Geology of the Cerro Jumil gold skarn deposit, Mexico

Abstract

The Cerro Jumil deposit is a recently discovered gold skarn located in the state of Morelos, Mexico. The deposit is comprised of 1.47 million ounces Au and 16.01 million ounces Ag in the measured and indicated categories at a 0.3 g/t gold equivalent cut-off. Mineralized skarn zones have been developed in limestone of the Mesozoic Morelos-Guerrero Platform in response to the intrusion of a 34.3 Ma feldspar porphyritic granodiorite. The feldspar granodiorite intrusion is associated with an alteration halo that shows a pronounced mineralogical zonation from distal marble to proximal pervasive calc-silicate alteration. This zoning occurs along the flanks of the granodiorite and above the cupola of the intrusion, but is also developed along fault planes and bedding planes away from the intrusive contact. Petrographic and mineral chemical investigations permitted a detailed reconstruction of the skarn paragenetic sequence and also provided important constraints on the relative timing of gold mineralization. Textural evidence suggests that wollastonite formation preceded development of clinopyroxene, vesuvianite, and garnet during the prograde stage of skarn formation. Wollastonite skarn is well developed along former limestone bedding planes. Vesuvianite and garnet skarn occurs as massive zones, containing subordinate amounts of clinopyroxene. Formation of garnet skarn, dominated by grossular and andradite series garnets, is interpreted to record peak metamorphic conditions. Minor pyrite formed in association with the garnet skarn. Garnet skarn formation was followed by the onset of retrograde metamorphism. Actinolite and tremolite are common and pervasive talc pseudomorphs prograde and retrograde alteration minerals. Minor plagioclase, calcite, quartz, epidote, and chlorite are observed to form interstitial grains between talc. A second generation of pyrite formed during the retrograde evolution of the exoskarn. Due to supergene weathering, this pyrite is now largely replaced by Fe oxides and hydroxides. These limonitic intervals commonly show elevated gold grades, suggesting that gold mineralization was primarily associated with this sulfidation event. Bismuth shows a moderate to strong correlation with Au. The location of faults appears to have controlled the subsequent formation of fine-grained, massive, hematite- and silica-rich jasperoid. Although jasperoids are commonly barren, high gold grades have been observed for at least some intersections of jasperoid in drill core, suggesting that fluid pathways played a role in the late remobilization and concentration of gold. The formation of retrograde alteration assemblages was followed by the intrusion of a late quartz porphyritic granodiorite. The quartz porphyritic intrusion crosscuts the different skarn mineral assemblages and the marble. Skarn development around this intrusion was not observed although this granodiorite has also been affected by hydrothermal alteration. This intrusive phase does not appear to have contributed to the overall metal content of the Cerro Jumil deposit. The intrusion of the quartz porphyritic granodiorite broadly coincided with widespread faulting and a second, hematite-rich though weakly mineralized jasperoid at Cerro Jumil. U/Pb age dating of zircon grains separated from the two granodiorite intrusions suggests that the magmatic-hydrothermal activity at Cerro Jumil lasted between 0.4 and 0.6 million years. The prograde and retrograde skarn assemblages at Cerro Jumil are overprinted by pervasive supergene alteration, causing the observed extensive replacement of sulfides by limonite. Supergene alteration seems to have been important by redistributing gold on a local scale, making Cerro Jumil an attractive bulk-mineable deposit. Fluid inclusion investigations revealed that the prograde and retrograde stages of skarn formation occurred from a single-phase, intermediate-density hydrothermal fluid, suggesting that intrusion of the granodiorite occurred at an intermediate crustal depth (3.0 to 4.1 km) at lithostatic pressure conditions. Due to the comparably deep environment of skarn formation, mixing of the magmatic-hydrothermal fluids with meteoric water was probably insignificant. Although Cerro Jumil shares a number of deposit characteristics with other gold skarns in the Morelos-Guerrero Gold Belt, the results of the present study highlight some important differences such as the chemistry and alteration state of the skarn-related intrusion, the chemistry of the hydrothermal fluids, and resultant mineralogy, lack of exoskarn, depth of the intrusion and corresponding lack of pervasive retrograde alteration overprint, and age of formation. In particular, the age of Cerro Jumil has significant implications for regional exploration, as Tertiary intrusions into the Mesozoic Morelos-Guerrero Platform are not necessarily barren as previously suggested.