Concepts and Revised Models for Phanerozoic Orogenic Gold Deposits

Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: 1) crustal scale fault associated (CSF) subtype, 2) sediment-hosted orogenic gold (SHOG) subtype, 3) forearc (FA) subtype, and 4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.

Mining Associated Malaria Epidemics

Tropical alluvial gold and gem miners are often an especially at-risk population for malaria infection. Geographical areas of mining-associated malaria epidemics in the recent past include Southeast Asia (Cambodia, Thailand, and Myanmar); the Amazon basin (Brazil, French Guyana, Suriname, Columbia, and Peru); and tropical Africa. Mobile populations of young adult men engaged in the hard labor of mining may experience severe malaria especially if they lack preexisting immunity and are irregularly consuming antimalarial drugs. Particular problems occur because much of this informal mining activity is illegal and done in isolated areas without access to health services and with evidence of emerging antimalarial drug resistance. Concentrating vulnerable populations in an ecologically disturbed landscape is often conducive to epidemics, which can then spread as these highly mobile workers return to their homes. Mining-associated malaria endangers malaria elimination efforts and miners need to be addressed as a group of particular concern.

Program for determination of zones of gold concentration on sluices of washing devices

Currently sluice washing devices are the most common in alluvial gold mining. Their use provides a sufficiently high performance, relatively low power consumption, and acceptable recovery of valuable components. The theoretical provisions of traditional hydraulics make it possible to determine all the main parameters of the movement of particles of rocks and gold in the pulp, however, in real operating conditions of the sluice box, their actual values will differ greatly from the calculated ones, especially if there are solid fractions in the pulp with a particle size of more than 20 mm. This is explained by significant fluctuations in the values of the surface, average and bottom velocities of the two-phase flow, vertical pulsation velocity in conditions of constrained movement of the different fractional composition of rocks. The article presents the results of experimental studies to identify the dependence of the distance traveled by an individual gold particle and host rocks in a two-phase flow through a sluice, the bottom of which is lined with trapping coatings, on the design and technological parameters of the flushing device. The mathematical model for determining this distance formed the basis of the Gold Enriching program. The program allows, in a wide range of initial data, to determine the zones of concentration of gold of a certain size at the sluice boxes.

Study of gold-bearing ores material and mineralogical composition in Lower Angara region

In connection with the involvement in new deposits exploitation, represented by refractory oxidized ores with a fine-dispersed phase and valuable component low content, there have been trends in the processing of this type of raw material. The alluvial gold-bearing sands of Lower Angara region are of practical interest. The results of the material and mineralogical composition of gold-bearing ores made it possible to establish that most rock fragments are quartz-mica schists, which are dilapidated by weathering up to 15% of the sample mass and clay inclusions. Ore by 61.5% is presented with grain size of more than 20 mm. Gold is distributed almost evenly by size class. Basically, gold is found in intergrowths with quartz, mica, pyrite, and clay components. The size of the gold grains ranges from 0.03 to 0.05 mm to several microns. The content of the valuable component was about 3-5%. The result of gravity concentration of the original ores showed the possibility of obtaining gravity concentrates with a metal content of 220-250 g/t, with extraction up to 93-95% and obtaining tailings from 0.3-0.5 g/t, followed by leaching of cyanide and sulfur-alkaline solutions. The extraction of the valuable component into the solution was 80% and 98%, respectively. The results obtained indicate the possibility of processing these types of ores in Lower Angara region.

Forest restoration after alluvial gold mining can recover vegetation structure. A case study in Colombia

Technical reclamation and spontaneous succession (passive restoration) are the two main approaches for restoring post-mining sites worldwide. Despite substantial differences between both approaches, little is known regarding how they differ in terms of ecological outcomes. We assessed and compared the vegetation structure and composition in one spontaneous succession forest that is 29 years old, two technically reclaimed forests that are 2 and 10 years old after alluvial gold mining, and one old-growth reference forest in northeastern Antioquia, Colombia. We sampled trees and saplings establishing three Modified-Whittaker Plots in each site. We measured tree basal area, canopy cover, vegetation height, tree density, and species richness. Vegetation structure and composition differed considerably among restoration approaches. Species richness was significantly greater in the spontaneous succession forest than at the other sites. Similarly, technical reclamation significantly increased the vegetation attributes after 10 years, reaching values similar to the reference forest. We underscore the importance of fast-growth planted species (Acacia mangium, Samanea saman, and Schizolobium parahyba) to revegetate mining degraded lands.

Management and restoration of degraded areas by gold mining activity in madre de dios

This document synthesizes a model for the restoration of degraded areas by gold mining, developed between 2008 and 2016 by the Consortium Madre de Dios CMDD with the Florida University, and the support of the Government and People of the United States of America. This successful reforestation experience, continued by CMDD and CEDE, as well as by CINCIA, is part of an environmental management proposal, which with the support of communication tools and knowledge management has included the education of children with mental disabilities due to effects mercury contamination in alluvial gold mining; and since 2016 with CMDD CEDE and the Ministry of Energy and Mines MINEM, through effective interaction between stakeholders has strengthened governance and incorporated an environmental mitigation component with clean technologies. In the context of the current controversial and polemic political situation in Peru (2021) 21 miners' associations in process of being formalized in Madre de Dios are considering their implementation. Este documento sintetiza un modelo de restauración de áreas degradadas por la minería de oro, desarrollado entre 2008 y 2016 por el Consorcio Madre de Dios CMDD con la Universidad Florida, y el apoyo del Gobierno y del Pueblo de los Estados Unidos de América. Esta exitosa experiencia de reforestación, continuada por CMDD y CEDE, así como por CINCIA, es parte de una propuesta de gestión ambiental, la cual con el apoyo de herramientas de comunicación y gestión del conocimiento ha incluido la educación de niños con discapacidad mental por efectos de contaminación por mercurio en la minería aluvial aurífera; y desde 2016 con CMDD CEDE y el Ministerio de Energía y Minas MINEM, mediante efectiva interacción entre las partes ha fortalecido la gobernanza e incorporado un componente de mitigación ambiental con tecnologías limpias. En el marco de la actual controversial y polémica coyuntura política del Perú (2021) 21 asociaciones de mineros en proceso de formalización en Madre de Dios están considerando su implementación.

EARLY THEORIES AND PRACTICALITIES ON GOLD OCCURRENCE IN AUSTRALIA

The discovery of gold in Australia forced many changes to theory on the occurrence and origin of gold deposits. Initial discoveries appeared to confirm existing ideas on the global distribution of gold-bearing terrains. Later discoveries and research would show that this confirmation was largely coincidental, but nevertheless helpful in early prospecting. Prior to the first Australian gold rush, theoretical predictions of payable gold were made by Sir Roderick Murchison and Rev. W. B. Clarke based on knowledge of accidental gold finds and geological analogy with known areas of significant gold occurrence, particularly the Ural region in Russia. These predictions were overwhelmed when Edward Hargraves, realised he might be able to spark a gold rush that would prove the existence of payable gold. Hargraves travelled to the Bathurst region of New South Wales where numerous gold finds had already been made and with local guides, prospected Lewis Ponds Creek and the Macquarie River. He demonstrated the methods of alluvial mining, to John Lister and William and James Tom enabling them to find sufficient alluvial gold to initiate a gold rush. The crowd of attracted diggers demonstrated the existence of a payable goldfield. The unstoppable first rush resulted in the pragmatic introduction of government regulation and administration to allow alluvial gold mining. Other discoveries of payable goldfields quickly followed. As the local scientific expert on gold, W. B. Clarke was commissioned to conduct two extensive surveys of the goldfields between 1851 and 1853. Clarke also drew on his geological knowledge to provide practical advice to the thousands of prospecting gold diggers. Gold-bearing quartz reefs and lodes were discovered, but it was predicted that these could not be mined economically. Theory also predicted that the reef gold would not continue to depth. Practical observations and mining experience from the numerous discoveries led to revision of the widely held dicta on gold occurrence. Alluvial gold was found in a range of settings, including the recent drainage and ancient and buried leads. A wider variety of rock types was recognised as favourable for gold. Different styles of reef gold were identified and found to be economically mineable to great depth. Evolving ideas on the origin of gold deposits were widely discussed, tested, and refined. Of the many players involved in the early discovery of gold in Australia, Clarke, Hargraves and Murchison probably had the greatest overall influence in terms of theoretical predication and practical outcomes that initiated the Australian gold-mining industry.