THE EDIE CREEK PROJECT
Geology & Mineralisation
Regional mineralisation for gold in the Morobe Goldfields has been broadly described as being in three categories dependant on the temperature range of primary formation; Hypothermal or high temperature, Mesothermal or moderate temperature, and Epithermal or low temperature.
The Edie Creek project comprises epithermal systems and subsequent oxidised weathering products. These epithermal systems are related to near-surface volcanic and magmatic release of silica – carbonate – sulphide – gold - silver - bearing aqueous fluids, which deposit silica, as various varieties of quartz, carbonate, sulphides, silver and gold as the fluids cool, boil due to pressure release mechanisms, and / or mix with meteoric groundwater. The relative proportions of quartz, carbonate, sulphides, silver and gold vary substantially from place to place dependant of the variable physical and chemical conditions. Surface expression of these deposits can be as hot springs, sinters and steam eruption craters, and indications of such features in surface rock exposure can be a pointer to gold – silver deposits at shallow depths below.
Three styles of gold deposit are recognised in the Edie Creek Field; diatremes, lodes and stockworks. Diatremes are inverted conical-shaped columns of extremely fractured and milled rock fragments and rock flour formed as a result of steam explosion due to flash boiling of fluids due to near-surface pressure release. Lodes are a composite of planar sheet-like bodies, known as veins and stringers, formed as a result of infill of elongate cavities formed by fault movement. Stockworks are a network of small planar veins and can comprise both randomly oriented and sheeted swarms of veinlets.
Lodes tend to be narrow with high gold - silver grades and are therefore amenable to underground mining methods. Diatremes and stockworks tend to comprise larger tonnage deposits with disseminated lower grades of gold and silver, which are often amenable to open pit mining methods.
Diatremes were only recently recognised as having substantial economic potential within the Edie Creek mining leases. They were recognised by Renison Goldfields Consolidated in the 1980s as being of major importance to gold mineralisation in the nearby Wau area deposits of Golden Peaks, Golden Ridges and Upper Ridges. Two such bodies are currently defined, are known as the Enterprise Diatreme and Alpha South Diatreme, and are located at the intersections of the main NW lode corridor with Slaty Creek and Edie Creek respectively. In addition to their potential as bulk tonnage deposits in their own right, they are also be an important fluid source and thereby may be a factor in controlling the location of higher grade sections of adjacent lode and stockwork systems.
Two substantial stockworks are currently defined; the Karuka Stockwork extending south from the Enterprise diatreme and the Enterprise Stockwork extending to the north of the Enterprise diatreme. These are on the same trend and together extend over a strike length of 600 metres at widths of between 50 and 70 metres. This system has potential for around 75,000 tonnes per vertical metre and represents a major target. Continuous bench channel sampling has produced results of up to 70 metres at 1.4 grams per tonne. The Karuka Stockwork occupies a central zone between the Edie and Karuka Lodes, while the Enterprise Stockwork occupies a central zone between the Enterprise and Karuka North Lodes.
The lode deposits are planar fissure veins, usually between 1-1.25 meters thick. They have been oxidised to varying depths of up to hundreds of meters. The main lode systems within the mining leases are from north to south the Enterprise, Karuka North, Edie 1, Edie 2, Karuka North, Alpha, Mounts, Surman, Midas and Kia Ora.
The non-oxidised zone at depth contains primary sulphide minerals, quartz and carbonates. The gold at the Edie Lodes is often in carbonate ore associated with sulphides. At the Enterprise mine, gold has also been found in quartz gangue where the associated sulphide was pyrite. The grade of gold in the primary zone is lower than the oxidised and can range from trace up to 100 grams per tonne. The gold is usually free milling and very fine grained. The silver in the primary, or non-oxidised zone, is alloyed with the gold (electrum), in complex sulphide minerals, and at Upper Edie Creek is at rare times seen as native silver.
The oxidised zones of lode deposits at Upper Edie Creek, are soft manganese oxides and quartz often with alternating layers of fine quartz, crystalline quartz, iron and manganese oxides. Coarse native gold occurs in rich quartz leaders associated with the lodes, however within the main lodes of quartz-manganese oxides, the gold is very fine grained and floury.
The recovered silver from the oxide zones has primarily been from the silver that has been alloyed within the recovered gold grains. Very high grade silver occurs in the secondary manganese oxides mostly as an amorphous chemical deposition or sorption. In certain instances actual manganese-silver minerals are formed such as aurorite and todorokite. The grade and total amount of silver occurring in the manganese oxides cannot be ignored. Traditionally this silver has not been targeted for extraction but at current prices, head grades of 250 grams per tonne silver will require consideration.
The other major minerals in the oxidised zones at Upper Edie Creek are limonite and goethite, the available literature does not indicate the gold and silver values in these iron oxide minerals however, it is well documented that hydrous oxides of both iron and manganese are good scavengers of trace metals, particularly silver.
When process feed mineralogy is considered it becomes obvious that feed mineralogy will vary in accordance with the degree of oxidation at the point of mining. The Edie Lodes exhibit low-grade leached material at the surface but high-grade secondary enrichment at the lower limit of complete oxidation. The material beneath the lower limit of partial oxidation, ie the commencements of the primary zone exhibit grades of < 7.6 g/t gold and < 300 g/t silver. The bonanza grades at the lower limit of complete oxidation in the Edie mine reported grades of >30 g/t gold and >900g/t silver.
The Enterprise Mine did not exhibit the same secondary enrichment as the Edie Mine and was developed over 4 levels through to 1940 before mining was terminated due to the onset of war. No stoping was completed before termination and a resource of some 30,000 ounces of gold was defined by underground channel sampling.
Graphic presentation of Edie Creek operations