Benjamin F. Walter

Fluid mixing from below in unconformity-related hydrothermal ore deposits

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.

The Petrology of the Kaiserstuhl Volcanic Complex, SW Germany: The Importance of Metasomatized and Oxidized Lithospheric Mantle for Carbonatite Generation

The Miocene Kaiserstuhl Volcanic Complex (Southwest Germany) consists largely of tephritic to phonolitic rocks, accompanied by minor nephelinitic to limburgitic and melilititic to haüynitic lithologies associated with carbonatites. Based on whole-rock geochemistry, petrography, mineralogy and mineral chemistry, combined with mineral equilibrium calculations and fractional crystallization models using the Least Square Fitting Method, we suggest that the Kaiserstuhl was fed by at least two distinct magma sources. The most primitive rock type of the tephritic to phonolitic group is rare monchiquite (basanitic lamprophyre) evolving towards tephrite, phonolitic tephrite, phonolitic noseanite, nosean phonolite and tephritic phonolite by fractional crystallization of variable amounts of clinopyroxene, amphibole, olivine, spinel/magnetite, garnet, titanite, plagioclase and nosean. During this evolution, temperature and silica activity (aSiO2) decrease from about 1100 C and aSiO2 1⁄4 0 6–0 8 to 880 C and aSiO2 1⁄4 0 2. At the same time, oxygen fugacity (fO2) increases from DFMQ* 1⁄4 þ2–3 to DFMQ* 1⁄4 þ3–5, with DFMQ* being defined as the log fO2 deviation from the silica activity-corrected FMQ buffer curve. Nephelinitic rocks probably derive by fractionation of mostly olivine, spinel/magnetite, melilite, perovskite and nepheline from an olivine melilititic magma. The nephelinitic rocks were formed at similarly high crystallization temperatures (>1000 C) and evolve towards limburgite (hyalo-nepheline basanite) by an increase of silica activity from about aSiO2 1⁄4 0 4–0 5 to aSiO2 1⁄4 0 5–0 9, whilst redox conditions are buffered to DFMQ* values of around þ3. Haüyne melilitite and the more evolved (melilite) haüynite may equally be derived from an olivine melilitite by more intense olivine and less melilite fractionation combined with the accumulation of haüyne, clinopyroxene and spinel. These rocks were crystallized at very low silica activities (aSiO2 0 2) and highly oxidized conditions (DFMQ* 1⁄4 þ4–6). Even higher oxygen fugacities (DFMQ* 1⁄4 þ6–7) determined for the carbonatite suggests a close genetic relation between these two groups. The assemblage of carbonatites with highly oxidized silicate rocks is typical of many carbonatite occurrences worldwide, at least for those associated with melilititic to nephelinitic silicate rocks. Therefore, we suggest that the existence of highly oxidized carbonatebearing sublithospheric mantle domains is an important prerequisite to form such complexes.