The Mount Morgan area is in east-central Queensland about 25 miles southwest of Rockhampton. The climate is humid mesothermal, and the average rainfall is about 31 inches annually. Soil horizons are poorly developed or absent in the environs of the Mount Morgan mine. There exists a correlation between several plants and topography, but mineralization-indicator plants are absent. The scarcity of water has been an industrial and civic problem since the inception of mining.
Placer deposits of gold were worked in the district as early as 1858 with the first hard rock venture in Queensland being initiated in 1866 on a quartz vein about 12 miles northeast of Mount Morgan. A smelter operated during 1876-1877 on copper carbonate ore at the Dee Copper Mine, 9 miles southwest of Mount Morgan; and Chinese miners worked the small alluvial deposits in Linda Gully and Mundic Creek in 1869 within a hundred yards of the auriferous gossan on Mount Morgan before gold was discovered there in 1882.
The active mining of the supergene gold deposit from the surface was begun by a Syndicate in 1882; then in 1886, the Mount Morgan Gold Mining Company Limited was formed. Under the guidance of J.W. Hall, it became the premier gold mine in the world by 1890. A smelter was blown in to produce matte copper from the sulphide ore in 1906 but the company was forced into liquidation in 1927 through attempting to mine low-grade ore by underground methods and by a fire underground. Openpit mining was inaugurated in 1932 by Mount Morgan Limited. In 1967, Mount Morgan Limited merged with Peko-Wallsend Investments Limited.
The total production from the Mount Morgan mine for the period 1884 to June 27, 1965 amounted to 6,873,318 ounces of gold; 683,599 ounces of silver; 487,145 tons of pyrite concentrate; and 261,671 tons of metallic copper: with a total value of $A263,000,000. In total production of metallic copper Mount Morgan ranks third in Australia; in 1964 it ranked eighth in gold production. Mount Morgan has been the leading gold producing base metal mine in Australia for nearly half a century. From 1934 to 1968, the price of copper has largely controlled the prosperity of Mount Morgan Limited.
The Coast Ranges are the chief physiographic features in the Mount Morgan district. These tectonic mountains are essentially parallel to the coast line. In the environs of the mine a dendritic drainage pattern is commonly developed, and the local relief is moderate — as much as 800 feet within a square mile of the mine.
The Mount Morgan batholith is an igneous complex of Upper Devonian age. The core of the batholith is composed of a differentiated, phaneritic series comprising of noritic gabbro, diorite, quartz diorite, trondhjemite, granite, and alaskite species. The rapidly crystallized roof and flanks of the batholith formed an aphanitic association of intrusive rhyolite and quartz latite masses beneath the batholith's own cover of pyroclastic and flow rocks.
The phaneritic series commonly occur as curved integrals suggesting the magma was in a static state during the various stages of differentiation. Hydrothermal alteration associated with ore mineralization caused biotitization, silicification, sericitization, and pyritization of the quartz diorite boss exposed in the open-pit. In addition trondhjemite and alaskite rocks have been silicified and pyritized within each of a cluster of weakly mineralized breccia and shatter pipes a mile south of the mine. Deuteric alteration is best developed in trondhjemites, accounting for about 3 percent of their bulk.
The intrusive rhyolite unit is host for about half of the ore deposit. Around the core of massive sulphide mineralization, two zones of hydrothermal alteration are recognized in the rhyolite — quartz with lesser amounts of disseminated sulphides in the inner zone, and sericite and quartz in the outer zone. The two zones of mainly noneconomic alteration are restricted to a narrow aureole generally less than 300 feet wide around the massive sulphide cores. Even in areas contiguous to massive sulphide ore, the quartz latite unit is weakly mineralized and uneconomic to mine.
The bulk of the massive sulphide ore has been mined from the tuff and lava unit that formed a crust over the intrusive rocks of the batholith. Less than 70 acres of these rocks remain, having been virtually removed by erosion. The Dee Volcanics form part of the volcanic pile that overlies the intrusive series of the batholith. They are unrelated to ore mineralization at Mount Morgan. This formation covers at least 50 square miles in the district.
The Devonian igneous rocks at Mount Morgan are shown to be closely related in time and comagmatic on the basis of their high Na/K ratio. Thus they define a petrographic province. This series of rocks is not considered as the source of ore metals that have been concentrated in the Mount Morgan cluster of ore pipes.
Dilation dykes have intruded the older Palaeozoic rocks in Permian time. They constitute about 3 percent of the area delineated in plate 1. Chemical analyses of these post-ore intrusions show they are unrelated to the Mount Morgan petrographic province.
Mesozoic rocks vinconformably overlie Palaeozoic rocks in the proximity of the mine. These non-marine strata (the Razorback Beds) are about 200 feet thick and form mesa-butte topography. Several million ounces of gold, released by erosion of the ore body, may be trapped as buried placer deposits in the Razorback Beds. From the initial dips of cross-strata the direction of the palaeocurrents has been established. Accordingly, areas of potential placer gold production are now known. Seismic data may be utilized to plot the ancient watercourses on the elevated Jurassic landscape.
Small alluvial deposits of Quaternary age have yielded minor amounts of gold eroded from quartz veins in the headwater reaches of the Dee River.
The northwest-trending Gracemere Anticline forms the dominant structure in the district. There is close spatial relationship between the distribution of Devonian intrusions and this tectonic structure. Nevertheless, these structures appear to be independent features.
Rose diagrams of joint and dyke patterns in Palaeozoic rocks show close similarity. Because the very numerous dykes are almost vertical and major faults are the normal type, the district is inferred to have been deformed by vertical uplift in accordance with Anderson's theory. The Slide Fault Zone comprises many parallel and subparallel normal faults along which a reversal in the strike slip direction has occurred, the direction of the strike slip movement in the Permian rocks being opposite to that of the Devonian. The total strike slip movement along this fault zone since Devonian time approximates 700 feet, 330 feet in one direction in the interval Devonian to Permian and 370 feet in the opposite direction between the Permian and Jurassic; since deposition of the Razorback Beds in Lower Jurassic time there has been no strike slip or dip slip displacement.
Deflection in the strike of a northwest-trending set of dykes is inferred to have been caused by the Mount Morgan ore deposit, because less energy is required to propagate fractures in mineralized rock than in its unaltered equivalent. Thus the strike of the deflected dykes defines the horizontal projection of the plunge of mineralized rock (though not its angle) and this deflection may serve as a guide to ore.
A cluster of five mineralized breccia pipes and shatter pipes outcrops about a mile south of the Mount Morgan ore pipes. Four of these hydrothermal structures occur within embayments formed by the phaneritic members of the Mount Morgan batholith. The Talban Hill breccia pipe has all the features of an explosive pipe without exhibiting the effects of "milling" or fluidization as exemplified by the post-ore hydrothermal pebble dykes associated with the Mount Morgan ore pipes.
The embayment in the igneous rocks bordering the Mount Morgan ore deposit is the prime governing influence controlling the spatial relations of the ore pipes. The regular concavity of the embayment is interrupted by a quartz diorite apophysis projecting to the east. At the place where the apophysis leaves the main mass of the pluton the dip of the embayment is reversed from southeast (towards the ore body) to northwest (away from the ore body). Moreover at this same place the Porphyrite dyke intersects the apophysis and the embayment. Thus there is a spatial correspondence of the inflection of the embayment, the quartz diorite apophysis, the Porphyrite dyke, and the distribution of the ore pipes.
The abstract for the ore deposit is given in Volume 2.