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| Titel |
Magma Emplacement Rates and Porphyry Copper Deposits: Thermal Modelling of
the Yerington Batholith, Nevada, USA |
| VerfasserIn |
Anne Schöpa, Catherine Annen, John H. Dilles, R. Stephen J. Sparks, Jon D. Blundy |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250148834
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| Publikation (Nr.) |
 EGU/EGU2017-13127.pdf |
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| Zusammenfassung |
| Many porphyry copper deposits are associated with granitoid plutons. Their genesis is
attributed to the degassing of pluton-forming intermediate to silicic magma chambers. These
plutons are commonly envisioned as resulting from the slow cooling and crystallization of
large magma chambers. Most of the models combine the formation of ore deposits
and the cooling of a magma chamber. However, they do not consider neither how
typically hundreds of cubic kilometres of magma were emplaced into the upper
crust, nor the prolonged growth of plutons involving simultaneous cooling and
crystallization together with the release of exsolved volatiles, which may contribute to ore
formation.
We use numerical simulations of thermal evolution due to pluton growth to investigate the
links between pluton construction, magma accumulation, solidification, volatile
exsolution, volatile release and porphyry copper formation. The Jurassic Yerington
batholith in western Nevada, USA, is used as a case study because it is associated with
economic porphyry copper deposits, it shows an exceptional exposure revealing the
geometry of the intrusion, and petrological and geochronological analysis have
shed light on its emplacement style and duration. Our conductive heat flow model
simulates the growth of the ∼1000 km3 batholith emplaced at 2-8 km crustal depth by
step-wise intrusions of vertically stacked sills. Different emplacement rates and
repose times of no melt injection between the three main Yerington intrusions were
tested.
Our numerical simulations show that to comply with the conceptual model
linking porphyry copper deposits with the presence of large, highly molten magma
chambers, magmas must be emplaced at a high rate of several cm/yr. In plutonic
records, such high rates are uncommon. It follows that either the current conceptual
model is incorrect or that porphyry copper deposits are only produced by the rare,
rapidly emplaced plutons. The fact that many granitoid plutons are barren might
be because they emplaced too slowly. Those slowly emplaced granitoids do not
accumulate melts, release volatiles early during emplacement, and are unlikely to
form cupolas that channel the volatiles. In the case of the Yerington batholith, the
formation of a magma chamber requires a magma emplacement rate higher than 4
cm/yr and repose periods between the three intrusions exceeding 100 kyr. Since
only the last emplaced intrusion is associated with copper deposits, we speculate
that this fertile unit might have been emplaced more rapidly than the former units. |
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