Contrasting Porphyry Cu Fertilities in the Yidun Arc, Eastern Tibet: Insights from Zircon and Apatite Compositions and Implications for Exploration
Authors: Cao Kang , Yang Zhi-Ming , Hou Zeng-qian , White Noel C. , Yu Chao 
Published: October 2022

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), ΔFMQ (0.8–2.4; where ΔFMQ is the log fO2 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 Fe2O3/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 ΔFMQ (–3.3 to +0.5), and low whole-rock Fe2O3/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 SO3 contents (0.07–0.79 wt %) than those in the barren suites (&lt;0.04 wt % SO3). 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 (&gt;20), V/Sc (&gt;32.5–0.385 × wt % SiO2), Eu/Eu* (~1) and 10,000*(Eu/Eu*)/Y (&gt;400) ratios, zircon Eu/Eu* (&gt;0.4) and ΔFMQ (&gt;1), and apatite SO3 contents (&gt;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 (Ce4+/Ce3+, 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.