Palladian Gold Research Articles

Palladian Gold: Chemical Composition, Minerals in Association, and Physicochemical Conditions of Formation at Different Types of Gold Deposits

This paper reviews and summarizes the available information on the composition of palladian gold with various contents and sets of isomorphic impurities (Ag, Cu, Hg) at 50 deposits and ore occurrences with Au-Pd mineralization. It is revealed that Palladian gold is represented by the systems Au–Pd, Au–Pd–Hg, Au–Pd–Cu, and Au–Pd–Ag–Hg, but more frequently corresponds to Au–Pd–Ag, Au–Pd–Ag–Cu, and Au–Pd–Ag–Cu–Hg. Objects with palladian gold belong to different types of gold deposits and to the deposits at which the main components of ores are PGE, Cr, Cu, Ni, V, and Ti. We propose a classification of the types of deposits with palladian gold: (1) PGE ore deposits related to mafic–ultramafic magmatic complexes (two subtypes—(a) low-sulfide-grade (less than 2%–5% sulfides) Alaskan, and (b) high-sulfide-grade (more than 5% sulfides) Norilsk); (2) orogenic gold deposits (OG); (3) epithermal (porphyry) gold–copper deposits (EPGC); (4) iron oxide copper gold deposits (IOCG); (5) ferruginous quartzite deposits; (6) volcanic exhalation; and (7) gold-PGE placers of five subtypes corresponding to the types of 1–5 primary sources. Physicochemical conditions of the formation of palladian gold at some deposits of type 1 cover two areas—magmatic high-temperature and hydrothermal low-temperature. At the majority of deposits of types 2–4, its formation proceeds with the participation of hydrothermal fluids (300–60 °C) of various salinities (0.2–30 wt.% NaCl eq.). Palladian gold is mainly high-fineness (910‰–990‰), is less frequently medium-fineness, and contains Ag and Cu, but does not contain Hg at the deposits of types 1, 3, and 4. The only exception is the Au-Pd-Hg Itchayvayam ore occurrence (Kamchatka, Russia), for which two varieties of Pd,Hg-bearing native gold (fineness 816‰–960‰ and 580‰–660‰) are determined. Low-fineness palladian gold with the major content of Ag is typical of OGD deposits. Medium-fineness palladian gold occurs at ferruginous quartzite deposits and in volcanic exhalations. Hg, Ag, Cu-bearing high-fineness palladian gold is present mainly in placer deposits (type 7). The most common minerals in association with palladian gold are arsenides, stibioarsenides, sulfides, stannides, bismuthides, tellurides, and selenides of Pd and Pt. These are typical of deposit types 1 and 7. The minerals of Au, Ag, and Cu (tetra-auricupride, aurostibite, chalcopyrite, bornite, chalcocite, eucairite, etc.) are in association with palladian gold at OG, EPGC, and IOCG deposits. Hg minerals (cinnabar, tiemannite, coloradoite, potarite) are at some deposits (types 1, 2, 7-1, 7-4). Cu, Fe, and Pd oxides (tenorite, hematite, magnetite, PdO, (Pd,Cu)O) and Fe and Pd hydroxides (goethite, (Fe,Pd)OOH) occur at the deposits of the 3, 4, and 7 groups and indicate the highly oxidizing conditions of ore formation. The most common minerals among host minerals are quartz and muscovite, including fuchsite (Cr-Ms), chlorite, albite, K-feldspar, hornblende, and carbonates (calcite, siderite, etc.). The fineness, content, and set of impurities in palladian gold and minerals in association with it reflect the mineralogy of Au-Pd ores and allow them to be used as indicators for the deposit types.

Palladiferous placer gold from the Paraíba do Sul River in northern Rio de Janeiro (Brazil) and its geological implications

ABSTRACT Placer gold recovered from the Paraíba do Sul River, which flows along a high-grade shear zone in northern Rio de Janeiro, Brazil, has a distinctive component of palladiferous gold. Its finding, reported for the first time in the state of Rio de Janeiro, implies a hydrothermal system of high salinity at relatively low temperatures, in which metal transport is facilitated by hematite-rich rocks and generally precipitated at redox fronts. Such a hydrothermal system is not geologically manifested in the currently exposed high-grade shear zone. The finding of palladiferous gold suggests that its source was eroded during periods of increased denudation rates since the Lower Cretaceous, as indicated by apatite fission-track (AFT) data that have been reported for the region.

Ore mineralogy of the Serra Pelada Au-Pd-Pt deposit, Carajás, Brazil and implications for ore-forming processes

Serra Pelada is a world-class hydrothermal Au-Pd-Pt deposit located at the eastern border of the Amazon craton, northern Brazil. The rocks at Serra Pelada have experienced intense tropical weathering for about 70 Ma, but drill core samples preserve the primary mineralogy and hydrothermal alteration features, with extreme grades of Au, Pd and Pt individually reaching hundreds of parts per million (ppm) by weight. Mineralization at Serra Pelada occurs in hydrothermally altered metasiltstones and dolomitic metasandstones at the hinge zone of a recumbent syncline, comprising zones of hematite, chlorite-carbon, argillic, and siliceous alteration. The main hydrothermal gangue minerals are quartz, kaolinite, sericite, amesite, hematite, monazite, florencite and variable amounts of highly reflective carbonaceous matter. Hydrothermal carbon input is evident from precipitated carbon occurring along crenulation planes and veinlets associated with the precious metals. Ore and accessory minerals include a variety of sulphide, selenide, arsenide, sulphate and oxide minerals, including gold with variable metal contents, palladian gold, fischesserite, sudovikovite, sperrylite, selenian braggite, isomertieite, mertieite-II and secondary Au-Pt-Pd alloys. The composition of fischesserite varies from the ideal formula (Ag3AuSe2) towards a more Ag-rich composition, indicating a disordered solid solution form that is stable only above 260 °C, consistent with the high thermal maturity of associated carbonaceous matter approaching graphite. Primary ore and gangue minerals at Serra Pelada comprise a suite of elements that are best transported in oxidising conditions and precipitated upon reduction. This suggests that fluid mixing between a highly oxidised (metal carrier) and a reduced fluid was a key process for high-grade noble metal precipitation at Serra Pelada.

LATE CAMBRIAN Au-Pd MINERALIZATION AND Fe ENRICHMENT IN THE ITABIRA DISTRICT, MINAS GERAIS, BRAZIL, AT 496 Ma: CONSTRAINTS FROM U-Pb MONAZITE DATING OF A JACUTINGA LODE

Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in the province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilatero Ferrifero of Minas Gerais, the ~2.0 Ga Transamazonian event and the ~0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceicao Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaco, along the platiniferous Au-Pd belt of Minas Gerais.

Geology, mineral chemistry and tourmaline B isotopes of the Córrego Bom Sucesso area, southern Serra do Espinhaço, Minas Gerais, Brazil: Implications for Au–Pd–Pt exploration in quartzitic terrain

The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au–Pd–Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to ~ 6 wt.% TiO 2). The latter minerals connect the alluvial mineralisation to the rutile–hematite–quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg + Fe) and X/(X + Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~ 1 to − 10.4‰ δ 11B. The B-isotope range, in conjunction with the Na–Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500 °C to ~ 350 °C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age.

LATE CRETACEOUS BONANZA-STYLE METAL ENRICHMENT IN THE SERRA PELADA Au-Pd-Pt DEPOSIT, PARA, BRAZIL

A 40 Ar/ 39 Ar age of 75 ± 6 Ma (2 σ ) was obtained for aggregates of Mn-Ba oxide intergrown with palladiferous gold and Pd-Pt minerals, representative of near-surface bonanza Au-Pd-Pt mineralization that triggered the gold rush to Serra Pelada in northern Brazil. The mineralization occurs in goethite-rich tectonic breccia and records recurrent fault displacement in the still active Cinzento strike-slip fault system. The goethite-rich tectonic breccia is truncated by the bonanza-style Au-Pd-Pt-Mn-Ba enrichment, with which tabular hematite is spatially associated on a micrometric scale. The tabular hematite is coarsely crystalline, >5 μ m in grain size, and lines cavity walls in the goethite-rich breccia. The presence of coarsely crystalline tabular hematite constrains the metalliferous overprint to temperatures in excess of about 100°C. We suggest that the bonanza mineralization is a Late Cretaceous, epithermal-like, fault-breccia system that overprinted Paleoproterozoic intrusion-related Au-Pd-Pt mineralization preserved at depth.

THE PLATINIFEROUS Au-Pd BELT OF MINAS GERAIS, BRAZIL, AND GENESIS OF ITS BOTRYOIDAL Pt-Pd AGGREGATES

We delineate an approximately 240-km-long, north-trending Au-Pd-Pt belt in Minas Gerais, Brazil, which hosts hydrothermal Pd-Pt-bearing gold mineralization in its southern part (Quadrilatero Ferrifero and Itabira district) and alluvial Pd-Pt-Au mineralization in its northern part (southern Serra do Espinhaco). The hydrothermal Pd-Pt-bearing gold mineralization, locally known as jacutinga, occurs as late-orogenic quartz-hematite-(talc-kaolinite) veins, which truncate the Brasiliano regional tectonic foliation of the itabiritic host rocks of the Paleoproterozoic Itabira Iron Formation. The alluvial Pd-Pt-Au mineralization occurs as botryoidal Pd-Pt nuggets and delicate palladiferous gold aggregates in clastic and organic-rich valley infill on quartzite and hematitic rocks of the Paleo- to Mesoproterozoic Espinhaco Supergroup. The Au-Pd-Pt belt follows the regional trend of major thrust faults related to the ~0.6 Ga Brasiliano orogeny, and the hydrothermal jacutinga mineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum. Secondary botryoidal and arborescent Pt-Pd aggegates and primary (detrital) Au-Pd grains are peculiar to the alluvial platiniferous deposits of the northern part of the belt in the Serra do Espinhaco. We suggest that their source was jacutinga-style vein mineralization, as deduced from abundant detrital hematite and rutile, and the discovery of detrital hongshiite, a Pt-Cu alloy that forms bonanza pockets within jacutinga at Itabira. Reconnaissance laser ablation-inductively coupled plasma-mass spectrometry analyses for trace metals in the botryoidal Pt-Pd grains show that they have elevated Tl and Se contents of 0.9 to 354 and 54 to 2,700 ppm, respectively. These elements correlate positively, with a Tl/Se ratio of about 0.08, a value that contrasts with Tl/Se >1 for hongshiite from the hydrothermal jacutinga system at Itabira. The low and roughly constant Tl/Se ratios of the alluvial samples are close to the average value of 0.12 for fluvial waters. We suggest that the botryoidal Pt-Pd aggregates grew in situ by electrochemical metal accretion from dilute solutions carrying Pt and Pd from the dissolution of detrital Pt-Pd minerals from jacutinga. The connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend of about 240 km for Pd-Pt-bearing gold mineralization reaching from Ouro Preto to Diamantina. The very high oxidation state required for efficient low-temperature aqueous mobilization of Pt, Pd, and Au was apparently controlled by the quartzitic and hematitic rocks in the Serra do Espinhaco, but not in the southern belt where greenstone rocks buffered fluids to moderate pH and lower oxidation states.

Arborescent palladiferous gold and empirical Au2Pd and Au3Pd in alluvium from southern Serra do Espinhaco, Brazil

Arborescent palladiferous gold and empirical Au2Pd and Au3Pd in alluvium from southern Serra do Espinhaco, Brazil

Palladiferous Gold Mineralisation (ouro preto) in Brazil: Gongo Soco, Itabira and Serra Pelada.: ALEXANDRE RAPHAEL CABRAL. Softcover. Pp. 115. E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers. 2006. ISBN 3-510-95954-X/9783510959549. Price $29.99 GBP.

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 ore-bodies, within the Quadrilatero Ferrifero, is used to compare these systems with the bonanza grade Au-Pd-Pt mineralization of the infamous Serra Pelada deposit in the Carajas mineral province. The volume also briefly describes deposit geology and …

Seleniferous minerals of palladium and platinum from ouro preto-bearing mineralisation in Brazil

Selenium is a trace, but characteristic element in the bonanza-style mineralisation of palladiferous gold that triggered two major gold rushes in Brazil (Minas Gerais and Serra Pelada). The auriferous mineralisations of Gongo Soco and Itabira, Minas Gerais, and Serra Pelada, Pará, have Pd–O-bearing, dark-coloured ferruginous coatings on gold ( ouro preto). Depending upon the amount of palladium alloyed with gold, a number of seleniferous minerals of palladium and platinum can be found: chrisstanleyite, Ag 2Pd 3Se 4, a phase analogous to tischendorfite, Pd 8Hg 3Se 9, an argentiferous palladseite-like phase, empirical Pd 5(Hg,Sb,Ag) 2Se 6 and (Pd,Sb,Ag,Hg) 5Se 4, at Gongo Soco; palladseite, ideally Pd 17Se 15, and sudovikovite, PtSe 2, at Itabira; and, exclusively to Serra Pelada, empirical Pd 9PtSe 2, Pd 9Se 2, Pd 3(Se,Bi) and (Pd,Hg,Pb) 3Se. This mineralogical signature of selenium is typical of hydrothermal vein-type deposits formed at relatively low temperatures. Although the synthetic Pd 9Se 2 equivalent is stable at temperatures over 390 °C, fluid inclusion data suggest lower temperatures of formation (< 150 °C). The exclusiveness of Se-bearing, alloy-like phases of palladium to Serra Pelada testifies to its very Pd-rich hydrothermal system.

Evidence for metre-scale variations in hematite composition within the Palaeoproterozoic Itabira Iron Formation, Minas Gerais, Brazil

AbstractHematite is a mineral the chemical composition of which rarely differs significantly from stoichiometric Fe2O3. As such, little attention has been paid to the mineral chemistry of hematite in Precambrian iron formations, where hematite forms monomineralic high-grade orebodies. Electron microprobe analysis of hematite from two iron-ore deposits, Cauê (Itabira district) and Gongo Soco, in the Palaeoproterozoic Itabira Iron Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil, has revealed distinct variations in chemical composition with respect to Ti and Cr. Hematite containing Ti and/or Cr is of very local occurrence in the itabirite unit and shows a spatial relationship to hematitic, palladiferous gold-bearing veins (known as ‘jacutinga’), occurring either within the veins (adjacent to, or included in, palladiferous gold grains) or in their vicinity. Where present, titaniferous hematite (to ∼1.3 wt.% TiO2) is lepidoblastic and defines a pervasive tectonic foliation (S1). In contrast, Ti-free, chromiferous hematite (to ∼6.4 wt.% Cr2O3) characteristically occurs as inclusions in palladiferous gold within S1-truncating ‘jacutinga’. Replacement of granoblastic, Ti-free, chromiferous martite with relicts of magnetite by lepidoblastic, Cr-depleted, titaniferous hematite proves that Cr and Ti were mobile during metamorphism. Chromium was ultimately fractionated into the hematite found in auriferous aggregates within cross-cutting ‘jacutinga’. A positive correlation between Cr and Pt in bulk-rock samples from the Itabira district suggests that Cr is a potential prospective guide for Au-Pd-Pt-bearing hematitic veins (‘jacutinga’).

A two-stage process of native palladium formation at low temperatures: evidence from a palladian gold nugget (Gongo Soco iron ore mine, Minas Gerais, Brazil)

AbstractNative palladium is rare and occurs in oxidizing environments (hematite stability field) and consistently has very low contents of other metals. On the other hand, gold may contain significant amounts of alloyed palladium. A nugget of palladian gold, obtained from a specularite-rich auriferous vein (‘jacutinga’) of the Gongo Soco iron ore mine, Quadrilátero Ferrífero, Minas Gerais, allows the definition of a paragenetic sequence of native palladium formation. The cm-sized nugget is brecciated and consists of fragments of palladian gold [Au0.79Pd0.09Cu0.07Ag0.04Hg0.01] with inclusions of isomertieite [Pd11Sb2As2], cemented by kaolinite-hematite-palladian gold infill. The isomertieite inclusions are preferentially fractured and display various degrees of alteration to a Pd-O phase. Native palladium occurs as a crystallized component within the Pd-O phase, with microstructural and chemical features suggesting a desiccation process. It is proposed that native palladium forms by (1) oxidation of a pre-existent palladium mineral to a Pd-O phase, and (2) subsequent crystallization of native palladium from the Pd-O phase, by oxygen loss and dehydration. This process operates in a low-temperature hydrothermal or weathering environment and leads to strong fractionation of palladium and gold.

ON THE ASSOCIATION OF PALLADIUM-BEARING GOLD, HEMATITE AND GYPSUM IN AN OURO PRETO NUGGET

An ouro preto (black gold) nugget from Gongo Soco, Minas Gerais, Brazil, has a mineral assemblage of hematite and gypsum hosted by Pd-bearing gold. The hematite inclusion is microfractured and stretched. Scattered on the surface of the gold is a da rkcolored material that consists partially of Pd–O with relics of palladium arsenide-antimonides, compositionally close to isomertieite and mertieite-II. The Pd–O coating has considerable amounts of Cu, Fe and Hg, and a variable metal:oxygen ratio, from O-deficient to oxide-like compounds. The existence of a hydrated Pd–O compound is suggested, and its dehydration or deoxygenation at low temperatures may account for the O-deficient Pd-rich species, interpreted as a transient phase toward nati ve palladium. Although gypsum is a common mineral in the oxidized (supergene) zones of gold deposits, the hematite–gypsumbearing palladian gold nugget was tectonically deformed under brittle conditions and appears to be of low-temperature hydrother mal origin.

PALLADIUM AND PLATINUM MINERALS FROM THE SERRA PELADA Au-Pd-Pt DEPOSIT, CARAJAS MINERAL PROVINCE, NORTHERN BRAZIL

The Serra Pelada garimpo (1980–1984) was the site of the most spectacular gold rush in recent history, but the mineralogy of the bonanza-style mineralization has not so far been documented in detail. Rediscovery of an early drill-core, recovered in 1982 from the near-surface lateritic portion of the garimpo area, has provided coarse-grained gold aggregates for this study. The centimeter-long aggregates of gold occur in powdery, earthy material. They exhibit a delicate arborescent fabric and are coated by goethite. Four compositional types of gold are recognized: palladian gold with an atomic ratio Au:Pd of 7:1 (“Au7Pd”), Hgbearing palladian gold (Au–Pd–Hg), Pd-bearing gold with up to 3 wt.% Pd (Pd-poor gold) and pure gold. A number of platinumgroup minerals (PGM) are included in, or attached to the surface of, palladian gold: “guanglinite”, Sb-bearing “guanglinite”, atheneite and isomertieite, including the noteworthy presence of Se-bearing PGM (Pd–Pt–Se, Pd–Se, Pd–Hg–Se and Pd–Bi–Se phases, and sudovikovite and palladseite). They define an As–Sb–Hg–Se mineral assemblage typical of hydrothermal vein-type deposits formed at relatively low temperatures. Native palladium, characteristically situated in the goethite coating, is intimately associated with a Pd–O phase. The remarkable occurrence of native platinum associated with Pd-bearing gold, PGM (compositionally close to mertieite-II and isomertieite) and berzelianite from a nearby prospect (Elefante prospect) is also recorded.

The Serra Pelada Au-Pd-Pt Deposit, CarajasMineral Province, Northern Brazil: Reconnaissance Mineralogy and Chemistryof Very High Grade Palladian Gold Mineralization

A historic drill core from the Serra Pelada open pit was only recently assayed and has spectacular gold, palladium, and platinum grades over a 43-m-depth interval (4,709 g/t Au, 1,174 g/t Pd, 204 g/t Pt). The Au-Pd-Pt mineralization in a bonanza-grade interval (54.5–55.0 m @ 132,000 g/t Au, 11,400 g/t Pd, 359 g/t Pt) consists of coarse-grained, up to several centimeter-large, dendritic palladian gold aggregates (Au7Pd) with abundant inclusions of guanglinite (Pd3As), stibio-guanglinite (Pd3[As,Sb]), sudovikovite (PtSe2), palladseite (Pd17Se15), and an unnamed Pd-Pt-Se alloy. The palladian gold aggregates are often coated by goethite and are embedded in a powdery, ferruginous, clay-rich matrix with fragments of vein quartz. Iron and manganese oxides occur as vug fillings in palladian gold and are associated with native palladium and Pd-oxygenated compounds. The near-surface bonanza palladian gold mineralization is part of a larger, deeply weathered, hydrothermal system which, in deepest drill holes (>300 m), has a relict sulfide assemblage with a variable degree of overprint by iron oxides, sericite, and kaolinite. The deep system still has gold, palladium, and platinum grades in the g/t range and displays erratically high contents in light rare earth elements, bismuth, and selenium. Drill core samples from intermediate depth (157–275 m) are high in gold, palladium, and platinum and show a very pronounced bismuth enrichment, as well as enrichment in iridium, mercury, rare earth elements, uranium, copper, and lead. The shallow drill core is extremely enriched in gold, palladium, platinum, mercury, and manganese. The occurrence of gold dominantly as an Au-Pd alloy, the abundant selenide and arsenide inclusions in palladian gold, and the characteristic mercury signature point to an origin of the near-surface bonanza ore from a sulfur-deficient hydrothermal system, with apparently only minor supergene precious metal enrichment.

HONGSHIITE, PtCu, FROM ITABIRITE-HOSTED Au Pd Pt MINERALIZATION (JACUTINGA), ITABIRA DISTRICT, MINAS GERAIS, BRAZIL

Hongshiite occurs associated with palladian gold, and more rarely with native platinum, in the sulfide-free, hematite-rich jacutinga mineralization of the Quadrilatero Ferrifero, central Minas Gerais, Brazil. Hongshiite occurs in grains up to about 1.5 mm in cross-section. The crystals (>0.1 mm) are generally covered by a porous rim from which copper is preferentially leached, eventually forming a rind of nearly pure native platinum. The morphological and chemical features of hongshiite are similar to features caused by the weathering of primary gold in a lateritic environment. Submicrometer-sized crystals, acicular in shape, replace an octahedral precursor phase, interpreted to be magnetite. The magnetite could have acted as a local redox barrier to the passage of oxidizing hydrothermal fluids leading to hongshiite precipitation. Sperrylite, isomertieite, atheneite, sudovikovite, a “guanglinite”-like phase, an undefined copper selenide, and barite occur as euhedral to anhedral inclusions in the larger crystals of hongshiite. Patches of a Au–Cu–Pt alloy within hongshiite have a composition nearly intermediate between tetra-auricupride and hongshiite, and could be called platinian tetra-auricupride, suggesting an extension of a solid solution toward hongshiite. The presence of tiny rosette-like crystals of native palladium in the auriferous jacutinga mineralization is also documented.

Palladseite and its oxidation: evidence from Au-Pd vein-type mineralization (jacutinga), Cauê iron-ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

AbstractPalladseite (palladium selenide) and palladian gold occur as euhedral inclusions in specularite and as infill of microfractures and interstices in magnetite aggregates (host rock fragments) within specularite-rich auriferous veins (jacutinga). Palladseite has minor amounts of Pt (0.6–3.5 wt.%), Cu (2.9–3.5 wt.%), Hg (0.9–1.9 wt.%), and Ag (0.3–0.5 wt.%). The palladian gold contains up to 6 wt.% Pd, and minor Cu and Ag. Isomertieite (Pd11Sb2As2) and sudovikovite (PtSe2) are also recorded. The veins cross-cut the main tectonic foliation of the wallrock (itabirite). The feature of fracture infill and the Se-Sb-As-Hg-Cu-Ag geochemical/mineralogical signature of the Au-Pd mineralization point to a post-ductile deformation and possibly low-temperature (epithermal) origin of the jacutinga mineralization style.Palladseite is replaced locally by a Pd-rich oxidation phase. Electron microprobe analysis of alteration halos around palladseite yields 76–80 wt.% Pd. Oxygen calculated by stoichiometry accords with a monoxide compound of the type PdO. However, the measured oxygen content is lower than expected for the PdO stoichiometry, suggesting loss of volatile species, and a more ‘hydrated’ compound, such as an oxyhydroxide.

Palladian gold and palladium arsenide–antimonide minerals from Gongo Soco iron ore mine, Quadrilátero Ferrífero, Minas Gerais, Brazil

The present-day Gongo Soco iron ore deposit was one of the most famous gold mines in Brazil in the nineteenth century. The iron ore mining has exposed crosscutting, quartz–specularite auriferous ore shoots within Palaeoproterozoic banded iron formation. The coarse-grained gold particles are characteristically palladium-bearing and commonly have inclusions of palladium arsenide-antimonides, whose compositions are close to mertieite-II and isomertieite. Composite crystals with a mertieite-II core and isomertieite rim are interpreted as precipitates from a hydrothermal fluid of variable Sb : As ratio. The palladium content in gold appears to be correlated with the platinum content in the associated palladium arsenide-antimonide. Its platinum content, up to about 1 wt% Pt, seems to be related to the amount of arsenic. A myrmekitic intergrowth of an Fe-rich, goethite-like phase in a mertieite-II-bearing palladian gold nugget is recorded and is interpreted to be of low-temperature hydrothermal origin. A late-stage event, possibly of a low-temperature hydrothermal nature, deposited Pt–Pd alloy locally.

The Au-Pd Mineralization at the Conceicao Iron Mine, ItabiraDistrict, Southern Sao Francisco Craton, Brazil: An Example of aJacutinga-Type Deposit

The Conceicao mine, which is located in the Itabira district, southern Sao Francisco craton, is a rare example of gold and palladium mineralization hosted by jacutinga, an altered Lake Superior-type iron-formation. The mine sequence consists of three metamorphosed lithostratigraphic units, which are from the oldest to the youngest: (1) an Archean volcano-sedimentary sequence, (2) a Paleoproterozoic iron-formation, and (3) a Paleoproterozoic quartzite. These units are cut by mafic intrusive rocks and, with the exception of intrusive contacts, all contacts between the various units are structural and interpreted as thrust faults. The rocks were subjected to three phases of folding: (1) D1, characterized by tight asymmetric and isoclinal folds, which develop locally into sheath folds (the D1 fabrics are the S1 foliation and elongation lineation); (2) D2, represented by tight, large-scale folds and associated parasitic folds; and (3) D3, characterized by open folds with an associated crenulation cleavage. The D1 and D2 deformation are interpreted to be products of simple shear with a transport direction consistently east over west. These rocks were metamorphosed to typical mineralogy of amphibolite facies conditions during the D1 phase of folding. The Conceicao orebody consists of a single quartz vein surrounded by jacutinga. The vein is parallel to the S1 foliation, which is locally oriented west-southwest and dips 55° to 75° NW, and was folded by D2 deformation. It extends 100 m downdip in the direction of the Le elongation lineation and 400 m along strike. The jacutinga consists of centimeter-scale layers of muscovite, specular hematite (hematite I), and octahedral hematite grains (hematite II) with remnants of magnetite, and a finely laminated envelope around the quartz vein, which is composed mainly of Fe hydroxides, hematite (hematite III), kaolinite, muscovite, and tourmaline. The quartz vein is dismembered and quartz grains are partially corroded and filled by Fe hydroxides and kaolinite. The ore minerals, interpreted to have formed during the main hydrothermal event, are palladian gold (up to 3.2 wt % Pd) and an unknown Pd-Sb-As mineral (tentatively identified as mertierite II), which are typically included in hematite II octahedra. A few grains of palladian gold (up to 0.9 wt % Pd) were found mantling hematite II grains. Native gold is commonly associated with Fe hydroxide in the finely laminated alteration envelope around the quartz vein and is interpreted to have precipitated during weathering. The similarities between the Au-Pd orebodies at the Conceicao and Caue deposits (referred to as jacutinga-type deposits) suggest that they were formed by the same processes and their characteristics favor an epigenetic model for gold and palladium concentration in the iron-formation. Since the Cretaceous, these Archean and Proterozoic rocks have been weathered, and the hydrothermal silicates and oxides have been replaced by kaolinite and Fe(Mn) hydroxides; gold and palladium have been remobilized and reprecipitated as pure metals or Pd oxides.

The first occurrence of platinum group minerals (PGM) in a chromite deposit in the Eastern Desert, Egypt

The platinum-group mineralogy (PGM) of the chromitite from Gebel Lawi, in the southeastern desert has been investigated. The most abundant base metal Sulfides (BMS) associated with the Lawi chromite are pentlandite, millerite and heazlewoodite. The major platinum-group minerals identified were as follows: laurite (IrOsRu)S2, osmian iridium (OsIr), hollingworthite (RhAsS), tellurian arsenopalladinite (PdTeSbAs), polarite (PdHg) besides cuprian palladian gold (CuPdAu), a Pd-Sb-Hg and HgTe phases. Laurite and osmian iridium occur preferentially in chromite. Os-Ir commonly forms composite PGM with laurite. Hollingworthite and tellurian arsenopalladinite are included within serpentine and, close to the base-metal sulfides, the cuprian palladian gold shares boundaries with chromite. Polarite together with the Pd-Sb-Hg and HgTe phases are embedded in serpentine. Palladium is the most abundant PGE in the Gebel Lawi chromite.