Summary:
The Grasberg minerals district is situated on the crest of the Sudirman Mountain Range at elevations over 2,500 meters with the highest peak, Puncak Jaya at 4,884 meters. Ore deposits in the Grasberg minerals district have economic copper, gold, and silver mineralization in porphyries and skarns. The primary sulfide mineralization is chalcopyrite, with lesser bornite, chalcocite, and covellite. Gold concentrations usually occur as inclusions within the copper sulfide minerals although in some parts of deposits gold can also be strongly associated with pyrite.
There are five major ore bodies all within the center of the Grasberg minerals district:
• Grasberg Intrusive Complex (GIC) includes the GBC and the depleted GRS_OP.
• EESS includes DMLZ and depleted GBT, IOZ, DOZ block caves.
• BG.
• KL (development stage mineral reserve).
• Dom (classified as mineral resource).
The ore bodies are located within and around two main igneous intrusions: the Grasberg monzodiorite and Ertsberg diorite. The host rocks of these ore bodies include both carbonate and clastic rocks that form the ridge crests and upper flanks of the Sudirman Range and the igneous rocks of monzonitic to dioritic composition that intrude them. The igneous-hosted ore bodies (GBC and portions of the DMLZ) occur as vein stockworks and disseminations of copper sulfides, dominated by chalcopyrite and to a lesser extent bornite. The sedimentary-rock hosted ore bodies (portions of the DMLZ, KL and all of the BG) occur as “magnetite-rich, calcium/magnesian skarn” replacements, whose location and orientation are strongly influenced by major faults and by the chemistry of the carbonate rocks along the margins of the intrusions.
Rock Types
The sedimentary rock units of the Grasberg minerals district are classified into the two main groups of formations from oldest to youngest as described below Kembelangan Group is approximately 3,400 meters thick, consists largely of siliciclastic rocks, and is divided into four formations:
• Middle to Upper Jurassic Kopai.
• Upper Jurassic to Lower Cretaceous Woniwogi.
• Lower to Middle Cretaceous Piniya.
• Upper Cretaceous Ekmai.
New Guinea Limestone Group is approximately 1,700 meters thick, is carbonatedominated, and is divided into four formations:
• Paleocene Waripi.
• Eocene Faumai.
• Oligocene Sirga.
• Upper Oligocene to Middle Miocene Kais.
The northern and central portions of the Grasberg minerals district are dominated by the 1.7 kilometer thick, largely carbonate rocks of the Lower to Middle Cenozoic New Guinea Limestone Group. The Kais formation is the uppermost part of the New Guinea Limestone Group and is a 1.2 kilometer thick fossiliferous limestone. The Kais limestone is underlain by the 30-meters-thick quartz-carbonate sandstone of the Sirga formation, a Grasberg minerals district stratigraphic marker. Below the Sirga formation is the 150-meters-thick, massive-bedded, clean limestone of the Faumai formation. The lowermost New Guinea Limestone Group and Cenozoic unit is the Waripi formation, a 300-meters-thick anhydrite nodule-bearing dolomitic limestone, containing thin but laterally continuous quartz sandstone beds. All New Guinea Limestone Group carbonate formations can host highgrade copper and gold skarn mineralization, but the most susceptible unit is the lower part of the Waripi formation.
Alteration and Mineralization
Mineralization occurred in a short timeframe between 2.5 to 3.5 million years, shortly after the emplacement of the metal-laden associated igneous rocks. All the porphyry and skarn copper ore bodies in the area contain gold and silver as additional metals. The structurally controlled porphyry systems in the Grasberg minerals district are derived from potassiumrich magmas, monzodioritic to monzonitic in composition. Exoskarn and associated mineralization is mainly present in the New Guinea Limestone Group rocks, particularly within the dolomite of the Waripi formation, but also within the Cretaceous Ekmai limestone and sandstone.
The bulk of the mineralization in the Grasberg minerals district occurs within potassicaltered intrusive rocks or within prograde skarn assemblages. Chalcopyrite is the dominant ore mineral in all ore bodies. Bornite prevalence increases with depth in both the Grasberg porphyry and the Ertsberg skarns, but rarely dominates. Covellite is a common constituent in phyllic-altered (sericite-pyrite) zones. It is the dominant copperbearing mineral in distal portions of KL and is common in the very local, argillic alteration zones at Grasberg and KL, along with other high sulfidation state sulfides. Supergene chalcocite was a minor constituent of the low-grade Grasberg ores to as much as 300 meters below original topography within the highly permeable, anhydrite-depleted “poker chip” zone. Oxide copper minerals are insignificant in all ore bodies, except for Dom where malachite and chrysocolla are abundant.
In the EESS, approximately 80 percent of gold in the prograde skarn and potassic alteration-hosted ores in the deposits occurs as free inclusions in chalcopyrite, bornite, and digenite; the remainder occurs in pyrite and silicate gangue minerals. In zones with intense phyllic and/or advanced argillic alteration, early formed, gold bearing chalcopyrite is converted to covellite with pyrite and gold is taken up in the pyrite lattice. This is especially the case in the highly altered ores in parts of KL and at the deeper margins of the Grasberg deposit. Gold is typically fine-grained and not visible. Silver appears to be contained largely within the crystal lattice of the copper-bearing sulfides.
Lead (as galena), zinc (as sphalerite), and arsenic (as arsenopyrite and enargite) generally occur in low concentrations and limited locations. These accessory sulfide minerals are most common at margins of mineralization (BG and KL) and in distal faultfracture systems, commonly accompanied by anomalous gold values and generally in areas of elevated pyrite and/or pyrrhotite. Pyrite (and lesser pyrrhotite) is relatively uncommon in the potassic alteration and prograde skarn-hosted ores, but can reach high concentrations in the lower grade phyllic and retrograde skarn alteration zones at ore body margins, such as the Heavy Sulfide Zone at GBC, KL, and on margins of DMLZ and BG. Thermal metamorphism of the carbonate rocks is evident in the occurrence of marble aureoles around all the intrusions in the Grasberg minerals district. Marbleization extends outward from the contact between 50 and 1,000 meters. The inner boundary is commonly sharp, beginning at either the igneous/sedimentary rock contact or at the skarn front. The shale member of the Ekmai formation tends to alter to hornfels outward from the igneous contacts (Leys et al., 2012).