Hollister mine site.
The overall contained gold-equivalent ounces at its Hollister in Nevada’s Carlin Trend have increased by 27% from 2.3 million in June 2008 to 2.9 million, Great Basin Gold Ltd. said this morning.
At a cut-off grade of 0.25 opt, the combined measured and indicated mineral resources contain 1.45 million gold-equivalent ounces grading 1.167 opt for gold and 8.59 opt for silver. A further 1.43 million gold-equivalent ounces are contained in inferred mineral resources of 1.03 million tons at a grade of 1.340 opt for gold and 2.72 opt for silver, the company said. Gold-equivalent ounces in the estimated inferred resources increased 111% from June 2008.
The resource estimate has benefited significantly from over nine months of trial mining, which generated data from channel sampling and delineation drilling for stope development, the company said.
As a result of the integration of empirical data from geological observations and ore control sampling into the vein model, more stringent parameters have been applied to measured and indicated classifications. In addition, 161 additional boreholes (totaling 51,430 feet) completed in the period from April 2008 to March 31, 2009 were integrated into the model.
Most drilling from underground
“The drilling program, mostly conducted from underground, has provided infill data to delineate stopes for trial mining and significantly improved our understanding of the lateral and vertical geological continuity of the vein system,” Great Basin Gold reported.
The resource estimate reflects depletion of material mined in 2008 and 2009 (up to March 31 2009), totaling 103,746 gold-equivalent ounces at an average grade of 1.55 opt.
The outcome of changes made in determining resources is a more conservative approach to resource classification that more closely reflects what is observed empirically underground, the company said.
Consequently, the tonnage in the measured category has decreased but grade has increased. Tonnage in the indicated category has increased due to reclassification and tonnage in the inferred category has increased due to re-classification and additional drilling testing the extent of mineralization.
More evaluation planned
As the mine development progresses, the evaluation and upgrade of the mineral resources from the inferred and indicated to a measured category will provide data to assess the potential for reducing cut-off grade, the company said.
It added that the current testing of various mining technologies may reduce stoping costs and expand the opportunity to exploit certain narrow veins that otherwise would not be considered in the mine plan. Employing a lower cut-off grade would allow lower-grade material in the indicated category to be included in the resource.
The depth extent of the vein system is yet to be fully established, Great Basin Gold said, so to be conservative, inferred resources have been constrained to a depth of 1,200 feet below surface. Certain areas of previous indicated resource have been re-categorized into inferred because of lesser density of sampling data.
Inferred resource grades have increased from 0.510 to 1.34 opt for gold and from 1.43 to 2.72 opt for silver due to the localized extrapolation of well defined areas of higher-grade pay shoots in the measured and indicated categories.
Gwenivere, Clementine veins
The ongoing program will continue to focus primarily on extensions of the Gwenivere and Clementine vein system to the west/northwest and at depth, the company said. Further investigation of Blanket zone mineralization hosted in Tertiary volcanics above the Gwenivere/Clementine vein system is planned.
Miners heading underground at Hollister.
Detailed reassessment and modeling of all Blanket zone intersections is currently under way, as there is potential for a significant resource in this area as indicated by previous surface drilling, the company said.
Great Basin Gold currently has three drill rigs in operation underground, undertaking stope delineation, resource infill and exploration drilling. Surface exploration is currently focused on collating and reviewing all geophysical, geological and drilling data for the property, with the intent of better delineating basement structures that control mineralization.
Additional drilling at Hollister is being considered for the Hatter Graben area during 3Q and 4Q.
Dippenaar’s comments
“Our ongoing exploration drilling programs and results from trial mining continue to confirm the prospectivity of the Hollister property, evidenced in the substantial increase in the reported resources,” said Ferdi Dippenaar, president and CEO.
“The tighter geological controls constraining the estimate are also improving trial stope tonnage and grade estimates and, as a consequence, our mine planning is benefiting from more accurate information.
“As the underground development continues, there will be further opportunities to drill-test the depth extensions of a number of high-grade zones that are emerging from the evaluation. The continued exploration drilling from underground is bearing success with tracking the lateral extensions of the Hollister veins both eastward towards Hatter Graben, but also stepping northwestward into the major Butte Fault bounding structure,” Dippenaar said.
“Furthermore, the review of Blanket style mineralization in the overlying volcanics may provide an interesting upside to the current resource profile, as it could be mined from the current infrastructure.”
