- UW Department of Geology and Geophysics
The Bonito Lake stock is one of three stocks that intrude the Sierra Blanca Volcanics in the Nogal-Bonito mining district of New Mexico. The 26.6-Ma stock consists primarily of porphyritic syenite with subordinate quartz syenite and monzonite. SiO2 concentrations range from 54 to 66 weight percent. Na2O exceeds K2O and combined alkalis average 9.4 weight percent. The rocks are alkaline and metaluminous and plot in the syenite and syenodiorite fields on an R1R2 multicationic plot. The chondrite-normalized extended trace element diagram has a negative slope with Ba values 167 to 286 times chondritic values and Yb values eight to 14 times chondritic values. The chondrite-normalized REE plot has a negative slope with mild LREE enrichment. (La/Yb)N values range from 15 to 22 and the mean Eu/Eu* is 0.91. Clinopyroxene, zircon, and apatite fractionation, along with feldspar accumulation were the most important factors in controlling elemental abundances. Quasi Pearce Element Ratio (PER) analysis confirms the role of clinopyroxene fractionation, as well as indicates that an increase in K and a loss of Ca ± Na occurred during alteration. The quasi PER analysis also shows that the rocks are slightly metasomatized. Whole-rock δ18O values for the Bonito Lake stock range from 2.0 to 8.4 per mil. Oxygen isotope values for the more permeable surrounding volcanic rocks range from 1.0 to 7.8 per mil. The lower values of the volcanic rocks are reflective of meteoric-hydrothermal water interaction. These data help us better understand not only this stock, but also the Sierra Blanca igneous complex and the Lincoln County Porphyry Belt. This study also contributes to our understanding of the Nogal-Bonito mining district and the larger Rocky Mountain alkalic province.