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Ok Tedi Mine (Papua New Guinea): Fluid Paths, Magnetite Skarn and Sulfide Deposition in a Porphyry Cu-Au Deposit

Roberto Weinberg, Monash University, Australia

Michiel van Dongen, Monash University, Australia




This web page documents the styles of hydrothermal deposits of magnetite and sulfide 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 to form the youngest exposed porphyry Cu-Au deposit.
This page is divided for clarity and simplicity into two parts. One dealing with the range in styles of magnetite alteration found in the mineralized Cu-Au skarn around the twin stocks, the other dealing with sulfide mineralization, which commonly overprints magnetite alteration.
Four dominant styles have been recorded with important implications for the genesis of the mineralization, partly controlling rock behaviour during deformation and partly controlling the chemical environment experienced by mineralizing fluids. The main types are:
a) passive substitution of limestone
b) passive substitution of mudstone
c) hydrothermal brecciation of siltstone, or monzonitic rocks
d) magnetite skarn as breccia clasts photos 359 sample ddh 894



a) Passive substitution of limestone by magnetite skarn

primary bivalve bivalve in magnetite skarn, substitution
Figure 1a: Bivalve in limestone from the Berlin skarn on the western pit wall. Figure 1b. Bivalve preserved in a magnetite skarn where the clay-rich surrounding limestone has been passively substituted.


limestone features preserved in magnetite skarn
Figure 2: a) Irregular, elongated patches of white within grey limestone (above). b) comparable feature in magnetite skarn, preserved from alteration (below).



b) Passive substitution of mudstone by magnetite skarn


mudstone sustituted by magnetite, skarn
magnetite skarn structure
Figure 3a: Original sedimentary structure in mudstone from the Berlin wall, W mine wall. Figure 3b. Same structure, at a similar scale in magnetite skarn, where zeolite blocks have been substituted by tremolite-actinolite plus....(72.2m, DDH947).


sedimentary structure in mudstone
sedimentary structure in mudstone substituted by magnetite, skarn
Figure 4a: Original sedimentary structure in mudstone. Figure 4b. Same structure, at a similar scale in magnetite skarn, where zeolite blocks have been substituted by tremolite-actinolite plus....(87-89m, DDH894).


sedimentary structure in mudstone magnetite substitution, skarn

magnetite substitution, skarn

Figure 5a: Original sedimentary slump structure in mudstone, Berlin pit wall. Figure 5b. Reminescent structure in magnetite skarn
(top 75.4m, DDH947; bottom 88m, DDH947)



c) Bedding?


magnetite-rich, and py-cpy-rich skarn magnetite substitution, skarn
Figure 6a: Banded magnetite-rich and py-cpy-rich skarn, approximately parallel to the Parrots Beak thrust (base of the thrust ~284m DDH961 click for cross section). Figure 6b. (~284m DDH961 click for cross section)



d) Other substitution fabric


Pyrite dissemination in magnetite, skarn
Layer parallel Pyrite dissemination in magnetite, skarn
Figure 7a: Dissemnitated py in mag (168.1m DDH947). Figure 7b. Layer paralle Py+cpy dissemination through magnetite skarn (174m DDH947).


Massive pyrite
Sulfide-filled fracture cross-cutting preserved original
mudstone bedding in magnetite skarn
Figure 7c: Massive pyrite showing large grains in finer matrix (178m DDH947). Figure 7d. Sulfide-filled fracture cross-cutting preserved original mudstone bedding in magnetite skarn, and linke with layer-parallel sulfide alteration of pre-existing magnetite (98 m, DDH894).



e) Hydraulic fracturing in skarn: Interaction between bedding and cross-cutting fractures


Sulfide-filled fracture sulfide vein in magnetite skarn
Figure 8a: Sulfide-filled fracture cutting across faint bedding and stalling along a more pronounced bedding heterogeneity (76.5m, DDH947). Figure 8b. Inferred bedding parallel sulfide vein in magnetite skarn linked to a pervasive pyritization of magnetite (DDH958 282m, parallel to the Parrots Beak Thrust).


pyrite veins

pyrite veins
Figure 8c: (98.6m DDH947). Figure 8d. Bedding parallel, narrow and long pyrite veins, physically and continuously linked with cross-cutting wider and shorter veins containing py-cpy-trm-act (78m, DDH947).



f) Higher energy hydraulic fracturing in skarn


sulfide veins in magnetite skarn
Figure 9a: Radial fracturing around a wide and zoned sulfide alteration in pre-existing magnetite skarn: py+qz in center surrounded by cpy. The main length of the sulfide alteration parallels bedding (78.2m DDH947).



g) Timing of magnetite versus sulfide alteration


The two photographs show two sections of the same drill hole spaced by 10 m.


lamination in magnetite skarn
lamination in magnetite skarn
Figure 10a. Fine lamination in magnetite skarn interpreted to represent original mudstone lamination (87 to 89 m, DDH894). Figure 10b: Fine lamination in magnetite skarn fractured by and altered to late stage sulfides (98 m, DDH894).


vein zonation, magnetite-sulphides
Figure 10c. Zonation in a vein, with magnetite surroudning a wedge of pyrite, surrounding an internal wedge of chalcopyrite. (sampled, 101.5 m, DDH908).


Angular magnetite skarn in sulhides matrix
brecciation, skarn
Figure 10d. Angular magnetite skarn blocks in a matrix rich in sulfides (sampled, 99.6 m, DDH894). Figure 10e. Brecciation and alteration of endoskarn by irregular magnetite-rich (diagonally from upper left), cut across by narrow veinlets of cpy+py+mag (109m, DDH908).