Short life-span of the ore-forming system at the Porgera gold deposit, Papua New Guinea: laser 40Ar/39Ar dates for roscoelite, biotite, and hornblende

摘要

The Porgera gold deposit, Papua New Guinea, is associated with sodic-alkalic, hypabyssal intrusions of alkali basaltic to mugearitic composition. The intrusions were emplaced into Cretaceous mudstones and siltstones in the latest Miocene, and both igneous and sedimentary rocks are mineralized. Three types of veins occur: (1) magnetite–sulfide ± Au-carbonate veins; (2) base-metal sulfide ± Au-carbonate veins; and, economically most important, (3) quartz–roscoelite-Au veins and breccias. Magmatic hornblende and biotite, hydrothermal biotite from alteration selvages associated with the magnetite-sulfide ± Au veins, and roscoelite were dated using the laser 40Ar/39Ar step-heating technique. Magmatic biotite yielded a date of 5.99±0.11 Ma (2σ error; MSWD=3.7), and two samples of magmatic hornblende provided dates of 6.35±0.23 Ma (MSWD=1.0) and 6.3±0.7 Ma (MSWD=2.6). Hydrothermal biotite provided a date of 5.98±0.13 Ma (MSWD=2.7), and analyses of two roscoelite samples yielded dates of 5.92±0.09 Ma (MSWD=2.0) and 5.92±0.17 Ma (MSWD=2.0). The date of the magmatic biotite is interpreted to represent the age of the intrusions more accurately than the hornblende dates because small amounts of excess 40Ar were present in the hornblende samples. The date for hydrothermal biotite is interpreted to be the age of the alteration associated with the magnetite–sulfide ± Au veins (the paragenetically earliest veins), and thus reflects the age of the onset of the mineralizing activity at Porgera. Based on the two dates for roscoelite, the age of the main ore deposition event is interpreted to be 5.92±0.08 Ma. The apparent ages for the intrusive and the mineralizing events are thus identical within error, and suggest that the magmatic and ore-forming system at Porgera was short-lived.