Mass balance constraints on gold placers; possible solutions to "source area problems"

Loen, J.S.

doi:10.2113/gsecongeo.87.6.1624

Cited by 19 publications

(12 citation statements)

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“…McConnell (1905) studying the gold placers in the Klondike suggested that the original gold content of the quartz was probably no more than a few cents to the ton and •hence the veins might not be productive even if discovered. Similar conclusions were reached by Loen (1992) in studying mass-balance constraints on gold placers of varying ages and in a variety of locations such as Fairbanks, Alaska (Pliocene to Quaternary), Sierra Nevada, California (Eocene to Quaternary), and Witwatersrand, South Africa (Archean). Several of Loen's conclusions are presented herein because they might have bearing on the Fortymile River area placers for which no lode source has been identified.…”

Section: Eagle Quadranglesupporting

confidence: 69%

“…It is an instructive and interesting exercise to calculate what might be the expected amount of placer gold deposited in the Fortymile River area using Loen's (1992) mass-balance equation and making some assumptions on the efficiency of the gold release system, erosion rates, and duration of erosion. The mass-balance equation, (Loen, 1992) is: P=(D X Ad XC X T X R) X E 100 where P =total mass of gold in the placer deposit (tons Au), D =mean density of the source rocks (t I m 3 or tons per cubic meter), Ad= drainage basin area (km 2 or square kilometers), C =mean abundance of gold in source rock (ppb or parts per billion), R = mean denudation rate (cm/ka or centimeters per thousand years), T =time constraint on erosion of source rocks (My or millions of years), and E =efficiency of weathering and concentration processes (in percent). Efficiencies of placer-forming systems would be expected to be low because a substantial portion of the heavy minerals are either not released by weathering or are chemically and mechanically lost from the system (Loen, 1992).…”

Section: Eagle Quadranglementioning

confidence: 99%

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Gold placers of the historical Fortymile River region, Alaska

Yeend¹

1996

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Section: Eagle Quadranglesupporting

confidence: 69%

Section: Eagle Quadranglementioning

confidence: 99%

Gold placers of the historical Fortymile River region, Alaska

Yeend¹

1996

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“…Frimmel et al (2005) suggest that the Witwatersrand gold, with anomalously high Re and Os concentrations and 187 Os/ 188 Os of 0.108, equivalent to the Os isotope composition of the 3.0 Ga mantle (e.g., Kirk et al, 2002), was likely derived from dispersed magmatic phases in mantle-derived mafic and/or ultramafic rocks of now eroded granitoid-greenstone terranes in the hinterland to the Central Rand basin. A source region with a bulk concentration between about 0.5 and 7 ppb Au, in agreement with a value of 4 ppb Au for Barberton-type granitoid-greenstone crust (Robb and Meyer, 1990), is all that is required to form the Witwatersrand deposits (Loen, 1992), provided that concentration mechanisms were very effective (Frimmel et al, 2005). High background gold values in the early Archean komatiites may have correlated with mantle plumes derived from the mantle boundary with the gold-rich core (Kerrich, 1999).…”

Section: Placer and Paleoplacer Goldmentioning

confidence: 54%

100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits

Groves¹,

Condie²,

Goldfarb³

et al. 2005

Economic Geology

197

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Mineral deposit types commonly have a distinctive temporal distribution with peaks at specific periods of Earth history. Deposits of less redox-sensitive metals, such as gold, show long-term temporal patterns that relate to first-order changes in an evolving Earth, as a result of progressively declining heat production and attendant changes in global tectonic processes. Despite abundant evidence for plate tectonics in the early Precambrian, it is evident that plume events were more abundant in a hotter Earth.Episodic growth of juvenile continental crust appears to have been related to short-lived (<100 m.y.) catastrophic mantle-plume events and formation of supercontinents, whereas shielding mantle-plume events correlated with their breakup. Different mineral deposit types are associated with this cycle of supercontinent formation and breakup. Broadly synchronous with juvenile continental crust formation was the development of subcontinental lithospheric mantle, which evolved due to progressively declining heat flow and decreasing plume activity. Archean subcontinental lithospheric mantle has a distinct mineralogical composition and is buoyant, whereas later lithosphere was progressively more dense. Changes in the buoyancy of both oceanic lithosphere and subcontinental lithospheric mantle led to evolution of tectonic scenarios in which buoyant, roughly equidimensional, early Precambrian cratons were rimmed by Proterozoic or Phanerozoic linear elongate belts of neutral to negative buoyancy.Orogenic gold deposits, which formed over at least 3.4 b.y., had the highest preservation potential of any gold deposit type. The pattern of formation and preservation, from episodic to more cyclic, broadly mirrors that of crustal growth. Early Precambrian (mostly ca. 2.7 and 2.0-1.8 Ga) deposits, protected from uplift and erosion in the centers of buoyant cratons, are rare between ca. 1.7 Ga and 600 Ma due to the change to more modernstyle plate tectonic processes, with nonpreservation of deposits of this age due to uplift and erosion of more vulnerable orogenic belts. Volcanic-hosted massive sulfide (VHMS) deposits were accreted into the convergent margin terranes in which orogenic gold deposits were forming. Their temporal distribution, from strongly episodic to more cyclic peaks, also supports a model of selective preservation.The first appearance of iron-oxide copper-gold (IOCG) deposits at ~2.55 Ga closely follows development of early Precambrian subcontinental lithosphere mantle. Their genesis involved melting of metasomatized subcontinental lithosphere mantle, so they could not form until such metasomatized mantle evolved below cratons with buoyant lithosphere. Giant Precambrian paleoplacer gold deposits probably formed by effective fluvial sorting under extreme climatic conditions but were largely preserved due to early buoyant subcontinental lithospheric mantle below hosting foreland basins. Unequivocal intrusion-related gold deposits are related to complex felsic intrusions with a mixed mantle-crustal signature, which...

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“…Loen (1992) determined the gold redistribution from source rocks to placer deposits. The material balance was computed as follows:…”

Section: Governing Equations Mass Balancementioning

confidence: 99%

Chemical modelling on the bare rock or forested watershed scale

Bassett

1997

Hydrol. Process.

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The simulation of weathering, solute distribution or acidi®cation at the catchment scale is predominantly done with either a mass balance or process level model. The former redistributes total elemental concentrations between known points with measured total concentration, but does not explicitly include catchment hydrology. The latter includes compartmental hydrological models and detailed descriptions of spatially averaged chemical reactions. Interestingly, the model applications tend towards hydrologically dierent watershed structures: mass balance modelling favours bare rock watersheds similar to the Apache Leap Research Site, whereas process level models are applied most often to forested watersheds, among which the Hubbard Brook Experimental Forest is an example. Although constrained either by mineral or water compositions on the one hand, or calibrated against stream or lake water chemistry on the other, both approaches basically ®t parameters to the geochemical circumstances of the speci®c watershed of interest. Limited success is attained if the hydrological conditions remain within the circumstances of the parameter ®tting. The principal dierences in model formulation and approach to mass balance modelling are discussed. Without advancements in model calibration and rigorous model testing, and the development of methods for optimizing the important reactions and pathways, the transportability of models between watersheds or the simulation of extreme events will continue to be inadequate.

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Mass balance constraints on gold placers; possible solutions to "source area problems"

Cited by 19 publications

References 0 publications

Gold placers of the historical Fortymile River region, Alaska

Gold placers of the historical Fortymile River region, Alaska

100th Anniversary Special Paper: Secular Changes in Global Tectonic Processes and Their Influence on the Temporal Distribution of Gold-Bearing Mineral Deposits

Chemical modelling on the bare rock or forested watershed scale

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