The Shasta Ag-Au deposit, located in the Toodoggone River area of north-central British Columbia, has many characteristics of the low sulphidation, adularia-sericite class of epithermal deposit. It consists of quartz-calcite stockworks and breccias, associated with potassic (K-feldspar + sericite) alteration of dacitic tuff host rocks. Ore grade mineralization occurs mainly in Bonanza-style breccias which host acanthite, electrum and native silver, associated with pyrite, sphalerite, chalcopyrite, galena, chlorite and hematite. However, Shasta lacks vein-hosted adularia, and the highest grades of ore are associated with calcite-rich breccias, which are features atypical of the adularia-sericite class. / Hydrothermal breccias, chalcedonic quartz and abundant vapour rich fluid inclusions are strong evidence for boiling of the vein fluid. Fluid inclusion evidence indicates that ore was deposited between 280$ sp circ$ and 225$ sp circ$C at a maximum depth of 750 m, from a relatively dilute hydrothermal fluid. Oxygen and hydrogen isotopic data suggest that the fluid was meteoric in origin. Initial log fO$ sb2$ - pH conditions were estimated to have been between $-$33 to $-$31, and 4.25 to 6, respectively, and to have increased during mineralization. / A model is proposed in which quartz, and subsequently calcite and silver-gold minerals, were deposited due to boiling of the fluid. The consequent decrease in temperature and increase in pH controlled deposition of quartz and calcite, respectively. Precipitation of silver and gold was caused by a combination of increased pH and oxygen fugacity, and a decrease in H$ sb2$S fugacity.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.69669 |
| Date | January 1993 |
| Creators | Thiersch, Peter C. |
| Contributors | Jones, Williams (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Master of Science (Department of Earth and Planetary Sciences.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001357777, proquestno: AAIMM91766, Theses scanned by UMI/ProQuest. |
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