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Komatiites h Eruption and flow
- Journal of Petrology
, 1985
"... Because of their high eruption temperatures and ultrabasic composition, komatiite lavas had low viscosities, which typically ranged from 0-1 to 10 Pa s. A major consequence of this low viscosity is that most lavas erupted as turbulent flows. An analysis of their ascent through the lithosphere sugges ..."
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Because of their high eruption temperatures and ultrabasic composition, komatiite lavas had low viscosities, which typically ranged from 0-1 to 10 Pa s. A major consequence of this low viscosity is that most lavas erupted as turbulent flows. An analysis of their ascent through the lithosphere suggests ascent velocities in the range of 1 to over 10ms"1 and Reynolds numbers much greater than the critical value of 2000. The lavas would have remained turbulent for most or all of their subse-quent flow and emplacement. Typical horizontal flow rates are estimated to range from 0-5 to 100 m2 s~ ' per unit width of flow. Such turbulent lava flows would have lost their heat by convection to the surroundings, at rates which are orders of magnitude greater than the rates for laminar flows, which cool by conduction. A quantitative analysis of the cooling of komatiites indicates cooling rates from over 1000 °C hr" ' to a few °C hr"1, while the flows remained turbulent. These rates are in an appropriate range to cause phenomena such as high nucleation rates, strong supersaturation of the lava, delayed nucleation of olivine, and skeletal or dendritic crystal morphologies. Komatiites often flowed over ground composed of rocks with lower melting temperatures. It is
The Proximal Volcanic Environment of the Scotia Nickel Deposit
"... Detailed exploration of the stratigraphy associated with the Scotia nickel sulfide deposit has led to the identification of a succession of u!tramafic flows displaying decreasing relative magnesium content both up the pile and along strike from the deposit. In addition, vent facies ultramafic pyrocl ..."
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Detailed exploration of the stratigraphy associated with the Scotia nickel sulfide deposit has led to the identification of a succession of u!tramafic flows displaying decreasing relative magnesium content both up the pile and along strike from the deposit. In addition, vent facies ultramafic pyroclastics and massive olivine peridotite flows dominate the locally thickened pile immediately overlying the ore. Interflow sediments are absent from this vent facies material but are present both higher in the sequence and along its flanks. The relatively undeformed and weakly recrystallized stratigraphy at Scotia affords an opportunity to recon-struct the development of an ultramafic succession proximal to a volcanic vent. The presence of ultramafic pyroclastics, amygdaloidal units, and a crosscutting ultramafic intrusive char-acterize the vent environment and distinguish the overlying pile at Scotia from that at other Western Australian nickel sulfide deposits.
FIELD CHARACTERISTICS AND EROSIONAL PROCESSES ASSOCIATED WITH KOMATIITIC LAVAS: IMPLICATIONS FOR FLOW BEHAVIOR
"... Although komatiitic lavas have long been depicted as turbulent flows, especially near the vent, field characteristics indicate that many komatiitic lavas did not flow turbulently, or only initially so. The bases of komatiites are commonly conformable with their substrate, including fine pelitic sedi ..."
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Although komatiitic lavas have long been depicted as turbulent flows, especially near the vent, field characteristics indicate that many komatiitic lavas did not flow turbulently, or only initially so. The bases of komatiites are commonly conformable with their substrate, including fine pelitic sediments, and the margins of komatiites are overwhelmingly coherent, or marked by local quench-fragmented hyaloclastite breccia. Autobreccias are notably missing. These characteristics are not consistent with turbu-lent flow, but clearly indicate conditions of laminar flow. If komatiitic flows were turbulent, they should commonly have scoured into substrate sediments through a variety of physical erosion processes, including foundering into underlying seafloor sediments, because of density inversion, and turbulence-induced scouring of sediments. These features are not commonly developed, also indicating that generally komatiites were emplaced under tranquil, laminar-flow conditions. Trough-like structures that com-monly host nickel sulfide mineralization have commonly been interpreted to originate by thermal erosion of substrate by the komatiitic lava. The evidence supporting thermal erosion is not strong, and commonly ambiguous. Trough structures at Kambalda, Western Australia, are fault bounded, as noted by several previous investigators. However, there is a common, but not universal, antithetic relationship between trough presence and sediment absence. Removal of sediment from troughs could be explained by physical erosion, with an initial narrow, turbulent flow-head scouring a channel in the underlying sediments. As the lava flows spread laterally, their flow-front velocity decreased, and flow became laminar, so explaining the conformable contacts with
©2015 Society of Economic Geologists, Inc. Economic Geology, v. 110, pp. 505–530
"... The remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes, the recognition of which could potentially enlarge the detectable footprint of this ore type. The Miitel komatiite-hosted nickel sulfide deposit in Western ..."
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The remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes, the recognition of which could potentially enlarge the detectable footprint of this ore type. The Miitel komatiite-hosted nickel sulfide deposit in Western Australia was used as a case study to investigate the nature and 3-D geometry of the geochemical halo created by the remobilization of base metals, gold, and platinum group elements (PGE) into the rocks surrounding the mineralization. At Miitel, anomalous metal enrichment is found in the country rocks surrounding the massive sulfides, up to 250 m away from the ore. This enrichment, detected using portable X-ray fluorescence (pXRF) and backed up by labora-tory analyses, occurs in the Mount Edwards footwall basalt within decimeters of the contact with the overlying Widgiemooltha komatiites. It is associated with the presence of nickel arsenides. Gersdorffite and minor nick-eline are concentrated within small quartz and carbonate veinlets, and are interpreted to form during the circu-lation of arsenic-rich hydrothermal fluids. Results of lead fire assay analyses and in situ laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) analyses indicate high PGE concentrations (Pd and Pt) and minor gold associated with the observed nickel and arsenic enrichment. Results from a larger, regional-scale study, combined with this PGE enrichment, suggest that the massive nickel sulfides from the Miitel ore are
Geochemical and Fluid Dynamic Modeling of Compositional Variations in
"... Stratigraphic, volcanological, petrological, and fluid dynamic studies suggest that Archean komatiite-associated nickel sulfide deposits in Western Australia formed by thermal erosion of unconsolidated sulfidic sediments beneath channelized lava flows. Geochemical and Nd isotope data for the type ar ..."
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Stratigraphic, volcanological, petrological, and fluid dynamic studies suggest that Archean komatiite-associated nickel sulfide deposits in Western Australia formed by thermal erosion of unconsolidated sulfidic sediments beneath channelized lava flows. Geochemical and Nd isotope data for the type area at Kambalda suggest that mineralized lava channels are less contaminated than barren flanking segments of the flow, owing to high discharge rates and continuous replenishment within the channel. This interpretation has several implications for the mode of emplacement of sulfides, their degree of equilibration with the host komatiite, and the compositions of sulfide ores: (1) the solubility of sulfur in komatiite is relatively low, so most of the sulfide in eroded sediments will be melted and remain as a dense layer at the base of the flow or be incorporated into the turbulently flowing komatiite as immiscible sulfide droplets; (2) sulfide ores will not necessarily be in equilibrium with overlying koma-tiites but will record equilibration with variably contaminated lavas during initial stages of emplacement and crystallization; and (3) ore compositions will vary from channel to channel (shoot o shoot) depending on (i) the composition of the hybridized komatiite, (ii) the fluid
Weathering, element distribution and geochemical dispersion at Mt Keith, Western Australia: implication for nickel sulphide exploration
"... ABSTRACT: The komatiite–dunite-hosted Ni sulphide deposit at MKD5 (Mt Keith) has a complex regolith consisting of a residual profile (>75 m depth), partly collapsed towards the top, overlain by 40 m of exotic sediments. The weathered komatiites are composed predominantly of degraded serpentine, n ..."
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ABSTRACT: The komatiite–dunite-hosted Ni sulphide deposit at MKD5 (Mt Keith) has a complex regolith consisting of a residual profile (>75 m depth), partly collapsed towards the top, overlain by 40 m of exotic sediments. The weathered komatiites are composed predominantly of degraded serpentine, neo-formed hydrated Mg-silicates, carbonates, silica and Fe oxides. Magnesium concentrations decrease upwards through the lower regolith, whereas those of Fe and Si increase, reflecting degra-dation of Mg-bearing minerals. A sharp decline of Mg within the profile marks the Mg discontinuity, separating Mg-saprolite from the overlying Fe saprolite. The Fe saprolite, dominated by Fe oxides and silica, has pseudo-sedimentary structures indicating partial collapse of the profile. At the unconformity with the sediments, eluviation, mixing and churning have produced zones with chemical and mineralogi-cal characteristics of the underlying, overlying and adjacent lithologies. Nickel, Co, Mn, Cr and PGE are dominantly hosted by secondary Fe oxides in the regolith, but above the Mg-discontinuity, Ni and Co are locally enriched by co-precipitation with Mn oxides. The oxidation potential, controlled by ferrolysis,
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"... IN KOMATIITE-HOSTED Ni-Cu-(PGE) deposits the primary dis-tribution of ore is controlled by volcanological processes (e.g., ..."
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IN KOMATIITE-HOSTED Ni-Cu-(PGE) deposits the primary dis-tribution of ore is controlled by volcanological processes (e.g.,
©2005 Society of Economic Geologists, Inc. Economic Geology, v. 100, pp. 1441–1455 Structural Setting and Shape Analysis of Nickel Sulfide Shoots at the Kambalda Dome,
"... KAMBALDA is a type locality for Archean komatiite-associated nickel sulfide deposits (class 1A deposits of Lesher, 1989) and a world-class mining district, with a premined reserve of ~35 million metric tons (Mt) containing 3 percent nickel (Gresham, ..."
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KAMBALDA is a type locality for Archean komatiite-associated nickel sulfide deposits (class 1A deposits of Lesher, 1989) and a world-class mining district, with a premined reserve of ~35 million metric tons (Mt) containing 3 percent nickel (Gresham,
©2015 Society of Economic Geologists, Inc. Economic Geology, v. 110, pp. 1157–1191
"... The Kalgoorlie goldfield (~50 Moz Au produced), famous for its long mining history and diversity of precious metal telluride minerals, is a world-class Neoarchean Au-Ag-Te district, which includes the Golden Mile Super Pit, the largest single gold deposit in the Eastern Goldfields of Western Austral ..."
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The Kalgoorlie goldfield (~50 Moz Au produced), famous for its long mining history and diversity of precious metal telluride minerals, is a world-class Neoarchean Au-Ag-Te district, which includes the Golden Mile Super Pit, the largest single gold deposit in the Eastern Goldfields of Western Australia, and the smaller but nonethe-less significant Mount Charlotte deposit, 3 km to the north. The gold ore at Kalgoorlie is of two types—Au- and Te-rich first stage (Golden Mile, or Fimiston, ore), which constitutes the bulk of the Au endowment, followed by a relatively Te-poor second stage (Mount Charlotte ore). Fimiston-stage ore is characterized by deformed quartz-carbonate structures termed “lodes: ” thin (1–10 cm) zones of quartz/ankerite/gold/telluride-rich vein breccias with halos of fine-grained pyrite, muscovite, ankerite, and tourmaline. Charlotte-stage ore is primarily hosted by ankerite-pyrite-rich alteration selvages around flat-sided, undeformed bucky quartz veins and is the only ore style present in the Mount Charlotte mine itself. The primary host unit for both mineralization styles is the Golden Mile Dolerite, one of several dolerite intrusions in the mafic-ultramafic volcanic succession of the Kalgoorlie terrane. Along with the large amount of mafic metavolcanics, consistent with typical greenstone belt stratigraphy, the Kalgoorlie goldfield contains at least three fine-grained carbonaceous (meta)black shale units (from oldest to
