GEOCHRONOLOGY OF THE PORGERA GOLD DEPOSIT, PAPUA-NEW-GUINEA - RESOLVING THE EFFECTS OF EXCESS ARGON ON K-AR AND 40AR/39AR AGE ESTIMATES FOR MAGMATISM AND MINERALIZATION

Mesothermal/epithermal gold mineralization at Porgera in the highlands of Papua New Guinea (PNG), occurs in structurally controlled veins and disseminations, which overprint and cross-cut a suite of shallow-level, comagmatic, mafic alkaline stocks and dykes (the Porgera Intrusive Complex, PIC) and their sedimentary host rocks. Conventional K-Ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 Ma, but four biotite separates are concordant at 6.02 ± 0.29 Ma (2σ). 40Ar 39Ar step-heating experiments on six of the hornblende separates reveal saddleshaped age spectra, which indicate the presence of excess 40Ar. One of these samples yields a well-defined plateau with an apparent age of 5.96 +- 0.25 Ma (2σ). This age is identical to that of the biotites and suggests an age of 6.0 ± 0.3 Ma (2σ) for emplacement of the PIC; older apparent ages obtained from conventional K-Ar and 40Ar 39Ar analyses are explained by excess 40Ar contamination. Conventional K-Ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5.1 and 6.1 Ma and indicate that ore deposition occurred within 1 Ma of magmatism at Porgera. Evidence for the evolution of a magmatic volatile phase, and the presence of excess 40Ar both in the intrusives and in hydrothermal fluids associated with the orebody, suggest that magmatic fluids may have had some involvement in metallogenesis, but the exact nature of this involvement is not yet clear. Late Miocene magmatism and mineralization at Porgera are thought to have occurred shortly prior to or during the initiation of continent/arc collision and to pre-date associated Pliocene uplift and foreland deformation in the highlands. © 1990.