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Paragenetic Relationships and Timing of Orogenic Gold, Klondike District, Western Yukon

Part of the Tintina Gold Province, Western Yukon is host to many economical mineral deposits of various styles. Specifically, the Klondike is one of the world’s richest and most productive placer gold districts, with over 20 million ounces recovered. Efforts made to determine and analyze the lode sources of these placer deposits has lead to the discovery of a widespread episode of orogenic gold mineralization in the Middle to Late Jurassic within the Klondike and other nearby mineral districts in the Yukon Tanana terrane. A general understanding of the orogenic vein distribution, structural control, and correlation with regional cooling ages has been established. However, the occurrences remain ambiguous in terms of their detailed timing of veining/mineralization relative to deformation and genesis. Additionally, no knowledge exists on how gold was introduced into the veins and if U-Th-Pb geochronometers can reliably determine an absolute age of the mineralizing event(s). Through detailed petrographic and analytical analysis, this study will establish controls on vein/alteration paragenetic relationships and gold mineralization. Wherever possible, the absolute age(s) of mineralization will be determined by U-Th-Pb dating of hydrothermal monazite.

Project Information

Ore Deposit:
Commodity:
Location:
Klondike, Yukon
Project Status:
Researchers:
  • Craig Hart

Students:
  • Brodie Stroh

Start Date: 2016
End Date: 2019

The objectives for the study are:

  • Establish controls on vein/alteration paragenetic relationships and gold mineralization
  • Determine the composition of the fluid(s) responsible for vein formation and mineralization
  • Define the temporal framework for mineralization
  • Interpret the results to define similarities/differences at various scales, including district wide, within Western Yukon, regionally, and the global orogenic gold model

This study was part of the Yukon-Alaska Metallogeny Project, supervised by Dr. Murray Allan and Dr. Craig Hart.

Stroh, Brodie. 2019.  Resolving monazite growth mechanisms in orogenic gold settings : a study from the Klondike Gold District, Western Yukon.  MSc thesis, The University of British Columbia.

Abstract

Orogenic gold deposits form some of the world’s largest gold sources; however, the timing of gold mineralization is difficult to constrain. The ~20 Moz of placer gold mined in the Klondike was derived from orogenic, gold-bearing quartz veins where the exact timing of gold mineralization is unknown despite previous age data from hydrothermal micas and rutile. U-Th-Pb dating of hydrothermal monazite can be a robust alternative to constrain the timing of orogenic gold deposits and associated geological events that has not been attempted in the Klondike until now. In orogenic gold settings or fluid-affected metamorphic terranes in general, monazite can grow by metamorphic and hydrothermal processes. The objective of this research is to distinguish between these monazite growth processes to provide constraints on orogenic gold mineralization in the Klondike. Vein material, altered wall-rock, and unaltered host-rock samples were investigated at the Virgin, Mitchell-Sheba, and Lone Star gold occurrences. Detailed petrographic analyses were integrated with LA-ICP-MS Th-Pb monazite dates and ThO₂ concentrations to identify metamorphic and hydrothermal monazite. The age of metamorphic monazite in the Klondike is between 189 and 151 Ma, with possible discrete pulses at approximately 175 and 160 Ma. These ages overlap with existing ⁴⁰Ar/³⁹Ar mica cooling ages and represent retrograde monazite growth during crustal exhumation. The age of hydrothermal monazite is between 178 to 117 Ma, with possible discrete pulses at approximately 169, 148, and 128 Ma. These ages represent episodic vein formation and provide approximate constraints on gold mineralization. The methods developed can be used to identify metamorphic and hydrothermal monazite globally to provide robust time constraints on metamorphism, vein formation, and gold mineralization. Metamorphic monazite occurs regardless of proximity to veins, is typically adjacent to or intergrown with other metamorphic minerals, and has variable ThO₂ concentrations that depend on the host-rock composition and metamorphic grade. In contrast, hydrothermal monazite occurs in or adjacent to veins, is typically adjacent to or intergrown with hydrothermal minerals, and has a distinctly low ThO₂ concentration <2.00 wt.%

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