Structural controls of kimberlite and lamproite ...

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Journal of GeochemicalExploration 53 (1995) 245-264

JOURNAL OF GEOCHEMICAL EXPLORATION

Structural controls of kimberlite and lamproite emplacement S.H. White a, H. de Boorder ", C.B. Smith b a Geology Department, Utrecht University, Utrecht, Netherlands b CRA Exploration Pty. Ltd., Belmont, W.A. 6104, Australia

Abstract In this article we review the regional and local structural controls on the emplacement, in the crustal environment, of kimberlites and lamproites after the generation of the magmas at depth. We find that there is good evidence that they are related to major deep faults or shears (mobile zones) that traverse the entire crust and may even traverse the lithosphere. Kimberlite and lamproite emplacement is favoured by transcurrent or extensional reactivation of these. The transcurrent reactivation may be a part of continental rifting. The extensional reactivation forms characteristic linear grabens (aulacogens). The emplacement of the kimberlites and lamproites appears to predate or, especially, postdate the peak tectonism, but this last point may be largely apparent because any sedimentation associated with the tectonism will tend to cover syn- or pre-tectonic intrusives. The kimberlites and lamproites within or adjacent to the mobile zones preferentially occur at intersections between conjugate zones or along internal and splay faults and shears which have a dilational or a conjugate orientation. The orientation of these depends upon kinematics of the transcurrent movement at the time of emplacement. Intrusives in the aulacogens are preferentially sited in the hanging wall of the major graben forming zone and tend to occur along oblique transfer structures which in turn are usually old basement shears or faults and may extend into the basement. The sedimentary cover may obscure the deep basement structures of aulacogens to such an extent that they may be seen only as widely spaced fault braids, fractures or joints in the cover. Diamondiferous kimberlites occur when the above structures form on Archean basement, whereas diamondiferous lamproites appear to be favoured by old reworked basement. In both cases the lithosphere should be abnormally thick.

I. Introduction The main regional and local structural controls on the emplacement of kimberlites, lamproites and related rocks are reviewed. W e are chiefly concerned with kimberlites and lamproites with an emphasis on those which are likely to be diamondiferous. The genesis of these rocks is deep within the mantle and the processes which trigger their generation are discussed in this volume by Helmstaedt and Gurney and by Morgan. W e are solely concerned here with structures which allow the magmatism to penetrate to surface through the crust. The deeper mantle processes also serve to localize the regional distribution of these rocks. W e find that the main regional controls involve deep fractures, faults 0375-6742/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10375-6742(94)00033-6

or shear zones in the continental crust that should traverse deep into the lithosphere (see for example Crocket and Mason, 1968, and Dawson, 1970). However, the existence of such structures was, in the past, doubted, the exceptions being Hills (1961) and O'Driscoll ( 1981 ). Even as late as 1986 deeply penetrating faults within the continental lithosphere were a controversial issue (see papers in Reading et al., 1986). In the case of diamond studies, the deep structures are in the crystalline basement which is often covered by a veneer of sediments, often kilometres thick. The veneer will obscure any direct structural relationship between the siting of pipes and an underlying basement structure. The deep structures often give rise to fractures, fold warps or monoclinal flexures in the sedimentary veneer

S.H. White et al. / Journal of Geochemical Exploration 53 (1995) 245-264

246

or to basement highs and arches, features long associated with the preferential location of kimberlites by Russian geologists (see Erlich, 1985). A change in attitude to deep faults or shear zones has recently come about with the advent of deep crustal seismic reflection studies (Blundell et al., 1992, Goleby et al., 1989, Drummond et al., 1989) which show that such features are commonplace. The findings of the deep seismic traverses give credibility to those geologists who maintained that deep basement structures exist and might influence the location of diamondiferous pipes. It is interesting to compare the most recent Russian studies of the East Siberian province (Kushev et al., 1992) with the earlier ( see Erlich, 1985 ). Kushev et al. (1992) discuss the siting of the kimberlites in terms of deep structures. We first present an overview of the subject based entirely on the published literature and consequently the most recent North American finds are not considered. We identify the major types of regional controls, the local controls within the regional ones and then associated factors. In following sections we discuss specific examples, illustrating both the regional and local structural controls we have identified.

berlite pipes, is preferentially associated with major in linear zones. These have generally been associated with plume migration (Morgan, 1972; Gass et al., 1978). Nevertheless, there has been a persistent argument that such controls are secondary or passive to lithospheric structural controls (Sykes, 1978; Bonin and Lameyre, 1978). The most well known examples of the linear distribution of alkaline intrusives is within the African continent. Marsh (1973) related the linear zones of alkaline intrusives (cf. carbonatites and kimberlites) in Angola and SW Africa to the on-land extension of oceanic transform fracture zones. Sykes (1978) further noted that the effects of the transforms were to reactivate old continental structures. The relationship was highlighted in general for alkaline intrusives in W. Africa (Williams and Williams, 1977) and recently reiterated by Haggerty (1992) and is shown in Fig. 1. Stracke et al. (1978) found a similar structural relationship between the transform fracture zones of the Southem Ocean and the Late Jurassic kimberlites in the Adelaide Geosyncline area of South Australia. In this last instance three major oceanic fracture zones, all with a dextral offset of the mid-ocean ridge, converge on the major fault structures in the central zone between the Adelaide Geosyncline and the Gawler Craton. The major faults form the Torrens Hinge Zone or the Torrens Fault Corridor. The kimberlite fields occur, to a broad approximation, in areas of splay faults off the Zone (Fig. 2). Splays with a more northerly trend appear to be preferred. Such splays would be in a dilational orientation if the dextral movement of the fracture zones is transferred to the faults in the Torrens Fault Corridor. In this case, the kimberlite intrusions

2. Overview 2.1. Regional structural controls

Black et al. (1985) in a review of the structural controls of alkaline intrusives and complexes reiterated that alkaline magmatism, including lamproite and kimK

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247

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Fig. 2. Relationship between kimberlite fields in South Australia, the main faults in the Torrens Fault Corridor or Hinge Zone and the fracture zones of the Southern Ocean (based on Stracke et al., 1978 and Preiss, 1987).

pre-dated the main phase of oceanic rifting. The faults of the Torrens Hinge Zone are associated with the Late Proterozoic Delamarian Orogeny and formed well before the breakup between Antarctica and southern Australia. It appears that rather than the transform structures reactivating an old structure in a rather coincidental fashion (Sykes, 1978) it is the old continental structures that control the position of the transforms (Lister et al., 1986). The importance of old structures being reactivated by the transforms is also illustrated by the alkali intrusives (cf. carbonatites) of the Pan African Damara Mobile Belt which again can be related to a Mid-Atlantic transform fracture system (Sykes, 1978). The Damaran Mobile Zone which can be traced across Africa (Daly, 1986) is a long established, major zone of crustal weakness with a counterpart in Brazil. Although its last major phase of tectonism was in the Pan African, it is believed to have been active in earlier Proterozoic times (Daly, op. cit.).

The relationship between transforms and old continental faults and mobile zones and belts is clearly demonstrated for Mesozoic or younger pipes and dykes and present day oceanic transforms. The relationship becomes impossible to establish for older pipes that might be related to transforms of older oceans. Nor can older pipes be clearly related to modem transforms. Stracke et al. (1978) went on to argue that the many Mesozoic and Cenozoic "kimberlites" (now recognised to be mantle-xenolith-bearing nephelinitic diatremes) in Victoria and New South Wales could also be related to transforms in the Tasman Sea, but their data in these areas seem less convincing. The same holds for the kimberlites in South Africa. In this area older Proterozoic pipes are associated with the younger ones and the basement is often covered by sediments. Nevertheless, it has been argued that the kimberlitic and related alkali intrusives in southern Africa, and also in eastern Africa, are related to fractures, faults or lineaments that reflect deep basement structures (Crocket and Mason, 1968; Dawson, 1970; Edwards and Howkins, 1966), but the deep structures themselves have not been identified. The existence of a second structural environment for Mesozoic or younger intrusives, that is not directly influenced by transforms, is seen in the south Kimberley area of NW Australia. Here the young Miocene lamproite pipes of the Ellendale Province of NW Australia (Fig. 3) are associated with an older PalaeozoicMesozoic graben centred on a yet older Proterozoic Mobile Zone (White and Smith, 1992; Atkinson et al., 1984). The Mesozoic pipes of the eastern USA also lie within an older, linear Palaeozoic graben, the Rome Trough (Parrish and Lavin, 1982). The alkaline intrusives, cf. carbonatites, that are associated with the East African Rift fall within a graben type setting (Dawson, 1970). There appear to be two structural environments prone to hosting Mesozoic and younger alkali intrusives and kimberlites and lamproites in particular. These are: A. The landward extension of a transform fault onto a pre-existing mobile belt or zone or a fault/fracture corridor, and B. linear grabens. The two are not exclusive because in eastern Angola (see below) the kimberlites occur within the Lucapa Graben which is related to an earlier structural corridor,


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Fig. 3. The structural setting of kimberlitesand lamproitesin the Kimbereyarea of NW Australia (after White and Smith, 1992). the Lucapa Corridor or Lineament that is the continuation of an earlier oceanic transform (Sykes, 1978; Reis, 1972). The intrusives appear to be associated with features in the cover sediments that relate to deep seated basement structures. Unfortunately, as in southern Africa, the craton is overlain by sediments that partly obscure basement features. It is not possible to directly relate older pipes to oceanic transforms. However, there is a relationship between major mobile zones and alkaline intrusives. The best known example is of the Argyle lamproite pipe within the Halls Creek Mobile Zone in NW Australia and the kimberlite pipes and dykes along its border zone with the Kimberley Block (Fig. 3). Another is the 550 Ma Venetia Pipe of southern Africa which occurs within the Limpopo Mobile Zone. Similar controis have been proposed for kimberlite pipes of Cretaceous age in Brazil (Tompkins, 1992) and on the Sino-Korean Platform (Wu et al., 1992). Kimberlites and carbonatites occur within the mobile zones surrounding the cratonic blocks of the Yakutian area of the Siberian Plaform (Kushev et al., 1992). The best examples of linear grabens hosting kimberlitic, lamproitic and carbonatitic intrusives are the aula-

cogens of the East European Platform (Fig. 4) which are associated with pipes in the Archangel area of NW Russia. These appear to be expressions of underlying earlier mobile zones that host alkaline intrusions on the Baltic Shield (Sinitsyn et al., 1992; Skorospelkin, 1992), and which may apply to pipes within the whole of the East European Platform (Smirnov, 1992). That is, the aulacogen structures are expressions of older mobile belts or zones which were later reactivated under extension to form linear grabens (Fig. 4). The on-craton kimberlites of the Yakutian area of East Siberia are controlled by structural lineaments parallel to aulacogens on the craton. Again the aulacogens formed as a result of the extensional reactivation of older deep structures (Kushev et al., 1992). These settings are very similar to that at Ellendale (see above). We conclude that the major regional structural controis for alkaline intrusives, especially kimberlites and lamproites, are: A. deep seated basement mobile zones or fracture/ fault corridors, and B. linear grabens, and in particular a linear graben above a deep-seated mobile zone.


For young pipes (Mesozoic or younger) there is often an association with an ocean transform structure but this is not an exclusive requirement.

2.2. Local structural controls

Local structures commonly control the siting of pipes in major structural lineaments or mobile zones. They are clearly seen in the Lucapa Lineament in Angola. Intemal shears (Reis, 1972) and cross cutting old structures ( De Boorder, 1982) are important. Dawson (1970) noted the importance of the intersection of major fractures in the Kaapvaal Craton and also that intersecting trends were important in Tanzania, an observation earlier recorded by Edwards and Howkins (1966). Haggerty (1992) emphasizes the same local controls for the West African kimberlites. White and Smith (1992) and Deakin and White (1991) have emphasized the role of internal structures within the Halls Creek Mobile Zone in the siting of the Argyle Pipe in NW Australia. Similar controls occur in the Yengema area of Sierra Leone (Deakin and White, 1991 ). In many of the above instances it is only some of the internal structures that are locii for the intrusives. Which these are is determined by the kinematics of the movement at the time of emplacement (Deakin and White, 1991) and will be discussed below. It also appears that internal, cross cutting, subsidiary structures may be important in locating alkaline intrusives in linear grabens. Parrish and Lavin (1982) observe such a control for the Appalachian Plateau kimberlites and similar internal influences may apply in the East European grabens (Sinitsyn et al., 1992). Within cratons with no directly visible major regional mobile zone or linear graben, it is often the intersection of major fractures that forms a favourable site. Examples are the Kalahari Craton (Dawson, 1970) and the Tanzania Craton (Edwards and Howkins, 1966). In the case of the Kalahari Craton, it will be argued later that there is a major regional graben structure affecting at least part of the kimberlite distribution and that the above fractures are the subsidiary controls. This again highlights the problem of sediments covering the basement structures.

249

2.3. Geodynamic controls Finally, the question of the geodynamic history of major structures and the kinematics of the internal movements during the right geodynamic environment must be considered. The alkaline intrusives associated with the mobile zones and fault/fracture corridors of the Atlantic side of Africa are thought to have been preferentially emplaced when these structures underwent strike-slip movements, or were reactivated as transcurrent zones as a result of the final phases (drift phase, Haggerty, 1992) of movements associated with the opening of the Atlantic (Marsh, 1973; Sykes, 1978; Haggerty, 1992). The kimberlites of the Adelaide Geosyncline of S. Australia predated the active rifting. The conclusion to be reached is that emplacement does not occur during peak rift activity. This means intrusion did not take place during peak strike-slip activity either. This type of scenario also applies to the Argyle lamproite within, and kimberlites adjacent to, the Halls Creek Mobile Zone of the Kimberley area of NW Australia (White and Smith, 1992). They were emplaced during a later strike-slip reactivation of an earlier formed feature. At first glance, the alkali intrusives associated with linear grabens represent a different environment, one of aborted continental rifting. Again, the diamondiferous intrusives consistently postdate the rifting. In the case of the Ellendale lamproites in NW Australia they intruded when the deep basin forming faults were reactivated during later strike-slip movements (White and Smith, 1992). In the Appalachian Plateau the kimberlites are Mesozoic in age and related to Atlantic opening (Parrish and Lavin, 1982) but occur in a Palaeozoic basin and are associated with Palaeozoic age basin faults. The pipes within, or associated with, the linear grabens of the East European Platform and the Siberian Platform appear to have been emplaced in the last stages of a major period of rift formation. We conclude from the above that the fault activity which favours the emplacement of alkaline intrusives in both mobile zones/fracture corridors and the linear grabens either during a late phase or more probably during a reactivation well after a major period of activity. In the case of the mobile zones/fracture zones it may also predate the major activity. Unfortunately, any intrusives emplaced at the corresponding time in a linear basin will be covered by later sediments.


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Fig. 4. (a) East European Platform with the distribution of kimberlites, Archean cratons, mobile zones and aulacogens. The Archangel Tectonic Trend (ATT) which bounds the Archangel kimberlite fields is shown (based on Sinitsyn et al., 1992). (b) East European Platform showing the distribution of aulacogens, Archangel Tectonic Trend (ATT) and the Archangel kimberlite fields (based on Sinitsyn et al., 1992). (c) Details of the northern East European Platform showing the interrelationship between Archean major cratonic areas, mobile zones, grabens and the kimbedite fields in the Archangel area. The fields are clearly bounded by the Archangel Tectonic Trend ( A T f ) . The section, below, across the Kandalaksha Graben (just west of Archangel), shows a diamondiferous kimbedite pipe on the graben shelf, intruding along an extensional fault, and also illustrates the importance of extensional faults in the siting of the shelf fields (based on Sinitsyn et al., 1992).

252


2.4. Additional factors related to the structural controls

We have concluded that there are specific regional and local structural controls on the crustal emplacement of kimberlites, lamproites and related intrusives. Structural controls on (i) the type of intrusion, and (ii) on its potential to host diamonds in economic quantities, are yet to be determined. It appears that the age of the cratonic basement hosting the structure and the extent of reworking by the hosting structure are important. The structure itself does not seem to be important as both diamondiferous kimberlites and lamproites together with carbonatites may be associated with the same major structure. This is clearly seen within the Halls Creek Mobile Zone of NW Australia, the Lucapa Corridor in Angola, and the Reelfoot Rift in Arkansas, USA. Clifford (1966) put forward the generally accepted view that economic kimberlites are associated with old cratonic areas that were stabilized in the Archean (Janse, 1992) and is best demonstrated in southern Africa. Those kimberlites off the craton tend to be noneconomic and to be accompanied by carbonatites. The further the alkaline intrusives are from the craton the more they are likely to be a carbonatite or form alkaline volcanic ring complexes (see Sykes, 1978; Sinitsyn et al., 1992). The diamondiferous Venetia kimberlite in the Limpopo Mobile Zone, between the Archean Kaapvaal and Zimbabwe Cratons, might at first sight appear to be an exception. However, the Limpopo Belt was initiated in the Middle to Late Archean (approx. 3.0 b.y.) and stabilized by 2.5 b.y. (Light, 1982). On the other hand, lamproites appear to characterize mobile belts and zones within areas of Archean basement, where reworking has been more extensive, probably in the Early Proterozoic. In addition to the above requirement of old Archean basement for diamondiferous kimberlites it is also necessary to have thick lithosphere (Helmstaedt, 1991, 1993; Helmstaedt and Gumey, this volume). The lithosphere underlying the East European Platform that hosts the NW Russian diamondiferous kimberlites has been shown by tomography to be in excess of 200 km thick - - considerably thicker than the West European lithosphere which is circa 100 km (see Blundell et al., 1992) and which does not have known diamondiferous

alkaline intrusives. In fact, the thickness of the East European lithosphere is at least equal to, if not greater than, the thickness of the root zone under the Alpine Mountain Chain of Switzerland and Italy. 2.5. Conclusions

The main conclusions from this overview are: Kimberlites, lamproites and related alkaline intrusives are preferentially associated with deep basement structures that extend into the upper mantle. These deep structures may form fracture/fault corridors or mobile zones or may be a part of linear grabens (aulacogens). 2. Diamondiferous kimberlites occur where there is an association between the above and old (Archean) crust within an area of abnormally thick lithosphere. Diamondiferous lamproites occur where old basement has been reworked in the Early Proterozoic. 3. Pipes are associated with minor or lower order crustal structures within the major corridors or zones, with internal crustal structures orthogonal or oblique to the main trend of the linear grabens or at crustal fracture and fault intersections in those cratonic blocks not directly influenced by fault corridors and mobile zones or aulacogens. 4. The intrusives are favoured by transcurrent or extensional tectonic activity usually following a period of major oceanic or continental rifting. 1.

3. Examples of regional and local structural controls of some known diamondiferous pipes Three categories of diamondiferous pipes have been identified on the basis of major regional controls. They are as follows: I. Examples associated with linear grabens, 2. Examples associated with mobile zones or fracture corridors, 3. Examples with no known regional control but with local controls. Where possible we illustrate each with examples from both young and old diamondiferous provinces.


3.1. Kimberlites and Lamproites associated with linear grabens There are two prime examples of this type of association, the Palaeozoic diamondiferous pipes on the East European Platform, chiefly in the Archangel area of Russia and the Miocene Ellendale Pipes within the Fitzroy Trough (Graben) adjacent to the King Leopold Mobile Zone in NW Australia. The on-craton kimberlites within the Yakutian area of Siberia are also stated to be examples (Kushev et al., 1992) but no details have been published.

A. Structural controls of diamondiferous pipes on the East European plaOeorm The major structures on the East European Platform are an interconnected series of mainly northwesterly and northeasterly trending grabens on an Archean basement. (Fig. 4a and b). Kimberlites are associated with areas of graben development and occur both in the grabens and on the interleaving craton areas (Fig. 4a) The grabens are characteristically narrow (ca. 50 km) and can be traced over hundreds of kilometres. They occur as mobile zones in the basement where they extend onto the Baltic Shield. These long grabens rotate into each other rather than one set cutting the other and, therefore, form an orthogonal set of syntectonic structures. Some have a Devonian to Carboniferous history, others also had a rifting history in the Late Proterozoic (e.g. Sinitsyn et al., 1992). Most are demonstrably underlain or bordered by Proterozoic mobile zones between Archean Cratonic blocks (Fig. 4c). Within the East European Platform, the kimberlites are generally controlled by the northwest and northeast faults associated with the grabens (Smirnov, 1992). The major cluster of kimberlite pipes occurs on the East European Platform in the Archangel area and has associated melilitites and alkaline complexes. Major structural elements of the Precambrian basement in the Archangel area are the Proterozoic White Sea Mobile Zone between the Archean Kola-Kuloi Craton to the north and the Archean Karelian Craton to the south (Fig. 4c). The mobile zone is a major structure that extends from the western edge of the Shield adjacent to the Caledonides in the northwest to the Central Russian Aulacogen in the southwest, a distance of about 1000 km. It hosts the Kandalaksha and Keretsk-Leshukonskii Grabens in the Archangel area

253

(Fig. 4c). They are located on, or bordered by, mobile zones (Fig. 4c). The kimberlite pipes occur on both shelves of the Kandalaksha Graben and the adjacent Archean Kola-Kuloi Craton and especially on a basement high in the Keretsk-Leshukonskii Graben (Fig. 4). A cross section by Sinitsyn et al. (1992) shows that growth faults in the shelf areas are important as conduits for pipes. The pipes on the shelf are not diamondiferous but those on the adjacent craton are diamondiferous or potentially diamondiferous (Sinitsyn et al., 1992). Rifting is presumed to have taken place in the Proterozoic and again in the Devonian to Carboniferous, although the presence of both rift events in all of the grabens is controversial. The pipes have an estimated age between Devonian and Middle Carboniferous (Sinitsyn et al., 1992), but no isotopic dates are available. The estimated dates indicate they intruded after the Late Proterozoic rifting that occurred in the mobile zones but at the same time as rifting or tectonic activity was occurring in the aulacogens elsewhere on the East European Platform. The major Archangel diamondiferous province, to the east of the White Sea, although associated with the northwest trending grabens and mobile zones (Fig. 4c), is bounded by a northeast-southwest trending structure, the Archangel Tectonic Trend (ATT), extending about 1000 km between Lake Onega in the southwest and Cape Kanin in the northeast (Fig. 4c). It is orthogonal to the mobile zone and graben axes and to the trend of the major Archean cratonic blocks (Sinitsyn et al., 1992) which parallel that of the grabens. So far, six fields of kimberlite pipes and associated rocks have been found along the trend, both within the bordering cratons and within the grabens. The Archangel Tectonic Trend appears to be the significant controlling structural element. However, the tectonic nature of the ATT is not immediately clear. Cross sections based on seismics, gravity and magnetics (Egorov et al., 1992) indicate that the ATT can be traced into the lithosphere where it may intersect deep lithospheric structures underlying the Mezen Depression (Fig. 5). There is unfortunately no deep structure shown for the White Sea Mobile Belt. The ATT also constitutes the western boundary of the Late Proterozoic-Palaeozoic basin of the East European Platform (Sinitsyn et al., 1992) which, to the east of the White Sea, is represented by the Mezen Depression and Pre-Timan Basin.

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The structure therefore had a marked period of tectonic activity in the Late Proterozoic and Palaeozic. Summarising, the main structural controls are the northwest grabens and their underlying mobile zones and orthogonal or oblique structures that would be in a transfer orientation during graben formation. The same controls appear to apply to other fields on the East European Platform (Skorospelkin, 1992).

B. Structural controls of diamondiferous lamproite pipes in the Ellendale area within the Fitzroy Graben and adjacent shelf King Leopold Mobile Zone, NW Australia The regional setting of the lamproites of the West Kimberley Province within the King Leopold Mobile Zone of NW Australia is shown in Fig. 3. They form three main clusters of pipes spread along a northerly trending zone extending from the Lennard Shelf to the Fitzroy Trough. The fields are from north to south, the Ellendale Field, the Calwynyardah Field and the Noonkanbah Field, of which the Ellendale Field is by far the most diamondiferous (White and Smith, 1992). These fields occur in the hanging wall of a major shear that reflection seismics show to form the contact between the King Leopold Mobile Zone and the Lennard Shelf, which is the shelf to the deep (8 km) Fitzroy Graben of the Canning Basin. The major shear is believed to be a reactivated part of the King Leopold Mobile Zone ( White and Muir, 1989; Tyler and Griffin, 1990) which has been traced into the upper mantle by deep seismics

(Drummond et al., 1989). An interpretation (White and Smith, 1992) is shown in Fig. 6 and resembles the deep section across the Archangel Tectonic Trend in northwestern Russia in Fig. 5. Again, the most diamondiferous pipes are those nearest the craton, the Kimberley Block. It can be seen that the zone of lamproite intrusions stretches from the outcrop of the Oscar Shear System to a position that corresponds to the upward projection of the point at which the Oscar Shear System enters the mantle. The scatter of the pipes within each cluster increases to the south as the thickness of sediment above the basement increases. The general north-south trend of the three lamproite fields appears to follow the continuation of a major northtrending fault structure out of the Kimberley Block into the Fitzroy Trough and which is likely to have acted as a major transfer fault during the sedimentation in the Fitzroy Graben. The increased flowering effect of the basement transfer structure, as commonly occurs in basins ( Gibbs, 1987), is the likely cause of the increasing spread of the pipes within the clusters as sediment depth increases (see Fig. 7). It also enhances the possibility that fractures and joints in the sediments are the final channelways for intrusions. The basin infill within the Fitzroy Graben is mainly from Ordovician to Triassic. Both earlier and later movements are recognised (White and Muir, 1989; Tyler and Griffin, 1990) within the King Leopold Mobile Zone. The pipes are Miocene in age and are associated with one of the later movements, which was

S.H. White et al. / Journal of Geochemical Exploration 53 (1995) 245-264 I~

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Fig. 7. Cartoon illustrating the possible relationships between basement faults, basin structures and kimberlites in a graben. The same relationship will hold for lamproites. The cartoon shows that as the depth of sediment infill increases, the basement faults or shears become less distinct and more dispersed and consequently the kimberlite clusters also become wider and more dispersed.

256

S.H. White et al. /Journal of Geochemical Exploration 53 (1995) 245-264

a strike-slip transfer movement to an extension in the off-shore Browse Basin. There is no information available on detailed structures within the lamproite fields.

.

Buried basement faults formerly active as growth faults may be marked by fault strands, fractures or monoclinal warps and which may control intrusive emplacement.

C. Summary Striking similarities exist when the East European Platform kimberlite data and the Lennard Shelf-Fitzroy Graben lamproite data are compared. In both cases the diamondiferous pipes occur in the shelf or cratonic area of the grabens, with less economic pipes occurring in the deep parts of the grabens where sediment accumulation is thickest (see Fig. 7). The role of cross structures is clearly important in the East European Platform and evidence suggests a similar control in the Lennard Shelf-Fitzroy Graben. Both show a relationship to major growth faults, which in the Fitzroy Graben have been traced directly into the upper mantle and which are indicated to be lithospheric on the East European Platform. Both are related to a major cross fault, which in the Archangel area also appears to relate to deep crustal and possibly deep lithospheric structures. In both areas the pipe emplacement postdates the main sedimentation. These features are summarized in cartoon form in Fig. 7, as follows: 1. Within the graben, alkaline intrusives occur in the hanging wall of a major growth fault that can be traced into the upper mantle. The growth fault is an old basement shear structure that was reactivated. 2. Intrusives tend to follow major deep transfer faults that are associated with the deep growth fault. These also appear to be old basement features. The intrusives may spread out into the footwall of the graben/shelf system and onto basement highs. 3. The diamondiferous pipes occur on the shelf or on cratonic basement. Pipes within the graben itself do not appear to be economically mineralised, and those on the shelf appear to be less mineralised than those on the craton. 4. The diamond pipes exploit the above faults/ shears, not when they are active as major basin forming structures but, later, when they are weakly reactivated during a subsequent event. 5. Deep transfer faults in the basement become dispersed flower structures in the basin sediments. This will cause dispersal of the intrusives from a single basement structure into the braids of the flower or into associated joints and fractures.

3.2. Kimberlites and lamproites related to fault/ fracture corridors and mobile zones The best known examples of this type of structural control are the Cretaceous kimberlite pipes and dykes along the Lucapa Corridor within the Central African or Congo Craton of Angola and the Proterozoic Argyle lamproite pipe and Palaeozoic kimberlites associated with the Halls Creek Mobile Zone, in the Kimberley area of NW Australia.

A. The Lucapa Corridor, Angola The Angolan kimberlites occur along a major basement structure that trends in a NE direction. It has a width of circa 50-90 km (De Boorder, 1982) and can be traced for over 1600 km across the Congo Craton into Zaire (Reis, 1972). The zone corresponds to a northeast trending corridor with local basin development (Lucapa "Graben" ) in the northeast of Angola. It can be traced to the southwest to the transforms of the Mid Atlantic Ridge which have dextral offsets totalling 300 km (Sykes, 1978). There are four Cretaceous kimberlitic provinces (I-IV) known along the structure (Fig. 8). The one nearest to the Atlantic (IV) is mainly carbonatitic and non-diamondiferous, the number of carbonatites increasing from Province III to IV. Fig. 8 shows that two subsidiary trends are important in locating the kimberlite provinces. The first are northwest to north-northwest fault and fracture corridors and the other is an east-northeast fault and fracture trend inside the Lucapa Corridor and which corresponds to the internal R-shear for a dextral sense of displacement. The Lucapa Zone itself also has east-northeast inflexions (Fig. 8) and the main kimberlite field in NE Angola (Province I, the Lunda field) and part of Province IV are in areas in which the east-northeast subsidiary structures dominate. They also give rise to the local grabens (Reis, 1972). The Provinces II and III occur where conjugate corridors cut the Lucapa Corridor and where, again, the east-northeast trends are present (Fig. 8). A detailed structural analysis of Province III in Fig. 8 is presented in Fig. 9 which is based on the work of

S.H. Whiteet al./ Journal of GeochemicalExploration 53 (1995) 245-264

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Fig. 8. Kimberlitesand carbonatitesassociatedwiththe LucapaCorridorin Angola(fromReis, 1972and De Boorder, 1982).The maindiamond provinces are markedI-IV. De Boorder (1982). It summarizes the local structural controls for the kimberlites in the Lucapa Corridor as a whole. Kimberlites and carbonatites are concentrated where a north-northeast trending conjugate zone cuts the main Lucapa Corridor. They follow the main shears and the internal fractures and faults within the Lucapa Corridor and within the conjugate shear. In both shears, the internal structures fit a Riedel geometry (Fig. 9) in which R-shears and R'-shears are pre-eminent. The area is one in which tensile fractures have also developed. The kimberlites cluster at the intersection of the two structures. The carbonatites have a similar relationship to the main corridors but spread to the southwest along the Lucapa Corridor. A cluster occurs where an internal conjugate occurs in the main trend. Finally, the economic pipes are situated where the Lucapa Corridor crosses the Congo Craton. As the trend comes off the Craton and enters the Pan African West Congo Mobile Belt (Province IV), the number of kimberlites decrease and carbonatites, nepheline

syenites and alkaline extrusives become dominant. Again diamondiferous kimberlites occur on the craton. B. The alkaline intrusives associated with the Halls Creek Mobile Zone, N W Australia

The Halls Creek Mobile Zone (HCMZ) is a structure that can be traced for over one thousand kilometres from its junction with the King Leopold Mobile Zone northwards into the Arafura Sea (Fig. 3). The HCMZ hosts the Argyle lamproite and has kimberlites associated with splays off its northwestern margin. The width of the Zone varies from 50 to ca. 80 km. It is a zone in which basement reworked in the Early Proterozoic was uplifted along with folded Middle Proterozoic sediments (Carr Boyd sediments) which are limited to the HCMZ. It is overlain by Late Palaeozoic sediments, especially in the north. The HCMZ has a total cumulative sinistral displacement of 110 km (Plumb, 1968 ). The sinistral movements, of which three phases are postulated (White and Muir, 1989), overprint an ear-

258

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Fig. 9. (a) Detailsof the structuralcontrolsof ProvinceIII alongthe Lucapa Corridor (after De Boorder, 1982). The internal faultshave a Riedel geometry(b). The kimberlitescluster where a majorconjugate corridor cuts the Lucapa Corridor. A cluster of carbonatites have formedin the southwest where an internal conjugate fault has developed within the LucapaCorridor. lier dextral phase of movement (see White and Smith, 1992). Tectonic activity commenced, at the latest, in the circa 1850 Ma Barramundi Orogeny and has continued into post-Devonian times. The internal structure of the HCMZ is characterized by discrete second order faults/shear zones which have a consistent Riedel geometrical relationship to each other and to the major shears that form the margins of the HCMZ. This is best seen in the vicinity of the Argyle lamproite pipe (Fig. 10a) where the HCMZ is confined between the Greenvale Fault and Halls Creek Fault. The internal shears fit an overall Riedel pattern (Fig. 10b) with the Argyle Pipe intruding an area in which transtensional R shears (Carr Boyd, Revolver Creek and Glenhill Faults) dominated and which produced local extension during sinistral slip movements (discussed in White and Smith, 1992, and Deakin and White, 1991) in the Middle Proterozoic and again in

the Devonian. White and Muir (1989) have identified an intervening period of strike-slip in the Late Proterozoic. The pipe was intruded during the localized Middle Proterozoic extension and its position appears to have been influenced by the local R-shears. A number of small lamproite dykes occur in northwest trending tension gashes along an east-northeast trending fault, the Razor Ridge Fault, in the area adjacent to the Argyle Pipe. They have an orientation consistent with R'shears associated with the conjugate X-shear. They occur at the termination of a 3rd order shear (the Razor Ridge Shear) and their detailed structural setting is discussed in Deakin and White ( 1991 ). The root to the Argyle Pipe itself is sited in the extensional intersection of the Razor Ridge Fault and the continuation of the Glenhill Fault through Devonian cover sediments; the pipe occurs in Middle Proterozoic sediments (Deakin and White, 1991). Splay shears along the northwestern edge of the Halls Creek Mobile Zone terminate in the Kimberley Block (Fig. 3). They have an influence on the siting of the kimberlites along the southeastern margin of the Block. These kimberlites are thought to be the same age as kimberlites along the northern margin of the Block (White and Smith, 1992), namely 800 Ma, i.e. again at a time when sinistral strike-slip movements may have been occurring. The splay shears would have acted as dilational R-shears during such a movement.

3.3. Areas with direct local controls but lacking obvious regional controls

The examples cited above have strong regional and local structural associations with their alkaline intrusives. However, there are areas in which local controls are well established but regional controls are less apparent. This generally reflects a buried or poorly outcropping basement. In such cases the basemen~ features can be identified through geophysics, cf. gravity and aeromagnetics, or inferred from the kinematics of the local contols. An example of each is discussed. The first is South Africa in which geophysical data indicate that deep basinal structures have at least influenced the emplacement of some intrusives. The second is West Africa where kinematics support basement structures related to oceanic transforms.

S.H. White et al. / Journal of Geochemical Exploration 53 (1995) 245-264 Halle C r e e k Mobile z o n e

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A. Southern Africa A total of 580 kimberlites occurs within southem Africa. They occur as two distinct types based on isotopic and petrographic differences (Skinner et al., 1992). The Group 1 kimberlites occur at 1600 Ma, 1200 Ma, 500 Ma and between 100 and 70 Ma. The Group 2 kimberlites were emplaced between 200 and

110 Ma. This highlights one of the problems associated with the study of the structural setting of the southern Africa kimberlites - - there are at least four periods of kimberlite emplacement. Group 1 kimberlites have a wide distribution, with most occurring in a wide NE trending belt from Sutherland to Pretoria (Figs. 11 and 12). The same belt almost exclusively hosts the


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younger Group 2 kimberlites. In each case the diamondiferous kimberlites occur on the Kalahari Craton, including those at Venetia within the Limpopo Mobile Belt which, as stated previously, marks a major collisional zone stabilized in the Late Archean (Light, 1982). Carbonatites, nephelinites, melilitites and nondiamondiferous kimberlites occur off the craton, in and towards the Cape Fold Belt. Dawson (1970) recorded that the kimberlites are associated with fractures and dykes that have the trends shown in Fig. 12. They are

northeast-southwest, northwest-southeast, east-west and north-south. In addition to the above, kimberlites outside the Sutherland-Pretoria Belt have a W N W ESE trend. Clusters of kimberlites follow all of the above trends (Fig. 12). The long northeast trending zone follows the trend of the Ventersdorp Graben as proposed by Clendenin et al. (1988), the position and structure of which was identified through gravity, magnetic and seismic studies. The outline of the graben is shown in Fig. 11. These

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