Abstract A model is presented for the generation of natural hydrogen from cratonic basement rocks and its migration into the sediments of overlying cratonic basins. It is based on the ‘discovery’ of hydrogen at Bourakebougou in the Taoudeni Basin of Mali. In the ‘Cratonic Greenstone Model’, hydrogen is generated by the serpentinization of olivine-rich, ultramafic rocks contained within Precambrian ‘greenstones’. The model requires a protolith (in greenstones), a supply of water (from groundwater), connecting faults to act as a plumbing system and an indurated sediment cover to retard hydrogen movement. Hydrogen is expelled into the overlying basin sediments, which form the host for hydrogen accumulations. The model describes a continental ‘hydrogen system’, which can form the basis for petroleum-type play-based hydrogen exploration in cratonic settings. Using play elements derived from the model, the Bourakebougou play fairway can be extended across the Taoudeni Basin >700 km northwards of the discovery.

Abstract At La Libertad, located in central Nicaragua, the Cerro Mojon gold deposit occurs in andesites and basaltic andesites of the Miocene Lower Coyol Group. Cerro Mojon shows characteristics typical of low-sulfidation epithermal gold deposits. Mineralization occurred in massive and stockwork quartz veins showing open-space filling textures, including colloform banding, comb structure, and crustification. Alteration surrounding the quartz veins consists of kaolinite-illite near the surface and adularia-illite at depth. Gold mineralization was accompanied by pyrite. Subsequent oxidation to depths of up to 250 m has destroyed the pyrite. Construction at Cerro Mojon was initiated in March 1996, and the mine went into production in March 1997. At the time of writing, the mine was projected to reach target production rates in 1998, with an annual output of 180,000 oz of gold from 3 million metric tons (Mt) of ore. Geologic gold reserves were 1.8 Moz, but ongoing exploration at Cerro Mojon will increase these reserves.