Fluid inclusion systematics in porphyry copper deposits: The super-giant Grasberg deposit, Indonesia, as a case study

Abstract

Fluid inclusion data provide a unique insight into the behaviour of magmatic fluids as they expand from source to surface to form porphyry copper deposits. For example, saline, liquid phase inclusions in combination with vapor phase inclusions help to define the environment of mineralization in terms of large scale, two-phase fluid flow within a magmatic vapor plume. However, the pressure at which ore formation occurs within the plume is controversial because inclusion-based estimates are commonly much higher than those based on geological estimates. Uniquely, the Grasberg supergiant porphyry copper deposit occupies the breccia-filled diatreme underlying the crater of a maar volcano and preserved a lacustrine-volcaniclastic sequence that marks the paleo-surface to within a few hundred meters. Interpreting fluid inclusion data with respect to this paleo-surface shows that porphyry mineralization extended from a paleo-depth of about 500 m to over 1500 m depth, as defined by drilling.The Grasberg deposit has three distinct populations of fluid inclusions (low salinity, hypersaline liquid and multi-solid inclusions) that are strongly clustered with respect to salinity and trapping temperature. Similar clusters of co-existing liquid- and vapor-rich inclusions are also common to porphyry copper deposits worldwide. Applying the principles of energy and mass conservation, a simple model of isenthalpic phase separation by condensation may be derived to account for these data clusters and how they are related. Vapor- and liquid-rich hypersaline inclusions may be shown quantitatively to be the consequence of expansion of a continuum low salinity, low density magmatic vapor phase to a two-phase fluid where the liquid fraction is minor (<2 vol%) and is best considered to be an aerosol or fog. The clustering of the inclusion data is the product of the different ranges of pressures that fluids encountered from the initiation of individual stockwork fractures through to their resealing.The application of mass and energy conservation principles also shows that, even though metal partitioning to the liquid phase may be favored, the proportion of total metal fractionated to the small mass of liquid is low relative to that remaining in the vapor phase. Therefore, while partitioning of metals between liquid and vapor phases may be a contributing process for metal deposition in porphyry copper deposits, other vapor-phase processes are required to efficiently deposit these major metal resources.