The Nena high sulphidation deposit, located in the Sandaun
Province, PNG, contains a measured and indicated resource of
51 Mt of copper ore grading 0.5% Cu and 0.6 g/t Au at a 0.5% Cu
cut-off. The copper orebody is overlain by an 18Mt oxide gold
resource grading 0.1 % Cu and 1.4 g/t Au at a 0.6 g/t Au cut-off.
The deposit is hosted in advanced argillic altered pyroclastics
interstratified with lavas, volcanoclastics, epiclastics and
sediments of the Debom Volcanics within the Frieda River
Intrusive Complex. Foraminiferal sediments indicate the Debom
Volcanics to be Early-Middle Miocene in age. Alteration is elongated along, and concentric about a northwest
trending structure (Nena Structural Corridor) which is about 7km
in length. The inner two alteration types, residual and massive
silica-alunite are strongly brecciated and host the bulk of later
hypogene Cu-Au mineralisation. Weakly mineralised weak silica- alunite and barren peripherally zoned clay alteration surround
the inner mineralised alterations. The hypogene sulphide deposition was polyphasal, episodic and
is zoned from north to south from covellite-enargite chalcocite
(5600N to 5200N) to a mixed enargite-luzonite (5200N to 4700N)
to luzonite-stibioluzonite (4700N to 4400N) in the south within a
1200m long by 300m diameter sub horizontal pipe like body.
Paragenesis studies indicate that pyrite marcasite were
introduced early with barite and black native sulphur before
deposition of banded pyrite (melnicovite) with barite during a
period of brecciation. Continued brecciation allowed bladed
covellite chalcocite followed by enargite-luzonite-stibioluzonite
pyrite (brassy) and minor acicular covellite deposition. Yellow
native sulphur and trace amounts of galena were deposited in
remaining pores. Pre-acid alteration intrusives domed the mainly volcanic
stratigraphy in the southwest and post-mineralisation porphyries
in the northeast truncate mineralisation. Supergene processes and erosion have modified the top of the
deposit, creating a 45m thick goethitic-scoroditic-haematitic gold
cap above a 40m thick (on average) chalcocite rich supergene
blanket. Lesser amounts of digenite, covellite native copper and
rarely seen malachite occur in the supergene copper zone.