Paragenesis and geochemistry of scheelite deposits at Glenorchy, West Otago

Abstract

Scheelite (CaWO4) is a major tungsten ore mineral that has commercial and industrial uses. Widespread scheelite deposits occur as mineralised quartz veins filling extensional faults in the Glenorchy region, NW Otago. Historic mining occurred from the 1880’s in several locations, including the Glenorchy State Mine, which exploited the Glenorchy lode on Mt Judah, and the Bonnie Jean Mine. Scheelite-bearing samples were collected from the waste piles of these two lodes. The mineralised veins are composed of large blocky white quartz with sporadic patches of scheelite that are ≤ 30 mm and fluoresce bright blue under short wave UV. Minor arsenopyrite and pyrite are also present in veins and are associated with inclusions of host schist. Calcite occurs in several samples where it replaces scheelite. Electron backscatter diffraction reveals that the scheelite masses are composed of multiple smaller grains with different crystallographic orientations. Cathodoluminescence imaging shows variations in intensities between the smaller grains. These variations correlate to changes in the rare earth elements (REE). Variations in normalised REE represent three types of scheelite, broadly defined by varying MREE. These variations are interpreted to represent MREE fractionation within the mineralising fluid. 87Sr/86Sr in-situ analyses, measured by laser ablation inductively coupled plasma mass spectrometry, reveal that the scheelite is more radiogenic (~0.7070) than the immediate Caples Terrane host rocks (whole rock ~0.7050, age corrected to 135 Ma), which indicates that the fluid source must be more radiogenic. The most likely source is the underlying Aspiring Terrane; greyschists from the Aspiring Terrane have radiogenic isotopic signatures of ~0.7074-0.7086 (age corrected to 135 Ma). The Glenorchy scheelite samples however, appear to display some mixing trend between Aspiring Terrane-sourced mineralising fluid with the Caples Terrane. The volume of fluid that precipitated scheelite would have had to have been large to offset overprinting of the fluid 87Sr/86Sr by the isotopic signature of the Caples Terrane. The geochemical and isotopic compositions of minerals in the Glenorchy W systems therefore require large scale redistribution of hydrothermal fluids from depth. Calcite REE are dependent on fluid-mineral ratios. Where these are high, calcite reflects the fluid chemistry, and where they are low, calcite incorporates the REE signature of scheelite. Measured 87Sr/86Sr of calcite reflects the textural association. Calcite replacing scheelite inherits the 87Sr/86Sr of the scheelite, while calcite filling fractures in scheelite retains the fluid 87Sr/86Sr. The later stage Ca- bearing fluid that deposited calcite inherited an 87Sr/86Sr signature from the scheelite-bearing veins. The scheelite isotopic signatures of the Bendigo and Macraes deposits in East Otago indicate that the mineralised fluids for these deposits have been sourced from the Rakaia Terrane, which hosts both of these deposits. However, Waipori scheelite, which is hosted in the Caples Terrane, has an 87Sr/86Sr that is more radiogenic than the Caples Terrane. Mineralising fluids for this system were sourced from the Rakaia Terrane, rather than the Caples itself, similar to Glenorchy. Scheelite therefore is a powerful tracer of fluid source in orogenic deposits.