Gold, iron and manganese in central Amapá, Brazil

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.摘要：含有金、铁、锰矿床的绿岩带（Greenstone belts）在圭亚那地盾（Guiana Shield）的古元古代马罗尼-伊塔卡伊纳斯构造省（Paleoproterozoic Maroni-Itacaiunas Tectonic Province）中分布广泛。在巴西阿马帕州（Amapá）及帕拉州（Pará）西北部，这类绿岩带以维拉诺瓦群（Vila Nova Group）为代表，该群由一套玄武质变质岩（metabasalts）基底单元构成，其上覆有碎屑成因与化学成因的变质沉积岩（metasediments）。这套基底变质沉积岩即塞拉杜纳维奥组（Serra do Navio Formation），由旋回层（cyclothem）系组成，每个旋回层顶部均发育锰质大理岩（manganese marbles）透镜体。在亚马逊地区强烈风化作用下，这些透镜体被氧化形成大型高品位氧化锰矿床。对这类氧化锰矿床的开采后遗留下了锰碳酸盐矿物与低品位氧化锰矿石。上覆的塞拉达坎加组（Serra da Canga Formation）发育钙镁质域，并逐渐过渡为含铁域，其中包含条带状硅酸盐铁建造与氧化物铁建造，这些铁建造是铁矿石的开采对象。叠加构造与变质重结晶现象表明该区经历两期动热变质作用（dynamothermal metamorphism）：第一期变质轴为北东向，第二期为北西向，其间还发育一期与同构造花岗岩侵入体（syntectonic granitic intrusions）相关的热变质作用。南北向剪切带（shears）极可能形成于第一期动热变质作用，并在第二期活动中被重新活化，这类剪切带是热液作用（hydrothermalism）与金矿化（gold mineralization）的理想场所，在铁建造与含碳酸盐岩地层中金矿化作用更强，图卡诺矿（Tucano mine）的成矿情况便是典型实例。绿岩带中的层状镁铁质-超镁铁质侵入体（mafic-ultramafic intrusions）具备铬铁矿（chromite）与铂族元素（platinum group elements）的成矿潜力。伟晶岩（pegmatites）是锡石（cassiterite）与钽铁矿（tantalite）的物源，相关矿物可从冲积矿床（alluvial deposits）中开采获取。