Gold, iron and manganese in central Amapá, Brazil

Abstract

ABSTRACT:Greenstone belts with deposits of gold, iron and manganese are common in the Paleoproterozoic Maroni-Itacaiunas Tectonic Province of the Guiana Shield. In Brazil, in the State of Amapá and northwest of Pará, they are represented by the Vila Nova Group, constituted by a basal unit of metabasalts, covered by metasediments of clastic and chemical origin. The basal metasediments, the Serra do Navio Formation, are made of a cyclothem with lenses of manganese marbles at the top of each cycle. Under the intense weathering of the Amazon, these lenses were oxidized to large deposits of high-grade manganese oxides. The exploitation of these oxides left behind the manganese carbonates and low-grade oxides. The overlaying Serra da Canga Formation presents a calcium and magnesium domain grading to an iron domain with banded silicate and oxide iron formations, mined for iron ores. Overlapping structures and superposed metamorphic crystallizations indicate two phases of dynamothermal metamorphism, the first one with axis to north-northeast and the second one to northwest, with an intermediate phase of thermal metamorphism related to syntectonic granitic intrusions. Shears oriented north-south, possibly formed during the first dynamothermal metamorphism and reactivated in the second, are ideal sites for hydrothermalism and gold mineralization, which is greater when occurs in iron formation and carbonate-bearing rocks, as it happened at the Tucano mine. Layered mafic-ultramafic intrusions in the greenstones represent a potential for chromite and platinum group elements. Pegmatites are source of cassiterite and tantalite exploited from alluvial deposits. KEYWORDS:Group Vila Nova greenstones; Manganesiferous Precambrian cyclothem; Precambrian iron formation; Precambrian mafic-ultramafic complexes; Superposed metamorphic cycles. RESUMO: A Província Tectônica Paleoproterozoica Maroni-Itacaiunas estende-se ao longo de toda a área costeira do Escudo das

Chromium, iron, gold and manganese in Amapá and northern Pará, Brazil

“…We revise the published information, updating it with observations recently made in the field by one of the authors. We show that exposures at new mining excavations and aero-geophysical information demonstrate that the mafic-ultramafic complex is intrusive in the Vila Nova Group, similarly to what we saw at the area of the Serra da Canga (Scarpelli & Horikava 2017), and that the auriferous quartzite-conglomerate unit of the area occurs at the top, and not at the base, of the column of metasediments, actually overlaying the Vila Nova Group. Examining the Água Boa auriferous deposit, we saw a very good degree of similarity between the principal structural control of the gold mineralization of this deposit with that of the Tucano Mine, practically outlining a pattern worth to follow in general exploration for gold in the MIP.…”

“…A detailed petrographic study of the Amapari Greenstone belt at Serra do Navio has shown that it was subject twice to regional metamorphism, with an intermediate episode of thermal metamorphism caused by syntectonic granitic intrusions. Local fold axis and elongation of greenstone to north and northeast suggest that these structures might be remnants of the first regional metamorphism (Scarpelli & Horikava 2017).…”

“…These rocks are covered by a well foliated biotite schist and, closing up the cycle, with graphite schist locally with a lens of fine to medium-grained spessartite-bearing rhodochrosite marble. Gondites, formed by coarse-grained spessartite, manganese silicates and graphite, appear in the transition zone between the manganesiferous marble and the graphite schists (Scarpelli & Horikava 2017). This formation was only seen around Serra do Navio.…”

“…The Jornal Formation orthoamphibolites are massive, medium to coarse grained, nematoblastic without a clear layering, formed mostly by hornblende, plagioclase, quartz and accessory magnetite. Occasionally, there are intercalations of quartz-mica schist, well banded actinolite schists and other schists (Scarpelli & Horikava 2017).…”

“…Following our description of the geology of the Amapari Greenstone Belt near Serra do Navio, Amapá, and its deposits of manganese, iron and gold (Scarpelli & Horikava 2017), we now expand laterally the limits of our area of observation and describe other areas of the Paleoproterozoic Maroni-Itacaiúnas Tectonic Province (MIP) presenting similar geology and mineralization. Actually, the area object of the present note covers the MIP from the coast of Amapá, at east, to the headwaters of the Ipitinga River, in the north of the Pará State, at west.…”