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| Titel |
Generation of porphyry copper deposits by gas-brine reaction in volcanic arcs |
| VerfasserIn |
Jon Blundy, John Mavrogenes, Brian Tattitch, Steve Sparks, Amy Gilmer |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093844
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| Publikation (Nr.) |
 EGU/EGU2014-8962.pdf |
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| Zusammenfassung |
| Porphyry copper deposits (PCDs) are characterised by a close spatial and temporal
association with small, hypabyssal intrusions of silicic magmas in volcanic arcs. PCD
formation requires elevated chlorine and water to concentrate copper in magmatic hypersaline
liquids (or brines), and elevated sulphur to precipitate copper-rich sulphides. These twin
requirements are hard to reconcile with experimental and petrological evidence that
voluminous chlorine-rich, hydrous silicic magmas, of the variety favourable to copper
enrichment, lack sufficient sulphur to precipitate directly the requisite quantities of sulphides.
These features are, however, consistent with observations of active volcanic arcs whereby
PCDs can be viewed as roots of dome volcanoes above shallow reservoirs where silicic
magmas accumulate over long time spans. During protracted periods of dormancy
metal-enriched dense brines accumulate in and above the silicic reservoir through slow,
low-pressure degassing. Meanwhile cogenetic volatile-rich mafic magmas and their exsolved,
sulphur and CO2-rich fluids accumulate in deeper reservoirs. Periodic destabilisation of
these reservoirs leads to short-lived bursts of volcanism liberating sulphurous gases,
which react with the shallow-stored brines to form copper-rich sulphides and acidic
vapours.
We test this hypothesis with a novel set of “porphyry in a capsule” experiments
designed to simulate low-pressure (1-2 kbar) interaction of basalt-derived, sulphur-rich
gases with brine-saturated, copper-bearing, but sulphur-free, granite. Experiments
were run at 720-850 °C in cold-seal apparatus with basaltic andesite, loaded with
H2O and S, situated below dacite, loaded with H2O, Cl and Cu. At run conditions
both compositions are substantially degassed and crystallized. S-rich gas from the
basaltic andesite ascends to react with Cu-rich brines exsolved from the dacite,
Our experiments reveal the direct precipitation of copper-sulphide minerals, in
vugs and veins within the dacite, at magmatic temperatures and support previous
suggestions of gas-brine interaction as a fundamental ore-forming process. The
simultaneous production of acid (HCl) during sulphide precipitation drives mica-producing
alteration reactions in brine-hosting granites and their wall-rocks that replicate
observed associations of sulphides and mica-rich alteration haloes around PCDs. |
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