Metavolcano-sedimentary Sequence Research Articles

Gold deposits of the Carajás mineral province: deposit types and metallogenesis

The Carajas mineral province is located in the south-eastern part of the Amazon craton and is divided into the southern and northern tectonic blocks, which are referred to as the Rio Maria granitoid–greenstone terrane and the Itacaiunas shear belt, respectively. The geological evolution of this province occurred almost entirely in the Archean, with no tectonism from ~2.4 to 1.9 Ga and then an episode of very extensive intraplate granitic magmatism. Distinct types of gold deposits, most associated with Archean metavolcano-sedimentary sequences, including greenstone belts as old as 2.9 Ga, comprise (1) Fe-oxide-poor Cu–Au, (2) Fe-oxide–Cu–Au–U–REE, (3) shear-zone related lode-gold, and (4) sedimentary rock-hosted Au–PGE. The shear-zone related deposits are more typical of the Rio Maria granitoid–greenstone terrane, whereas the other types are more abundant in the Itacaiunas shear belt. The first type, represented by the Serra Verde deposit, shows most characteristics of volcanogenic massive sulfide (VMS) deposits, except for the dominance of chalcopyrite over pyrite and/or pyrrhotite. Some deposits of the second type (Igarape Salobo and Igarape Bahia) are also syngenetic, but their ore mineralogy and paucity of pyrite and/or pyrrhotite are inconsistent with, or at least unusual for, classic VMS deposits. Other Fe-oxide–Cu–Au–U–REE deposits, such as the Cristalino and Sossego, occur as epigenetic stockwork mineralization related to Archean granitoid stocks. Most shear-zone related lode-gold deposits are classified as orogenic gold deposits (e.g. Sapucaia, Babacu and Tucuma) that cut greenstone belt rocks. The Cumaru deposit is also hosted by a shear zone, but may have a genetic association with a granitoid intrusion. The Aguas Claras deposit, although also hosted in a shear zone and perhaps intrusion-related, differs from the orogenic gold deposits by its extensional tectonic setting, lack of CO2 in the ore fluids, and voluminous chalcopyrite. The sedimentary rock-hosted Serra Pelada Au–PGE deposit is structurally controlled and occurs in an Archean clastic-chemical sedimentary rock sequence.

Geochemical Characterization of Granites of Distinctly Different Ages in the Alto Pajeu Terrane, Borborema Province, Northeast Brazil

This work presents geochemical data for granitic plutons that intruded the Alto Pajeu terrane, Borborema Province, northeastern Brazil, during several distinct tectonic cycles. The Alto Pajeu terrane is represented in the study area by metasedimentary (São Caetano Formation) and metavolcanosedimentary (Irajai Formation) sequences overlying a gneissic-migmatitic basement. Several granitic plutons of different ages, mineralogical proportions, and chemical compositions have invaded the supracrustal and basement rocks. Two deformational events, one tangential (older) and the other transcurrent (younger), were identified. The metamorphic grade determined for the metasedimentary rocks varies from amphibolite to high amphibolite facies, with temperatures ranging from 600 to 680°C and pressures from 6.0 to 7.5 kbar. Three magmatic events with distinct ages were identified for the tectonic evolution of the Alto Pajeu terrane in the study area. The older magmatic event is related to the Transamazonico orogenic cycle (∼ 2.0 Ga), the intermediary magmatic event to the Cariris Velhos cycle (∼0.97 Ga), and the younger magmatic event to the Brasiliano orogenic cycle (∼0.55 Ga). The Cariris Velhos- and Brasiliano-age granites are geochemically classified as peraluminous, subalkalic granites of the calc-alkalic series, whereas the Transamazônico-age granite is metaluminous and alkaline. Geochemical data presented and discussed here suggest that the magmatism related to the three tectonic events had distinctly different sources and petrogenetic evolutions.

THE MARA ROSA GOLD DISTRICT, CENTRAL BRAZIL

In the Mara Rosa District, three different magmatic series have been identified and all of them host gold mineralization. The Mara Rosa Metavolcano-Sedimentary Sequence constitutes a poorly differentiated calc-alkaline magmatic series typical of arc settings. It is arranged into three belts, forming synclinal structures and was affected by high-grade amphibolite facies metamorphism. Early intrusive rocks of the Posse Unit show an alkaline affinity and host the epigenetic Posse Gold Deposit, which is associated with a late hydrothermal event, preferentially where earlier shear zones are present. Early hydrothermal alteration has produced an aluminous, sulphide rich greisen rock, which is barren of gold. Slices of an allochthonous and more magnesian sequence are associated with metachert and graphitic schists that host the syngenetic Au-Ag Zacarias mineralization and extensions. Gold-bearing quartz veins were emplaced before the second main isoclinal folding event. Supergene ore has played an important role in the mining activity of the Mara Rosa Region, since it represents the majority of the ore exploited. It is possible that yet undiscovered deposits lay hidden underneath the extensive lateritic cover.

THE IGARAPÉ BAHIA CU-AU MINERALIZATION, CARAJÁS PROVINCE

The IGARAPE Bahia Cu-Au mineralization is hosted by an Archean, low-grade metavolcano-sedimentary sequence. The orebodies define an ellipsoidal structure and are associated with subvertical breccia units located at the contact between two distinct units of the host sequence. Mineralized breccias include fragments of both footwall and hanging wall, which are cemented by variable amounts of chlorite, siderite, magnetite, chalcopyrite, K-bearing phases and minor U-REE-minerals. Quartz diorite dikes that disrupt the host rocks show a variety of textures, ranging from weakly altered granophyric terms to intensely venulated and brecciated rocks. Hydrothermal alteration of dikes includes propylitization, potassification and local albitization. Based on ore petrology and geochemical data (major elements, REE, oxygen and carbon stable isotopes) it is proposed that the progressive interaction of a hot saline and acid, deep-seated fluid with a low-temperature less saline and oxidizing meteoric solutions is the most likely depositional mechanism of the IGARAPE Bahia mineralization. The resemblance of the alteration styles of mineralized breccias and dioritic dikes suggest that both have interacted with the same hydrothermal fluid. Thus, the dioritic dikes could have been the source of heat and of magmatic fluids during the final stages of epicrustal emplacement.

Chemical deposits associated to metavolcanosedimentary sequences of the central portion of the São Francisco Craton the state of Bahia, Brazil: a review

Supracrustal suites of metavolcanosedimentary rocks (SC) occur in the central portion of the Sao Francisco Craton in the State of Bahia. They were primarily formed by calc- alkaline to perialkaline volcanic flows and tuffs, clastic and chemical sediments. The latter are referred to the banded iron (BIF), iron-manganese (BIMF) and manganese (BMP) formations, iron-poor magnesian-rich carbonate formation (IPMRCF), and minor metacherts. BIF and IPMRCF are present in all the reported SC. BIF is typified by the presence of the oxide, carbonate and silicate facies. Occurrences of the sulfide facies are reported for the Sobradinho-Salitre and the Boquira SC. The burial metamorphism and thermal effects pronounced by the Transamazonian migmatization and granitization are accountable for the generation of amphibolites, epidosites, steatite and talc schists, phyllites, sandstones, quartzites, itabirites, metamorphic limestones to include banded carbonates, marbles and calc-silicate rocks. It is suggested that the deposition was similarly processed throughout an extensive crustal segment and that it was part of the cratonic evolution occurred during the end of the Archean to the beginning of the Proterozoic.

THE IGARAPÉ BAHIA CU-AU MINERALIZATION, CARAJÁS PROVINCE

The Igarapé Bahia Cu-Au mineralization is hosted by an Archean, low-grade metavolcano-sedimentary sequence. The orebodies define an ellipsoidal structure and are associated with subvertical breccia units located at the contact between two distinct units of the host sequence. Mineralized breccias include fragments of both footwall and hanging wall, which are cemented by variable amounts of chlorite, siderite, magnetite, chalcopyrite, K-bearing phases and minor U-REE-minerals. Quartz diorite dikes that disrupt the host rocks show a variety of textures, ranging from weakly altered granophyric terms to intensely venulated and brecciated rocks. Hydrothermal alteration of dikes includes propylitization, potassification and local albitization. Based on ore petrology and geochemical data (major elements, REE, oxygen and carbon stable isotopes) it is proposed that the progressive interaction of a hot saline and acid, deep-seated fluid with a low-temperature less saline and oxidizing meteoric solutions is the most likely depositional mechanism of the Igarapé Bahia mineralization. The resemblance of the alteration styles of mineralized breccias and dioritic dikes suggest that both have interacted with the same hydrothermal fluid. Thus, the dioritic dikes could have been the source of heat and of magmatic fluids during the final stages of epicrustal emplacement.

The Mara Rosa gold district, Central Brazil

In the Mara Rosa District, three different magmatic series have been identified and all of them host gold mineralization. The Mara Rosa Metavolcano-Sedimentary Sequence constitutes a poorly differentiated calc-alkaline magmatic series typical of arc settings. It is arranged into three belts, forming synclinal structures and was affected by high-grade amphibolite facies metamorphism. Early intrusive rocks of the Posse Unit show an alkaline affinity and host the epigenetic Posse Gold Deposit, which is associated with a late hydrothermal event, preferentially where earlier shear zones are present. Early hydrothermal alteration has produced an aluminous, sulphide rich greisen rock, which is barren of gold. Slices of an allochthonous and more magnesian sequence are associated with metachert and graphitic schists that host the syngenetic Au-Ag Zacarias mineralization and extensions. Gold-bearing quartz veins were emplaced before the second main isoclinal folding event. Supergene ore has played an important role in the mining activity of the Mara Rosa Region, since it represents the majority of the ore exploited. It is possible that yet undiscovered deposits lay hidden underneath the extensive lateritic cover.

THE MESOPROTEROZOIC VOLCANO-SEDIMENTARY SERRA DO ITABERABA GROUP OF THE CENTRAL RIBEIRA BELT, SÃO PAULO STATE, BRAZIL: IMPLICATIONS FOR THE AGE OF THE OVERLYING SÃO ROQUE GROUP

One of the fundamental problems to understanding the evolution of volcano-sedimentary sequences in southeastern Brazil is constraining their depositional ages. Brasiliano tectonic and metamorphic overprinting either obscured or destroyed primary features, such as unconformities, as well as other geologic relationships. This problem is exemplified by the Serra do Itaberaba and Sao Roque groups, where the lack of data about the timing of their deposition has prevented resolution of proposed one- and two-stage geotectonic/depositional models. Recent U-Pb zircon data obtained from metavolcanic rocks in the Sao Roque Group indicate that it was deposited between 628 and 607 Ma. New U-Pb zircon data of 1395 + 10 Ma for a metandesite in the basal Morro da Pedra Preta Formation (Serra do Itaberaba Group) indicate the maximum age for the beginning of the deposition of the pelites overlying MORB-like basalt. A metarhyolite of the upper unit, the Nhangucu Formation, contains two zircon populations. One yielded an age of 619 + 3 Ma, which defines the crystallization age of the rock, and the other an age of 1449 ± 3 Ma, interpreted as inherited xenocrystal grains from older units of the Serra do Itaberaba Group. The younger metarhyolite was affected only by the S, foliation, generated during the Brasiliano orogenesis, whereas the Middle Proterozoic metavolcano-sedimentary sequence records additional metamorphic and deformational events, confirming the presence of two different geotectonic cycles.

THE MESOPROTEROZOIC VOLCANO-SEDIMENTARY SERRA DO ITABERABA GROUP OF THE CENTRAL RIBEIRA BELT, SÃO PAULO STATE, BRAZIL: IMPLICATIONS FOR THE AGE OF THE OVERLYING SÃO ROQUE GROUP

One of the fundamental problems to understanding the evolution of volcano-sedimentary sequences in southeastern Brazil is constraining their depositional ages. Brasiliano tectonic and metamorphic overprinting either obscured or destroyed primary features, such as unconformities, as well as other geologic relationships. This problem is exemplified by the Serra do Itaberaba and São Roque groups, where the lack of data about the timing of their deposition has prevented resolution of proposed one- and two-stage geotectonic/depositional models. Recent U-Pb zircon data obtained from metavolcanic rocks in the São Roque Group indicate that it was deposited between 628 and 607 Ma. New U-Pb zircon data of 1395 + 10 Ma for a metandesite in the basal Morro da Pedra Preta Formation (Serra do Itaberaba Group) indicate the maximum age for the beginning of the deposition of the pelites overlying MORB-like basalt. A metarhyolite of the upper unit, the Nhanguçu Formation, contains two zircon populations. One yielded an age of 619 + 3 Ma, which defines the crystallization age of the rock, and the other an age of 1449 + 3 Ma, interpreted as inherited xenocrystal grains from older units of the Serra do Itaberaba Group. The younger metarhyolite was affected only by the S 2 foliation, generated during the Brasiliano orogenesis, whereas the Middle Proterozoic metavolcano-sedimentary sequence records additional metamorphic and deformational events, confirming the presence of two different geotectonic cycles.

GEOLOGIA E GEQQUÍMICA DOS ELEMENTOS MAIORES DOS DEPÓSITOS DE MAGNESITA PRÉ-CAMBRIANA (~1.8Ga) DA FAIXA MÓVEL ORÓS ( CEARÁ)

The five most important magnesite mines and some small occurrences of the Ceara State form an array of discontinuous lenses extending for about 140 km. The magnesite-rich rocks are hosted in metadolomites with lutecite, fibroradiated quartz pseudomorph after sulfate nodules, scapolite and dissolution breccias. The marbles are hosted by a metavolcanosedimentary sequence of the greenschist to amphibolite facies, and were intruded by basic sills and granites of size, form, composition and age that vary from the Meso to Neoproterozoic. Two types of magnesite marbles may be distinguished: (1) magnesite marble of medium grain-size (1 to 9mm) and (2) sparry magnesite marble (1 to 15cm). The sparry magnesite marble has porphyric, rosette, layered, and palisade textures, the latter having remains of the original sedimentary features. In spite of deformation, the sparry crystals are hipidiomorphic and pinolitic. The rocks vary from white to light gray or dark gray or even red. The dark banded terms have traces of microfossils and stromatolitic structures. The texture and color of the medium-grained magnesite marbles are more homogeneous and the crystals are more anhedral. The sparry magnesite marble is in general poorer in SiO 2 , Fe 2 O 3 , Al 2 O 3 and CaO and richer in MgO as compared to the medium-grained types. The paleogeographic interpretation suggests that these rocks formed near a paralic system (lagoon), with strong evaporitic conditions. The depressions were of variable depth and length and probably isolated by stromatolite barriers, dried and flooded by seawater with continental inflows. The largest and deepest deposits correspond to the sparry magnesite. Regionally, the magnesite deposits give place to dolomitic marbles, which grade into almost pure calcitic marbles, indicating a chemical differentiation within the paralic system, by which the lagoon waters turned progressively poorer in Ca by the precipitation of calcium carbonate, thus increasing the Mg/Ca ratio and leading to the precipitation of magnesite. The magnesite marble is than interpreted as of sedimentary origin, and underwent strong diagenesis before metamorphism and Neoproterozoic deformation.

Geoquímica e Petrologia das rochas máficas e ultramáficas do complexo estratiforme de Cana Brava-GO, e das suas encaixantes

The Cana Brava body is a 2.0 Ga, anorogenic, stratiform complex whose layers dip from 30º to 50º NW. The massif is made up of five units, composed of layers containing several associations of cumulus phases and variable amounts of inter-cumulus minerais. Transitions between units are characterized by changes in the composition and/or in the relative abundance of these phases. Stratigraphically, the lowest sequence is composed of amphibolites (PICB1), overlain by serpentinites (PICB2), metawebsterites (PICB3) and metagabbroic rocks (PICB4 and PICB5) at the top. This sequence was originally formed by microgabbros, peridotites, websterites and gabbroic rocks. The rock sequence of the complex can be attributed to a single magmatic fractionation. There is no evidence of multiple injections. Metamorphic events affected the different units heterogeneously. The gabbroic rocks (PICB4 and PICB5) were the least transformed; hence their chemical compositions are close to the original ones. Units PICB1 to PICB3 were the most affected by post-magmatic processes. Geochemical evidences suggest that PICB1 may be the chilled border of the massif. However, due to post-magmatic phenomena, the composition of the parent magma was calculated using the chemical compositions and inferred volumes of units PICB2 to PICB5. For PICB2 and PICB3 the compositions of mineral phases were utilized. The possible Cana Brava parent magma is olivine-tholeiite, similar to the majority of the complexes utilized for comparison. Its amount of normative olivine is within the range for liquids derived from 25 to 35% of mantle melting. Phase equilibria condictions point to pressures under 6-7 Kbar during crystallization. The possible parental magma of the Cana Brava complex is compared with the alcali-olivine basaltic magma of the Palmeirópolis metavolcano-sedimentary sequence, and with the possible parental magma of the Serra da Bota satellite body.

SUPERPOSIÇÃO DESTRAL EM AMPLA ZONA DE CISALHAMENTO SINISTRAL: CINTURÃO DE CISALHAMENTO SÃO PAULO, IMEDIAÇÕES DE PIRACAIA - SP

The shear zones in question are roughly NE-SW and intersect gneissic-migmatitic terrains of the Piracaia Complex to the northwest and low- to medium-grade metavolcano-sedimentary sequences of the Sao Roque and Serra do Itaberaba Groups to the southeast. The early Sao Bento do Sapucai and Sertaozinho shear zones (SBSSZ and SSZ) comprise a large mylonitic belt that records sinistral lateral displacement that took place under low to medium metamorphic conditions. This movement generated both widespread subsidence toward the SE and transtensile domains in which less metamorphic rocks and granitic bodies were emplaced. This arrangement (including a probable decrease in the metamorphic grade toward the SE) was disrupted by later dextral displacement of the Jundiuvira Shear Zone (JSZ), a narrower zone of mylonites and ultramylonites with a foliation that is distinguished from the former by retrograde minerais. A SSW to ENE inflection of the JSZ produced both transtensile and transpressive domains, the latter being characterised by asymmetric cylindrical, northwest-verging folds in the SBSSZ region. Lattice preferred orientation (LPO) analysis of quartz C-axes suggests distinct deformational histories for two different parts of the area, whose boundaries are related to the JSZ transpressive domain. In the northwest (SBSSZ) the most typical LPO diagrams show small-circle girdles with asymmetric maxima, which are thought to represent non-coaxial deformation followed by flattening (transpression), while in the southeast (SSZ) the patterns are well-defined sinistral single girdles, typical of non-coaxial deformation (simple shear). The general decrease in metamorphic grade observed in the area is also suggested by the quartz C-axes patterns, which display a reduction in the basal slip (indicative of low temperatures) from the border to the centre of the JSZ.

O SISTEMA ORÓS-JAGUARIBE NO CEARÁ, NE DO BRASIL

The Oros-Jaguaribe System is a peculiar geotectonic unit in the Borborema Province (NE Brazil) framework. It contains two linear mobile belts, whose evolution began c.a. 1.9 Ga ago, overlying an older basement characterized by high amphibolite metamorphism and tangential tectonics. In contrast with its basement, the System contains essentially metavolcano-sedimentary sequences intruded by acid and basic magmatic rocks metamorphosed under low greenschist to granulite facies and displays a strike-slip deformation pattern of brasiliano age (c.a 600 Ma). The metasediments are essentially pelitic with important intercalation of semi-mature lenticular quartzite, carbonatic quartzite, calcsilicate rocks and calcitic/dolomitic/magnesitic marbles. The Oros Belt exhibitsan inverted distribution: volume and extension of detritic sediments, essentially quartzite, decrease south-westward, whereas carbonates, essentially calcitic marbles, show a significant increase. These ones become more, magnesitic north-eastward by a differentiation chemical process (elimination of Ca that precipitate as calcic, calci-magnesitic carbonates or sulfates). These relationships are consistent with a paralic/deltaic system. The acid metaplutonic and metavolcanic rocks are dominantly alkaline or sub-alkaline, characteristic of an intracratonic setting.The lithologic association and distribution of the metasediments (essentially the basal ones) are shelf type, preceding rift stage, which is associated with an intense magmatism.

GEOLOGIA DA SEQÜÊNCIA VULCANO-SEDIMENTAR DE ALPINÓPOLIS (MG)

The Alpinopolis Belt is located in the southern part of the State of Minas Gerais. It is one of several meta volcano-sedimentary sequences of the Morro do Ferro Greenstone Belt. As a result of surface geological mapping, petrological studies and subsurface exploration work conducted on the Alpinopolis region, a group of four rock suites have been recognized. The most important features of these four rock suites can be summarized as follows: The first suite comprises (a) chlorite-actinolite-tremolite schists with spinifex texture and minor olivine pseudomorphous; and (b) serpentinites with variable proportions of actinolite-tremolite-chlorite-talc-oli vine relicts and hornblende schists. The combined evidence of mineral assemblages, preserved primary textures and trace element chemistry indicate that these rocks were originally peridotites and pyroxenites of komatiitic affiliation. The second suite consists of a variable assemblage of plagioclase-homblende-actinolite-tremolite schists and amphibolites. The chemical and mineralogical features of this suite are consistent with a tholeiitic basalt origin. The third suite consists of metasedimentary rocks such as gamet-biotite schists, actinolite-tremolite-biotite schists, sillimanite-staurolite-biotite schists, gamet-antophylite schists, quartzites and minor iron formation units. The schists are interpreted to be isochemically metamorphosed tuffaceous, argillaceous and impure Fe-Mg-carbonate sediments. The aforementioned supracrustal rocks have been intruded by a number of granitoid rocks that consist of quartz, sodic-plagioclase and biotite. Near to and at the contact with the ultramafic flows, marundite rocks are observed. The policyclic tectonic-metamorphic and deformational processes which affected the area have given place to a complex, large scale structurally deformed framework and imprinted metamorphic patterns on the existing rock units and the associated Ni-sulphide ore bodies.

GEOLOGIA DA SEQÜÊNCIA VULCANO-SEDIMENTAR DE ALPINÓPOLIS (MG)

The Alpinópolis Belt is located in the southern part of the State of Minas Gerais. It is one of several meta volcano-sedimentary sequences of the Morro do Ferro Greenstone Belt. As a result of surface geological mapping, petrological studies and subsurface exploration work conducted on the Alpinópolis region, a group of four rock suites have been recognized. The most important features of these four rock suites can be summarized as follows: The first suite comprises (a) chlorite-actinolite-tremolite schists with spinifex texture and minor olivine pseudomorphous; and (b) serpentinites with variable proportions of actinolite-tremolite-chlorite-talc-oli vine relicts and hornblende schists. The combined evidence of mineral assemblages, preserved primary textures and trace element chemistry indicate that these rocks were originally peridotites and pyroxenites of komatiitic affiliation. The second suite consists of a variable assemblage of plagioclase-homblende-actinolite-tremolite schists and amphibolites. The chemical and mineralogical features of this suite are consistent with a tholeiitic basalt origin. The third suite consists of metasedimentary rocks such as gamet-biotite schists, actinolite-tremolite-biotite schists, sillimanite-staurolite-biotite schists, gamet-antophylite schists, quartzites and minor iron formation units. The schists are interpreted to be isochemically metamorphosed tuffaceous, argillaceous and impure Fe-Mg-carbonate sediments. The aforementioned supracrustal rocks have been intruded by a number of granitoid rocks that consist of quartz, sódic-plagioclase and biotite. Near to and at the contact with the ultramafic flows, marundite rocks are observed. The policyclic tectonic-metamorphic and deformational processes which affected the area have given place to a complex, large scale structurally deformed framework and imprinted metamorphic patterns on the existing rock units and the associated Ni-sulphide ore bodies.

Formation of sulfide deposits and its relation to sodic and potassic alteration of Proterozoic metabasites in the Sax� rift basin, Bergslagen, Sweden

A number of Cu-Zn-Fe-Pb-As sulfide mineralizations spatially associated with metabasic sills, dikes, and flows of originally continental tholeiitic composition occur in the 1.9–1.8-Ga-old metavolcano-sedimentary sequence in the Sax» sedimentary rift basin. In higher stratigraphic levels weak sulfide mineralizations occur just below Narich, albite-chlorite-calcite-quartz-actinolite-bearing metabasic flows; in the lower stratigraphic levels sulfide mineralizations occur (a) spatially associated with potassiumrich, microcline-scapolite- or biotite/phlogopite-rich metabasic sills and dikes, and (b) below metabasic flows in which albite and calcite are replaced by microcline and scapolite. The sodium- and potassium-rich metabasites are demonstrated to represent products of spilitization and potassic alteration, respectively. During spilitization Fe, Cu, Co, and HREE are leached from the basic flows, whereas during potassic alteration Fe, Cu, Y, and possibly Zn and Zr are leached from the basic rocks, indicating a genetic relationship between the metabasites and the Cu-Fe-sulfide mineralizations. Spilitization probably took place due to interaction of the flows with descending, sea-water-dominated fluids in a subseafloor hydrothermal system with temperatures roughly between 250° and 400°C. Potassic alteration probably took place by ascending fluids at higher temperatures than for spilitization.

The Precambrian metavolcano-sedimentary sequence east of Ife and Ilesha/SW Nigeria. A Nigerian ‘greenstone belt’?

The metavolcano-sedimentary sequence east of Ife and Ilesha southwest Nigeria has previously been classified as a proterozoic mobile belt. Field observations indicate, however, that this sequence has a typical, for Archaean, granite-greenstone rock succession. Geochemical data could support this opinion. In spite of the lack of age determinations, it is possible to place these rock successions in the early Archaean. This new classification of the metavolcano-sedimentary sequence of the Ife-IIesha area is, therefore, of far-reaching economic importance. The known mineralizations of gold, cassiterite, niobite, tantalite and tungsten in this area and its surroundings call for further prospection, not only in southwest Nigeria, but also in the other mobile belt region of Nigeria. One important result is the fact that geochemical methods are a tool for the stratigraphic classification and rock genesis in areas with strong lateritic weathering or metamorphic overprint.