New metamorphic constraints on the Nova-Bollinger Ni–Cu deposit, Fraser Zone, Western Australia

Metamorphic investigations were conducted using mineral chemistry, phase equilibria modelling and conventional thermobarometry on four metamorphic country rock samples from the Nova-Bollinger Ni–Cu deposit, Western Australia. New <i>P</i>–<i>T</i> constraints obtained from phase equilibria modelling of garnet-bearing granulites and cordierite–sillimanite-bearing metasedimentary rocks indicate that the Nova-Bollinger deposit formed at mid-crustal depths (0.75–0.76 GPa and 880–920 °C), along high thermal gradients around 1200 °C/GPa. Peak metamorphic conditions are consistent with those obtained from the Nova-Bollinger intrusive rocks, and with metagabbros from the southern Fraser Zone, but are slightly elevated relative to metapelites examined in the southern Fraser Zone. The peak metamorphic conditions at Nova-Bollinger reflect the combined effects of regional-scale high<i>-T</i> conditions during orogenesis superimposed by contact metamorphism in a thermal aureole adjacent to the Nova-Bollinger mafic–ultramafic magmas. Thermal metamorphism may have been further enhanced by local heat transfer from intruding sulfide liquid, which pervasively infiltrated country rocks on scales of tens to hundreds of metres beneath the orebodies. This additional heat source may be locally significant, despite being volumetrically limited. Retrograde <i>P</i>–<i>T</i> conditions were determined by garnet–biotite conventional thermobarometry of garnet-core–matrix biotite pairs (0.29–0.45 GPa and 550–640 °C). These <i>P</i>–<i>T</i> conditions suggest that the Nova-Bollinger country rocks cooled along a near-isobaric cooling path and reflect the effects of thermal decay following emplacement of voluminous magmas into the Fraser Zone during Stage I Albany–Fraser Orogeny. We present the first quantitative P–T constraints for host rocks of the intrusion-hosted Nova-Bollinger Ni–Cu deposit in the Fraser Zone, Western Australia. Peak metamorphism at Nova-Bollinger occurred at 0.75–0.76 GPa and 880–920 °C (1200 °C/GPa). GBAQ-GB thermobarometry records re-equilibration conditions at 0.29–0.45 GPa and 550–640 °C, indicating near-isobaric retrograde cooling. Infiltration of sulfide liquids into host country rocks may provide a volumetrically low but significant contribution of heat to high-T metamorphism during formation of orthomagmatic deposits in mid-crustal terranes.

针对西澳大利亚诺沃-博林格（Nova-Bollinger）镍铜（Ni–Cu）矿床的4件变质围岩样品，本研究采用矿物化学、相平衡模拟（phase equilibria modelling）及常规温压计（conventional thermobarometry）开展了变质作用研究。通过对含石榴子石麻粒岩（garnet-bearing granulites）及含堇青石-矽线石变沉积岩（cordierite–sillimanite-bearing metasedimentary rocks）开展相平衡模拟，获得了全新的温压（P–T）约束结果，表明诺沃-博林格矿床形成于中地壳深度（0.75~0.76 GPa、880~920 ℃），且处于约1200 ℃/GPa的高热梯度环境中。峰期变质条件与诺沃-博林格侵入岩（intrusive rocks）、弗雷泽带（Fraser Zone）南部变辉长岩（metagabbros）的峰期变质条件一致，但相较于弗雷泽带南部研究的变泥质岩（metapelites），其峰期变质条件略高。诺沃-博林格的峰期变质条件，反映了造山作用（orogenesis）期间区域尺度的高温环境与诺沃-博林格镁铁质-超镁铁质岩浆（mafic–ultramafic magmas）相邻接触带热晕（thermal aureole）中的接触变质作用（contact metamorphism）的联合效应。侵入的硫化物熔体（sulfide liquid）可通过局部热传递进一步强化热变质作用，这类熔体可在矿体下方数十至数百米尺度上广泛渗透至围岩中。尽管该热源的体积占比较小，但其局部热效应不容忽视。通过石榴子石核-基质黑云母对的石榴子石-黑云母常规温压计（garnet–biotite conventional thermobarometry）分析，限定了退变质温压条件（retrograde P–T conditions）为0.29~0.45 GPa、550~640 ℃。该温压条件表明诺沃-博林格围岩沿近等压冷却路径（near-isobaric cooling path）降温，并反映了第一阶段阿尔巴尼-弗雷泽造山运动（Albany–Fraser Orogeny）期间大规模岩浆侵位（emplacement）至弗雷泽带后，热衰减（thermal decay）过程的影响。本研究为西澳大利亚弗雷泽带内赋存于侵入体中的诺沃-博林格镍铜矿床的赋矿围岩（host rocks），提供了首组定量温压约束结果。诺沃-博林格的峰期变质条件为0.75~0.76 GPa、880~920 ℃（热梯度约1200 ℃/GPa）。GBAQ-GB温压计记录的再平衡条件为0.29~0.45 GPa、550~640 ℃，指示了近等压退变质冷却过程。硫化物熔体渗透至赋矿围岩中，可为中地壳地体（mid-crustal terranes）内岩浆熔离型矿床（orthomagmatic deposits）形成过程中的高温变质作用，提供一种体积占比低但热贡献显著的额外热源。