Scheelite (CaWO4) is a hydrothermal mineral that occurs in numerous deposit types, which include skarn, greisen, veins, and porphyry deposits, and in orogenic gold deposits as well. The scheelite crystal lattice hosts several trace elements (REE, Mo, Sr, Y, Nb, Na), which provide valuable information about physico-chemical conditions, pathway and source of the hydrothermal ore fluids. Recently, many studies have had focused on understanding the factors that control the incorporation of trace elements into scheelite lattice, and thus using its composition as a discriminant between different types of deposits, focused especially on tungsten skarn-type, and orogenic gold deposits. In this study, we compare the composition of scheelite derived from reduced intrusion-related gold and tungsten skarn-type deposits from the Tombstone-Tungsten Belt (TTB). The goal is to use scheelite as an indicator mineral, not only for tungsten deposits, but also for intrusion-related gold deposits.
The TTB is located at the Northern Canadian Cordillera, between Yukon and Northwest Territories border. Three mid-Cretaceous plutonic suites, from the youngest to the oldest, form the TTB: Tombstone, Mayo and Tungsten. Each suite has distinctive metallogeny that relates to the source and redox conditions of the magmas from which they crystallized. Reduced intrusion-related gold and the tungsten skarn deposits are associated with Mayo and Tungsten suites, respectively. Mayo suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, contains felsic and mafic components, and moderately reduced. The Tungsten suite peraluminous, felsic, highly fractionated and reduced ilmenite dominant. Both suites share a similar magma oxidation state, which is an important factor in the formation of ore deposits, as well as a control on the concentrations of trace elements in scheelite.
Scheelite from three reduced intrusion-related gold and from three skarn-type deposits were analysed by EPMA and LA-ICP-MS. Scheelite from reduced intrusion-related gold deposits is constrained to quartz veins, whereas scheelite from tungsten skarn occurs disseminated in different alteration skarn facies (garnet, pyroxene and amphibole/biotite). Cathodoluminescence images reveal that scheelite from reduced intrusion-related gold deposits is homogeneous, whereas scheelite from skarn deposits is variably zoned. Preliminary LA-ICP-MS results reveal that scheelite from reduced intrusion-related gold deposits contains more than 100 ppm of Sr, 1200 ppm Mo in average, and 600 ppm of REE. By contrast, scheelite from tungsten skarn contain less than 100 ppm Sr, and wide range of Mo and REE contents (ranging from 70 to 5000 ppm, and from 50 to 4000 ppm, respectively). Chondrite-normalized REE patterns of scheelite from gold deposits display a negative slope, with positive and negative europium anomalies. However, patterns for scheelite from skarn deposits have three different shapes: i) negative slope, ii) flat, and iii) bell shape, all with mainly negative europium anomalies. Currently, we are working with these data and statistical methods in order to discriminate scheelite from both deposits types.