| The El Teniente Cu deposit, located in the Andes of central Chile (34°S), is associated with a variety of late Miocene to early Pliocene hydrothermal breccias. Two types of anhydrite-bearing igneous breccias previously named by mine geologists as "Andesite" and "Microdiorite" Igneous Breccia (AIB and MIB, respectively) are located at the center of a hydrothermal breccia complex on the East side of the deposit. Both breccias include clasts of the rocks into which they intruded. However, the MIB surrounds an ∼100m wide, clast-free stock that is referred to as "Porphyry A", although it is not porphyritic. The MIB is temporally and genetically related to the hydrothermal anhydrite breccia that surrounds it. The AIB contains clasts of the MIB, and therefore intruded subsequent to the MIB.; Major minerals in the MIB matrix are sodic plagioclase (An4-7), quartz, K-feldspar, poikilitic anhydrite (>30 modal %), Cu-sulfides, and biotite. The AIB matrix is a fine-grained, equigranular igneous rock that contains abundant biotite, sodic plagioclase (An25-32), quartz, interstitial anhydrite (<10 modal %) and Fe-oxides. Except near later hydrothermal veins, matrix minerals of both breccias exhibit planar crystal boundaries suggesting that they are primary phases that formed in the open space created by brecciation and not secondary alteration phases. Disparate crystallization temperatures obtained using Ti-in-biotite and S and O isotope geothermometers suggest various degrees of S and O isotopic disequilibrium. From textural, mineralogical, and chemical evidence, anhydrite in the AIB is a magmatic phase, while anhydrite in the MIB crystallized from an aqueous fluid/vapor exsolved from the same magma that formed the rest of the rock.; Major and trace element chemistry of these rocks suggest that they were formed from highly evolved magmas enriched in S and Cu; however, Nd and Sr isotope data imply their derivation from the same long-lived, magmatic source as all the other igneous rocks in the deposit. Processes that may have contributed to the formation of the high fO2, Cu-, S-, and volatile-rich magmas that formed the AIB and MIB igneous breccias include igneous fractionation involving fluid/vapor-phase transport, magma mixing, and possible assimilation of the associated hydrothermal anhydrite breccia. |
