Fluids associated with the Au(-Cu) skarn in La Joya, Mezcala district, Guerrero, Mexico: regional implications for deposits formed from calkalkaline vs. adakitic rocks

Abstract

The La Joya prospect is found in the Mezcala district, Guerrero, Mexico. The gold-bearing mineralization is associated to jasperoids that form a small lenticular body near the surface. The ore is related to a granodioritic intrusive with a skarn aureola with grossular formed on recrystallized limestones from the Morelos Formation. An argillic alteration zone located between the skarn and the recrystallized limestones contains small copper sulfide lenses, that predate the formation of Au-bearing jasperoids. The formation of presently mineable skarns at the Mezcala district is related with adakite intrusions during the Paleocene. However, the area contains several skarn deposits associated with calc-alkaline intrusives. Most of these, such as La Joya, have a subeconomic character. We performed a microthermometric study of the fluid inclusions contained in samples from (1) late-magmatic early-hydrothermal stringers in the granodiorite, (2) the argillic alteration zone and skarn, (3) jasperoids, and (4) recrystallized limestones. The fluid inclusions from late-magmatic stringers, jasperoids and recrystallized limestones are biphasic (liquid+vapor) at room temperature, whereas the fluid inclusions from the skarn and argillic alteration zones are multiphasic (liquid+NaCl+vapor). The fluid inclusions in quartz from late-magmatic stringers in the granodiorite display calculated salinities between 15.5 and 17.3 wt. % NaCl equiv., and homogenization temperatures (Th) between 360 and 413 ºC. The fluid inclusions in quartz from the skarn and argillic alteration zones display higher salinities, between 32 and 42 wt. % NaCl equiv., and Th between 306 and 348 ºC. The fluid inclusions in calcite from recystallized limestones display salinities between 5 and 15 wt. % NaCl equiv., and Th between 172 and 205 ºC. Finally, the fluid inclusions in quartz from the gold-bearing jasperoids display the lowest salinities and Th of the deposit, between 0.3 and 1 wt. % NaCl equiv., and between 160 and 170 ºC, respectively. We interpret that boiling occurred in the copper sulfide-rich zone, and early gold transport by chloride complexes, considered as “moderately” saline initial magmatic fluids. The brine diluted towards the surface in the jasperoid zone, allowing gold to precipitate. The characteristics of the initial mineralizing fluids would partially explain the relatively low gold content of the La Joya deposit, in contrast with the deposits associated to Paleocene adakitic magmas of the Mezcala district, formed by oxidizing and hypersaline initial mineralizing fluids.