The Role of Magmatic Sulfides and Mafic Alkaline Magmas in the Bingham and Tintic Mining Districts, Utah

Keith, Jeffrey D.

doi:10.1093/petrology/38.12.1679

Cited by 38 publications

(43 citation statements)

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“…56,72,76,87 , as long as sufficient interaction between sulfide and silicate melt (such as by convection) occurs. This process may also be expected to deplete more strongly those metals with a greater affinity for the sulfides (e.g.…”

Section: Jj Wilkinson Imperial College Londonmentioning

confidence: 99%

“…Although sulfide saturation could be viewed as destroying ore potential, the concentration of ore metals by saturation of a sulfide melt may in fact be a highly effective mechanism for converting a magmatic system with a dispersed, low concentration of metals into one with pockets of highly enriched sulfide melt. As long as the sulfide melt, or its crystallized products, are accessible to -and soluble in -fluids that are subsequently exsolved from the magma 72,76,88 , then the potential exists to produce anomalously metal-rich hydrothermal fluids 47 . Evidence for the operation of this process has been described from the active Merapi Volcano in Indonesia where it was inferred that the injection of sulfide-saturated mafic magma into a felsic chamber triggered volatile exsolution, dissolution of sulfide into the volatile phase and caused explosive eruptions 75 .…”

Section: Box 2 | Trigger 2 -Magmatic Sulfide Saturationmentioning

confidence: 99%

“…It is suggested here that such intrusions, and release of their contained volatiles, may trigger sudden, local sulfide undersaturation in the chamber, producing a flush of metalliferous, sulphur-rich volatiles and, at the same time, initiating emplacement of a porphyry intrusion 60,74 . A number of previous workers have proposed that such mafic intrusions could themselves add sulfur and metals during such a process 61,[74][75][76][77][78][79] . …”

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Triggers for the formation of porphyry ore deposits in magmatic arcs

2013

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Porphyry ore deposits source much of the copper, molybdenum, gold and silver utilized by humankind. They typically form in magmatic arcs above subduction zones via a series of linked processes, beginning with magma generation in the mantle and ending with the precipitation of metals from hydrous fluids in the shallow crust. A hierarchy of four key "triggers" involved in the formation of porphyry deposits is outlined. Trigger 1 (10 2 -10 3 km scale) is a process of cyclic refertilization of magmas in the deep crust. Trigger 2 (10 1 -10 2 km scale) is the process of sulfide saturation in magmas that can both enhance and destroy ore-forming potential. Trigger 3 (10 0 -10 1 km scale) relates to the efficient transfer of metals into hydrothermal fluids exsolving from porphyry magmas. Trigger 4 (~10 0 km scale) identifies processes that lead to the final precipitation of ore minerals. Although all processes are required to a greater or lesser degree, it is argued that trigger 3, as an over-riding mechanism, can best explain the restriction of large deposits to specific arc segments and time periods. Consequently, recognition of the fingerprint of sulfide saturation in igneous rocks may help mineral exploration companies to identify parts of magmatic arcs particularly predisposed to ore formation.Ore deposits are scarce. Their discovery consumes considerable time and resources and only about one in every one thousand prospects explored by companies is eventually developed into a mine. Deposits often occur in clusters and formed within specific time intervals. This non-uniform pattern provides keys to understanding how and why metals accumulate in some places and not in others and its explanation is fundamental for mineral exploration.The most important metalliferous ore deposits are hydrothermal deposits, formed from hot waters circulating in Earth's crust. Such deposits represent a highly efficient trap where fluids were focused into a limited volume of rock, became supersaturated and precipitated ore minerals. In theory, this does not require unusual fluid compositions 1,2 as long as fluid focusing, sufficient fluid flux and efficient precipitation conditions can be maintained. Recently, this paradigm has been challenged by the recognition that ore fluids in sediment-hosted 3-5 , epithermal 6,7 and porphyryrelated [8][9][10] hydrothermal deposits can carry orders of magnitude more metal than previously considered probable, or measured in modern fluid samples. This suggests that our understanding of metal extraction and transport processes is incomplete.Here, this theme is explored by consideration of porphyry ore deposits [11][12][13][14][15] , remarkable geochemical anomalies that can contain up to 1 Gt of sulphur 16 , 200 Mt copper, 2.5 Mt molybdenum and 2600 t gold 17 . The most copper-rich examples include the 4-5 million-year old El Teniente and Rio Blanco-Los Bronces deposits in Central Chile, and the most gold-rich is the 3 million-year old Grasberg deposit in Irian Jaya, Papua New Guinea 17 .Porphyry deposits...

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Section: Jj Wilkinson Imperial College Londonmentioning

confidence: 99%

Section: Box 2 | Trigger 2 -Magmatic Sulfide Saturationmentioning

confidence: 99%

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Triggers for the formation of porphyry ore deposits in magmatic arcs

2013

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“…Sulfide melt inclusions are generally absent from any phenocrysts in the extrusive andesites, suggesting that these magmas never exsolved a sulfide melt on a broad scale. Rapid degassing upon magma mixing suppressed the formation of a sulfide melt (Keith et al 1997) and induced eruption selectively at times when volatile exsolution occurred locally in the magma chamber.…”

Section: Melt Inclusions and Magma Mixingmentioning

confidence: 99%

Magma evolution and the formation of porphyry Cu?Au ore fluids: evidence from silicate and sulfide melt inclusions

2005

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Silicate and sulfide melt inclusions from the andesitic Farallo´n Negro Volcanic Complex in NW Argentina were analyzed by laser ablation ICPMS to track the behavior of Cu and Au during magma evolution, and to identify the processes in the source of fluids responsible for porphyry-Cu-Au mineralization at the 600 Mt Bajo de la Alumbrera deposit. The combination of silicate and sulfide melt inclusion data with previously published geological and geochemical information indicates that the source of ore metals and water was a mantle-derived mafic magma that contained approximately 6 wt.% H 2 O and 200 ppm Cu. This magma and a rhyodacitic magma mixed in an upper-crustal magma chamber, feeding the volcanic systems and associated subvolcanic intrusions over 2.6 million years. Generation of the ore fluid from this magma occurred towards the end of this protracted evolution and probably involved six important steps: (1) Generation of a sulfide melt upon magma mixing in some parts of the magma chamber. (2) Partitioning of Cu and Au into the sulfide melt (enrichment factor of 10,000 for Cu) leading to Cu and Au concentrations of several wt.% or ppm, respectively. (3) A change in the tectonic regime from local extension to compression at the end of protracted volcanism. (4) Intrusion of a dacitic magma stock from the upper part of the layered magma chamber. (5) Volatile exsolution and resorption of the sulfide melt from the lower and more mafic parts of the magma chamber, generating a fluid with a Cu/Au ratio equal to that of the precursor sulfide. (6) Focused fluid transport and precipitation of the two metals in the porphyry, yielding an ore body containing Au and Cu in the proportions dictated by the magmatic fluid source. The Cu/S ratio in the sulfide melt inclusions requires that approximately 4,000 ppm sulfur is extracted from the andesitic magma upon mixing. This exceeds the solubility of sulfide or sulfate in either of the silicate melts and implies an additional source for S. The extra sulfur could be added in the form of anhydrite phenocrysts present in the rhyodacitic magma. It appears, thus, that unusually sulfur-rich, not Cu-rich magmas are the key to the formation of porphyry-type ore deposits. Our observations imply that dacitic intrusions hosting the porphyryCu-Au mineralization are not representative of the magma from which the ore-fluid exsolved. The source of the ore fluid is the underlying more mafic magma, and unaltered andesitic dikes emplaced immediately after ore formation are more likely to represent the magma from which the fluids were generated. At Alumbrera, these andesitic dikes carry relicts of the sulfide melt as inclusions in amphibole. Sulfide inclusions in similar dykes of other, less explored magmatic complexes may be used to predict the Au/Cu ratio of potential ore-forming fluids and the expected metal ratio in any undiscovered porphyry deposit.

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“…Needless to say, such a link is hard to establish because it requires access to synchronous gas and rock samples, a combination not often met in the context of ore deposits related to magmatism. Previous petrological studies on ancient magmatic intrusions have suggested that sulphides could supply the magmatic ore fluid with metals, but in such an ancient example the accompanying gas obviously could not be collected 10 .…”

Section: Introductionmentioning

confidence: 99%