The contact breccia is associated with the West edge of the main volcanic edifice.
The contact breccia is very well exposed only in the West Quarry section, with a maximum thickness of 17.
Because the water vapour has high pressure and temperature during the thermohydraulic phreatomagmatic explosion, it rapidly drives the expansion of gas to the surface, moving out great amounts of clasts from the contact breccia (Lorenz et al.
This is deduced from: a) the tectonic structure of the Sierra de las Cabras, b) the highest values of dipping of the phreatomagmatic breccia at the southwest edge, c) the highest content of contact breccia on the West edge (Fig.
The crystalline lamproitic dome of the volcano is surrounded by a ring constituted by the phreatomagmatic-effusive complex, composed by contact breccia, phreatomagmatic breccia (with pyroclast ash, lapilli and bombs, and white carbonate clasts from the host rock) and lava interlayers (massive clastogenics and banded vesiculars).
In this sector, contact breccia and host rock xenoliths are more common.
The extent and nature of alteration and mineralisation styles observed to date at Oomargi all suggest the presence of a large mineralised porphyry system, with potential for higher grade copper-gold mineralisation in contact breccia zones based on megascopic observations of various mineral species.
Drill holes OM 001 and OM 002 intersected alternating zones of altered diorite porphyry, mineralised contact breccia zones, and phyllitic metasediments.
Some diorite porphyry clasts within the contact breccia zone in drill hole OM 001 contain abundant disseminated chalcopyrite.
Within the contact breccia zones strong magnetite and actinolite also occur within the propylitic assemblage.
The strongest concentrations of copper minerals occurs in marginal contact breccias where a high chalcopyrite to pyrite ratio has been observed locally, with strong biotite dominated potassic alteration.