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Causes and time scales of fluid release from magma chambers related to ore-forming magmatic-hydrothermal systems

English title Causes and time scales of fluid release from magma chambers related to ore-forming magmatic-hydrothermal systems
Applicant Driesner Thomas
Number 125307
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geochemie und Petrologie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Geochemistry
Start/End 01.12.2009 - 30.11.2012
Approved amount 185'825.00
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All Disciplines (3)

Discipline
Geochemistry
Geophysics
Other disciplines of Earth Sciences

Keywords (9)

ore deposits; geochemistry; magmatism; fluid flow; economic geology; convection; magma chamber; fluids; time scales

Lay Summary (English)

Lead
Lay summary
The world's most important type of copper deposits - so-called porphyry type deposits - is connected to the release of metal-bearing fluids from magma chambers at high temperatures. Conventionelly, this has been related to the gradual crystallization of the magma chamber on time scales of tens of thousands of years. Based on an interpretation of field observations, it has recently been proposed that such deposits may form on much shorter time scales, i.e., decades to hundreds of years, implying that they are deep-seated, confined analogues to volcanic explosions. This would have a number of physical consequences: in particular, any process leading to the rapid fluid release must obey rather tight constraints on the redistribution of heat and at the same time must provide a mechanism for focusing the released fluids into sites of metal deposition. This study will investigate, by means of numerical simulation, the hypothesis that convection within the magma chamber will lead to focused release on fluids near its top when upward moving batches of magma become fluid-saturated in the pressure gradient that exist in the magma chamber. The computations will be based on a generic model of magma properties without detailed phase petrology. The chamber-scale simulations will be complemented by numerical studies on the intrusion behavior of such magma when ascending in the Earth's crust.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Name Institute

Publications

Publication
Mafic injection as a trigger for felsic magmatism: A numerical study
Schubert M., Driesner T., Gerya T. V., Ulmer P. (2013), Mafic injection as a trigger for felsic magmatism: A numerical study, in GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, 14(6), 1910-1928.

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
AGU Fall Meeting Poster 03.12.2012 San Fracncisco, United States of America Driesner Thomas;
9th Swiss Geoscience Meeting Talk given at a conference 19.11.2011 Zuerich, Switzerland Schubert Maike;
12th International Workshop on Modeling of Mantle Convection and Lithospheric Dynamics Talk given at a conference 20.08.2011 Gross Doelln, Germany Schubert Maike;
Joint Annual Meeting of the Swiss Physical Society and the Austrian Physical Society Talk given at a conference 15.06.2011 Lausanne, Switzerland Schubert Maike;
EGU General Assembly Poster 04.04.2011 Wien, Austria Schubert Maike;


Associated projects

Number Title Start Funding scheme
143778 Causes and time scales of fluid release from magma chambers related to ore-forming magmatic-hydrothermal systems, II. 01.12.2012 Project funding (Div. I-III)
116693 Metal transport and ore deposition: Quantitative modelling of magmatic-hydrothermal systems 01.04.2007 Project funding (Div. I-III)

Abstract

Magmatic-hydrothermal ore deposits such as porphyry Cu (-Au, -Mo) deposits are thought to form when magmatic fluids are released from a batholith-sized magma body and become focused into a much smaller apical dike- or stock-like intrusion. Strong gradients in temperature and pressure lead to phase separation of the fluid and metal precipitation. For decades, the conventional model of porphyry Cu deposit formation has been that the magmatic fluid is released as a consequence of gradual crystallization of the larger magma chamber, leading to “second boiling” over time scales of typically tens of thousands of years. This view has recently been challenged by CATHLES and SHANNON (2007) who suggested that the formation of porphyry copper type ore deposits happens on time scales no longer than hundreds of years and may even represent deep, fast, “nearly explosive” magmatic fluid release. This raises the question which processes would allow to extract a large part of the fluid inventory of a batholith in a period of probably hundreds of years or even less.Building on an earlier model by SHINOHARA et al. (1995), we propose to test by numerical simulation the feasibility of a process in which magma is fluid-undersaturated in most of the batholith and reaches fluid saturation only transiently when encountering lower pressures in the top-most part of the magmatic system during convection. This would not only explain the strong localization of fluid release as evidenced by most porphyry-Cu systems, but also seems physically possible as hydrous felsic to intermediate magmas experience only little change in viscosity when losing aqueous fluids in access of 2 wt.% (estimates of initial water contents in ore-forming systems typically indicate 5 wt.% H2O or more), and may lead to a self-stabilizing or even self-accelerating convective process as the fluid-depleted magma should become denser and hence sink down, allowing lighter fluid-rich magma to ascend convectively. We propose a PhD project that comprises three consecutive steps, building upon each other:1. (Supervised mostly by Peter Ulmer) Building on published models and data, parameterize a simplified “model magma” whose behavior with respect to (a) fluid saturation as a function of temperature and pressure and (b) viscosity, density and degree of crystal content as a function of temperature, pressure and fluid concentration that resembles as closely as currently possible some average intermediate to felsic calc-alkaline magma as found in typically porphyry-copper forming systems. As we expect to test the physics of a process that is likely to happen on time scales much shorter than magmatic differentiation and fractionation, we currently do not plan to include any complex petrologic phase relations into the model.2. (Supervised mostly by Taras Gerya). Using Taras Gerya’s unique modeling software I2ELVIS, study the crustal scale magma ascent and intrusion dynamics of this model magma at high resolution (i.e., down to 50x50 m resolution in a 2D model), in particular with respect to time scales, rates and steady vs. pulsating magma supply.3. (Supervised mostly by Thomas Driesner) On the scale of the batholith, study the first order geological and physical factors that control whether convection of a magma body may lead to wholesale fluid release in a short period of time as a consequence of fluid saturation when magma convects through the intrusion’s apical parts. Test the hypothesis that the formation of dikes and stocks in these apical parts may be a necessary consequence of this process, immediately preceding the rapid release of large volumes of metal-laden magmatic fluids.
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