[1] Magma mixing plays a prominent role in the origins of intermediate magmas in subduction zones. However, the conditions and time scales of magma mixing and how these are linked to subsequent eruption are unclear. Mount Tauhara is the largest dacitic volcanic complex in the Taupo Volcanic Zone, New Zealand. Dacites from Tauhara volcano have a complex petrography (Qtz 1 Plag 1 Amph 1 OPx 1 CPx 1 Oxi 6 Oli) that can only have been produced by magma mixing and offer an ideal opportunity to investigate the processes and time scales involved in assembling dacite magmas in a continental subduction zone. Here, we present whole-rock and mineral-specific major and trace element and isotopic data for the Tauhara dacites in order to identify the magma mixing end-members, constrain the physical conditions of mixing, and estimate the time scales and relationships between magma mixing, ascent, and eruption. These data reveal that four separate mixing events between crystal-rich rhyolites (77-80 wt % SiO 2 ; 40 ppm Sr) and crystal-poor mafic magmas of basaltic (48 wt % SiO 2 ; 1340 ppm Sr) to andesitic (55-59 wt % SiO 2 ; 490-580 ppm Sr) composition occurred to produce the Tauhara dacites. Mixing took place in well-stirred magma chambers located at midcrustal depths (8-13 km) at temperatures from 840 to 900 C. The time scales of magma mixing obtained from Ti diffusion in quartz appear to be largely dependent on the temperature and viscosity contrast between the end-members as andesite and rhyolite magma mixed on time scales of 2-7 months, whereas basalt and rhyolite magmas mixed on time scales of 1-2 years. The short magma mixing time scales, combined with the physical properties (e.g., viscosity and density) of the mixed dacite magmas, as compared with those of the end-member magmas, facilitated the ascent and eruption of dacite magmas at Tauhara volcano.
The amphibolite-facies, Au-mineralized mafic rocks at the Plutonic Gold Mine are intruded by a suite of dolerite dykes of unknown age. The zones between these intrusive units often host significant Au mineralization. It is unclear whether this enrichment in Au mineralization is a function of the intrusion of the dolerites themselves or the influence of pre-existing structures (e.g. faults or shears). Geochemical characterization of the different microcrystalline dolerite units is important to the understanding of the structural architecture of the deposit and to the possible relationship of the dolerites to Au mineralization. The collection of a large geochemical dataset ( n = 497) from the dolerite dykes from across the deposit using portable X-ray fluorescence technology allows us to break them into four distinct geochemical groupings. Thus we can define their geometries with greater confidence than was possible using lithology alone. Traverses across individual dolerite dykes indicate that the chill margins are the most geochemically homogenous and most likely to represent the chemistry of the source magma. Plots of Ti v. Zr combined with principal component analysis (PCA) define four geochemically distinct suites of dolerites. By applying this understanding to dolerites in a small area of the deposit, a new interpretation was generated whereby significant amounts of rock that were previously modelled as being dolerite were reclassified as potential host-rock, thus increasing the potential for Au in this area.
Since 2009 all underground face samples and diamond-drill core samples at the Plutonic Gold Mine (Plutonic), Marymia Inlier, Western Australia have been analysed by portable X-ray fluorescence (pXRF) following a systematic approach. This method is rapid and cost-effective and provides analyses of a large suite of chemical elements which can be used to characterize lithology and alteration. The delivery of a comprehensive workflow for sample preparation, analysis, results correction, and rapid processing enabled a quantum leap in the way mine geologists use geochemistry in modelling the ore body. Interpretation of the mine-site dataset of over 200 000 multi-element analyses has resulted in significant improvements in the understanding of the Plutonic deposit. In this contribution, we review how our understanding of Plutonic has been significantly improved through the use of pXRF geochemical analyses. Incorporation of pXRF data into routine geological modelling at Plutonic has resulted in improved confidence in the models of the ore bodies themselves and late-stage dolerite intrusives. It has allowed better management of milling processes through the development of metallurgical proxies and for significant insights into the role of stratigraphy in controlling the location of gold mineralization. We highlight the potential that a systematic approach to collecting pXRF data can have in a mining environment. These same techniques could be adapted and used in other mine and/or exploration settings.
<p>Mount Tauhara is the largest dacitic volcanic complex of onshore New Zealand and comprises seven subaerial domes and associated lava and pyroclastic flows, with a total exposed volume of ca. 1 km3. The dacites have a complex petrography including quartz, plagioclase, amphibole, orthopyroxene, clinopyroxene, olivine and Fe‐Ti oxides and offer an excellent opportunity to investigate the processes and timescales involved in assembling dacitic magma bodies in a continental subduction zone with in situ and mineral specific analytical techniques. Whole rock major and trace element data and Pb isotopes ratios define linear relationships indicating that the dacites are generated by mixing of silicic and mafic magmas. Two groups of samples define separate mixing trends between four endmembers on the basis of La/Yb ratios, 87Sr/86Sr ratios and Sr contents. The older Western and Central Domes have low 87Sr/86Sr (0.7042‐0.7046) and high LREE/HREE (LaN/YbN = 8.0‐11.5) and Sr (380‐650 ppm) compared to the younger Hipaua, Trig M, Breached and Main Domes, which have higher 87Sr/86Sr (0.7047‐0.7052) and lower LREE/HREE (LaN/YbN = 6.5‐7.5) and Sr (180‐400 ppm). In situ mineral major and trace element chemistry of mineral phases, as well as Sr and Pb isotope ratios of mineral separates have been used to: (i) fingerprint the origin of each crystal phase; (ii) constrain the chemistry of the four endmembers involved in the mixing events and; (iii) estimate the timing of mixing relative to eruption and the ascent rate of the dacitic magmas. The presence of quartz and analyses of quartz‐hosted melt inclusions are used to fingerprint the chemistry of the silicic endmembers, which is a rhyolitic melt with a major element chemistry similar to that of either the Whakamaru Group Ignimbrite melts (Western, Central and Trig M Domes) or intermediate between that of the Whakamaru and the Oruanui Ignimbrite melts (Hipaua, Breached and Main Domes). Similarly, Ba‐Sr concentrations and Sr isotopic signatures of plagioclase show that this phenocryst phase also predominantly crystallized from the rhyolitic melt. Variations in the Mg# and trace element chemistry of clinopyroxenes suggest they were formed both in the mixed dacitic melts and in a mafic endmember. The chemistry of the mafic endmembers have been traced using a combination of back‐calculated Sr melt concentrations from clinopyroxene with the highest Mg# in each sample group, and the linear trends between whole rock SiO2 content and most elements. These results indicate that dacites erupted from the Western and Central Dome were generated by the mixing of a high alumina basalt and a rhyolitic melt and Trig M Dome dacites were generated by the mixing of an andesite with a rhyolitic melt. Magmas erupted from Hipaua, Breached and Main Domes were also produced by the mixing of an andesitic melt and a rhyolitic body with a composition intermediate between that of the Whakamaru and the Oruanui melt bodies. Trace element data and 87Sr/86Sr ratios of amphibole demonstrate that it crystallized from the mixed dacitic melt. Thermobarometric conditions obtained from amphibole indicate that the magma mixing event that produced the dacites occurred within a magma chamber located at ca. 9 km depth and ca. 900°C with the exception of Trig M Dome which occurred deeper at 13 km and 950°C. Diffusion profiles of Ti in quartz and Fe‐Mg in clinopyroxene indicate the magma mixing events occurred < 6 months prior to eruption. Amphibole reaction rims show the magma to have ascended over 2‐3 weeks for each dome, with the exception of Main Dome where reaction rims were not present in the amphibole, suggesting the ascent rate was faster than 0.2 m/s (< 6 hours).</p>
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