Navegando por Autor "SILVA, Evaldo Raimundo Pinto da"

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Estudo das ocorrências auríferas da área de Natividade-GO.
(Universidade Federal do Pará, 1987-06-26) SILVA, Evaldo Raimundo Pinto da; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
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Geologia e geoquímica das mineralizações supergênicas de ouro das áreas Salobo e Pojuca-Leste, Serra dos Carajás
(Universidade Federal do Pará, 1996-11-29) SILVA, Evaldo Raimundo Pinto da; KOTSCHOUBEY, Basile; http://lattes.cnpq.br/0096549701457340
In Salobo sector, the primary gold and copper mineralization occurs in two types of iron formations, both of the silicate facies: Type I composed of magnetite>50% + fayallite + grunerite and Type II constituted of magnetite<50% + biotite + grunerite + almandine, which occurs interlayered in barren metagraywacks. In iron formations gold is present as: i) microscopic (5-14m) and submicroscopic (<0,1 µm) inclusions in magnetite; ii) microscopic and submicroscopic inclusions in primary copper sulphides (mainly chalcopyrite; subordinatelly bornite and chalcosine); and iii) as rare visible gold particles in quartz-carbonate-clorite veins which cut the Type II iron formation. Magnetite is the major gold-bearing mineral at Salobo. In the veins, gold is rich in silver and copper (16,5% and 8,4%, respectively) and exhibits increasing fineness from the border to the center of the particles. In these latter, a negative correlation between silver and copper contents, and exsolution textures suggest a separation between Au-Cu and Au-Ag during gold deposition in veins. The Salobo weathering profile is truncated and developed upon a thick metagraywacke sequence with interlayered iron formations lenses. From bottom to top, the weathering profile consists of a transition zone, a saprolitic zone and a colluvium horizon. The transition zone and the saprolite exhibit textural and mineralogical variations that reflect the heterogeneous composition of the bedrock. Weathering of metagraywacke resulted in a clay-mineral rich saprolite -hidrobiotite, smectite, kaolinite- derived principally from biotite. On the other hand, the alteration of mineralized rocks resulted, from bottom to top of the profile, in a supergene sulphide zone, an oxidized zone and a ferruginous saprolite. The most important supergene sulphides are digenite and covelline, while in the oxidized zone occur minerals such as cuprite, malaquite, azurite, native copper and native silver. In the ferruginous saprolite copper is enriched in clay-minerais (smectite) and in iron oxi-hydroxides. The secundary gold mineralization at Salobo occurs only in the oxidation products of iron formations. Gold exhibits enrichment in two levels of the weathering profile: in the supergene sulphide and oxidized copper minerals zones, and in the intermediate zone of the ferruginous saprolite. In these alteration products, gold occurs as: i) tiny inclusions (7-30m) in refractory magnetite crystals and in fratures of this mineral; ii) as submicroscopic particles inclosed in martitized magnetite crystals; and iii) as submicroscopic particles adsorbed ("invisible gold") in goethite. At Pojuca-Leste the primary gold mineralization is present in quartz-tourmaline-flurite-copper sulphides veins which cut a thick quartz-biotite-schists sequence and in iron formations lenses interlayered in these rocks. Gold occurs as: i) submicroscopic inclusions in chalcopyrite of veins, schists and iron formations; ii) as submicroscopic inclusions in magnetite of iron formation; and iii) as rare microscopic inclusions in quartz of veins. In the veins gold particles shows low fineness 809). The weathering profile at Pojuca-Leste consists of a thick saprolitic horizon and a ferro-aluminous duricrust. The saprolite exhibits compositional variations related to the nature of the parent rocks. It is generally composed of kaolinite, goethite and hematite and its iron contents are higher when derived from iron formations and sulphide veins. The secundary gold mineralization at Pojuca-Leste occurs in alteration products of sulphide veins, in the ferruginous saprolite derived from iron formations and in ferro-aluminous duricrust. In these secondary products, gold is present as: i) rare particles in relict quartz-tourmaline±sulphide veins; ii) as submicroscopic inclusions in martitized cristais of magnetite; iii) as submicroscopic particles adsorbed in goethite from the veins; and as iv) submicroscopic particles adsorbed in Fe-Al oxi-hydroxides of the duricrust. The supergene concentration of gold in Salobo and Pojuca-Leste sectors, envolved principally relative enrichment, and only subordinatelly absolute enrichment. In these areal, gold enclosed in refratary cristais of magnetite and quartz, favoured a relative enrichment during alteration of the primary mineralization. On the other hand, a minar amount of gold enclosed in copper sulphides (mainly chalcopyrite) was released, mobilized and reprecipited in the sulphide oxidation zone during several phases of the lowering of water table. Such evolution resulted in individualization of several enrichment levels, which shows variable lateral extension in the weathering profile. The main factor which controlled the secondary concentration of gold in Salobo and Pojuca-Leste areas was the forro of gold occurrence in the primary ore. In these sectors, gold armoured as tiny inclusions in magnetite and quartz crystals, dificulted the supergene remobilization process and resulted in a high relative enrichment.