Great Basin Gold’s (GBG) Bonanza Grades – A Closer Look
November 9th, 2010
By: Marco G.

http://goombarhsedge.blogspot.com/

Great Basin announced the highest “Bonanza Grades” of Gold found, 2560 ounces per ton, that the author has ever encountered this morning:
Great Basin Encounters Bonanza Grades in Blanket Style Mineralization at Hollister
VANCOUVER, Nov. 9 /CNW/ – Great Basin Gold Ltd. (“Great Basin” or the “Company”), (TSX: GBG; NYSE Amex: GBG; JSE: GBG) announces that trial mining in the Blanket Zone above the Main Clementine vein (number sign)18 at its Hollister project in Nevada has encountered bonanza grades of gold and silver. The Company cautions investors and readers that we are making this announcement out of an abundance of concern over interpretation of this information and, as the information may be known locally in the region of the mine site, the Company felt obligated to make it public.
Channel sampling carried out in conjunction with trial mining in the Blanket Zone has encountered the bonanza grades over a strike distance of 170 feet (57 meters).Channel samples taken every 10 feet (3 meters) gave values ranging from a low of 1.5 oz/ton(52.0g/t) Au and 3.2 oz/ton(111.9 g/t) Ag to a high of 2,560.4 oz/ton (88,845.9 g/t) Au and 1,829.8 oz/ton (63,494.1 g/t) Ag over channel widths from 0.3 to 2 feet wide. The current stope is continuously mineralized along its 180-foot (60-meter) length. Diluted over 3.5 feet (the width of the stope development), the average sample values were 66.4 oz/ton (2,404 g/t) Au and 78.5oz/ton (2,723.9 g/t) Ag. Muck piles have also been sampled; grabs are taken over the pile to collect as representative a sample as possible (between 10-15 lb. are collected every 10 feet). The fully diluted value of the muck samples taken from the stope to date averages 22.3 oz/t (773.8 g/t) Au and 23.4 oz/ton (811.9 g/t) Ag.
The Blanket style mineralization at Hollister is typified by very fine grained disseminated gold hosted by tuffaceous horizons in the Tertiary (10-15 million years old) volcanics that lie unconformably on the basement Ordovician (~430 million year old) metasediments. These zones of mineralization are thought to be “mineralization plumes” directly related to the activity of fluid which has focused in structures that control the underlying epithermal quartz – adularia veins, and propagated into the Tertiary volcanic pile.

Blanket mineralization was previously exploited by opencast methods during 1990-1992 by the Touchstone – Galactic Joint Venture. According to historic records, 115,000 ounces of gold were produced by a heap leach operation that treated low grade ore (~0.003 oz/ton or 1 g/t Au). Great Basin modeled all 46 drill intersections above the Tertiary unconformity, and +1 g/t grade shells generally locate above known mineralized quartz – adularia veins. In general, this style has been located in the first ~30 feet (10 meters) above the unconformity, and may have dimensions in excess of 150 feet (50 meters) long and 60 feet (20 meters) wide. Grades from these 46 drill intersections average 0.45 oz/ton Au (15.4 g/t) and 1.7 oz/ton Ag (59 g/t).

Extrapolation of stope 3000N 1E to surface (approximately 200 feet or 67 meters vertically above), places this zone 300 feet (100 meters) west of the historical Clementine mercury mine. It supports the near surface working metal zonation and gold deposition model for the Hollister mine, and indicates additional exploration potential.
Ferdi Dippenaar, President and CEO, commented: “In the past, we have identified the Blanket Zone as a target area worth exploring, and trial mining at the top of vein (number sign)18 has turned out to be a great way to test the prospective nature of this style of mineralization. Although we have encountered a limited amount of this high grade material through trial stoping, drilling is underway to determine the full extent of mineralization. More information will be made available as and when it becomes available. Based on our experience in the Main Clementine vein (number sign)18, we are evaluating the possibility of returning to previously stoped out areas above the Gwenivere high grade veins.”
Great Basin Gold’s Hollister Mine in Nevada is already one of the highest grade Gold producing mines in the world, with 1.64 million ounces of Gold estimated at an average grade of measured and indicated resources at 1.3 ounces of Gold per ton. This morning’s news of such bonanza grades certainly bears some examination.
From previous background knowledge and from researching the terms within the news release, the author pieces together what this may mean. For help in understanding the basic geology, the author had his assistant “My Right Hand With Mouse” put together a rough diagram displayed following:

Figure 1: Model of Great Basin Gold’s bonanza gold grades found above their Clementine Vein.
With reference to the above diagram, Great Basin’s Hollister mine is constructed to mine the underground Gold veins. These veins are in the basement sedimentary rocks, which refers to the lower layer of rock geology, the “Ordovician” which are 439 million years old. This basement is covered with a more recent geological layer of volcanic rocks named the “Tertiary”, which are only 10 – 15 million years old. The meeting point of these two rock layers is termed the “Unconformity”, or joining between the two rock types.
The Gold veins that Great Basin are mining are formed from magmatic fluids that originated deep in the Earth’s crust and flowed through faults and fissures in the basement rocks coming up to the surface. There are two main types of deposits formed from these magma fluids, high sulphidation and low sulphidation deposits.
Great Basin’s geologists believe that Hollister is an example of low sulphidation deposits. As the fluids left the Unconformity and entered the Tertiary volcanics, the fluids encountered groundwater. The magma fluids then interacted with the ground water causing violent boiling and depositing of the metals and minerals. The act of depositing minerals seals off the fault and so the magma fluids seek another way to surface. Again, more groundwater is encountered and more furious boiling occurs. The boiling drops the minerals and seals the fissure again. These cycles of furious action results in more and more of the minerals being deposited in the fissures within this area of volcanics above the Unconformity.
Eventually the magmatic fluids reach near surface and dissipates and mixes thoroughly with the surface rocks. This results in a broad disseminated layer of mineral deposits that is termed the “Blanket Zone” at Great Basin’s Hollister mine.
In this style of low sulphidation depositing at Hollister, the minerals deposited by the repeated action of the magma encountering ground water has resulted in very high Bonanza grades of Gold and Silver that Great Basin has just announced. These high grades appear to be concentrated above the existing Hollister Gold veins, between the existing mine tunnels and the surface Blanket Zone. These high grades positioned where they are gives Great Basin a large amount of bonanza mineralization in a location where it will be easy to mine.
Another way of looking at this is the average grade of their “muck”. Muck is the broken rock ore, that will be refined in the mill circuit. Their average muck grade is spectacular at:
The fully diluted value of the muck samples taken from the stope to date averages 22.3 oz/t (773.8 g/t) Au and 23.4 oz/ton (811.9 g/t) Ag.
Compare this grade of 22 ounces of Gold per ton with anything else, you read about gold miners anywhere. Compare this with the grade at their Burnstone mine in South Africa, which has an average grade of 5 grams (.15 oz) Gold per ton, which is considered high grade. This grade is at least two orders of magnitude or 100 times higher.
Great Basin are understating in saying, that this model, ” indicates additional exploration potential”.
Disclosure: The author holds shares of Great Basin Gold (GBG).

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