Variable modes of formation for tonalite-trondhjemite-granodiorite-diorite (TTG)-related porphyry-type Cu ± Au deposits in the Neoarchean southern Abitibi subprovince (Canada): Evidence from petrochronology and oxybarometry
Variable modes of formation for tonalite-trondhjemite-granodiorite-diorite (TTG)-related porphyry-type Cu ± Au deposits in the Neoarchean southern Abitibi subprovince (Canada): Evidence from petrochronology and oxybarometry
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新太古代南部阿比蒂比省（加拿大）与英闪长岩-长角闪长岩-花岗闪长岩-闪长岩（TTG）相关的斑岩型铜±金矿床的不同形成模式：来自岩石年代学和氧气压测量的证据
DOI：
10.1093/petrology/egab079
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发表时间：
2021
影响因子：
3.9
通讯作者：
P. Jugo
中科院分区：
文献类型：
--
作者：
Xuyang Meng;J. Richards;D. Kontak;A. Simon;Jackie Kleinsasser;J. Marsh;R. Stern;P. Jugo
大多数已知的斑岩型Cu ± Au矿床与中生代中氧化、富硫、钙碱性至弱碱性的弧相关岩浆有关。与此相反，富钠英云闪长岩-奥长花岗岩-花岗闪长岩-闪长岩（TTG）岩浆主要在太古代被假设为未氧化和硫贫，这共同排除斑岩铜存款形成。在这里，我们测试这一假设的成因岩浆的~2.7 Ga TTG相关的辉长岩金，圣裘德，和克利福德斑岩型铜± Au存款设置在新太古代南部Abitibi次省。新的和以前发表的地质年代学结果限制的成因岩浆的侵位年龄在~2.74 Ga，~2.70 Ga，和~2.69 Ga，分别。闪长质和奥长花岗质岩浆与金矿床和圣裘德矿床有关，它们沿沿着以斜长石为主的分馏趋势演化，与克利福德英云闪长质岩浆以角闪石为主的分馏形成对比。对采自Alkanté Gold、St-Jude和Clifford火成岩的锆石颗粒的分析表明，εHf（t）± SD值分别为4.5 ± 0.3、4.2 ± 0.6和4.3 ± 0.4，δ 18 O ± SD值分别为5.40 ± 0.11 ‰、3.91 ± 0.13 ‰和4.83 ± 0.12 ‰。这些同位素特征表明，虽然这些岩浆是地幔来源的地壳污染最小的圣裘德和克利福德设置的岩浆或其来源可能经历了可变的热海水或流星流体的改变。
原生气压矿物（即，锆石、角闪石、磷灰石和磁铁矿-钛铁矿），用于估算成因岩浆的fO 2。利用锆石地球化学估算的岩浆fO 2值（相对于铁橄榄石-磁铁矿-石英缓冲层报告为ΔFMQ）表明，St-Jude岩浆、Alcanté Gold岩浆和Clifford岩浆的fO 2值分别从ΔFMQ-0.3 ± 0.6、ΔFMQ +0.8 ± 0.4增加到ΔFMQ +1.2 ± 0.4。与此相反，角闪石化学系统地产生了较高的fO 2值，分别为ΔFMQ +1.6 ± 0.3和ΔFMQ +2.6 ± 0.1，这与以前的研究一致，表明角闪石可能高估侵入岩的fO 2高达一个对数单位。微X射线吸收近边结构（μ-XANES）光谱法测定硫（即，St-Jude和Clifford的原始磷灰石中的S6+/S1 S）分别产生≥ΔFMQ-0.3和ΔFMQ +1.4 - 1.8。Clifford英云闪长岩中的磁铁矿-钛铁矿矿物对在634 ± 21 °C的平衡温度下的ΔFMQ为+3.3 ± 1.3，记录了岩浆结晶后期的氧化还原状态。电子探针显微分析表明，来自Clifford的磷灰石颗粒富含S（高达0.1重量%）。%），这归因于Clifford英云闪长岩的相对氧化或富硫特征。
我们将这些结果解释为表明，Alfreté Gold和Clifford的矿床是由轻度（~ΔFMQ +0.8 ± 0.4）至中度（~ΔFMQ +1.5）氧化岩浆形成的，其中大量早期硫化物饱和度可能有限，而St-Jude存款代表了一种罕见的情况，即外部热液流体的进入促进了相对减少的岩浆房（~ΔFMQ +0）中的金属丰度。此外，我们得出结论，这些矿床的形成模式，此外，在太古代的斑岩型铜-金矿床的明显罕见性可能是由于有利的成矿条件，和/或保存，或勘探的偏见的局部限制。
Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite-trondhjemite-granodiorite-diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2.7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2.74 Ga, ~2.70 Ga, and ~2.69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4.5 ± 0.3, 4.2 ± 0.6, and 4.3 ± 0.4, and δ18O ± SD values of 5.40 ± 0.11 ‰, 3.91 ± 0.13 ‰, and 4.83 ± 0.12 ‰, respectively. These isotopic signatures indicate that although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids.
Primary barometric minerals (i.e., zircon, amphibole, apatite, and magnetite-ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite-magnetite-quartz buffer as ΔFMQ, using zircon geochemistry indicate that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ -0.3 ± 0.6, ΔFMQ +0.8 ± 0.4, to ΔFMQ +1.2 ± 0.4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1.6 ± 0.3 and ΔFMQ +2.6 ± 0.1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate amphibole may overestimate the fO2 of intrusive rocks by up to one log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e., S6+/ΣS) in primary apatite yielded ≥ΔFMQ -0.3 and ΔFMQ +1.4–1.8 for the St-Jude and Clifford, respectively. The magnetite-ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3.3 ± 1.3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0.1 wt. %) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite.
We interpret these results to indicate the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0.8 ± 0.4) to moderately (~ΔFMQ +1.5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu-Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.