EXPLORATION
IN THE SHADOW OF THE HEADFRAME: GYMPIE
GETTING MINING
ENGINEERS AND ACCOUNTANTS TO HELP FUND OREBODIES.
Gympie Eldorado Gold Mines
Pty Ltd
Gympie Qld 4570
Key Words: Gympie, gold, mining
Discovered in 1867 and in
continuous production for 60 years to 1927, the Gympie Goldfield is the sixth
largest historical gold producer in Australia.
Over an area of 10 kilometres by 4 kilometres, over 1,500 shafts were
sunk to a maximum depth of 1.0 kilometre, from which 150 kilometres of workings
were developed. A total of four million
ounces at an average grade of 29 g/t Au was produced up until 1927.
Modern exploration began at
Gympie between 1980 and 1995 by Freeport then BHP under joint venture with
Gympie Eldorado Gold Mines (GEGM). Work focussed on definition of the Inglewood
Reef. In 1995 GEGM started a
small-scale mining operation with the objective of providing the basic mining
infrastructure and cash flow to fully explore the goldfield.
Since 1995 total production
has been 266,000 ounces.
Current resources and
reserves stand at 783,000 ounces and 198,00p ounces respectively.
The Gympie Goldfield covers
an area of 10km x 4km and consists of an extensive mesothermal quartz-vein
system hosted within the Permo-Triassic volcanics and sediments of Gympie
Group. Bonanza grades occur where the
quartz veins are in contact with carbonaceous sediments.
Three styles of quartz
veining are recognised:
1. Inglewood
Northwest
stacking sub-vertical structures 1.8-6.0 metres wide with gold grades 6.0 –
10.0 g/t Au.
Feeder
structures.
2. Gympie
Veins
Narrow
(0.2 – 2.0 metre wide) tension veins striking north south and dipping west,
perpendicular to bedding. Historically
averaged 34 g/t Au when in contact with the carbonaceous sediments and
containing bonanza pods with several percent gold.
3. Stockworks
Gympie
Veins where the vein density is sufficient to mine in bulk. Up to 100 metres wide with grades from 8.0 –
12.0 g/t Au.
Orebodies
at Gympie occur as an inter-relationship of three parameters:
·
A north-south corridor
of mineralisation that can be traced through all rock types over a strike
length of 20 kilometres. The Gympie
Goldfield sits in the centre of this corridor.
·
Northwest structures
cutting across the north-south corridor.
These are the Inglewood feeder systems.
·
East-dipping
carbonaceous sediments. All veins
passing through these sediments are enhanced in grade. The best orebodies (size and grade) occur at
the juncture of these parameters.
Exploration up until 2000 has
focussed on the mining environment in order to establish sufficient reserves
for sequenced production and to gain a basic understanding of the geological
controls on mineralisation.
Since 2000 exploration has
steadily extended further into the goldfield.
The Gympie Goldfield contains
considerable alluvial and stratigraphic cover.
The full extent of the goldfield is not understood or contained. Potential exists for the discovery of large
ore systems under shallow cover adjacent to historical and current mining
operations.
For exploration beneath
alluvial cover GEGM has devised a process of air core drilling to fresh rock
with parts per billion assays. In
addition to geochemical anomalies, 50 metre spaced air core drilling provides
the required bedrock geology and structure for diamond drilling to follow upon
a prospect scale.
Exploration prospectivity of
the goldfield is such for the exploration budget to be increased to $7 million
over the 2002-2003 financial year, with an intention to spend $23 million over
the next three years.
With the styles of orebodies
at Gympie and the strong focus on exploration, the integration of exploration
and mining activities is essential.
GEGM has addressed this issue by having all geological activities
(mining, production and exploration under the direct charge of the Chief
Geologist. The Chief Geologist
therefore has line accountability for exploration and support accountability
for production. The General Manager of
the mine in contrast has line accountability for production and support
accountability for exploration. The
intent of this management structure is to marry the benefits of geology with
mine planning and production.
Gold is hard to find and at
$US300 per ounce is a valuable commodity.
Successful exploration therefore enhanced by access to good mine
planning and production personnel. This
is particularly true at Gympie where the exploration and mining functions are
intertwined.