Kimberlites in a Karoo graben of Northern Mozambique: tectonic setting, mineralogy and Rb-Sr geochronology

Kimberlites occur as dykes up to 3 m thick and isometric bodies in the southern explored part of a NE-SW trending Karoo graben across the Mozambique Belt in northern Mozambique. The kimberlites are reported along four NW-SE-trending zones, and one ENE-WSW-trending zone. The kimberlites are macrocrystic hypabyssal Group Ia kimberlites based on their mineralogy and whole rock geochemistry. The kimberlites show a typical crystallization history with early olivine, perovskite, ilmenite and phlogopite growth succeeded by rutile and spinel Kimberlites of northern Mozambique by Key & others 10/06/2009 2 crystallization. Serpentine and calcite are the youngest primary minerals although there is also secondary growth of these two minerals. Pyrope occurs as rounded xenocrysts. SEM analyses of various macrocrysts and groundmass minerals, as well as Sr isotope data confirms the Group I classification of these kimberlites. A phlogopite Rb-Sr mineral isochron provides a Lower Cretaceous estimate of 138 ± 8.5 Ma for emplacement of one of the isometric body. The two principal trends of the dykes, together with their absolute age, strongly suggest that their emplacement was controlled by stresses associated with Lower Cretaceous break-up of Gondwana, involving dextral offset of Madagascar with respect to mainland Africa. These stresses reactivated bounding faults to the Karoo graben hosting the kimberlites. Dextral shearing along the bounding faults produced Riedel fractures within the graben that were infilled by kimberlite dykes. U-Pb zircon dates from the crystalline basement of northern Mozambique indicate that the Karoo graben hosting the kimberlites covers a major NNE-SSW trending tectonic boundary in the basement. To the NW of the graben, Palaeoproterozoic (>1.95 Ga) crust is recorded while Mesoproterozoic (<1.07 Ga) crust is widespread to the SE. Insufficient mineral-chemistry data is presently available to decide whether the kimberlites are likely to be diamondiferous. The northern Mozambique kimberlites form the NE extremity of a trans-continental NE-SW kimberlite corridor that stretches from southern Namibia across Botswana and central Zimbabwe into northern Mozambique. Introduction Kimberlites were found in northern Mozambique by a group of Soviet geologists in the 1970s (Afonso & Marques, 1993, quoted in Lächelt, 2004). The kimberlites Kimberlites of northern Mozambique by Key & others 10/06/2009 3 intrude Karoo sedimentary rocks which define a major graben cutting across Proterozoic metamorphic rocks of the Mozambique Belt. The field relationships of the northern Mozambique kimberlites are documented in original reports and maps in Russian. However, accompanying laboratory work is not published, and thus the diamond potential of these kimberlites is largely unknown. Several of the kimberlites were re-sampled in 2003, as part of a regional bedrock-mapping program. The present account summarizes available information on the kimberlites and their geologic setting, and reports new data, including field observations, petrographic data, SEM mineral analyses, and Rb-Sr isotopic data. The objective is to characterize these kimberlites and provide a framework to assess their diamond potential. Geological setting The studied kimberlites are hosted in a ca. 140 x 60 km, NE-SW tending graben filled with Permian to Jurassic Karoo sedimentary strata in northern Mozambique. The graben is situated immediately to the east of the East African Rift System (Figure 1), and extends to the NE in Tanzania. In the literature, it is variously referred to as the Lunho River Graben, Maniamba Graben or Metangula Graben (Karakoulov, 1983, 1984; Verniers & others, 1989) in northern Mozambique and the Selous Basin in Tanzania. The Karoo strata include a lower sequence with ‘coal measures’ of Permian age attributed to the Ecca Group. The overlying upper Karoo siliciclastic sediments are of probable upper Permian/Triassic to Jurassic age (Verniers & others, 1989). The graben system developed by rifting throughout Karoo deposition and is up to about 10 km deep in its centre. Karoo strata dip to the NW in the southernmost part of the graben where Lower Karoo strata are exposed. Elsewhere Upper Karoo strata are exposed. They are flat-lying in the centre of the graben and define a broad NNEtrending anticline in the north (on the north side of the major NW-trending Moola Kimberlites of northern Mozambique by Key & others 10/06/2009 4 River Fault). It has been proposed that the total thickness of the Karoo sequence increases from about 1650 m in the south to about 6000 m along the Tanzania border. No extrusive volcanics (of Jurassic age) are preserved at the top of the Karoo sequence in contrast to the voluminous flood basalts that cap Karoo successions further to the south throughout southern Africa. The Karoo graben overlies Proterozoic rocks of the Mozambique belt. Investigations in progress suggests that the two shoulders of the Karoo graben are distinct, and consequently that the graben covers a major Pan-African or older lithospheric boundary. To the NW of the graben, the Ponta Messuli gneiss complex (outcropping along the shores of Lake Niassa) is associated with metasedimentary rocks of the Txitonga group. An amphibolite-facies migmatitic paragneiss from the Ponta Messuli Complex contains detrital zircon cores ranging in age between 2.20 and 1.99 Ga and metamorphic zircon overgrowths dated between 1.98 and 1.95 Ga (unpublished SIMS data). U-Pb dating of monazite in this sample (unpublished LA-ICPMS data) constrains the timing of migmatitization at 1954 ±15 Ma. Available zircon and monazite data demonstrate that the Ponta Messuli complex contains a basement older that 1.95 Ga affected by an amphibolite-facies overprint at 1.95 Ga. The data suggests that the Ponta Messuli complex forms an extension of the Palaeoproterozoic Usangaran belt of Tanzania. Whole-rock Sm-Nd isotopic data are available on three samples from the Ponta Messuli complex (Saranga, 2004). The samples are garnetbiotite gneiss, biotite-epidote gneiss and a charnockite, mapped by the authors as part of units of amphibolite and granitic gneiss. The three samples yield Archaean depleted mantle model ages (TDM) ranging from 3.01 to 2.64 Ga. The data indicate that the extraction of the protoliths to these gneisses from the depleted mantle took Kimberlites of northern Mozambique by Key & others 10/06/2009 5 place during the Archaean. The data suggest the presence of an Archaean crustal component in the crust forming the Ponta Messuli complex (sedimentary material or magmatic basement). To the SE of the Karoo graben, the Unango complex consists mainly of granitic and charnockitic orthogneisses formed between 1.07 and 0.99 Ga. These gneisses are intruded by 0.80 Ga alkaline granite and Pan-African plutons. The Unango complex is affected by Pan-African high-grade metamorphism, though this metamorphism is poorly dated as of today. Previous studies on the Kimberlites Oberreuter & Pilale (1998) provide an overview after Stajilo-Alekseev (1983) of the diamond exploration work undertaken by Soviet geologists in the late 1970s and early 1980s in the NW part of Niassa Province. The first kimberlite was discovered in 1979 at Lufutiche by Jakovenko & others (1979). These workers estimated that the area with potential for kimberlites covered about 1100 km in the basins of the rivers Lunho, Fúgòe, Luile and Chissongo. Another 43 kimberlite dykes, as well as an unknown number of kimberlites and 4 isometric bodies (diatremes or pipes) were subsequently discovered in the SW part of the Karoo graben in the basins of the Lunho and Fúgòe rivers. As of today, the remainder of the graben remains unexplored for kimberlites. Following Oberreuter & Pilale (1998), brecciated kimberlites were not found as dykes but confined to pipes. The kimberlites are associated with low (positive and negative) magnetic anomalies. No kimberlites were found in the Proterozoic basement surrounding the Karoo graben. An investigation of stream sediments in the basement areas also failed to uncover any kimberlite indicator minerals. Available reports therefore concluded that there was a direct link between the formation of the graben and the emplacement of the kimberlites. A comparison Kimberlites of northern Mozambique by Key & others 10/06/2009 6 was made between the kimberlite occurrences in northern Mozambique and kimberlites in northern Tanzania, confined to the Ruhuhu Graben within the East African Rift System (McKinlay, 1955). Five zones with kimberlites were identified in northern Mozambique, labelled 1 to 5 from west to east (Fig. 2). The three westerly zones (1-3) have parallel NW-SE trends. These are the ‘LefululutxeFúgòe Rivers’ Zone (1), the ‘Upper course of the Fúgòe River’ Zone (2) and the ‘Tulo-Namango Rivers’ Zone (3). The fourth zone, called the ‘Luimba River’ Zone is short and trends ENE-WSW. The fifth zone, called the ‘Micuela’ zone is the shortest. It trends NW-SE and is situated directly to the east of zone 4. Kimberlite dykes within each zone trend parallel to the trend of the zones. The ‘LefululutxeFúgòe Rivers’ Zone (1) is located between 4 and 10 km east of Lake Niassa and is about 28 km long and about 5 km wide with a trend of 145-325o. Nine dykes were identified as well as a kimberlite lode with the biggest dyke on the left side of the Lefululutxe River. Ground investigations were curtailed due to security problems. The ‘Upper course of the Fúgòe River’ Zone (2) is narrow (1-2.5 km wide) and was only traced for 12 km. It nevertheless probably continues towards the north out of the area studied by the Soviet geologists. Eleven dykes were found but no detailed ground studies were completed. The ‘Tulo-Namango Rivers’ Zone (3) was traced for 25km NW from the southern edge of the graben and is about 4 km in width across the Lunho River valley. Nineteen dykes and 4 isometric bodies were discovered. The 4 isometric bodies were found along the Namango River valley in the only area prospected in any detail. One isometric body (‘TX3’ Pipe) underwent detailed magnetic studies and a 1:500 scale map of its outcrop was produced. Kimberlites of northern Mozambique by Key & others 10/06/2009 7 The ‘Luimba River’ Zone (4) is ca. 500 m wide and trends 065-245o for about 2.5km. Four very thin subvertical dykes were discovered. The ‘Micuela’ Zone (5) was followed by ground magnetic surveys for about 600m in a 140/150o 320/330o direction with a width of 200m. Kimberlites were found in this zone, which is thought to be larger than suggested by the initial surveys. Most kimberlite dykes are up to 3m in thickness but can be 15m thick. The dykes of the ‘Luimba River Zone’ (4) are the thinnest (<0.2m). The TX3 pipe of the ‘TuloNamango Rivers Zone’ (3) is about 60 by 70 m in cross section. There is a large block of thermally metamorphosed Karoo strata in the centre of the pipe. Adjacent kimberlite pipes are circular to elongate (e.g. with ca. 15 x 60 m cross sections). Kimberlite dykes are locally displaced (up to 50m) by NE-trending faults. The dykes are mostly subvertical although dips as low as 65o were recorded. Jakovenko & others (, 1979) suggested that about 1500m of erosion took place in the Namango River region based on the attenuated (lower) Karoo sequence of only 150m in total thickness. They suggested that this erosion affected the kimberlites so that only their roots are preserved. This would imply that the kimberlites are of Karoo or immediate post-Karoo age. An overview map at a scale of 1:125000 prepared by the Soviet geologists shows kimberlite dykes cutting through the entire Karoo sequence, implying that the dykes post-date deposition of all of the Karoo sedimentary rocks. However, Lächelt (2004) records various kimberlitic minerals (pyrope garnet, Mgmagnetite, Mg-ilmenite as well as fragmented diamonds in the basal conglomerates (K7 unit) of the upper Karoo. This observation implies that there are pre-upper Karoo kimberlites within the graben. It is therefore possible that more than one generation of kimberlite dykes is present in the graben. Kimberlites of northern Mozambique by Key & others 10/06/2009 8 The primary surveyors noted that more kimberlites are likely to be found outside of the investigated area within the graben structure. Prospective areas include the northern sides of the Lunho and Fúgòe rivers and the Messinge River basin where indicator minerals have been found. They suggested that the amount of erosion decreases towards the source of the Messinge River where there is a better chance of finding less eroded (and therefore larger) kimberlite pipes. Olivine, phlogopite, perovskite, serpentine, carbonate, monazite, and traces of pyrope garnet are the main indicator mineral phases recorded in the stream sediments. Pyrope occurs as pale (low chrome) to dark (high chrome) violet grains. Brecciated kimberlite pipes are associated with the largest number of indicator minerals (60% contain pyrope) and magnetite-bearing kimberlites have the lowest number of indicator minerals. Lithic as well as crystal (olivine, garnet and picrite-ilmenite) clasts are set in a matrix of serpentine and carbonate with sodalite, perovskite and chromediopside. The exploration area was divided into the following three sub-areas based on the results of panning in the streams, • Upper courses of the Lucumba and Mecondece rivers with samples characterised by ilmenite-zircon-rutile assemblages with minor clinopyroxene, almandine, tourmalines, epidote and apatite. • Fúgòe River Basin and right margin of the Lunho River with samples characterised by pyrope-almandine-ilmenite assemblages with minor rutile, zircon, clinopyroxene, limonite and hornblende. • Left margin of the Lunho River with samples characterised by ilmenitelimonite-almandine assemblages with minor clinopyroxene, zircon, hornblende etc. Kimberlites of northern Mozambique by Key & others 10/06/2009 9 Oberreuter & Pilale (1998) noted that there are a number of discrepancies and omissions in the three original reports prepared between 1979 and 1983 for the Mozambique Geological Survey Department (DNG). In particular no results were given for the analyses of most of the 5000+ pan and geochemical samples. Most of the detailed (1:500) maps reporting geophysical surveys were not provided. It is therefore not clear as to whether the Soviet geologists found any diamonds. New results Fieldwork The kimberlite pipe TX3 from the ‘Tulo-Namango Rivers’ Zone (3) was accurately located between UTM (WGS84) grid reference 36L 0719537 8610862 and 0719493 8610891. Four other trenches across minor kimberlite dykes close to TX3 were also examined (e.g. at 36L 0717946 8613395 and 36L 0719490 8610422). Two near vertical, khaki green, weathered kimberlitic dykes were also observed in a small road cut on the main road to Cobue (36L 0714109 8617410). These dykes are 0.1-0.2 m and 0.8 m in thickness. They are separated by a thin screen of baked sedimentary rock (mudstones from the Beaufort Group). The dykes’ trend about 054o i.e. at right angles to the main dyke trend in ‘Tulo-Namango Rivers’ Zone (3), and it is possible that the dykes represent a south-westerly continuation of the ‘Luimba River’ Zone (4) (Figure 2). At the site of kimberlite TX3, several exploration pits with diameters of about 1m and depths of several metres are still open. Adjacent spoil heaps display fresh, massive, blue-black (with a faint green tint) kimberlite with mm-thick weathered rinds that vary from dull grey to blue-grey in colour (Plates 1 and 2). All of the examined kimberlites are weakly magnetic with kappameter readings between 9 and 12 x 10 SI units. Fresh kimberlite is overlain by pale yellow clayey soil between 1 and 2m thick. Kimberlites of northern Mozambique by Key & others 10/06/2009 10 An uppermost veneer of impure grey calcrete over TX3 forms a slightly elevated mound above surrounding Karoo sandstone outcrop. Large crystals (macrocrysts) up to about 1 cm in length of olivine (common), garnet, phlogopite, diopside, magnetite, calcite and ilmenite are noted in the TX3 kimberlites in a fine-grained matrix. Olivine and garnet grains have rounded outlines and commonly have very fine-grained alteration rims up to about 1 mm in thickness. Garnet varies in colour from a very dark red to orange to pale purple and all with a glassy aspect. Olivine grains are variably altered to serpentine. Calcite occurs interstitially in the kimberlite matrix and also as individual large grains or as clusters of large grains. Some of this rhombic carbonate may be dolomite. The amount of country rock xenoliths is highly variable, although the majority of examined rocks from the spoil heaps are clast deficient. The most common exotic components noted in the field are angular clasts of indurated Karoo sedimentary strata up to several centimetres in length, and angular to rounded quartz grains. The studied kimberlites intrude into well-exposed essentially horizontal sandstone of the Ecca Group (lower Karoo). Abandoned core from a borehole drilled into lower Karoo strata several kilometres from TX3 (at GR 0717644 8613912) was found during the fieldwork. Thinly interbedded arkosic sandstones, siltstones, quartz-pebble conglomerates and carbonaceous mudstones comprise the core and this sequence is interpreted as fluviatile in origin. Methods Five kimberlite samples were analysed at the Geological Survey of Norway including whole rock analyses by XRF on 4 samples (Table 1). Petrographic studies, performed with optical microscopy and a scanning electron microscope (SEM) coupled with an energy dispersive X-ray spectrometer (EDS) were undertaken on nine polished thin Kimberlites of northern Mozambique by Key & others 10/06/2009 11 sections of the 5 kimberlite samples. Rb-Sr isotopic data on phlogopite separates were performed. The Rb-Sr data provides more precise information on the emplacement age of the kimberlites. Petrography The examined rocks are all Group 1 macrocrystic hypabyssal kimberlites with inequigranular textures. Olivine is the dominant primary macrocrystic phase with isolated Mg-ilmenite, pyrope and phlogopite macrocrysts. The groundmass comprises intergrown primary calcite and serpentine with ubiquitous olivine grains and disseminated opaques with common phlogopite and traces of apatite and clinopyroxene. SEM data shows that the opaques comprise perovskite, ilmenite (including Mg-ilmenite) and various spinels including chromite and magnetite. Nickel sulphides (millerite and nickeliferous pyrrhotite) form clusters of (exsolved) acicular grains as inclusions in the macrocrysts. Enstatite forms rare rounded inclusions in Mg-ilmenite macrocrysts. Barium and strontium sulphate/carbonate are ‘late’ replacement minerals identified by the SEM analyses. Olivine grains vary from rounded, strongly fractured macrocrysts with embayed margins to small, unfractured euhedral grains in the groundmass. Serpentine infill cracks in olivine macrocrysts and also forms corona structures (with calcite and opaques) around individual grains. Olivine grains remain unaltered away from the serpentine infilled cracks. The macrocrysts are variably replaced (even in a single thin section) by both calcite and serpentine with tiny sulphide needles, and in extreme cases are completely pseudomorphed. Modal amount of phlogopite is highly variable. Phlogopite occurs as corroded macrocrysts, as well as more common small, embayed and corroded flakes in the groundmass. Rounded and fresh (but internally fractured) pyrope macrocrysts have Kimberlites of northern Mozambique by Key & others 10/06/2009 12 thin alteration (kelyphitic) coronas with an innermost, very thin layer (see SEM data,) and an outer dark layer with finely disseminated opaques. The garnet macrocrysts are embayed and are clearly not in equilibrium with the enclosing groundmass i.e. they are xenocrysts. Internal cracks are partly infilled by ‘late’ mineral phases that include carbonates and sulphates. Small olivine grains, carbonate (probably calcite), serpentine and disseminated opaques with variable amounts of tiny phlogopite flakes form a fine-grained groundmass. Euhedral carbonate rhombs (? calcite and/or dolomite) are locally present, as are irregular patches (? amygdales) of calcite in serpentine and calcite veinlets. Some calcitic segregations in the groundmass are cored by an unidentified yellowish, cryptocrystalline mineral (serpophite?). Spinels, ilmenite and perovskite are very common as equant grains with a bimodal grain size distribution. Perovskite grains in the groundmass are embayed with thin alteration rims. Lithic (mostly sandstone) clasts are strongly indurated and locally replaced by calcite and serpentine. A relict trachytic texture was noted in one clast and an oval-shaped clast has a fern-like internal texture defined by radiating olivine grains from a central ‘stem’. The texture may be a fragment of a spinifex fabric within a mantle-derived xenolith. Whole rock chemical analysis of one kimberlite sample (31990 in Table 1) that contains indurated sandstone clasts indicates considerable contamination probably due to assimilation of country rock material into the kimberlite. SEM petrology data Examined garnet macrocrysts (Plate 3) have well-defined outer alteration (kelyphitic) rims that are about 200 μm thick. Internal fractures are partly infilled by barium sulphate. Oval shaped millerite inclusions in garnet are about 50 μm long and nickeliferous pyrrhotite forms tiny needles. A very narrow strip (ca. 5-10 μm) of Kimberlites of northern Mozambique by Key & others 10/06/2009 13 strontium sulphate and calcite was identified along the edge of one garnet macrocryst. A similarly narrow strip of barium sulphate was identified between a garnet xenocryst and its kelyphitic corona. Barium sulphate also occur as tiny grains in the groundmass. Spinels and perovskite grains are typically about 50 μm in diameter. Perovskite grains are rounded and embayed and replaced by rims of ilmenite with outermost rutile rinds. The centre of altered perovskite grains is commonly infilled by calcite aggregates with or without rutile. Chromite forms tiny grains cutting into the margins of altered perovskite grains as well as embayed (frayed) cubes and embayed elongate laths with strongly fractured rims of less Cr-rich spinel. All spinels, including Timagnetite, occur as tiny cubes that commonly amalgamate, as well as more ragged grains. Ilmenite forms embayed, subhedral laths with strongly fractured spinel rims. Mgilmenite forms macrocrysts with very narrow alteration rims with rutile. A dropshaped inclusion of enstatite was found in one Mg-ilmenite xenocryst. Apatite forms embayed subhedral laths, locally with inclusion of Ca-ilmenite. Crystallization history The examined rocks are all Group 1 macrocrystic hypabyssal kimberlites with inequigranular textures. Macrocrysts noted in thin sections are mostly olivine with isolated Mg-ilmenite, pyrope and phlogopite. The garnets are xenocrysts although it is not clear whether the olivine macrocrysts are early primary (cognate) kimberlitic minerals or xenocrysts. However, at least some of the groundmass olivine is an early crystallization phase of the kimberlite magma. Some olivine macrocrysts have angular shapes to suggest that they are fragments of larger megacrysts. Kimberlites of northern Mozambique by Key & others 10/06/2009 14 Opaque minerals appear to have crystallised at different stages with embayed and altered ‘early’ perovskite and ilmenite grains and ‘late’ spinel grains. Alteration rims around perovskite grains formed during resorption in the latest stages of magma crystallization and typically include rutile (Mitchell, 1986). Phlogopite also appears to have crystallised at different stages with embayed ‘early’ flakes and ‘late’ poikilitic flakes with inclusions of other minerals including spinel. Serpentine and calcite are the youngest primary phases and variably replace olivine. They are interpreted as mostly ‘late stage’ primary kimberlite minerals (equivalent to glass in more felsic magmatic rocks i.e. residual liquid crystallization products) as opposed to secondary alteration phases. However, there is also the possibility that some carbonate, especially in strongly altered samples (e.g. 31990) is secondary and related to groundwater infiltration. Barium and strontium sulphate/carbonate are ‘late’ replacement minerals identified by the SEM analyses. This sequence of mineral crystallization accords with the typical crystallization history for kimberlites recorded by Mitchell (1986) Reconnaissance SEM chemistry results Selected partial mineral analyses performed with SEM + EDS are reported in Tables 2 to 5 inclusively. Very little zoning was noted in any of the analysed grains. Pyrope garnets (Table 3) plot in the G9 (lherzolitic) garnet field on the classical Cr2O3 against CaO diagram (Figure 4; Gurney & Zweistra, 1995). The retrograde kelyphitic rims as seen in the present study around garnet xenocrysts (provide minimum PT conditions for garnet growth) and typically form at temperatures of between 900 and 1300o C at 10-12 Kbars according to Garvie & Robinson (1982). An even higher temperature of formation of the garnets is indicated by their NiO values of up to 0.036% with a mean of 0.014% (110 ppm Ni). This mean value indicates formation Kimberlites of northern Mozambique by Key & others 10/06/2009 15 temperatures of about 1350° C using the nickel thermometer in Ryan & Griffin (1996). However, lower temperatures of between about 800 and 950 oC are indicated by plotting the garnet’s mean chrome content (5-6 %) on the Ryan & Griffin (1996) chrome against nickel thermometer, using the geotherm for Group 1 Kaapvaal Craton kimberlites. This would suggest that there were different (higher) geothermal gradients during Group 1 kimberlite emplacement in the Mozambique Belt than into the Kaapvaal Craton. Nyblade & Brazier (2002) show that the thick depleted lithospheric mantle underneath the Tanzania Craton does not extend under the Mozambique Belt. This observation strongly suggests higher geothermal gradients under this belt and under adjacent cratons. A range of spinels is present in the kimberlites based on the SEM data (Figure 6) although all occur as tiny grains in the groundmass. Chromites plot outside of the field of chromite inclusions in diamonds, but do overlap with the chromites associated with the Argyle lamproite field of northern Australia (Figure 5). It is accepted that chromites with more than 60 % Cr2O3 are associated with diamondiferous kimberlites. The maximum chrome content of the (five) analysed chromites from the northern Mozambique kimberlites is 55.62 % (Figure 5). Analysed olivine grains are fosteritic with over 45% MgO and less than 14% total iron as FeO (Figure 7 and Table 2). The single analysis of a groundmass clinopyroxene is typical of the ‘pure’ diopside found in kimberlite groundmasses, as recorded by Mitchell (1986). The very high Ca but low Al, Cr and Na contents distinguish these clinopyroxene grains from clinopyroxene megacrysts. Similarly the single orthopyroxene analysis is closest to those for the Ti-bronzites found as megacrysts in kimberlite or intergrown with ilmenite (Mitchell, 1986). Mica analyses (Table 4) show a range from phlogopite (9-10%K2O, 6-7% FeO) to hydro-phlogopite Kimberlites of northern Mozambique by Key & others 10/06/2009 16 with low K2O (8.4%) and totals (88 wt%). Also present in 31990 is hydro-biotite (K2O 3%, FeO 16.9%), possibly xenocrystal. Whole rock geochemistry Whole rock analyses were completed on 4 kimberlite samples from northern Mozambique (Table 1). Three of the analysed rocks are uncontaminated (based on the Contamination Index of Clement, 1982). The fourth has a high C.I. but this is due to a high calcite content and thereby low MgO content. The whole rock geochemistry from the three uncontaminated samples confirms the petrographic conclusion that they belong to Group I kimberlites, and probably part of Subgroup Ia (Smith & others, 1985). For example, the analyses plot in the field of Group I kimberlites on plots of TiO2 vs. K2O and Pb vs. SiO2 (Smith & others, 1985), and SiO2 vs. MgO (Skinner, 1989) with Al2O3, Fe2O3, MgO, Sr, Rb, Th, Cr, Zn and Cu values comparable to Subgroup Ia kimberlites. Only the light REEs were analysed and these show typical kimberlite patterns when plotted on a chondrite normalized REE (Masuda-Coryell) diagram with enhanced values for the Rb to La elements and depleted values for the Ce to Y elements (Smith & others, 1985). K2O contents of up to 1.58 % are typical for kimberlites but low for other types of ultramafic rocks (Table 1; Dawson, 1980, Table 6; Kirkley & others, 1991; Mitchell, 1986). Conversely the SiO2 values of between 27 and 30% for the least carbonated samples are very low for ultramafic rocks but typical for uncontaminated kimberlites (25-35% SiO2). The potassium contents of the northern Mozambique kimberlites are higher than kimberlite analyses shown in Mitchell (1986) but other elements fall within the range of kimberlite values quoted by Mitchell (1986). High TiO2, Fe oxide and V2O5 in all of the analysed kimberlites reflect the common presence of spinels, ilmenite and perovskite. Similarly high values of K2O, Al2O3, Rb and Sr are all linked Kimberlites of northern Mozambique by Key & others 10/06/2009 17 to the common presence of phlogopite. Nickel values fall within the 710-1600ppm range for kimberlite (Dawson, 1980, Table 8) and nickel-bearing sulphide needles (including exsolved needle clusters) were detected during the SEM analyses. High barium and strontium values can be explained by the presence of sulphate and carbonate rims around some macrocrysts. Cerium is highest in sample 31991, correlating with high phosphorus values due to relatively abundant tiny apatite laths in the sample. Smith & others (1985) conclude that the geochemical signature of Group I kimberlites suggests that they had an asthenospheric-like source in contrast to Group II kimberlites that are inferred to have originated from sources within ancient stabilized subcontinental lithosphere (i.e. Archaean cratons). This would accord with the tectonic setting of the northern Mozambique kimberlites that are set within a rift into Proterozoic crust. Rb-Sr geochronology and other isotope studies Rb-Sr analyses of phlogopites separated from one sample of the TX3 kimberlite (Plate 4) yield an isochron age of 138 ±9 Ma (Figure 8), i.e. in the Lower Cretaceous. The isochron has an MSWD of 8.5 and an initial Sr/Sr ratio of 0.7052±0.0026. This age is interpreted as the time of closure/crystallization of the phlogopite synchronous with the emplacement of the kimberlite. C and O isotope values from carbonate from the studied kimberlites are typical mantle values. The values are as follows: δC = -7.8 per mill vs. V-PDB, δO = 13.8 per