Antonio Arribas

Contemporaneous formation of adjacent porphyry and epithermal Cu-Au deposits over 300 ka in northern Luzon, Philippines

There is commonly a close spatial relation between porphyry Cu (± Au) and high-sulfidation epithermal Cu-Au deposits throughout the world, although a genetic association has not been proven. Nowhere is this spatial association better seen than in northern Luzon, Philippines, where the Lepanto epithermal Cu-Au deposit overlies the Far Southeast (FSE) porphyry Cu-Au deposit, both world-class orebodies. Fresh rock and hydrothermal mineral separates yield K/Ar ages indicating that premineralization and postmineralization volcanism occurred at 2.2–1.8 Ma and 1.2–0.9 Ma, respectively, and that the hydrothermal system was active from ∼1.5 to ∼1.2 Ma. K/Ar ages of alunite from Lepanto have the same range as those of hydrothermal biotite and illite from the FSE deposit, indicating that both epithermal and porphyry mineralization formed from an evolving magmatic-hydrothermal system that was active for about 300 ka. This temporal relation strengthens the argument for a genetic link between these two styles of ore deposit, and has implications for exploration. Where one style of mineralization is found, there is potential for the other nearby.

Mineralized and unmineralized calderas in Spain; Part II, evolution of the Rodalquilar caldera complex and associated gold-alunite deposits

The Rodalquilar caldera complex is located in the western part of the Cabo de Gata volcanic field in southeastern Spain and is the first documented example of epithermal gold-alunite mineralization within a caldera in Europe. The Rodalquilar caldera is an oval collapse structure having a maximum diameter of 8 km and formed at 11 Ma from eruption of the Cinto ash-flow tuff. The oval Lomilla caldera, with a diameter of 2 km, is nested within the central resurgent dome of the older Rodalquilar caldera. The Lomilla caldera resulted from the eruption of the Lazaras ash-flow tuff which was ponded within the moat of the Rodalquilar caldera. The last phase of volcanic activity in the caldera complex was the emplacement of hornblende andesite flows and intrusions. This magmatic event resulted in structural doming of the caldera, opening of fractures and faults, and provided the heat source for the large hydrothermal systems which deposited quartz-alunite type gold deposits and base metal vein systems. The gold-alunite deposits are enclosed in areas of intense acid sulfate alteration and localized in ring and radial faults and fractures present in the east wall of the Lomilla caldera. Like other acid-sulfate type deposits, the Rodalquilar gold-alunite deposits are closely related in time and space to porphyritic, intermediate composition magma emplaced along caldera structures but unrelated to the caldera forming magmatic system.