ROLE OF STRATIGRAPHIC INHERITANCE IN CONTROLLING STRATABOUND BASE METAL CONCENTRATIONS IN CARBONATE STRATA: MVT TO SKARN SYSTEMS
In more dynamic tectonic settings, reactive sedimentary successions adjacent to plutons commonly are responsible for the development of skarn-hosted metal deposits, but seldom are these relationships pursued from the context of detailed analysis of unaltered units that can be correlated with the altered and mineralized intervals. The Ertsberg East ore zone in the Ertsberg-Grasberg district in Papua, Indonesia, includes one of the world’s largest skarn-hosted Cu-Au orebodies with current reserves totalling nearly 700 Mt at 0.77% Cu and 0.7 g/t Au. The skarn ores are hosted by mixed assemblages of lower Paleogene siliciclastic and dolomitic carbonate strata that have been altered to Mg-rich calc-silicate assemblages. Unaltered strata from an equivalent interval have been characterized with regard to their petrographic features, major element compositions, and petrophysical properties. The dominant prograde skarn assemblages are controlled by protolith composition, notably the relative abundance of quartz, dolomite, and calcite. Models based on isochemical metamorphism of mixed assemblages of quartz and dolomite suggest that the formation of forsterite-diopside-dominant skarn assemblages resulted in the greatest amount of premineralization pore space.
In all of these settings, an analysis of stratigraphic inheritance not only guides the selection of exploration targets, but assists in the challenge of assessing continuity and grade of mineralized zones from drilling information as is necessary for the current requirements of resource definition.