Research School of Earth Sciences, Australian National University, Canberra, ACT, 2601, Australia.
Sci Rep. 2023 Mar 28;13(1):5078. doi: 10.1038/s41598-023-31736-z.
The world's largest current Cu resource is volcanic arc-hosted, porphyry copper deposits. Whether unusual parental magmas or fortuitous combinations of processes accompanying emplacement of common parental arc magmas (e.g., basalt) is required for ore deposit formation, remains unclear. Spatial and tectonic associations of adakite (andesite with high La/Yb, Sr/Y) with porphyries exist, but genetic links are debated. Delayed saturation with Cu-bearing sulfides consequent to elevated redox state seems essential for late-stage exsolution of Cu-bearing hydrothermal fluids. Partial melting of igneous layers of subducted, hydrothermally altered oceanic crust in the eclogite stability field are invoked to account for andesitic compositions, residual garnet signatures, and the putative oxidised character of adakites. Alternative petrogeneses include partial melting of lower crustal, garnet-bearing sources and extensive intra-crustal amphibole fractionation. Here we demonstrate mineral-hosted, adakite glass (formerly melt) inclusions in lavas erupted subaqueously in the New Hebrides arc are oxidised relative to island arc (and mid-ocean ridge) basalts, are HO-S-Cl-rich, and moderately enriched in Cu. Polynomial fitting of chondrite-normalised, rare earth element abundance patterns shows the precursors of these erupted adakites were unequivocally derived from partial melting of subducted slab, and represent optimal porphyry copper progenitors.
世界上最大的当前铜资源是火山弧型斑岩铜矿。矿床形成是否需要异常的母岩浆或伴随常见母弧岩浆（例如玄武岩）就位的偶然过程组合仍不清楚。与斑岩相伴存的埃达克岩（高 La/Yb、Sr/Y 的安山岩）具有空间和构造联系，但遗传联系存在争议。由于氧化还原状态升高而导致含铜硫化物的延迟饱和似乎对含铜热液流体后期离溶至关重要。俯冲的、水热蚀变的大洋地壳的火成岩层的部分熔融，在榴辉岩稳定场中，用来解释安山岩的成分、残留石榴石特征和埃达克岩的假定氧化特征。替代的岩石成因包括下部地壳、含石榴石源的部分熔融和广泛的地壳内部角闪石分异。在这里，我们证明了在新赫布里底弧水下喷发的熔岩中存在矿物包裹的埃达克玻璃（以前是熔体）包裹体，与岛弧（和大洋中脊）玄武岩相比，这些包裹体是氧化的，富含 HO-S-Cl，并且 Cu 中度富集。球粒陨石归一化的稀土元素丰度模式的多项式拟合表明，这些喷发的埃达克岩的前体明确来自俯冲板块的部分熔融，并且代表了最佳的斑岩铜矿前体。
