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Geochemistry of the Bonito Lake stock, Lincoln County, New Mexico

Petrogenesis and hydrothermal alteration

Department of Physical Sciences, Eastern New Mexico University, Portales, NM 88130, U.S.A.

email: jim.constantopoulos{at}enmu.edu

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 ¹⁸O 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.

Key Words: alkaline rocks • Bonito Lake stock • geochemistry • hydrothermal alteration • New Mexico • REE • syenite