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

Author : Rogerson, R. & Williamson, A.
Publication : Bulletin of the Geological Society of Malaysia
Page : 487-502
Volume Number : 20
Year : 1987

Bulletin of the Geological Society of Malaysia, Volume 20, August 1986, pp. 487 – 502


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


Geological Survey of Papua New Guinea, P.O. Box 778, Port Moresby, Papua New Guinea


Abstract: Numerous Neogene porphyritic basic to intermediate intrusive rocks crop out as batholiths, 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% SiO2 compared with the wide range of SiO2 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% SiO2+.

However, at 60% SiO2, Elandora-type intrusives have higher Sr (700 ppm) and lower K2O (< 2%) than Akuna-type intrusives. These empirical geochemical features, coupled with field criteria, serve to distinguish the two intrusive types.