Primary and Secondary Gold Sources of Quaternary Placers of Western Spain: a Morphotextural and Compositional Analysis of the Fresnedoso Deposit.

<p>In this contribution we have investigated the Fresnedoso Quaternary gold placer (Western Spain), analyzing the morphotextural and microchemical evolution of gold particles. The statistical analysis has revealed the presence of two populations of particles being consistent with primary sources situated at a distal [20 - 50 km] and a proximal [2.5 - 10 km] range. The gold morphology and chemistry point to a recycling (and potentially undiscovered) Tertiary paleoplacers. The discovery of primary laminar morphologies points to lode deposits in small-flat veins hosted in Precambrian metasediments (Schist Greywacke Complex). All these findings suggest that the Fresnedoso gold deposit is formed by mono and polycyclic particles. We have tested previous transport distance vs Flattening indexes (CFI, Shilo) models resulting in useful framework for exploration of undiscovered ores, even with a small sample dimension. Chemical analysis of the different gold morphologies depicted that the Fresnedoso gold is a AuAg bimetallic alloy. Three groups were identified based on the texture and composition of the gold particles: Type 1 (Au1= Au89-94Ag11-6), Type 2 (Au2= Au99 Ag1) and Type 3 (Au3~ Au >99). Particle's cores (gold Type 1) show a compositional range that could be interpreted as differences in primary sources, spatial dispersion of sources or the actuation of secondary processes, probably in an orogenic gold context. Microchemical heterogeneity in the particles is probably due to secondary processes. A conceptual model has been elaborated to explain particle's microchemical domains represented by gold Type 2 (rim) and Type 3 (micro-aggregates) as the result of two different de-alloying stages: A) initial Ag-leaching at the rim and/or through microcracks and grainboundaries (Type 2), B) Total reset of the primary chemical fingerprint, with porous microtexture and the precipitation of gold with iron oxyhydroxides and clays (Type 3). This model suggests a silver de-alloying mechanism favored in a chlorine-iron-rich environment as in the case of laterites. Deformation and eventually recrystallization mechanisms associated with the fluvial transport (mechanical cold-work), cooperated in the evolution of the particles (Dos Santos et al. 2020).</p><p>References </p><p>Dos Santos Alves, K., Barrios Sánchez, S., Gómez Barreiro, J., Merinero Palomares,R. and Compaña Prieto, J.M. (2020). "Morphological and compositional analysis of alluvial gold: The Fresnedoso gold placer (Spain)." Ore Geology Reviews : 103489.</p>

Current methods of technogenic gold placer deposit mining with small size gold extraction technology

The purpose of this article is to conduct industrial tests of spiral separation technology on technogenic refractory raw materials. The object of the study is the Yurskoye gold placer deposit in the Republic of Sakha (Yakutia). An OKV-100 benefication complex with the productive capacity of 100 m3 /h by initial sands was set up in order to conduct pilot mining of the fine-grained tailings dumps of the Yurskoye deposit with the maximum extraction of small size gold. Sands supplied from the different sections of the technogenic deposit had different gold content, granulometric characteristics, and mineralogical composition. The study included experiments carried out at different initial feed; equipment was adjusted for specific raw material depending on productive capacity, liquid/solid ratio, product yield of the small size gold recovery module. The complex productivity was from 90 to 150 m3 /h. The yield and extraction of gold-bearing concentrate, as well as its gold content were determined. The pilot washing with the use of small size gold recovery technology resulted in the average additional gold recovery from sluice tailings of 19.07 %, which is a high indicator, given the low content of gold in technogenic placers. The technology under consideration can be used for the development of both technogenic and operated gold placer deposits.

Typomorphism of сhromespinelides from the Holocene psephites of the Kyvvozhskoye gold-placer field (Volsko-Vymskaya ridge, Middle Timan)

The results of the study of accessory chrome spinels from heavy mineral concentrates of gold-bearing psephites of the Kyvvozhsky gold-placer field (Volsko-Vymskaya ridge, Middle Timan) are presented. As a result of the study of the typochemistry of chromspinelides, their genetic relationship with rocks of mainly alkaline-ultrabasic composition — analogues of Timanian Late-Riphean-Vendian ultramafites and Early-Middle Devonian alnoites is substantiated. The formation of magmatic sources of chrome spinels occurred in a wide range of geotectonic settings — from passive (platform) to active (island-arc subduction zones), which are more characteristic of the bulk of chrome spinels. The crystallization of chrome spinels, found as microinclusions in lherzolite pyropes in the same shlich samples of the Kyvvozhskaya gold-bearing area, is associated with the conditions of rift zones (spreading of the seabed). We made a conclusion about probable local (intra-valley) position of main magmatic sources of chrome spinels. The revealed chrome spinels, containing microinclusions of nickel and copper sulfides, are considered by the authors as indicators of igneous rocks containing platinum group minerals.

Determining the source of placer gold in the Anaconda metamorphic core complex supradetachment basin using detrital zircon U-Pb geochronology, western Montana, USA

Despite the widespread occurrence and economic significance of gold placer deposits, modern provenance studies of placer sediments remain largely qualitative. This study applies detrital zircon (DZ) geochronology to determine the source of zircon in placer deposits. We then evaluate the provenance of the zircon to assess whether the gold might have been derived from the same sources, thereby providing a case study of the use of DZ geochronology applied to placers. We present a new set of DZ U-Pb ages (n = 1058) and Lu-Hf (n = 61) isotopic data from four placer deposit samples collected from the Pioneer District of western Montana (USA). Each of the four samples yielded similar age spectra, with a range of U-Pb ages between 3000 and 25 Ma. We interpret that ≥250 Ma zircons were recycled from the Mesoproterozoic Belt Supergroup, Paleozoic–Mesozoic sedimentary rocks, and the Upper Creta­ceous–Paleocene Beaverhead Group. Our 237 DZ U-Pb ages ≤250 Ma reveal two prominent age-probability peaks centered at ca. 69 Ma and ca. 26 Ma, which we interpret to record first-cycle derivation from the Royal stock and nearby Dillon Volcanics, respectively. We evaluate these data using an inverse Monte Carlo DZ unmixing model that calculates relative contributions from plausible source units, determining a 12% contribution from the Royal stock and a 43% contribution from the Beaverhead Group. A current absence of the Beaverhead Group in the hypothesized source region suggests complete erosion of the unit into the placer-bearing basin. Detrital zircon geochronology, Hf isotopic data, and the unmixing mod­eling results offer the first zircon-based support for previous interpretations that the Late Cretaceous Royal stock precipitated gold along its contact with overlying Proterozoic–Mesozoic sedimentary strata. Subsequent exhuma­tion and erosion of the lode source led to gold deposition in the Anaconda metamorphic core complex supradetachment basin during the late Oligo­cene–late Miocene. The worldwide occurrence of gold placer deposits with unknown source areas provides abundant opportunity to apply these tech­niques elsewhere.

Micro-Raman—A Tool for the Heavy Mineral Analysis of Gold Placer-Type Deposits (Pianu Valley, Romania)

In the current study, different heavy minerals typical of gold placer deposits were identified by means of micro-Raman spectroscopy, and their chemical composition analyzed and discussed (garnet, kyanite, staurolite, zircon, allanite, monazite, xenotime, rutile, anatase, cassiterite, titanite, barite). Even complex solid solution series, such as those of garnets, can be deciphered with the aid of systematic trends observed in Raman line frequencies. The ν1 mode in garnets will shift from high to low frequencies as a function of the ionic radius of the X2+ cation, from Mg2+, to Fe2+ and Mn2+, while the presence of Ca2+ will make the band to be shifted strongly to even lower wavenumbers. This approach has successfully been taken to differentiate between polymorph triplets such as kyanite-sillimanite-andalusite and rutile-anatase-brookite. Minerals under consideration with high contents of REE, U and Th are affected by intensive metamictization, particularly zircon and titanite. Raman peak features, such as shape, symmetry and intensity, respond to this radiation damage of the lattice and enable fine-tuning of these heavy minerals, such as in the case of fluorite (fetid fluorite).

Mineralogy and geochemistry of chloritholites from the Nepryakhino gold field, South Urals

The chlorite rocks (chloritolites) exposed in a bedrock of the Mokhovoe boloto (Moss swamp) gold placer (East Uralian Megazone, South Urals), which occurs on ultramafc rocks, are studied. The trace element composition of chloritolites is characterized by elevated contents of Mn, Ti, V (hundreds of ppm), Cu, zn, Ni, Co, Cr, zr, Li, Sc (tens of ppm), w, zr, Y and REE. Chloritolites contain up to 3 vol. % of disseminated magnetite, ilmenite and accessory minerals (rutile, xenotime, monazite, zircon, apatite, scheelite, U-bearing thorite) from a mineral assemblage, which cocrystallize with the main volume of chlorite. The mineralogical and geochemical features of the Mokhovoe boloto chloritolites and gold-bearing chloritolites of the Karabash massif in the Main Uralian Fault zone are slightly similar. The elevated Ti and P contents of the studied chloritolites, the level of REE contents corresponding to mafc rocks, and the lack of relict chromite indicate their possible metasomatic formation after dolerite dikes known within Chebarkul-Kazbai ultramafc complex. Figures 8. Tables 7. References 14.

Demantoid from the Telyansky Klyuch deposit (Middle Urals) – is the place of the first discovery in the Urals

The relevance of the work is due to the need to study the Ural gems, including those discovered in the Urals. This is important not only from a scientific point of view, but for the history of science and the development of geological tourism in our region as well. Purpose of the work: study of the chemical composition of demantoid from the Telyansky Klyuch deposit, which is the site of the first find of this gem in the Urals. Research methodology. Demantoid (green jewelry andradite) was washed by the author in the Telyansky Klyuch gold placer; the quantitative analysis of the chemical composition of garnet was performed using the JXA-733 X-ray electron probe microanalyzer. Results. A historical study of the sites of finds of demantoids in the Middle Urals was carried out. It was established that precious garnet was found not in placers of the Bobrovka river located south of Nizhny Tagil, but in the gold placer of the Telyansky Klyuch to the north of Nizhny Tagil. Microprobe analysis of individual demantoid crystals showed that garnets with different green color saturation according to the main components are andradites with average contents: CaO – 33,18%; Fe2 O3 – 30,74%; SiO2 – 35,02% (for 6 analyzes). The feature of the chemical composition of Telyansky demantoids is the absence or insignificant amount of aluminum impurity in them – not more than 0.02% Al2 O3 , which is practically at the level of microprobe analysis sensitivity for this element. The amount of chromium varies from 0.02% Cr2 O3 in a light green demantoid to 0.5–0.74% Cr2 O3 in green and bright green andradites. Conclusions. The place of the first find of low-impurity and chromium-containing andradites in the Urals with the name of “diamantoite” (proposed by N. G. Nordenskiöld ) is the Telyansky Klyuch gold deposit on the East Tagil dunite-harzburgite massif in the Middle Urals