Placer Deposits

The Precambrian shield of eastern Bolivia occupies some 220,000 km2, consisting of granulite, gneiss, syn- and post-kynematic granitoids and a number of elongated low to medium grade schist belts. The latter are at least of lower Proterozoic age, cover 10%
of the region and exhibit in places volcano-sedimentary and structural units of weak to high primary gold content. Al1 gold placer deposits are spatially related to these schist belts. The
different types developed during the geomorphological evolution of the shield since late-Miocene time. The mmbined effects of early lateritization, tectonical uplift and erosion under
semi-arid climate and savanna vegetation have led to the formation of eluvial-lateritic, coluvial and different types of alluvial placer deposits. In the last 300 years principally small coluvial and headwater gulch placersw ere mined, yielding a total productiono f about 16 tons of gold. This quantity is relatively low in comparison with other Precambrian gold placer districts. Medium-size placer deposits of economic grade may have formed where several
auriferous gulches join to form a higher orderS tream or in sediment traps of the larger rivers. These deposits are found near the shield border in paleo-channels which lie under argillaceous
fluvio-lacustrine sediments. The region as a whole is considered as a minor gold placer province of Bolivia.

RESUMEN: El escudo Precámbrico de Bolivia oriental abarca un área de 220.000 km2 Consiste de granulita, gneiss, granitoides syn- y post-cinemáticos y varias fajas elongadas de esquistos de metamorfismo bajo a medio. Estos tienen una edad mínima de Proterozoico inferior, cubren 10% de la región y están compuestos frecuentemente de unidades volcanosedimentarias y estructuras con bajo a alto contenido de Oro primario. Todos los placeres auríferos de la región están relacionados a las fajas de esquistos. Los tipos diferentes-fueron formados durante la evolución geomorfológica del escudo desde el Mioceno superior. La combinación de los efectos de una lateritización temprana, ascenso tectónico, erosión en clima semiárido y cobertura de sábana resultó en la formación de placeres eluvio-lateriticos, coluviales y aluviales. En los últimos 300 años se explotaron principalmente placeres coluviales y las cabeceras secas de quebradas. La producción total esta estimada en unas 16 toneladas de Oro. Esa cantidad es considerada pequeña comparada con otros distritos de placeres de zonas Precámbricas. Placeres de dimensión media y tenor económico se formaron probablemente en sitios donde varias quebradas auríferas se juntaron en un canal de mayor orden, o en trampa sedimentarias de los rios mayores. Esa situación se encuentra con preferencia cerca del borde sudoeste del Precámbrico dentro de paleo-canales debajo de una cobertura de sedimentos fluvio-lacustres. La región es considerada según sus características como una provincia aurífera de menor importancia  de Bolivia.

The processes leading to the formation of beach placer deposits generally begin inland and terminate at the coast, including source rocks being weathered, eroded and then transported by streams and rivers to the coast, where the sediments are deposited in a variety of coastal environments. The coastal sediments are reworked by the action of waves, tides, longshore currents and wind, which are effective processes for sorting the mineral grains based on differences in their size and density, resulting in laminated or lens-shaped packages of sediments up to tens of meters thick that are rich in heavy minerals. Detailed studies of sedimentary basins, as well as peripheral (paleo-)valleys that drained sediment source areas, are important tools in the exploration for heavy mineral resources. Knowledge of the (paleo-)basin, associated valley architecture and the source of heavy minerals concentrated in the shorelines and valleys are useful guides to the potential for, and location of, economic deposits. Evidence from sedimentology can be combined with that from other geological and geophysical characteristics to arrive at a general reconstruction of basin and paleovalley architecture and depositional environments. Complex paleogeography of the shorelines can influence or determine the sites of heavy mineral concentration. Heavy mineral sands tend to concentrate in certain shoreline settings during storm activity. Repeated storm erosion and reworking over centuries (e.g., the southeastern coast of Australia) or millennia (e.g., the Eucla and Murray Basins of Australia) can progressively enrich heavy mineral sand deposits. Preservation of these deposits over a geological timeframe of millions of years can ensue through subsidence of coastal sediments, and during sea-level change that results in either shorelines migrating inland (marine transgression) or seaward (marine regression), potentially burying or stranding earlier deposits or reworking them to form younger deposits. Refinements in remote sensing and geophysical techniques, data processing, sedimentology and computer-aided interpretations provide effective, economic and efficient methods for modeling coastal reconstructions and for exploring provinces and terrains prospective for heavy mineral sand deposits. Landscape topography analysis, combined with geophysical methods that can resolve physical property contrasts between the shoreline sediments and underlying sequences, are increasingly used in mineral exploration to locate and to reconstruct paleoshorelines and paleovalleys. Australia has modern and ancient beach-placer deposits, both of which show many similar geologic features. The formation of these heavy mineral deposits provides one of the best examples of applying knowledge of modern systems (e.g., the west and east coasts of Australia) as an analogue to interpret and understand the geology and form of ancient deposits (e.g., the Eucla and Murray Basins of southern Australia). This study provides descriptive and exploration models of Australia's heavy mineral sand deposits formed in coastal environments, which can be applied to similar settings worldwide.

GOLD PLACER OF THE MILANO SUBSOIL

Gold is a mineral widespread throughout the Alps and in the northern Apennines, but economically interesting concentrations are very scarce and are found only in the central-western Alps: they generally consist of quartz vein with native gold or gold-containing sulphides. The Po plane placers are made up of recent alluvial sediments of many rivers, containing very fine dust and scales. On the Po river southern side, the Orba is the only rich stream; on the northern side there are the rich rivers Orco, Dora Baltea, Elvo, Ticino and Adda, and many minor tributaries, which do not take gold directly from primary gold ores and related eluvial and alluvial deposits, but from the glacial sediments that carried them for many kilometers to the south. In addition, all these rivers received, and transport downstream, the gold particles lost during the Celtic and Roman exploitations of the highest gold-bearing alluvial terraces.

The city of Milan is located at the confluence of three small streams (Olona, Seveso, Lambro) responsible for the considerable thickness - almost 400 feet - of the alluvial layers below, made up of sand and gravel rich in aquifers: the entire requirement water supply of the metropolitan area is completely satisfied by many wells.

The systematic collection of samples, in large sand quarries operating around the city, has made it possible to obtain interesting samples of natural sands and its concentrates composed of heavy minerals. The estimated gold content in the extracted sediment, up to 30 m deep, is a few centigrams per cubic meter. Gold is made up of thin flakes, some ten millimeters large, of an intense yellow color, with a rough surface and irregular edges: the average composition is 92.3% pure gold, 6% silver, 1.7% other metals. The concentrates are mainly composed of garnets (30-50% by weight), magnetite (20-40%) and ilmenite (12-20%), with a gold content rancing from 0,1 to 0,4%.

Gambar berbagai macam ukuran dan bentuk emas aluvial (placer). Ukuran, Kehalusan, dan Bentuk Emas Aluvial serambigeologi.blogspot.co.id /2016/02/ukuran-kehalusan-dan-bentuk-emas-aluvial.html Ukuran Emas Aluvial (Placer) Sebagian besar emas aluvial (placer) terdapat dalam bentuk butiran-butiran berukuran halus yang disebut " dust " (debu), tetapi oleh para penambang yang beruntung kadang ditemukan juga gumpalan-gumpalan atau bongkah-bongkah yang lebih besar yang disebut " nuggets ". Secara umum kisaran ukuran nugget emas aluvial (placer) mirip biji-bijian sampai mirip kacang-kacangan, tetapi nuggets yang beratnya 1-10 pon (0,5 – 5 kg) juga cukup umum ditemukan.

The collected wadi deposit samples (24 samples) from Gabal El-Dob area, at the
northern part of the Eastern Desert of Egypt are subjected to mineralogical and chemical
analyses. Generally, they are classified into light and heavy minerals. The light minerals are
mainly quartz and feldspars. The heavy minerals are divided into magnetic and non-magnetic
minerals. The magnetic minerals are represented mainly by magnetite, however the nonmagnetic
ones are classified into metallic and non-metallic minerals. The metallic minerals
comprise hematite, ilmenite, rutile, wolframite and cassiterite. However, the non-metallic
minerals are represented by zircon, apatite, monazite and garnet with addition to mafic
minerals (biotite, hornblende, epidote and chlorite). The occurrence and distribution proved
also that they had been formed by weathering action of fresh water (rivers) rather than
beaches, nearby the outcrops of rock sources.

The accidental discovery of a 17-pound gold nugget by a trio of adolescents in Cabarrus County, NC in 1799 spurred a spasmodic gold rush that spread throughout the American Southeast. From the early 1800s to 1849 the search for the precious metal fomented exploration and various industries along the gold-bearing regions of the Piedmont and eastern Appalachian Mountains from Alabama to Maryland. The focus of the nascent gold industry was off-season, haphazard placer mining by individuals and small family groups who used primitive medieval mining techniques and backbreaking physical labor. By the 1820s, the part-time, untrained farmers, slaves, and "boomers" had depleted the easily found nuggets and lodes in the ‘branch’ streams. New techniques and innovations in machinery were developed by a set of diverse, yet interconnected, events and individuals to bring forth the golden metal. Thus, the southern gold fields of the southeastern United States became a proving ground for placer mining techniques and technology that fostered an economic and industrial expansion in the agriculturally dominated region as well as providing a substantial proportion of the wealth of the new nation. This 50-year period of placer mining innovation and practical application in the southern gold fields provided the skills and basic equipment that promoted the whirlwind of mining frenzy that was the 1849 California Gold Rush.

The induced polarization (IP) response of economically valuable placer
deposits of altered ilmenite and other heavy minerals in -fossil beach-complex sediments of northeastern Florida is unusually strong. This suggests that mineral-resource exploration and evaluation could be made much more efficient by use of the IP method. Field and laboratory studies conducted on detrital heavy-mineral deposits and on commercial concentrate stockpiles in Florida indicate that of the two main titanium ore minerals, altered ilmenite has a strong IP response while that of rutile is weak.
Induced polarization spectra for altered ilmenite were acquired by a
unique laboratory sampling method and are distinctly different from those of pyrite. Placer ilmenite consistently shows a phase-angle maximum at about 10 Hz, whereas pyrite has a phase-angle maximum that varies with grain size but generally centers at around approximately 0.1 Hz. This differences along with the unusually strong amplitude and phase variations observed in the IP response of altered ilmenite should permit separation of this altered ilmenite from pyrite in the field under typical northeastern Florida survey conditions. In northern.Florida» our experiments suggest that one volume percent of ilmenite corresponds to about 5-6 millivolt-seconds/volt inherent chargeability (or about 6-7 milliradians phase shift at 1.0 Hz). This permits quantification of ilmenite reserves by use of surface IP measurements.
The magnetic susceptibility of altered placer ilmenite concentrates was
significant (ranging -from 0.0025 to 0.0045 SI units), yet substantially below that reported in the literature for stoichiometric ilmenite. Field magnetic measurements have not been diagnostic of the presence of ilmenite due in part to the surprisingly low susceptibilities as well as to the low concentrations found in southeastern U.S. economic deposits. High-resolution aeromagnetic surveys however have been successfully used regionally in outlining target areas for drilling. Other geophysical exploration techniques for ilmenite-bearing placers such as gamma-ray spectrometry and Side-Looking Airborne Radar (SLAR) are dependent on geophysical signatures of associated minerals and/or geomorphologic associations and detect only surface characteristics. Consequently
quantification of resources with field measurements is not possible with
these methods.
The studies carried out so far encourage us to think that IP might be
the most efficient means to locate and delineate analogous ilmenite
concentrations in submerged U.S.  Continental Shelf sediments. If on-going research proves, this to be the case IP will become a major marine exploration tool in the evaluation and exploration for placer mineral
resources within the U.S. Exclusive Economic Zone.

The accidental discovery of a 17-pound gold nugget by a trio of adolescents in Cabarrus County, NC in 1799 spurred a spasmodic gold rush that spread throughout the American Southeast. From the early 1800s to 1849 the search for the precious metal fomented exploration and various industries along the gold-bearing regions of the Piedmont and eastern Appalachian Mountains from Alabama to Maryland. The focus of the nascent gold industry was off-season, haphazard placer mining by individuals and small family groups who used primitive medieval mining techniques and backbreaking physical labor. By the 1820s, the part-time, untrained farmers, slaves, and "boomers" had depleted the easily found nuggets and lodes in the ‘branch’ streams. New techniques and innovations in machinery were developed by a set of diverse, yet interconnected, events and individuals to bring forth the golden metal. Thus, the southern gold fields of the southeastern United States became a proving ground for ...

... High Mg/Fe and Ni values in the latter approach those for corundum in diamond, and are a possible diamond indicator. ... Refereed: Yes. Authors: Sutherland, FL, Zaw, K., Meffre, S., Giuliani, G., Fallick, AE, Graham, IT and Webb, GB. Subjects: Q Science > QE Geology. ...