Recent Exploration per 20-F:
Recent Exploration
The goal of Great Basin's exploration program at Ivanhoe is to locate high-grade gold deposits that are mineable by underground methods. In general, exploration will be based on comparisons to the Ken Snyder Mine at Midas, located further to the northwest on the Carlin Trend, and the Goldstrike area (i.e. Post-Betze and associated deposits). The comparison with Ken Snyder is based on the similarity in style and mineralogy of the feeder veins that have been intersected beneath the Hollister deposit to those at the Midas discovery. The comparison with Goldstrike is based on the similarity of the size and tenor of the gold leakage anomalies in both the Hollister and Hatter areas; the ore controlling structures; the Lower Palaeozoic stratigraphy; and the association of mineralization with an intermediate composition intrusive body (i.e. the Hatter Stock).
The best exploration potential for the more traditional Carlin-style deposits in the lower plate rocks occur at depths ranging from 5,000 to 6,000 feet (1,524 to 1,829 metres) in the silty carbonate host rocks of the Rodeo Creek, Popovich and Roberts Mountain Formations. Deep drilling by Newmont in 1996 on the northern margin of the Little Boulder Basin stock east of Goldstrike discovered high-grade gold zones at over 6,000 feet (1,828 metres) below the surface. Two intercepts from this zone averaged 206 feet of 0.146 oz. Au/ton (62.8 metres of 5.0 g/tonne) including 22.5 feet of 0.570 oz Au/ton (6.8 metres of 19.54 g/tonne). The Little Boulder Basin discovery may provide the best analogy to the deep exploration planned for the Hatter area and the Ivanhoe Property as a whole, as it demonstrates that high-grade gold mineralization exists over a considerable range of vertical depth throughout the Carlin Trend.
An initial drilling program by Great Basin at Hollister in January 1998 tested one of the vein systems containing a 1994 Newmont core intercept of +30 oz. Au/ton (+1,029 g Au/tonne). Using a N40E oriented fence of six vertical core holes at 25 foot (7.6 metre) spacing, Great Basin's drilling crosscut high-grade intercepts in the northern area of the deposit. Hole IH004 pierced a very high-grade vein zone of 4.6 feet grading 11.13 oz. Au/ton and 103.4 oz. Ag/ton (1.4 metres grading 381.6 g Au/tonne and 3,545.17 g Ag/tonne) within a thicker interval of 10.6 feet assaying 4.96 oz. Au/ton and 47.8 oz. Ag/ton (3.23 metres at 170.06 g Au/tonne and 1,638.87 g Ag/tonne). Unlike the Newmont drilling, this hole was not lost and another intercept of 12.6 feet grading 1.64 oz. Au/ton and 39.0 oz. Ag/ton (3.84 metres at 56.23 g Au/tonne and 1,337.15 g Ag/tonne) was intersected further down the hole.
The 1998 drilling program encountered the first reported quartz-silver-selenide-electrum veining reported in the Valmy Formation and the Ivanhoe area. The first hole in the fence of drilling, IH002, intercepted a very thick banded, brecciated and cockscomb-textured micro to cryptocrystalline quartz vein zone with minor disseminated sulphides. This style of veining had not previously been demonstrated in the upper plate Valmy rocks anywhere on the Carlin Trend. Smaller but similar vein intercepts were found in 5 of the 6 holes, with the higher-grade zones in 1H004 displaying the most interesting vein and sulfide mineralogy. Great Basin's geologists noted several similarities with high-grade core samples from the nearby Ken Snyder Mine in the Midas District, owned by Franco-Nevada Mining Corp. These include visible electrum, a black silver selenide series mineral naumannite-aguilarite, and quartz vein banding with quartz pseudomorphs after bladed calcite; the presence of electrum, naumannite-aguilarite, pyrite, and kaolinite - an assemblage similar to that in the Midas area - was later confirmed by petrographic work. The vein zones at Midas are hosted in volcanic rocks, which are age-equivalent to the Tertiary bi-modal suite overlying the Ivanhoe area. These same volcanic rocks do not appear to host vein zones at Hollister, but rather, occur as diffuse argillic-altered zones of low-grade gold mineralization. The Midas-style veining at Hollister is hosted predominantly in the underlying Ordovician-aged upper plate Valmy Formation.
The similarities between Hollister and Midas prompted Great Basin to design an exploration program to test for lateral continuity of the newly discovered veining along a postulated N47W strike to the vein zones, and other potential vein directions. Significant results were obtained in all four holes drilled. Great Basin planned and implemented a two phase $5.0 million exploration program in 1999 to test the newly interpreted and defined multiple vein systems beneath the Hollister deposit. This program included drilling angled core holes to establish the presence and continuity of this high-grade gold-silver system.
Drill holes tested projected intersections of anticipated major ore-controlling structures trending northeast, east and northwest beneath the Hollister deposit; 59 holes totalling 47,827 feet (14,578 metres) were drilled. Geological mapping and compilation of all existing data at Hollister and Hatter was also done that generated additional high-grade structural targets for drill-testing.
Beginning in 1999, reverse circulation precollars were drilled through the Tertiary strata to reduce overall drilling costs, increase the average penetration rate, and to avoid "squeezing ground" where the pervasive argillically-altered tuffaceous rocks made it difficult to keep the hole open.
Problems with drooping or steepening in the RC precollars was minimized through constant experimentation with different types of downhole tools and stabilizers in the early drilling phase, and casing and gyro-surveying to ensure that hole had not deviated too far for core drilling to hit its target.
The 1999 drilling program resulted in the discovery and initial delineation of two high-grade gold-silver vein systems, Clementine and Gwenivere, in the Valmy Formation beneath the Hollister deposit. At the end of 1999, these vein systems had been drilled over respective east-west strike lengths of 1,800 feet (550 metres) and 1,000 feet (305 metres).
Approximately $10.0 million was spent on exploration in 2000, including 143 drill holes, totalling 141,832 feet (43,230 metres). Drilling in 2000 in the Hollister area extended the strike length and number of gold-silver drill intercepts on the Clementine and Gwenivere vein systems: the Clementine system was outlined over a strike length of 3,000 feet (914 metres), and the Gwenivere system over a strike length of 2,000 feet (610 metres). In addition a third system, called South Gwenivere, was discovered and traced over at least 500 feet (152 metres) (see Figure 3). Analytical results from holes drilled in 1998-2000 are tabulated in the 2000 Annual Information Form and in a 2000 report by Akright.
Exploration in 2000 also took place outside of the immediate Hollister area. Drilling at Velvet, an area located 1,400 feet (425 metres) north of the Clementine vein system, encountered high-grade gold mineralization in a new vein system. Rotary hole 204 returned 15 feet of 0.56 oz. Au /ton, (4.6 metres of 19.20 g Au /tonne) including 5 feet of 1.04 oz. Au /ton (1.5 metres of 35.65 g Au/tonne) in a banded quartz vein intersection in Valmy Formation quartzites.
At Hatter, located to the east of the Hollister area, the enclosing stratigraphy is composed of quartz-rich clastic sediments and a composite granodiorite intrusion. Holes in the Hatter Stock were drilled at shallow 45-degree inclination to give maximum exposure to shallow, epithermal targets at about 1,300 feet (396 metres) below surface. Although one of these holes hit narrow-banded and/or sulphidic veins, only modest gold values were encountered. Historical gold intersections at Hatter are either (a) proximal-internal to the stock or (b) proximal-external to the stock in an area that is cut by several dykes compositionally similar to the Hatter Stock. Significant gold intersections have been obtained from depths up to 2,450 feet (747 metres) below surface, including 2.39 oz. Au/ton over 1 foot (81.94 g Au/tonne over 0.3 metres) in hole IH174. Gold mineralization at Hatter appears to lies at significantly lower elevations than the veins successfully drilled in the Hollister area.
Preliminary three dimensional (3D ESCAN) processing of a 1990 resistivity field data set revealed a distinctive resistive envelope signature enclosing the Clementine-Gwenivere vein systems. A new ESCAN survey was undertaken in 2000 to complement and expand the 1990 data coverage, and extended to the north from the Velvet-Butte trend and to the south and east over the Hatter stock area. Over 1,600 potential fields originating from over 1,600 current injection points were measured; these are defined by over 84,000 individual potential field measurements. Three anomalies were identified for testing. Soil and lithogeochemical surveys, totalling 2,325 soil samples and 654 rock samples, were carried out over the grid-area established for the 3D ESCAN surveys. Weak pathfinder-element geochemical anomalies were outlined that correspond with the three ESCAN targets. These targets have not yet been tested.
Drilling through 2001 continued to delineate the strike length and number of gold-silver drill intercepts on the Clementine and Gwenivere vein systems. To December 31, 2001, Great Basin completed a total of 205,443 feet (62,619 metres) of drilling in 216 combined reverse circulation precollar and core tail holes. The last hole completed in 2001, IH-214, was designed to test for the presence of Devonian carbonates (Popovich and Roberts Mountain Formations) beneath the Roberts Mountain Thrust in the Hollister area. A precollar was drilled and cased to 1,400 feet (425 metres) where it was determined that the casing would seal the hole and eliminate any drilling fluid losses. The core drilling proceeded to 5,500 feet (1,675 metres) using HQ equipment, was changed to NQ equipment with a 20-foot core barrel and the hole completed to total depth of 6,940 feet (2,115 metres).
The deep hole shows that the platform carbonates of the Roberts Mountain Formation extend west under the Hollister deposit. Samples from the base of the Rodeo Creek Formation immediately above the Roberts Mountain Formation (the location of many of the Carlin Trend deposits) are anomalous in Au, Ag, As and Sb, although the intersection is not of ore grade. However, the presence of mineralization in stratigraphy indicates that many of the criteria contributing to the formation of a "Carlin-type" deposit could be present beneath the Hollister-Hatter area.
A re-interpretation of exploration information for the eastern half of the property resulted in the recognition of a second major structural trend on the Ivanhoe property. The north-northwest trend of the newly identified major fault structures is the same as the principal ore-bearing structure at Ken Snyder and other mines in northeastern Nevada. This new structural trend provides potential to find significant, additional high-grade gold vein systems.
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