Chapter 6: Mechanical Regimes of Hydrothermal Gold Mineralization

Pore fluid pressure and differential stress are among the most important controls on the mechanical behavior of mineralizing systems. Their separate influences can be readily identified on failure mode diagrams, which show failure envelopes for pore fluid factor, or pore fluid pressure at failure, against differential stress. The effect of the intermediate principal stress can be shown on such diagrams by using a failure criterion that includes all three principal stresses, such as the Murrell extension to the Griffith criterion. The effect is apparent from the significant variation of the position of the failure envelope as a function of the ratio between the three principal stresses, which is therefore another important control on failure. Characteristic regimes for different gold deposit types occur as distinctive fields on failure mode diagrams. Carlin, Epithermal and Volcanogenic Massive Sulfide deposit types have low absolute pore fluid pressures. Iron Oxide Copper Gold, Intrusion Related Gold and Porphyry deposits encompass low to intermediate values of pore fluid pressure, while the field of Lode Gold deposits may extend to the highest pore fluid pressures. Lode Gold, IOCG and Epithermal deposit types may have the largest values of differential stress. Carlin and VMS deposits are associated with normal stress regimes: the other deposit types may have structures that that formed in either normal or reverse stress regimes. Exploring the effects of the stress ratio, and refining these currently broadly-defined regimes for the mechanics of mineralization, are important future directions for research.