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Ok Tedi Mine (Papua New Guinea): Breccias in a Porphyry Cu-Au Deposit

Roberto Weinberg, Monash University, Australia

Michiel van Dongen, Monash University, Australia



Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL:


I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.


DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.


This web page documents brecciation related to hydrothermal deposits in sedimentary rocks surrounding the twin monzonitic body of Fubilan and Sydney at Ok Tedi in Papua New Guinea fold and thrust belt. These styles reflect the hystory of interaction between hydrothermal fluids emanating from these stocks emplaced at shallow crustal-levels into limestone and siltstone layers and tectonics, to form the youngest exposed porphyry Cu-Au deposit. See twin page (Ok Tedi Sulfide Alteration) for description of alteration.



a) Magnetite breccia in skarn.


Drill hole DDH908 shows the brecciation and substitution of endoskarn by mag-py-cpy.


Radial fracturing
Figure 1a: Radial fracturing of endoskarn accompanied by deposition and substitution by magnetite giving rise to irregular, poorly defined boundaries (sample, 116m DDH908).
Brecciation and alteration of endoskarn
Figure 1b. Brecciation and alteration of endoskarn by irregular magnetite-rich (diagonally from upper left), cut across by narrow veinlets of cpy+py+mag (109m, DDH908).


brecciation of endoskarn
Figure 1c:  In situ, hydraulic brecciation of endoskarn by veins of mag+py+cpy. Note epidotization in endoskarn (sample, 111m DDH908).



b) Brecciated magnetite skarn


In contrast to the breccia above, DDH894 shows the brecciation of a pre-exisiting magnetite skarn in a silt-sand matrix,
which is possibly contemporaneous with sulfide alteration. A thick body of massive magnetite skarn immediately below this breccia
was not brecciated.


Figure 2a. Limestone clasts in silt-sand matrix (80-81 m, DDH894) below the breccia containing siltstone clasts .


Figure 2b.  Carbonatic silt-sandstone in a silt-sandy matrix originated from the break-up of the clasts, best shown by the gradual transition between clast and matrix on the right-hand side (see Fig. 2b). Matrix has py+cpy alteration (sample, 73.5 m, DDH894).
clast disaggregation
Zoom on the right-hand side (sample, 73.5 m, DDH894).


Figure 2c. Four meters away from Fig. 2a within the same breccia body, angular clasts of magnetite skarn in carbonatic sandy matrix  (sample, 77.5 m, DDH894). This breccia lies above a body of well-preserved (not brecciated) finely laminated magnetite (see Fig. 10a)  skarn interpreted to represent subsitution of mudstone, that resisted post-skarn brecciation.
Angular magnetite skarn blocks
Figure 2d. Angular magnetite skarn blocks in a matrix rich in sulfides (sample, 99.6 m, DDH894). This breccia is below the thick body of finely laminated magnetite skarn and massive magnetite skarn with disseminated sulfides.


Angular magnetite skarn blocks, breccia
Figure 2e. Magnetite skarn clasts in matrix which contains py+cpy and fibrous actinolite. Fractures filled with py cut across magnetite skarn clasts.



c) Taranaki Thrust Breccia at Berlin


In the Berlin (W) wall of the mine a thick region of brecciated rocks is known as the Taranaki thrust. This thrust separate normal stratigraphic sequence, with Darai Limestone on top of Ieru Formation. In this region the Ieru  Formation is characterized by interlayerd mudstone and siltstone packages.. Mudstone tends to behave viscously forming the matrix of breccias, fault gouge. The limestone clasts dominate on the upper part of the breccia, close to the massive limestone, whereas siltstone clasts dominate in the lower part of the breccia. In this area, steep and sharp faults separate breccias of different natures, indicating considerable movement on these post-breccia steep planes.


taranaki breccia
Figure 3a. Taranaki thrust indicated by mixed breccia with clasts of both limestone (light grey) and siltstone (medium grey) and occasional irregular mudstone clast (dark grey on the middle right-hand-side).
siltstone breccia
Figure 3b. Siltstone clasts in mudstone matrix.


Siltstone breccia
Figure 3c. Siltstone breccia in mud matrix.
Fault gouge and breccia
Figure 3d. Fault gouge and breccia.



d) Parrots Beak Thrust Breccia (DDH958)


Parrots Beak breccia
Figure 4a. Parrots Beak thrust: limestone breccia.



e) Fubilan Breccia


This breccia occus is a magmatic-hydrothermal breccia inside the Fubilan monzonite porphyry, N contact of the stock.


Fubilan breccia
Figure 4a. Fubilan breccia with clasts of granite and altered mudstone, with 0.5 cm wide biotite veins.
Quartz core
Figure 4b. Quartz core is a zone inside the Fubilan porphyry with reduced copper. Small black minerals in veins are chalcocyte.