Accessory Pyrite Research Articles

Geochemical evolution of laterites from two areas of the semiarid region in Bahia State, Brazil

Lateritic covers developed during the Pleistocene in two distinct areas (Gentio do Ouro District and Sento Sé) of the State of Bahia, Brazil, have been investigated geochemically. Samples of the bedrock (gabbros, from the Gentio do Ouro District, and silicate facies iron formation and associated iron-rich banded carbonates, from the Sento Sé Area) and the lateritic products (latosols, cuirasses and pisolites) were analyzed for major and selected trace elements (Co, Cu, Cr, Ni, V, Pb, Ba, Au and ree). The purpose of the research was to study the geochemical evolution of the laterites and the relationship of weathering processes to secondary gold mineralization. Four types of element mobilities are recognized, defined by progressive depletion (type A) or enrichment (type C) in the sequence latosol-cuirasse-pisolite or by extreme enrichment (type B) or depletion (type D) in the cuirasse. Cr and V have similar behavior (type C), whatever the type of parent rock: however, when the original rocks are gabbros, Co, Cu and Ni demonstrate behavior A, B and A, respectively. Also Ti has the same type of mobility (type A), whatever the type of parent rock. The concentrations of some residual elements, e.g., Ti, V, Mn, Fe and Au, are inherited from the original rocks, that is, they exhibit higher values in the lateritic materials on parent rocks in which their contents are high. The enrichment of ree in the lateritic materials on iron-rich carbonates is attributed to relatively basic and reducing conditions at depth. This environment allowed the ions of these elements (Ce, Sm, Eu and Yb in the reduced ionic form) to stay in solution along with Fe 2+. At shallow depht, under oxidizing conditions, the iron oxyhydroxides adsorbed the ree ions. This adsorption was more effective for La, Ce and Yb ions. The ree contents in the lateritic products from gabbros indicate a depletion of hree plus Eu, Nd and Sm are also depleted in cuirasses and Ce is depleted in pisolites. The loss of these ree and the slight concentration of lree in the red latosol of the Gentio do Ouro District are attributed to partial leaching of ree by a relatively acidic solution. The adsorption of ree by iron oxyhydroxides was less intense here than in the weathering of the iron-rich carbonates at Sento Sé Area. It was also more effective for La and Ce (ions). This type of weathering solution may be generated from oxidation of the accessory pyrite present in the gabbros of the Gentio do Ouro District. It may dissolve primary gold which then precipitates on encountering basic conditions at depth.

Petrology and Petrography of Neogene Sandstone Reservoirs of the East Slovakian Basin, Czechoslovakia

The Neogene East Slovakian basin forms the northwest depocenter of the Transcarpathian depression. It is a back-arc transtensional basin, approximately 100 km long with up to 7 km of sedimentary fill consisting of shallow-marine to continental clastics and locally abundant volcanities, volcaniclastics, and tuffs. Volcanism was most active during the Miocene (late Badenian to early Sarmatian), with subsidence and sedimentation rates reaching their peak during this period. The sandstones within the basin are primarily immature lithic arenites, commonly up to 8 m thick, derived from the Inner West Carpathians, from local volcanics, and from earlier flysch and molasse sediments. Sandstones are typically composed of very fine- to fine-grained subangular quartz, lithic fragments, and feldspars with accessory micas, chert, pyrite, clays, heavy minerals, and calcite cement. Carbonaceous material is present in many of the sandstones. Porosity has been severely reduced in many of the sandstones by carbonate cementation and alteration of unstable grains. Authigenic clay minerals and zeolites further reduce porosity. A complete paragenetic model has yet to be defined; however, the complexity of the diagenetic/paragenetic relationships suggests multiple episodes of alteration.

Precious and base metal selenide mineralization at Hope's Nose, Torquay, Devon

AbstractPrecious and base metal selenide minerals have been identified in gold-bearing carbonate veins cutting Middle Devonian limestones of the Torquay Limestone Group at Hope's Nose, Torquay. The selenide assemblage consists of clausthalite (PbSe), tiemannite (HgSe), klockmannite (CuSe), umangite (Cu3Se2), tyrrellite (Cu,Co,Ni)3Se4, trustedtite (Ni3Se4), penroseite (NiSe2), naumannite (Ag2Se), eucairite (AgCuSe) and fischesserite (Ag3AuSe2), only clausthalite having previously been reported from Britain. They are associated with palladian gold, gold, hematite, and accessory pyrite and chalcopyrite in a gangue consisting predominantly of calcite; alteration products include cerussite, malachite, aragonite and goethite.The relative abundance of Au, Ag, Hg and Se is a characteristic feature in the uppermost parts of some precious metal ‘epithermal’ systems. The occurrence at Hope's Nose is related to both structural and lithological factors: a deep-seated NW-SE structural lineament, the Lundy-Sticklepath-Lustleigh-Torquay fault; local thrusting, and to an association of basic-intermediate igneous rocks with a sedimentary sequence including carbonaceous shales and limestones. The mineralization is considered to be post-Variscan, probably Permo-Triassic in age.

Discovery of a sedimentary Ba(Fe, Zn, Pb) ore body of Frasnian age at Chaudfontaine, province of Li�ge, Belgium

In the light of recent drill holes, located in northern and north-eastern Belgium, the metallogenic importance of a stratigraphic horizon of upper Devonian age is pointed out. Indeed, in 4 drill holes, this horizon is weakly mineralized in pyrite, galena, sphalerite and barite Furthermore, at Chaudfontaine (a well-known spa) 3 other boreholes have intersected, at the same stratigraphic level, a barite ore body with accessory pyrite, sphalerite and galena. The main features of this ore body are described in this note. Numerous observations are mentioned which suggest that all the mineralized occurrences here considered are of sedimentary origin.

天草下島かやの木付近の変質流紋岩（陶石）脈および周縁変質岩中にみられる粘土鉱物

There are two groups of rocks in the Kayanoki area, Amakusa County; the Cretaceous sediments of the Himenoura group, and rhyolite dikes of Miocene epoch. The Himenoura group consists principally of shale, sandstone, and conglomerate. The rhyolite dikes intruding the Himenoura group are partly or entirely altered and have been mined as potter's clays. The potter's clays are composed of quartz, sericite, kaolinite, plagioclase (Ab95-Ab85), potassium feldspar, and accessory pyrite, siderite, calcite, and apatite. Fine-grained sericite is a mixture of 1M and 2M1 polymorphs; coarse-grained muscovite is 2M1 polymorph. The latter is found in the intensively altered rhyolite dikes. The sedimentary rocks in contact with the altered rhyolite dikes contain more sericite and kaolinite than the other sedimentary rocks and are associated with the interstratified sericite-montmorillonite and chlorite-expandable layer.