Award Date

5-1-2014

Degree Type

Thesis

Degree Name

Master of Science in Geoscience

Department

Geoscience

First Committee Member

Jean S. Cline

Second Committee Member

Adam C. Simon

Third Committee Member

Rodney V. Metcalf

Fourth Committee Member

Barbara Luke

Number of Pages

262

Abstract

The Estelle Property, located 170 km northwest of Anchorage, Alaska in the South Central Alaska Range, has been the focus of recent exploration activity for gold. Geological and geochemical investigations indicate that gold is spatially associated with felsic end-members of a series of zoned, Late Cretaceous intrusions that were emplaced into the Kahiltna terrane. A detailed study of outcrop and drill core samples was conducted to characterize host rock lithologies, intrusion compositions, and four main vein types including their mineralogy as well as their associated alteration mineral assemblages that are restricted to narrow selvages adjacent to veins. This study was also conducted to determine if the geology is consistent with a proposed reduced intrusion-related gold system (RIRGS) model.

Examination of core samples indicated felsic lithologies associated with gold vary from a biotite granite/granodiorite at the Oxide Prospect to a quartz monzonite at the Shoeshine and Shadow Prospects to a quartz monzodiorite at the Discovery Prospect. Examination of vein assemblages and cross-cutting relationships, using transmitted and reflected light petrography as well as scanning electron microscope and electron probe microanalyses, identified minerals associated with gold mineralization and determined the relative timing of mineral and vein formation. Early Type 1 quartz veins were followed by Type 2 quartz-sulfide-Au-feldspar veins with albite-sericite alteration selvages and locally sheeted Type 3 quartz-sulfide-Au-chlorite veins, which appear to host most of the gold at the Estelle Property, with chlorite-sericite alteration selvages; Type 4 calcite veins formed last. The most significant gold mineralization occurs as inclusions of gold, bismuth, and tellurium within arsenopyrite that are inferred to be exsolution products from the replacement of loellingite and pyrrhotite by arsenopyrite. The variability of ratios between Bi and Te ± Au within inclusions is consistent with a Bi-Te melt scavenger model of formation, which suggests that a Bi-Te melt phase can extract up to 43 wt. % Au from a hydrothermal fluid.

Fluid inclusions were analyzed to identify pressure-temperature-chemistry conditions of the hydrothermal system at the time of vein formation at Estelle. Generally, three types of fluid inclusion assemblages were identified in Type 2 and Type 3 veins from the Discovery and Oxide prospects respectively. Class 1 assemblages contain irregular to sub-rounded two-phase, liquid-vapor inclusions with < 10 vol% vapor, and the inclusions have a diameter of 1-10 um. Class 2 fluid inclusion assemblages contain irregular to sub-rounded ≥ three-phase, liquid-vapor-halite inclusions with 1-10 vol% vapor, and the inclusions have a diameter of 2-20 μm. Class 3 fluid inclusion assemblages are 5-10 um diameter two-phase, liquid-vapor inclusions with 60-85 vol% vapor. The majority of inclusions from Type 3 veins in the Oxide prospect homogenized to liquid between about 225 and 350°C and had salinities of 5 to 30 wt.% NaCl equiv. The majority of inclusions from Type 2 veins in the Discovery prospect homogenized to liquid between about 275 and 400°C and had salinities of 15 to 35 wt.% NaCl equiv. The presence of coexisting vapor-rich and brine inclusions in some Type 2 veins from the Discovery Prospect indicates trapping conditions of some fluids at approximately 250 bars and 400°C.

Study results suggest that the 1) association of Au with Bi-Te, 2) association of Au with sheeted veins containing arsenopyrite, and 3) restriction of alteration to narrow selvages adjacent to veins at the Estelle Property are consistent with the genetic deposit model for RIRGS.

Keywords

Alaska--Alaska Range; Bismuth mines and mining; Gold mines and mining; Gold ores—Geology; Prospecting

Disciplines

Geology

Language

English


Included in

Geology Commons

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