Gold mobility during Palaeoarchaean submarine alteration

- Gold mobility has been evaluated for Palaeoarchaean seafloor alteration.
- Non-actualistic process of silicification resulted in transfer of Au to the ocean.
- Gold became enriched in carbonaceous chert.
- Results help to explain the secular evolution of gold deposits.

Seafloor alteration provides large amounts of solutes to the hydrosphere. In order to investigate gold mobility during water-rock interaction prior to 3-billion-years ago, low detection limit analysis of Au concentrations was carried out on rocks from marine alteration zones. Stratiform zones recording low-temperature (≤150 °C) seafloor alteration are a characteristic feature of greenstone belts older than 3.0 Ga. Hydrothermal processes were operating on, and immediately below, the seafloor, giving rise to extensive silicification of sub-seafloor volcanic rocks and silicification of seafloor sediments. In order to investigate gold mobility during silicification, unaltered and variably silicified volcanic rocks and associated cherts from Palaeoarchaean greenstone successions (c. 3.4 Ga) of South Africa were analyzed. Results show mobility of gold during silicification of mafic/ultramafic rocks and transfer to the Archaean ocean. Some gold was incorporated into carbonaceous marine sediments overlying the alteration zones. A combination of pervasive silicification, rarity of black shales, and low gold content in komatiites can explain the low mineralization potential of Palaeoarchaean greenstone belts for orogenic gold deposits.