THERMAL HISTORY OF THE GRASBERG PORPHYRY COPPER DEPOSIT: IMPLICATIONS OF RAPID COOLING

Classical porphyry copper deposit genetic models assume that they form beneath 2+ km tall volcanic cones, where magmatism and hydrothermal fluid flow takes place over millions of years. In this study, we present 20 apatite and zircon (U-Th)/He ages from a 2.5 km vertical profile through the center of the supergiant Grasberg porphyry copper deposit. Near-surface samples cooled below ~210°C (effective zircon closure temperature based on He-diffusion studies) immediately following crystallization of the host intrusion at 3.1±0.1 Ma (3.1±0.2 Ma zHe age). Samples at 2.5 km depth cooled more slowly (2.1±0.3 Ma zHe age). Throughout the vertical profile aHe ages are less than 0.6 m.y. younger than zHe ages. The minimum cooling rate from ~700°C to 210°C was ~25°C/10 kyr near the surface and ~4°C/10 kyr at 2.5 km depth. Coeval zircon U/Pb crystallization and He cooling ages from near-surface samples precludes the presence of a tall volcano over the orebody. These results indicate Grasberg ore formation followed maar volcanism, was short-lived, and the system cooled rapidly.

Rapid cooling at depth in the GIC indicates locally steep thermal gradients, which is only possible if the intrusions were emplaced into cold country rock. Major implications are: (1) tightly spaced vertical isotherms near the surface focus precipitation of copper sulphide minerals into a restricted volume, a critical factor contributing to the formation of the extraordinarily high grade Cu-Au ore zone. (2) Steep lateral thermal gradients at depth cause faster heat loss and more rapid crystallization of anhydrous mineral phases along the margins of the parental stock. When crystallization rate governs the rate of fluid exsolution steady and prolonged bubbling can occur. Once the water saturation front reaches depths of 6+ km (pressures ~2 kbar), chlorine and thus copper will partition strongly into the fluid phase. Bubble bearing magma will eventually become sufficiently buoyant to rise along the walls of the stock and discharge expanding bubbles, ultimately resulting in copper-rich fluids collecting beneath the cupola. The efficiency of deep-seated fluid generation, cupola charging, and localization of copper sulphide deposition into a high grade porphyry copper-type ore body is greater the higher the thermal gradients.