BIOTITE ALTERATION IN THE PRECAMBRIAN HAIB PORPHYRY COPPER DISTRICT, NAMIBIA – HYDROGEN ISOTOPE CONSTRAINTS ON FLUID ORIGINS

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. δD_biotite 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 δD_biotite 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 δD_H2O values between -68 and -52‰, averaging -61‰, based on δD_biotite values of hydrothermal biotite in the Main Haib deposit and isotope fractionation factors of Taylor (1979) at T=300-450ºC 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 (600-1055ºC).

The spatial trend of the δD_H2O 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 δD_H2O = -65‰ and external formational waters of δD_H2O = -20‰ could produce the observed spatial trend, with the external formational water source peripheral to the district. Alternatively, a homogeneous hydrothermal fluid with δD_H2O = -45‰ fractionating along a thermal gradient could also produce the observed spatial pattern over a temperature range of 155 to 685ºC.

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).