Testing Established Models of Hydrothermal Fluid Distribution Around Porphyry Deposits: The Application of Fluid Inclusion Research to Porphyry Deposit Exploration

The Kabba porphyry copper prospect in northwestern Arizona contains two zones of porphyry style mineralization and alteration, which are spatially separated by a north-south trending normal fault. These two zones are referred to as the Kabba footwall and hanging wall and are hypothesized to represent the root and intermediate depth levels of the same porphyry-like hydrothermal system, respectively. If this hypothesis is correct, then fluid inclusions found in the hanging wall and footwall should have the physical and chemical characteristics of fluid inclusions in the root and intermediate depth levels of other known porphyry deposits. Vein and alteration petrography, fluid inclusion petrography and microthermometry, and Raman and LA-ICP-MS analyses of individual fluid inclusions from Kabba footwall and hanging wall samples lead to the following observations: 1) the textures and paragenetic sequence of alteration and mineralization in the Kabba hanging wall is almost identical to the textures and paragenetic sequence of alteration and mineralization in the footwall, 2) the mineralization and alteration in the Kabba prospect is consistent with the footwall and hanging wall having formed in the deep roots and intermediate zones to a porphyry-like hydrothermal system, respectively, 3) the distribution of fluid inclusion types in the footwall and hanging wall broadly matches the distribution of fluid inclusion types in the deep roots and intermediate zones of other porphyry systems, 4) the chemical and physical conditions of fluid inclusions in the Kabba system are very similar to the chemical and physical conditions of fluid inclusions in the Butte porphyry system in Montana, 5) Cu concentrations in fluid inclusions from Kabba are highest in inclusions with the lowest Sr, Rb, and Ba, and 6) Cu concentrations were consistently high in vapor-rich fluid inclusions. These observations strongly suggest that the mineralization and alteration in the Kabba system formed at great depth in a porphyry-like hydrothermal system, and that the hanging wall and footwall represent the root and intermediate depth levels of that system. Based on the microthermometric behavior of fluid inclusions from the hanging wall and footwall it is interpreted that, 1) the mineralization and alteration in the Kabba system formed via 5 episodes of hydrothermal fluid flux through the footwall and hanging wall, 2) immiscibility may have played a role in forming the mineralization in the hanging wall, 3) fluids went from being H2O to CO2-rich over time, 4) the Kabba footwall and hanging wall formed at pressures between 2500-3000 bars (10-12 km) and 1500-2000 bars (6-8 km), respectively, 6) fluids in the footwall and hanging wall were trapped betwee