Source of scheelite in the turbidite-hosted orogenic au deposits of Otago, New Zealand: an integrated metamorphic source model explaining the presence or absence of scheelite in turbidite-hosted orogenic au deposits
Creators: name: Cave, BJ
Publisher: name: University Of Tasmania
Year: 2023

description: The Otago Schist in the South Island of New Zealand is a Mesozoic-aged accretionary belt comprised predominately of metaturbiditic graywackes and argillites, with subordinate metabasic horizons and ultramafic slices. A complete crustal section is exposed from prehnite-pumpellyite facies on the flanks to upper greenschist facies (biotite-garnet-albite) in the center of the belt. Quartz veins are abundant throughout the schist, some of which are enriched in a variety of metals including W, Au, As, Ag, Hg, and Sb. Historically, W (scheelite; CaWO4) has been exploited from some of these veins, with in excess of 3,000 tons of scheelite concentrate having been produced. Previous research on metal sources for these deposits has defined the sources for most of these metals. Sources are well-constrained for Au, As, Ag, Hg, and Sb, with the metamorphic recrystallization of sedimentary pyrite (to metamorphic pyrrhotite) having been shown to mobilize these metals. The source for W, however, has not been constrained. The primary aim of this thesis is to investigate the source of W (scheelite) in the turbidite-hosted orogenic Au deposits of the Otago Schist. Major lithologies have been sampled and analyzed from the lowest metamorphic grade to the highest metamorphic grade observed in the Otago Schist. Combined laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) traverses, images, and spot analyzes, show that detrital rutile is the most important host mineral for W in the subgreenschist facies rocks, and that its prograde metamorphic recrystallization to titanite releases significant amounts of W. Mass balance calculations indicate that this mineral reaction has potential to mobilize over 1,100 tons of W from every 1 km3 (0.41 g of W per ton of rock) of subgreenschist facies rock metamorphosed to greenschist facies conditions. Scheelite development closely follows the progression of this W-liberating reaction, with early formation of scheelite micrograins within the fabric of the rock evolving to locally and regionally sourced scheelite-bearing veins. Scheelite is present in the Otago Schist in two major vein forms, that is, synmetamorphic and late-metamorphic veins. Syn-metamorphic veins sampled at Fiddlers Flat and Lake H¿ívÖwea show distinct differences in chemical composition to scheelite from late metamorphic veins at the Macraes Mine, with the Macraes scheelite being enriched in REEs, Y, and Sr. This enrichment in the Macraes scheelite is likely sourced from the breakdown of the Ca-silicate minerals epidote and titanite by the ore forming fluid. Tungsten enrichments (in the form of scheelite) are common in Archean to Cenozoic aged turbidite-hosted orogenic Au deposits, worldwide. The source model developed for the W-bearing turbidite-hosted orogenic deposits of the Otago Schist was evaluated through investigating the source and mobilization mechanism in two additional turbidite-hosted orogenic Au provinces, one containing orogenic Au mineralization with associated subordinate W (Meguma Terrane, Canada) and the other containing orogenic Au mineralization without associated W (Bendigo-Ballarat Terrane, Australia). Similar to the Otago Schist, in both of these terranes, detrital rutile is the most important host mineral for W in the lowest metamorphic grade rocks, and its prograde metamorphic recrystallization (to ilmenite) releases significant amounts of W (1.9 g and 0.18 g of W per ton of rock, Meguma Terrane and Bendigo-Ballarat Terrane, respectively). This release of W in the Meguma Terrane is likely the source of W in the orogenic Au deposits. The lack of W in the orogenic Au deposits of the Bendigo-Ballarat Terrane possibly reflects the Au in these deposits as being sourced from lower greenschist facies metasediments (Castlemaine metasediments), and further potentially precludes previous models that have suggested source rocks for Au in these deposits as being either Castlemaine metasediments at the upper greenschistamphibolite transition or underlying Cambrian metavolcano-sedimentary rocks. However, the lack of scheelite in the Bendigo-Ballarat Terrane could also result from other factors, such as variations in the conditions of formation of the mineralized veins (P, T, pH, redox, host-rock composition) that may not favor the transport and/or precipitation of W. These factors were unable to be resolved in this study. The results presented within this thesis support recent models for turbidite-hosted orogenic Au mineralization, whereby prograde metamorphic recrystallization of diagenetic or detrital metal-rich mineral phases [pyrite to pyrrhotite, Au, As, Ag, Hg, and Sb; rutile to titanite or ilmenite (this study), W] can release significant amounts of these metals into the concurrently developing metamorphic fluids that can be subsequently focused into regional structures and form orogenic Au ¬¨¬± W deposits.