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| Titel |
Parametric investigation of a brine lens formation above degassing magma chamber |
| VerfasserIn |
Andrey Afanasyev, Oleg Melnik, Ivan Utkin, Yulia Tsvetkova |
| 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 |
250141932
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| Publikation (Nr.) |
 EGU/EGU2017-5491.pdf |
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| Zusammenfassung |
| Formation of porphyry-type ore deposits is associated with degassing of crustal magma
chambers. Saline, metal-rich magmatic fluid penetrates into a shallow region saturated with
cold meteoric water where the metals concentrate in brine lenses. The formation of the lenses
and, thus, of the deposits occurs due to phase transitions [1]. The evaporation of H2O
results in enrichment of residual fluid in NaCl. At a depth of 1–2 km precipitation of
solid halite blocks the pore space and facilitates formation of concentrated brine
lenses.
In order to investigate lens formation, we developed an extension of our multiphase
simulator MUFITS [2] for NaCl–H2O mixture flows. We applied the code in a simple
axisymmetric scenario with a high permeability zone in the central part of the domain
surrounded by low permeable rocks. The high permeability zone simulates a volcanic conduit
above a magma body. The degassing of magma is simulated with a point source of hot
supercritical fluid that ascends rapidly up the conduit, undergoing phase transitions en route.
Evaporation and rapid ascend of vapor results in increasing from bottom to top salinity of
the fluid. As temperature and pressure decline closer to the surface, solid halite
precipitates blocking the conduit. Convection of meteoric water in surrounding rocks
favors compact location of the brine lens beneath the region of precipitation. Typical
temperature in the lens is 450–550∘C and overpressure above lithostatic is a few
MPa.
We conducted a parametric analysis, investigating the influence of model parameters on
accumulation of halite and metals. We found that a higher permeability in the conduit, a
smaller permeability in the surrounding rocks and a higher salinity of magmatic fluid favor
larger lenses. A smaller magmatic fluid temperature T , i.e. temperature in the chamber,
results in a smaller lens that disappears abruptly at a threshold value Ta≈ 650˚ C, and it
does not form at T <Ta. This is because at T <Ta the thermodynamic conditions at
shallow depth do not favor halite precipitation that blocks the flow up the conduit.
Finally, we conclude that at T >Ta the lens parameters are most sensitive to the
permeability of rocks surrounding the conduit, because the permeability influences a lot
intensity of meteoric water convection that blocks spreading of brine at a depth of 1–2
km.
This work was supported by the Russian Science Foundation under grant
RSF-16-17-10199.
References
[1] Weis, P. (2015) The dynamic interplay between saline fluid flow and rock permeability
in magmatic-hydrothermal systems // Geofluids, 15, 350–371.
[2] Afanasyev, A. MUFITS Reservoir Simulation Software. www.mufits.imec.msu.ru. |
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