New Mexico Mineral Symposium — Abstracts

The Pilares wollastonite deposit is 55 km northwest of Hermosillo, Sonora, Mexico and was first identified in 1981. Cerro Pilares, the most prominent feature in the area, is a mountain rising 300 m out of the flat Sonoran desert and covering about 5 sq. km. Geologic examination has shown that most of this mountain is wollastonitic marble. Current investigations have focused on the westernmost flank of the mountain where several tens of millions of tons of easily accessible, high-quality wollastonite has been identified by geologic mapping and diamond drilling.

The deposit is owned and is being developed by Minera NYCO S.A. de C. V., a wholly-owned subsidiary of NYCO Minerals, Inc. of Willsboro, New York, the world's largest wollastonite producer.

Wollastonite is a white, acicular, calcium metasilicate [CaSiO₃] mineral typically formed in contact metamorphic (dry) and metasomatic (wet) environments where intrusive igneous rocks have come in contact with carbonate rocks. Calcite from the limestone combines with silica carried from the intrusive in hot, aqueous solution to form wollastonite and CO₂ gas by this equation
CaCO₃ + SiO₂ (aq.) --> CaSiO₃ + CO₂ t .
Its usefulness as an industrial commodity is largely due to its acicularity. Applications for wollastonite include ceramics, brake linings, plastic reinforcement, thermal insulation board, and protective coatings.

The Pilares deposit is a huge, contact metasomatic or skarn deposit formed from the intrusion of a large, Cretaceous, granodiorite stock into a Paleozoic limestone. Remnant beds of thin, pure-white quartzite indicate that some, or even most, of the silica required for the formation of the wollastonite may have been available in the carbonate sequence, and the two rocks would have reacted under the influence of the heat and fluids from the intrusion to form the skarn. A few discontinuous green hornfels beds, ranging from 0.5 to 3 m thick, also occur in the sequence. Wollastonite occurs predominantly as clusters of radiating, white to pale-cream needles, typically ranging from 1 to 2 cm long. The other major mineral in the deposit is medium- to coarse-grained white calcite [CaCO₃]. These two minerals make up 95% of the deposit. Commercial development will occur where wollastonite grades exceed 60% over tens of meters in thickness. Locally, beds 5 to 10 m thick can average 80+ % wollastonite. The deposit is extensively brecciated on a megascale, probably due to the explosive release of CO₂ during formation.

The most common minor mineral in the deposit is pale- to medium-green diopside [CaMgSi₂O₆], which is disseminated throughout the formation. Other minerals include quartz [SiO₂], garnet [Ca₃(Al,Fe^3±)₂Si₃O₁₂], vesuvianite [Ca₁₀Mg₂Al₄(SiO₄)₅(Si₂O₇)₂(OH)₄], epidote [Ca₂(Fe³⁺,Al)₃(SiO₄)₃(OH)], tremolite [Ca₂Mg₅Si₈O₂₂(OH₂)], orthoclase [KAlSi3O₈], clinozoisite [Ca₂Al₃(SiO₄)₃(OH)], zeolites [hydrous and hydrated, alkali and alkaline earth, aluminum silicates], and chalcopyrite [CuFeS₂].

Acknowledgments—The authors would like to express their gratitude to Minera NYCO S.A. de C. V. because without their cooperation and assistance this presentation could not be given. We also would like to acknowledge the assistance of Prof. Albert M. Kudo of the Department of Earth and Planetary Sciences of the University of New Mexico for his excellent and detailed petrographic studies of selected specimens from the Pilares deposit.

* Part of this work was performed at Sandia National Laboratories, operated for the U.S. Department of Energy under contract number DE-AC04-94AL85000.