country rock xenoliths. It is probable that this

rock is a mobilized product of feldspathization, as

observed in the Chilwa province of Malawi (Gar-

son and Smith, 1958; Garson, 1962) instead of an

igneous syenite. The crater lake deposit and pyro-

elastic breccias of the Central Zone are superficial

products of the explosive eruption of a carbonatite

magma that penetrated the alkaline plug along its

central axis. The crater is probably underlain by

carbonatized breccia intruded by and passing

downward into an extensive network of carbona-

tite veins. This network appears to be connected

to carbonatite dykes filling radial and conical

fractures in the country rock and injected pas-

sively. One of the final episodes, not unusual in

alkaline complexes, was silicification of the brec-

cia and limited hydrothermal mineralization.

Despite superficial similarities to certain eroded

impact and/or cryptoexplosion structures, little

doubt remains that the Salpeterkop structure is

endogenic and only moderately explosive. Careful

mapping of many more radial and concentric dy-

kes is required to substantiate the presence of

several centres of intrusion and of foci from which

conical fractures may have developed at depth.

Detailed microscopic investigation of the breccias,

bedded tuff and shattered country rocks should be

undertaken in order to characterize any micro-

deformation associated with carbonatite-type vol-

canism.

Acknowledgements

Rand Mines Ltd. granted access to the area

where they hold the mineral rights. The aerial

photograph is published under Government Print-

ers’ copyright authority No. 8813 of 8 January

1988. Messrs. L.J. Jordaan and P.V. Storm are

thanked for assistance in the field.

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Documents

The Salpeterkop ring structure, Cape Province, South Africa