Scheelite trace element and isotope geochemistry reveals fluid pathways in orogenic and magmatic W + Au systems

Provinces and Case Studies

Dr James Scott

University Of Otago

Tracing pathways of mineralizing fluids can be difficult because common vein minerals carrying readily measurable isotopes, such as carbonates, are often affected by wall-rock interaction at the
deposit scale. Here, results are presented for an on-going systematic in-situ trace element and 87Sr/86Sr laser ablation study of scheelite (CaWO4) from W + Au-bearing orogenic (Macraes,
Glenorchy, Bendigo, Barewood, Waipori, Huxley) and magmatic (Batemans, Canaan) deposits in New Zealand. Trace element data require that scheelite, even within single grains, often
precipitates in stages from a progressively rare earth element-depleted fluid or from separate pulses of variably REE-depleted fluid. NZ orogenic and magmatic scheelite grains can be
distinguished by their Mo concentrations and LREE abundances. Since the Sr concentration of scheelite is high (usually > 1000 ppm), extensive fluid-rock reaction is required to affect the
87Sr/86Sr and meaning this isotope system can be useful for tracing scales of fluid flow. For example, scheelite 87Sr/86Sr from the orogenic Macraes, Barewood and Bendigo deposits overlap
with 87Sr/86Sr of the host Rakaia/Aspiring terranes, whereas scheelite 87Sr/86Sr from Glenorchy and Waipori is more radiogenic than the host Caples Terrane. Since the Caples Terrane was thrust over the Rakaia/Aspiring terranes, this Sr isotope discovery requires that the mineralising fluids precipitated in the Caples Terrane were derived from dehydration reactions within the underlying radiogenic terranes and/or that the fluids acquired their isotope signatures through fluid-rock reaction with these more radiogenic terranes during ascent. Either way, the W + Au-bearing fluids were evidently mobile over at least km-scale vertical distances and probably greater. Magmatic 143Nd/144Nd analyses were in progress at the time of writing but are expected to show a closer relationship to the immediate wallrock as a result of the less mobile nature of REE.

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