Abstract

The 40Ar/39Ar ages of 10 magmatic and hydrothermal micas from the Grasberg Igneous Complex range from 3.33 ± 0.12 to 3.01 ± 0.06 Ma. The ages of intrusive rocks and the paragenetic relationships between intrusive rocks and hydrothermal alteration and mineralization indicate that the Grasberg Igneous Complex formed during several cycles of intrusion and hydrothermal alteration. These include the Dalam and Main Grasberg intrusion and alteration cycles (3.33 ± 0.12–3.19 ± 0.05 Ma), a Kali intrusion and alteration cycle (3.16 ± 0.06–3.06 ± 0.03 Ma), and a post-Kali intrusion and Grasberg mineralization cycle (3.06 ± 0.03 and 3.01 ± 0.06 Ma). Each cycle of intrusion and alteration appears to have lasted around 0.1 m.y. or less and indicates that the huge size and high grade of Grasberg did not result from an unusually prolonged period of hydrothermal activity.

A sample of phlogopite predating magnetite from the Kucing Liar Cu-Au deposit adjacent to Grasberg has an age of 3.41 ± 0.03 Ma. This is within error of the age of a Dalam intrusive rock from the Grasberg Igneous Complex and suggests formation of the calc-silicate skarn part of Kucing Liar at an early stage in the development of the complex. The ages of the equigranular diorite from the Ertsberg intrusion (2.67 ± 0.03 Ma), phlogopite from an endoskarn vein in the intrusion (2.71 ± 0.04 Ma), and phlogopite from the Ertsberg Cu-Au deposit (2.59 ± 0.15 Ma) indicate that intrusion and alteration and/or mineralization at Ertsberg are younger than the intrusions and mineralization in the Grasberg Igneous Complex. The Ertsberg therefore represents at least one additional cycle of approximately 0.1 m.y. of intrusion and alteration and/or mineralization in the district.

The intrusions that make up the Grasberg Igneous Complex and Ertsberg and the hydrothermal fluids responsible for much of the alteration and mineralization appear to have been derived from a deeper level magma chamber. The youngest dated intrusive phase in the Grasberg Igneous Complex is a post-Kali diorite dike that is more basic than the preceding Kali quartz monzodiorite intrusions. This, together with the presence of mafic xenoliths in the Kali and Ertsberg intrusions, suggests that the magma chamber from which the intrusions and fluids were sourced was periodically replenished by basic magma. This process may also have triggered release of magma to form the shallow-level intrusions now exposed at the surface. The basic magmas also may have contributed components including fluids, metals, and/or sulfur to the Cu-Au deposits in the Ertsberg district.

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