The Bolivar Cu-Zn skarn deposit is located in the Sierra Madre Occidental mountain range in the state of Chihuahua, Mexico. Most skarn alteration and mineralization is hosted by a sequence of sedimentary rocks that occurs as a lens or layer within the andesitic Lower Volcanic Sequence (LVS) and is intruded by the Piedras Verdes Granodiorite (all late Mesozoic to early Cenozoic in age). Composition of the pluton is consistent with worldwide average for Cu skarns. The mineralogy and size of the metamorphic aureole suggest a depth of formation for the skarn is between 5 and 7 km. Current reserves are 322,400 tons of 5.16% CuEq with very high grade ore (e.g. drill core intercept of 31m of 4.03% Cu, 22.75% Zn, and 37.4 g/t Ag). The sedimentary rocks are comprised of carbonate and clastic layers that lie subparallel to Piedras Verdes intrusive contact. Mineralization primarily occurs in two distinct carbonate horizons - the lower skarn altered from dolostone and the upper skarn altered from limestone. Mineralogy of the lower skarn is typical of magnesian skarns with pyroxene, olivine, relatively scarce garnet, massive magnetite, phlogopite, talc, tremolite and serpentine. In contrast, the mineralogy of the upper skarn reflects the composition of the more calcic host with relatively abundant garnet, lesser pyroxene, no magnetite, and none of the Mg-silicates observed in the lower skarn. Mineralogy, pyroxene composition, and relative metal abundance all serve as important indicators of proximity to a hydrothermal center. Proximal zones are characterized by light green and Fe- and Mn-poor pyroxene and relatively abundant Cu. In distal zones, pyroxene is dark green-brown and Fe- and Mn-rich and Zn is relatively more abundant. In very distal zones skarn alteration minerals are absent and marble is observed. These zonation trends indicate a hydrothermal center in the Bolivar Sur area. Regional zonation is more accentuated in the upper skarn than the lower skarn. Copper is more abundant in the lower skarn and zinc in the upper skarn, suggesting that stratigraphy may have a strong control on mineralization. Fluid inclusion data are consistent with mineralogical zonation trends. Prograde pyroxene has homogenization temperatures and salinities ranging from very high in proximal zones (475-590°C; 72-76 wt. percent NaCL+KCl with as many as 9 daughter minerals) to much lower in distal zones (250-330°C; 37-41 equiv. wt. percent NaCL with no daughter minerals). These temperatures are at the high end for copper skarns and much higher than typical zinc skarns. Retrograde minerals such as epidote and quartz are evidence of later, lower temperature and lower salinity alteration events (160-495°C,8-41 equiv. wt. percent NaCl). Sphalerite is also lower temperature with homogenization temperatures randing from 230° to 365°C.
Reynolds, Merilie, "Stratigraphy, mineralogy, and geochemistry of the Bolivar Cu-Zn skarn deposit, Chihuahua, Mexico" (2008). Honors Project, Smith College, Northampton, MA.
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