LITHOCHEMICAL EXPLORATION TOOLS REVISITED: MNO AND REE

Lithogeochemical fingerprinting of productive calc-alkaline intrusions (associated with mineralization) is important, yet diagnostic criteria for chemical prospecting and predicting mineralization are scarce. Previous discrimination attempts have used halogens, water contents, K contents, Al2O3/(K2O+Na2O+CaO) ratios, Rb, Sr and transition metal contents and REE fractionation. Most have met with mixed success. Fluid release from the lower crust and the presence of amphibole seem to be key ingredients for ore formation. We review the MnO and REE contents of Jurassic to recent central Andean igneous rocks as a potential discriminant. 85% of the total estimated copper deposits for the American Cordillera (~850 million tonnes) are concentrated in Late Cretaceous to early Pliocene porphyry intrusive rocks. Arc crustal thicknesses during this time almost doubled from ~35 to 70 km, and the geochemical signatures of rocks in this episode show that amphibole played an important role in petrogenesis. Sm/Yb and La/Sm ratios of these rocks indicate increasing pressures in the residual and/or source mineralogy from gabbroic to amphibolite to eclogite, consistent with major arc crustal thickening.

Amphibole can both contain significant Mn concentrations, and be crystallized from magmas and concentrated in the residue during partial melting. The resultant magma should be depleted in MnO. Hydrothermal replacement of hornblende also causes MnO to decrease. Most hydrothermal fluids contain significant Mn concentrations and most porphyry Cu deposits are surrounded by Mn-rich halos. The middle to heavy REE are not strongly partitioned into volatile phases, but into amphibole (Sm) and/or garnet (Y,Yb). Thus, low middle and heavy REE ratios and low MnO contents indicate amphibole involvement during the magmatic history and can be diagnostic of productive intrusions. Low MnO may indicate extensive loss of magmatic fluids from the magma and could be directly related to mineralization events.