Evolution of volcanic and tectonic features in caldera settings and their importance in localization of ore deposits

Abstract

Many calderas are located along regionally important fault zones that are intermittently active before and after the caldera cycle. In mineralized calderas, the ore deposits are controlled by structures developed during caldera formation and by regional faults which intersect and reactivate the caldera-related structures. In the earliest phase of the caldera cycle, tumescence occurs over the shallowly emplaced magma chamber and normal faults develop within the structural dome above the magma chamber. This early phase of faulting is disrupted by caldera formation but some of the faults are preserved outside the caldera margin and may become important ore-controlling structures such as the Ruja mercury deposit in the McDermitt caldera complex, Nevada, and the precious metal deposits in the Atlanta district, Nevada, outside the Indian Peak caldera. Caldera-related structures such as ring fracture faults are commonly the most important in controlling the emplacement of resurgent magma, as well as postcollapse intrusions and associated ore deposits. During the resurgence phase of the caldera, faults related to an apical graben, and radial and ring faults and fractures form and may extend outside the collapse structure. Radial and ring faults and fractures provided the structural control for gold alunite-type deposits in the Rodalquilar caldera complex, Spain.At the end of the caldera cycle, the area over the batholith related to the caldera-forming magma chambers is uplifted as isostatic compensation occurs, or subsides if the volcanic field is developed within a rapidly subsiding graben. Volume change resulting from crystallization will also contribute to subsidence. Regional subsidence of the Lake Owyhee volcanic field in southeastern Oregon at the end of the caldera cycle is reflected by arkosic sedimentation in the caldera basins and throughout the eastern part of the volcanic field. Crystallization of the underlying batholith permits the reestablishment of brittle deformation along the regional faults which initially may have controlled magma emplacement. This tectonic activity may reactivate caldera structures and these typically become important for localization of hydrothermal activity. Hot spring-type gold deposits in the Lake Owyhee volcanic field formed when a regional northwest-trending structure reactivated the caldera ring fracture fault zone of the Three Fingers caldera. The formation of hydrothermal ore deposits at the end of the caldera cycle is often related to tectonic reactivation of caldera structures by regional faults and emplacement of small volumes of magma as stocks and vent complexes along these structures.