Recent Placers Research Articles

Extreme fractionation of selenium isotopes and possible deep biospheric origin of platinum nuggets from Minas Gerais, Brazil

Abstract Platinum-rich nuggets offer an opportunity for understanding how precious metals accumulate. We analyzed the selenium (Se) isotopic composition of Se-rich (102–103 µg g−1) platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas Gerais, Brazil, for which a biogenic origin has been inferred. We obtained Se isotopic values with a relatively narrow range (δ82/76SeNIST3149 = −17.4 to −15.4 ± 0.2, two standard deviations [2 SD]). The Pt-Os age of the nuggets is 181 ± 6 Ma (2 SD). The data indicate that the nuggets did not form in the recent placer deposit, but by replacement of hydrothermal vein minerals at ~70 °C and at least 800 m below the surface. The high abundance and extreme isotopic composition of Se as well as the presence of other biophilic elements like iodine, organic carbon, and nitrogen within the nugget matrix are consistent with a microbial origin. Although abiogenic reduction of Se oxyanions cannot be ruled out, the nuggets plausibly record Se-supported microbial activity in the deep biosphere.

Comparison of Diamonds from the Rassolninskaya Depression and Modern Alluvial Placers of the Krasnovishersky District (Ural Region)

Three hundred thirty-six diamonds from deposits of the Rassolninskaya depression and 144 crystals from recent alluvial placers of the Krasnovishersky district were studied by IR absorption and photoluminescence spectroscopy. It is shown that crystals from the Rassolninskaya depression have a close-to-normal distribution for the nitrogen concentration. The average nitrogen content is 725 ppm, and no nitrogen-free crystals were detected. A sampling from recent alluvial placers contains 25% crystals with a nitrogen concentration smaller than 150 ppm; 3% of them are nitrogen-free. Among crystals from the Rassolninskaya depression, 12% are octahedral, 80% rhombododecahedral, and only one crystal has relicts of cubic faces. The collection from recent placers contains 3% cubic crystals, 10% individuals with relicts of cubic faces, 16% octahedroids, and 66% dodecahedra. Alluvial diamonds are often encountered with crescent-shaped cracks; however, they were observed only on a single crystal from the Rassolninskaya depression. It has been revealed that among alluvial placer diamonds, up to 95% of crystals contain nitrogen in the form of B1 defects. Thus, first, in morphological and structural-mineralogical features, diamonds from the Rassolninskaya depression differ from crystals of the nearest recent alluvial placers; second, they may belong to primary deposits based on the set of their characteristics.

Architecture‐aware routability‐driven placer for large‐scale mixed‐size designs

In this study, the authors have presented a simple but robust routability-driven placement for the mixed-size designs. The proposed technique is implemented through (i) look-ahead legalisation-based global placement, (ii) congestion removal and (iii) detailed placement stages. A balanced clustering technique has been proposed to group the circuit blocks into clusters based on the types of circuit blocks and their connectivity. A 0-1 integer programming-based global placement method is framed, which performs look-ahead legalisation. A new site information table concept is introduced to keep the information about each placement cell. Based on the divide conquer strategy, placement area is divided into a region to reduce problem size. A force-directed method has been conceived to select an appropriate region for global placement for the blocks of a cluster. A new congestion removal approach substitutes the legalisation stage to remove circuit block congestion and pin density in different regions. Finally, a gain-based strategy has been introduced for routability-driven detailed placement. The proposed technique is implemented and tested on ICCAD 2012 benchmark circuits. It achieves 1.92 and 0.13% improvements in terms of half perimeter wirelength and routing congestion w.r.t. recent placers.

X-ray spectroscopic identification of garnet from the placer deposits of the Taman peninsula

Garnet from recent placer deposits of the Taman peninsula has been investigated by energy-dispersive X-ray fluorescence microanalysis and X-ray absorption near-edge structure (XANES) spectroscopy. Energy-dispersive X-ray fluorescence microanalysis showed that the chemical composition of the garnet under study corresponds to pyrope-almandine-spessartine series. The Fe K-edge X-ray absorption spectra of garnet have been recorded using a Rigaku R-XAS laboratory spectrometer. Iron K-edge XANES spectra for two iron-containing garnet minals (components), almandine and andradite, have been calculated using the full multiple-scattering and finite-difference methods. Based on a comparison of the experimental and theoretical Fe K-XANES spectra, it is concluded that recent magnetite-garnet placer deposits of the Taman peninsula contain garnet in the form of almandine.

Mineralogical siting and distribution of gold in quartz veins and sulfide ores of the Ashanti mine and other deposits in the Ashanti belt of Ghana: genetic implications

The Ashanti belt of Ghana constitutes a gold province which has produced a total of about 1500 t of gold historically. Gold mineralization is found in steep, NNE-SSW to NE-SW trending shear zones predominantly transecting metasediments of the Palaeoproterozoic Birimian Supergroup (2.2–2.1 Ga), disseminated in ca. 2.1 Ga granitoids, in paleo-conglomerates of the Tarkwaian Group (< 2135 Ma), and in recent placers. The distribution of gold, its chemistry, paragenesis and mineralogical siting in the mesothermal ores of the major mines in the Ashanti belt, namely Konongo, Ashanti, Bogosu and Prestea mine, are the subject of this study. At the localities studied, gold is present in two main types of ores: 1. Quartz veins with free-milling gold. The gold is relatively silver-rich (true fineness values from 730 to 954) and is accompanied by a distinct suite of Cu, Pb, Sb sulfides. 2. Sulfide ores, consisting of arsenopyrite, pyrite and rarer pyrrhotite and marcasite, with refractory gold. The ores have apparent fineness values larger than 910. Arsenopyrite and locally (at Bogosu) pyrite were identified as the hosts of submicroscopic gold. Mean concentrations of gold in arsenopyrite in various samples from the different mines, obtained by secondary ion mass spectrometry (SIMS), range from 67 to 314 ppm Au. Gold concentration mapping in individual arsenopyrite crystals from the different deposits revealed similar patterns of gold distribution: the grains possess a gold-poor core, and elevated gold contents are present along distinct crystal growth zones towards their rims. The outermost crystal layer is usually gold-poor. The well-preserved distribution patterns also indicate that remobilization of gold from the sulfides played an insignificant role in the ores of the Ashanti belt. Multiple quartz veining and growth zoning of the sulfides are interpreted as manifestations of multiple episodes of fluid infiltration, fluid flow and mineral deposition. The bimodal occurrence of gold in spatially closely associated quartz vein and sulfide ores indicates a genetic link between these ore types. A model implying a grossly coeval formation of the ores from mesothermal fluids is proposed.

Ore Deposits of Gold and Silver

Abstract As in the past, gold and, to a lesser extent, silver are attracting considerable attention from the mineral industry. Expenditure on gold exploration dominates exploration budgets worldwide. Gold is most common in the native form, but also occurs as tellurides. Although silver does occur as the native metal, it more often exists as sulfide minerals and sulfosalts, or as minor constituents of common sulfide minerals such as galena, sphalerite, and chalcopyrite. The inertness and high specific gravity of gold account for its concentration by mechanical processes as a heavy mineral in stream and beach environments (placers). Gold and silver are both mobilized by hydrothermal fluids as sulfide or chloride complexes, and precipitated in response to changes in solubility caused by decreasing temperature, or variations in fluid composition, pH, oxygen fugacity. or sulfur fugacity. Gold and silver occur in a diverse range of mineral deposits including recent placers and paleoplacers of gold, epithermal gold-silver vein and disseminated deposits, stratabound (iron formation-hosted) and vein gold in Archean greenstone belts, turbidite-hosted gold, skarn gold, Cordilleran vein silver deposits, veins of silver, cobalt-nickel-arsenides, and sediment-hosted copper-silver deposits.

Vein and stratabound gold deposits of northern Zaire

Seventy years of gold mining in the North Zaire region resulted in a total output of ca. 350 t of metal, half of this amount coming from recent placer deposits. All deposits are linked to volcano-sedimentary series and associated granitoid bodies constituting the granite-green-stone terrane of northern Zaire. The actual metallotects are tectonic shear structures cutting across various rock types and giving rise either to quartz veins or to quartz-free tabular deposits, thereby obscuring the original stratabound (or rock-type bound) character of the subeconomic mineralization. Both the Archaean magmatic activity and the late Precambrian tectonics have played a role in gold concentration.

Geochemical and morphological characteristics of gold particles from recent river deposits and the fossil placers of the Witwatersrand

A relationship has been established between morphological features and fineness of gold particles and the distance over which they have been transported in recent alluvial placer deposits, such as the rivers of the Barberton Mountain Land, South Africa and the river Rhine in Germany. It was possible to show that most gold particles from the Witwatersrand conglomerates retained their detrital morphology and by comparing them with particles from recent alluvial gold deposits it was possible to estimate the distance of transport for the Witwatersrand gold, which in most cases ranged from 10 km to 30 km. Gold particles in recent placers show a characteristic increase in fineness with increasing distance of transport because of the leaching of the silver from them. The Witwatersrand gold particles on the other hand, have retained their primary fineness, because leaching of silver in the oxygen-deficient Precambrian atmosphere was not feasible chemically.

Epigenetic, diplogenetic, syngenetic, and lithogene deposits

Much of the disagreement over and deposits is semantic; some ores and altered rocks are in part syngenetic and in part epigenetic and for them the term diplogenetic is proposed. All these terms are primarily time terms related to the contemporaneity of the deposit and the enclosing rock, but in a secondary sense they are space terms; they do not, however, imply process of formation or source of the chemical constituents. Process or source terms such as magmatic, sedimentary are generally satisfactory, but the process term lateral secretion is now vague in meaning. A more precise term is needed for the process of mobilization of elements from a solid rock and their transportation and redeposition elsewhere; for this process I propose the term (stone-born), to stress the source and process as do magmatic and sedimentary. Such deposits could be derived from syngenetic, diplogenetic, or epigenetic deposits through the action of metamorphic, hydrothermal, supergene or other solutions. If one wishes, however, to emphasize the distance the mobilized elements have moved from the source rock, the resulting deposit can be described best as a locally derived lithogene deposit (lateral secretion), or as a regionally derived lithogene deposit (product of regional metamorphism).The classification of many deposits such as epigenetic replacement bodies of magnetite in limestone, and syngenetic gold in recent placers is not controversial; however, where large parts of the mineral deposit are made up both of syngenetic elements and introduced elements, the term diplogenetic is appropriate. Examples include mineral deposits in which syngenetic cations are united with epigenetic anions, as in fluorite replacements in limestones, or where the reverse relationship holds and epigenetic cations are united to syngenetic sulfur or other anions--as may be true of certain black cupriferous shales of the Mansfeld type. Guides for distinguishing the various types of mineral deposits are discussed at length and include: mineralogy; texture; isotopic composition of hydrogen, carbon, oxygen, sulfur, and perhaps other elements; form; age relations; alteration; zoning; position and quantitative adequacy of supposed source rocks; lithology; structural geology; paleogeography; and regional geologic relations. The criteria developed are used in a critical review of several examples of controversial deposits--including the African Copperbelt deposits and those of the graphitic schists of Fenno-Scandia.

Nature and origin of central Idaho blacksands

Fifty or more heavy minerals occur in blacksand placers adjacent to the Cretaceous Idaho batholith. They contain Nb (columbium), Ta, Y, Zr, Hf, U, Th, Ti, and the rare earths. These minerals are of present or future potential value as sources of metals characterized by versatility, including strength and resistance to heat and chemical corrosion. Natural concentrations of blacksand involve mineral hardness, texture, chemical composition, and specific gravity; mineral size, shape, and degree of subroundness; and major alterations in stream regimens affecting competency, capacity, and interruptions of transportation. The latter include climatic changes, lava damming, crustal warping, faulting, and mass-wasting phenomena. Local deep disintegration of bedrock is an important adjunct to blacksand concentration. The original source of heavy mineral elements is believed to be Precambrian placer accumulations. These concentrations were later partially dispersed by metasomatic reconversion during diastrophic maxima, and indistinct swarms of accessory minerals were produced. New placers were formed again later through the operation of erosional and depositional agents. Recent blacksand placers derived their heavy minerals from quartz monzonite, granodiorite, diorite, and metamorphic remnants of original country rock commonly found in adjacent exposures of the Idaho batholith. The batholith may appear deceptively normal in these areas. When present, pegmatite and aplite dikes provide an additional source of heavy detrital minerals. The domination of one or more minerals over others in the more recent placers, appears to be related to the domination of similar elemental constituents in earlier original placer deposits. Thus recent data on both the petrography of the Idaho batholith and the nature and origin of blacksand placers lead to the following conclusions: erosional processes concentrated the first placer ore deposits in the Precambrian; and subsequent events both geomorphic and metasomatic probably played more important roles in the genesis of all types of ore deposits, including placers, than previously has been supposed.