The Kimberlites and Related Rocks of the Kuruman Kimberlite Province, Kaapvaal Craton, South Africa. (Donnelly et al.)
Electronic Supplementary Material 1 - Classification and Petrography of the Kuruman Intrusions
Classification:
Group I and Group II ‘kimberlites’ have been given separate definitions (e.g. Mitchell 1995; Woolley et al. 1996; Le Maitre 2002) reflecting the general consensus that they are derived from different magmas and sources. However, an agreement has yet to be reached on terminology. Mitchell and Bergman (1991) and Mitchell (1995) have suggested that as Group II kimberlites are not actually ‘kimberlites’ they should have separate terminology and have proposed the term “orangeite” in recognition of the distinct character and occurrence of these intrusions within the Orange Free State in South Africa. The current International Union of Geological Sciences (IUGS) classification of igneous rocks adopted the Group II kimberlite terminology but follows the orangeite definition given Mitchell (1995). Tappe et al. (2005) recently proposed an update to the IUGS classification system due to the omission of ultramafic lamprophyres (UML) from this scheme. This modification adopts the terms kimberlite and orangeite in place of the current IUGS terminology of Group I and Group II kimberlite, respectively. The proposed modification utilizes groundmass mineral modal abundances and compositions to discriminate between the macroscopically similar kimberlites, orangeites, ultramafic lamprophyres (UML) and olivine lamproites. Following this scheme, nine of the intrusions from this study have been classified as kimberlite, one as orangeite and two as ultramafic lamprophyres (aillikites and mela-aillikites).
The Kuruman intrusions can be further subdivided according to texture into macrocrystic and aphanitic varieties (Clement and Skinner 1979, 1985; Clement et al. 1984; Mitchell 1986, 1995). The non-genetic textural term ‘macrocrystic’ indicates the presence of large crystals (>0.5 mm), predominantly anhedral olivine crystals and broken and/or rounded phlogopite laths, that may have been derived from the disaggregation of mantle xenoliths (e.g. Mitchell 1986, 1995; le Roex et al. 2003). In this work samples containing >10 vol% macrocrysts are classified as macrocrystic. More euhedral olivine and phlogopite crystals are termed phenocrysts (>0.5 mm) and microphenocrysts (<0.5 mm). The groundmass is characterized by small (typically <0.1 mm in size) randomly oriented crystals set in a uniform-textured matrix, with subordinate regions of segregationary-textured (non-uniform distribution of groundmass and mesostasis) groundmass. Mineralogical abundances and rock classifications are summarized in Table 1.
Kimberlites
The majority (nine) of the intrusions from this study are kimberlites (Table 1). These intrusions are classified as kimberlites based on the absence of groundmass clinopyroxene (Mitchell 1995; Tappe et al. 2005), the presence of groundmass monticellite (Mitchell 1995) and Al- and Ba-enriched phlogopite (Mitchell 1995; Tappe et al. 2005). The Kuruman kimberlites are predominantly macrocrystic hypabyssal-facies kimberlites, although regions of Bathlaros are aphanitic. The kimberlites are characterized by macrocrysts and phenocrysts/microphenocrysts of olivine and phlogopite set in a matrix dominated by variable proportions of carbonate, phlogopite and serpentine. Accessory groundmass phases include spinel, perovskite, apatite, ilmenite, rutile and altered monticellite (Helpmekaar, Toxteth 02 and White Ladies). The Kuruman kimberlites have unusually high modal abundances of primary groundmass and phenocrystal phlogopite (up to 30 vol% in White Ladies), relative to other kimberlites (e.g. Group I kimberlites; Skinner 1989).
Orangeites
Two of the Kuruman kimberlites, Cox’s Mine and X007 are mica-rich orangeites (De Beers database, and the present study). No samples from Cox’s Mine were available for this study. X007 was classified as orangeite based on the abundance of micas (up to 60 vol%) from the tetraferriphlogopite trend (Mitchell, 1995; Tappe et al. 2005) and the presence of Cr-rich spinels (atomic Cr/(Cr+Al) >85). X007 is a macrocrystic to aphanitic, hypabyssal-facies orangeite that is comprised of macrocrysts of altered olivine and rare phlogopite and subhedral phenocrysts and microphenocrysts of olivine and phlogopite set in a matrix dominated by small (generally < 0.1 mm) stubby phlogopite laths with lesser carbonate and serpentine. Additional matrix phases include fine-grained (0.01 to 0.03 mm) magnesiochromite, perovskite and apatite.
Ultramafic Lamprophyres
Aalwynkop and Clarksdale are classified as UML (aillikite end-member) following Tappe et al. (2005) based on the absence of melillite, nepheline and alkali-feldspar, and the presence of mica following the tetraferriphlogopite evolution trend (Mitchell 1995; Tappe et al. 2005), spinels from the titanomagnetite trend with Cr# <85, and Al- and Ti-enriched clinopyroxene (Tappe et al. 2005). Aalwynkop and Clarksdale are characterized by macrocrysts and phenocrysts/microphenocrysts of olivine and phlogopite set in a matrix of phlogopite, carbonate, serpentine, spinel and apatite. Aalwynkop dominantly has a macrocrystic texture, although it contains some carbonate-rich aphanitic zones that are typically 1 to 5 cm wide. Aalwynkop also contains macrocrystal and groundmass ilmenite, trace clinopyroxene phenocrysts and zircon. Clarksdale is termed a mela-aillikite as it contains >70 vol% mafic silicate minerals (Tappe et al. 2004, 2005), while the more carbonate-rich Aalwynkop dike is an aillikite.
Groundmass Phases (corresponding mineralchemistry data presented inElectronic Supplementary Material 2)
Olivine
Olivine is present in varying abundance in all of the Kuruman intrusions, comprising up to 50 vol% (e.g. Elston). There are three populations of olivine; anhedral to rounded macrocrysts (>0.5 mm) and more euhedral-to-subhedral phenocrysts (>0.5 mm) and microphenocrysts (<0.5 mm). Most of the macrocrystal olivine crystals are 2 to 3 mm in size, but reach a maximum length of10 mm. The proportion of macrocrystic olivine varies greatly between the different rock types (Table 1) with kimberlites typically having higher olivine macrocryst contents (5 to 25 vol%) relative to orangeite (5 to 10 vol%) and the aillikites (2 to 5 vol%). Variations in the olivine macrocryst content can also be considerable within intrusions (e.g. 15 vol% variation in Elston). Olivine phenocrysts and microphenocrysts (reported together in Table 1) are present in similar abundances in the kimberlites and aillikites (15 to 40 vol%), with lower olivine phenocrystal content in the X007 orangeite (15 to 20 vol%).
The degree of alteration of olivine to serpentine and calcite varies between intrusions, with the colour of the olivine crystals ranging from dark green, to pale green, to greenish white. In some instances olivine is partially replaced by fine, needle-like opaque minerals along internal fractures. Complete pseudomorphing of olivine crystals by serpentine is common. Partial pseudomorphs, where the outer portion of the grain is replaced by serpentine and carbonate while core compositions are preserved, are observed in Elston, Helpmekaar, and Zero.
Phlogopite
Phlogopite is present in all of the Kuruman intrusions but abundances are highly variable with ~ 5 vol% in the Belle Isle kimberlite up to 35 vol% in the White Ladies kimberlite, 60 vol% in the X007 orangeite and 30 to 40 vol% in the aillikites (Table1). Phlogopite macrocrysts (>0.5 mm, maximum size of 4.2 mm) are relatively rare in the kimberlites (<5 vol%) and orangeite (1 to 2 vol%), compared to the aillikites (5 to 10 vol%). The phlogopite macrocryst population is characterized by dark orange-brown irregular or fragmented laths, the majority of which are likely derived from larger crystals. The macrocrysts are commonly weakly pleochroic and often possess thin, irregular rims of later-stage phlogopite. The phlogopite macrocrysts occasionally show evidence of strain deformation (e.g. kink banding), undulose extinction, and are frequently replaced along cleavage planes by calcite and chlorite.
Phlogopite phenocrysts and microphenocrysts are typically characterized by weakly-pleochroic, randomly oriented, euhedral to subhedral poikilitic plates and laths that are pale-yellow to light-brown in colour. The phlogopite phenocrysts and microphenocrysts are late-stage phases that commonly contain inclusions of groundmass spinel, perovskite and occasionally olivine microphenocrysts.
Groundmass phlogopites (typically <0.1 mm) in kimberlites generally occur as colourless to light-brown to orange-brown subhedral to euhedral poikilitic laths that range from long, slender laths to stubby laths. Groundmass phlogopites in the X007 orangeite are typically darker orange-brown euhedral to subhedral laths. Groundmass phlogopites from the Aalwynkop aillikite are similar to those of the kimberlites, while Clarksdale phlogopites are characterized by extremely slender needle-like laths typically 0.1 to 0.2 mm in length. The type and intensity of phlogopite alteration is variable and pipe-dependent, typically characterized by partial chloritization of phlogopite (e.g. White Ladies, X007 and Clarksdale) and/or replacement of phlogopite by carbonate along grain margins and cleavage planes. Many of the phlogopites are colour-zoned, usually with darker yellow-brown cores and thin near-colourless rims. The cores typically exhibit normal pleochroism from colourless to pinkish-orange, while the rims often show reversed pleochroism from colourless to red-brown.
Apatite
Apatite is a late-crystallizing groundmass phase present in all of the Kuruman intrusions, except Helpmekaar. Apatite ranges in modal abundance from <1 vol% up to ca 5 vol% in Bathlaros (Table 1). Apatite is occasionally resorbed and replaced by calcite, but many unaltered grains are present. Apatite typically occurs as euhedral, prismatic crystals or more rarely as thin (<0.01 mm) needle-like laths. Acicular, radiating aggregates of apatite are observed in the Elston kimberlite.
Spinel
Spinel is ubiquitous in the Kuruman kimberlites, orangeite and aillikites. In the kimberlites spinel abundances range from approximately 1 to 2 vol% in Exit up to 15 vol% in Toxteth 02 and White Ladies. The X007 orangeite and the Clarksdale and Aalwynkop aillikites have much lower abundances of spinel at 1 to 2 vol%. Spinel is present in all size ranges from minute groundmass crystals to rare macrocrysts reaching a maximum size of 2.1 mm. Spinel macrocrysts occur only within the kimberlites and are present in trace amounts (<1 vol%) in all kimberlites, except Bathlaros in which they were absent. Spinel macrocrysts are typically rounded partial fragments of larger crystals and are often mantled by later groundmass spinel or, more rarely, they are surrounded by very fine-grained euhedral groundmass spinel crystals. Most groundmass spinels are homogenous euhedral to subhedral crystals that are typically 0.03 to 0.05 mm in size in the kimberlites and aillikites but are smaller (0.01 to 0.03 mm) in the orangeite. Spinels are occasionally found as “necklaces” around olivine crystals and as inclusions in late-stage phenocrystal phlogopite. Atoll-textured spinel, where chromite cores are surrounded by atolls of resorbed spinel, (e.g. Mitchell and Clarke 1976) is observed rarely and only in the kimberlites.
Perovskite
Groundmass perovskite is found in five kimberlites (e.g. Bathlaros, Elston, Helpmekaar, White Ladies and Zero) and in the X007 orangeite. The size and the morphology of the perovskite grains vary significantly between pipes. In X007 perovskite is rare and occurs as small (typically 0.01 to 0.03 mm) rounded crystals, while in Bathlaros it comprises up to 10 vol% (typically 0.2 to 0.4 mm). Pervoskite occurs as well-preserved euhedral grains that are 0.04 to 0.07 mm across in Helpmekaar and Elston and 0.02 to 0.04 mm across in the Zero pipe. The White Ladies kimberlite contains rounded grains, typically 0.03 to 0.05 mm across. Perovskite occurs as discrete crystals set in the uniform-textured matrix and smaller grains commonly form “necklaces” around olivine macrocrysts.
Ilmenite
Ilmenite is present in the Aalwynkop aillikite, dominating the opaque-mineral population (5 to 10 vol%), and in four kimberlites (Bathlaros, White Ladies, Toxteth 01 and Toxteth 02) where it comprises < 1 vol%. In kimberlites, ilmenite typically occurs as small (<0.02 mm) anhedral-shapedcrystals that possibly represent fragments of larger magnesian ilmenites from the megacryst suite. In the Aalwynkop aillikite ilmenite macrocrysts are variably abundant (1 to 5 vol%) and typically occur as anhedral crystals that have a continuous range of grain-sizes from 0.5 mm up to 3 mm. Groundmass ilmenites are common (5 to 8 vol %) in Aalwynkop and generally do not have well-formed crystal shapes and are typically <0.1 mm in size.
Additional Groundmass Phases (without corresponding mineral chemistry data)
Monticellite
The presence of monticellite (altered completely to carbonate) in the groundmass of three Kuruman kimberlites (e.g. Helpmekaar, Toxteth 02 and White Ladies) is suggested by the characteristic relict ‘sugary’ texture produced by small ~0.02 mm high-relief colourless crystals.
Carbonate
Carbonate is present as a late-stage groundmass phase in all of the Kuruman intrusions. It modally dominates the matrix of many of the kimberlites, varying in abundance from 5 to 35 vol% (Table 1). Carbonate is less abundant in the X007 orangeite (5 to 10 vol%). Considerable variation exists between the Aalwynkop aillikite (20 to 35 vol%) and the Clarksdale mela-aillikite (10 to 15 vol%). Carbonate is present in the uniform-textured matrix as inter-locking euhedral to anhedral crystals that are typically <0.1 mm in size. Calcite also occurs within the segregationary-textured regions of the matrix, commonly occurring as intergrowths with serpentine.
Serpentine
Serpentine is present in all of the Kuruman intrusions and varies considerably in abundance from 1 to 5 vol% (e.g. White Ladies) to 15 to 30 vol% (e.g. Toxteth 01 and Toxteth 02) in the kimberlites, with lower abundances in the X007 orangeite (5 to 10 vol%) and the Clarksdale and Aalwynkop aillikites (5 to 15 vol%). In all of the intrusion types, primary serpentine occurs as late-stage, green-brown, irregularly-shaped segregations which are commonly intergrown with carbonate.
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