Great Basin delves under Hollister

Deep drilling by Great Basin Gold (GBG-V) is testing for Devonian carbonate rocks below the Hollister deposit at the Ivanhoe property in Nevada’s Carlin trend.

Ivanhoe is midway between two major operations: Franco-Nevada Mining’s Ken Snyder and Barrick Gold’s vast Goldstrike property. A low-grade, disseminated deposit, Hollister was previously mined as an open pit. However, recent drilling has focused on high-grade, gold-silver veins similar to those being exploited at Ken Snyder.

The veins delineated to date occur immediately below the Hollister deposit and are hosted by Ordovician Valmy Formation rocks that have been thrust over younger carbonate rocks and form the upper plate of the Roberts Mountain thrust fault. The deep drilling is aimed at Devonian carbonate rocks, such as the Popovich and Roberts Mountain formations, below the Roberts Mountain fault. These formations host most of the large gold deposits in the Carlin trend.

Drilling by previous operators revealed Devonian fossils 1,600 ft. below surface at Hollister. Subsequent re-logging of these holes revealed that the strata containing the fossils are rocks of the Rodeo Creek formation, which overlies the main gold-bearing formations (Popovich and Roberts Mountain).

Great Basin hopes to intersect these favourable units beneath the Clementine-Gwenivere vein systems at a down-hole depth of 5,000-5,500 ft. It will be the first time that drilling at the property has tested such a high-risk, though potentially high-reward, target.

High-grade veins

Meanwhile, Great Basin will continue to advance the existing high-grade veins at Ivanhoe. Toward that end, the company has initiated permitting and technical work as part of a preproduction development program. Underground drill-testing will provide data on grade and geometry for final engineering and design.

Mining contractor Dynatec has already prepared a scoping study for a 2-stage development program at Ivanhoe. For US$27 million, the high-grade veins could be mined and processed under a toll-milling plan, thereby eliminating the cost of building a plant on site. In the next stage, the vein systems would be developed in order to boost overall production rates.

Earlier this year, Great Basin released drill results from a program focused on vein targets at Ivanhoe. Drilling in the South Velvet area, 1,400 ft. north of the Clementine vein system, encountered a new vein system and returned 15 ft. of 0.56 oz. gold, including 5 ft. grading 1.04 oz., from hole 204. A hole drilled in the North Velvet zone returned 6 ft. of 30.2 oz. silver. Other results from the North Rowena and Hatter areas include 5 ft. grading 0.58 oz. gold and 1 ft. of 2.39 oz. gold plus 1.4 oz. silver.

Since then, the new South Velvet vein trend — which strikes south-southeast, crosscutting the main east-southeast Clementine and Gwenivere vein systems — has been expanded with widely spaced holes over a strike length of 3,000 ft. A second trend, the new East Clementine vein trend, parallel to the South Velvet structures, is emerging and remains open for expansion. The extension of the East Clementine trend to the northwest appears to meet an area where overlying rocks are gold-enriched, suggesting that the fracture system may have provided some gold to the overlying rocks.

Great Basin says these new vein trends tie together previously unlinked drill results “and are the first indication at Ivanhoe of the more characteristic north-northwest orientation of other important epithermal gold-silver deposits along the Carlin trend, such as the nearby Ken Synder mine.”