The Italian Mountain Intrusive Complex lies within the Colorado mineral belt. It consists of Oligocene plutons, dikes, and associated hydrothermal lead-silver deposits. The rocks range in composition from quartz diorite to quartz monzonite. The core of the youngest intrusive mass is porphyritic and contains a central facies characterized by a partly aphanitic ground-mass, which was formed by quenching and represents a late-stage venting of the intrusive complex. Upon venting, the youngest plutonic rocks fractured, fluids in the core boiled and were introduced into fractured quartz phenocrysts, and quartz veins were formed.
The evolution of the late magmatic and postmagmatic fluids is inferred from fluid inclusions. Pressure constraints imposed by measured fluid compositions and homogenization temperatures indicate that a pressure of 250 bars existed on the fluids at the time of venting. Depth of emplacement was between 950 and 2,700 m.