Samples from drilling of the Bi'r Tawilah gold prospect in Saudi Arabia reveal the occurrence of a Ca–Fe arsenate phase, which is similar in appearance and chemistry to yukonite. Upon weathering of a granodiorite host, oxidation of arsenopyrite (0–25 m deep) leads to the formation of a very peculiar brown, amorphous to very poorly crystalline aggregate with cellular-like texture. This mixture consists of Ca–Fe arsenate and arsenic-rich ferric oxyhydroxide resulting from the oxidation of arsenopyrite. It is intergrown with colloform ferric oxyhydroxide, the latter resulting from the oxidation of pre-existing pyrite. The EMPA analyses indicate that the Ca-rich domain contains the maximum As2O5 content (up to 22.3 wt%) whereas the colloform ferric oxyhydroxide contains the highest amount of Fe2O3 among the sample studied (60.8–63.1 wt%) associated to higher H2O content (31.4–33.2 wt%) than in the case of common goethite and lepidocrocite. As far as typical yukonite, scorodite or arsenosiderite are absent in the studied weathered granodiorite, it is believed that oxidation took place at elevated pH (>7) and temperature up to ∼75 °C. The source of Ca2+ can be derived from alteration of plagioclase in the granodiorite but its possible derivation from strongly corroded marble bands cannot be discarded. It is evident that availability of Ca2+ and high pH buffered by the dissolution of calcite in the marble, in addition to the prevailing temperature upon weathering, played important roles in the formation of these pseudomorphs at Bi'r Tawilah.