Abstract The recent discovery of large Cenozoic porphyry copper deposits in the Tibetan Plateau has revealed atypical features. Their formation all postdate the India-Asia collision at 55 ± 10 Ma, and therefore they are not affiliated with normal arc magmatism. Three major nonarc porphyry copper belts or provinces in Tibet comprise the Gangdese porphyry Cu-Mo belt (>45 Mt Cu, 1.79 Mt Mo), the Yulong porphyry Cu-Mo belt (8.75 Mt Cu,1.04 Mt Mo), and the western Yunnan porphyry Cu-Mo-Au polymetallic province (~1 Mt Cu, ~1 Mt Mo, and 310 t Au). Alkaline volcanic rocks (lamprophyres, shoshonites, and potassic-ultrapotassic volcanic rocks) are common in these metallogenic belts and provinces, but the temporal, spatial, and genetic relationship between this magmatism and deposit formation remains enigmatic. There are two episodes of porphyry mineralization in the Tibetan Plateau, 45 to 35 and 22 to 11 Ma, and alkaline volcanic rocks are both contemporaneous with and spatially close to porphyry mineralization. Evolved Nd-Hf isotope compositions, and high Mg#, Cr, and Ni contents of Tibetan alkaline volcanic rocks suggest that they are derived from phlogopite-bearing lithospheric mantle, whereas the adakitic property and hybrid geochemical and isotopic features of the high Sr/Y granitoids suggest they are derived from partial melting of lower crust by mantle-derived alkaline mafic melt, with subsequent mixing. The mantle-derived alkaline magmas: (1) triggered water-flux melting of the thickened lower crust and generation of fertile high Sr/Y magmas with high water contents; (2) that dominate the source of ore-related magmas are more Au rich; (3) have variable oxidation states and some can oxidize residual sulfide in the lower crust to release Cu and Au for porphyry deposit formation; other lower crustal melts became oxidized via amphibole and/or garnet fractionation; and (4) provide higher S and Cl contents that are essential volatiles for deposit formation. We conclude that mantle-derived alkaline melts are vital to form porphyry deposits in nonarc settings, thus explaining the close spatial and temporal association of alkaline volcanic rocks and porphyry deposits in Cenozoic Tibet.

Abstract The Yidun arc, part of the Sanjiang Paleo-Tethyan orogenic belt in eastern Tibet, hosts several porphyry Cu deposits in its southern section, whereas abundant contemporaneous but barren granitoid intrusions occur in the northern section. Here we present an integrated, temporally constrained dataset of zircon and apatite compositions together with whole-rock geochemical results for both the fertile and barren suites in the Yidun arc. We investigate the probable factors leading to such contrasting porphyry Cu fertilities and also assess the application of geochemical and mineral proxies for porphyry Cu exploration. Both the fertile and barren suites in the Yidun arc share similar petrographic and geochemical characteristics typical of arc magmas. However, the two suites have distinct differences in certain trace elements and element ratios (e.g., Sr, Y, Sr/Y, V/Sc, Eu anomaly). The fertile suites have adakite-like character, with high Sr/Y, La/Yb, and V/Sc ratios, and show no or minimal negative Eu anomalies, indicating early dominant amphibole with limited plagioclase fractionation. By contrast, the barren suites have low Sr/Y, La/Yb, and V/Sc ratios, and display minimal to significant negative Eu anomalies. These barren suites probably formed by crystal fractionation dominated by plagioclase, with limited amphibole crystallizing from the same parental magma. Zircon geochemical data for both suites combined with Rayleigh fractionation modeling show that zircon compositions (e.g., Hf, Ti, [Yb/Dy] N , Eu/Eu*, Ce/Nd) are affected by the compositions, water content, and redox state of the parental magma, as well as by prior or concurrent crystallization of minerals (e.g., plagioclase, amphibole, apatite, titanite). For the fertile suites, the high zircon Eu/Eu* (0.43–0.91), Δ FMQ (0.8–2.4; where Δ FMQ is the log f O 2 difference between the sample value and the fayalite-magnetite-quartz mineral buffer), the presence of the assemblage amphibole + titanite + quartz + magnetite, and high whole-rock Fe 2 O 3 /FeO, Sr/Y and V/Sc ratios, collectively indicate that associated magmas were hydrous and oxidized. For the barren suites, the common presence of the assemblage amphibole + ilmenite, low zircon Eu/Eu* (0.01–0.34) and Δ FMQ (–3.3 to +0.5), and low whole-rock Fe 2 O 3 /FeO, Sr/Y, and V/Sc ratios, together indicate that the related magmas were hydrous but reduced. Magmatic apatites in the fertile suites have higher SO 3 contents (0.07–0.79 wt %) than those in the barren suites (<0.04 wt % SO 3 ). The estimated magmatic sulfur contents for the fertile suites are 35 to 160 ppm, whereas for the barren suites, their related magmas were sulfate poor. Compared to the hydrous, oxidized, and S-rich fertile suites in the southern Yidun arc, the reduced and sulfate-poor characteristics of the barren suites hinder the transport of adequate S and metals to form porphyry Cu deposits, even though they are hydrous; thus there is little potential for porphyry Cu deposits in the northern Yidun arc. Whole-rock Sr/Y (>20), V/Sc (>32.5–0.385 × wt % SiO 2 ), Eu/Eu* (~1) and 10,000*(Eu/Eu*)/Y (>400) ratios, zircon Eu/Eu* (>0.4) and Δ FMQ (>1), and apatite SO 3 contents (>0.1 wt %) can help to discriminate porphyry Cu intrusions from barren granitoids in the Yidun arc, indicating their usefulness as porphyry Cu fertility indicators. The zircon Ce anomaly (Ce 4+ /Ce 3+ , Ce/Ce*, Ce/Nd), however, overlaps between the oxidized fertile and reduced barren suites, hampering its use to estimate relative magmatic redox state and as a robust porphyry Cu fertility indicator. The combination of whole-rock analyses and zircon and apatite compositions helps focus porphyry Cu exploration on prospective areas, coupled with investigations of structural geology, geophysical surveys, and mapping of hydrothermal alteration.

Abstract The Machangqing porphyry Cu-Au deposit is located in the Sangjiang region, Jinshajiang-Ailaoshan metallogenic belt, southeastern Tibet. It has three main phases of felsic-mafic intrusions: barren granites, ore-forming porphyry intrusions, and mafic lamprophyres. U-Pb zircon dating shows that these intrusions were emplaced over a period of ~3 m.y., with lamprophyres at 36.50 ± 1.6 Ma (1 σ ), porphyry intrusion at 34.26 ± 0.22 Ma (1 σ ), and granite intruded at 34.00 ± 0.26 Ma (1 σ ). The in situ Rb-Sr analysis of phlogopite and amphibole, primary minerals in the lamprophyres, also gives a date of 36.5 ± 1.5 Ma (2 σ ), regarded as the emplacement age of the lamprophyre, earlier than the ore-forming porphyry intrusions. The magmatic phases have significantly different sulfur and chlorine contents. The SO 3 contents of igneous apatite microphenocrysts from the mineralization-related porphyry intrusions are higher (0.24 ± 0.14 wt %, 1 σ , n = 82) than those from the barren granites (0.08 ± 0.07 wt %, 1 σ , n = 30). The chlorine contents in apatite grains from the porphyry intrusions (0.18 ± 0.16 wt %, 1 σ ) are also higher than those from granites (0.04 ± 0.02 wt % Cl, 1 σ ). The apatite in lamprophyres have higher sulfur (0.68 ± 0.19 wt %, 1 σ , n = 40) and chlorine (0.48 ± 0.13 wt %, 1 σ ). The large difference of Cl and S in lamprophyres might suggest that elevated magmatic volatile contents derived from the mafic magma were important for ore formation in the Machangqing porphyry systems. The in situ Sr and O isotopes in apatite phenocrysts from the porphyry intrusions ( 87 Sr/ 86 Sr: 0.70593–0.70850; δ 18 O: 6.0–7.0) are similar to those in the lamprophyres ( 87 Sr/ 86 Sr: 0.70595–0.70964; δ 18 O: 5.4–6.9), consistent with similar origins for their volatile contents. These data may indicate that the deeper magma chamber was recharged by a relatively S-Cl-rich mafic magma similar to the lamprophyres, triggering the ore-forming magmatic event. This study also suggests that origin of apatite Sr and also volatile contents, combined with in situ Sr and O isotopes, could be useful for fingerprinting fertile intrusions associated with mineralization within drainage source areas or in outcrops.