R. J. Chapman

The use of microchemical analysis of alluvial gold grains in mineral exploration: experiences in Britain and Ireland

Abstract The examination of populations of gold grains by SEM and EPMA generates a microchemical signature based upon the assemblage of opaque mineral inclusions within the gold and the concentration of minor alloying metals. Duplicate samples of alluvial gold from the same locality have yielded the same microchemical signature independent of the field worker who collected the sample, or the date of collection. A study of five separate alluvial localities yielded a microchemical signature consistent with the mineralogy of the adjacent host mineralisation. This result has permitted informed speculation on the styles and complexity of gold mineralisation contributing to the alluvial population of gold grains where the style of source mineralisation is unknown. The technique can yield information about several aspects of the source mineralisation of an alluvial population at an early stage in the exploration process. Distinctive microchemical signatures can be used to evaluate the dispersion of gold either by fluvial or glacial action. The technique can identify gold derived from separate mineralising events either within a region or along a watercourse, thereby allowing speculation on the magnitude of the source mineralisation. It is also possible to establish whether specific types of gold mineralisation of potential economic interest have contributed to an alluvial population. In areas subjected to glaciation, analysis of the microchemical signature of populations of alluvial gold may indicate whether the gold grains are of proximal or distal origin. The volume of information made available through the study of alluvial gold grains has contributed greatly to the understanding of regional gold mineralisation. Interpretation of microchemical signatures of populations of alluvial gold from about 130 localities throughout Great Britain and Ireland has facilitated characterisation of gold in terms of the style of the source mineralisation. This information has augmented that available from about ten bedrock gold localities and has permitted a more complete classification of gold occurrences throughout the region. Quantitative analysis of some minerals within the opaque inclusion assemblage can provide information on the chemistry of the mineralising fluid, which may be related to the capacity for gold transportation. In Devon, South West England, a model for the emplacement of gold mineralisation was developed from interpretation of the microchemical signature of alluvial gold grains. The selenide-rich inclusion suite containing no sulphides suggested that oxidising conditions prevailed during precipitation. The model of transport of gold in solution through the oxidising environment of Permian red beds and its subsequent precipitation in underlying rocks due to reduction in Eh was consistent with the observed distribution of gold in alluvial sediment, residential overburden and drill core. Using this model other targets for gold mineralisation were successfully predicted.

The origin of alluvial gold in the Leadhills area of Scotland: evidence from interpretation of internal chemical characteristics

A detailed chemical study has been carried out of 1794 gold grains from 55 sites within an area approximately 7 km by 18 km around Leadhills in the turbidite-facies, wacke-dominant Southern Uplands terrane of Scotland, formed as an accretionary thrust complex at the Laurentian continental margin during Ordovician and Silurian subduction of the Iapetus Ocean. The Leadhills area has had a long history of alluvial gold working, since at least 1510, and probably much earlier, but few bedrock sources of mineralisation have been located. Quantitative electron microprobe point analyses were obtained of sectioned and polished gold grains and of the associated microscopic inclusions, which in total comprised 35 types of opaque minerals. Microchemical X-ray maps were also obtained using a fully automated electron microprobe to show internal chemical compositional variation, to resolve the components of multiple and composite inclusions and to help in the identification of the smallest examples. Silver is present within most grains but Cu, Hg and Pd were only detected in about 3% of the grains. The shapes of cumulative frequency plots of the Ag contents of grains from different sites allow recognition of populations from different sources and enables comparison and differentiation between sites. In one of the main rivers, the Shortcleugh Water, the nature of the gold is generally similar, both in terms of Ag content and inclusion type, but a further type of gold, relatively rich in Ag and containing distinctive Ni and Sb-rich inclusions, is also present in 4 adjacent samples in one sector of the watercourse. This component of the samples reflects downstream dispersion, of up to 400 m, of gold grains from a mineralised structure cutting the river almost perpendicularly. Elsewhere abrupt changes in the nature of the alluvial gold within streams reflect a series of different sources and indicate limited glacial and alluvial dispersion. Four main types of gold which are considered to be derived from different varieties of source mineralisation can be recognised on the basis of composition and inclusion assemblage. The commonest type (ca. 70%) contains 8.4–13.1% Ag and inclusions, mostly of arsenopyrite, pyrrhotite, pyrite, chalcopyrite, galena, sphalerite and cobaltite, characteristics typical of mesothermal gold from shear-zones within a turbidite environment, suggesting sources within the local Lower Palaeozoic sedimentary rocks. The second type is richer in Ag (15.8–31.3%) with a basic igneous signature indicated from inclusions of Sb-rich gersdorffite, pyrite, Ni arsenide and other Ni-rich minerals, and shows some spatial association with discordant Tertiary basaltic dykes which cut the sedimentary rocks. The third type, mostly found in an area mantled by glacial drift, is poor in Ag (<1.2%) but frequently Cu-bearing and with inclusions of platinoids and Cu oxide. Associated with this type of gold are grains of the intermetallic compound Au3Cu and platinoids such as isoferroplatinum and a complex Ir-rich phase. Its source is problematical and could partly reflect detrital material associated with ophiolitic debris, which is common in the northernmost turbidites, and partly a local source of lode gold. The fourth gold type is also Ag-poor (<3.2%), but frequently contains Pd, up to 6.3%, while the inclusions comprise selenides of Pb, Hg and Cu. In nearby areas this type of gold is associated with Permian red beds and associated basaltic volcanics and also structures in the Lower Palaeozoic into which oxidising solutions derived from the red bed sequence have penetrated.

Microchemical Characterization of Alluvial Gold Grains as an Exploration Tool

There is considerable variation in the composition of native gold and the nature of minerals co-existing with it, and this reflects differences in the geological environment and chemistry of ore-forming processes. In areas where gold-bearing mineralization is subject to active fluvial erosion, especially in temperate climatic regimes, any discrete grains of native gold pass into alluvial sediment with little modification. The chemical characteristics of alluvial grains and the nature of preserved mineral inclusions provide a signature which points back to the type of source mineralization. This signature may be established using electron probe microanalysis and scanning electron microscopy and can be interpreted to provide information about the original bedrock mineralization. Identification of the type of source mineralization using the technique at an early stage in regional exploration can help focus attention on targets with the most potential economic importance.

Solution and laser ablation MC-ICP-MS lead isotope analysis of gold

A new approach is presented for the analysis of lead isotopes in gold using solution and laser ablation multi-collector inductively-coupled plasma mass-spectrometry (MC-ICP-MS). Repeat analyses of two gold reference materials (RAuGP3: 34.1 ± 0.5 ppm Pb and RAuGP5: 129 ± 4 ppm Pb, from SPEX CertiPrep Ltd) yield an external reproducibility of 20. Instrumental mass bias is corrected by standard sample bracketing, using NIST SRM981 for solution analyses and NIST SRM610 for laser ablation analyses. Both techniques agree within error, and they have been further demonstrated through the analyses of both geological and archaeological samples: multiple grains from two Irish alluvial gold deposits and samples of two Early Bronze Age Irish gold lunulae. The applicability of these techniques to studies concerning the sources and age of gold mineralisation, and the identification of gold sources exploited in antiquity, is highlighted.

Regional variation in gold mineralization in the vicinity of the Glengaber Burn, Scottish Borders

Synopsis The alluvial gold at Glengaber Burn in the valley of the Megget Water is of interest because of its coarse size and historical working. A total of 350 alluvial gold grains from 17 sites in Glengaber Burn and the vicinity have been studied using the technique of microchemical characterization, and two major types of gold have been identified, each with well defined geographical distribution. Type 1a gold contains only pyrite inclusions, and has an Ag content of between 8% and 11%. Type 1b has a similar Ag content, but the inclusion assemblage contains galena together with pyrite. Type 2a gold typically contains between 3% and 7% Ag, and a suite of inclusions comprising chalcopyrite, sphalerite, tetrahedrite, arsenopyrite and gersdorffite. Type 2a gold occurs in a SW–NW trending zone which contains the catchments of the Glengaber and Craigierigg Burns. Type 2b gold is confined to the catchment of the Cramalt Burn and contains an inclusion suite similar to Type 2a, but is more silver-rich. The abundance of alluvial gold of all types falls dramatically to the south of Megget Water, although the occurrence to the north of the Glengaber Burn and Craigierigg Burns has not been established. The distribution of Type 1a gold is mutually exclusive with Type 2a and flanks the zone of Type 2a and 2b to both east and west suggesting zonation in a single mineralizing event. Type 1b gold occurs to the west of Type 1a and is probably unrelated to the other gold types. The mineralization acting as the source for the alluvial gold could be controlled by deep structures roughly perpendicular to the regional strike, revealed by prominent gravity lineaments, and their intersections with other structural features.

Palynological Evidence for a Warmer Boreal Climate in the Late Pliocene of the Yukon Territory, Canada

The Late Pliocene (3.6–2.6 Ma) was a period of significant global warmth, considered a potential analogue for future anthropogenic climate change. Newly discovered fine-grained sediments from between the gold-bearing lower and upper White Channel Gravels show the presence of a wetland or lake within Bonanza Creek, Dawson Mining District, Yukon. This environment was surrounded by a diverse Pinaceae-dominated boreal forest with significant stands of angiosperms in favourable sites. Quantitative climate reconstructions derived from pollen and spores reveal a mean annual temperature at least 6 °C warmer than today with warm summers and relatively mild winters. Finally, the new pollen assemblage is used to discuss the age of the White Channel Gravels.

The significance of copper concentrations in natural gold alloy for reconnaissance exploration and understanding gold-depositing hydrothermal systems

Analytical studies of placer (detrital) gold grains, intended to track bedrock sources and styles of mineralization, mainly consider alloy Ag content and inclusion mineralogy to generate ‘microchemical signatures’. Occasionally Hg and Pd help discriminate gold from different sources; Cu infrequently because electron microprobes seldom detect the low concentrations. Reporting of Cu has been largely confined to gold from porphyry–epithermal environments, and this has biased subsequent interpretations of observed Cu-bearing gold alloys. This study focuses on placer gold in the Mourne Mountains, Northern Ireland, where auriferous bedrock remains undiscovered and the complex geology is consistent with either orogenic or intrusion-related mineralization. Over 500 gold grains analysed average 0.17% Cu ranging to >1% Cu and show a wide variation in Ag content. Inclusion mineralogy mainly matches that of orogenic gold elsewhere in the host terrane; however, some signatures are suggestive of zonation within an orogenic hydrothermal mineralizing system, or possibly metamorphic-hydrothermal remobilized gold associated with subsequent intrusive activity. One locality in the study area has placer gold of a distinctive Cu-rich alloy composition containing Cu sulphide inclusions, an association noted elsewhere in gold derived from alkali Cu-Au porphyries. Consideration of Cu-bearing gold alloys worldwide indicates that previously proposed compositional correlations with deposit type are of limited value. We show that Cu contents to at least 0.8% are permissible within orogenic gold – the first time that such compositions have been clearly ascribed to orogenic mineralization. The result is particularly important considering on-going exploration in northern Canada which employs gold grain analysis to help define exploration targets.

A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada

Porphyry-epithermal and orogenic gold are two of the most important styles of gold-bearing mineralization within orogenic belts. Populations of detrital gold resulting from bulk erosion of such regions may exhibit a compositional continuum wherein Ag, Cu, and Hg in the gold alloy may vary across the full range exhibited by natural gold. This paper describes a new methodology whereby orogenic and porphyry-epithermal gold may be distinguished according to the mineralogy of microscopic inclusions observed within detrital gold particles. A total of 1459 gold grains from hypogene, eluvial, and placer environments around calc-alkaline porphyry deposits in Yukon (Nucleus-Revenue, Casino, Sonora Gulch, and Cyprus-Klaza) have been characterized in terms of their alloy compositions (Au, Ag, Cu, and Hg) and their inclusion mineralogy. Despite differences in the evolution of the different magmatic hydrothermal systems, the gold exhibits a clear Bi-Pb-Te-S mineralogy in the inclusion suite, a signature which is either extremely weak or (most commonly) absent in both Yukon orogenic gold and gold from orogenic settings worldwide. Generic systematic compositional changes in ore mineralogy previously identified across the porphyry-epithermal transition have been identified in the corresponding inclusion suites observed in samples from Yukon. However, the Bi-Te association repeatedly observed in gold from the porphyry mineralization persists into the epithermal environment. Ranges of P-T-X conditions are replicated in the geological environments which define generic styles of mineralization. These parameters influence both gold alloy composition and ore mineralogy, of which inclusion suites are a manifestation. Consequently, we propose that this methodology approach can underpin a widely applicable indicator methodology based on detrital gold.

Evidence for the historical exploitation of possible bedrock gold in the upper Mennock Water, Wanlockhead, Dumfries and Galloway

Synopsis Microchemical characterization of gold grains collected from a probable site of old gold processing at the confluence of the Mennock Water and Whitestone Cleuch in the Leadhills–Wanlockhead auriferous area has led to the identification of a type of gold previously unrecorded in the area. Within the Leadhills–Wanlockhead gold region the alluvial gold is dominated by a type which contains typically 10–12% Ag, little or no Cu and Hg, and an opaque inclusion suite containing sulphides (about 60%) and sulpharsenides (about 40%). Alluvial gold grains recovered from the Mennock Water and Whitestone Cleuch during this study are of this type. In contrast, gold grains recovered from the base of pits at the study site contain between 5.6 and 7.4% Ag, negligible Cu and Hg, and an opaque mineral inclusion suite characterized by pyrite, chalcopyrite, galena, sphalerite and tetrahedrite. Sulpharsenides are absent. This microchemical signature is more similar to that previously recorded in alluvial gold from two other sites in the Southern Uplands: the Tweed headwaters and the Glengaber Burn. The distinctive microchemical signature of gold from the study site has not been observed in any other gold grains recovered from the Mennock Water catchment (c. 20 km2) or elsewhere in the Leadhills–Wanlockhead region. These grains frequently exhibit textures incompatible with any fluvial transport, but characteristic of gold grains liberated through crushing ore. Thus, the evidence from gold composition and grain textures suggests that a distinctive bedrock source of gold was crushed and presumably beneficiated at this site. This discovery represents evidence for an unrecorded site of possible in-situ gold exploitation.

Characterisation of the crystallography of gold using EBSD

invoked as the source of the extensive placer deposits of the Klondike District. Exploration for plausible bedrock sources has proved challenging because of a lack of outcrop and the discontinuous nature of the veins, which are hosted in complex units of polydeformed schist. The discovery of a unit of disseminated gold in schist along the Lone Star ridge has raised the possibility that quartz veins may not be the sole source of the detrital gold. The Klondike Gold district is located in north-west Yukon and is part of the Tintina Gold belt which extends through the Yukon and into Alaska. The discovery of the White Gold District in 2004 precipitated an exploration boom accompanied by studies of regional metallogeny. Subsequently both the Klondike and White goldfields were ascribed to a major period of orogenic mineralisation in the Mid-Late Jurassic. Mineralised discordant quartz veins comprise subhedral milky quartz and contain visible gold associated with minor pyrite. The veining at Lone Star is interpreted to be part of a series of orogenic style veining across the Klondike which formed as a single stage (Rushton et al. 1993). Disseminated gold is present in a unit of schist thought to be a rhyo-dacite which also contains syngenetic sulphide lenses (Mortensen 1990). A model proposed by Mackenzie et al. (2007) suggests that the syngenetic layer may be a source for gold in the discordant veins (MacKenzie et al. 2007), but the genetic relationship between the auriferous schist and the auriferous veins has hitherto been unclear. Detailed petrographic studies revealed that the parageneis of gold is late and associated with a discrete phase of hyaolophanee (Ba Kspar), Fe–Mg carbonate and telluride minerals. Imaging of the quartz veins using cathodoluminescence revealed four stages of quartz emplacement (Q1–Q4) with gold occurring in Q3. This phase of quartz displays bright CL, forms less than 5% of the vein and fractures pyrite. Gold occurs along the fractures of pyrite with hyaolphane at a number of localities. The mineralogical association and late paragenetic timing of gold in the schist and veins suggests that they may be part of the same event. A new model is proposed whereby a single aurifierous fluid permeated pre-existing fractures in veins and the schist. Trace element analysis of the quartz and gold are currently being investigated to better characterise the auriferous fluid. The discovery of gold within the schist and a single auriferous quartz stage has permitted an informed critique of previous exploration strategies which have focussed on the (mostly barren) quartz veins whilst being unaware of a potential low grade high tonnage target. Additionally, the new model may provide a solution to discrepancies between gold inventories in auriferous quartz veins and their local placer expressions both at Lone Star and elsewhere.