Abstract:

Scheelite (CaWO4) is a common accessory mineral found in a variety of geologically diverse ore-deposit settings, including vein/stockwork, skarn, porphyry, epithermal and strata-bound. As part of the Geological Survey of Canada's (GSC) Targeted Geoscience Initiative (TGI 4) program, the project reported on here was developed to investigate the potential for discriminating scheelite originating from different ore-deposit types. The study investigated whether crystal-chemical features of scheelite, such as cathodoluminescence (CL), trace-element chemistry, and isotopic signature (O), could be used independently or together as deposit-type discriminators, thereby assessing the feasibility of using scheelite for provenance studies in regional till-sampling programs. Here we report on the geochemical data obtained using the LA ICP-MS method on scheelite to see if it could be used to geochemically fingerprint its environment of formation. The samples used come from the granite-related, world-class Sisson W-Mo porphyry-type deposit, NB, along with forty-one scheelite samples from a range of deposit types that constituted the suite used in the broader crystal-chemical study. The protocol used was twofold: (1) collect data using line traverses and integrate the data over intervals showing uniform chemistry; and (2) generate element maps for a select few scheelite grains which displayed complex zoning patterns revealed through CL imaging. Despite using an extensive element list (e.g., LILEs, alkalies, transition metals, HFSEs), only Mo, As and the REEs, which follow crystal growth patterns, showed significant levels of elemental enrichment (i.e., > 1.0 ppm). The correlation of As and Mo indicate only a small intra-deposit variance, but the large inter-deposit variation offers the potential to use this element pair to discriminate deposit types. The results for the REEs indicate: (1) a lack of apparent correlation between REEs and the type of CL observed despite previous suggestions to the contrary; (2) considerable variation in the ?REEs amongst the sample suite used; (3) most samples are dominated by a single chondrite-normalized (CN) pattern, but rarely a second pattern is present; although the type of CN REE patterns vary (e.g., convex MREE, LREE enrichment), there is a similarity among deposit types; and 5) both positive and negative Eu anomalies are observed. These initial results suggest that the minor and trace-element chemistry of scheelite along with CL imaging, may offer the potential to discriminate and identify deposit types based on its geochemical fingerprinting.