Dunedin, New Zealand
The Grasberg Cu-Au Porphyry deposit hosts 2.0 billion tons of ore with an average grade of 1.2% Cu and 1.2 g/t Au. Mineralization is hosted in the 3.0 Ma Grasberg Intrusive Complex, comprised of a suite of monzonitic to quartz monzonitic igneous intrusions, in a horse shoe shape around the youngest intrusion in the complex (Kali) (see figure). The focus of my thesis was to identify the alteration and mineralization extending southwest from the center of the GIC out into the sedimentary host rocks. Five stages of alteration were identified and follow in chronological order: 1. Early Intrusion Alteration, 2. Potassic Alteration, 3. Phyllic Alteration, 4. Sulfidation, and 5. Advanced Argillic Alteration. Alteration shows pronounced lithologic control.
Re/Os isotope data collected from the various stages of mineralization that accompanies the alteration, suggest that there are two sources of metal for mineralization in the GIC. The 187Os/188Os values of the porphyry style mineralization form an isochron, which gives an age of mineralization of 2.9±0.3Ma.
I am in the process of identifying a topic for my Ph.D. dissertation. Of particular interest to me is mineralization in the southwestern region of the US, in the Basin and Range Province. A significant volume of mineralization in the southwest is related to Laramide, and to a lesser extent Tertiary, magmatism. Extension in the province is often in excess of 100%, resulting in the tilting of crustal blocks to expose a cross sectional view to paleo-depths of ~ 12km. This exposure provides the opportunity to a) study in detail the hydrothermal systems associated with Laramide magmatism, and b) study the influence post-Laramide magmatism has had on mineralization.
Gibbins, S.L., 2000. Alteration in the Southwest Wall Rocks of the Grasberg Cu-Au Porphyry Deposit. M.S. Thesis, Unpub.
Mathur, R., Ruiz, J., Titley, S., Gibbins, S., Margotomo, W., 2000. Different Crustal Sources for Au-rich and Au-poor ores of the Grasberg Cu-Au Porphyry Deposit. Earth and Planetary Science Letters. V. 183, p. 7-14.