DEPOSIT DESCRIPTIONS - MODULE 2
Image: Mt Leyshon open pit, Queensland.
This module of the tour and the selected gold deposits included on the itinerary comprised:
Module 2 - Proterozoic & Phanerozoic,
Sat 29 November to Thurs 4 December 1997
Telfer, Western Australia.
Granites-DBS-Callie, Northern Territory.
Tennant Creek, Northern Territory.
Charters Towers Gold Field, Queensland.
Mt Leyshon, Queensland.
Cadia, New South Wales.
Bendigo Gold Field, Victoria.
Bendigo Seminar, Victoria.
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The Telfer orebodies are
hosted by late Middle Proterozoic to early Upper Proterozoic quartzites,
sandstones and siltstones. The structure in the mine area is dominated by
a broad gentle domal structure, with shallow bedding dips on its flanks.
The dome is intruded by 680 to 620 Ma granitoids, which on the basis of isotope
studies may be related to the emplacement of mineralisation. Ore deposits
occur as generally conformable reefs around 2 m thick within units of
claystone, mudstone, carbonaceous limestone and argillaceous siltstone, and
comprise pyrite and quartz with minor chalcopyrite, bornite and
chalcocite. Sulphides occur as disseminated blebs and euhedral crystals
replacing sediments or as disseminated and locally massive zones in quartz
veins which are both conformable and cross-cutting. The initial mining
exploited oxidised and higher grade supergene ore in the top 100 m below the
present surface, which were underlain by hypogene mineralisation that rarely
exceeded 3 g/t Au. More recently discovered underground, high grade
hypogene ore does exist, with individual reefs around 50 cm thick which have
grades of 60 g/t, diluted to 10 to 12 g/t Au, 0.8% Cu over mining widths.
In 1988 reserves + production accounted for 146 t Au at an average grade of
2.35 g/t Au. In 1996 open pit resources totalled 92 Mt @ 1.1 g/t Au,
while underground resources were 3.9 Mt @ 11 g/t (indicated) and 7.5 Mt @ 6 g/t
Au (inferred). In 1997, the total measured, indicated and inferred
resource was quoted at 173 Mt @ 1.4 g/t Au.
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Bullock Soak orebodies are grossly stratabound and hosted within a complexly
deformed Lower Proterozoic sequence of fine to medium grained clastic, and
minor chemical meta-sediments, which include a large body of semi-conformable
dolerite. The 5 to 35 m thick host unit is composed of a mixed facies of
banded iron formation, silicate facies, and silicate-sulphide facies, with
lesser carbonate and oxide facies. There are at least four distinct
suites of quartz-dominant veins observed within the gold deposits.
Sheeted quartz veins within a late cleavage are the main ore veins. The
Callie deposit veins are localised within an antiformal hinge zone.
Resources in 1997 totalled 16.8 Mt @ 5.56 g/t Au. At the Callie ore
deposit, cumulative production to December 1996 was 2.3 Mt @ 5.8 g/t Au, with
resources of 9.7 Mt @ 7 g/t Au. During 1994/95 production amounted to
1.35 Mt @ 5.05 g/t Au.
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The Tennant Creek district
had yielded 125 t Au to 1988 from a series of Au-Cu-Bi orebodies in
hematite-magnetite rich lodes. These deposits are hosted by the Lower
Proterozoic Waramanga Group which consists of interbedded greywacke, siltstone,
shale, felsic porphyroids and minor, thin, discontinuous argillaceous banded
iron formation or hematitic shale. The individual lodes vary from a few
tens of tonnes, to 15 Mt of ore, with irregular, but overall ellipsoidal
shapes. Long axes are normally oriented either near vertical or near
horizontal, generally being more pipe-like than tabular. The principal
ore minerals are chalcopyrite, native gold, bismuthinite and various bismuth
sulpho-salts. Gangue is magnetite, quartz, chlorite, talc, hematite,
dolomite, sericite, jasper, pyrite and pyrrhotite. Deposits
included WARREGO - 4.8 Mt @ 2% Cu, 8 g/t
Au; NOBLES KNOB - 2 Mt @ 17.2 g/t Au (incl.
0.5 Mt @ 50 g/t Au); JUNO - 0.45 Mt @ 0.33%
Cu, 56 g/t Au; PEKO - 3.7 Mt @ 4% Cu, 3.5
g/t Au; WHITE DEVIL - >0.53 Mt @ 11.6
g/t Au; ORLANDO - 0.68 Mt @ 4% Cu, 8.8 g/t
Au; ARGO - 0.3 Mt @ 3.8% Cu, 13.6 g/t
Au; GECKO - 4.9 Mt @ 3.8% Cu, 0.8 g/t
Au; ELDORADO - 0.15 Mt @ 22 g/t Au,
TC8 - 0.035 Mt @ 67 g/t Au.
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To 1987, the Charters
Towers Gold Field was the third largest producer in Australia with a total
cumulative output of 225 t Au. The deposits in the immediate Charters
Towers field were developed as a series of 0.25 to 1 m thick gold bearing
quartz veins with 10% sulphides (pyrite, galena, sphalerite, arsenopyrite,
chalcopyrite, tetrahedrite and tellurides) within the Devonian (395 Ma)
Ravenswood Granodiorite Complex batholith. The veins are flanked by green
muscovite and ankerite selvages, which pass laterally into propylitic
(montmorillonite-illite) alteration. The nearby Ravenswood lodes are
hosted by 405 Ma tonalites.
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The Mt Leyshon deposit,
which is 24 km south of Charters Towers, is hosted by the Permian Mt Leyshon
diatreme, formed at the contact of a Cambro-Ordovician volcano-sedimentary
sequence with the Ordovician to Devonian Ravenswood Granodiorite Complex
batholith. This diatreme is roughly circular with a diameter of 1.5 km,
and contains a diverse sequence of early breccias and tuffs related to a suite
of rhyolite porphyries. The principal host to ore is a 300 m diameter
phreatic breccia pipe, localised in the NW of the diatreme. This breccia
is polymict and matrix supported, with sub-rounded to angular clasts varying
from large blocks >1 m in diameter, through predominantly cobble sized
fragments to abundant pebble sized clasts. The clasts are set in a
grey-green to black-green matrix that constitutes 30% of the rock. Gold
mineralisation is predominantly within the breccia, but also occurs in adjacent
wall rocks. It is spatially associated with small scale fracture
patterns, fracture density and other permeability controls, and is present as
replacements of the matrix, cavity fill, disseminated and as veins. The
mineralogy comprises sulphides (pyrite, chalcopyrite, galena and sphalerite)
with free gold and electrum, predominantly in the 1 to 600 µm
range. In 1995 the total resource was 48.3 Mt @ 1.27 g/t Au. In the
same year production totalled 6.77 t Au from 5.34 Mt of milled ore averaging
1.44 g/t Au and 0.49 t Au from heap leaching, as well as 7.5 t
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The Pajingo group of ore
deposits are spread over 40 km2, hosted by the predominantly continental
sediments of the late Devonian to lower Carboniferous Drummond Basin.
These sediments are also intruded by high level, late Carboniferous, felsic
plugs, dykes and intrusive breccias which are locally hosts. Gold occurs
within steeply dipping quartz veins in fractures. Vein fill is dominantly
clear to milky, occasionally red-brown, chalcedonic, micro-crystalline silica,
with fine limonite or hematite bands. Gold is present as electrum and
free gold, while free silver is also obvious. The initial reserve at
Pajingo was 1.15 Mt @ 10 g/t Au, 38 g/t Ag. The recently discovered Vera
Nancy deposit at Pajingo contains 1.75 Mt @ 13.6 g/t Au.
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Cadia is a porphyry Au-Cu
resource associated with an Ordovician volcano-intrusive complex. The
mineralised intrusive has two phases, an inner magnetic diorite and an outer
porphyritic monzonite with coarse orthoclase phenocrysts. The intrusive
is unconformably overlain by Silurian sediments. There is a strong NW-SE
control on both the monzonite, and the veining of the ore deposit.
Mineralisation is developed over an area of 3.5 x 1.5 km, partly in the
marginal quartz-monzonite (adamellite) porphyry phase of the composite
intrusion, and partly in shoshonitic volcanic wall rock. An extensive
halo of sub-economic mineralisation extends to the NW and SE of the main
deposit, while the intrusive is accompanied by a small mineralised magnetite
skarn. The ore occurs predominantly as copper sulphides and gold within,
and disseminated around, sheeted quartz veins in a zone 100 to 350 m wide and 1
km long.. This zone dips at 65° to the SW, and is made up of veins
that are 1 to 20 mm thick, and present at a density of 2 to 5/m. Au is
associated with Cu sulphides in the veins but to a lesser extent in the
disseminated mineralisation. Alteration comprises pervasive
propylitisation, with weak potassic vein selvages, late, structurally
controlled phyllic over-printing and widespread hematite. The resource at
Cadia Hill totalled 352 Mt @ 0.63 g/t Au, 0.16% Cu at the end of 1996, and is
300 m thick, with a mine life stripping ratio of 1.25:1. The nearby Cadia
East resource has a further 150 Mt @ 0.44 g/t Au, 0.43 g/t Cu. Together
these account for 285 t Au. Drilling at the Ridgeway Prospect nearby has
encountered blind mineralisation with intersections including 170 m @ 3.68 g/t
Au, 1.03% Cu, 246 m @ 3.23 g/t Au, 0.94% Cu, 145 m @ 4.3 g/t Au, 1.2% Cu, and
111 m @ 7.05 g/t Au, 1.75% Cu. This represents a high grade core with a
strike length of at least 130 m. The top of the ore zone is at a depth of
500 m, and has (1997) been drilled over a vertical interval of 250 m, being
open at depth. This core is surrounded by a lower grade
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The Bendigo Goldfield is
the second largest producer in Australia after the Golden Mile at Kalgoorlie,
with a historical output of 540 t of primary gold from an 8 x 5 km
area. Of the 6380 t of gold produced in Australia up to 1987, some
2450 t originated from Victoria, 60% of which was from alluvial/deep lead
workings. The Bendigo gold field lies within the Broken Hill-Bendigo
tectonic corridor and is restricted to a tightly folded synclinorium of
monotonous, lower Ordovician turbidite slates and greywackes, which have been
metamorphosed to lower greenschist grade between two major faults. Some
twenty near parallel anticlines have been mineralised within this zone, with
80% of the production being from quartz bodies in anticlinal crests.
These reefs are composed of 'bucky' or laminated quartz with carbonate and
sulphides, mainly pyrite constituting between 0.5 and 2.5%. Gold is
present as 100 µm to 2 mm grains, and occasionally as large
nuggets. Many of the quartz reefs however, are barren.
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An underground tour was
conducted in the Central Deborah Tourist Mine at Bendigo where geological
relationships, saddle reefs, etc., were demonstrated, followed by a 'crib room'
dinner underground, and a seminar comprising presentations on:
The old Central Deborah, now a tourist mine, allows the appreciation of the hosts and the style of mineralisation worked in the past. To 1951 the Deborah Reefs had yielded 0.36 Mt of quartz for 6 t of Au at a recovered grade of 16 g/t Au.
- "An Overview of the Tectonics, Geology and Distribution of Gold in the Proterozoic and Phanerozoic of Central and Eastern Australia" by Dr Neil Phillips, Chief Geologist, Great Central Mines;
- "The History, Geology and Potential of the Victorian Gold Fields" by Tom Dickson, General Manager, Victorian Geological Survey
- "The Characteristics and Potential of the Bendigo Gold Field" and the plans of Bendigo Mining NL by Doug Buerger, a geologist and director of that company.
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Fosterville is located 20
km to the ENE of Bendigo. It comprises fault controlled deposits
developed along and adjacent to a series of major, steeply dipping mineralised
structures developed as shoots over intervals of up to 8 km. The ore is
associated with silicification, brecciation and carbonaceous zones within
Ordovician sandstones, siltstones and shales, with mineralisation in shears and
stockwork veins. At December 1995 the resources totalled 4.79 Mt @ 1.1
g/t Au as oxide ore and 5.90 Mt @ 2.7 g/t Au primary sulphide ore. The
ore is mined by open cut and treated by heap leach. In 1994/95 0.75 Mt of
ore was stacked for leaching.
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For more information contact:
T M (Mike) Porter, of Porter GeoConsultancy
This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.
Porter GeoConsultancy Pty Ltd|
6 Beatty Street
LINDEN PARK, 5065
Telephone: +61 8 8379 7397
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