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Alexandre Raphael Cabral:

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

2006. 115 pages, 52 figures, 19 tables, 21x30cm, 510 g
Language: English

(Sonderhefte Reihe D - Geol. Jahrb., Heft 8)

ISBN 978-3-510-95954-9, paperback, price: 29.90 €

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Keywords

palladiferous gold mineralisation ouro preto Brazil Gongo Soco Itabira Serra Pelada

Contents

Abstract top ↑

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 hydrothermal selenide vein-type deposits.

[Über das Vorkommen von Palladium-haltigem Gold (ouro preto) in Brasilien: Gongo Soco, Itabira und Serra Pelada]

Goldmineralisationen, in denen Gold mit Palladium legiert ist, sind außergewöhnlich und selten. In Brasilien jedoch war Palladium-haltiges Gold Auslöser mehrerer großer Goldräusche. So trug im brasilianischen Goldzyklus (1695-1785) eine gangartige, in einer palaeoproterozoischen, gebänderten Eisenformation (Itabirit) auftretende Mineralisation wesentlich zur Goldproduktion des Gebietes um Ouro Preto in Minas Gerais bei. Bemerkenswerte Beispiele dieses Mineralisationstyps sind Gongo Soco und Itabira. In jüngster Zeit erfolgte ein spektakulärer Goldrausch auf Reicherze von Palladium-haltigem Gold in der Serra Pelada der Carajás-Mineralprovinz im Bundesstaat Pará.

Gongo Soco ist eine Eisenerzgrube, in der Gold immer wieder aus querschlägigen, Hämatitreichen Gängen (jacutinga) oder Disseminationen in Itabirit und aus mürben Hämatiterzen gewonnen wird. Das Palladium-haltige Gold hat eine variable Zusammensetzung bezüglich der Gehalte an Pd, Ag und Hg und tritt in Form von Nuggets und Aggregaten mit Hämatit auf. Die Goldkörner haben typischerweise einen dunklen Überzug, der wesentlich aus Pd-O-Verbindungen und Eisenoxyhydroxiden besteht und der diesem Gold die Bezeichnung ouro preto (schwarzes Gold) verlieh. In den Goldnuggets wurden einige Platingruppen-Minerale (PGM) gefunden, nämlich Isomertieit, Mertieit-II, Chrisstanleyit und Selenide mit der empirischen Stöchiometrie [Pd5(Hg,Sb,Ag)2Se6] und [(Pd,Sb,Ag,Hg)5Se4].

In Gold eingeschlossener Eisenglimmer (Hämatit) ist generell chromführend. Gediegen Palladium bildet sich in einem zweistufigen Alterationsprozess: PGM →Pd-O →Pd. Querschlägige Beziehungen und das Vorhandensein von mechanisch verzwillingtem Hämatit weisen darauf hin, dass zumindest einige der Goldnuggets altersmäßig jünger sind als die Brasiliano-Orogenese (< 0.6 Ga). Mikrothermometrische Untersuchungen an Flüssigkeitseinschlüssen in kogenetischem Hämatit deuten auf Minimaltemperaturen von etwa 160 °C und mäßig salinare Fluide hin. Gesamtgesteinsanalysen ergaben, dass die goldführenden Gänge (jacutinga) im Vergleich zu frischem, nicht alterierten Itabirit deutlich an Au, Pd, Fe, Mn, Ba, Hg und Cr angereichert sind.

Itabira ist ein Eisenerzdistrikt der ``world-class''-Kategorie. Wie in Gongo Soco schneiden die goldführenden Gänge die folierten Nebengesteine (Itabirit) und belegen daher ein spät- bis post- Brasiliano Alter der Vererzung. Palladium-haltiges Gold und PGM treten als Rissfüllungen und seltener als Einschlüsse in Hämatit auf. Palladseit, Sudovikovit, Isomertieit, Atheneit und Hongshiit [PtCu] sind die wesentlichen PGM. Hongshiit-Körner besitzen einen porösen Saum von gediegen Platin, ganz ähnlich den Mustern residualer Goldpartikel in Seifen und Lateritprofilen, die durch Verwitterung erzeugt wurden.

Die Serra Pelada Au-Pd-Pt-Lagerstätte befindet sich in schwach metamorphen sedimentären Gesteinen spät-archaischen Alters. Ein erhaltener Bohrkern von 1982 erlaubte, Partien der oberflächennahen, völlig ausgeerzten ``Bonanza''-Erze zu untersuchen. Auffällig sind häufige grobkörnige Dentriten Palladium-führenden Goldes in einem völlig zersetzten, eisenhaltigen, tonigen Material. Die PGM liegen als Einschlüsse oder verwachsen mit Palladium-haltigen Gold vor und bestehen aus ``Guanglinit'', Sb-führendem ``Guanglinit'', Atheneit, Isomertieit, Selenführenden Phasen (Sudovikovit und Palladseit sowie Pd-Pt-Se, Pd-Se,- Pd-Hg-Se- und Pd-Bi-Se- Verbindungen), und gediegen Palladium. Eine Mineralvergesellschaftung, bestehend aus feinkörnigem Gold mit etwa 2 Gewichts-\% Pd, Isomertieit und einem Ba-haltigen Mn-Oxid (wahrscheinlich Romanèchit), ist jünger als die Golddentriten, die etwa 7 Gewichts-\% Pd führen. Flüssigkeitseinschlüsse in Quarz bestehen aus Ca-reichen Solen, die zwischen etwa 100 bis 175 °C homogenisieren. Im Vergleich zum Nebengestein zeigt der Bohrkern starke Anreicherungen an Ag, As, Ba, Cr, und Sb (um bis zu drei Größenordnungen) sowie an Au, Pd, Pt und Hg (um bis zu vier Größenordnungen und mehr).

Abgesehen von den unterschiedlichen geologischen Milieus und spezifischen Besonderheiten besitzen die drei Lagerstätten doch gemeinsame mineralogische und geochemische Merkmale, die auf stark oxidierende (Stabilitätsfeld des Hämatits) Absatzbedingungen in einem relativ seichten Niveau hinweisen, die ganz typisch sind für hydrothermale Selenid-Ganglagerstätten.

Review: Economic Geology vol. 103 top ↑

This book gives a descriptive insight into the weathered nature and oxide mineralogy and petrology of three epigenetic Au-Pd-(Pt) orebodies within two distinct geologic terranes of Brazil. A thorough and descriptive petrological review of the Gongo Soco and Itabira jacutinga-type palladiferous gold orebodies, within the Quadrilátero Ferrífero, is used to compare these systems with the bonanza grade Au-Pd-Pt mineralization of the infamous Serra Pelada deposit in the Carajás mineral province. The volume also briefly describes deposit geology and possible fluid sources and characteristics for the various Au-Pd-(Pt) orebodies. However, it is the detailed petrological descriptions of the oxidized orebodies that adds significantly to the limited literature covering these epigenetic Au-Pd-(Pt) deposits. These descriptions greatly advance our understanding of these unique epigenetic Au-Pd-(Pt) deposits, which are broadly spaced in different parts of Brazil.

The book is composed of three main bodies of work that cover individually the banded iron formation-hosted Gongo Soco and Itabira jacutinga-style Au-Pd-(Pt) deposits and the carbonaceous rock-hosted Serra Pelada Au-Pd-Pt orebody. Descriptions of the three individual deposits each include brief geologic overviews of the deposit host rocks, deposit histories, and, in the case of the Quadrilátero Ferrífero deposits, descriptions of the more typical palladiferous gold mineralization, or ouro preto (black gold). The ouro preto is located within numerous jacutinga-style (specular hematite-rich veins) within the Paleoproterozoic banded iron formations of the Quadrilátero Ferrífero, central Minas Gerais state. The strength of the individual deposit descriptions is the thorough and detailed petrographical studies of the secondary Au-Pd- (Pt) mineral characteristics that is eloquently displayed with scanning electron microscope imagery from each deposit. The petrological studies are accompanied by electron-microprobe data, fluid inclusion microthermometry, and bulk-rock geochemistry for each deposit to further characterize their individual natures. In conclusion, the author discusses the Au- Pd-(Pt) composition of each deposit, gold-related mineral assemblages and the formation of secondary auriferous and palladiferrous gold mineralization within the Quadrilátero Ferrífero, and possible sources of ore fluids. The regional aspects of palladiferous gold mineralization within the Quadrilátero Ferrífero and Carajás mineral province are also briefly presented.

The author is to be congratulated on his descriptive work that has focused on the secondary nature and petrology of these unique epigenetic Au-Pd-(Pt) deposits that have, to date, only been marginally described in the scientific literature. This work will add greatly to the geologic understanding of these unique deposits within Brazil and globally.

CHRISTIAN J. GRAINGER

Economic Geology vol. 103, page 456

Contents top ↑

Preface 11
1 Introduction 13
2 Gongo Soco: a Typical Case of Palladiferous Gold Mineralisation (ouro preto)
17
2.1 Geological setting and the Gongo Soco iron ore mine 19
2.2 Jacutinga: palladiferous gold mineralisation in Minas Gerais 22
2.3 The jacutinga-style mineralisation of Gongo Soco 24
2.4 The Sump orebody 24
2.5 Sample material and analytical techniques 26
2.6 Nuggets and aggregates of gold 27
2.7 Ore microscopy and mineral chemistry 30
2.8 Gold composition 30
2.9 Minerals included in nuggets of gold 36
2.10 Pd-O compounds and native palladium 44
2.11 Fluid inclusion microthermometry 49
2.12 Bulk-rock geochemistry 51
3 Itabira: Another Case of Palladiferous Gold Mineralisation (ouro preto) 56
3.1 Geological setting 57
3.2 The jacutinga-style mineralisation of Itabira 58
3.3 Sample material and analytical techniques 59
3.4 Gold and associated platinum-group minerals (PGM) from heavy-mineral 60
concentrates
3.5 Composition of gold from heavy-mineral concentrates 67
3.6 Gold-bearing aggregates of fine-grained magnetite 67
3.7 Bulk-rock geochemistry 71
4 Serra Pelada: a Startling Case of Palladiferous Gold Mineralisation 74
(ouro preto)
4.1 Geological setting and the Serra Pelada deposit 76
4.2 Sample material and analytical techniques 78
4.3 Near-surface bonanza ore 79
4.4 Reconnaissance fluid inclusion microthermometry 85
4.5 Bulk-rock geochemistry 86
5 Discussion and Conclusions 91
5.1 Gongo Soco gold composition 91
5.2 Gold-related mineral assemblage and timing of mineralisation at
Gongo Soco 92
5.3 Gongo Soco fluid inclusion microthermometry 92
5.4 Formation of native palladium at Gongo Soco 93
5.5 Ouro preto from Gongo Soco and Itabira 93
5.6 Gongo Soco bulk-rock chemistry 93
5.7 Itabira auriferous mineralisation 94
5.8 Hongshiite-hosted mineral assemblage 95
5.9 Weathering of Pd- and Pt-bearing minerals from Itabira 96
5.10 Possible sources of fluids and palladiferous gold mineralisation
in the Quadrilátero Ferrífero 96
5.11 Serra Pelada bonanza ore 98
5.12 Quadrilátero Ferrífero and Carajás: regional aspects of palladiferous
gold mineralisation 99
6 Concluding Remarks 101
7 Acknowledgements 103
8 References 104