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.

Abstract Zircon composition has great potential as a pathfinder for porphyry Cu ± Mo ± Au systems. The present study used a large integrated laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb age and trace element dataset for both infertile and fertile magmatic suites in order to elucidate distinctive zircon signatures diagnostic of metallogenic fertility of the parent magma. The infertile suites are defined as magmatic rocks that are absent of alteration and mineralization at any grade, whereas fertile suites refer to the causative intrusions leading to porphyry-type ore formation. The infertile suites are relatively reduced S- and A-type and relatively dry A- and I-type magmas, including the Yellowstone rhyolite (Wyoming), Bandelier rhyolite (New Mexico), Bishop tuff rhyolite (California), Lucerne reduced granite (Maine), and Hawkins S-type dacite and Kadoona I-type dacite (Lachlan belt, Australia). The fertile suites are more oxidized and hydrous and are selected from representative causative I-type intrusions from porphyry and high-sulfidation epithermal Cu-Au deposits (Batu Hijau, Indonesia, and Tampakan, Philippines), porphyry Cu-Mo-Au deposits (Sar Cheshmeh, Iran; Dexing, eastern China; and Jiama, southern Tibet), porphyry Cu-Mo deposits (Sungun, Iran, and Qulong, southern Tibet), and porphyry Mo deposits (Nannihu and Yuchiling, central China). The best fertility indicators are zircon Eu/Eu* and (Eu/Eu*)/Y ratios, whereas zircon (Ce/Nd)/Y and Dy/Yb ratios are moderately useful. In particular, fertile magmatic suites have collectively higher zircon Eu/Eu* ratios (>0.3), 10,000*(Eu/Eu*)/Y (>1), (Ce/Nd)/Y (>0.01), and lower Dy/Yb (<0.3) ratios than infertile suites. In fertile suites, zircon (Eu/Eu*)/Y ratios are positively correlated with (Ce/Nd)/Y ratios, but this correlation is lacking in the infertile suites. The distinctive zircon ratios in the fertile suites are interpreted to indicate extremely high magmatic water content, which induces early and prolific hornblende fractionation and suppresses early plagioclase crystallization. In addition, we found that Mo is able to substitute for Zr in the zircon lattice. The Mo-rich porphyry systems that were analyzed as part of this study tend to produce some zircons with a higher Mo content (>1-9 ppm) than Mo-poor porphyry systems and infertile suites, indicating that Mo content in zircon is a potential pathfinder to porphyry Mo ore deposits. The zircon Mo/Ti ratio has a broad positive correlation with the oxygen fugacity of the magma, indicating that this ratio may be potentially used as a proxy for the oxidation state of the melt. Analyzing the compositions of detrital zircons from an area with little geologic information or poor outcrop could efficiently and cheaply discriminate whether the drainage source area is dominated by unprospective A-, S-, and I-type granitoids or by prospective I-type granitoids, which could help focus exploration on prospective areas.