MINERALIUM DEPOSITA
Mineral Deposit Letters
9 Springer-Verlag 1994 Mineral. Deposita 29, 414-421 (1994)
The origin of diamonds in western Minas Gerais, Brazil G.M. Gonzaga 1, N.A. Teixeira 2, and J.C. Gaspar 3 1 SHIG-Sul, 714 B1. J Apt. 201, 70380-760 Brasilia, Brazil 2 NTX Consultoria SQN 215 BI. A Apt. 302, 70874-010 Brasilia, Brazil 3 GMP Instituto de Geoci6ncias, Universidade de Brasilia, 70910-900 Brasilia, Brazil Received: 14 May 1993/Accepted: 1 February 1994
Abstract. I m p o r t a n t diamond mineralized alluvium/colluvium occurs in western Minas Gerais (WMG) in the Brasilia Orogenic Belt (700-450 my). Diamonds are geographically related to glacial and periglacial sediments (Upper Proterozoic and Permo-Carboniferous), wadi and debris flow conglomerates (Lower Cretaceous and Upper Cretaceous), and ultramafic alkaline rocks (intrusive and extrusive, including "kimberlites", leucitites, kamafugites, carbonatites, etc.) of Cretaceous age. Petrological data indicate that the rocks originally petrographically classified as kimberlites present mineralogical, chemical, and isotopical differences with G r o u p I and II kimberlites and more closely resemble kamafugites. The Brasilia Orogenic Belt presents features of a Wilson Cycle implying that W M G does not fulfil the geotectonic and geothermometric requisites to host primary diamond sources. An analysis of field relations and sedimentology in W M G shows that the majority of the diamonds have been transported by glacial events from the Silo Francisco Craton further east.
The discussion about the origin of diamonds from western Minas Gerais (WMG), including the Coromandel area in the Trifingulo Mineiro (a region of important diamond production, with stones larger than 100 ct) and adjacent southeast Goifis, takes place under the following tricky geological scenario: D i a m o n d mineralization occurs in alluviums/colluviums spread over an area larger than 40000 km e, in the Barsilia Orogenic Belt (700 to 450 my) along the west margin of the S~o Francisco Craton (Fig. 1). As observed in Fig. 2, diamonds are geographically related to: (1) glaciogenic Upper Proterozoic sediments within the orogenic belt (Ibifi Formation; Dardenne et al. 1978); (2) Permo-carboniferous glaciogenic sediments of the Santa F6 G r o u p (Campos et al. 1991) belonging to the undisturbed sedimentary cover (not shown on Fig. 2); (3) conglomerates from an arid depositional system (Areado Formation, Lower Cretaceous); (4) diversified magmatic activity emplaced between 85 my and 119 my (Rb-Sr, Bizzi et al. 1991) including maars, dia
tremes, craters, pipes, plugs and flows of "kimberlites" (isotopically different from G r o u p I and II kimberlites), olivine melilitites, mafic-mineral-rich volcanics, leucitites, kalsilite-rich volcanics, ugandites, katungites, carbonarites, and alkali basalts (Danni 1985; Bizzi et al. 1991; Meyer et al. 1991; Sgarbi 1992); (5) conglomerates (mud and debris flow) from the Upper Cretaceous (Bauru Group, Barbosa et al. 1970). The Brasilia Orogenic Belt coincides with a strong Bouger gravimetric anomaly (Hasui and Haraly 1985) and hosts remnants of oceanic crust (Brod et al. 1991) and ophiolitic m61anges enclosed in metasediments of the Araxfi and Canastra Groups. Such features indicate, according to Strieder and Nilson (1991), subduction trough deposits involved in underthrusting of a wedge-shaped continental plate or trapping of small plates or magmatic arcs during the convergence of two large crustal blocks. After a long stable period the local lithosphere underwent incipient stretching (late Jurassic to early Cretaceous) resulting in the Paranaiba Uplift, important alkaline magmatic activity, and deposition of sedimentary cover.
A historical view Since the beginning of the century WMG has been well known for its good-quality diamond production. Some of the largest Brazilian gems, such as Presidente Vargas (726.6 ct), Darcy Vargas (460.0 ct), etc, were found in alluvium/colluvium in WMG (Reis 1959). In some cases diamonds are associated with pyrope (mostly corresponding to the lherzolitic G 9 garnet group of Dawson and Stephens 1975) and Mg-rich ilmenites. The occurrence of this association spatially related to ultramafic-alkaline pipes and volcanics led pioneer geologists to assume the existence of primary diamond sources in WMG (Rimann 1917). However, during the seventies, hundreds of"kimberlitic" intrusives were discovered and carefully evaluated by the De Beers Company. Except for Trfis Ranchos-4 (Fazenda Alagoinha Intrusion), that presented an extremely low diamond grade, all other pipes were revealed to be sterile (Gonzaga and Tompkins 1991). Despite the fact that for over a century the "garimpo" (artisan digger) and company activities in the area have never exploited any primary diamond source, the "kimberlitic" intrusives are still considered by many authors as the source of diamond mineralization in WMG (Barbosa 1991; Leonardos et al. 1991; Svisero 1992).
415 and/or lamproitic provinces situated in stable areas of the S~,o Francisco Craton, emplaced before __ 800 my, and the diamonds would have been transported to WMG by two glacial events: the first in the Upper Proterozoic (Jequitai Glaciation) and the second in the Permo-Carboniferous (Fig. 1).
Tompkins and Gonzaga (1989) and Gonzaga and Tompkins (1991) presented sedimentological and mineralogical evidence that is not compatible with the theory that the "kimberlitic" pipes are the sources of WMG diamonds. According to these authors the majority of the diamonds would be genetically related to kimberlitic 48 ~
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416 This glacial genesis proposed for WMG diamonds has not been well accepted (Barbosa 1991; Leonardos et al. 1991; Svisero 1992). This is not due to field evidence or new theoretical considerations but appears to be mainly due to the lack of knowledge about the transportation, concentration, and dispersion mechanisms of diamonds in glacial or periglacial systems. The very low petrological knowledge about the intrusives, that only recently has begun to increase (Danni et al. 1991; Meyer and Svisero 1991; Tompkins t991a; Ulbrich and Leonardos 1991; etc.), has also contributed to the authors' rejection of the glacial theory.
Geological evidence for glacial transportation The argument that glacial or periglacial processes do not allow the transport and concentration of diamonds is not based on actual or theoretical facts. Rouffaer (1988) states that: " . . . glacial conditions are like giant grinders milling away continuously until only the hardest elements remain in the pulp, which is flushed off by flowing water leaving behind a residue enriched in diamonds. It will all come out in the wash! The industrial application of this process is the ballmill which is extensively used in the "final recovery" of the diamonds. Does glacial transport favour larger diamonds? This is certainly contrary to fluvial transport whereby the smaller stones are carried farther because less energy is required for their mobilization. However, the push-broom action of ice would exert a greater grip on the larger stones. The analogy with trying to sweep dust into a dustpan, appears to be valid". Additionally, deflation processes in the arid conditions during the Lower Cretaceous were responsible for diamond concentration in WMG. Gonzaga (1978) showed that in some areas (e.g. Presidente Oleg~trio) deflation formed hamadas over the glacial sediments (Jequitai Formation). This process led to a residual concentration of clasts and heavy minerals. The hamadas were the source area for the wadi fan diamondiferous sediments of the Abaet6 Member (Areado Formation). The economic deposits of diamonds in the Elizabeth Bay area, Namibia, represent an example of the ability of aeolian processes to produce secondary diamond concentrations (Gurney 1986). Many worldwide diamond deposits are indeed associated with glacial and periglacial sediments: Tibagi River, Brazil (Maack 1968), Yuichi River, Bolivia (Oppenheim 1943), Bloemhof, South Africa (Wilson 1982), Somabula, Zimbabwe (Stagman 1978), North Central Region, USA (Schwarcz 1965; Bardet 1977), etc. An analysis of the geological framework of W M G tends to reinforce the genetic theory of Tompkins and Gonzaga (1989) and Gonzaga and Tompkins (1991), as follows: 1. There is a direct relationship between diamonds and the Upper Proterozoic glacial sediments. This is demonstrated by: (a) the existence of diamonds in glaciogenic sediments as shown by "garimpos" (artisan diggings) in the Jequitai and Macafibas Groups (chrono-stratigraphic units related to the Ibifi Formation in WMG) (Gonzaga and Dardenne 1991; Fleischer 1991); (b) the presence of faceted clasts (flat-iron) and rare striated clasts, similar to faceted clasts from the Ibifi Formation. Examples can be seen in the Gamela and Rufino "garimpos" (Fig. 2); (c) the
occurrence of diamonds on top of or downstream from the Ibifi diamictite outcrops. The Rufino "garimpo", in the headwaters of the Santo Antonio do Bonito River, and the Limfio "garimpo", in the Quebra-Anzol River, are typical examples (Fig. 2); (d) absence of kimberlitic minerals in the Gamela, Rufino, and other "garimpos". Their characteristic heavy minerals are zircon, tourmaline, rutile, etc., which are typical minerals in Upper Proterozoic glacial sediments (Gravenor and Gostin 1979). 2. The Santa-F6 tillites (Permo-Carboniferous, according to Campos et al. 1991)in the Canabrava region, MG, have been tested by BBM Ltda who found diamonds in them (Campos et al. 1993). 3. In some areas where glaciogenic sediments are absent (e.g. Arax~-Tapira), many pipes are present, but diamonds have never been found (Fig. 2). 4. In some places in W M G (e.g. Presidente Oleg/trio) there exist diamondiferous conglomerates (Abaet6 Member) in the Areado Formation, which is Lower Cretaceous (Hasui and Penalva 1970; Gonzaga and Tompkins 1991; Campos et al. 1992). It proves that even if the ultramafic-alkaline pipes were diamondiferous they would not have been the source of diamonds found in the Abaet6 conglomerates - the pipes were intruded during the Upper Cretaceous (Hasui and Cordani 1968; Bizzi et al. 1991). 5. Campos et al. (1992) explained the presence of diamonds in the Canabrava Formation, which is correlated to the Areado Formation, as the result of reworking of underlying sediments of the Santa-F6 glaciation (PermoCarboniferous), a hypothesis already advanced by Gonzaga and Tompkins (1991). The diamonds in the Canabrava region occur in Lower Cretaceous sediments. The glaciogenic Santa-F6 sediments present clasts of hematitic phylites, itabirites, metaconglomerates, quartzites, etc., indicating reworking of the Chapada Diamantina region (locally containing Middle Proterozoic diamondiferous conglomerates), in the Silo Francisco Craton. The presence of striated pavements (S 10-20~ and crescentic fractures indicate mass movement to the southeast (Fig. 1), supporting the idea of Chapada Diamantina reworking (Dardenne et al. 1990; Campos 1992). 6. The most probable source of diamonds in alluvium of the Franca (SP) and Cfissia (MG) region (gems up to 70 ct) are sediments of the Permo-Carboniferous glaciation, Aquidauana Formation (Itarar6 Subgroup) (Leite et al. 1984; Gonzaga and Tompkins 1991). Contrary to the sediments of the Canabrava region, these sediments were transported from southeast to northwest, a glacial lobe correlated to the Kaokoveld lobe, in the African continent (Frakes and Crowell 1970). Therefore, these diamonds have not originated in the Silo Francisco Craton. So far, no pipes or kimberlitic minerals have been found in "garimpos" of the Franca-Cfissia region (Fig. 1). 7. The Bauru Group (Upper Cretaceous - Barbosa et al. 1970) contains diamonds in many localities in WMG. In regions where "kimberlite" pipes are present, as in Romaria, pyrope (mainly G 9) and magnesian ilmenite are present. However, wherever the pipes are absent, as in the Pedrin6polis-Perdizes area (Fig. 2), the diamonds are not associated with pyrope and ilmenite suggesting that the diamonds have no genetic link to these minerals. In general, the heavy mineral concentrates from the Bauru
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418 Group contain abundant magnetite (70%), zircon, tourmaline, and rutile, among others (Barbosa et al. 1970). It is possible to conclude that the diamonds in the Bauru Group in W M G originated from reworking of glaciogenic sediments of the Ibifi Formation according to the following evidence: (a) zircon, tourmaline, and rutile, typically resistant to sedimentary recycling, plus opaques are the accessory minerals of the Ibifi Formation (Pereira 1992); (b) clasts from the Ibifi Formation are often found in the Bauru Group; (c) paleocurrent studies in the Bauru Group in the Romaria area indicate a provenance from the southeast (Barbosa et al. 1970; Gallo 1991) that is compatible with the position of the Ibi/t outcrops (Fig. 2); (d) the occurrence of diamonds is continuous in alluvium of the Quetbra-Anzol River, from the Pedrin6polis-Perdizes region upstream, until the contact of the Ibi/t Formation is reached (Fig. 2); (e) in Romaria, the Bauru Group contains a grade of 3-5 points/m 3 (Barbosa 1991) (and not 10--15 points/m 3 as suggested by Gallo 1991), which is easily attained by a sedimentary recycling. Discussion The ultramafic alkaline activity, considered by some authors as the diamond source of recent alluvial deposits (Barbosa 1991; Leonardos et al. 1991; Svisero 1992), occurs preferentially in the Paranaiba Uplift (lineament 125 ~ AZ; Bardet 1977), an extensional Cretaceous tectonic feature imprinted over the Upper Proterozoic orogenic Brasilia Belt. Where the lineament 125 ~ AZ intersects the southwestern corner of the Silo Francisco Craton, it controls the emplacement of few pipes, i.e. Boa Esperanca and Cana Brava (Tompkins 1991b). Recent petrological studies (Danni et al. 1991; Meyer and Svisero 1991; Ulbrich and Leonardos 1991; Bizzi et al. 1991) do not confirm a kimberlitic or lamproitic nature to the W M G ultramafic-alkaline magmatism. Despite the petrographic similarity of some of these rocks to kimberlites, the isotopic and mineral chemistry do not agree with typical kimberlites. All the varieties of alkaline manifestations in W M G present similar Nd and Sr signatures to each other, falling in between the group I and II kimberlites (Meyer et al. 1991; Bizzi et al. 1991). Even Limeira I and Indai/t I pipes that are petrographic and mineralogically similar to kimberlites, are also isotopically different from kimberlites but similar to all other W M G alkaline rocks (Meyer et al. 1991). A possible kamafugitic affinity has been proposed. Spinel lherzolite xenoliths from the Limeira "kimberlite" appear to have equilibrated at relatively low pressures (27kb) within the subcontinental lithosphere (Meyer et al. 1991). Bizzi et al. (1991), based on spinel lherzolite xenoliths and the absence of garnet hazburgite, suggest generation depths of less than 100 km for the W M G magmas, shallower than the diamond stability field. Leonardos et al. (1993) studied one garnet lherzolite xenolith from the Tr~s Ranchos 4/Fazenda Alagoinha intrusion and determined equilibrium conditions varying form 78 to 55 kb and t270 to 970 ~ Despite the fact that a better evaluation of non-equilibrium textures and calculation methods is necessary, this xenolith seems to come
from a deeper source that the spinel lherzolites studied by Bizzi et al. (1991). This deeper source for the Tr~s Ranchos 4/Fazenda Alagoinha intrusion would be in agreement with the fact that it contains diamonds, even if of an extremely low grade. Tompkins and Ramsay (1991) studied diamond indicator minerals from the Boa Esperan~a and Cana Verde pipes from the Bambui field, both intruded in the S~o Francisco Craton, near its southwest border. The authors state that Boa Esperan~a may be a kimberlite and Cana Verde may be a lamproite. They concluded that garnet lherzolite was sampled by these pipes and Ti-metasomatism was pervasive in both localities. Despite the fact that they have found no evidence for hazburgite sampling, and that the Ti-metasomatism may have decreased the diamond potential, the depths sampled by these two pipes indicate a much higher diamond potential than any other pipe emplaced in the Brasilia orogenic belt. Kimberlites, orangeites (Mitchell 1991), and/or lamproites and their correct emplacement into a thick and cool lithosphere (craton or ensialic mobile belt) are fundamental factors for the existence of primary diamond sources (Boyd et al. 1985; Nixon 1987; Haggerty 1989; 1991; Jaques et al. 1990; Helmstaedt 1991; Gonzaga et al. 1992). Gonzaga et al. (1992) consider that mobile belts developed from the Middle Proterozoic onwards started to present a tectonic evolution more similar to the Wilson cycles, a feature that reached its completion in the Phanerozoic, where oceanic crust generation, subduction, and high heat flow regimes are widespread. These conditions determined, in general, the shallower generation depths of lamproites, kimberlites, and orangeites emplaced in mobile belts younger than 1.7 my. The geotectonic environment for the ultramafic alkaline activity in WMG, the Brasiliano event (700-450 my), displays strong low-angle tectonics with vergence towards the craton, progressive metamorphism, emplacement of mantle wedges in deep fractures, and remnants of oceanic crust and magmatic arcs (Fig. 3). Such features are generated in extensional/compressional environments with high heat flow characteristic of the Wilson Cycle. Heat flow measurements in the area support it (Vitorello et al. 1980; Vitorello and Pollack 1980; Hanza 1982). Hanza (1982) states that "lower than normal values are obtained in the S~o Francisco Craton (39 mW/m 2) of Archean to early Proterozoic age while late Proterozoic folded regions (53 mW/m 2) and Phanerozoic sedimentary basins (58 mW/m 2) seem to be characterized by normal values". Despite the scarcity of these measured thermometric values, the existing data suggest that the W M G region does not fulfil the geotectonic and geothermometric requisites to host primary diamond sources. Some geologists argue that the Brasilia Fold Belt resulted from a thin-skinned tectonism and that, in the WMG, it is thrusted over the S~o Francisco Craton. According to these geologists, kimberlites or lamproites emplaced in W M G would, consequently, have the same diamond potential as "on craton" kimberlites. This conclusion, however, does not seem to be valid since there are many indications that the sialic substrate of the Brasilia Fold Belt was affected by the Brasiliano orogeny and underwent heating and tectonic reworking (Machado
419
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and Schrank 1989; Delhal and Demaiffe 1985; Teixeira and Figueiredo 1991).
Conclusions
The W M G does not seem to be an exception regarding the settings for p r i m a r y sources of diamond. It shows the worldwide characteristics of an area in which m a g m a t i c alkaline activity (kamafugites, carbonatites, "kimberlites") were emplaced in an U p p e r P r o t e r o z o i c mobile belt, in g e o d y n a m i c conditions that are not favourable for the generation of m a g m a s from inside the d i a m o n d stability field. We have no evidence to state that no d i a m o n d at all could have ever come from one of these pipes (e.g. Trfs R a n c h o s - 4 / F a z e n d a Alagoinha). But the distribution of diamondiferous alluvium, pales-currents, and heavy-mineral concentrates, indicate that the majority of the W M G d i a m o n d s come from stable portions of the Sao Francisco Craton, transported by glacial events. Exceptions are the d i a m o n d s from the Franca/C~tssia region that were transported by glacial and periglacial sediments during the P e r m o - C a r b o n i f e r o u s from an u n k n o w n p r i m a r y source from the southeast. The significance of glaciations in the dispersion of d i a m o n d s is fundamental to understanding the unsolved genesis of d i a m o n d s in W M G . These conclusions strongly agree with the previous propositions of T o m p k i n s and G o n z a g a (1989) and G o n z a g a and T o m p k i n s (1991).
Acknowledgement. We thank Dr. Chris Smith for his valuable comments of an early version of this manuscript. References
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