Age, petrology and mineralisation associated with two Neogene intrusive types in the Eastern Highlands of Papua New Guinea

Abstract

Numerous Neogene porphyritic basic to intermediate intrusive rocks crop out as batho­ liths, stocks and dyke swarms in the eastern highlands of Papua New Guinea between 144 °E and 146°E. Their radiometric ages range from 18 Ma to 7 Ma, and are consistent with known more definitive stratigraphic relationships. Our work has shown that within this time range, two distinct phases of plutonism occurred and that the second phase from 9 Ma to 7 Ma was often associated with magmatic­ hydrothermal Cu-Au-Ag mineralisation. We have named the earlier phase Akuna-type, and the later phase Elandora-type. Akuna-type intrusives tend to form large complexes (eg. Akuna Intrusive Complex, Bismarck Intrusive Complex) up to 800 km2 in outcrop area, displaying a wide variety of fractionated compositions from pyroxenite, gabbro, diorite to granodiorite. Other smaller intrusions (Oipo and Kimil Diorites) are more homogeneous. In contrast to Akuna-type, Elandora-type intrusives (Elandora Porphyry, Yandera porphyries) generally form microdioritic, often tabular stocks less than 10 km2 in area, dykes and dyke swarms, some of which intrude Akuna-type masses. Many bodies and parts of individual bodies display propylitic alteration assemblages, with argillic, phyllic and silicic alteration being locally dominant. Elandora-type intrusives are apparently associated with either areas of outcropping basement or areas underlain at shallow depth by basement. Exposure of many Elandora-type intrusions at the present erosion surface is commonly a function of two fault sets trending 120° and 040°, which intersect to form elevated fault-bound basement blocks and areas of cover underlain at shallow depth by basement. Some Elandora-type intrusions occur in erosional basement highs. The two intrusive types have calc-alkaline affinities. Analysed Elandora-type intrusives have a narrow compositional range between 60 and 70% Si02 compared with the wide range of Si02 values shown by analysed Akuna-type intrusives. Elandora-type analyses plot on or adjacent to Akuna-type major element variation diagram trend lines at 60% Si02• However, at 60% Si02, Elandora-type intrusives have higher Sr (700 ppm) and lower K,O (< 2%) than Akuna-type intrusives. These empirical geochemical features, coupled with field criteria, serve to distinguish the two intrusive types.