ROLE OF STRATIGRAPHIC INHERITANCE IN CONTROLLING STRATABOUND BASE METAL CONCENTRATIONS IN CARBONATE STRATA: MVT TO SKARN SYSTEMS

Many of the world’s major base metal orebodies occur as stratabound concentrations within limited portions of carbonate-rich stratigraphic successions. These restricted ore-hosting intervals typically are related to the susceptibility of certain units to post-depositional alteration by fluid systems ranging from near-surface to burial diagenesis, commonly inherited from their depositional framework. Common examples include stratabound breccia or other porosity zones resulting from differential removal of soluble components that provided permeable zones for subsequent fluid migration, including MVT mineralizing brines as in the Tennessee and Pine Point districts.

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.