Origin and pathways of pro- and retrograde fluids, PTt paths and fluid-mineral equilibria from Alpine veins of the Central Alps : case studies of the Fibbia and Amsteg areas

In order to constrain the conditions of and processes related to vein formation and the
origins of mineralising fluids, field work, multiple geochemical methods applied to whole
rocks, minerals, and fluid inclusions, as well as thermodynamic modeling on these veinwall
rock systems have been carried out. The focus of this study was on vein and fissure
systems from two particular areas in the external parts of the Central Alps: the Fibbia area
in the southern Gotthard massif and the NEAT-Cable tunnel (Amsteg) in the northern
Aar massif. The observations and results are divided into three chapters.
The first chapter focusses on fluid-rock interactions around five vein types found in the
Fibbia area. Fluid-rock interactions were not visible and measurable in the case of V1a,b
and V5 veins, very minor for V3-V4 veins, and significant for V2 veins. The host rock and
mineral alteration near V2 is documented geochemically and compared with the geochemistry
of the different fluid populations, that were measured with LA-ICPMS and crush
leach analyses. The host rock near V2 veins indicates silica depletion and a relative enrichment
in K, Na, and Al. During the early stages of V2 opening, biotite and quartz
were leached from the host rock, while during later stages pores formed by earlier leaching
were refilled with biotite, albite, quartz, albite, muscovite, and with hematite and chlorite
during V3 and V4 vein formation. The alteration is mainly iso-chemical and shows a
segregation of material from the host rock to the vein. Nevertheless, minor external input
of some components (CO2, SiO2) may also have occurred. The fluids in these five veins
contain Na, K, Cl and minor Ca, Li, Mg, Fe, Cu, Rb, Sr, Pb, Ba, Cs, Zn, As, Br, and
SO2− 4 . Thermodynamic speciation calculations on the Na and K concentrations and ratios
in the fluids in equilibrium with a rock of granitic composition using a modified version
of THERIAK yield an approximate comparison to the measured fluid chemistries up to 1
molal Cl concentrations. The Na-K concentrations and ratios are not related to the total
anion concentration in the fluid. Fe-Mg-Ca concentrations in the modelled fluids are lower
than the analysed concentrations. This is mainly related to possible over-saturation of
Ca, Fe, Mg or to the lack of Ca, Fe, and Mg species in the database used.
The second chapter concerns the origins and flow paths of pro- and retrograde fluids in
the Fibbia area. V1a veins are pre-Alpine and V1b veins are prograde Alpine older than
37 Ma. V2-V5 veins formed between 20–13 Ma during Alpine retrograde metamorphic
conditions at temperatures from 420 to ≤ 200 ◦C and fluid pressures of 4 to ≤ 1 kbar.
Vein formation was characterized by varying tectonic conditions: unknown for V1a veins
and related to S1 formation for V1b veins, V2 formed during near-vertical extension and
V3-V5 formed during horizontal NE-SW extension. The V1 and V2 vein fluids contain up
to 7 mol% CO2 and 1 mol% NaCl. Later retrograde fluids from V3 and V4 veins are CO2
depleted and contain up to 2 mol% NaCl. V5 fluids are CO2 free and have a salinity lower
than 0.6 mol%. Hydrogen isotope measurements on fluids and minerals from V1-V4 veins
give evidence supporting a metamorphic origin. In contrast, for V5 veins hydrogen isotope
analyses support a meteoric origin. δD values less than −130 for V5 fluids and hydrous
minerals indicate that the meteoric fluids originated from a high altitude mountainous
area ( ≥ 3000m) of 15–10 My. Stable isotope measurements on minerals from host-rock
and veins and fluid inclusions shows that the scales of fluid flow are constrained to 1–10
m for V1b-V4 and were ≥ 8 km for V5, as the overburden during meteoric fluid flow in V5
was at least 8 km. Middle Miocene orogen-parallel extension was important for facilitating
localized meteoric fluid ciruclation along large-scale V5 fracture systems. Before V5 veins
formed, meteoric fluid influence was not observable in the studied areas.
The third chapter focusses on Alpine fissures from the Amsteg area (northern Aar massif)
and the differences in vein formation that are observed between V2 veins from the
Fibbia area and the Amsteg area. The Alpine fissures in the Amsteg area formed between
19–14 Ma at temperatures between 350–250 ◦C and pressures between 3.5–2.5 kbar
by sub-vertical extension and SE-NW compression, with little fluid-rock interaction. All
three fluid populations are of the H2O-NaCl type and contain up to 5 wt% NaCl eq. The
mineralisation is similar to the V2 veins from the Fibbia area, although pyrrhotine and
pyrite are common accessory minerals. Small amounts of REE-containing carbonates were
observed as well. Hydrogen and oxygen stable isotope measurements on fluid inclusions
and vein minerals (quartz) indicate that the mineralising fluids were mainly metamorphic.
The change in mineralisation in the veins as well as slight salinity decreases of later
vein forming fluids suggests that fluids from lower temperatures flowed through the veins
systems.