Big or Bigger
The Carlin Trend
The Carlin Trend is North America's most prolific gold-producing area and hosts the second-largest known gold resources in the world, after the Witwatersrand in South Africa. More than 107 million ounces of known proven and probable reserves occur on the Carlin Trend. And there may be up to 180 million ounces of resources, depending on what you include in the number.
The trend is a 40-mile-long, northwest to southeast zone of low-grade, epithermal deposits, discovered in 1961 by John Livermore and Alan Cope, geologists then working for Newmont [NYSE:NEM]. The mineralization was near surface (only 25 meters down), but so finely disseminated that all traces were microscopic. In the areas that proved economic, assays averaged 6.2 g/t gold. Newmont opened the original Carlin open pit mine in 1965, before the introduction of heap leaching. When that technology matured in the 1980s, production ramped up to where the Carlin Trend now accounts for over 35% of all U.S. gold output.
In 1994, mining companies began moving underground to exploit the higher-grade ore down dip from existing open pits. These deeper deposits have been discovered at Rossi, Dee, Meikle, Gold Bug, Rodeo, Deep Post, Deep Star, Turf, Four Corners, West Leeville, Hardie Footwall, Deep Carlin, Mike, Rain, Tess and Rain Extension. Taken together, these recent underground discoveries contain a total of 42 million ounces of announced gold reserves, at an average grade of 10.8 g/t. Large quantities of additional unannounced gold resources are now drill indicated on the Carlin Trend.
The Cortez Trend
-------------------
The Cortez Trend is in the same area of Nevada and is similar in size to the Carlin Trend, but is displaced about 50 miles southwest. It includes the Pipeline Mine Complex (12 million ounces) to the north and the Gold Bar Mine (1 million ounces) to the south.
At a meeting of the Geological Society of Nevada in mid-2004, a representative of the CJV - the Placer Dome/Kennecott Cortez Joint Venture - announced that they had intercepted 1.5 ounces of gold per tonne (over 46 g/t) over an interval of 400+ feet. Some reports have it at almost 2 ounces per tonne - a staggeringly rich intercept. Current Proven and Probable reserves at Cortez/Cortez Hills, as announced by joint venture operator Placer Dome [NYSE:PDG; TSX:PDG] (now part of Barrick Gold, [NYSE:ABX; TSX:ABX]) stand at 10 million ounces, with another 10.1 million ounces in resources). The CJV has now approved mine construction. Since the much-discussed CJV announcement at the Geological Society of Nevada meeting, the Cortez Trend has become one of the most active prospecting areas in Nevada. The expectation - or hope - that the Cortez Trend is now an opportunity like the Carlin Trend was in the 1960s is clearly the driving force in the area.
But is this hope realistic?
Comparison
The Eureka-Battle Mountain region of Nevada has two layers of sedimentary rock: the “Upper Plate” and “Lower Plate.” The Upper Plate does not typically host higher-grade gold mineralization, but can contain indications of higher grades in Lower Plate rocks that have “leaked” upwards. Lower Plate mineralization (Silurian-age Roberts Mountains formation) correlates with the major gold deposits of the Carlin Trend, where a great deal of surface erosion and other geological activity has brought Lower Plate blocks ("horst blocks") within range of open pit mining in some places. "Carlin-style" deposits contain disseminated gold mineralization, usually structurally controlled (meaning, along fault lines, etc.). Mineralization may be predominantly oxides, sulphides, refractory or carbonaceous sulphides.
Cortez Trend deposits are replacements or disseminations in calcareous sediments and limestone strata, also in Lower Plate rock. As on the Carlin Trend, one of the keys to discovery is finding Lower Plate rocks that have been exposed in or through "windows" where the Upper Plate has been eroded.
The largest and highest-grade discoveries along the Carlin Trend are associated with major faults. These are very old faults, as are the Cortez faults, though there is considerable disagreement among geologists on numerous issues relating to these faults. Consulting geophysicist Hans Rasmussen, who once worked with Newmont on the Carlin Trend, believes there is a strong possibility that the Cortez Structural corridor system may be both older and bigger than the Carlin fault system. Being older is good: that allows more time for more geological events to occur. The reasoning is long and technical, but the bottom line is that if Rasmussen is right, the Cortez Trend could prove to be not only as big as Carlin, but bigger. |
