INCREASED MAGMATIC WATER CONTENT THE KEY TO PORPHYRY CU±MO±AU FORMATION IN THE GANGDESE BELT, TIBET

The Gangdese magmatic belt in Tibet preserves a series of Cenozoic collision-related igneous rocks that have formed since the start of the Indo-Asian collision at ~55 Ma. Oligocene and Miocene magmatic rocks crop out as small-volume intrusions, and are associated with several large porphyry ± skarn deposits. In contrast, earlier, more voluminous Eocene magmatism is rarely associated with mineralization. We argue that this uneven distribution of deposits in time is controlled by changes in magmatism and tectonics during the Cenozoic.

The Eocene magmatic rocks have calc-alkaline continental arc-like compositions with medium to high-K contents, intermediate [La/Yb]_N ratios (average = 9.8 ± 8.0, n = 50), intermediate-to-low Sr/Y ratios (mostly <40), and negative Eu anomalies (average Eu_n/Eu* = 0.79 ± 0.24). They show decreasing Sr concentrations and increasingly negative Eu anomalies with SiO_2. Additionally, the Y content of these rocks steadily increases until ~66 wt.% SiO₂, then decreases, which suggests that hornblende did not begin to fractionate until relatively late stages of magma evolution. Mafic Eocene granitoids are mainly composed of pyroxene and plagioclase, with little or no amphibole. In contrast, the Oligo-Miocene granitoids are mostly peralkaline, potassic calc-alkaline, to calc-alkaline in composition. They have high [La/Yb]_N ratios (average = 31.9 ± 12.3, n = 130), high Sr/Y ratios (>40), Y contents that decrease with SiO₂, and listric-shaped (MREE-depleted) rare earth element patterns. Their mineralogy consists of plagioclase, quartz, and amphibole. They start from much higher Sr contents and lower Y contents, suggesting early hornblende fractionation and delayed plagioclase fractionation. These geochemical and mineralogical characteristics suggest that the Eocene magmas were relatively anhydrous and primarily fractionated pyroxene and plagioclase, whereas the Oligo-Miocene magmas were more hydrous and fractionated significant amounts of hornblende.

High magmatic water content is essential for the formation of magmatic-hydrothermal ore deposits, and we therefore conclude that this increase in magmatic water content from the Eocene to the Oligo-Miocene in the Gangdese belt explains the relative scarcity of such deposits in the Eocene compared to the Oligo-Miocene.