Early sulfide saturation is not detrimental to porphyry Cu-Au formation

Ore-forming magmas are commonly considered to have been unusually metal rich. Because Cu and Au are strongly chalcophile, early sulfide saturation has been regarded as detrimental to porphyry Cu-Au mineralization. Here we demonstrate, based on amphibole-rich cumulate xenoliths and amphibole megacrysts from the Tongling porphyry(-skarn) Cu-Au mining district in southeastern China, that this view is not necessarily correct. Age data combined with petrological and geochemical evidence suggest that the mineralizing magmas at Tongling underwent significant fractional crystallization of amphibole, clinopyroxene, and magmatic sulfides in the middle to lower crust. The fact that the silicate melts nevertheless were able to produce substantial porphyry(-skarn) Cu-Au deposits implies that the formation of metal-rich cumulates at depth was not detrimental to their fertility. On the contrary, the common association of porphyry Cu (Au, Mo) deposits with high-Sr/Y magmas suggests that amphibole fractionation at depth even promotes the mineralization potential, despite the likely loss of metals.

Amphibole-rich cumulate xenoliths in the Zhazhalong intrusive suite, Gangdese arc: Implications for the role of amphibole fractionation during magma evolution

Amphibole fractionation during the early evolution of arc magmas has been widely inferred on the basis of distinctive geochemical fingerprints of the evolved melts, although amphibole is rarely found as a major mineral phase in arc volcanic rocks, so-called cryptic amphibole fractionation. Here, we present a detailed case study of xenoliths of amphibole-rich cumulate from the Zhazhalong intrusive suite, Gangdese arc, which enables an investigation of this differentiation process using a combination of petrological observations and in situ geochemical constraints. Evidence that the xenoliths represent fragments of igneous cumulates includes: (1) the presence of an amphibole-dominated crystal framework; (2) mineral and whole-rock Fe–Mg exchange coefficients; (3) rare-earth element patterns that are similar in the amphiboles and the xenoliths; (4) the compositions of basaltic to andesitic liquids in equilibrium with amphiboles; and (5) enrichment of the xenoliths in compatible elements and depletion in incompatible elements. The amount of trapped liquid based on La, Ce, and Dy abundances varies from ~12 to ~20%. Actinolitic cores within amphibole grains likely represent reaction between olivine precursor and hydrous melt, as evidenced by their high Cr and Ni contents. Amphibole thermometry and oxybarometry calculations indicate that crystal accumulation occurred over temperatures of 857–1014 °C, at mid-crustal pressures of 312 to 692 MPa and oxygen fugacity between 0.4 and 1.9 log units above the nickel–nickel oxide buffer. Quantification of the major-element compositions of the parent liquids indicates that the Zhazhalong amphibole cumulates crystallized from basaltic to andesitic magmas, probably with a shoshonitic affinity, and with SiO2 contents of 46.4–66.4 wt%. Appropriate partition coefficients, calculated using a parameterized lattice strain model and an empirical partitioning scheme, were employed to calculate the trace-element compositions of the liquids in equilibrium with amphibole. Our results confirm that Dy/Yb and Dy/Dy* ratios, which decrease with increasing degrees of differentiation, can be used as robust signatures of amphibole fractionation. This work presents a direct snapshot of the process of amphibole fractionation and provides a natural example of the hidden amphibole “sponge” in arc crust. In particular, this study also suggests that some appinites likely represent amphibole-rich cumulates, which may help to explain the genesis of other unusual but petrologically significant rocks.

Geochemistry and petrology of the Búzios Island alkaline massif, SE, Brazil

ABSTRACT: The Late Cretaceous Búzios Island alkaline massif intrudes Precambrian charnockites and consists dominantly of syenitic rocks that are cut by a large number of dikes, mostly NE-trending, and representing two distinct suites, a felsic one and a mafic-ultramafic one. Alkali feldspar is the most abundant mineral; other constituents are clinopyroxene, commonly replaced by amphibole/biotite, and opaques. Accessory minerals include occasionally rare phases bearing Zr, Ti, Nb and Rare Earth Elements (REE). The felsic dikes may also have nepheline (sodalite). The mafic-ultramafic suite, in particular the lamprophyres, shows a primary mineral assemblage with olivine, clinopyroxene and amphibole in addition to a groundmass having glassy material and carbonates (ocelli). The Búzios rocks are chemically evolved, mostly of potassic affinity and mainly belong to the miaskitic series. Variation diagrams for major and trace elements show a bimodal distribution, suggesting an origin from different magmatic pulses. The rocks are interpreted as having been derived by fractional crystallization processes from a basanitic parental magma. The SiO2-undersaturated and SiO2-oversaturated associations present in the massif are apparently not linked to a single magmatic source, and in the petrogeny residual system two trends are evident: the first one towards the phonolitic minimum and the second one towards the rhyolitic minimum, possibly pointing to amphibole fractionation.