Genetic Links Between Ash-Flow Calderas And Associated Ore Deposits As Revealed By Large-Scale Thermo-Mechanical Modeling

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Journal Article: Genetic Links Between Ash-Flow Calderas And Associated Ore Deposits As Revealed By Large-Scale Thermo-Mechanical Modeling

Abstract
In addition to a number of epithermal ore deposits, some porphyry-type ore deposits are also hosted by ash-flow calderas. Spatial correlation between ash-flow calderas and these deposits is observed in several parts of the world, but temporal correlation between the mineralization and the episode of caldera formation remains unclear, especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation. Because caldera collapse represents only a very short event in a long time-scale evolution between a silicic magma chamber and its host rocks, we have developed a thermo-mechanical model of upper crustal and magmatic system behavior during the pre- and post-caldera formation stages. Major petrophysical properties of ash-flow caldera related rocks are incorporated into the model, which accounts for brittle-elasto-ductile rheology and heat transfer, and reproduces fault initiation and various caldera collapse scenarios. Large-scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described. In particular, we found that the outer sides of caldera border faults are associated with both anomalous heat-transfer and a high fracture density. The finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults. In most cases, the model predicts "Y-shaped" fault geometry, which is surprisingly similar to that of mineralized faults in the field. Variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20 MPa, with deep overpressured zones close to the inner edge of the chamber corners. Despite the fact that hydrothermal convection and fluid geochemistry are not included in the model, these thermal and mechanical results are believed to constrain the conditions for mineralization processes. Our results show that in the presence of regional extension, the development of caldera border faults may be suppressed to the advantage of deep-seated fractures, clustered above the central part of the magma chamber, thus favoring conditions for the formation of porphyry-type ore deposits. Finally, we demonstrate that well before the eruption, the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust, resulting in breakup and pre-eruptive normal faulting and thus allowing for conditions of pre-caldera mineralization.

Authors 
L. Guillou-Frottier, E. B. Burov and J. P. Milesi








Published Journal 
Journal of Volcanology and Geothermal Research, 2000





DOI 
Not Provided
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Citation

L. Guillou-Frottier,E. B. Burov,J. P. Milesi. 2000. Genetic Links Between Ash-Flow Calderas And Associated Ore Deposits As Revealed By Large-Scale Thermo-Mechanical Modeling. Journal of Volcanology and Geothermal Research. (!) .