Download Overview of global Cu-Au porphyry

Overview of global Cu-Au porphyry-type deposits
Eric Jensen, Eurasian Minerals, Inc., Littleton, USA
David Johnson, Eurasian Minerals, Inc.
Porphyry-style copper and gold deposits occur in a wide variety of geologic settings around the
world, with examples that span much of geologic history from the Precambrian to Late Tertiary.
These can occur as large to supergiant (5 Gt) deposits of “disseminated” and veinlet-controlled
base and precious metals associated with large volumes of hydrothermally altered rocks.
Considering the inventories of metals contained in these deposits, and the large numbers
currently in production (120 active mines), these represent the most important class of
economic mineral deposits of base and precious metals.
Gold-rich porphyry copper deposits are genetically linked to a characteristic suite of arc-related
intrusive igneous rocks (more mafic or alkaline in composition than Cu-Mo type porphyry
deposits), but their overall characteristics in terms of host rocks, tectonic settings, and styles of
mineralization/alteration vary substantially. Thus, this clan is linked by common processes of
ore formation in specific types of magmatic-hydrothermal systems that can develop in a wide
variety of environments.
Since porphyry deposits tend to form at relatively shallow levels in the crust, preservation
potential is highest in young volcanic belts, which has skewed their distribution through
geologic time. However, erosion levels of one to several kilometers from former paleosurfaces
is advantageous to expose systems for discovery, exploration and development. Thus, systems
are best exposed in young arc environments that have been deeply incised by erosion or in
older arc environments that have been more broadly eroded to appropriate crustal levels.
When the right levels of the crust have been preserved and exposed, these deposits are
typically found in clusters and belts.
Further, the older the deposit, the more likely it has been deformed or reconfigured by postmineral structures. Many early Cenozoic and older deposits have been tilted substantially
and/or dismembered after formation. A key consideration for exploration is to understand their
system-scale zoning patterns and geomorphological and post-mineral histories. Many deposits
are explored with a preconceived notion that the deposit remains upright and intact, leading to
more protracted exploration histories or missed opportunities for discovery in some cases.
The anatomies of individual deposits, and this deposit class in general, have been described and
discussed in detail elsewhere. This talk will instead focus on considerations for exploration,
recent success and new discoveries around the world. Key issues for any company exploring for
these deposits include “what does commercial success look like”, and how exploration
expectations can be managed.
Giant porphyry Cu-Au deposits: Important global resources for the future