Metallogenic relevance of trace element systematics in skarn-related garnet – the Morelos district in Mexico as a case study

The Morelos district (Guerrero, Mexico) hosts several Au-Cu skarn deposits, i.e., the El Limon, Guajes and Media Luna deposits, that are centered around the El-Limon granodiorite. The three skarn deposits comprise a total Au endowment of 8.8 Moz at an average grade of 3.2 g/t (all classes). In addition, the Media Luna deposit (south) comprises 20.9 Mt at 1.07 wt.% Cu. This different metal tenor (Au-Cu vs. Au), has been interpreted to be related to higher Mg-concentrations in the sedimentary protolith at Media Luna.
Forty-two samples from eleven drill-cores from the El Limon (north) and Media Luna (south) deposits were selected along systematic depth profiles, including skarn samples in proximal, intermediate and distal position to the intrusion. The prograde skarn mineral association is characterized by mainly garnet, pyroxene and magnetite, whereas retrograde skarn minerals comprise amphibole, calcite and sulfide minerals (mainly pyrrhotite, arsenopyrite and chalcopyrite). Preliminary petrographic analyses of selected drill-core material reveal a systematic variation in garnet color from dark brown/red in proximal endo/exoskarn to green in intermediate exoskarn to pale green in distal marble-hosted garnet nodules.
After detailed mineralogical characterization (optical and scanning electron microscopy), we are planning to analyze major and trace element compositions of skarn-related garnet by electron microprobe analysis and laser ablation – inductively coupled- mass spectrometry. The goal of this study is to constrain trace element systematics in the prograde skarn stage along the fluid flow path as well as to assess the influence of different host rock compositions on garnet trace element chemistry.