The Haib district in southern Namibia hosts a well-preserved
porphyry copper deposit (Main Haib deposit) and several satellite
alteration zones. We analyzed samples of hydrothermal biotite in the
Main Haib deposit and from a 1km wide satellite alteration zone to
constrain the possible origin of potassium metasomatism in the
satellite deposit. δDbiotite values of hydrothermal biotite in the
satellite deposit ranged from -115 to -79, which was isotopically
heavier than hydrothermal biotite from the Main Haib deposit (-121
to -117). Biotite samples in the satellite deposit zoned spatially
toward heavier δDbiotite values to the southeast away from the
Main Haib deposit.
Hydrothermal fluids that precipitated biotite in the center of the
Main Haib porphyry have calculated δDH2O values between -68 and
-52, averaging -61, based on δDbiotite values of hydrothermal
biotite in the Main Haib deposit and isotope fractionation factors
of Taylor (1979) at T=300-450ēC at observed Mg/Fe ratios. If these
waters represent primary magmatic-hydrothermal fluids, hydrothermal
biotite with isotopic compositions observed in the satellite deposit
could only precipitate from such porphyry fluids at unreasonably
high temperatures (600-1055ēC).
The spatial trend of the δDH2O values in the satellite deposit
suggests either a mixing of heavier, externally-derived water with
lighter, primary porphyry-magmatic waters, or a fractionation of
externally-derived formational (non-porphyry) waters along a thermal
gradient. Isothermal mixing between calculated porphyry fluid with
δDH2O = -65 and external formational waters of δDH2O = -20 could
produce the observed spatial trend, with the external formational
water source peripheral to the district. Alternatively, a
homogeneous hydrothermal fluid with δDH2O = -45 fractionating along
a thermal gradient could also produce the observed spatial pattern
over a temperature range of 155 to 685ēC.
Isotopic studies of biotite from potassic alteration zones are a
potentially helpful tool in mineral exploration for evaluating
involvement of non-magmatic fluids in hydrothermal alteration, which
could suggest a potential for other ore types in that area (e.g.,
IOCG).
Biotite alteration in the Precambrian
Haib porphyry copper district, Namibia Hydrogen isotope
constraints on fluid origins