DOGWOOD PORPHYRY Cu-Mo PROSPECT, BENAMBRA TERRANE, VICTORIA

Simon Maher
Geological Survey of Victoria, P.O. Box 500, East Melbourne, VIC 3002
LOCATION to the Climax-type Mo intrusions that formed from lithospheric and
lower crustal melts broadly coeval with strong rift-related extensional
The Dogwood Prospect is 12 km SW of Buchan, Victoria, at 37°34’25”, faulting. Extension and rifting are thought to have occurred in response
148°00’31”; Bairnsdale 1:250 000 map sheet SJ55-07. to strike-slip faulting along the Ensay, Cassilis and Haunted Stream
DISCOVERY HISTORY dextral faults.

Australian Anglo American Prospecting Pty Ltd (AAAP) discovered REGOLITH

low-grade porphyry Cu-Mo mineralization at Dogwood during 1978-83 Weathering and tectonic history
while following up Cu anomalies in drainage sediment and soil samples The weathering of rocks of the Dogwood prospect is linked to the
(Orr, 1980). Supergene chalcocite overlying the mineralization was tectonic evolution of SE Australia. Weathering, initiated by the Middle
delineated by RC and diamond drilling by CRA Exploration Pty. Devonian Tabberabberan Orogeny, continued into the Late Mesozoic,
Limited (CRAE) during 1988-94 (Maher, 1994). Recognition of by which time it had produced a deeply weathered land surface of
supergene Cu mineralization in the Benambra Terrane, although in low relief. Break up of Gondwanaland in the mid-Cretaceous resulted
this instance uneconomic, highlights the potential of numerous other in major subsidence in the Gippsland Basin, combined with major
porphyry Cu ± Mo ± Au prospects in the region and the importance uplift of a broad belt to the N of the coastal plain. Uplift began a
of immobile pathfinder elements, like Mo, that can provide a vector to continuing cycle of incision and lateral erosion. The Dogwood prospect
undiscovered acid-weathered porphyry Cu deposits. lies between the subsided and uplifted landscapes; its mid-Palaeozoic to
PHYSICAL FEATURES AND ENVIRONMENT Late Mesozoic regolith is partially preserved in resistant rocks. Three
main regolith units have been recognized and their distribution is shown
The prospect lies on the southern margin of the Eastern Highlands, N in Figure 2A.
of the Gippsland coastal plains at between 240-440 m AHD. Relief is
mostly less than 100 m, but resistant contact-metamorphosed Pinnak Miocene-Pliocene fluvial sand and gravel
Sandstone, surrounding the Kaerwut Granodiorite, forms prominent Remnants of a Miocene-Pliocene fluvial sand and gravel sheet occur in
ridges up to 180 m. The climate is humid with annual rainfall around the northern part of the prospect. The sheet has a flat top at around 200
700 mm. Mean maximum and minimum monthly temperatures are m AHD. Clay, from decomposed lithic clasts, and localised ferruginous
25°C and 12°C for January and 14°C and 3°C for July (Bureau of cement are products of Quaternary weathering.
Meteorology, 2003). Recessive Kaerwut Granodiorite
GEOLOGICAL SETTING The largest granodiorite ‘outcrop’ (400 x 400 m) has been deeply
incised and lies at 280-260 m AHD. Incision commenced in the mid-
The prospect lies within the Benambra Terrane of the Lachlan Fold Cretaceous and stripped the mid-Palaeozoic to late Mesozoic regolith.
Belt; close to the Cassilis, Ensay, and Haunted Stream faults. The Cainozoic weathering has left a shallow but well developed pedolith
prospect occurs within the multiphase Kaerwut Granodiorite (Klicker, and saprolith. The granodiorite is covered by 4-10 m of colluvium shed
1984), one of a suite of granodiorite-tonalite stocks of probable Early from the surrounding high-relief contact-metamorphic aureole.
Devonian age that include the Bete Bolong Suite. These flank the
southern half of the Buchan Rift and, with Ordovician Pinnak Sandstone Resistant, leached Pinnak Sandstone
country rocks, host subeconomic porphyry Cu-Mo mineralization Pinnak Sandstone saprolith overlain by thin soil is widespread. The
(Figure 1). VandenBerg et al. (2000) considered that this mafic, pedolith has been eroded. The saprolith is pallid, fractured sandstone,
oxidized, high-level magmatism was linked to rifting and was similar siltstone and shale with abundant goethite, from intense leaching by
acidic groundwater. Reflectance spectrometry (Maher, 1994) identified
a supergene argillic assemblage of kaolinite-halloysite±illite. Acidic
weathering, due to weathering of abundant sulphide, began in the mid
Palaeozoic and continued into the late Mesozoic. The distribution of
acid-weathered rocks corresponds with abundant hypogene pyrite in the
sericitic alteration zone (Figure 2A). The saprolith has been protected
from erosion in resistant contact-metamorphosed Pinnak Sandstone that
forms a shell with an elevation of up to 440 m AHD around the Kaerwut
Granodiorite.
MINERALIZATION
The hypogene mineralization comprises chalcopyrite and molybdenite
as disseminations and stockwork veins centred on the Kaerwut
Granodiorite and its apophyses and extending into the surrounding
Pinnak Sandstone. The highest Cu grades rarely exceed 0.4% (best 0.5
m @ 1.65% from 168.0 m in DG1) and are centred on the granodiorite
and surround inferred NE sinistral faults. Highest Mo grades are
centred on the granodiorite and rarely exceed 1200 ppm (best 1 m @
2750 ppm from 209.0 m in DG6), but Mo concentrations are also high
around the faults.
Propylitic alteration (chlorite-epidote-pyrite) envelops some veins
close to the granodiorite and sericitic alteration (pyrite-sericite-quartz)
surrounds the granodiorite (Figure 2). Weber (1999) described
plagioclase cores consistent with restite mixing for one of the Bete
Bolong Suite and speculated that chalcopyrite-molybdenite along joints,
faults and in late dykes originated from a residual chlorine-rich fluid
Figure 1. Simplified Early Devonian regional geology around the formed during solidification.
Buchan Rift with porphyry copper prospects (after Rajagopalan, 1999).

© CRC LEME 2003 Dogwood Page 1

Figure 2. Geology (A); Cu, Pb and Zn soil geochemistry (B); Mo and Au soil geochemistry (C). Plans highlight samples containing >90th percentile of
elements. In B, Pb and Zn have similar distributions. Data illustrate concentric Mo, Cu-Au, Pb-Zn-Mn and Au zonations, centred on the granodiorite
(Maher, 1994).

© CRC LEME 2003 Dogwood Page 2

Figure 3. Section along the Polygam Fault showing down-hole Cu and Mo. Supergene Cu enrichment, as a chalcocite blanket, occurs above the
mid-Palaeozoic – Late Mesozoic acid-weathering front. The granodiorite is inferred from drilling, Rajagopalan’s (1999) magnetic interpretation and
the distribution of contact metamorphosed Pinnak Sandstone.
The most significant Cu grades are from a supergene chalcocite blanket Orr, D.B., 1980. Australian Anglo American Prospecting Limited. EL
developed over the sericitic alteration zone and preserved in contact- 671 Bruthen. Report for the period ending 31 March 1980.
metamorphosed Pinnak Sandstone. Highest grades were along the Minerals and Petroleum Expired Exploration Reports File
conjugate Polygam Fault (Figure 2). The best intersection was 24.4 (unpublished). 20 pp.
m @ 0.8% Cu as chalcocite from 65.6 m in OR46 (open at depth,
Rajagopalan, S., 1999. Porphyry-type copper deposits, eastern Victoria.
including 13.3 m @ 1.1% Cu) from a pervasively sericite-quartz altered
In A.J. Willocks, S.J. Haydon, M.W. Asten, and D.H. Moore
granodiorite apophysis along the fault. Grade and thickness of the
(Editors) Geophysical signatures of base metal deposits in
supergene mineralization, and grade of primary mineralization, decrease
Victoria. Report 119, Geological Survey of Victoria, pp.
sharply away from the fault (Maher, 1994).
113-127.
REGOLITH EXPRESSION
Vandenberg, A.H.M., Wilman, C.E., Maher, S., Simons, B.A., Cayley,
Pinnak Sandstone, containing only 800 ppm Cu, overlies the 24.4 m @ R.A., Morand, V.J., Taylor, D.H., Moore, D., Radojkovic, A.,
0.8% Polygam Fault intersection. Acidic groundwater has transported 2000. The Tasman Fold Belt System in Victoria. Geological
mobile metals and destroyed hypogene alteration. The sample media Survey of Victoria Special Publication, Melbourne, 462 pp.
summary table shows depletion of Cu and Zn in acid-weathered
Weber, G.B., 1999. PGM Management Pty Ltd. EL 3829 Yalmy.
rock compared to fresh rock. Molybdenum and Pb are relatively
Annual report for the period ending 20 December 1999.
immobile here and are little depleted. Molybdenum can be used to map
Minerals and Petroleum Unexpired Exploration Reports File.
hypogene mineralization in the regolith. Figures 2B and C show
(unpublished).
broad, granodiorite-centred, hypogene metal zonation in soil. Nearly
all soil and drilled samples lie within the Dogwood hydrothermal White, W.H., Bookstrom, A.A. and Kamilli, R.J., 1981. Character and
system. As a consequence, backgrounds and thresholds are not able origin of climax-type Mo deposits. In B.J. Skinner (Editor)
to be determined. The geometric mean is given as a measure of Economic Geology 75th Anniversary Volume, 270-316.
central tendency for pathfinder elements. Although the 90th percentile is
arbitrary, it accurately maps the metal zonation at Dogwood (see sample
media summary table and Figures 2B and 2C). The chalcocite blanket
is now perched at 250-390 m AHD. Incision of the blanket around the
altered Kaerwut Granodiorite has created one of the highest ranked Cu SAMPLE MEDIA—SUMMARY TABLE
drainage sediment anomalies in E Victoria.
Sample Indicator Analytic Detection Geometric 90th Max anomaly Dispersion
The chalcocite blanket was developed above the mid-Palaeozoic to medium elements methods limits (ppm) mean (ppm) percentile (ppm)
(ppm)
late Mesozoic weathering front, now between 250 and 390 m AHD. Fresh rock* Cu ICPAES 5’ 378 1296 16500

Chalcocite commonly coats hypogene pyrite, where present. It has Mo

Pb
ICPAES
ICPAES
5 & 1’
5
12
7
213
20
2750
1030
been preserved in resistant contact-metamorphosed Pinnak Sandstone Zn ICPAES 5 27 72 254
Au PM209 0.01 0.03 0.06 0.26
and has been eroded around the Kaerwut Granodiorite during uplift and
rejuvenation of drainage initiated in the mid-Cretaceous. Consequently, Acid-
weathered
Cu
Mo
ICPAES
ICPAES
5’
5 & 1’
240
9
584
26
860
275
high
low
supergene mineralization on the Polygam Fault lacks the strike extent [leached] rock^ Pb ICPAES 5 7 12 64 low
Zn ICPAES 5 9 22 87 high
to be economic (Figure 3). Au PM209 0.01 0.02 0.05 0.10 low

Residual soil# Cu ICPAES 5 98 280 922 high

Mo ICPAES 5 6 25 100 low
Pb ICPAES 5 10 23 404 low
Zn ICPAES 5 14 54 1100 high
Au PM205 0.001 0.002 0.007 0.313 low

ACKNOWLEDGEMENTS Drainage Cu AAS 5 22 160 1900 high

sediment+ Pb AAS 5 9 20 70 high
Interpretations were developed with D. Waterfield, I.C. Colliver and T. Zn AAS 5 24 50 255 high

Aravanis. Reviews were by A. Radojkovic, A.H.M. VandenBerg and R.

Buckley. ICPAES—mixed acid digest including HF; Inductively coupled plasma atomic Emission Spectrometry.
PM205—50 g sample: aqua regia digest—AAS/GFA.
PM209—50 g sample: fire assay—AAS.
REFERENCES * Includes fresh rock from CRAE and AAAP drill holes except DG3, 4 & 6. Large numbers of Kaerwut
Granodiorite samples from these holes were excluded so that the comparison between fresh rock and soil or
Bureau of Meteorology, 2003. www.bom.gov.au/climate. acid-weathered rock, that have few Kaerwut Granodiorite samples, was not biased.
‘ Samples from DG1 were analysed by AAS after perchloric acid digestion.
Klicker, T.M., 1984. Sedimentology, palaeovolcanology and ^ Only weathered samples without sulphide from CRAE and AAAP drill holes with substantial supergene
copper mineralization (>0.5% copper; OR9, 25, 40, 45, 46, DG1) were included in the dataset.
mineralization in an associated bedded volcaniclastic and # CRAE unsieved samples. Samples of transported colluvium and Miocene–Pliocene fluvial sand and gravel
intrusive complex, Devonian Snowy River Volcanics, Bruthen- were excluded.
+ AAAP -80# sieved samples. Insufficient samples were analysed for Mo for these results to be included in
Buchan area, eastern Victoria. B.Sc. (Hons) thesis, Monash the analysis. No samples were analyzed for Au.
University, Melbourne (unpublished). 376 pp.
Maher, S., 1994. CRA Exploration Pty. Limited. EL 3012 Breakfast
Creek. Final report. Minerals and Petroleum Expired
Exploration Reports File (unpublished). 16 pp
© CRC LEME 2003 Dogwood Page 3