Using zircon and apatite chemistry to fingerprint porphyry Cu – Mo ± Au mineralization in the Delamerian Orogen, South Australia
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Abstract To evaluate the fertility of porphyry mineralization in the Delamerian Orogen (South Australia), zircon and apatite from four prospects, including Anabama Hill, Netley Hill, Bendigo, and Colebatch, have been analyzed by LA-ICP-MS and electron microprobe. The zircon is characterized by heavy REEs enrichment relative to light REEs, high (Ce/Nd) N (1.3–45), and weak to moderate negative Eu/Eu* (0.2–0.78). The apatite has right-sloped REE patterns with variably negative to positive Eu anomalies. Low Mg (< 670 ppm) and Sr/Y ratios (< 5) in apatite likely illustrate fractional crystallization trends for the granitic melts in shallow crust. The Yb/Gb and Eu/Eu* in zircon reveal that intrusions at Anabama Hill, Netley Hill, and Bendigo underwent fractional crystallization controlled by amphibole (< 50–60%), garnet (< 15%), apatite (< 0.6%), and/or titanite (< 0.3%). These stocks have average f O 2 values reported relative to fayalite-magnetite-quartz buffer (ΔFMQ), from 0.7 ± 0.9 to 2.1 ± 0.4, ascribed to prolonged magmatic evolution or sulfur degassing during post-subduction processes. Our data imply that both Anabama and Bendigo complexes experienced prevalent (garnet-) amphibole crystallization from hydrous melts that have moderately high oxidation (ΔFMQ + 1 to + 3) and elevated sulfur-chlorine components (Anabama, 37 ± 9 to 134 ± 83 ppm S and 0.30 ± 0.24 to 0.64 ± 0.89 wt% Cl; Bendigo, 281 ± 178 to 909 ± 474 ppm S and 0.45 ± 0.47 to 3.01 ± 1.54 wt% Cl). These are crucial ingredients to form porphyry Cu–Mo ± Au ores with economic significance, which provides encouragement for mineral exploration in this orogen.Keywords:
Amphibole
Fractional crystallization (geology)
Titanite
Mineral redox buffer
but the results suggest that other mechanisms may also be involved. All four amphiboles exhibited systematically higher ferric-ferrous ratios with increasing 6, of equilibration. Equilibrium R values were achieved relatively rapidly and could be readily restored to onginal values by treatment at the appropriate buffer. In some cases, a metastable equilibrium of ferric-ferrous ratio was achieved before the amphibole decomposed to other Fe3*-bearing phases. Of the four amphiboles, grunerite is apparently the least able to accommodate Fe3* within its crystal structure and decomposes at relatively higher oxygen fugacities. The value of a sin B decreases systematically uniformly as the Fe3* content of tschermakitic hornblende, magnesio-hornblende, and riebeckite increases, reflecting increasing Fe3* in the octahedral cation sites. The variation in a sin B ofgrunerite is significantly less than for the other three amphiboles. The variation in b for the two hornblendes suggests that Fe3* produced by oxidation is not strongly ordered into the M(2) site.
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Mineral redox buffer
Hornblende
Formula unit
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Amphibole
Mineral redox buffer
Phenocryst
Liquidus
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Abstract Although the evolution of residual melts in magmatic systems controls their eruptability and ore-forming potential, their compositions are obscured in plutonic rocks by a protracted near-solidus evolution and the absence of interstitial glass. Here, we trace the evolution of residual melt compositions in rocks from the Strontian Intrusive Complex, Scotland, using the trace element chemistry of amphiboles, and titanites which are intergrown with amphibole rims. Laser ablation mapping reveals an abrupt change in certain trace elements in the amphibole rims, with sharp increases in Eu/Eu* and Sr/Y, and decreases in rare earth elements, Ta, Nb, and Ta/Nb ratios. Core-rim variations in these elements in titanite show the same variations as in amphibole, but are more gradual. By reconstructing the crystallisation sequence of the Strontian magmas using textural observations and thermobarometric estimates, we determine that amphibole cores crystallised prior to titanite saturation, but amphibole rims crystallised simultaneously with titanite. Using the trace element composition of the mineral phases and their modal abundance in the rock, with comparison to the whole-rock chemistry, we determine that titanite hosts the majority of the rare earth and high field strength element budget of the rocks. We therefore propose that the onset of titanite crystallisation had a profound effect on the trace element composition of late-stage residual melts at Strontian, which were inherited by the amphibole rims and subsequent titanites. This is supported by Rayleigh fractional crystallisation modelling, which demonstrates that the composition of amphibole rims cannot be explained without the influence of titanite. We therefore show that the saturation of trace element-rich phases in magmas represents a significant geochemical event in the petrogenesis of intermediate to silicic magmas. This has implications for provenance studies that attempt to reconstruct bulk rock compositions from mineral compositions, as the residual melts from which those minerals crystallise can be driven to significantly different compositions from the host magma by late-stage accessory phase crystallisation.
Titanite
Amphibole
Trace element
Incompatible element
Fractional crystallization (geology)
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Analyses of sodic amphiboles from a blueschist metamorphic terrain of constant grade in the Tavşanli area, Northwest Turkey, show that practically the whole range of sodic amphibole compositions is stable in the glaucophane-lawsonite facies. By analysing sodic amphiboles coexisting with magnetite and hematite, it has proved possible to plot for a constant pressure and temperature an oxygen fugacity isopleth. Through the analyses of sodic amphiboles and reference to the extended oxygen fugacity contours drawn on the Miyashiro compositional diagram, relative rock oxygen fugacities can be obtained. The oxygen fugacity isopleth corresponding to the magnetite/hematite buffer from two blueschist terrains with differing P-T regimes, namely Tavşanli in Turkey and Shuksan in Cascades, lies along the same loci on the amphibole diagram indicating that the intrinsic $$fO_{2}$$ slope of sodic amphiboles in the $$fO_{2}$$-T plane is approximately the same as the $$fO_{2}$$ slope of the magnetite/hematite buffer. Changes in the $$fO_{2}$$ during metamorphism may be recorded as zoning in sodic amphiboles, two examples of which are given.
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Mineral redox buffer
Blueschist
Doming
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Mineral redox buffer
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Oxidizing agent
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Amphibole
Mineral redox buffer
Fugacity
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Amphibole
Mineral redox buffer
Fugacity
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The stability of amphiboles on the join MgrSi.Orr(OH)r-FezSisorr(OH), has been hydrothermally investigated at 2 kbar fluid pressure as a function of oxygen fugacity and temperature. Atf;, defined by the MH buffer, the maximum extent of solution of Feend-member in amphibole is l4 and 22mole percent at725 and 630'C respectively; amphibole is unstable below 630'C, being replaced by the assemblage talc + quartz * magnetite + hematite. AtI), defined by the NNO buffer the extent of solid solution expands to 54,62, and 65 mole percent Fe end-member at 7250, 625', and 600C, respectively. Results obtained in this study have been combined with previously published data to produce a I-X section of the upper thermal stability of amphibole at2kbar andl,, defined by the FMQ buffer. Temperatures for the reaction: amphibole + pyroxene + quartz + vapor decrease from -765'C for the pure Mg end-member to -710'C for 62 mole percent Fe endmember. The breakdown reaction: amphibole - olivine + quartz + vapor, was observed for the more iron-rich amphiboles, and takes place at -675C for amphibole of 73 mole percent Fe end-member. Comparison of the experimental results to selected natural cases shows good agreement in maximum iron contents for the appropriate oxygen fugacity range. Estimates of temperatures of crystallization of metamorphic and igneous Fe-Mg amphiboles are also consistent with pnor reports.
Amphibole
Mineral redox buffer
Pyroxene
Fayalite
Fugacity
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Amphibole
Mineral redox buffer
Phenocryst
Liquidus
Fugacity
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