Academic literature on the topic 'Eldorado deposit'

Metamorphic processes play a key role in forming orogenic gold deposits. In this paper, we present new evidence that host schists of the two largest gold deposits of the Yenisey ridge (Russia) Olympiada and Eldorado underwent a single stage of metamorphism in contrast to surrounding blocks. This metamorphism is of moderate thermal gradient and belongs to the Barrovian type, which is typical for the collisional event in the time range 800–850 Ma. The new Ar/Ar age data presented in this paper and the review of magmatic and metamorphic events and ore-forming processes indicate that the most productive stage (gold-sulfide-quartz) correlates well in time with the regional metamorphism of the Barrovian type. This indicates that metamorphic processes can have a crucial role in forming gold deposits of the Yenisey ridge. Carbonaceous material thermometry indicates a wide range of obtained temperatures around 90–150 °C around the mean temperature for each sample. The highest temperatures are close to the peak metamorphic temperatures estimated by garnet-biotite thermometry.

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Este trabalho foi desenvolvido no garimpo do Araras, localizado na margem direita do rio Madeira, no Km 40 da BR 425, em Nova Mamoré, Rondônia, e originado a partir de uma ocupação, por garimpeiros, de uma área que estava sendo pesquisada para ouro por uma empresa mineradora. A partir da disseminação da notícia de que havia muito ouro no referido local, formou-se uma currutela, hoje Vila do Araras, que teve seu auge de produção aurífera e crescimento demográfico em 1992, quando havia aproximadamente três mil habitantes e cerca de trezentas dragas e balsas. A partir desta data, começou a fase de esgotamento dos depósitos auríferos aluvionares, dando início à saída da população para outras áreas de trabalho. Porém, uma parte dessas pessoas permanecem até hoje na vila do Araras, motivo pelo qual escolhemos o lugar para realizar a presente pesquisa, que objetiva detectar os níveis de impacto ambiental e em que condições sócio-econômicas vivem essas pessoas, propondo medidas visando reduzir os efeitos negativos produzidos durante mais de dez anos pela lavra do ouro.
This study was developed in the Araras gold garimpo, located along the right margin of the the Madeira River, km 40 of BR 425, in Nova Mamoré, Rondônia. This city was originally founded by prospectors and the locale was also investigated by a mining company. After word got out that there was alot of gold in this area, it quickly turned into a mining camp, today known as Vila do Araras. This town reached it's peak production in 1992 when there were approximately 3000 inhabitants and ca. 300 dredges and mining boats functioning. After this date, the alluvial gold deposits began to give out, thus the population of the region also decreased as many workers sought employment elsewhere. However, some of the original prospectors still remain in Vila do Araras. For this reason, we selected this place to carry out this study. The objective of this study has been to determine the levels of environmental impact and what the current socio-economic conditions of the remaining populace is. By doing this, we propose measures with the intention of reducing the negative effects created during more than ten years of alluvial gold mining.

Orientador: Gilda Carneiro Ferreira
Banca: Sandra Elisa Contri Pitton
Banca: Ene Glória da Silveira
Resumo: Este trabalho foi desenvolvido no garimpo do Araras, localizado na margem direita do rio Madeira, no Km 40 da BR 425, em Nova Mamoré, Rondônia, e originado a partir de uma ocupação, por garimpeiros, de uma área que estava sendo pesquisada para ouro por uma empresa mineradora. A partir da disseminação da notícia de que havia muito ouro no referido local, formou-se uma "currutela", hoje Vila do Araras, que teve seu auge de produção aurífera e crescimento demográfico em 1992, quando havia aproximadamente três mil habitantes e cerca de trezentas dragas e balsas. A partir desta data, começou a fase de esgotamento dos depósitos auríferos aluvionares, dando início à saída da população para outras áreas de trabalho. Porém, uma parte dessas pessoas permanecem até hoje na vila do Araras, motivo pelo qual escolhemos o lugar para realizar a presente pesquisa, que objetiva detectar os níveis de impacto ambiental e em que condições sócio-econômicas vivem essas pessoas, propondo medidas visando reduzir os efeitos negativos produzidos durante mais de dez anos pela lavra do ouro.
Abstract: This study was developed in the Araras gold garimpo, located along the right margin of the the Madeira River, km 40 of BR 425, in Nova Mamoré, Rondônia. This city was originally founded by prospectors and the locale was also investigated by a mining company. After word got out that there was alot of gold in this area, it quickly turned into a mining camp, today known as Vila do Araras. This town reached it's peak production in 1992 when there were approximately 3000 inhabitants and ca. 300 dredges and mining boats functioning. After this date, the alluvial gold deposits began to give out, thus the population of the region also decreased as many workers sought employment elsewhere. However, some of the original prospectors still remain in Vila do Araras. For this reason, we selected this place to carry out this study. The objective of this study has been to determine the levels of environmental impact and what the current socio-economic conditions of the remaining populace is. By doing this, we propose measures with the intention of reducing the negative effects created during more than ten years of alluvial gold mining.
Mestre

The Au-Cu-Bi- deposits of the Proterozoic Tennant Creek Inlier share geological and geochemical characteristics that indicate strong links in their genesis, yet the diversity in alteration assemblages, metal ratios and zonation patterns reflect variations in ore forming processes that previously have not been explained in detail. The West Peko deposit is representative of Cu-rich, pyrrhotite-bearing mineralisation with intermediate gold grades, in magnetite+ hematite-rich syntectonic ‘ironstones’. By contrast, the high grade Eldorado Au deposit contains minor sulfides and very low Cu grades, similar to several of the larger gold producers in the field (e.g. Juno, White Devil, Nobles Nob), and is also hematite-rich. Au, Chalcopyrite and Bi-sulfosalts were introduced into pre-existing ironside during progressive shearing, either late in the first regional deformation event (D1) or during a second phase of deformation. The occurrence of some Au zones outside ironstones suggests the ore fluids in part followed different flow paths to hose of the ironside-forming fluids. Three chemically and isotopically distinct fluids have been characterised. (i) Ironstone-forming fluids at West Peko and Eldorado were Ca-Na-Cl (-Fe?) brines containing 12-20 weight % total dissolved salts, and reached temperatures of 350-400°C during magnetite deposition. Oxygen and hydrogen isotope compositions of minerals formed at the ironside stage are consistent with an origin of ironstones from formation or metamorphic waters. (ii) The inferred Au-Bi+Cu transport fluid in he Cu- and sulphide-rich West Peko deposit was of low to moderate salinity (3-10 eq. wt. % Na Cl), ~300-350°C and N2 + CH4 – rich. Newly represented phase equilibria among the Fe-silicates stilpnomelane and minnesotaite, chlorite, biotite, sulfides, oxides and carbonates as well as fluid inclusion vapour compositions indicate that the Au-Bi+Cu transport fluid was relatively reducing with near-neutral pH and total dissolved sulphur contents of 0.001m to 0.01m. In the Eldorado Deeps Au- and hematite-rich deposit the Au-transporting fluid also may have been of low-moderate salinity, with Au deposition occurring at ~300°C. The reducing Au-Bi+Cu transport fluid at West Peko resembles primary magmatic or metamorphic water in oxygen and hydrogen isotopic composition. Carbon isotope ratios of Au-sulfide stage carbonates at West Peko point to involvement f organic carbon, probably sourced outside the host Warramunga Formation. (iii) A regionally distributed, oxidising Ca-Na-Cl brine with 20-35 weight percent total dissolved salts, was present prior to, after and probably during ore deposition. Mixing with lower salinity reducing Au-Bi+Cu transport fluid is inferred at West Peko and us suggested to have caused effervescence of N2+CH4 by ‘salting out’, relatively late in the Au depositional stage. An hypothesis of metal transport and deposition is proposed for the Tennant Creek deposits in which gold, copper and bismuth were transported in a reducing fluid and were deposited in the Cu- and sulphide-rich deposits dominantly by oxidation, desulfidation and initial pH increase as the reducing fluid reacted with magnetite+hematite ironstone. Mass transfer modelling indicates that relatively small amounts of ironstone are required to precipitate Au + Bi-sulfides, such as Eldorado, the oxidising brine may have played a significant role in ore deposition either by mixing with a reducing Au-Cu-Bi-transporting fluid, or by producing hematite oxidant additional to any already present in the ironstones. The greater extent of oxidation of the ore fluid in such deposits may have generally prevented saturation of copper minerals, resulting in low Cu grades. Gold is inferred to have been transported dominantly as uncharged bisulfide complexes, although biselenide complexes were potentially important. New thermodynamic data estimated for bismuth complexes are consistent with bismuth transport as uncharged S-H-O-bearing species in the Tenant Creek ore fluids. The existence of high grade Au-Bi deposits outside ironstones is predicted by chemical modelling of mixing between reducing and oxidising fluids, located where structures allowed focused flow of both fluids.