Journal
GEOLOGY
Volume 42, Issue 12, Pages 1035-1038Publisher
GEOLOGICAL SOC AMER, INC
DOI: 10.1130/G35708.1
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Funding
- German Research Foundation (DFG) [BO 1776/8]
- DGMK (German Society for Petroleum and Coal Science and Technology) - ExxonMobil Production Deutschland GmbH [718]
- GDF SUEZ E&P Deutschland GmbH
- RWE Dea AG
- Wintershall Holding GmbH
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Unconformity-related hydrothermal ore deposits typically form by mixing of hot, deep, rock-buffered basement brines and cooler fluids derived from the surface or overlying sediments. Current models invoking simultaneous downward and upward flow of the mixing fluids are inconsistent with fluid overpressure indicated by fracturing and brecciation, fast fluid flow suggested by thermal disequilibrium, and small-scale fluid composition variations indicated by fluid inclusion analyses. We propose a new model where fluids first descend, then evolve while residing in pores and later ascend. We use the hydrothermal ore deposits of the Schwarzwald district in southwest Germany as an example. Oldest fluids reach the greatest depths, where long residence times and elevated temperatures allow them to equilibrate with their host rock, to reach high salinity, and to scavenge metals. Youngest fluids can only penetrate to shallower depths and can (partially) retain their original signatures. When fluids are released from different levels of the crustal column, these fluids mix during rapid ascent in hydrofractures to form hydrothermal ore deposits. Mixing from below during ascent provides a viable hydromechanical mechanism to explain the common phenomenon of mixed shallow and deep fluids in the formation of hydrothermal ore deposits.
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