Date of Award

Spring 2019

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Christopher Gammons

First Advisor

Peter Knudsen

Second Advisor

Christopher Roos

Abstract

The McDonald gold deposit is located in Lewis and Clark County near Lincoln, Montana and is divided into two regions: East Butte and West Butte. Approximately 90% of gold mineralization at McDonald is hosted by quartz-adularia-altered lithic tuff. Low-grade, stratabound, disseminated mineralization within permeable volcanic tuff is the dominant style of mineralization at McDonald. Higher-grade mineralization at West Butte is interpreted to be controlled by subvertical, intersecting vein systems that occur along the 9800’ E-W fault. Preliminary gold estimates at West Butte indicated the possibility of a 500 koz gold orebody at an average grade of 7.2 grams per ton or 0.21 troy ounces per ton (opt). The purpose of this project is to develop a gold resource estimate and simulated model of West Butte at the McDonald gold deposit using Maptek Vulcan. A drillhole database was provided for this project, however, previous work indicated significant downhole contamination in reverse circulation (RC) drilling. Domains could not be defined based on logged geologic information, so a grade shell was used to define estimation domains. The grade shell was defined based on a 0.04 Au opt cutoff and only core drilling was considered. Resource estimation of gold at West Butte was conducted in three passes. At a cutoff of 0.06 Au opt, conservative estimates indicate the existence of a 2.75 Mt orebody at an average grade of 0.171 Au opt and a total of approximately 470 koz. Metal loss reports indicate potentially-substantial increases in total ounces if more core drilling takes place to replace contaminated RC drilling. Sequential Gaussian simulation (SGS) was conducted in conjunction with resource estimation of gold at West Butte. Visual discrepancies between the estimated model and the averages of the simulated models delineated regions of high uncertainty within West Butte. It is recommended that more core drilling takes place at West Butte. Angled core drilling should be conducted to better characterize subvertical orientations outlined by previous company geologists, and new exploration drilling should take place to the east and below the project area. It is likely that additional core drilling and replacement of contaminated RC drilling will result in a higher-tonnage orebody at West Butte through the expansion of the grade shell.

Comments

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geosciences

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