IDENTIFICATION OF CLAY WITHIN A BRECCIA PIPE FROM THE HILLTOP AU DEPOSIT, LANDER CO., NEVADA

This study focused on identifying the clay mineral(s) within the matrix of an unmineralized breccia pipe that mantles a 39.3 ± 0.4 Ma granitic stock located on the southeastern flank of the Hilltop Au deposit, Lander County, Nevada. Identification of the clay minerals may help delineate the conditions under which this breccia pipe formed, and the presence of a K-bearing clay phase may elucidate the temporal relationship between breccia pipe formation and the underlying granitic stock.

The Hilltop gold deposit, located 11 km southeast of Battle Mountain, Lander County, Nevada, is estimated to contain nearly 70 million short tons of ore averaging 0.028 ounces of gold per ton (opt) totaling 1.943 million ounces. Gold ore is hosted with silica, pyrite, and arsenopyrite within chert, argillite, siltstone, and quartzite of the Ordovician Valmy Formation (part of the upper plate sequence of the Roberts Mountains allochthon). Several genetically-different breccia pipes occur at Hilltop, and intermediate-felsic Tertiary intrusive rocks are also present.

The breccia pipe studied here consists of variably-sized, sub-angular clasts of bleached and recrystallized Valmy Formation quartzite, argillite, siltstone, argillite, and Tertiary intrusive igneous rock (quartz feldspar porphyry) in a matrix of rock flour + blue-green-colored clay. The clay fills isolated ≤ 3 cm vugs and cavities within the matrix.

Four separate clay samples were recovered from a cut piece of breccia pipe material. Each clay sample was gouged out of the rock sample, finely-ground, and analyzed via XRD both in an air-dried state and after ethylene glycol saturation. Preliminary XRD analysis shows that each sample is identical to the others and is comprised mostly of the same, non-expandable 7Å phase, probably kaolinite or dickite. Other XRD analyses are forthcoming, as is SEM analysis to qualitatively verify elemental composition and the presence/absence of a K-bearing phase for subsequent geochronological analysis.