Geotectonic controls of primary diamond deposits: implications for area selection

ELSEVIER

Journal of Geochemical Exploration 53 (1995) 125-144

JOURNALO[ GEOCHEMICAL EXPLORATION

Geotectonic controls of primary diamond deposits" implications for area selection H.H. Helmstaedt a, j.j. Gurney b a Department of Geological Sciences, Queen's University, Kingston, Ont. K7L 3N6, Canada b Department of Geochemistry, University of Cape Town, Rondebosch 7700, South Africa

Abstract

Area selection in modem diamond exploration should be based on ( 1) prediction of regions under which diamonds may have formed, (2) selection of those areas under which the diamonds may have survived until entrained by younger kimberlites or lamproites, and (3) identification of mantle-root-friendly structures which served as pathways for diamondiferous kimberlites or lamproites in the appropriate areas. Prediction of diamond-prospective areas should incorporate petrological, geochemical, geotectonic and geophysical studies to detect diamondiferous lithospheric roots under exposed and tectonically buried Archean cratons. Area selection must be guided by the concept of mantle-root-friendly and mantle-root-destructivestructures and processes to constrain the geotectonic environment of diamondiferous kimberlite magmatism.

1. Introduction "Mineral exploration is 'going and looking'; but today we must look with more than our eyes, and think before we go" (Woodall, 1993, p. 6). The "going" in this frequently restated prospectors' wisdom is the area selection of modern mineral exploration that is guided by sets of criteria also known as deposit models. Such criteria may be purely empirical, or they may be based on an understanding of the deposit genesis (Hodgson, 1987). Generally they evolve from the former to the latter with a better understanding of the deposit origin, putting deposit models on a more solid theoretical basis and making mineral exploration increasingly efficient. Historically, diamond exploration evolved from the search for alluvial diamonds to the use of alluvial diamonds and other indicator minerals as tracers to the principal primary source rock - - kimberlites. As reviewed by Atkinson (1989), significant modem dis0375-6742/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10375-6742(94)0001 8-2

coveries have been sizable primary deposits which are amenable to large-scale open-pit mining and guarantee a long mine life. However, the understanding of controis governing kimberlite location and emplacement did not advance significantly, and important exploration breakthroughs came in terms of new geographic areas (e.g., Siberia, Western Australia, Baltic Shield, Slave Province), refined search technology (e.g., aerial magnetic surveys), recognition of new types of host rock (i.e., lamproite), and the application of ever more sophisticated analytical methods to predict the host rock diamond potential from the indicator mineral getchemistry. The major guide line for area selection in exploration for primary diamond deposits has been the worldwide experience that economically viable deposits occur primarily on Precambrian cratons, particularly on those of Archean age (Kennedy, 1964). This empirical knowledge, also known as Clifford's Rule (Clifford, 1966; Janse, 1991), together with the notion that diamonds

126

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144 EMPLACEMENT structural controls of depositional site and diatreme formation

T

TRANSPORT factors controlling transport routes and fate of diamond during transport

I

ORIGIN OF TRANSPORT MEDIUM mode and depth of origin and tectonic controls on location of kimberlites and lamproites

PRESERVATION OF DIAMONDIFEROUS MANTLE ROOTS recognition of mantle-root-friendly and mantle-root-destructive events and structures

FORMATION OF DIAMOND SOURCE ROCK factors controning formation of diamond-bearing assemblages in ancient mantle roots Fig. 1. Summary diagram of tectonic and structural controls on the formation of primary diamond deposits.

are part of the igneous assemblage of their host rocks, directed efforts to improve area selection criteria mainly towards understanding the geotectonic and structural controls of kimberlites and lamproites. A major conceptual breakthrough was the recognition that kimberlites ( and lamproites), though our principal source for economic diamonds, are not primary diamond deposits in the sense that diamond originated within them. As diamonds are generally older then their igneous hosts (Richardson et al., 1984, 1993), they are not phenocrysts, crystallized from their host magmas, but xenocrysts, derived from physically disaggregated upper-mantle source rocks. The igneous host rock simply provided the transport medium from the uppermantle source to the surface. In common with other mineral deposits, tectonic and structural controls of primary diamond deposits can thus be analysed in terms of source, transport medium, and depositional site (Fig. 1). Judging from the regional distribution of diamonds in kimberlites, the diamond source rocks, the truly primary diamond deposits (garnet harzburgites, lherzolites, eclogites), are concentrated in the roots of Archean cratons (Gurney, 1990), where they are not directly accessible. Although, in exceptional circumstances, diamondiferous source rocks may reach the Earth's

surface by tectonic processes (see review of diamonds in metamorphic rocks by Nixon, 1995), such occurrences are as a rule not economically viable. Transport of economically significant quantities of sizeable stones requires kimberlites, lamproites, or any other magmatic rock type originating deep enough to sample the mantle source, and intruding fast enough for the diamonds to survive the transport to the surface. Unlike in most other mineral deposit types, where transport from the source to the depositional site involves concentration of the mineral commodity, the igneous recycling of diamonds from the mantle source into "primary" diamond deposits results in significant dilution of diamond grade (Helmstaedt, 1993) and deterioration of diamond quality. Realizing that an understanding of the geotectonic environment of diamond formation is an entirely separate problem from that of the geotectonic controls of formation of the transport medium (kimberlites or lamproites) (Fig. 1), we can formulate a geotectonically based diamond-exploration model which includes the following three steps (Helmstaedt and Gurney, 1994): 1. Predictions of regions under which diamonds have formed. 2. Selection of those areas where diamonds may have survived to be sampled by younger kimberlites or lamproites. 3. Establishment of mantle processes controlling kimberlite formation and regional tectonic and local structural controls for the emplacement of kimberlites (and related rocks) in the appropriate areas.

2. Diamondiferous mantle roots

Correlation between diamondiferous kimberlites and Archean cratons as well as Archean isotopic dates from southern African diamonds (Kramers, 1979; Richardson et al., 1984, 1993) show that substantial amounts of diamonds were formed during early lithosphere development and that these diamonds survived in the roots of Precambrian shields to be picked up by kimberlites and lamproites ranging in age from Late Archean to Cenozoic (Fig. 2). Studies of mineral inclusions in diamonds and of mineral assemblages in diamond-bearing xenoliths reveal that diamond formation in the subcratonic lithosphere worldwide is associated with two rock types, garnet peridotites in which harz-

127

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

Age

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1800

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Fig. 2. Age ranges of diamondiferous placer deposits, kimberlites, lamproites, and diamond-bearing metamorphic rocks compared to ages of mineral inclusions in E-type and P-type diamonds from kimberlites and lamproites. H = Harzburgitic, L = lherzolitic assemblage. From Helmstaedt (1993).

burgites predominate over lherzolites (yielding P-type diamonds with peridotitic inclusions) and eclogites (with E-type diamonds containing eclogitic inclusion minerals), and the relative proportion of P-type and Etype diamonds suggests a chemically highly depleted, peridotitic mantle source with lenses of eclogitized mafic rocks (Gurney, 1989). Chemical compositions of coexisting minerals in the peridotitic assemblages suggested until recently that most diamonds were stable at pressures corresponding to depths of 150 to 200 km and temperatures generally not exceeding 1200°C (Boyd and Gurney, 1986; Boyd et al., 1985). Archean vertical geothermal gradients

were thus locally comparable to those calculated for present Precambrian shields and, considering a generally hotter Archean Earth (Bickle, 1978), lateral temperature variations between cratons and oceanic lithosphere almost certainly exceeded those in the upper mantle today. Diamonds, therefore, are considered to have formed and survived in relatively cool, very reduced lithospheric roots with convex downward-deflected isotherms and a corresponding convex upward expansion of the diamond stability field (Fig. 3). More recently, a number of unusual inclusion minerals were identified in diamonds suggesting that not

H.H. Helmstaedt, J.J. Gurney /Journal of Geochemical Exploration 53 (1995) 125-144

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Fig. 3. (A) Simplified model of a diamondiferous lithospheric or mantle root (after Haggerty, 1986). The downward deflected dashed lines are the approximate shape of the 900 ° and 1200°C isotherms within the root. (B) Petrological basis for model in A based on geothermobarometry on xenolith suites from East Griqualand (EG), northern Lesotho (NL), Frank Smith (FS), Finsch (F), and Louwrencia (L) kimberlites (after Boyd and Gurney, 1986). Note that on-craton kimberlites (NL, FS, F) have xenoliths that are derived from the diamond stability field, whereas off-craton pipes (EG, L) do not have such xenoliths. Line A-A' represents points of inflection in xenolith "geotherms" interpreted as base of the lithosphere. (C) Same section as in (B), but drawn without any vertical exaggeration to give a more realistic impression of the approximate shape of the root. Convection currents are modelled after Ballard and Pollack (1987) (see Fig. 6).

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144 l

]Early

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km Fig. 4. Outline of Kaapvaal craton showing distribution of diamondiferous (filled dots ) and non-diamondiferous kimbedite clusters (open dots ) (after Gurney et al., 1991). Section in Fig. 3B and C goes from non-diamondiferous East Griqualand kimberlites (southwest of Durban) to Gibeon kimberlites in Namibia (westernmost non-diamondiferous cluster). Subdivision of Kaapvaal craton after De Wit et al. (1992).

all diamonds originated in the 150 to 200 km interval suggested by the common peridotitic inclusion minerals. Unusually high pressures (corresponding to depths greater than 300 km) must be inferred for diamonds from the Monastery kimberlite, South Africa, containing inclusions of high-silica (majoritic) garnets thought to represent pyroxene solid solution in garnets (Moore and Gurney, 1985). An ultra-high-pressure origin for such diamonds appears to be corroborated by the discovery, also at Monastery, of moissanite (natural SiC) inclusions, some of which occur in diamonds that also host "eclogitic" garnets showing the effects of pyroxene solid solution (Moore et al., 1986). The interpretation of majoritic garnet inclusions as pyroxene solid solution in garnet is also suggested by the discovery, in xenoliths from Jagersfontein, South Africa, of the corresponding clinopyroxene exsolution textures in garnets of lherzolitic affinity (Haggerty and Sautter, 1990). Further studies are needed to define the relationships between the ultra-high-pressure diamonds and the more common P-type and E-type diamonds, as this has major implications for our understanding of the depth of kimberlite formation

(Ringwood et al., 1992; see also section on tectonic controls of kimberlites).

2.1. Petrological signature of mantle roots As diamond-bearing kimberlites nearly always contain garnet and chromite xenocrysts that resemble the mineral inclusions of P-type diamonds, the presence of diamonds and the indicator minerals (subcalcic, chromium-rich garnets and high-chromium chromites) can be used as a mantle-root signature, suggesting that their hostrocks have penetrated a mantle root on their way to the surface. Such mantle-root signature is particularly strong in southern Africa, where the distribution of P-type diamond indicator minerals in primary diamond sources closely parallels the inferred outline of the Archean parts of the Kalahari craton (Gurney, 1984) (Fig. 4). A mantle root consisting of ancient diamond source rocks has thus survived only under the combined Kaapvaal and Zimbabwe cratons, where it was sampled by kimberlites of many different ages. In contrast, off-craton kimberlites have a low diamond potential, because mantle roots either never existed in

130

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

their intrusive paths, or such roots were destroyed prior to kimberlite emplacement by a variety of processes, including metasomatic events, thermal erosion, or tectonic displacements. Whereas in traditional diamond exploration the more resistant macrocrysts (garnet, chromite, ilmenite, etc.) have been used merely as indicators of a nearby kimberlite source, the use of these heavy minerals has been refined during the last two decades to allow an estimate whether such source may be diamondiferous. The method is based on the geochemical discrimination between minerals derived from diamondiferous peridotites and eclogites and those from non-diamondiferous varieties (for a review of the use of major and trace element studies on indicator minerals see the appropriate chapters in this volume). The diamond potential of entire regions may thus be assessed from the mantle-root signature of alluvial or kimberliteborne diamond indicator minerals. It is also possible to monitor the diamond potential of a craton with time by studying the mantle-root signature of successive kimberlite generations (see also section on Preservation of Mantle Roots).

2.2. Geophysical signature of mantle roots The petrological model of diamondiferous mantle roots, implying that the lithosphere under Archean cratons is cooler and more refractory than that of the adjacent upper mantle, is supported by seismological and geothermal data suggesting that plates in older continental regions are of greater than average thickness and are underlain by an extensive layer of anomalous mantle material. Jordan (1978, 1979, 1988) referred to these lithospheric roots as tectosphere and proposed that they remain attached to the continents during plate motions. On the basis of seismic tomography, Grand (1987) found that the shield and stable platform of North America coincide with a region of relatively fast shear waves and that deep, high-velocity mantle roots are situated beneath the Archean Superior and Slave provinces of the Canadian Shield (Fig. 5). As the roots are gravitationally stable, and thus must be composed of less dense material, the higher shear wave velocities within the roots require cooler temperatures relative to adjacent hotter asthenosphere. The seismic properties of mantle roots are thus consistent with the model of

relatively cool and chemically depleted, highly refractive lithospheric roots proposed on the basis of petrological studies (Fig. 3). The petrologic model of mantle roots is consistent also with heat flow models attempting to explain the differences in surface heat flow between Archean cratons and surrounding younger terrains. According to Ballard and Pollack ( 1987; see discussion by Morgan, 1995), a cratonic root of relatively cool, non-convecting and poorly conducting, depleted material, extending to depths of 200-400 kin, can divert enough heat away from the craton to account for 50-100% of the observed contrast in surface heatflow between the Archean Kaapvaal-Limpopo-Zimbabwe craton (about 40 m W / m 2) and the surrounding mobile belts (about 65 mW/m 2) (Fig. 6). The possible coincidence of petrological and geophysical mantle roots is of potential importance for diamond exploration, as it may enable us to predict the diamond potential of a craton on the basis of geophysically detectable roots. For example, the limited shearwave-velocity high under the southern Slave Province (Fig. 5), was used to suggest a better diamond potential for this region (Helmstaedt, 1991 ) without the knowledge that indicator minerals with a strong mantle-root signature had been picked up there and were being traced to their source by the Diamet exploration crews who announced the discovery in the Fall of 1991. Although the present horizontal resolving power for teleseismic tomography is still low (approximately 400 km), newer worldwide data by Anderson et al. (1992) warrant close attention by explorationists. It is suggested that exploration models could be improved significantly by establishing the geophysical response of the petrologically identified mantle roots under the diamond-rich southern African and Siberian cratons and comparing it to that of mantle roots identified by seismic tomography under other cratons.

2.3. Age and origin of mantle roots Presently available data suggest that geophysically detectable, deeper mantle roots are more common under Archean cratons (as compared to Proterozoic shields and Phanerozoic orogenic belts), though they are not equally developed and present under all Archean crust (Hoffman, 1990). Roots are present also under Proterozoic orogenic belts, such as the Penokean,

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

Depth = 140 to 235 km

De ~th = 235 to 320 km

A

B

131

De ~th = 320 to 405 km

C Fig. 5. Positive shear wave velocity perturbations under North America from seismic tomography (after Grand, 1987). Note that for clarity only the + 3% contour is indicated at shallower depths (A, B). Contour for 320 to 405 km (C) is + 1.5%. Note that below 235 km fast zones exist under Archean Slave and Superior provinces, but not under the Nain and Wyoming provinces.

Trans-Hudson and W o p m a y orogens, but these are shallower and less well developed than those under adjacent Archean cratons. As economic quantities of diamonds are also concentrated under Archean cratons, models explaining the formation of diamondiferous mantle roots must appeal to uniquely Archean conditions in order to satisfy the observed secular controls.

The question of the formation of mantle roots is thus closely linked to that of Archean lithosphere evolution, a topic that is still highly controversial, especially regarding the Archean global thermal budget (Vlaar et al., 1994) and the problem of the significance of plate tectonics in the Archean (Hamilton, 1993). As a full review of this topic is beyond the scope of this chapter,

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

132

Post-Archean Crust

I

Archean Crust

Lithospheric Mantle

Convecting Mantle

im

±

\

Fig. 6. Cross-section sketch illustrating possible diversion of upwardflowing mantle convection currents by lithospheric root of an approximately Kaapvaal-size craton (radius 400 kin) (after Ballard and Pollack, 1987).

we confine our discussion to possible explanations of the depleted nature and the relatively low geothermal gradient, the two characteristics of diamondiferous mantle roots most pertinent for the purpose of the tectonic argument in this chapter. Two main hypotheses have been proposed to explain the depleted nature of the diamond-beating peridotitic mineral assemblages: ( 1 ) The harzburgitic assemblages formed as residues from a normal mantle-peridotite source by extraction of either komatiites (Boyd et al., 1985; Boyd, 1989; Boyd and Nixon, 1989; Haggerty, 1986; O'Hara et al., 1975; Richardson et al., 1985), or a much thicker basaltic crust (Bickle, 1986; McKenzie and Bickle, 1988; Vlaar et al., 1994). (2) The depleted roots are the result of tectonic underplating by imbricated slabs of subducted oceanic lithosphere containing a significant proportion of graphite-bearing metaserpentinites (Abbott, 1991; B

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Helmstaedt and Schulze, 1989; Ringwood, 1977; Schulze, 1986). The komatiite residue hypothesis was reviewed recently by Herzberg (1993), who concluded that it fails to explain the relatively high SiO2 content of many of the xenoliths from the kimberlite-derived mantle sample of the Kaapvaal craton. According to Herzberg, the harzburgites could have formed as cumulates from peridotite magmas higher in SiO2 than most komatiites; magmas that could have formed by extensive melting ( > 50%) of normal mantle peridotite, either in a gigantic plume or from a high-SiO2 terrestrial magma ocean. Alternatively, the widespread lithospheric depletion may have been a function of higher Archean mantle temperatures, leading to an increased thickness of the upper mantle melt zone and extraction of a much thicker basaltic crust (Bickle, 1986). Experimental studies (Canil and Wei, 1992) support the komatiite-residue hypothesis only for the Cr-poor population ( < 4 wt% Cr203) of low-Ca garnets, suggesting that the more Cr-rich garnets of this population ( > 3 wt% Cr203) would require multiple melt extraction, if they originated by such a process. However, the low-Ca garnets with more than 4 wt% Cr203, typical of diamond inclusions, coexist with spinel and require very depleted, Cr-rich protoliths, such as harzburgites or dunites found only in ocean-ridge environments or ophiolite complexes. Recrystallization of such rocks in the diamond stability field is thus compatible with the subduction hypotheses of Ringwood (1977) and Schulze (1986). Neither the extensive melting model, nor the komatiite extraction model, appear to explain the development of the relatively low T-high P gradients necessary for diamond formation. In a generally hotter Archean mantle, the lower temperatures in the lithospheric roots

A

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Fig. 7. Thermal model for Archean lithosphere with high geothermal gradients under midocean ridges (A) and cooler gradients under continents (B) leading to the emergence of plate tectonics by rapid convective overturn over a hotter (and possibly molten ) Archean asthenosphere (after Nisbet, 1986).

H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

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Fig. 8. (A) Model of formationof cool, depleted lithosphericroot by low-anglesubductionand imbricationof oceaniclithosphereprior to collisionbetweentwo Archeanprotocontinents(after Helmstaedt and Schulze, 1989). Blacklenses are disrupted and eclogitizedoceanic crust which may be diamondiferous (see also Fig. 9). (B) Thermal erosion of imbricated lithospheric root, leaving a remnant of the depleted zone in which diamondsmay have survived. must have caused the development of strong lateral temperature gradients which, in turn, may have led to a proto-plate-tectonic regime in which cooling of the Earth was accomplished by rapid convective overturn beneath the ocean basins (e.g., Fig. 7). Moreover, while the komatiite extraction hypothesis may be appealing for models explaining the evolution of the km

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Fig. 9. Thermalmodelof subductedoceanicslab,showingdownward deflection of isotherms to bring eclogitized oceanic crust into the diamond stability field (after Kesson and Ringwood, 1989). Rapid imbrication of such slabs, as depicted in Fig. 8A, would lead to formationof cool subcontinentallithosphere.

133

Kaapvaal craton, where komatiites are common, it appears to be less plausible for the Archean Slave Province, in the Canadian Shield, where field evidence for the presence of true komatiites is lacking. Unless it is assumed that ultramafic liquids were formed, but were too dense to be erupted (e.g., Boyd, 1989), the extraction hypothesis should not be invoked as an explanation for the presence of a depleted root in this area. In the subduction model, the secular control on the formation of mantle roots and Archean diamonds is thought to be a consequence of the Archean tectonic environment, in which a buoyant, shallow mode of subduction was predominant (Abbott and Hoffman, 1984) allowing continental nuclei to become tectonically underplated by depleted oceanic lithosphere. The model presented by Helmstaedt and Schulze (1989) for the underplating of the Kaapvaal craton (Fig. 8A) depicts a subducted slab consisting of highly imbricated and boudinaged slivers of ocean-floor rocks and oceanic upper mantle containing all the protoliths for the xenolith types found in the kimbeflite-borne, southern African upper mantle sample, probably including carbon-rich metaserpentinites (Schulze, 1986). Assuming a generally hotter Archean mantle, it is most plausible that the relatively high P-low T gradient required for diamond formation was achieved by the depression of isotherms within the subducted slab (Fig. 9). This gradient was maintained by the successive underplating of cool oceanic slabs, resulting in an imbricated geometry similar to that observed on deepseismic-reflection profiles under the convergent plate margin of western North America (Monger et al., 1985; Page et al., 1986). As pointed out by Helmstaedt and Schulze (1989), this relatively shallow and low-angle subduction geometry was probably the rule from midto late Archean times. Subduction zones became generally steeper in post-Archean times, when the shallow mode was restricted to regions of exceptionally fast plate convergence and/or subduction of young ocean floor. Although the processes of diamond formation in mantle roots are not yet fully understood, exploration models must take into account that diamond-beating eclogites in southern Africa, belonging to Group I eclogite xenoliths as defined by McCandless and Gurney (1989), and eclogitic diamond inclusions mirror the distribution pattern of G10 garnets and P-type diamonds. In spite of the fact that some eclogitic inclusions

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H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

indicate a deeper origin, possibly from a depth exceeding 300km (Haggerty, 1986; Moore and Gurney, 1985), and that some have yielded younger dates than those of the oldest reported, early Archean G 10 garnets in diamonds (Richardson, 1986; Richardson et al., 1984), these diamondiferous eclogites occur within the same vertical upper mantle sections and mantle root sampled by southern African on-craton kimberlites. They are absent off-craton (e.g., Karroo and Gibeon kimberlites), where only non-diamondiferous eclogite xenoliths have been found. Gurney (1990) has suggested, therefore, that an early Archean harzburgitic keel of the craton was underplated periodically by diamondiferous eclogites ranging in age from late Archean to Proterozoic (Smith et al., 1989). Whether some or all of these eclogites represent fragments of subducted oceanic crust or products of highpressure magmatic crystallization is the subject of an ongoing debate. This is complicated by the fact that the diverse ages for eclogitic diamonds have not yet been matched by a corresponding diversity in ages of peridotitic diamonds. It also remains to be seen whether the southern African example can be applied elsewhere, i.e., whether the formation of Proterozoic eclogitic diamonds under other cratons (e.g., Northwestern Australia; Richardson, 1986) is the result of underplating of a pre-existing, cool Archean mantle root, or the formation of an entirely new, Proterozoic mantle root.

3. Survival of Archean diamondiferous mantle roots

Whereas economic quantities of diamonds are restricted to cratons with ancient mantle roots, the distribution of the generally much younger kimberlites and lamproites does not appear to show a direct correlation with such roots. This explains why so many kimberlites and lamproites do not contain diamonds, although they are identical to their diamondiferous counterparts in most other respects. Large-scale area selection for kimberlite search should therefore concentrate on regions in which Archean mantle roots have survived either into the Recent, or long enough to have been sampled by at least one kimberlite event. Preservation of the diamonds requires that the refractive, relatively cool and low-density peridotitic roots remain insulated against reheating and excessive tec-

tonic reworking and stay attached to the cratons during subsequent plate motions. Hoffman (1990) discussed the geological constraints on the formation and preservation of mantle roots and pointed out that roots under Archean cratons survived the emplacement of extensive mafic dike swarms, but were thermally eroded near the sites of mantle plumes. The challenge for the diamond explorationist is thus to distinguish between "mantle-root-friendly" structures (horizontally intruded dike swarms, thin-skinned thrust belts, etc.) and "mantle-root-destructive" structures (mantle plumes, rifts, collision zones). Examples of such structures were discussed by Helmstaedt and Gurney (1994) who used the abundance of diamonds and the composition of diamond indicator minerals in kimberlites as measure of the"mantle-root-signature" of subcratonic lithosphere at the time of kimberlite eruption. Comparisons of the strengths of mantle-root signatures in successive kimberlite generations would make it possible to monitor the effect of post-mantle-root tectonic and magmatic events of individual cratons on the diamond potential of their hosts. In traditional diamond exploration, the diamond potential of a region would be assessed from the mantle-root signature of alluvial or kimberlite-bome diamond indicator minerals. However, as discussed in the section on the geophysical signature of mantle roots, it should be possible to predict this potential also on the basis of seismic data and thermal signature. On cratons with seismological evidence for mantle roots, all kimberlites postdating the formation of these roots could be expected to have a stronger mantle-root signature and to have tapped the mantle from within the diamond stability field. On the other hand, kimberlites on cratons without geophysical evidence of mantle roots would have diamond potential only if they were emplaced prior to the destruction of an earlier mantle root. A careful study of the orogenic and magmatic processes that have affected the craton is thus necessary to assess whether they have destroyed or preserved earlier formed mantle roots.

3.1. North American examples The North American craton is underlain by a general zone of high shear-wave velocity (Fig. 5A), and highvelocity mantle roots extending to depths of approximately 400 km can be recognized under much of the

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135

Archean Superior Province and the southern half of the Slave Province (Grand, 1987) (Fig. 5B,C). Bothprovinces are underlain by Archean rocks that have not undergone major basement reactivations since about 2.4 Ga. As noted earlier by Hoffman (1990), a highvelocity root is conspicuously absent from the Archean Wyoming Province. To illustrate our mantle-root approach to area selection, we will discuss the examples of the Slave and Colorado/Wyoming provinces and attempt an explanation of why a mantle root is preserved only under the southern part of the former, and appears to be absent altogether under the latter. As a third North American example we discuss the tectonic setting of the Saskatchewan kimberlites which were discovered in flat-lying Mesozoic and Paleozoic strata along covering the southern extension of the Proterozoic Trans-Hudson orogen. Slave Province The discovery of diamond-beating kimberlites near Lac de Gras, in the Archean Slave Province (Fig. 10), is an excellent example of the convergence of the petrological and geotectonic/geophysical approaches to establishing the existence of an ancient, diamondiferous mantle root. The original heavy mineral trail, picked up by Chuck Fipke and his co-workers from Diamet, showed mineral compositions with a strong mantle-root signature, suggesting derivation from a source with outstanding diamond potential. The discovery is precisely in the region of the Slave Province described earlier in this chapter as being diamond prospective on geophysical and tectonic grounds. Interestingly, this region is not within the oldest part of the Slave Province, the Anton terrane, which includes > 3 Ga rocks and is located to the west (Fig. 10). The kimberlites of the Lac de Gras area are located at the northwestern margin of a shear-wave-velocity high which includes the southeastern Slave Province and adjacent Churchill Province (Fig. 5B). Although the area of this high approaches the present horizontal resolving power of the tomographic inversion (approximately 400 km), it can be seen that the higher velocity region is clearly to the south of the middle Proterozoic Coppermine plume (Fig. 11), a potentially mantleroot-destructive feature. This plume is inferred to be the cause of the Mackenzie igneous event comprising a widespread episode of mafic magmatism in the northwestern Canadian Shield that occurred within a time-

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span of only a few million years at 1270 Ma (LeCheminant and Heaman, 1989) and produced the Copper Mine River flood basalts, the Muskox layered intrusion, and the extensive, fan-shaped Mackenzie dyke swarm (Fig. 11, top). The short time-span, large volume and specific focus of the event are attributed to magmatism above a large, plume-generated hotspot that led to rifting and ocean opening (Fahrig, 1987). Flow pattems in the dykes of the Mackenzie swarm, determined by measurements of the low-field anisotropy of the magnetic susceptibility, show a tightly constrained transition from vertical to horizontal flow between 500 and 600 km away from the apex that is believed to map the outer boundary of the plume (Ernst and Baragar, 1991) (Fig. 11). It is interesting to note that the southern margin of the mantle-root-destructive plume in the northern Slave

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H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

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137

H.H. Helmstaedt, J.J. Gurney/ Journal of Geochemical Exploration 53 (1995) 125-144

Province is located near the northern end of Contwoyto Lake, well to the north of the northern margin of the observed high-velocity mantle root in this area (compare Figs. 5B, 10, 11). South of the plume margin (approximately 200 km south of the Copper Mine River flood basalts), the Mackenzie dikes change into mantle-root-friendly structures, as they were intruded laterally, probably entirely within the brittle upper part of the lithosphere, leaving the lithospheric root beneath this part of the swarm intact (Fig. 11, bottom). The diamondiferous kimberlites of the Lac de Gras area are located well to the south of the plume margin, in an area where a potential Archean mantle root should have survived. Although kimberlites may be found also further north, within the perimeter of the Copper Mine plume, only those pre-dating the MacKenzie dikes should be diamond prospective. Although there has been much discussion about possible "direct" relationships between the kimberlites and the MacKenzie dikes (Schiller, 1992), spatial correlations between the dikes and the late Cretaceous/ early Tertiary kimberlites should not be overemphasized. Outside the plume perimeter, the dikes were intruded horizontally and thus are likely to have a restricted depth extent. Kimberlites intruding from greater depth may or may not encounter these dikes when penetrating the crust. As diabase dikes commonly utilize pre-existing fracture sets, the same sets may of A_n:hcanWyoming Block Chcy~

course be utilized by later kimberlites along the final part of their ascent route. As pointed out by Helmstaedt and Gurney (1994), the fan-shaped, late Archean Matachewan dyke swarm may have had a similar influence on the southern Superior Province as the MacKenzie dike swarm in the Slave Province. If the plume which generated the Matatchewan swarm was located south of the present margin of this province (Fahrig, 1987), the dikes should become increasingly "mantle-root-friendly" towards the north, and it could be expected that kimberlites in the James Bay region, north of the 50th parallel, should show a stronger mantle-root signature than the Kirkland Lake kimberlites further south (Brummer et al., 1992). Colorado~Wyoming kimberlite province

As discussed by Eggler et al. (1988) and Hoffman (1990), a mantle root is conspicuously absent under the Archean Wyoming province (Fig. 5), and to account for its absence, Hoffman (1990) suggested that an earlier root was destroyed either by Proterozoic collisional orogeny (Karlstrom and Houston, 1984) or by early Tertiary shallow subduction related to the Laramide orogeny (Bird, 1988; Helmstaedt and Doig, 1975). The presence of diamonds in the Devonian State Line kimberlites and a medium-strong G10 garnet signature in these kimberlites (Eggler et al., 1988; Helmstaedt and Gurney, 1994) are a clear indication

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H.H. Helmstaedt,

138

J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

that diamond-bearing source rocks of Archean age indeed existed near the southern rim of the Wyoming craton during the Paleozoic. Together with Eggler et al. (1988) we thus suggest that the root was destroyed during processes related to the Laramide orogeny. The State Line kimberlites pose an interesting problem, as they occur in Proterozoic rocks of the Colorado Front Ranges that are thought to represent juvenile Proterozoic crust accreted to the Archean Wyoming province of Laurentia between about 1800 and 1600 Ma (Karlstrom and Houston, 1984). The occurrence of diamondiferous kimberlites in such a Proterozoic terrain (Eggler et al., 1987) would clearly be an exception to Clifford's Rule, according to which diamondiferous kimberlites should be restricted to Archean ArcheanBasementWindows alongIstrike

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150200-krn Fig. 13. (Top) Schematic cross-section through Trans-Hudson orogen based on LITHOPROBE seismic reflection data (after Lucas et al., 1993) showing tectonically buried Archean terrane under Glennie Domain. (Bottom) Speculative interpretation of diamond prospectivity of above section, suggesting that the tectonically buried terrane may have a lithospheric root separate from that of the Superior Province.

H.H. Helmstaedt, J.J. Gurney/ Journal of GeochemicalExploration53 (1995) 125-144 later (Fig. 12B), because Tertiary kimberlites and lamproites in the Wyoming Province have not yielded any diamonds and it has not been detected by seismic tomography. Saskatchewan kimberlites The diamond-bearing kimberlites in the Fort a la Come region of central Saskatchewan are another apparent exception to Clifford's Rule, as they are located in the Trans-Hudson orogen, a Proterozoic orogenic belt between the Archean Superior and Rae/ Hearn provinces. The geological setting of these kimberlites has been described by Lehnert-Thiel et al. (1992) and Strnad (1993). The Fort a la Come kimberlite field lies within the southerly extension of the Glennie Domain, a region that was recently shown as consisting of an allochthonous Proterozoic thrust sheet over Archean basement (Collerson et al., 1990). The tectonic boundary with the underlying Archean basement was spectacularly imaged on LITHOPROBE seismic reflection profiles (Lucas et al., 1993), though it is uncertain whether the basement represents part of the Superior Province or a separate Archean microcontinent with its own lithospheric root (see also Fig. 13). The kimberlites of the Fort a la Come region and of the State Line district at the Colorado/Wyoming boundary are examples where too strict an adherence to Clifford' s Rule during area selection could have led to rejection of these areas as diamond prospective. However, as tectonic and seismic reflection studies (described in the preceding paragraphs) showed in both areas that the Proterozoic rocks outcropping at the surface are in thrust contact with underlying Archean rocks, these examples suggest that modern diamond explorationists should utilize such methods to identify thrust-buried Archean cratonic areas and thus add to the diamond-prospective land area.

4. Geotectonic controls of the transport medium After predicting areas under which diamond-bearing source rocks may have survived, the major challenge remains the identification of geotectonic and structural factors affecting the formation and distribution of the generally much younger kimberlites and lamproites. The role of these rocks as transport vehicle for diamonds is unique when compared to that of magmatic

139

or hydrous transport media in the formation of most other mineral deposits. Unlike typical magmatic mineral deposits, a genetic relationship between the mineral commodity and the magmatic host does not exist, i.e. the diamond does not form and settle out in the magma but is incorporated as a finished product from peridotitic and eclogitic country rocks and transported in suspension, either as xenocrysts or within xenoliths. Unlike most magmatic or hydrothermal deposits, the transport process does not lead to concentration but involves dilution in the depositional site (Helmstaedt, 1993). The transport is unusually fast and does not involve vast quantities of magma. The transport medium can be observed after formation of the deposit and carries with it clues about its source and depth of origin (deep-seated xenoliths, etc.). From an explorationist's point of view, the requirements for the formation of economic "primary" diamond deposits are now well known and may be summarized as follows: 1. The host rock must originate in or below a diamondrich source region of the upper mantle. 2. The host rock must ascend fast enough for diamonds to survive the transport to the surface. 3. Enough host magma must reach the surface and encounter emplacement sites where conditions are conducive to the formation of sufficiently large pipes or bodies. 4. The post-emplacement geological environment must favor preservation of the pipes. However, as very little is certain about the geotectonic factors governing the formation of kimberlites and lamproites, it is not yet possible to predict their locations. The problem of predicting the location of diamondiferous host rocks is compounded by the fact that kimberlites and lamproites, though both products of intraplate magmatism, are not confined to the Archean parts of cratons. Nevertheless, explorationists must be aware of the constraints on depth and geological environment of kimberlite and lamproite formation. The brief review of these problems in the following section is restricted to kimberlites. 4.1. Depth of origin ofkimberlites Assuming that diamonds have survived in their stability field, the minimum depth of origin of diamondbearing kimberlites was estimated as 150 km (Dawson,

140

1-1.14.Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

1984). A maximum depth is not as easily established, but Dawson inferred from the abundance of both xenocrystal and phenocrystal olivine in kimberlite that it should not be greater than 400 km. Below this depth, olivine is thought to invert to its high-pressure phase, beta-olivine, which has not yet been identified in kimberlites. Unless beta-olivine inverts very efficiently back to alpha-olivine, even when included in diamonds, the estimated depth range of 150 to 300 km for diamondiferous kimberlites is very plausible and in agreement with experimental evidence reviewed by Eggler (1989) and Wyllie (1989). A case for a much deeper origin of kimberlites has been made by Ringwood et al. (1992) on the basis of majoritic garnet inclusions identified in diamonds from the Monastery kimberlite (Moore and Gurney, 1985). The majoritic garnet and other ultra-high-pressure mineral inclusions (see also Haggerty, 1994) are inferred to have originated at pressures characteristic of the transition zone (ca. 400-670 km), and it was proposed initially that the diamonds containing them were carried from this depth by rising plumes or convection currents to the base of the lithosphere (ca. 180 km), where they were entrained by the kimberlite. Alternatively, the kimberlite magma itself may have been generated within the transition zone and carried the diamonds with their ultra-high-pressure inclusions directly to the surface (Moore et al., 1991). On the basis of this assumption, Ringwood et al. (1992) conducted melting experiments with kimberlites of average Group I composition and showed that, at 16 GPa ( 160 kb) and 1650°C, majorite garnet and beta-spinel crystallized on the liquidus. They also showed that the kimberlite composition used in the experiments could not have been produced by partial melting of garnet peridotite at a depth of 300 km or less, because its 10 GPa ( 100 kb) liquidus is not multiply saturated with olivine, orthopyroxene, and garnet. The experiments imply that kimberlites of Group I composition could have been derived from source regions in the transition zone, casting doubts on conventional hypotheses of kimberlite genesis. Ringwood et al. (1992) thus concluded that the ultimate source of Group I kimberlites lies in the transition zone, at a boundary layer comprised of mixed domains of subducted former harzburgites and asthenospheric pyrolite that was fertilized by partial melts of garnetite representing former subducted oceanic crust. The

ascending kimberlite magmas collected xenoliths and diamonds mainly in the subcontinental lithosphere but retained trace amounts of ultra-high-pressure phases as evidence of its deeper-seated origin. 4.2. Geotectonic controls on locations of kimberlites

In spite of numerous attempts to explain the spatial and temporal distribution of kimberlites, no concensus has been reached with regard to their geotectonic controls (see also Dawson, 1989; Janse, 1984; Mitchell, 1991). As summarized by Helmstaedt (1993), kimberlites have been related to: ( 1) regional uplifts above upwelling convection currents (Dawson, 1970; Janse, 1975; Milashev, 1974), (2) mantle diapirs (Green and Guegen, 1974; Mercier, 1979; Wyllie, 1980), (3) mantle hot spots (Crough, 1981; Hastings and Sharp, 1979; Le Roex, 1986; Skinner, 1989), (4) rifting of continents (Le Bas, 1971), (5) flat-dipping subduction zones (Helmstaedt and Gurney, 1984; Sharp, 1974), (6) non-laminar flow above subduction zones (Anderson and Perkins, 1975 ) and (7) transform faults ( Haggerty, 1982; Marsh, 1973; Stracke et al., 1979; Williams and Williams, 1977), but so far none of these models can explain all of the aspects of the problem. Haggerty (1994) has made an attempt to correlate peaks of kimberlite magmatic activity with normal and reverse superchron and subchron behavior of the geomagnetic field and claims to have detected a time lag between magnetohydrodynamicactivity in the core and kimberlite cycles of the order of 25-50 Ma, consistent with traveltimes of plumes from the core-mantle boundary to the subcontinental lithosphere. Much of the uncertainty in these models derives from the fact that our knowlegde about virtually every aspect of the complex process of kimberlite formation and ascent to the surface is still very speculative. In addition, there is the problem of correlation between the mainly sublithospheric processes involved in kimberlite formation and the geotectonic environment in the upper parts of the lithospheric plates through which the kimberlites erupt. This includes the question of the role of lithospheric extension in triggering the kimberlite magmatism. Although the larger-scale geotectonic controls for kimberlite magmatism are still elusive, exploration models must take into account that only relatively mantle-root-friendly processes may be invoked as causes

H.H. He lmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

for diamond-beating kimberlite magmatism. For example, the fact that successive generations of kimberlites on the southern African craton are diamondiferous and carry strong mantle-root signatures suggests that kimberlites are essentially mantle-root-friendly features. It is unlikely that diamonds would have survived if mantle-root-destructive hotspots or plumes that raise the ambient temperature up to 200°C above that of a lithospheric conductive gradient (Hill et al., 1992) would have impinged repeatedly on the roots of this craton. Although some workers have proposed a temporal relationship between plume activity and kimberlite magmatism (Haggerty, 1994), an examination of the geographic distribution of plume-generated Mesozoic large igneous provinces (LIPs) (Coffin and Eldholm, 1994) and Mesozoic diamondiferous kimberlites worldwide shows a definite negative correlation. Even continental flood-basalts are generally located near the edges of cratons (Anderson, 1994) and at best may have exerted a secondary control on kimberlite formation by causing metasomatic enrichment of adjacent mantle. The Jurassic Karroo igneous province (Cox, 1988), for example, is centered at the southeastern margin of the Kaapvaal craton, whereas the bulk of the Cretaceous diamondiferous kimbedites is located within the craton (Gurney et al., 1991), well to the west and northwest of the inferred plume centre. Where flood basalts occur within a diamondiferous kimberlite province, such as the Triassic Siberian Traps, only the kimberlites predating the flood basalts are diamondiferous (the Mid-Paleozoic provinces), whereas younger kimberlites are not known to contain economic amounts of diamonds (Milashev, 1974). Following Anderson (1994), a major kimberlite event, such as occurred during the Cretaceous, may thus be related to events resulting from global plate reorganization following the breakup of a supercontinent (Pangea) rather than to superplume tectonic s (Haggerty, 1994). On the regional scale, within cratons, examples abound where the location of kimberlites is controlled by regional fractures or fault structures, though it is rarely possible to determine whether these structures were active during kimberlite intrusion or merely served as passive pathways. During area selection, a careful assessment should be made whether such "kimberlite-controlling" structures have a mantle-rootfriendly geological history. For example, although Mitchell (1986) found no correlation between kim-

141

berlite magmatism and rift structures, a number of recent kimberlite discoveries occur along rift systems (e.g., the kimberlites of the Lake Timiskaming Structural Zone of Brummer et al., 1992). However, those kimberlites, known to have intruded into regions of noticeable previous or contemporary rifting, generally have weak or no mantle-root signatures (Helmstaedt and Gurney, 1994), suggesting that rifting is a progressively mantle-root-destructive process (see also Hoffman, 1990).

5. Conclusions 1. First-order controls for the location of primary diamond deposits are the sites of diamond source rocks which are located in the well-preserved lithospheric roots of Archean cratons. 2. The source rocks for economic diamond deposits consist mainly of gametiferous peridotites (mainly low-Ca garnet harzburgites) and subordinate eclogites that are in general much older than the host kimberlites and lamproites. 3. Area selection for diamond exploration should be based on geotectonic and geophysical studies aimed at predicting regions under which diamondiferous lithospheric roots are preserved or were present at the time of kimberlite or lamproite emplacement. Prospective areas for exploration may be enlarged significantly by considering those parts of Archean cratons that have been tectonically buried by Proterozoic or later orogenic belts. 4. The locations of magmatic host rocks (kimberlites and lamproites) in prospective areas are controlled by tectonic and structural conditions at the time of magmatism. As these conditions are not well understood, predictions of the locations of prospective host rocks remain unreliable. Area selection should thus concentrate on identifying mantle-root-friendly structures which may have served as pathways for the emplacement of diamondiferous kimberlites or lamproites.

Acknowledgements We thank the editor of this volume, W.L. Griffin, for inviting us to contribute this paper. Our research was supported, respectively, by NSERC Operating Grant

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H.H. Helmstaedt, J.J. Gurney / Journal of Geochemical Exploration 53 (1995) 125-144

#A8375 and the South African Foundation for Research and Development. Nancy Thomas is thanked for preparing the figures.

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