Gold has been exploited intensively in the Brazilian Amazon during the past fifteen years using garimpo methods (small-scale gold mining). In this study, two gold mining areas were investigated, the municipalities of Pocone and Alta Floresta located in the state of Mato Grosso, Central Brazil. The elemental mercury (Hg) used in amalgamating the gold, the fir.al stage of the ore dressing process, has caused abnormal Hg concentrations in waterways. This has occurred principally in the Amazon region, where most of the ore prospected is alluvial. Background levels of metals were determined by analyzing sediments and soils located upstream of the anthropogenic inputs and unaffected by mining activities. The study aimed to evaluate the pollution level in sediments and soils, taking into account drainage waters directly affected by gold mining. 'Geoaccumulation indexes' (Igeo) of Hg in sediments from both study areas were used to assesses the pollution level in the aquatic environment. The geoaccumulation indexes of Hg in sediments of the Bento Gomes River in Pocone indicate a relatively high degree of pollution at some sites, even reaching class 4 (1.85 mg/kg). However, when the river reaches the Pantanal swamp, Hg concentrations drop considerably to 0.30 mg/kg. This drop seems to be due to accumulation of metals in the sediments of a lake (sampling site PG-24), which retains most of the sediments transported by the Bento Gomes River. Accumulation of metals in the lake also occurred for Cu, Pb, Zn, Fe and Mn. In the region of Alta Floresta, total Hg concentrations in sediments of the Teles Pires River were studied in the grain size fractions < 74 /urn and > 74 /xm. Hg concentrations in bottom sediments of this river were higher than those found in the Pocone region, with increases of 1.5 to 30 times the background, and thus reaching an Igeo up to class 5.

The Kokchetav microcontinent or the Kokchetav terrain distinguished by E.D. Shlygin [1] is a Precam-brian subplatform structure within the completed and uncompleted Caledonides. The Precambrian Kokchetav microcontinent is well distinguished in the geophysical fields, where the thickness of the Earth’s crust within the Continent ranges from 42 to 45 km [2], and the bounds of the microcontinent are traced by deep tectonic structures. Metamorphic rocks, developed within the microcontinent are divided into two series. The thickest lower (Zerendinskaya) series [3] consists mainly of metamorphic rocks of amphib-olite facies metamorphism. The upper (Sharykskaya) series is composed of Late Precambrian rocks metamorphosed under the green schist metamorphism. Both these series are separated by metamorphic and stratigraphic unconformity [3]. Precambrian rocks of both series were intruded by anomalously large masses of the Caledonian granitoids [4, 8]. The age of the rocks of the Kokchetav microcontinent remains a debatable topic. The analysis of the model age of the metaterrigenous rocks of the Zerendinskaya series (2.1–2.5 Ba) [4] and diamond-bearing gneisses of Kumdy-Kul (2.2–2.3 billion years) [5] allows us to conclude that the sedimentation of terrigenous material occurred as a result of the erosion of the Earth’s crust mainly in the Paleoproterozoic.

Plate tectonics, as it operates in the modern earth, represents in a fundamental sense a mechanism by which excess thermal energy from the mantle is dissipated (Sclater et ai., 1980). A substantial data base now exists concerning the operation of plate tectonics during Phanerozoic time, and evidence for similar tectonic activity during Proterozoic time appears to be steadily growing, both in quantity and quality.

The report that follows presents a first statement of the results of a study sponsored by the European Economic Community and the Department of Trade and Industry as communicated to a meeting of the Institution of Mining and Metallurgy Midlands Section at which the President (Dr A. J. Robinson) and the Section Chairman (Professor N. A. Warner) were present. It was my privilege to attend and to make some concluding remarks; these form the basis of this preface. The Geological Survey, now in its 154th year, has as one of its principal objects the provision of geological maps based on surveying in maximum detail at a scale of 1:10 000. In areas where extractive or other engineering industry is important, the surveys have been revised more than once as new subsurface data became available.