MULTIPULSED MAGMATISM AND DURATION OF THE HYDROTHERMAL SYSTEM OF THE GIANT JIAMA PORPHYRY Cu SYSTEM, TIBET, CHINA

Abstract

Abstract Jiama is the largest porphyry-skarn ore system in the Gangdese metallogenic belt, Tibet. It is composed of porphyry Cu-Mo, Cu-polymetallic skarn, Cu-Pb-Zn manto, and distal vein Au orebodies with associated Ag, W, and Bi. However, the precise timing of the magmatism and hydrothermal events at Jiama remains obscure. Here, using high-precision chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon dating of ore-bearing and post-ore intrusions, we accurately constrain the lifespan of magmatism and hydrothermal mineralization. Monzogranite porphyry dikes are cut by Cu-Mo vein mineralization in the deeper part of the system, indicating that they were emplaced pre-ore, and yield a crystallization age of 15.534 ± 0.007 Ma (mean square of weighted deviates [MSWD] = 0.99, n = 6). A granodiorite porphyry that cuts the monzogranite porphyry and hosts vein and disseminated chalcopyrite and molybdenite is considered synchronous with ore and yields a crystallization age of 15.368 ± 0.007 Ma (MSWD = 1.01, n = 5). These two phases of intrusions are cut by quartz-diorite porphyry bodies, which yield a crystallization age of 15.076 ± 0.006 Ma (MSWD = 0.13, n = 6) and contain weak, subeconomic Cu and almost no molybdenum mineralization. A post-ore barren quartz monzonite porphyry yields a crystallization age of 14.925 ± 0.006 Ma (MSWD = 1.12, n = 6). The lifespan of magmatism at Jiama is thus about 0.61 m.y. The difference with previous molybdenite Re-Os isochron ages from the porphyry (14.7 ± 0.3 Ma), hornfel (14.7 ± 0.4 Ma), and skarn (15.4 ± 0.2 Ma) suggests that high-precision chronology is required to decipher accurate timing of mineralization in porphyry systems such as Jiama. The 40Ar/39Ar ages of hydrothermal biotite coexisting with molybdenite in monzogranite porphyry and hornfels are 15.25 ± 0.17 Ma (MSWD = 1.6) and 15.25 ± 0.24 Ma (MSWD = 0.14), respectively, slightly younger than the granodiorite porphyry and older than weakly mineralized quartz diorite porphyry, which represents the time of the ore-forming hydrothermal event. Thus, Jiama is the product of pulsed magmatism during which a short-lived hydrothermal event formed the giant Cu polymetallic system.