Geological and geochemical zoning of the Grasberg Igneous Complex, Irian Jaya, Indonesia

Abstract

The Grasberg Igneous Complex is host to a porphyry CuAu deposit whose hypogene mineralization is of remarkably high grade. The complex occupies the central portion of the Ertsberg Mining District, in the central highlands of New Guinea in the province of Irian Jaya, Indonesia. Disseminated and stockwork mineralization is confined to intrusive rocks emplaced in tightly folded carbonate strata of Tertiary age. Mineralization extends from the surface at 4,200 m elevation to the deepest drill penetrations at 2,700 m elevation and includes 700 million tonnes, mineable by open pit, at an average grade of 1.4 wt.% Cu and 1.8 ppm Au. Two distinct stages of spatially coincident intrusion produced two nested coaxial porphyry orebodies having significantly different styles of mineralization as well as sulfide-rich skarn at the margin of the igneous complex. The intrusive rocks and their attendant mineralization have been radiometrically dated at 2.7 to 3.3 Ma. The earliest intrusive stage, the Dalam Diatreme (DD), hosts the deep, largely disseminated CuAu mineralization of the weak stockwork zone. The Dalam Diatreme is highly fragmental, with clasts and matrix both of dioritic composition. Mineralization is chalcopyrite dominant and has an average grade of 1.2% Cu and 0.5 ppm Au. The top of the Dalam mineralized zone lies 800 m below the present topographic surface. Based on the occurrence of volcaniclastic and pyroclastic assemblages noted there, the topographic surface may be the erosional remnant of a maar volcano. The diatreme exhibits well developed, sharply defined silicate zoning and has a distinctive minor and trace element signature. The middle intrusive stage, the Main Grasberg stock (MG), is the source of an intensely developed quartz magnetite dilational stockwork that carries high grade veinlet-controlled CuAu mineralization. Grades averaging 1.5% Cu and 2 ppm Au have been intersected in drill core between 4200 and 2700 m, and are expected to continue with depth. Rocks of this intrusive stage are non fragmental, porphyritic monzodiorites. Main Grasberg stage mineralization is hosted by the MG stock as well as the Dalam Diatreme which completely encloses it. Silicate alteration is concentrically developed around the MG, and overprints that of the earlier Dalam mineral phase. The third and final intrusive stage associated with intrusion of the South Kali Dikes, was the weakest of the mineralizing pulses. The South Kali intrusion is a tabular, structurally controlled quartz-bearing monzonite porphyry body. It breaches the originally continuous cylinder of mineralization formed by the earlier Dalam and MG stages, resulting in a body of low grade mineralization which extends from the central area outward into the limestone on the southeast margin of the Grasberg Igneous Complex. Mineralization associated with the South Kali (SK) intrusive stage averages 0.3% to 0.5% Cu and l ppm Au in an early phase; the final phase is barren. Silicate alteration extends relatively short distances beyond the contacts of the South Kali dikes, and is significantly less intense than that associated with the earlier stronger phases of mineralization. Because of its recent formation and because of rapid rates of erosion, the mineralized system has suffered only minor structural dislocation, and the effects of weathering extend only to shallow depth. Supergene oxidation and related clay formation is confined to the upper 30 m of the intrusive rocks. Copper enrichment as chalcocite and covellite is rarely more than 1.3 times the original hypogene grade and extends less than 100 m below the pre-mining topographic surface.