The Mount Morgan copper-gold deposit is located on the Tropic of Capricorn in the coastal ranges of eastern Queensland, 36 km north-west of Rockhampton. The deposit was discovered in 1882 and was mined by underground and glory hole methods to 1927, and as an open pit from 1932, up to 1981 when the ore was exhausted.
The host sequence belongs to the Calliope island arc which extends along the east coast from Rockhampton to Warwick on the New South Wales border. In the Mount Morgan District this sequence, which has been folded into a 70 km long SE trending anticline and comprises the Early Devonian Mount Holly Beds, Middle Devonian Mine Corridor Volcanics and the Middle Devonian Capella Creek Beds. An acid volcanic unit, the Mount Warner Volcanics which is found locally below the Capella Creek Beds is equated with the Mine Corridor Volcanics.
In the mine area the Mine Corridor Volcanics occur as a roof pendant within the Mount Morgan Tonalite and comprise from the base:
Lower Mine Pyroclastics, +850m thick - clean coarse grained quartz-feldspar crystal-lithic tuff and some coarse fragmentals,
Graben Sequence, 200m thick - interbedded fine ashy sediments and muddy coarse quartz-feldspar crystal-lithic tuff,
Banded Mine Sequence, 170-200m thick - thinly interbedded chert and quartz-feldspar crystal tuff with jasperoid at the top and a 15m thick limestone at the base,
Upper Mine Pyroclastics, 750m thick - clean coarse grained quartz-feldspar crystal-lithic tuff and some coarse fragmentals,
Arnold's Ridge Felsite, 60m thick - fine grained crystal ash tuff,
Baree Felsite, +400m thick - siliceous aphanitic rocks and quartz-feldspar crystal-ash tuff.
The mine exploited the Main Pipe body of pyritic massive sulphides and the adjacent siliceous stringer ore (10% sulphides) of the Sugarloaf orebody. When the host sequence is re-oriented to a horizontal position, compensating for fault offsets, folds, etc., the orebody is shown to have a pipe like geometry at a high angle to stratigraphy with maximum dimensions of 750 x 250 x 300 m. Minor conformable mineralisation adjacent to the stratigraphic top of the massive sulphides grades down dip away from the ore to a sequence of interbedded crystal tuff and cherty rocks which include some jasperoids of the Banded Mine Sequence.
The principal minerals in the ore zone are pyrite, quartz, pyrrhotite, chalcopyrite, magnetite and sphalerite. Gold, gold tellurides and silver minerals, together with chalcopyrite and pyrite were commercially important. Most of the recoverable gold occurred as disseminations of native gold of ultrafine to sub-microscopic dimensions.
The accompanying halo of alteration (an inner zone of silicification adjacent to ore, surrounded by quartz-sericite with an associated broad pyritic halo which is pronounced within 50 m of ore) has been cut by the late Devonian Mount Morgan Tonalite. This has in turn been overprinted by late stage mineralisation and alteration associated with the tonalite.
Total production by closure was 50 Mt @ 0.72% Cu, 4.75 g/t Au for 238.9 tonnes of gold and 360 616 tonnes of copper.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2005)