ORE CONTROLS FOR THE DEEP MLZ CU-AU SKARN DEPOSIT, ERTSBERG DISTRICT, PAPUA, INDONESIA

The Deep MLZ Cu-Au skarn deposit is developed in an Upper Cretaceous to Lower Tertiary siliciclastic and carbonate succession adjacent to the 3-Ma Ertsberg pluton in the Central Range of New Guinea. The Deep MLZ is the deepest explored part of the Ertsberg East Skarn System that consists of the surface Gunung Bijih Timor (GBT) deposit at 4200 m elevation, through the Intermediate Ore Zone (IOZ), Deep Ore Zone (DOZ), Mill Level Zone (MLZ) and Deep Mill Level Zone (DMLZ) at 2900 to 2550 m. The DMLZ skarn is hosted by mixed assemblages of siliciclastic and dolomitized carbonate strata that produce Mg-rich skarn assemblages. Prograde alteration can be divided into types based on dominant mineralogy as 1) forsterite-diopside, 2) anhydrite, 3) heavy sulfide, 4) magnetite, 5) hornfels, and 6) endoskarn within the Ertsberg diorite.

Forsterite is the dominant olivine phase with an average of Fo₉₇Fa₃Te₀. Forsterite is commonly altered to serpentine and is associated with magnetite, anhydrite and phlogopite. Magnetite skarn contains more Fe-rich olivine phases. The average composition of clinopyroxene is Di₉₃Hd₆Jo₀. Diopside is locally altered to tremolite and is associated with anhydrite, talc, and carbonate minerals. Garnet compositions vary, with an average of Ad₇₉Gr₂₀Py₂. Garnet in hornfels shows compositional zoning. Periclase after dolomite is present locally. Secondary Fe- and Mg-rich carbonate phases are common as disseminated crystals or rims on existing crystals.

Cu-Au mineralization consists of chalcopyrite and bornite predominantly within magnetite and anhydrite skarn; pyrite and pyrrhotite are abundant locally. The dominant prograde skarn assemblages appear to be controlled by protolith composition, notably the relative abundance of quartz, dolomite and calcite within the host Waripi Formation that determine the formation of forsterite- and diopside-bearing skarns. Skarn type based on position within the Waripi stratigraphy is directly associated with major ore zones.