U-Pb GEOCHRONOLOGY AND MINERALOGY OF HEMATITE FROM MANTOVERDE AND CARMEN DE COBRE, NORTHERN CHILE: CONSTRAINTS ON ANDEAN IOCG MINERALIZATION

The first U-Pb hematite dates are reported for the Mantoverde and Carmen de Cobre iron oxide copper-gold (IOCG) deposits in the northern segment of the Coastal Cordillera, Chile. Samples contain low U, low radiogenic Pb, and relatively high common Pb components. Nevertheless, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data, normalized using a newly developed hematite reference mate-rial, yield intercept dates of 108.01 +/- 2.25/4.69/6.66 Ma at Carmen de Cobre and 105.61 +/- 6.71/14/19.39 Ma at Mantoverde (uncertainties at 2 sigma given as analytical/+95% confidence interval/+factor of mean square of weighted deviates [MSWD]). Complementary micron-to nanoscale mineralogical characterization of hematite in the two IOCG systems was conducted to understand the formation conditions of hematite and assess the implications of obtained ages. The dated hematite is Ti-rich (up to 10.8 mol % ilmenite; similar to Fe1.89Ti0.11O3) but lacks ilmenite exsolutions. This implies minimum crystallization temperatures of similar to 500 degrees C, above the solvus of the FeTiO3-Fe2O3 system, supporting a magmatically derived fluid source. Nanometer-scale, K-Si-Al-Cl-bearing inclusions in hematite suggest an alkali alteration event during hematite formation. Alongside measurable U, the Ti-bearing hematite also contains growth zones with W and Sn, elements which are part of a "granitophile" signature analogous to that reported from IOCG systems elsewhere, notably in the Gawler craton, South Australia. Apart from some older IOCGs north of Taltal, the IOCG deposits in Northern Chile are considered to have been formed between similar to 125 and 110 Ma. Younger published dates, similar to those determined here for Mantoverde and Carmen de Cobre, overlap with those from younger porphyry systems following a shift from transpressional to contractional tectonics. These new dates support an interpretation in which Carmen de Cobre belongs to a younger episode (less than or similar to 100 Ma) of IOCG mineralization in the area, whereas the new date for Mantoverde, coupled with published data, could indicate superposition of several mineralizing events. At Carmen de Cobre, the dated Ti-bearing hematite is younger than the spatially associated, crosscut magnetite bodies. Systematic application of hematite geochronology can play an important role in constraining the timing of IOCG mineralization along the Coastal Cordillera in the context of panregional metallogeny and temporal development of the Atacama Fault System, its subordinate structures, and associated magmatism. Our data suggest that the IOCG deposits studied are unrelated to andesitic melts, but they could be related to more evolved granitic melts.