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 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 magmatichydrothermal 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 km 2 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 km 2 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% Si0 2 compared with the wide range of Si0 2 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% Si0 2 • However, at 60% Si0 2 , 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.

Copper‐gold mineralisation in New Guinea: Tectonics, lineaments, thermochronology and structure

“…(1987) inferred that the changing composition through time reflected the gradual evolution and maturation of a volcanic arc. East of the northern Bosavi Lineament, the arc is manifested in the Akuna and Bismarck intrusives (Figure 1), which have a calc‐alkaline to alkaline affinity, consistent with subduction beneath thinned continental crust (Watmuff 1979; Rogerson & Williamson 1985; Forwood 1990).…”

“…Significantly, the Cu–Au‐bearing Upper Miocene Yandera Porphyries, within the eastern Bismarck Intrusives, are low‐K tholeiitic to calc‐alkaline rocks for which Watmuff (1979) postulated an upper mantle source. Watmuff (1979), Mason and Heaslip (1980) and Rogerson and Williamson (1985) believed their emplacement was due to deep crustal fracturing.…”

Geology, Wall‐rock Alteration and Vein Paragenesis of the Bilimoia Gold Deposit, Kainantu Metallogenic Region, Papua New Guinea

“…The 18 -12-Ma subduction generated calc alkalicalkalic, ultramafi c -intermediate intrusions as plugs, stocks and batholiths. The 17.1 -13.9-Ma Akuna and 13 -12-Ma Bismarck intrusive complexes (900 km 2 ) are eroded volcano-plutonic centers ( Page, 1976;Rogerson & Williamson, 1985 ). Reconfi guration of plate motions in Late Miocene halted the subduction, forcing the Solomon Sea Plate to be doubly subducted beneath the GBBT and accreted volcanic arc ( Cooper & Taylor, 1987 ).…”

“…Late Miocene ( ca 9 -7 Ma) intermediate intrusions and felsic porphyries with no coeval extrusive phases occur as swarm of dikes, discrete dikes and small (<4 km 2 ) stocks ( Page, 1976 ). The intrusions contain high Sr (700 -1400 ppm) and low K 2 O (<2 wt%) contents at 60 -70 wt% SiO 2 and are considered to be responsible for the bulk of the mineralization in the goldfi eld ( Page, 1976;Tingey & Grainger, 1976;Rogerson & Williamson, 1985 ). for the Kainantu -Goroka Goldfi eld.…”

“…Analysis of regional whole-rock geochemical data shows that the Late Miocene intrusions are alkalicsubalkalic, metaluminous, sodic, andesitic -dacitic phases ( Rogerson et al , 1982;Rogerson & Williamson, 1985 ). The compositions of the felsic intrusions plot in the adakite fi eld defi ned by Defant and Drummond (1990) .…”