Early Proterozoic ensialic spreading-subsidence: evidence from the Garpenberg enclave, Central Sweden

The Early Proterozoic metavolcanic complex in the Garpenberg enclave of central Sweden displays bimodal basaltic-rhyolitic volcanism reflecting alternating compressional and tensional tectonic regimes. The acid metavolcanics show a continuous calcalkaline differentiation trend geochemically unrelated to the associated interbedded metabasalts. A dacitic horizon indicates mixing of the two end-member magmas during a compressional stage. Bimodal volcanism is characteristic of the Early Proterozoic supracrustals (Leptite formation) throughout the central Swedish province of the Baltic shield. The paucity of ocean floor rocks and high-pressure metamorphism together with the lack of abundant andesites suggest that this province does not represent an island arc. Rather the combination of an ensialic edifice with alternating compressional and tensional tectonic regimes suggests Andinotype, subduction-coupled spreading-subsidence within the margin of a continental plate

Genesis of an early Proterozoic zinc deposit in high-grade metamorphic terrane, Saxberget, central Sweden

The Saxberget strata-bound Zn-Pb-Cu sulfide deposit of early Proterozoic age (ca. 1.9 Ga) is located in central Sweden. It is hosted by a supracrustal sequence of quartz-feldspar and quartz-mica gneisses, with minor intercalations of amphibolite, marble, and calc-silicate gneiss (skarn). The area has undergone several metamorphic events subsequent to syndepositional hydrothermal alteration: regional high-temperature and moderate- to low-pressure metamorphism in the upper amphibolite facies up to granite melting; medium-grade, low-pressure metamorphism associated with granite intrusions; and retrograde metamorphism in the greenshist facies probably related to a burial metamorphic event. The metamorphism was essentially isochemical and bulk chemical changes to the host rocks were premetamorphic and probably prelithification.Geochemical and petrographic evidence indicates a volcanic protholith for the gneisses and amphibolite. They are the metamorphic equivalents of mainly felsic pyroclastic rocks, whereas amphibolites are metamorphosed mafic volcanic rocks of basaltic composition. The quartz-mica gneisses represent the metamorphic equivalents of hydrothermally altered felsic volcanic rocks. The intercalation of metavolcanoclastic and metasedimentary rocks (marble, skarn, and reworked volcanics) indicates a shallow-water submarine environment for this ore-bearing sequence. Geochemically the present composition of the metavolcanic rocks displays calc-alkaline affinity, and their bimodal character suggests a tensional environment of deposition.The Zn-Pb-Cu sulfide mineralization at Saxberget is mainly bounded to a calc-silicate horizon which laterally grades into dolomitic marble. The orebodies trend in a general east-west direction and dip 35 degrees to 45 degrees S, with a horizontal extension on the surface of about 500 m and more than 1,500 m in the pitch direction (southeast); the thickness can reach up to 40 m. From the footwall to the hanging-wall side, the ores include stratiform massive ball ore, semi-massive to disseminated strata-bound Zn-Pb ores in calc-silicate gneiss ("skarn ore"), and disseminated Cu-rich ore in quartz-mica gneiss. Large-scale zoning is observed in the mineralization, the massive Zn-Pb-rich ball ore dominates in the western part of the mine and disseminated skarn ore and Cu-rich ore are more common in the eastern part of the mine. The present footwall rocks are banded felsic metavolcanics which are normally less altered and deformed than the hanging-wall rocks. The contact between the footwall rocks and the ores is partly defined by the ball ore, which appears to represent a tectonically deformed sulfide body along a significant thrust. Upward, the ball ore grades into or is in contact with the skarn ore, which shows intense small-scale folding. The hanging wall of the ore-bearing horizon consists of strongly altered metavolcanic and metasedimentary rocks. Geochemically, the hanging-wall rocks are depleted in Na and Ca and are enriched in Mg and Fe, displaying a geochemical zoning relative to the orebodies. This geochemical zoning is mineralogically expressed in the hanging-wall rocks by orthoamphibole-bearing and spinel-bearing assemblages close to the ore and followed outward by the garnet-bearing quartz-mica gneisses which in turn are succeeded by the cordierite-bearing quartz-mica gneisses.The geochemistry and interrelationships of the various petrologies strongly suggest that the stratigraphic sequence at Saxberget is inverted and that the ore represents a proximal exhalite located on a subsurface alteration pipe. The deposit is classified with the Broken Hill type of middle Proterozoic stratiform sulfides, which is represented in the Phanerozoic by the familiar sedimentary exhalative group of stratiform zinc-lead-copper deposits in sedimentary rocks

Gold dispersion in Andean desert environments (Atacama, Chile)

The mechanism of Au dispersion in sediments of mountainous desert environments has been studied in two different areas of the Chilean Andes. The San Pedro de Cachiyuyo placer (ca. 1800 m a.s.l.) consists of alluvial fans and ephemeral stream sediments deposited in a stable piedmont. The primary Au source is Cu-Au-bearing hydrothermal tourmaline breccia. The relief of this area is low (1850 m) and the average annual rainfal is ca. 20 mm. The La Coipa area (ca. 4000 m a.s.l.) is characterized by a rugged relief. The annual average rainfall is ca. 100 mm. The sedimentary deposits are less sorted than in San Pedro de Cachiyuyo and consist of ephemeral stream alluvium and slope deposits formed in a periglacial environment. The primary Au sources are two volcanic-hosted epithermal precious metal deposits. At San Pedro de Cachiyuyo, the halo is less than 1 km in length and the highest Au concentration ware observed at the break in slope between the hillside and the piedmont. In the La Coipa area, Au was only detected by chemical analysis in the 125-63 micrometers and less than 623 micrometer fraction ; however,the dispersion halo extends for over 10 km from the source. In both cases the geochemical signal of gold was strongest in the fraction less than 63 micrometers. (Résumé d'auteur

Fluid inclusion studies of epithermal gold-bearing quartz veins in the La Libertad district, Nicaragua

The La Libertad mining district in central Nicaragua, is an epithermal deposit of Au-bearing quartz veins hosted in Tertiary basic to intermediate volcanic rocks. Fluid inclusion studies in quartz from the epithermal veins show that the ore forming solution varies in temperature between 172 and 316°C with a mode around 230-240°C. The salinily of the was in the, range of 0.98-2.10 eq. wt.% NaCl. The original ore solution involved is defined as dominated by meteoric water and sodium chloride (NaCl), as the main component, no CO2 was detected. The minimum depth of emplacement of the deposit was probably between 370-570 m. This gives an estimated erosion rate of 2.4-4.4 cm/1000 years and an estimated thermal gradient for the district of greater than 150°C/km. Due to the close relationship between the ore forming processes and volcanism in the district, the epithermal deposit at La Libertad appears to be the result of magmatic hydrothermal systems.El distrito minero de La libertad es un yacimiento epitermal con vetas de cuarzo aurífero dentro de rocas volcánicas terciarias de composición básica hasta intermedia. Los análisis microtermométricos de las inclusiones fluidas dentro del cuarzo indican que la temperatura de las soluciones hidrotermales varía entre 172 y 316° C, con una promedio alrededor de los 230-240° C. La salinidad de las soluciones varía entre 0,98 y 2,10 %eq. peso NaCl. La solución metalífera consistió principalmente de aguas meteóricas con NaCl como sal principal; no se detectó CO2. La profundidad del yacimiento era probablemente entre 370 y 570 m. La tasa de erosión estimada era de 2,4- 4,4 cm/1000 años y el gradiente geotérmico era mayor de 150° C/km. Se supone que el yacimiento epitermal de La Libertad se originó por sistemas hidrotermales relacionados con la actividad volcánica