Tungsten skarn deposits from the Canadian Cordillera: paleogeographic and geochemical controls on ore distribution
Creators: Elongo, Vanessa M
Publisher: name: University of Alberta Library
Year: 2022

<p>Tungsten is a critical metal. Critical metals are strategic metals with associated economic, social, energy, geostrategic and environmental issues. As a result, the need to monitor and to secure local supplies of critical elements, including tungsten, has become a strategic priority for many countries. The Canadian Cordillera hosts one of the largest tungsten-producing provinces in North America. Tungsten mineralization in the Cordillera occurs mostly as skarns, which are calc-silicate rocks resulting from the hydrothermal alteration of carbonate rocks by fluids exsolved from an intrusive body. The most significant among the tungsten deposits known in the Canadian Cordillera as well as in the North American Cordillera broadly, include the Cantung and Mactung skarn deposits. Because of their major tungsten endowments, these deposits provide an ideal framework to study the parameters that govern tungsten mineralization.<br>
In this dissertation, (1) we evaluate the paragenesis and mineral compositions in tungsten deposits from the Canadian Cordillera to determine the parameters that control the distribution of high- grade tungsten mineralization locally. (2) We constrain the magmatic sources of ore fluids in these deposits and the effects of fluid-rock interaction on the mineralization process and geochemical signatures in the Canadian Cordillera. And (3) we further evaluate the geochemical and paleogeographic controls responsible for the irregular distribution of tungsten in the North American Cordillera.<br>
(1) The composition of minerals in tungsten skarn deposits from the Canadian Cordillera are indicative of reduced fluids, which is characteristic of high-grade tungsten deposits. We document that the main tungsten-bearing mineral in these deposits, scheelite (CaWO4), precipitated throughout the entire evolution of the hydrothermal alteration but peaked at different stages for different deposits. Also, the preferential distribution of scheelite at the deposit scale was controlled<br>
by parameters such as the permeability and porosity of the host rock. (2) Strontium isotopes and chemical variations in the host rocks reflect fluid-rock interaction processes. Although the geochemical variability of the host rock precludes quantification of chemical exchange between ore fluids and host rock, the distribution of the ore within preferential areas in the host rock suggests a lithologic control on the tungsten mineralization. In addition, strontium isotope data indicates that crust-derived fluids, likely from the granitoids adjacent to the deposits, are the main contributors to the ore fluids. (3) Geochemical fingerprinting using neodymium isotopes reveals that the ore fluids that precipitated tungsten in the North American Cordillera are specifically associated with source materials of Mesoarchean to Paleoproterozoic average age. We propose that tungsten enrichment in the North American Cordillera is the product of intense weathering of the supercontinents Columbia and Rodinia, followed by chemical transport and accumulation of tungsten-rich materials along the Ancient North American craton margin. Subsequent melting of these enriched sediments caused by orogenic heating produced reduced melts that scavenged tungsten and formed the major tungsten deposits found today in North America.</p>