Document Type

Thesis

Date of Degree Completion

Spring 2023

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Hannah Shamloo

Second Committee Member

Angela Halfpenny

Third Committee Member

Christopher Mattinson

Fourth Committee Member

Carey Gazis

Abstract

The Swauk Mining District in Central Washington state is a historic mining community with active small scale mining dating back to the 1870’s. Gold has been mined from both lode and placer deposits. The placer deposits of Liberty area are unique due to the variety of gold textures, most notably are the nuggets of tangled masses of dendritic crystals and wire gold.

This research aims to determine the paragenic sequence that led to the formation of the lode and placer gold deposits using a combination of field work and laboratory analyses. Field work consisted of small-scale detailed mapping, recording of fault measurements, identifying cross cutting relationships of rock types and veins, and sampling of oriented in-situ and unoriented ex-situ samples. Samples of country rock, gold bearing vein material and gold nuggets were prepared for laboratory optical and scanning electron microscope imaging and chemical analysis via multiple techniques.

Chemical analysis has determined that gold is not present in the non-hydrothermally altered country rock of the district and has identified two gangue minerals associated with the gold mineralization which are laumontite and illite. Cross cutting relationships of the major mineralization events has also been established.

From this research it has been concluded that Liberty is best characterized as a low-sulfidation epithermal deposit with the veins currently exposed at the surface forming between 49 Ma and ~ 44 Ma at a depth of ~ 300 m below the paleo surface. Two events formed the in-situ deposits. The first mineralization event is characterized as silica dominated gold bearing hydrothermal fluid which travelled to the surface via high permeability zones caused by faulting which subsequently deposited free milling gold into quartz breccia veins with minor illite from wall rock alteration. The second event is characterized by a carbonate dominated fluid brecciating the initial quartz and gold mineralization veins. Subsequent dissolving of the gold and precipitation new large dendritic masses of gold occurred in structural traps in the carbonaceous shale. Weathering of this in-situ gold deposit and the geomorphology of the Swauk Mining District led to the formation of the placer deposits of the district.

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