Erschienen: 11.01.2007 Abbildung von Cabral | Palladiferous Gold Mineralisation (ouro preto) in Brazil: Gongo Soco, Itabira and Serra Pelada | 2006

Cabral

Palladiferous Gold Mineralisation (ouro preto) in Brazil: Gongo Soco, Itabira and Serra Pelada

lieferbar, ca. 10 Tage

2006. Buch. 115 S. 19 s/w-Tabelle, 52 Statistiken. Softcover

E. Schweizerbart. ISBN 978-3-510-95954-9

Format (B x L): 21 x 30 cm

Gewicht: 460 g

In englischer Sprache

Das Werk ist Teil der Reihe: Sonderhefte Reihe D - Geol. Jahrb.; 8

Produktbeschreibung

Gold mineralisation in which gold is alloyed with palladium is unusual. In Brazil, however, palladiferous gold is linked to large-scale gold rushes. In the Brazilian Gold Cycle (1695-1785), a Palaeoproterozoic banded iron formation (itabirite)-hosted, vein-style mineralisation presumedly accounted for much of the gold production from the Ouro Preto area, Minas Gerais. Remarkable examples of this mineralisation are Gongo Soco and Itabira. In recent times, a spectacular gold rush was triggered by bonanza-style ore of palladiferous gold at Serra Pelada, Carajás mineral province, Pará. Gongo Soco is an iron ore mine where gold is recovered, though intermittently, from cross-cutting, specular hematite-rich veins (jacutinga) in itabirite and soft hematite ore. Palladiferous gold, of variable composition in terms of Pd, Ag and Hg, occurs as nuggets and aggregates with hematite. Gold grains typically have a dark coating composed distinctively of Pd-O species together with iron oxyhydroxide, i.e. the so-called ouro preto (black gold). A number of platinum-group minerals (PGM) are included in nuggets of palladiferous gold: isomertieite, mertieite-II, chrisstanleyite and selenides of empirical stoichiometry of Pd5(Hg,Sb,Ag)2Se6 and (Pd,Sb,Ag,Hg)5Se4. Specular hematite included in gold is generally chromiferous. Native palladium is formed by a two-stage process of alteration: PGM ? Pd-O ? Pd. Cross-cutting relationships and the presence of mechanically twinned hematite suggest that at least some of the nugget gold is post-Brasiliano Orogeny in age (< 0.6 Ga). Fluid inclusion microthermometry in cogenetic specular hematite points to minimum temperatures of about 160°C and moderately saline fluids. Bulk-rock chemical analyses of auriferous (jacutinga) veins indicate enrichment in Au, Pd, Fe, Mn, Ba, Hg and Cr compared to fresh, non-altered itabirite. Itabira is a world-class iron ore district. Like Gongo Soco, auriferous veins truncate the foliated host rock (itabirite) and, consequently, a late- to post-Brasiliano age is inferred. Palladiferous gold and PGM occur as fracture infill and, occasionally, as inclusions in specular hematite. The PGM include palladseite, sudovikovite, isomertieite, atheneite and hongshiite (PtCu). Grains of hongshiite are haloed by a porous rim of native platinum, resembling the weathering-derived, coreand- mantle pattern of residual gold particles in placers and lateritic profiles. The veins are enriched in Au, Pd, Pt, Cr, Mn and Hg compared to host itabirite. The Serra Pelada Au-Pd-Pt deposit is hosted by weakly metamorphosed sedimentary rocks of Late Archaean age. Remains of its near-surface bonanza ore, now completely mined out, were studied in a drill core, recovered in 1982, with abundant coarse-grained dendrites of palladiferous gold in a completely disaggregated, ferruginous, clayey material. PGM, occurring as inclusions in, or attached to, palladiferous gold are "guanglinite", Sb-bearing "guanglinite", atheneite, isomertieite, seleniferous phases (sudovikovite and palladseite, as well as Pd-Pt-Se, Pd-Se, Pd-Hg-Se and Pd-Bi-Se phases), and native palladium. A mineral assemblage of fine-grained palladiferous gold (about 2 wt \% Pd), isomertieite and a Ba-bearing Mn oxide (possibly romanèchite) post-dates the dendrites of palladiferous gold (about 7 wt \% Pd). Fluid inclusions in quartz consist of Ca-rich brines that homogenise from about 100 to 175 °C. The drill core shows enrichments of up to three orders of magnitude for Ag, As, Ba, Cr, and Sb, and of four orders and more for Au, Pd, Pt and Hg compared to the country rock. Regardless of different geological settings and intrinsic particularities, all three deposits have mineralogical and geochemical features in common that point to highly oxidising (hematite stability field) and relatively shallow-level conditions, typical of hyd

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