Geochemical characterisation of scheelite from New Zealand

• Main Author: Palmer, Marshall Croft
• Other Authors: Scott, James Morfey
• Format: Thesis
• Language: English
• Published: University of Otago 2021
• Subjects: Scheelite; LA-ICPMS; New Zealand; Nd + Sr isotopes; REE; trace elements; Davidson; Greenland; Wall Rocks
• Online Access: http://hdl.handle.net/10523/12254

The trace element and isotopic compositions of scheelite CaWO4 from a variety of deposits in New Zealand were measured using LA-ICPMS and LASS-ICPMS. As part of this, a new method for determining in-situ Sm-Nd isotopic compositions of scheelite using a beam diameter of 193 μm was developed and used to broadly date the timing of scheelite mineralisation at Barrytown and gain insights into fluid-rock interaction at Batemans Creek and Canaan Downs. This method used Tory Hill Titanite as a primary calibration standard and was able to replicate the Sm-Nd isotope compositions of Tory Hill Apatite and two well characterised scheelite crystals (OU15013 and OU13940) showing that matrix matching was not an important source of variation in the results. The scheelite crystals were used as secondary, in-house standards as reconnaissance work found small portions of some fragments with significant isotopic variations. Metamorphic scheelite from W + Au deposits within the Otago Schist did not contain enough Sm or Nd for in-situ isotope analyses via LA-ICPMS. At Barrytown the trace element, REE and Sm-Nd isotopic compositions of scheelite were determined using LASS-ICPM. Variations in trace element and REE compositions corresponded to primary growth textures revealed by CL imaging. With an analytical resolution of 193 μm, individual grains showed chondrite normalised REE compositions that mostly ranged from n- to u-shaped and corresponded to variations in Sm-Nd isotope compositions. The Sm-Nd isotope compositions plotted as linear arrays on isochron diagrams and provided robust regression ages that overlap with the emplacement of the Barrytown Granite pluton. Due to geological and analytical limitations, multiple grains across an outcrop were required to construct statistically reasonable isochrons; however, multiple analyses within a single grain of scheelite from the Archean Young Davidson Gold mine gave an isochron age that is consistent with the known age constrains of scheelite mineralisation in the deposit. At Canaan Downs and Batemans Creek scheelite trace element, REE and Sr, Sm, Nd isotopic compositions were determined by LA-ICPMS. The variable initial isotopic compositions from the deposit to the grain scales in these two deposits meant that dating the timing of scheelite mineralisation via the Sm-Nd isochron method was not possible. Instead, insights into the sources of components in the mineralising fluids were established. At Canaan Downs, the compositions are generally considered to represent variable fluid compositions due to interaction of magmatic-derived hydrothermal fluids with blocks of marble that are mapped within the contacts of the granite. At Batemans Creek, the elemental and isotopic compositions of scheelite varied according to the host rock compositions and the textural context (disseminated versus vein) of the scheelite grains. These variations were considered to arise due to variable contribution of components from compositionally distinct wall rocks or preferential breakdown of minerals by the hydrothermal fluids. For orogenic scheelite, trace element, REE and Sr isotope compositions were determined by LA-ICPMS and found to be distinctive between Type 1 and Type 2 mineralisation styles in the Otago Schist. Scheelite within Type 1 mineralisation has heterogenous REE compositions and variable Sr isotope compositions, from the deposit to the grain scale. These highly variable compositions are proposed to reflect localised sourcing of components by the mineralising fluids, resulting in a greater sensitivity of scheelite compositions to fine-scale variations in host rock compositions. On the other hand, Type 2 deposits all have a set of n- to u-shaped REE patterns and more homogenous Sr isotope compositions at the deposit scale, which are proposed to reflect the regional source of the fluids, the larger size of mineralised structures, more extensive wall-rock interaction and greater availability of transporting ligands that promotes mixing and homogenisation of fluid inputs. Finally, fluid inclusions in vein-quartz from Boanerges Peak and magmatic scheelite from Canaan Downs, Barrytown and Batemans Creek were investigated. Inclusions from Boanerges Peak had very similar properties to vein-quartz inclusions from Lake Hawea and reflect similar styles of mineralisation at these two localities. The similarities were used to loosely infer the conditions of mineralisation at 350 – 400 oC and 4.1 – 6.0 kbar. Inclusions in scheelite from the magmatic deposits had salinities between 3.5 to 5.6 wt% NaCl equivalent and Batemans Creek had around 10 mol% CO2 in inclusions from Dunphy Granite-hosted and Greenland Group-hosted veins. These inclusion compositions are broadly consistent with fluid compositions of magmatic W-Sn deposits from around the world. Homogenisation and decrepitation temperatures were generally between 200 and 300 oC and possibly represent minimum temperatures of hydrothermal fluids that moved through these deposits.