The close association of Pd with Au has been previously reported in highly oxidized chloride hydrothermal systems from several regions, including Australia, Brazil, Poland, and the United Kingdom. The distinctive mineralization precipitated usually as a consequence of contact with reducing environments is characterized by Au alloys containing low Ag ± Pd ± Hg ± Cu, associated with selenides ± tellurides ± antimonides ± arsenides. Placer gold grains derived from this style of mineralization inherit these chemical and physical characteristics which are shown to be distinct from those placer grains derived from other styles of mineralization. The discovery of placer gold grains of this type in areas remote from known in situ mineralization initiated the present project and provided a methodology for the study of a relatively large number of occurrences.

Data describing alloy chemistry and mineralogy of polished sections of 957 placer grains from 19 localities in the United Kingdom and 1 locality in the Czech Republic have augmented previously published data for 303 gold grains from 10 localities in the United Kingdom. Six generic gold alloy types have been identified based on the relative proportions of Au, Ag, Pd, and Hg. Mineralogical descriptions of similar gold alloys from localities worldwide usually conform to this classification, but slight deviations are noted where P-T conditions differ from those previously identified in Devon. Alloy types may occur either as discrete grains or as phases within heterogeneous grains. Type 1 gold (defined as >99 mass % Au), type 2 gold (Au-Pd), and type 3 gold (Au-Ag) alloys (with selenide inclusions) are the most common compositions and occur at most localities, although in different proportions. Type 4 gold is also an (Au-Ag) alloy, but contains sulfide and sulfarsenide inclusions. Gold alloys or intermetallics containing Pd-Hg and/or Pd-Sb alloys (type 5), and Cu (type 6) are found only at some localities, as is a variety of type 3 gold (± selenide inclusions), which contains Hg.

The spatial arrangement of the main alloy types within heterogeneous gold grains shows a consistent precipitation order of type 1, type 2, and type 3, independent of locality. Interpretation of the chemical controls on alloy precipitation indicates that ƒO2 controls whether type 1 or type 2 gold precipitates initially. The formation of type 4 gold (Au-Ag alloys with sulfide inclusions) is ascribed to a fall in ƒO2 to around the hematite-magnetite buffer where HS− (aq) replaces Cl−(aq) as the aqueous gold complex. Types 3 and 5 gold commonly mantle earlier alloys, and show evidence for aggressive replacement of the core. This texture is interpreted as evidence for influx of a more oxidizing fluid which also accounts for the abundance of selenide and telluride mineral inclusions within this gold type.

This approach provides a plausible explanation of the complex heterogeneity within and between gold grains from the localities and accounts for the common sequence of alloy precipitation, without invoking multiple episodes of hydrothermal activity to account for different alloys. However, the absence of thermodynamic data describing the Au-Cu, Au-Ag-Cu, and Au-Ag-Pd-Hg systems prevents interpretation of the significance of some alloy types.

This study has demonstrated that gold mineralization associated with oxidizing strata is more widespread than currently appreciated. The methodology described here not only permits identification of gold from this style of mineralization but allows initial evaluation of the economic potential of the source through the study of the alloy types. This information may be gained at an early stage in the exploration process from gold grains collected during routine reconnaissance.

Chemical and Mineralogical Signatures of Gold Formed in Oxidizing Chloride Hydrothermal Systems and their Significance within Populations of Placer Gold Grains Collected during Reconnaissance

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.

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

Gold grade variation and particle microchemistry in exploration pits of the Batouri gold district, SE Cameroon

Low-temperature Pt enrichment is illustrated with reference to two hypogene platiniferous minerals: (i) a Pd–Cu-bearing ‘Pt2HgSe3’, an unnamed phase; and (ii) hongshiite, PtCu, with up to 0.7 wt% Hg. The former has patches oxidized to a Pt–O species with curvilinear microcracks originated by desiccation. The latter displays parallel sets of Cu-depleted, Pt–Hg-enriched patches, along which microfractures occur. Such microstructures are interpreted to arise from two processes: (i) oxidation and formation of a hydrated Pt–O compound, which tends to succumb to a Pt-rich alloy by deoxygenation and dehydration, leaving behind curvilinear desiccation cracks; (ii) metal leaching conducive to native Pt by removal of Cu from hongshiite via micropores that coalesce along crystallographic planes to result in reticulated microcracks. Platinum enrichment either by oxidation or metal leaching ideally produces characteristic microcracks, which may be indicative of the origin, i.e. detrital vs. supergene, of alluvial and eluvial Pt-rich alloys.

Platinum enrichment at low temperatures and related microstructures, with examples of hongshiite (PtCu) and empirical ‘Pt2HgSe3’ from Itabira, Minas Gerais, Brazil

Iodine in alluvial platinum–palladium nuggets: Evidence for biogenic precious-metal fixation

The Corrego Bom Sucesso alluvial deposit near Serro, Minas Gerais, probably provided the specimens from which the element palladium was first discovered. Its Pt–Pd nuggets are characteristically botryoidal, arborescent and coralloidal, and exhibit an external halo with the composition of palladiferous platinum to virtually pure platinum. X-ray mapping of an arborescent Pt–Pd nugget from the historical occurrence documents selective palladium depletion, similar to the high-fineness gold haloes developed on detrital Au–Ag grains under supergene conditions. The Pd-depleted alteration zone truncates inclusions of crystals stoichiometrically close to PdPt within the Pt–Pd aggregate. Selective metal leaching in the weathering environment can account for the frequently observed Pt-enriched rims on alluvial platiniferous alloys which may lead to the formation of platinum nuggets at an advanced degree of weathering.

J. C. Polônia

Papers

Supergene leaching and formation of platinum in alluvium: evidence from Serro, Minas Gerais, Brazil