AGE OF SANIDINE AND COMPOSITION OF MELT INCLUSIONS IN QUARTZ PHENOCRYSTS FROM VOLCANIC ROCKS ASSOCIATED WITH LARGE MO AND BE DEPOSITS IN THE WESTERN UNITED STATES

Rhyolitic melt inclusions (MIs) in quartz phenocrysts separated from volcanic rocks associated with the Henderson (HP) and Pine Grove (PG) porphyry Mo deposits and the Spor Mountain (SM) epithermal Be deposit were analyzed by EMP and LA-ICP-MS and the results compared to whole compositions to evaluate post eruption changes to elemental concentrations. Results on MIs from PG were compared to previous work to validate our analytical techniques. Sanidine phenocrysts were dated by 40Ar-39Ar methods to refine the age of the volcanic rocks and key them into each system. These deposits are products of hydrothermal systems associated with silicic magmatic centers that formed after the transition from contraction to extension and calcalkaline to bimodal volcanism in the western U.S.

Our date on Hideaway Park (HP) tuff of 27.6±0.2 Ma is the same as ⁴⁰Ar-³⁹Ar dates on the youngest intrusions at Henderson. At PG, our 24.57±0.11 Ma date on the air fall tuff is ~ 0.6 Ma older than a K-Ar date on the overlying pink tuff. At SM, our dates on the 21.73±0.19 Be tuff and a crosscutting 21.56 ±0.07 Ma plug are ~0.4 Ma older than a K-Ar date on the Be tuff.

Inclusions in quartz phenocrysts are comprised of crystals+vapor in HP tuff, glass+vapor or glass1+ glass2+ vapor in PG tuff, and glass+vapor in SM tuff and plug. MIs in HP tuff were rehomogenized to glass+vapor. Oxides, F, Cl, and S were determined by EMPA with a 1- to 3-micron beam at 15 kV, 20nA and counting times of 20-400 seconds. Li, Be, B, Na, MG, Al, Si, P, K, Ca, Sc, Ti, Mn, Fe, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Cs, Ba, La, Ce, Nd, Sm, Eu, Dy, Er, Yb, Ta, W, Re, Au, Hg, Tl, Pb, Bi, Th, U were analyzed by LA-ICP-MS with normalization to Al concentrations determined by EMPA .

With the exception of Li, Mo, Sn, Ba, W, and U, our average EMP and LA-ICP-MS data are within 30% of previous average EMP and SIMS data on MIs in the air fall unit at Pine Grove. Most elements showed no mobility due to devitrification of the whole rock, including Mo. Consequently, petrogenic plots (including REE’s) for MI’s and whole rocks are comparable.

The amount of metal in the deposits and metal in MIs suggest the following km3 of rhyolite were required to form each deposit: Henderson ~125-250, Pine Grove ~32-64, and Spor Mtn. ~0.1-0.2.