The Fazenda Largo off-craton kimberlites of Piauí State, Brazil

Journal of South American Earth Sciences 28 (2009) 288–303

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The Fazenda Largo off-craton kimberlites of Piauí State, Brazil Felix V. Kaminsky *, Sergei M. Sablukov, Ludmila I. Sablukova, Olga D. Zakharchenko KM Diamond Exploration Ltd., 2446 Shadbolt Lane, West Vancouver, BC, Canada V7S 3J1

a r t i c l e

i n f o

Article history: Received 26 November 2008 Accepted 2 June 2009

Keywords: Kimberlite Off-craton Pyrope Diamond Piauí Brazil

a b s t r a c t In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area – Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos – this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the ‘Transbrasiliano Lineament’. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In the late 1990s, the new cluster of kimberlites, termed the ‘Fazenda Largo kimberlites’ were discovered in Piauí State, Brazil. As with earlier known kimberlites, in this area: e.g., Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos, this cluster is located within the ‘Transbrasiliano Lineament’ (Fig. 1). The Redondão kimberlite pipe (centered at 439270E and 8977210 N) was the first to be discovered, not only in Piauí State but also in all of Brazil. This kimberlite was recognised in the 1960s during the course of 1:1,000,000 scale geological mapping, in the upper course of the Ribeirão do Mateiro, the right tributary of the Rio Parnaiba (Melo and Porto, 1965; Ellert, 1971). Being very large (approximately 1 km in diameter), the Redondão pipe is discernible both on airborne photo images and on satellite Landsat imagery (Almeida and Castelo-Branco, 1992). This pipe has been the focus of numerous descriptive accounts in the literature (e.g., Svisero et al., 1977, 1984; Svisero and Meyer, 1986; Castelo Branco,

* Corresponding author. Tel.: +1 604 925 8755. E-mail address: [email protected] (F.V. Kaminsky). 0895-9811/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2009.06.003

1986; Castelo Branco and Lasnier, 1991). The other kimberlite localities, Santa Filomena-Bom Jesus (Gilbues) and Picos are described by Tompkins (1994) and Svisero (1995). Diamonds have been reported in alluvial deposits of Ribeirão do Mateiro, which washes out the Redondão pipe (Mapa geológico, 1995). However, only limited prospecting works were carried out there. Two of the diamonds that were recently reported upon from the Fazenda Largo kimberlites in this region, have been supplied to us for study. The newly discovered kimberlite group, known as the ‘Fazenda Largo kimberlites’, occurs approximately 95 km NE of the Redondão pipe, within the basin of Riacho do Loco, a right-side tributary of Rio Uruçui Preto belonging to the Rio Parnaiba system (Fig. 1). In the ‘Mapa de Distribuição geoquimica’ compiled by Projeto Platina Nacional, several localities of Jurassic-Cretaceous ‘ultramafic rocks’ were mapped in a number of local areas. During the 2001–2002 field works, igneous rock occurrences were found in three of these local areas, and all these rocks have subsequently been identified as epiclastic kimberlites of crater facies. The main objective of this work, therefore, was to perform a comprehensive study of these new Brazilian off-craton kimberlites.

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lite. Shaft #2 was terminated at a depth of 12 m in weathered kimberlite. 3. Samples and methods of study

Fig. 1. Location and tectonic position of Fazenda Largo kimberlites (after Tompkins (1994), with corrections for kimberlite localities).

2. Area description Tectonically, the main part of Piauí State, which includes the newly discovered kimberlites, is located within the Palaeozoic Parnaiba Sedimentary Basin, separating two cratons with Archaean nuclei: the São Francisco Craton with a cratonization age of 3.5– 3.0 Ga, in the southeast, and the Amazonian Craton (Guaporé Shield) with a cratonization age of 3.0–2.5 Ga, to the west (Cordani and Teixeira, 2007) (Fig. 1). A regional Transbrasiliano Lineament Zone intersects the area from SW to NE (Tompkins, 1994); this was probably created during the Neoproterozoic Brazilian Orogeny, when the Amazonian and São Francisco cratons collided (Cordani and Teixeira, 2007). Besides the Redondão and Fazenda Largo kimberlite clusters, this Zone controls two others, namely Santa Filomena-Bom Jesus (Gilbues) and Picos (Tompkins, 1994). Extensional reactivation of the fault zone in Proterozoic times resulted in the formation of a graben structure which subsequently controlled the positioning of the Palaeozoic (Silurian to Cretaceous) sediments. Geophysical data suggest that, locally, a sedimentary sequence of Upper Proterozoic age lies beneath the Palaeozoic rocks, extending to a depth of 4500 ms. These are in-turn underlain by Precambrian crystalline basement. The three Fazenda Largo kimberlites that form the basis of this study, along with several other inferred kimberlites (10 in all) form a cluster with a locus at approximately 8°510 S and 44°510 W. The kimberlite bodies form a chain, approximately 14 km in length that extends in a NNW direction; it is perpendicular to the Transbrasiliano Lineament Zone. The kimberlites intrude the Carboniferous Piauí Formation. Two of the studied kimberlites (that are named after the Fazenda owners) are of several hundreds of metres in diameter: the Domingo pipe (centred at 518410E and 9018415 N; all data falling within UTM Zone 23) and the José Milhudo pipe (centred at 519310E and 9015150 N). Each exhibits evidence of the previous prospecting. The third, the Young pipe (the northernmost of the three, located near Fazenda Largo, and centred on 516940E and 9024690 N), is newly discovered. This kimberlite is approximately 530  410 m in size, elongated NNW, in accordance with the general orientation of the kimberlite cluster. The localisation of this pipe was verified by the excavation of four trenches of up to 4–5 m in depth, and two shafts with a 2  2 m cross section. Shaft #1 was sunk to a depth of 41 m; at a depth of 26 m, it encountered ground water and at 36 m it entered fresh kimber-

For this study, we collected kimberlite samples from all four above-mentioned pipes (labelled ‘R’ – for the Redondão pipe, ‘JM’ – for the José Milhudo pipe, ‘D’ – for the Domingo pipe, and ‘Y’ – for the Young pipe). The samples from the Young pipe shaft represent relatively fresh kimberlite, whereas samples from the remaining three pipes are very intensely weathered rocks. In addition to the kimberlite rock samples, two pyrope-peridotite xenoliths from the Young pipe were also sampled and studied. Panning samples of kimberlite eluvium, collected from each pipe and panned at the sample site, have also been subjected to mineralogical analysis. Furthermore, two diamonds, reportedly from the Young kimberlite, were recently supplied to us by BrazDiamond Mining Co., and form part of this study. Our studies of the rocks were based on visual examination of the collected rock samples, microscopic examination of sixteen thin and polished sections, silicate whole-rock analysis, inductively coupled plasma-mass spectrometry (ICP-MS) analysis, phase analysis by X-ray diffractometry, mineralogical analysis, and determination of the chemical composition of rock-forming and kimberlite indicator minerals (KIMs), utilising electron probe microanalysis (EPMA). Whole rock analysis for 16 components was performed on six samples using wet chemical methods in the Analytical Centre of the Russian Academy of Sciences. For the ICP-MS analysis, the samples were ground to powder and then dissolved in acids, in an autoclave. The analysis was performed using a PLASMA QD analyser in the laboratory of IMGRE. Nine samples, including five analyses of autoliths, were analysed for 41 elements. X-ray diffractometric semi-quantitative phase analysis of representative samples was performed in the laboratory of VIMS, using an ADP-1 X-ray diffractometer (Cu Ka; U = 40 kV; I = 40 mA). Eight analyses were made, including three analyses of autoliths. The material subject to phase analysis was selected from a homogenised, weighted sample of the bulk rock. The procedure of phase analysis included the following operations: (1) In the first instance, the sample was examined so as to identify all mineral phases present within. In this analysis, we calculated the concentrations (in percent) of all identified minerals, with the exception of minerals from the layered clay fraction, which were taken as a residuum. (2) The clay-sized fraction (particles approximately 0.001 mm in size) was separated from the initial sample by gravitational methods, using the following procedure: a sample of up to 5 g was comminuted to 0.5–1 mm and then placed into a porcelain mortar with distilled water. The sample was then pounded, rubbed and stirred to form a stable suspension of clay particles, whereupon the suspension was poured into a test tube. While the suspension settled, we prepared the substrates for the analytical specimens (slides degreased with alcohol). Upon complete settling of the particles with densities of greater than 2.5 g/cm3, which is necessary for the separation of the fine fraction for the analysis, the suspension was sampled using a pipette, placed onto the slides and allowed to dry for 12 h, to obtain oriented specimens. Once completely dried, the analytical specimens (slides with settled layers of oriented clay particles, four slides were prepared for each sample) were ready for further examination. (3) For precise identification of clay minerals we recorded diffractograms of oriented analytical specimens saturated with ethylene glycol; specimens were calcinated for one hour at 550 °C. The duration of saturation with ethylene glycol was one day. Identification of the clay minerals present in the samples was based on joint interpretation of the compositions

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of the air-dried, oriented, organic liquid-saturated and calcinated analytical specimens. (4) Detection limits were: quartz 0.5%, goethite 1–3%, montmorillonite 5%, illite 1–2%, hematite 1–3%, feldspar 1%, chlorite 1%, siderite 1%, pyroxene 1%, and pyrite 1%. The procedure for mineralogical analysis of the heavy mineral fractions of crushed and panned samples, which was carried out at our laboratory, included the following operations: (1) Grinding of crushed samples to –1 mm and separation of clay-sized particles. (2) Examination of the coarse (+1 mm) fraction of panned samples using the MBS-1 binocular microscope. (3) Heavy medium separation of the –1 mm size fractions of crushed and panned samples in CHBr3 (specific gravity 2.9) and fractionation of the heavy minerals. (4) Paramagnetic separation of the magnetic, paramagnetic and nonmagnetic fractions. (5) Weighing of all the resulting fractions. (6) Examination of the –1 mm size fractions using binocular and polarizing microscopes. (7) Semi-quantitative determination of the mineral composition of the various heavy fractions. (8) Picking of minerals for the microprobe analysis. In all, 12 mineralogical analyses were performed. The chemical composition of the minerals was determined using a Cameca Micro Beam Camebax X-ray spectral microanalyser; optimum mode: U = 15 kV, I = 15 nA. The minerals were analysed for the following oxides: SiO2, TiO2, Al2O3, Cr2O3, FeO, MgO, MnO, CaO, Na2O, K2O, ZnO, and NiO. The results were averaged from 2 to 3 determinations for each grain. In all, 687 mineral grains were analysed. 4. Petrology and petrography The Fazenda Largo kimberlites are redeposited volcano-sedimentary rocks (RVK, according to the classification scheme of Kjarsgaard (2007)) with varying ratios of host, mineral fragments, and country rocks. They are very intensely weathered, almost completely replaced (argillised or, in the case of the Young pipe, talcisized), kimberlitic tuffs and xenocrystal tuffs with a clayey, basaltype groundmass. In outcrop exposures and excavated trenches, the kimberlites show typical volcano-sedimentary bedding and, locally, cross-bedding characteristic of epiclastic facies deposits. Whereas the Redondão kimberlites are commonly fine-grained ash-tuffs, the Fazenda Largo kimberlites are, by contrast, coarsely bedded, ash-lapilli tuffs. In hand specimen, the studied kimberlites are blotchy and porous. They are light grey with a greenish-yellow hue, rather homogeneously coloured, with small inclusions of altered olivine and xenoliths; these are discernible as various yellowish-grey and greenish-grey shades against a homogeneous yellowish-light-grey background matrix. The rock texture is litho-crystalloclastic (or crystallo-lithoclastic, for the José Milhudo pipe), with small, nearly equigranular grains of altered olivine, autolithic kimberlite inclusions, country rock xenoliths and quartz grains, that are cemented in a microscaly saponite or talcose (Young pipe) matrix of a basal-type (Fig. 2A). The structure of the rocks is irregular and massive. The clastic material present within the kimberlites consists of magmatic rock and mineral fragments, fragments of country rocks, and a terrigene admixture of quartz sand. The magmatic component (15–65% of the rock volume) consists essentially of crystalloclasts and lithoclasts. Crystalloclasts (15–40 by vol.%) are mostly completely altered olivine grains or grain fragments falling into two groups with differing grain-size (Fig. 2A): 1. Large (1–5 mm), oval or irregular macrocrysts (olivine-1) with a zoned coloration (dirty-green for the marginal zones and yellowish in the central parts).

Fig. 2. Microstructure of kimberlitic rocks. A – Kimberlitic lapilli-ash crystallolithoclastic tuff from the José Milhudo pipe with fragments of kimberlitic autoliths (K) and quartz (Q) grains in a saponite matrix. Sample #JM/1; cross-polarised light; scale bar is 0.5 mm. B – Moderately sorted kimberlitic tuff from the Young pipe with a fine-grained lapilli-ash, litho-crystalloclastic texture. Top, right: pyrope grain with a kelyphitic rim (Py). Sample #Y/1; cross-polarised light; scale bar is 0.5 mm. C – Kimberlitic litho-crystalloclastic ash tuff from the Redondão pipe with numerous quartz grains (Q) and a small kimberlite fragment (K) with an ‘atoll-like’ zoned structure. Sample #R/1; plane polarised light; scale bar is 0.2 mm. D – Autolith from the Redondão pipe. A large pseudomorph consists of tobacco-coloured, lamellar saponite aggregates. Sample #R/2a; plane polarised light; scale bar is 0.5 mm.

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2. Small (0.1–0.7 mm), idiomorphic, subidiomorphic or irregularly shaped grains, completely altered with a homogeneous, greenish-grey or pale yellowish-grey coloration. These represent phenocrysts of second- and/or third-generation olivine (olivine-2 and olivine-3). Many olivine-2 and olivine-3 grains contain inclusions of brown spinel (the Redondão pipe) or idiomorphic microcrystals of yellowish-brown rutile (the José Milhudo pipe). In the Redondão pipe, olivine-2 and -3 strongly predominate over olivine-1, whereas in the Fazenda Largo kimberlites the reverse is true. Olivine grains are predominantly completely replaced by a finelamellar or fine-scaly aggregate of light-tobacco-coloured saponite, or in some cases with a relict parquet-like structure, typical of kimberlite-hosted serpentinised olivine. More rarely, microcrystalline carbonate aggregates occur in the cores of pseudomorphs after olivine. In the Domingo pipe, the central parts of large olivine-1 grains are replaced by a lamellar aggregate of a dirty-green-coloured clay mineral with low interference colours. According to X-ray diffractometry, this may be volkonskoite (Cr, Fe, Al)4 [Si4O10] [OH]8. In the Young pipe, olivine-1 grains commonly show a concentric zonal replacement pattern (from core-to-margin): lamellar saponite – lamellar serpentine – lamellar talc – scaly talc (in some cases, only the outer part of these zones is present) (Fig. 2B, centre). In this pipe, small grains of olivine-2 and -3 are, for the most part, replaced by a scaly (or more rarely, lamellar) aggregate of talc, not uncommonly occurring with serpentine and carbonate in the central parts of grains. Many of the olivine grains are surrounded by thin films of solidified melt, and occur as intergrowths with ‘dust-like’ opaque minerals. They thus have a ‘transitional’ state of aggregation, intermediate between crystalloclasts and lithoclasts. In addition to olivine crystalloclasts, the Redondão and the Young kimberlites contain a minor amount (0.2–1 vol.%) of chloritised (sometimes only partially) phlogopite laths, some of which are intergrown with olivine. Lithoclasts (5–25 vol.%) in the kimberlites vary in size (0.1– 8 mm, occasionally up to 15 mm) and in shape (oval or subangular, with uneven outlines). The majority of these are pseudomorphs after olivine (similar in morphology and replacement pattern to the olivine-1, -2 and -3 crystalloclasts described above), enclosed in rims or thin films of apohyaline groundmass.

Fig. 3. Kimberlite indicator minerals from the Redondão pipe (sample #R/1): purplish-red pyrope, picroilmenite (angular grain fragments with leucoxene rims), idiomorphic chromian spinel crystals and single grains of emerald-green chromian diopside. Scale bar is 0.5 mm.

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More rarely occurring are autoliths (commonly 2-4 mm, but locally up to 6 cm in size) representing fragments of porphyritic kimberlite, that hosts phenocrysts consisting of saponite pseudomorphs after olivine (Fig. 2A and B). The autoliths are also surrounded by an apohyaline groundmass. It was not possible to identify some of the lithoclasts. These likely comprise apohyaline, replaced by fine-scaly aggregates of saponite, that contain disseminated perovskite, replaced by leucoxene and octahedrite (0.01-0.04 mm segregations), and partially oxidised opaque minerals (chromian spinel and magnetite grains of 0.002-0.03 mm in size) which occur as idiomorphic crystals, some of which have well defined, characteristic, ‘atoll-like’ zoned structures (Fig. 2C). Autoliths are much fresher than host kimberlitic rocks (Fig. 2D). They contain irregular, subangular olivine-1 porphyritic phenocrysts and small, subidiomorphic olivine-2 and olivine-3 grains which are completely replaced by light green, lamellar serpentine aggregates, locally with peculiar ‘loop’-shaped or parquet-like replacement structures. In some of the larger pseudomorphs, the central parts of the ‘loops’ consist of tobacco-colored, lamellar saponite aggregates. The autolith matrix consists of a poikiloblastic carbonate aggregate with disseminated, small, idiomorphic, opaque minerals and apatite grains. In a few lithoclasts, the groundmass contains up to 10% small (0.05–0.1 mm), chloritised mica laths. A peculiar group of lithoclasts from the Young pipe includes 3– 5 mm (locally up to 25 mm) fragments of well sorted, fine-grained

Fig. 4. A large inclusion of serpentinised pyrope dunite in kimberlitic tuff from the Young pipe (sample #Y/2 k). A – General appearance. Reflected light. Scale bar is 1 cm. B – Structural detail. The rock consists of completely serpentinised olivine and large, irregular, subangular inclusions of purplish-red pyrope with thin, brown, cross-radial kelyphitic rims. Plane polarised light. Scale bar is 0.5 mm.

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sandstone (with a sand grain-size of 0.2–0.4 mm, locally up to 0.6 mm), consisting of subidiomorphic and oval olivine grains (replaced by talc) which are cemented with carbonate, locally exhibiting a crustification structure. Xenogenic material consists of irregular fragments of altered claystone, siltstone, sandstone, and, more rarely, crystalline schists and granitoids, 0.2–25 mm in size (1–8 vol.%), and grains of quartz (Fig. 2A and C) and feldspar, 0.1–0.5 mm in size (20–60 vol.%). Deep-seated material occurs in sizable amounts (1–2 vol.%) only in the Young kimberlite. It is dominated by pyrope grains of red, pale red, pale violet, purplish-red and orange colour, 0.4–3 mm in size, some of which have kelyphitic rims (Figs. 2B and 3). In addition, there occur intergrowths of chromian diopside with pyrope and with olivine, and irregularly shaped magnesian ilmenite (picroilmenite) grains bearing leucoxene rims. Microxenoliths of pyrope peridotite also occur. In the José Milhudo kimberlite, microxe-

noliths of up to 1–3 mm in size were also identified. Among these are pyrope grains with bright green rims of amphibole (fragments of amphibolitised pyrope peridotite), garnetised spinel peridotite, ilmenite peridotite fragments, and intergrowths of pyrope, chromian diopside and chromian spinel. In addition, several large pyrope dunite inclusions with coarsegrained, crystalline structures were identified in the Young pipe. One of these inclusions, 7 cm in size, is talcisized, while another inclusion, 11 cm in size, is serpentinised (Fig. 4). Rock groundmass (17–40 vol.%) is of basal (the Redondão pipe) or porous (Fazenda Largo) type. It consists of a fine-scaly aggregate of saponite with minor admixtures of finely dispersed iron hydroxides. Most probably, the groundmass comprises argillised, fineash-sized volcanic material, with a common pattern of replacement by saponite for both the volcanic inclusions and the tuff matrix.

Fig. 5. Cr2O3 vs. CaO (wt.%) compositional plot for pyrope garnets from the Fazenda Largo kimberlites: A – from the Redondão pipe, B – from the José Milhudo pipe, C – from the Domingo pipe, and D – from the Young pipe. Lherzolitic trend, bordered with a dotted line, after Sobolev (1971) and Sobolev et al. (1973); G-10/G-9 discrimination solid line after Gurney (1984).

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5. Mineralogy and mineral chemistry

5.2. Kimberlite indicator minerals (KIMs)

5.1. Bulk mineral composition

Deep-seated, or ‘kimberlite indicator minerals’ (KIMs) are the most important in the search for and characterisation of kimberlites. They include pyrope garnet, picroilmenite, chromian spinel, and chromian diopside (Fig. 5). Among KIM, the following three groups of minerals are present (following the classification proposed by Kaminsky et al. (2002)):

According to microscopical study and X-ray diffractometry data, among the minerals comprising the Fazenda Largo kimberlites, post-magmatic mineral phases predominate, such as saponite (predominant), carbonate (calcite and dolomite), talc, octahedrite, hematite, hydromica, chlorite and serpentine. The remainder of the minerals comprises terrigene, predominantly sedimentary quartz material and primary kimberlitic minerals. Saponite in rock samples from different pipes is of uniform composition, with the exception of that in the Domingo pipe, where this mineral has a high Cr content and likely is identified as volkonskoite. Among other layered silicates, talc (up to 20%), hydromica (up to 6%), and chlorite (up to 5%) were identified. The tuff from the Young pipe comprises up to 40% talc with an uncommon, strongly dispersed structure lacking any preferred orientation. Among other minor epigenetic minerals in the studied kimberlites, are octahedrite (probably developed after perovskite, 2–3%), hematite (up to 2–3% in the Domingo pipe), and calcite and dolomite. A large autolith from the Redondão pipe (sample #R/2a) has a distinctly different bulk mineral composition, with a predominance of serpentine (40%), saponite (24%) and talc (10%). Kimberlitic minerals, including deep-seated ones, comprise approximately 2% and are represented by pyrope garnet, picroilmenite, chromian spinel, and chromian diopside.

Table 1 Mineralogical composition of Fazenda Largo kimberlites (wt.% of heavy fraction). Mineral

Sample, rock R/1 Tuff

JM/1 Xenotuff

D/1 Tuff sandstone

Y/1 Tuff

Magnetite Martite Hematite Iron hydroxide Mn hydroxide Chrome spinel Ilmenite Picroilmenite Garnet Pyrope Clinopyroxene Chrome-diopside Orthopyroxene Hornblende Pargasite Epidote Mica Zircon Rutile Apatite Barite Dolomite Leucoxene Mineral intergrowths, debris

12.4 gr gr 25.0 – 5.4 0.4 2.1 tr 5.2 2.8 tr – 3.6 – 0.1 gr gr tr 0.3 20.7 3.0 0.2 18.8

3.8 – gr 18.5 11.6 7.1 tr 15.8 tr 25.2 0.1 0.7 – – 0.7 gr – gr 0.1 gr – tr 0.1 16.3

5.4 gr gr 16.8 19.9 14.5 tr 11.0 – 29.2 – 3 gr – – – – – gr tr gr – tr 0.3 2.9

0.1 – – tr – 3.5 tr 10.8 – 70.4 – 9.1 gr – 2.0 – gr gr gr gr – tr tr 4.1

Total KIMs Total sample weight, g Weight of heavy fraction, g Weight of heavy fraction per 1 kg of rock, g KIMs abundance, wt.%

100.0 12.7 541.0 5.1 9.4

100.0 48.8 807.0 5.8 7.2

100.0 54.7 657.0 2.2 3.3

100.0 93.8 1280.0 17.9 14.0

0.12

0.35

0.18

1.31

1. Minerals of the ultramafic Cr-association are most abundant. They comprise violet and purplish-red pyrope, chromian spinel, and chromian diopside, with a significant predominance of pyrope. 2. Minerals of the ultramafic Ti-association consist of numerous picroilmenite grains and single grains of orange pyrope. 3. Eclogitic association minerals of the mafic suite occur in very minor amounts. This group consists of rare grains of pyropealmandine, identified only from X-ray spectral microanalysis.

5.2.1. Pyrope garnet Pyrope is the most abundant deep-seated mineral. Its concentration accounts for between 5.2 and 70.4 wt.% (in, respectively, the Redondão and Young pipes) of the heavy minerals (Table 1). Pyrope occurs mostly as angular fragments and fractions of grains and, more rarely, as rounded (oval), full grains. The surfaces of pyrope grains bear discontinuously distributed, Type-1 hydrothermal corrosion-related surface features (primary magmatic microrelief features, after Afanasyev et al., 1979), that are mostly vuggy, comb-undulating, and pitted, forming an irregular surface fabric. Despite the high degree of alteration of the host rocks, pyrope grains are almost free of hypergene alteration, with the exception of corrosion-induced cracking. Only a few pyrope grains have proto-spallation surfaces with a very fine, dissolution-related, microrelief. Some of pyrope grains (up to 3%, for the Young pipe) bear kelyphitic rims. Pyrope grains display a broad spectrum of coloration varieties: yellow–orange and orange (1–3%), reddish-purple (5–10%), purplish-red (75–84%), violet (5–16%), and lilac (single grains). Chemical compositions of the pyrope grains from the four pipes, based upon 250 microprobe analyses, are presented in Table 2 and Fig. 5. In general, pyrope is characterised by a quite constant composition, whereas some oxide contents vary rather widely: Cr2O3 (0.03–10.39 wt.%), CaO (3.82–13.43 wt.%), FeO (5.25–14.25 wt.%) and Mg# from 71.7% to 87.7%. By CaO/Cr2O3 ratio, most of the analysed pyropes fall within the compositional field of the lherzolite paragenesis, as it was determined by Sobolev (1971) and Sobolev et al. (1973) (Fig. 5), predominantly representing a medium-Cr pyrope variety with Cr2O3 contents of <5 wt.% (varying between 2 and 5 wt.%). Only single grains have a higher Cr2O3 content (up to almost 11 wt.%). In general, the analysed pyropes have an above average content of TiO2 (up to 1 wt.%). According to the classification scheme outlined by Dawson and Stephens (1975), among the analysed pyrope grains are the representatives of the following varieties:  Cr-pyrope (G9 group) is predominant and occurs as pale purplish-red and violet grains with variable Cr2O3 (2–7 wt.%) and CaO contents (4–7 wt.%) and a lower than average content of TiO2 (<0.4 wt.%). In the CaO–Cr2O3 diagram (Fig. 5), G9 pyrope forms a linear trend in the compositional field of the lherzolite paragenesis. Only a few figurative points of pyrope from this group, having above average CaO contents (more than 5 wt.%), deviate from the trend into the compositional field of pyrope of the wehrlite paragenesis.

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Table 2 Representative EPMA compositions of pyrope garnet (wt.%). Grain

SiO2

TiO2

Al2O3

Cr2O3

Redondao pipe 7 41.42 14 41.23 25 40.08 31 40.87 36 39.98 55 40.19

0.03 0.04 0.64 0.36 0.23 0.14

20.50 21.81 15.84 22.21 20.77 18.63

4.94 2.96 10.39 1.81 3.18 6.69

Jose Milhudo pipe 2 43.01 8 41.21 20 41.44 23 41.58 32 41.77 47 41.27

0.01 0.23 0.21 0.26 0.02 0.04

20.64 21.78 21.95 21.54 21.41 19.29

Domingo pipe 8 42.21 16 41.32 22 42.27 30 41.01 34 41.35 35 41.42

0.21 0.03 0.05 0.02 0.07 0.35

Young pipe 6 10 37 39 50 69

0.48 0.01 0.09 0.05 0.25 0.25

41.34 41.98 41.02 41.07 41.04 41.63

FeO

MnO

MgO

CaO

Na2O

Total

Mg#

6.04 8.23 6.34 8.43 9.59 7.38

0.23 0.43 0.29 0.34 0.39 0.28

20.81 19.63 18.53 21.11 19.85 19.05

5.40 5.32 7.77 4.47 5.10 6.70

0.05 0.05 0.06 0.00 0.03 0.04

99.42 99.70 99.94 99.60 99.12 99.10

86.0 81.0 83.9 81.7 78.7 82.2

4.19 2.11 2.01 2.59 3.56 5.84

5.48 10.03 8.89 8.48 5.74 7.87

0.25 0.47 0.48 0.38 0.30 0.41

21.82 18.73 19.63 19.85 22.64 18.17

3.99 4.83 5.11 4.88 3.91 6.82

0.04 0.06 0.04 0.07 0.04 0.00

99.43 99.45 99.76 99.63 99.39 99.71

87.7 76.9 79.8 80.7 87.6 80.5

23.58 19.02 21.46 18.64 22.23 19.08

0.19 6.34 3.70 7.09 1.77 5.85

8.62 6.78 7.22 7.30 9.42 6.65

0.32 0.32 0.31 0.40 0.54 0.31

19.77 19.31 20.15 18.33 18.89 20.29

4.47 6.08 4.18 6.93 5.23 5.98

0.01 0.01 0.01 0.01 0.01 0.04

99.38 99.21 99.35 99.72 99.51 99.97

80.4 83.6 83.3 81.8 78.2 84.5

19.71 21.22 17.43 19.28 23.01 22.10

5.69 3.73 8.33 5.90 0.10 1.73

5.96 5.81 6.36 7.58 12.33 8.64

0.28 0.32 0.29 0.42 0.35 0.44

21.11 22.23 19.10 18.99 17.53 20.04

5.20 3.82 7.16 6.67 4.90 4.71

0.07 0.00 0.02 0.00 0.09 0.05

99.84 99.12 99.81 99.96 99.61 99.59

86.3 87.2 84.3 81.7 71.7 80.5

(0.64 wt.%) and low MgO (9.85 wt.%) and Cr2O3 (0.1 wt.%) contents. This group includes single grains of orange garnet.  Pyrope-almandine (G3 group, observed only in the José Milhudo kimberlite) with a high FeO content (14 wt.%), a lower than average MgO content (15.6 wt.%), and a low Cr2O3 content (0.03 wt.%). This group includes single grains of pale orange garnet.  Low-Ca Cr-pyrope (G10 group). Eleven grains, a few of which are included in this group with some degree of uncertainty. Most were identified in the José Milhudo kimberlite. The grains have lower than average CaO contents (3.8–4.1 wt.%) and moderate

 Orange Ti-pyrope (G1 group, observed only in the Redondão and the Young kimberlites) with a moderate Cr2O3 content (0.1–4 wt.%) and an above average TiO2 (0.6–1 wt.%) and FeO content (>10 wt%).  Uvarovite-pyrope (G11 group) with high Cr2O3 and CaO contents, an above average TiO2 content (>0.5 wt.%) and a lower than average FeO content (<6.3 wt.%). This group includes the single deeply coloured grains.  Ti- and Ca-rich pyrope-almandine (G4 group, observed only within the Redondão kimberlite) with high CaO (13.4 wt.%) and FeO (14.1 wt.%) contents, an above average TiO2 content

Table 3 Representative EPMA compositions of clinopyroxene (wt.%). Grain

TiO2

Al2O3

Cr2O3

FeO

MnO

MgO

CaO

Na2O

K2O

Total

Mg#

Redondao pipe 1 52.83 2 53.46 4 53.70 5 53.89 11 54.07

SiO2

0.27 0.05 0.14 0.18 0.27

4.51 1.88 2.82 2.62 2.50

1.72 0.83 2.43 2.45 1.67

1.60 1.43 2.36 2.33 3.08

0.02 0.05 0.07 0.07 0.04

15.01 16.59 16.01 15.70 15.66

21.83 24.39 19.73 19.93 20.57

1.81 0.54 2.31 2.04 1.89

0.00 0.00 0.00 0.00 0.00

99.60 99.22 99.57 99.21 99.75

94.4 95.4 91.7 92.3 90.1

Jose Milhudo pipe 1 52.72 2 54.41 3 53.11 10 54.02 19 52.11

0.36 0.08 0.48 0.27 0.15

4.18 0.83 4.47 2.92 3.75

1.62 2.37 1.96 1.93 1.51

1.44 2.58 1.49 2.75 2.19

0.06 0.10 0.00 0.07 0.08

15.45 15.94 14.36 14.98 16.44

22.29 21.97 21.16 20.82 21.25

1.55 1.60 2.31 2.01 1.66

0.00 0.00 0.00 0.01 0.05

99.68 99.87 99.34 99.78 99.19

95.0 91.7 94.5 90.7 87.6

Domingo pipe 1 54.24 2 54.02 3 53.89

0.26 0.16 0.12

1.18 3.05 1.83

1.43 2.74 2.17

4.05 2.81 3.04

0.05 0.09 0.13

16.08 15.61 16.45

20.99 19.15 19.53

1.30 2.23 1.89

0.00 0.00 0.02

99.59 99.87 99.07

87.1 90.3 90.9

Young pipe 2 5 12 13 14

0.12 0.17 0.28 0.24 0.07

2.72 3.33 0.88 1.04 2.32

2.06 2.88 1.48 1.28 1.65

2.89 2.55 3.07 3.99 2.30

0.09 0.09 0.10 0.10 0.07

15.70 15.26 16.94 15.82 16.25

19.30 18.59 21.00 21.23 21.45

2.23 2.52 1.13 1.32 1.55

0.00 0.01 0.04 0.00 0.00

99.56 99.19 99.81 99.00 99.75

90.6 91.4 90.8 87.6 92.6

54.45 53.79 54.88 53.98 54.09

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Cr2O3 contents (3.6–4.2 wt.%). These grains are characterised by lower than average FeO (<6 wt.%) and TiO2 contents (0.00–0.04 wt.%).  Cr-association pyropes (G9 and rare G10 grains) predominate, while pyrope grains from the different pipes studied, are generally similar to each other (Fig. 5A–D).

5.2.2. Chromian spinel Chromian spinel accounts for between 3.5% and 15.5% of the heavy fraction (in the Young and the Domingo kimberlites, respectively). Chromian spinel grains vary in size from 0.2 mm to 0.7 mm, with the majority falling into the 0.25–0.55 mm size fraction. The following morphological varieties of chromian spinel occur in the examined kimberlites: 1. Flat-faced octahedra with varying degrees of distortion. 2. Octahedra with rare vicinal faces developed instead of some edges and apices. 3. Combination-type, ‘transitional’ (octahedral to myriohedral) crystals with octahedral faces still discernible but occupying less than 30% of the crystal surface area. 4. Myriohedral crystals with numerous vicinal faces (with complex symbols) imparting a rounded shape to them. 5. Fragments and fractions of grains with an irregular shape and uncertain habit. The second and third varieties are most abundant. Many of the chromian spinel crystals have marks of Type-I and Type-II magmatic corrosion (after Afanasiev et al., 2001). Type-I corrosion shows up as parallel-step-like microrelief features devel-

oped at crystal edges and apices. Type-II corrosion is exhibited as a homogeneous surface fabric covering the entire grain surfaces, imparting a dull appearance to them. A minor proportion of the chromian spinel grains (mostly of the third and fourth morphological varieties) have smooth, glancing surfaces. In addition, many chromian spinel grains bear marks of hypergene alteration, manifested as small joints. This jointing occurs both in the outer zones of crystals (forming loose rims around solid cores) and in the entire grain volume. Results of X-ray spectral microanalysis of the chromian spinel grains are given in Table 4 and shown in Fig. 6. In all, 259 grains were analysed. The analysed chromian spinels are characterised by the following compositional features:  A wide variation in Al2O3, Cr2O3, FeO, MgO and TiO2 contents.  A predominance (in some cases, present in equal amounts) of medium-Cr (40–50 wt.% Cr2O3) and high-Cr (50–60 wt.% Cr2O3) chromian spinel varieties, corresponding in composition to Mg-chromite and Mg-alumochromite.  The presence of high-Fe chromian spinel varieties with an above average Ti content (up to 3.65 wt.% TiO2) and lower than average MgO and Al2O3 contents (below 10 wt.%, down to 6.24 wt.% Al2O3), corresponding in composition to ferrichromite.  The presence of low-Cr chromian spinel varieties with an above average Al (up to 35–55 wt.% Al2O3) content corresponding in composition to chrompicotite and picotite.  The presence of two chromian spinel groups with Cr3+ M Al3+ isomorphism (peridotite trend) and Cr3+ M Fe3+, Ti4+ isomorphism (picritic trend), which are typical of chromian spinels of kimberlitic origin.  The absence of diamond association chromian spinels.

Table 4 Representative EPMA compositions of chromian spinel (wt.%). Grain

TiO2

Al2O3

Cr2O3

FeO

MnO

MgO

ZnO

Total

Mg#

Cr#

Redondao pipe 1core 1rim 2core 2rim 3 12 14 16 17 18

3.47 3.17 0.25 0.25 3.63 0.09 0.11 0.47 3.34 1.60

11.90 12.86 16.75 17.04 6.24 36.87 35.74 17.06 12.16 9.34

49.89 52.01 44.14 47.05 51.34 27.30 32.68 48.24 48.12 42.61

18.67 18.37 22.71 22.39 26.48 16.42 15.14 18.22 19.99 36.56

0.18 0.20 0.23 0.13 0.32 0.25 0.19 0.21 0.23 0.36

15.05 12.41 14.33 12.24 10.65 17.91 15.20 15.02 14.95 7.95

0.00 0.10 0.08 0.12 0.16 0.00 0.37 0.06 0.14 0.23

99.16 99.12 98.49 99.22 98.82 98.84 99.43 99.28 98.93 98.66

59.0 54.6 52.9 49.3 41.7 66.0 64.1 59.5 57.1 27.9

73.8 73.1 63.9 64.9 87.4 33.2 38.0 65.5 72.6 75.4

Jose Milhudo pipe 4 3.66 5 0.16 11 3.61 50 0.75 58 3.25 60 0.06

12.24 20.86 9.99 19.94 9.49 55.47

48.61 47.06 50.65 45.12 52.10 12.42

24.39 18.31 19.84 17.93 19.82 12.68

0.19 0.24 0.18 0.18 0.26 0.12

9.72 12.42 14.28 15.00 13.85 18.52

0.10 0.13 0.15 0.03 0.00 0.22

98.91 99.18 98.70 98.95 98.77 99.49

41.5 54.7 56.2 59.9 55.5 72.2

72.7 60.2 77.3 60.3 78.6 13.1

Domingo pipe 3 20 22core 22rim 27 28 37 39 50

0.18 0.13 3.77 2.61 0.65 0.34 0.21 4.44 1.62

13.94 23.35 11.50 12.54 16.97 20.94 12.41 9.56 5.60

53.10 46.40 48.68 53.76 48.11 39.20 38.16 52.29 55.97

19.37 13.44 20.57 17.85 18.47 23.44 32.94 17.69 25.64

0.19 0.15 0.17 0.20 0.29 0.17 0.19 0.23 0.29

12.33 15.88 14.13 12.59 14.36 14.42 13.23 14.51 9.85

0.09 0.15 0.01 0.03 0.02 0.10 0.05 0.00 0.15

99.20 99.50 98.83 99.58 98.87 98.61 97.19 98.72 99.12

53.1 67.8 55.0 55.7 58.1 52.3 41.7 59.4 40.6

71.9 57.1 73.9 74.2 65.5 55.7 67.3 78.6 87.0

Young pipe 21 48 50 54 58 72

0.28 0.40 4.25 1.36 1.40 0.09

19.04 18.06 18.00 18.01 9.09 9.07

48.17 48.08 38.87 47.89 51.98 60.47

16.47 18.59 22.86 16.30 26.01 16.14

0.20 0.22 0.25 0.14 0.27 0.19

14.94 13.95 14.64 15.35 10.10 13.36

0.00 0.15 0.02 0.01 0.18 0.05

99.10 99.45 98.89 99.06 99.03 99.37

61.8 57.2 53.3 62.7 40.9 59.6

62.9 64.1 59.2 64.1 79.3 81.7

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In each kimberlite, zoned chromian spinel grains were identified. They exhibit a slight core-to-rim increase in Cr2O3 content and decrease in MgO content. 5.2.3. Chromian diopside Chromian diopside occurs in the heavy fraction much more rarely than pyrope (from single grains in the Redondão and the Domingo kimberlites up to 9% in the Young kimberlite). In contrast to the large pyrope grains, many of the chromian diopside grains are small, 0.25–0.5 mm in size. Most of the chromian diopside grains are carbonatised, with carbonate either developed along cleavage fractures or covering the grain surfaces as a carbonate ‘crust’, imparting a whitish shade to the grains. Carbonatisation of chromian diopside results in the formation of pyramidal-columnar microrelief on grain surfaces. The chemical composition of chromian diopside (from 56 microprobe analyses) is presented in Table 3. All analysed chromian diopside grains are characterised by variable but, in general, low Na2O (usually below 2.5 wt.%), Al2O3 (usually below 4.5 wt.%) and FeO (usually below 3 wt.%) contents and high MgO contents (greater than 14 wt.%). The sets of chromian diopside from the four kimberlites compositionally are very similar to each other, with only minor differences. At the same time, chromian diopside from the José Milhudo kimberlite has grains with the lowest concentrations of Al2O3, while chromian diopside from the Young pipe is the richest in terms of Cr2O3 (up to 2.88 wt.%). This may be considered as evidence that these grains formed at different depths, under conditions varying from the low-pressure spinel-pyroxene depth facies to the high-pressure, pyrope depth facies (predominantly grospydite and coesite subfacies). 5.2.4. Picroilmenite Picroilmenite concentration in the Fazenda Largo kimberlites varies from 2.1 (in the Redondão kimberlite) to 15.8% of the heavy fraction (in the José Milhudo kimberlite). Picroilmenite occurs as 0.35–1 mm irregular grains, most of them falling into the 0.5–1 mm size fraction.

Table 5 Representative EPMA compositions of picroilmenite (wt.%). Grain

Fig. 6. MgO vs. Cr2O3 (wt.%) composition of chromian spinel from the Fazenda Largo kimberlites: A – from the Redondão pipe, B – from the José Milhudo pipe, C – from the Domingo pipe, and D – from the Young pipe. Dotted line contours the field of kimberlitic chromian spinels after Sablukov et al. (2000); shaded areas are diamond association composition fields after data by Fipke (1994).

Al2O3

Cr2O3

FeO

MnO

MgO

ZnO

Total

Mg#

Redondao pipe 1 49.86 2 52.41 4 55.47 8 54.81 9 52.20

TiO2

0.10 0.45 0.69 0.40 0.78

2.66 0.12 4.12 2.77 6.25

37.15 36.63 24.87 28.24 26.51

0.34 0.28 0.19 0.28 0.16

9.45 9.64 14.55 13.12 13.17

0.02 0.09 0.03 0.00 0.00

99.58 99.62 99.92 99.62 99.07

31.0 31.8 50.9 45.1 46.8

Jose Milhudo pipe 3 54.33 4 53.20 6 55.85 8 55.58 16 46.93

0.54 0.19 0.57 0.19 0.11

2.79 0.64 1.11 1.05 4.14

27.49 34.84 27.22 30.66 39.49

0.18 0.31 0.22 0.42 0.31

13.35 10.35 14.32 11.30 8.18

0.10 0.00 0.09 0.04 0.10

98.78 99.53 99.38 99.24 99.26

46.3 34.4 48.2 39.3 26.8

Domingo pipe 1 54.95 3 55.05 5 54.53 14 49.09 15 55.32

0.63 0.24 0.28 0.48 0.48

0.39 2.06 2.71 0.83 1.49

30.13 29.19 28.40 39.46 27.78

0.34 0.34 0.28 0.27 0.33

13.14 12.87 13.39 8.31 13.74

0.07 0.00 0.00 0.11 0.00

99.65 99.75 99.59 98.55 99.14

43.5 43.7 45.6 27.2 46.6

Young pipe 1 50.06 6 44.71 7 54.35 16 51.11 17 48.97

0.64 0.19 0.24 0.13 0.22

2.87 0.49 0.78 1.24 1.26

35.54 47.90 31.58 36.76 41.62

0.23 0.28 0.33 0.33 0.33

9.60 5.28 11.35 9.72 7.11

0.00 0.01 0.00 0.02 0.01

98.94 98.86 98.63 99.31 99.52

32.4 16.4 38.8 31.9 23.2

F.V. Kaminsky et al. / Journal of South American Earth Sciences 28 (2009) 288–303

297

Most of the picroilmenite grains are angular fragments of subangular grains with some surface areas having granulose microrelief features. Polygonal fragments of picroilmenite grains with an ‘aggregate-like’ (fractured) structure are minor. The usual feature of picroilmenite is the presence of leucoxene rims on most of the grains (Fig. 3). These rims are developed upon the granulose surface microrelief, replacing very small (hundredths of a millimetre) octahedral crystals (probably of titanomagnetite). Results of the X-ray spectral microanalysis of picroilmenite are given in Table 5 and shown in Fig. 7. In all, 71 grains were analysed. In general, picroilmenite is characterised by a rather wide variation in MgO (7–15 wt.%) and TiO2 content (45–55 wt.%), which is typical of kimberlite-hosted Mg-ilmenites worldwide. Cr2O3 content of picroilmenite also varies rather widely (0.12–6.22 wt.%), with a few grains having an above average Cr2O3 content (more than 4 wt.%). Al2O3 and MnO content of the picroilmenites are generally moderate (below 1 wt.%), which is also typical of kimberlitic picroilmenite. Picroilmenite from all pipes studied are quite similar in terms of composition, with the exception of those from the Young kimberlite, which is significantly poorer in chromium and magnesium (Fig. 7D). Figurative points of picroilmenite grains from the Young kimberlite in Fig. 7D form a narrow zone stretched along the MgO axis, by contrast to figurative points of picroilmenite form other examined pipes, which form equidimensional zones in the higher-Mg and higher-Cr part of the diagram. 5.2.5. Rutile In the José Milhudo kimberlite, in addition to the above-mentioned KIM, ‘magmatic’ rutile can also be included in this group of minerals. ‘Magmatic’ rutile differs from xenogenic rutile primarily in its morphology: it mostly occurs as irregular grain fragments (>0.25 mm size fraction) that are black in colour with a bright red reflection from granulose spalled surfaces. The lustre of these rutile grains is pitchy, sometimes metallic; small fragments, when viewed in powder under a microscope, are almost transparent, with a diamond-like luster, while grain surfaces bear very thin leucoxene films (consequently, this rutile variety may be mistaken for picroilmenite). Most probably, this rutile is genetically related to deep-seated rock xenoliths. Similar rutile grains have been previously identified in the Grib kimberlite (Arkhangelsk Province), where they occur in association with picroilmenite (S. Sablukov, unpublished data). 5.2.6. Local features In addition to these common features, some of the studied kimberlites have their own mineralogical features:  The Young kimberlite is characterised by higher concentrations of KIM than other kimberlites.  The Domingo kimberlite is characterised by a higher concentration of chromian spinel xenocrysts and a very low chromian diopside content, compared to the other kimberlites.

6. Chemical composition of Fazenda Largo kimberlites 6.1. Preliminary notes

Fig. 7. TiO2 vs. MgO (wt.%) composition of picroilmenite from the Fazenda Largo kimberlites: A – from the Redondão pipe, B – from the José Milhudo pipe, C – from the Domingo pipe, and D – from the Young pipe. Solid line borders kimberlite composition field after Wyatt et al. (2004).

Petrographic examination of the Fazenda Largo kimberlites revealed that they are largely contaminated by terrigene quartz material and almost completely argillised and talcisized, under crustal weathering conditions. As a result, the present chemical characteristics of these rocks only reflect, to a minor extent, the

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initial magmatic composition. As such, any attempt to use traditional petrochemical calculation methods and/or diagrams would prove ineffective in the identification of rock types and their probable relationship to kimberlites. However, previous geochemical studies of kimberlites with varying degree of alteration have shown that there is a small group of elements, the concentration of which, in kimberlites, is almost independent of the degree of secondary alteration of the kimberlite rocks. This group includes the so-called ‘index elements’: Fe, Ti, Al,

Cr, Na, K and Zr (Milashev, 1965), and ‘index ratios’, such as MgO/ FeO, in addition to some other elements with low mobility under hypergenic conditions (Nb, Ta, Zr, and some others). In this study, we could rely only upon the distribution of precisely these major and minor elements in the rocks being studied. For a more accurate assessment of the geochemical characteristics of the studied kimberlite rock samples, however, we have chosen to additionally analyse a number of autolith samples recovered from the rocks, reasoning out that the composition of the autoliths

Table 6 Chemical composition of Fazenda Largo kimberlites. Componenta

R/1a Autolith

R/2a Autolith

JM/1 Xenotuff

JM/1a Autolith

D/2 Xenotuff

D/1a Autolith

Y/1 Tuff

Y/1a Autolith

Major elements (wt.%) 55.75 SiO2 TiO2 0.62 3.39 Al2O3 4.65 Fe2O3 FeO 0.24 MnO 0.14 MgO 15.53 CaO 2.77 0.06 Na2O 0.33 K2O 0.27 P2O5 LOI 15.93 Total 99.68 – 8.91 H2O 4.65 H2O+ 2.29 CO2 Stot <0.05

– 1.20 – – – – – – – – – – – – – – –

33.38 1.42 2.71 7.64 1.19 0.22 26.74 7.80 0.05 0.21 1.29 17.00 99.65 3.05 8.55 5.17 <0.05

61.88 1.25 4.38 6.62 0.28 0.19 10.80 1.08 0.06 0.57 0.26 12.61 99.98 7.47 4.90 0.21 <0.05

44.97 2.02 4.78 9.73 0.27 0.23 18.11 0.99 0.07 0.43 0.70 17.65 99.95 9.60 7.66 0.29 <0.05

54.24 1.19 6.39 11.54 0.14 0.22 7.29 0.27 0.07 0.50 0.03 18.56 100.44 10.27 7.70 0.40 <0.05

– 4.71 – – – – – – – – – – – – – – –

48.79 0.60 2.18 5.19 1.34 0.21 25.77 2.41 0.10 0.39 0.18 12.20 99.36 3.61 5.37 3.17 0.08

– 1.59 – – – – – – – – – – – – – – –

Trace elements (ppm) Be 0.706 Sc 6.43 Ti 3306 V 63.9 Cr 539.6 Mn 207.7 Co 34.46 Ni 569.8 Cu 18.2 Zn 37.5 Ga 5.02 Rb 19.62 Sr 147.7 Y 9.62 Zr 78.77 Nb 52.5 Mo 0.392 Cs 0.707 Ba 1949.0 La 42.0 Ce 78.9 Pr 8.60 Nd 30.8 Sm 4.49 Eu 1.33 Gd 3.53 Tb 0.459 Dy 2.06 Ho 0.365 Er 0.900 Tm 0.123 Yb 0.783 Lu 0.118 Hf 2.10 Ta 3.07 W 0.873 Tl 0.103 Pb 6.01 Bi 0.064 Th 7.00 U 1.06

1.358 14.91 7206 165.4 1306.1 460.3 58.38 630.3 30.1 65.6 6.51 22.75 219.2 13.85 158.09 170.0 0.695 0.545 1513.3 48.5 83.8 9.04 32.5 5.07 1.49 4.17 0.572 2.76 0.522 1.286 0.184 1.175 0.163 3.73 9.46 1.489 0.141 16.57 0.096 19.01 2.80

1.117 15.83 7268 101.6 1773.8 943.2 78.19 1013.2 37.2 59.1 4.26 12.74 793.1 13.84 165.15 199.1 5.138 0.434 811.9 67.4 117.0 11.77 41.5 6.39 1.85 5.25 0.688 3.10 0.530 1.188 0.157 0.934 0.138 3.59 12.18 0.743 0.739 6.00 0.037 24.97 3.95

2.731 9.67 6626 63.5 711.8 319.2 47.50 3821.4 25.9 157.0 6.51 28.52 24.4 20.63 172.42 186.7 1.553 2.419 214.2 75.0 124.7 14.53 52.5 7.47 1.79 6.13 0.776 3.44 0.623 1.509 0.195 1.173 0.181 4.38 24.85 0.992 0.178 4.57 0.070 12.35 1.93

7.571 18.81 10282 89.3 1550.2 536.6 60.44 6771.6 41.7 392.3 11.68 21.32 93.4 43.15 293.35 288.3 2.760 1.012 469.3 234.9 339.2 42.73 148.7 19.73 4.56 15.41 1.852 7.28 1.226 2.828 0.341 1.960 0.287 7.94 17.86 1.651 0.106 9.21 0.124 37.24 4.27

6.957 14.52 6481 82.0 1389.4 965.5 86.52 2757.8 58.0 116.2 10.05 28.70 25.5 35.65 182.96 104.9 3.085 1.573 361.3 164.6 190.6 26.05 92.9 13.06 3.06 11.46 1.393 6.24 1.129 2.679 0.336 1.989 0.294 4.83 6.57 2.015 0.354 8.77 0.096 14.84 2.78

6.729 26.90 28231 120.4 4966.8 543.0 41.77 3289.0 83.3 196.6 11.35 6.39 24.7 26.39 488.15 536.1 8.244 0.244 193.0 75.8 87.7 13.92 49.6 7.78 2.01 7.24 0.964 4.89 0.927 2.238 0.282 1.680 0.234 15.08 33.27 3.746 0.113 5.52 0.121 69.55 6.32

0.418 6.03 3163 67.5 971.7 630.0 70.31 982.5 16.7 45.1 3.11 20.32 291.6 5.66 77.11 54.3 1.216 0.761 526.1 35.9 63.0 6.68 23.1 3.16 0.82 2.42 0.306 1.29 0.210 0.505 0.068 0.398 0.060 1.93 3.20 0.342 0.092 3.13 0.026 6.75 1.41

0.601 17.24 9522 113.2 2216.0 514.8 99.55 1233.5 44.2 71.5 5.74 11.97 303.8 8.11 242.49 208.3 2.234 0.475 319.6 107.0 197.4 18.91 60.9 7.26 1.62 5.34 0.643 2.31 0.329 0.665 0.081 0.468 0.062 6.09 12.31 0.923 0.153 4.80 0.050 26.64 4.53

a

R/1 Tuff

Major oxides – wet silicate analysis, trace elements – ICP-MS analysis.

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must closely represent the unaltered magmatic component of these kimberlite rocks. As such, two silicate analyses and five ICP-MS analyses have been carried out. Thus, we examined the geochemical characteristics of the studied samples taking into account both data on bulk rock composition and data on the chemical composition of the autoliths present in these rocks. 6.2. Major elements Major components content of the analysed samples determined by chemical analysis is presented in Table 6. Chemical peculiarities of the samples studied, in general, agree with their petrographic and mineralogical features. In particular, all samples show a much higher SiO2 content than is usual for kimberlite. However, while the volcano-sedimentary rocks (tuffs and xenotuffs) of the Redondão, José Milhudo and Domingo pipes correlate well with the high degree of rock contamination by terrigene quartz material, the entirely magmatic tuffs of the Young pipe have an extremely high SiO2 content (49 wt.%, which is very high for a kimberlitic tuff) for a very different reason. Namely, this chemical trait reflects the almost complete talcisization of the rocks, requiring a strong influx of silica likely from a magmatic source during post-magmatic and hypergene rock alteration. Further, the Al2O3 content of the studied samples is generally only slightly elevated above average values for kimberlite, with the exception of xenotuffs from the Domingo pipe (6.39% Al2O3). In these rocks the xenogenic admixture consists predominantly of clayey siltstone and claystone xenoliths rather than of quartz material. Fe and Ti contents of the studied Fazenda Largo kimberlites are moderate (4.43–6.24% FeOtot and 0.60–1.25% TiO2), with most iron occurring in its ferric state (Fe2O3/FeO = 19–82; only the Young pipe tuffs have Fe2O3/FeO = 3.9), highlighting the high degree of oxidation of the analysed rocks, due to hypergene transformation. Again, only the Domingo pipe xenotuffs exhibit some differences; these have an above average iron content (10.53% FeOtot). In spite of having unfavourable structural characteristics (see above), the rocks being studied have a very high Mg index (Mg# = 75.5–88.4; only in the case of the Domingo pipe rocks is Mg# lower = 55.2), which is typical of kimberlites. Alkali content in the rocks is very low, with a sharp (4–9 times) predominance of K over Na (0.33–0.57% K2O and 0.05–0.10% Na2O). CaO content of the rocks is moderate (up to 2.7% in Redondão pipe), with all Ca and some Mg occurring within the carbonate component of the rocks (there are no Ca-silicates), which again is a typical feature of kimberlites. Autoliths differ from their host volcano-sedimentary rocks by having higher concentrations of magmatic, petrogenic elements (Fe, Ti, Mg, and P) and a lower SiO2 content, thus generally having a more ‘typical’ kimberlitic composition. Accordingly, the correlation of chemical composition of the studied Fazenda Largo kimberlites to a certain rock type is most pronounced in the autolith inclusions (which is not surprising), with the autolith composition generally corresponding to the composition of Group 1 South African kimberlites (Smith et al., 1985) and Fe–Ti series kimberlites of the Arkhangelsk diamond province (Sablukov, 1995).

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6.3.1. Compatible trace elements Of great interest is the distribution of compatible trace elements and, primarily, of the ‘ultramafic’ compatible elements, namely, Ni, Co and Cr. The concentrations of these elements in the rocks studied are high to very high, which is typical for kimberlitic rocks. Moreover, while Ni and Cr contents of autoliths from the Redondão and Young pipes (respectively, 630–1233 ppm and 1306– 2216 ppm) are within the limits of values typical for kimberlite, Ni content of the José Milhudo and Domingo kimberlites (3289– 6771 ppm) and Cr content of Domingo kimberlites (4966 ppm) greatly exceed values reported for kimberlites (Mitchell, 1986). This may reflect the effects of weathering under crustal conditions, which could have promoted Cr and Ni accumulation and the development of Cr-saponite (volkonskoite, as detected by X-ray diffrac-

6.3. Trace elements Trace element contents of the studied kimberlite samples, as determined by ICP-MS analysis, are given in Table 6. An assessment of the specific behaviour of different trace elements, in the rocks being studied, must be performed taking into account their very different mobilities in metasomatic and hypergene rock transformation processes (e.g., Rae et al., 1996).

Fig. 8. Chemical composition of the Fazenda Largo kimberlites. A – Sc vs. Ta; B – Zr vs. Nb; C – La vs. Th. Solid diamonds – kimberlites; open diamonds – autoliths. Field limits for kimberlites – after data by Smith et al. (1985) and Sablukov (1995), for lamproites – after data by Jaques et al. (1986).

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tometry in the Domingo kimberlite) and Ni-nontronite, in these rocks. The concentrations of V and Sc (‘mafic’ compatible elements, usually occurring in elevated concentrations within basalt, according to Sun and McDonough, 1989) in the studied Fazenda Largo kimberlites and in their autoliths are moderate to above average (63.5–165.4 ppm and 6.03–26.9 ppm, respectively), which agrees with varying iron content in these rocks (because V and Sc are geochemically cognate with Fe). In the rocks being studied, both ‘ultramafic’ and ‘mafic’ compatible trace elements show concentration predominantly in the autoliths (relative to the host volcanoclastic rocks); however this is based on a limited number of autoliths (two) analysed. 6.3.2. Incompatible trace elements The examined kimberlites are characterised by heterogeneous but, in general, moderate to above average (for kimberlitic series

rocks) concentrations of incompatible elements, which is evident from Fig. 8. The concentrations of LILE are close to or just below average for kimberlitic series rocks, in particular, rare alkaline elements contents: Rb (6.39-28.7 ppm), Cs (0.43-2.42 ppm), and alkaline-earth elements contents: Sr (less than 303 ppm), Ba (526 ppm). Only in the Redondão pipe tuffs is the Ba content much higher than average (up to 1949 ppm), reflecting the high proportion of barite in this sample; in a carbonatised autolith from the same pipe (sample #R/2a), Sr content is also well above the average (793 ppm). The concentrations of high-charge elements (HFSE) is variable, being mostly moderate, with the only exception those of an autolith from the Domingo pipe (sample #D/1a): Zr (79–293 and 488 ppm, respectively), Hf (1.93–7.94 and 15.01 ppm), rare metals Nb (52–288 and 536 ppm) and Ta (3.1–24.9 and 33.3 ppm), and Ti which is usually related to these rare metals (0.6–2.02 and 4.71 wt.% TiO2); radioactive elements Th (6.8–37.2 ppm and 69.6 ppm) and U (1.6–4.53 ppm and 6.32 ppm) and, lastly, LREE.

Fig. 9. Chondrite normalised (after McDonough and Sun, 1995) REE distribution patterns for the Fazenda Largo kimberlites: A – Individual analyses. Kimberlites – dotted lines with solid marks, autoliths – solid lines with open marks; B – Range of REE normalised concentrations (shaded) against average concentrations of REE in kimberlites and lamproites (after Rock (1991)).

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The concentrations of REE in the kimberlitic rocks studied is moderate to high, with REEtot = 138–515 ppm (in the autolith from the José Milhudo pipe, sample #JM/1a, REEtot is as high as 821 ppm, probably due to the presence of apatite and perovskite replaced by octahedrite). The plots of REE distribution (normalised to chon-

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drite) of the analysed rocks with differing structural characteristics are quite similar (Fig. 9A). They have a steep slope in the range of LREE (which reflects their enrichment) followed by a gentle slope in the range of HREE (starting with Ho). The autoliths’ samples are located in the upper part of the plot, while the kimberlitic

Fig. 10. Composition of the Fazenda Largo kimberlites: A – Primitive mantle normalised (after McDonough and Sun, 1995) and plotted from left to right in order of increasing compatibility in a small fraction melt of the mantle; B – Fazenda Largo kimberlites, normalised to average kimberlite (after Rock (1991)). Kimberlites – dotted lines with solid marks, autoliths – solid lines with open marks.

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samples trend to its lower part reflecting the contamination of epiclastic rocks and their secondary alterations. All examined rocks, except for those of the Redondão pipe, have a well defined negative Eu-anomaly (that is relatively small in the case of rocks from the Young pipe). Domingo and José Milhudo kimberlites have an additional negative Ce-anomaly, which may be due not only to intense hypergene transformation (i.e., oxygen fugacity) of the rocks, but also to certain variations in the primary composition of kimberlites, either of which that may have favoured Ce in its divalent state. In general, the Fazenda Largo kimberlites are similar in terms of REE distribution to kimberlites from different worldwide diamond provinces and even to lamproites, according to Rock (1991) (especially true for the autolith sample from the José Milhudo pipe) (Fig. 9B). It is noteworthy that, in the majority of the rocks studied, some of the incompatible elements are accumulated in autoliths relative to host volcanoclastic rocks (e.g., HFSE and LREE, with the exception of those in the Domingo pipe), whereas other elements tend to concentrate in the host rocks (alkaline elements). Again, this likely relates to the conditions under-which the rocks experienced alteration and/or weathering. 6.4. General In general, the studied Piauí kimberlitic rocks are characterised by moderate to higher than average concentrations of ‘ultramafic’ (Co, Cr, and particularly, in some samples, Ni) and ‘mafic’ (Fe, V, and Sc) compatible trace elements, as well as of most incompatible elements, with the exception of the alkali elements (Fig. 10). Besides alkalis, Sr shows very low concentrations as compared with both average kimberlite and primitive mantle values. There are no significant differences between the compositions of kimberlites and autoliths; only pairs of incompatible elements, Th–U, Nb–Ta and Hf–Zr exhibit depleted concentrations in kimberlites, reflecting their mobility during the course of secondary alterations of epiclastic kimberlitic rocks (Fig. 10A). The significantly above average concentrations of some elements (Ni, Cr, Sc, Ti, Nb, Ta, U, and Th in Domingo kimberlite, Ba and Sr in Redondão kimberlite, and REE in José Milhudo kimberlite) may be due to some specific features in the primary composition of the kimberlitic magma, alternatively, this may be related to intense alteration. The overall distribution of elements in the Fazenda Largo kimberlites is shown in Fig. 10. As can be seen from the figure, the kimberlitic rocks are similar to average kimberlite (Rock, 1991). Prominent negative peaks for the alkaline elements and for Sr, and positive Ni and Cr peaks in some samples might reflect redistribution of elements during intense hypergene transformation. In terms of chemical composition, the Fazenda Largo kimberlites are most similar to Fe–Ti series kimberlites of the Arkhangelsk diamond province (Sablukov, 1985; Sablukov et al., 2002).

7. Diamonds ‘from the Young pipe’ Diamonds reportedly from the Young kimberlite are a colourless, partly broken, dodecahedroid (28 mg) and a combination-type twin (ocatehedron + dodecahedroid) crystal (31 mg). The most characteristic feature of these crystals is the presence of rhombic patterns developed on their surfaces which may be considered as the initial stage of polished surface. These patterns are formed by numerous, tiny, surface, linear and crescentiform cracks (Fig. 11A). In addition, there are natural, fresh spallation surfaces on the crystals (Fig. 11B). These features point to a complex, post-genetic history for the stones: after erosion of the primary source (kimberlite?), these

Fig. 11. Diamond #1. A – A web of rhombic patterns with crescentiform cracks (shown by arrow) on the primary surface. A dark-brown spot of pigmentation is also present. Scale bar is 0.1 mm. B – A fresh, step-like spallation surface on the front of the crystal. Scale bar is 1 mm.

crystals were deposited within a coastal-marine environment where they gained (as a result of polishing) the surface rhombic patterns. The stones were then preserved in an ancient intermediate collector where they underwent high temperatures and gained brown pigmentation spots, and afterwards were transported into the alluvial environment where they were partly broken and obtained fresh spallation surfaces. Hence, the submitted diamond crystals were recovered not from kimberlites but, most likely, from alluvial sediments. They are similar to stones from the alluvial deposits of the Riacho do Contrato and the Rio Piripiri basins in the vicinity of the studied Fazenda Largo kimberlites (Kaminsky and Zakharchenko, unpublished data).

8. Conclusions The Fazenda Largo kimberlites are intensely weathered, almost completely altered (argillised or talcisized) rocks with a finegrained, clastic structure, containing variable amounts of terrigene admixture of quartz sand. These rocks represent near-surface volcano-sedimentary deposits from the crater parts of kimberlite pipes.

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Abundance of saponite among secondary minerals is the characteristic of kimberlites from Minas Gerais State, Brazil (F. Kaminsky and S. Sablukov, unpublished data), from Angola (pipes Katoka, Kamafuka, Kamachia and some others), and of kimberlites from the Archangelsk diamond province (Kharkiv et al., 1998). Talcisized kimberlite rocks occur in some kimberlites from the Yakutian diamond province and in South Africa (Kharkiv et al., 1998). The composition of the deep-seated material present within the kimberlites is quite diverse, particularly in the case of the José Milhudo and Young pipe rocks, where among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and (within the Young kimberlite) large xenoliths of pyrope dunite. High-pressure minerals are predominantly Cr-association minerals of the ultramafic suite, according to the classification by Kaminsky et al. (2002) (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Pyrope prevails over picroilmenite. Compositionally, high-pressure minerals are dominated by low-Cr and medium-Cr varieties with a minor proportion of high-Cr varieties. Rocks from all four studied pipes contain rare grains of G10 pyrope (Gurney, 1984) of the diamond association, but chromian spinel of the diamond association was not encountered. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to Group I South African kimberlites (Smith et al., 1985). There are, however, some distinctions between rocks from the different Piauí State pipes. In particular, while idiomorphic olivine-2 and olivine-3 crystals from the José Milhudo pipe rocks frequently contain inclusions of brownish-yellow rutile microcrystals (which is one of the diagnostic characteristics of Fe–Ti series Arkhangelsk kimberlites), in the Redondão rocks these inclusions consist of brown chromian spinel octahedral crystals, which is more typical of the Arkhangelsk Al-series kimberlites. The Fazenda Largo and Redondão kimberlites, by their tectonic position, are typical representatives of off-craton kimberlites. Like most other off-craton kimberlites, they contain mainly low-pressure, low- and medium-Cr KIM grains with only a few G10 pyropes and no diamond-association spinel grains. Based upon their tectonic position, their geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic. Acknowledgements The study was financially supported by BrazDiamond Mining, Inc. The authors are thankful to two anonymous reviewers whose notes helped in improving the manuscript, and to Ian Coulson for his help with editing the English. References Afanasiev, V.P., Zinchuk, N.N., Pokhilenko, N.P., 2001. Morphology and morphogenesis of kimberlite indicator minerals. Novosibirsk, 276pp. Afanasyev, V.P., Kharkiv, A.D., Sokolov, V.N., 1979. Morphology and morphogenesis of garnets from Yakutian kimberlites. Geologia i Geofizika (Russian Geology and Geophysics) 20 (3), 88–99 (in Russian). Almeida, F.R., Castelo-Branco, R.M.G., 1992. Location of kimberlites using Landsat Thematic Mapper images and aerial photographs; the Redondão diatreme, Brazil. International Journal of Remote Sensing 13 (8), 1449–1457. Castelo Branco, R.M.G., 1986. Aspectos geologicos de kimberlites Brasileiros com enfase nas ocorrencias do sudoeste do Estado do Piaui. Revista Escola de Minas 39 (2), 21–26. Castelo Branco, R.M.G., Lasnier, B.M., 1991. Geology and geophysics of the Redondão kimberlite diatreme, Northeastern Brazil. In: Fifth International Kimberlite

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