A COMPARATIVE STUDY OF PORPHYRY CU-MO DEPOSIT MINERALOGY

Porphyry-type deposits represent the most economically significant source of copper worldwide and are important sources of gold, molybdenum, tin, and tungsten. These are associated with magmatic arcs related to subduction processes along convergent plate boundaries. A comparative study using petrographic examination was performed on 31 samples from 21 porphyry copper-molybdenum deposits of the Western Cordillera. This was completed using optical microscopy methods under reflected and transmitted light as well as portable x-ray fluorescence spectrometry (pXRF). Accuracy of estimated mineralogical modes was assessed by calculation of whole rock chemical composition from those modes based on optical microscopy and then comparing modes to average composition as determined by pXRF.

Most of the samples assessed were veined porphyritic quartz monzonite that is characteristic of these deposits. Sulfide+silicate vein stockworks are pervasive in most of the samples as well as dissemination of sulfides outwards from veinlets. The main copper ore-bearing mineral present was chalcopyrite with lesser bornite, chalcocite, digenite, and covellite. Molybdenite was the only molybdenum-bearing mineral present. Petrographic examination showed generational changes in mineralogy within these rocks that was consistent with alteration zone sequencing of previous studies for both gangue and ore bearing minerals. Chalcocite, digenite, and covellite were copper-bearing minerals found associated with secondary alteration of these deposits caused by leaching leading to supergene enrichment.

Comparison between pXRF assays and chemical compositions calculated from petrographic modes show reasonably good correlation for most elements across most samples. S, Si, Ca, and K all were underestimated, and Cu, Mo, Al, Fe, Mg, and Ti were overestimated by optical mineralogy when compared with pXRF results. This suggests an overestimation of dark minerals and an underestimation of lightly colored minerals in thin section. The overestimation of Al may be due to improper assumptions of solid solution in feldspar and clay minerals, whereas a non-sulfide phase such as anhydrite may be an important host of S in some samples.