The occurrence of microdiamonds in Mesoproterozoic Chapada Diamantina intrusive rocks – Bahia / Brazil

The origin of diamonds from Serra do Espinhaço in Diamantina region (State of Minas Gerais) and in Chapada Diamantina, Lençóis region (State of Bahia) remains uncertain, even taking into account the ample research carried out during the last decades. The lack of typical satellite minerals in both districts makes a kimberlitic source for these diamonds uncertain. In mid 18th century the occurrence of a metamorphosed igneous rock composed of martite, sericite and tourmaline was described in Diamantina region and named hematitic phyllite, considered by some researchers as a possible diamond source. Similar rocks were found in Lençóis and examined petrographically and their heavy mineral concentration was investigated by means of scanning electron microscopy (SEM). Petrographic analyses indicated an igneous origin for these rocks and SEM analyses showed the discovery of microdiamonds. Geochronological studies using the Ar/Ar technique in muscovites yielded minimum ages of 1515 ± 3 Ma, which may correlate with 1710 ± 12 Ma from U-Pb method in igneous zircons from the hematitic phyllites. Both rock types also have the same mineral and chemical composition which leads to the conclusion that the intrusive rocks were protolith of the hematitic phyllites. This first discovery of microdiamonds in intrusive rocks opens the possibility of new investigation models for diamond mineralization in Brazilian Proterozoic terrains.


INTRODUCTION
The origin of the presence of diamonds in Proterozoic sedimentary rocks along the Serra do Espinhaço in Minas Gerais and Bahia States has been controversial since their discovery in the 1790's and the pioneer papers from Spix and Martius (1828) and Eschwege (1833).None of the several suggestions to explain the origin of the gem proved to be conclusive.Some authors (Pflug 1965, Chaves 1998, Chaves et al. 1993, 2000, 2001) proposed that diamonds found in Mesoproterozoic conglomerates originated from an covite and martite, which occurs in the Diamantina district in Minas Gerais State and is named hematitic phyllite (Walls 1919, Guimarães 1930, Correns 1932, Moraes and Guimarães 1930, 1931, Moraes 1934, Barbosa 1951, G. Herrgesell, unpublished data, Almeida-Abreu 1996, Almeida Abreu and Renger 1999, 2001).The second hypothesis seems more likely to be confi rmed, because a possible diamond source can be easily found in the diamond district of Diamantina, in Minas Gerais and is usually associated with the main mining activities of the area.
In 1984, Herrgesell (G.Herrgessell, unpublished data) for the fi rst time described rocks similar to the hematitic phyllite of Minas Gerais in Chapada Diamantina in Bahia.The main difference between the Minas Gerais hematitic phyllites and the rocks found in Bahia is the metamorphic grade.In Chapada Diamantina, Bahia, the igneous rocks are intruded in sedimentary rocks of the Proterozoic Tombador Formation.Crystallinity studies carried out by Battilani et al. (2000) in illites from these sedimentary rocks revealed that the maximum temperature reached the anchimetamorphic facies, whereas in the Diamantina region of Minas Gerais the metamorphic grade corresponds to the upper greenschist facies.
In order to obtain more information on these enigmatic intrusive rocks and their potential as the primary sources of diamonds, fi eld, mineralogical, heavy mineral, geochronological and geochemical works were carried out in samples of intrusive rocks from Chapada Diamantina in Bahia.

THE STUDY AREA
The study area is located in the central part of Bahia State, between the geographic coordinates 12 • 00 / 13 • 00 S and 42 • 00 /41 • 00 W. It encompasses a small part of Chapada Diamantina (Fig. 1), and is crossed by the highway that links Salvador to Brasília (BR 242).The main rivers present in the area are the Paraguaçú, Santo Antônio and São José, whose beds have been exploited for diamonds.

REGIONAL GEOLOGY
The Espinhaço Supergroup is ca.1200 km long, cropping out from the Quadrilátero Ferrífero in Minas Gerais to the boundary of Piauí-Pernambuco States.
In Bahia State, the Espinhaço Supergroup is divided into Espinhaço Setentrional and Chapada Diamantina, respectively west and east of the Paramirim River.In the Chapada Diamantina area, the Espinhaço Supergroup is divided, from base to top, into Rio dos Remédios, Paraguaçú and Chapada Diamantina groups (Inda and Barbosa 1978).The schematic representation of the stratigraphic sequence of the Espinhaço Supergroup in Chapada Diamantina was proposed by Rocha (1993) as illustrated on Figure 2. The Chapada Diamantina Group, according to Guimarães and Pedreira (1990), is composed of continental (Tombador and Morro do Chapéu formations) and marine (Caboclo Formation) sequences.The Tombador Formation composed of sandstones, conglomerates and pelites is the basal unit of the Chapada Diamantina Group.The outcropping area exceeds 20,000 km 2 and at least two of its conglomeratic levels are known to be diamond-bearing.
According to Torquato and Fogaça (1981) the metamorphic grade in Chapada Diamantina increases from north to south and does not exceed the greenschist facies, confi rmed by Varajão and Gomes (1998) and Battilani et al. (2000), on the basis of crystallinity studies of illite from the Tombador Formation sediments.
In addition to the sedimentary sequences, dikes and sills of volcanic rocks named muscovite-martite fels (G. A. Battilani et al., unpublished data) and diabase dikes occur associated with basal sediments of the Tombador Formation.Volcaniclastic rocks crop out in the Morro do Pai Inácio, intercalated within Tombador Formation siliciclastic sequences (Battilani et al. 1998).
The Tombador Formation was affected by hydrothermal events that caused intense tourmalinization.Tourmalinite veins that crosscut the sedimentary rocks are up to 30 cm thick and are composed of quartz and tourmaline and subordinate muscovite, rutile, barite and zircon.This hydrothermal event also caused intense alteration and tourmalinization of the rocks that compose the dikes and sills.

MATERIALS AND METHODS
Detailed sampling of sills and dikes, totaling 150 samples for petrographic studies and chemical analyses, was carried out in 90 days of fi eldwork.Approximately 600 kg of fresh igneous rocks were collected from nine outcrops (Fig. 1) for separation of heavy minerals.
The samples were ground in a ball mill and sieved.Fine material under 400# was discarded.Seven fractions (35, 48, 65, 100, 150, 200, 270#) resulted from this fi rst stage of sample preparation and the heavy minerals were concentrated in a KNELSON Concentrator, model KC-MD3, series MD39907504903197, which belongs to the Gorceix Foundation.
After concentration, the material was leached with hot 1M HCl for Fe removal and then dried.Panning and dense liquids (Bromoform, D = 2.85 and 2-iodine methane, D = 3.325 at 99%) were used in the second stage of mineral separation.In the last stage, the heavy minerals were handpicked under the microscope.Whenever identifi cation of heavy minerals was not possible under optical microscope or the stereomicroscope, EDS analyses were carried out in the microanalysis laboratory of the Geology Department of the Ouro Preto Federal University, equipped with a JEOL-JSM - 5510 with a EDS Thermo-NORAN detector.Minerals were fi xed to a sample holder and covered with a thin layer of carbon.The operation conditions were an acceleration voltage of 20kv and a current of 6mA.

THE INTRUSIVE ROCKS
The intrusive rocks or muscovite-martite fels are dense (2.9 g/cm 3 ), fi ne grained, grey and slightly red when altered.In the fi eld, the thickness varies from 0.8 to 30 m (Fig. 3).The NNW-SSE-striking dikes and sills cross- Both on outcrops and under the microscope, spherical structures similar to amygdales were observed.These are fi lled with muscovite or a mixture of muscovite and tourmaline.
An Acad Bras Cienc (2007) 79 (2) Despite an exhaustive search, using petrographic and SEM investigations, only one remnant feldspathoid could be identifi ed from the analyzed samples.EDS analyses of the small crystals within the muscovite aggregates showed the presence of O, Na, Al, Si, S, Cl, K, Ca, which allowed the identifi cation of the feldspathoid remains of the sodalite/lausurite isomorphic series.According to Tröger (1969) all of the sodalite series members alter to sericite under hydrothermal conditions.
Tabular phenocrystals, up to 0.5 mm long, of supposedly former feldspar or feldspathoid were pseudomorphically replaced by fi ne muscovite aggregates.Muscovite occurs in the following forms: 1) concentrations of small tabular shaped crystals oriented according to the preferential-flow direction; 2) randomly distributed crystals, associated or not with tourmaline; 3) euhedral crystals surrounded by iron oxide minerals or rutile; and 4) fi lling pseudo-amygdales, associated or not with tourmaline.
Martite occurs as pseudomorphic hematite aggregates crystals after igneous magnetite.The pseudomorphic crystals with an average size of 0.3 mm, are octahedrical, in part intensely fractured and corroded and associated with muscovite and quartz.The rims of several crystals were altered to limonite.Besides typical octahedral forms, the SEM/EDS analyses showed a close association of martite with rutile, which concentrates at the rims or along cleavage planes of the primary crystals.Rutile was probably generated by hydrothermal alteration of Ti-magnetite.
Tourmaline crystals vary from 0.05 to 0.10 mm in size, are euhedral to subhedral, sometimes zoned, and intergrown with each one.In general, tourmaline is closely associated with muscovite, as well as with opaque minerals or disseminated in the thin section.The largest concentration of tourmaline is found along the weakness directions of the rock as veinlets that can be identifi ed in outcrops.Microprobe analyses showed that the tourmaline composition falls in the schorl-dravite series.
Quartz is probably of secondary origin and occurs as small crystals associated with martite, disseminated together with muscovite, or as veinlets that crosscut the rocks.The rims of the quartz crystals are generally corroded.They are mostly monocrystalline and exhibit straight extinction.
On-going geochemical studies have revealed that the muscovite-martite fels has an alkaline composition typical of intraplate tectonic environments.The following tables show results of geochemical analyses of the intrusive rocks from Chapada Diamantina-BA as well as four samplesizes of the hematitic phyllite from Diamantina-MG.The data were plotted (Figure 4) and is possible to see the similar behavior between the geochemical results from Chapada Diamantina samples and Diamantina samples.

HEAVY MINERALS
The study of heavy minerals of igneous rocks showed the presence of zircon, baddeleyite, Ti and Fe oxides, tourmaline, magnetite, pyrite and two kinds of colorless minerals with adamantine luster, imbricated, or showing quadratic etching fi gures, as well as negative and positive trigons and grooves (Fig. 5).The colorless crystals were selected with a binocular microscope, eliminating the grains with oxide inclusions or with iron coatings.The clear grains without inclusions were selected for the EDS analyses.Lack of impurities was confi rmed by EDS observation.EDS analyses revealed that most of the grains are composed solely of SiO 2 .EBSD analyses allowed the identifi cation of a preserved coesite nucleus in the samples.
EDS analyses showed that fi ve colorless mineral grains from 80 µm to 200 µm in size, are composed of pure carbon (Fig. 5) and were interpreted as microdiamonds.Detailed EDS analyses in fragments of the fresh intrusive rock also revealed the presence of an euhedral mineral with cubic habit, composed solely of carbon (Fig. 6), which was also interpreted as diamond.

DISCUSSION
Field data clearly indicate an igneous intrusive origin of the so called " muscovite-martite fels" (Battilani et al. 2005) because they form subvertical dikes intruding subhorizontal sandstones layers.The sandstones do not display any evidence of metamorphism or deformation as supported by illite crystallinity determinations  I.The sample GCD is the average for all samples from Chapada Diamantina -BA.Major elements (wt %), trace and REE elements (in ppm).Observe that the elements behavior of the Chapada Diamantina samples is similar to the elements behavior of the hematitic philite from Diamantina -MG.  in these rocks, that estimated temperature conditions between high diagenesis to anchimetamorphism.Although the studied intrusive rocks are of Proterozoic age they do not show any evidence of deformation or metamorphism.

An Acad Bras Cienc
The mineral composition of the muscovite-martite fels is incompatible with that of an igneous rock.According to Trögger (1969), hematite does not form at temperatures higher than 600 • C in magmatic rocks, occurring as a product of magnetite autometasomatism under deuteric conditions and is, therefore, named martite and muscovite is totally unstable at temperatures higher than 750 • C.
According to Deer et al. (1992), magnetite is one of the most abundant oxides in igneous rocks and usually occurs as an accessory mineral and only occasionally at higher concentrations.Magnetite in several igneous rocks, and particularly in mafi c rock types, may be enriched in Ti, thus being named titanomagnetite.It is suggested that the martite in the studied rocks is a result of transformation of magnetite, titanomagnetite or both, which is confi rmed by the octahedral shape of the crystals and by the presence of rutile along the rims and cleavage planes of martite crystals.Muscovite is clearly a product of alteration or transformation of feldspar or feldspathoid.
The presence of tourmaline (up to 17%) is another important factor to confi rm the transformation of the primary minerals in the studied rocks.Tourmaline found in the studied intrusive rock samples occurs mainly fi lling microfractures, indicating a hydrothermal origin.The similarity of the chemical composition of the muscovite-martite fels from Bahia and of the hematitic phyllites from Minas Gerais corroborates the supposition that the muscovite-martite fels from Bahia, comprise a non-metamorphic equivalent (protolith) of the hematitic phyllites like those that occur in Minas Gerais, and that both are hydrothermally altered igneous source rocks of the diamonds.Trögger (1969) points out that the reaction necessary for graphite formation occurs only under pegmatitic conditions.Another possibility could be the assimilation of organic carbon by the magma.Besides, the high density of the crystal aggregates and their physical properties (habit, color, luster) lead to the conclusion that the crystals identifi ed are in fact microdiamonds.However, it is possible that these microdiamonds are fragments of larger diamonds or carbonados, generated during the intense grinding stage of sample preparation.Diamond, despite its high degree of hardness, is easily fragmented.
The features observed in minerals composed solely of SiO 2 and C were also identifi ed in diamond microcrystals that form carbonados and in " quartz inclusions" in these carbonados (G. A. Battilani et al., unpublished data).Another on-going study concerns the grains composed exclusively of SiO 2 .Their density is higher than 3.3, which rules out quartz, which has a density of 2.65.Coesite, a quartz polymorph, is dense and typical of rocks that underwent high-pressure conditions.According to Souza (M.M. Souza, unpublished data), coesite has been described in association with diamond-rich deposits in Venezuela.Iwanuch et al. (2005) describe the occurrence of coesite associated with diamond in kimberlite pipes of Juína, Mato Grosso State, Brazil.EBDS analyses showed preserved core of coesite in ten colorless grains that were analyzed in this study.
Fieldwork showed unconformities in the Tombador Formation, expressed by erosive contacts on top of some sills.Erosion and reworking of these microdiamondbearing intrusive rocks can constitute a mechanism of diamond concentration in the Lençóis, Bahia conglomerates.These diamond-rich conglomerates are younger and were not crosscut by intrusive rocks.

CONCLUSIONS
Dikes and sills of muscovite-martite fels were observed restricted at the lower and barren levels of the Tombador Formation, never underlying course clastic diamondbearing levels.
Textural analyses and chemical composition of these dikes and sills in the Tombador Formation, indi-cate an igneous origin.The mineralogical composition although, is supposed to be the result of the action of hydrothermal solutions, also responsible for the high concentration of tourmaline, which reaches 17.6%.
The mineral composition of the intrusive rocks of Chapada Diamantina is identical to the hematitic phyllites of Minas Gerais, suggesting that these phyllites could be a product of regional greenschist metamorphism of similar hydrothermally altered igneous rocks.Geochronological data obtained with the Ar/Ar technique applied to secondary metasomatic muscovites from the intrusive rocks could be correlated with U-Pb data yielded by igneous zircons from the hematitic phyllites.
The presence of upper erosive contacts in intrusive sills suggests evidence of unconformities in Tombador Formation.Erosion and reworking of the intrusive rocks could represent a source of diamonds on the Tombador Formation upper levels.
Micro diamonds would represent pulverized fractions of larger diamonds, carbonados, or both, generated during the mechanical treatment of samples.This occurrence of microdiamonds in the intrusive rocks shades light on new investigation models for diamond mineralizations in Brazilian Proterozoic terrains.
The authors thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for a Ph.D. scholarship, Conselho Nacional de Desenvolvimento Científi co e Tecnológico (CNPq) also for a Ph.D. scholarship, sandwich program (process number 200636/01-4) and Gorceix Foundation, for fi nancial support to this work.

Fig. 1 -
Fig. 1 -Regional geologic map of the Espinhaço Range and the Chapada Diamantina area showing the location of the studied outcrops (modifi ed from Barbosa and Dominguez 1996).Detailed geologic map with the location of the samples analyzed in this study (modifi ed from Bonfi m and Pedreira 1990 and Guimarães and Pedreira 1990).

Fig. 2 -
Fig. 2 -Schematic representation of the stratigraphic sequence of the Espinhaço and São Francisco Supergroups in the Chapada Diamantina, Bahia area (modifi ed from Rocha 1993).

Fig. 3 -
Fig. 3 -Outcrop of dike cross-cutting sandstones of Tombador Formation.The fi eld relationships show clearly the dike has igneous origin.

Fig. 4 -
Fig. 4 -A plot of geochemical analyses showed in TableI.The sample GCD is the average for all samples from Chapada Diamantina -BA.Major elements (wt %), trace and REE elements (in ppm).Observe that the elements behavior of the Chapada Diamantina samples is similar to the elements behavior of the hematitic philite from Diamantina -MG.
(2007)  79 (2) MICRODIAMONDS IN MESOPROTEROZOIC INTRUSIVE ROCKS -BAHIA 327 TABLE I Fig. 5 -(A) SEM images and EDS spectrum of grains composed exclusively of SiO 2 with dissolution structures.(a) corrosion structures and negative trygons; (b) negative trygons.The trygons were originated by dissolution.X = analized points.(B) SEM images and EDS spectrum of grains composed exclusively of carbon identifi ed in heavy mineral concentrates from the intrusive rocks.X = analized points.

Fig. 6 -
Fig.6-SEM image and EDS spectrum of a carbon mineral, showing cubic habits identifi ed as diamond from a hand sample of intrusive rock.This mineral is associated with the same minerals identifi ed in thin sections of rocks containing martite and muscovite.X = analized points.