The Carlin Trend has been studied intently by geologists for over 50 years, but it is not yet fully understood. It is arguably the most important gold-bearing formation in the United States.
Northeast Nevada has a complex geological history with several periods of contractional and extensional deformation. The area is a major overthrust belt where older cherts and mudstones of Early Paleozoic age have been thrust over geologically younger Silurian and Devonian sediments. The Carlin Trend, which is several miles wide and at least 60 miles long, is a zone of crustal weakness intruded by felsic dykes and dioritic stocks. The rocks exhibit various forms of alteration; decalcification, silicification, argillization, and pyritization are all present. Pyritization occurs in both silicified and argillized zones and is associated with the gold mineralization. Finely disseminated gold mineralization is found mainly in sedimentary rock and is most often structurally controlled.
Surface weathering reaches depths of 600 to 700 feet, creating oxide deposits, which have been exhausted. Below that, the bulk of the mineralization is refractory sulphide and carbonaceous orebodies. The ore type determines the method of recovery, the sulphide ore being processed by autoclave and the carbonaceous ore by roaster, prior to cyanidation.
The potential of the rich Carlin Trend did not escape Barrick’s exploration department. When the company bought Goldstrike, gold reserves stood at 600,000 ounces. At December 31, 2000, proven and probable reserves contained nearly 24.5 million ounces, and additional resources may contain over 7.2 million ounces… all this after producing 20 million ounces of gold.
This year’s US$4-million drill program includes 40,000 feet underground and an equal amount from surface.
Three major underground areas of interest are being investigated: Meikle, Griffin and Rodeo at depth; the Barrel area west of Rodeo; and the Banshee deposit north of Meikle. These areas lie to the west of the Post Fault, along its footwall. The fault is the boundary to known gold mineralization, with a 2,000-ft downthrow to the east. Interpretation at depth has so far been conjectural.
There is no shortage of targets to be drilled from surface. When Barrick and Newmont made their asset change, Barrick’s geologists gained access to new areas to explore and a lot of drillhole data. The data are being reinterpreted and added to, with special attention being paid to the mineralization and distribution of geochemical indicator elements. Geophysical surveys of the consolidated Goldstrike property are also being conducted this year.
A closer look is being taken at the Dee and Rossi properties, which lie north of Goldstrike. Barrick is earning a 60% interest in both holdings. Dee is a former producer owned by Glamis Gold. Over 47,000 feet of underground drilling was carried out in a number of promising zones last year, and a further 23,000 feet will be drilled this year. Mineralization plunges off at depth from the Dee property north to the Rossi ground belonging to Meridian Gold. Barrick completed a geophysical survey last year, and its drilling program successfully extended the mineralized zone. This year the resource is being updated, and a 32,000-ft drill program will target areas outside known mineralization.
Barrick explorationists are not slowing down. Goldstrike has more promising zones to be defined, and there are at least two other properties nearby to be investigated. All geologists will have promising targets to keep their skills sharp.