Sherwood Copper, a Vancouver-based junior mining company, has found success with a little-used geophysical method known as Gradient Array Induced Polarization survey at its Minto Mine in central Yukon Territory.
GAIP is less expensive and provides quicker results than other IP surveys, but has the drawback of not being able to provide information on the depth of the mineralization found. This is not a problem in an area like Minto where the geology is well known, according to Brad Mercer, exploration manager at Minto Mine.
“The GAIP is a relatively quick and inexpensive geophysical survey method. It provides a good picture of mineral signatures on a north-south and east-west planes but does not provide data on depth,” Mercer said.
With GAIP, current electrodes are placed on the perimeter of a block that is being surveyed. Two additional electrodes can be moved freely within the block to measure resistance between them. The degree of resistance indicates the mineralization in the ground between the electrodes. Other IP methods that provide depth data on mineralization must be done in a straight line, which requires moving the outside electrodes for each line surveyed. This is more time consuming and costly.
According to Dave Hildes, of Yellowknife, NWT-based Aurora Geosciences, GAIP is only used about 10 percent of the time by his firm due to its limitations in collecting depth data.
“The technique works well for Minto because we know from extensive drilling and geological modeling that the principal ore horizons are both sub-horizontal in orientation and they are stacked vertically upon each other; somewhat like a stack of pancakes,” Mercer said. “Since we plan our first-pass exploration vertical drill holes to reach at least 300-350 meters (deep), we don’t much care about the depth of the anomalies initially, as we will drill test beyond the maximum theoretical reach of the Gradient IP survey anyway.”
GAIP exploration expanded for 2007
Sherwood first used GAIP at Minto as part of its 2006 exploration program. Aurora Geosciences was hired to conduct the survey, completing about 20 miles that season. A portion of Sherwood’s 2007 drilling was to further explore areas of interest indicated by the results of the 2006 GAIP survey. What was found by the drilling coincided with the findings of the survey and prompted Sherwood to expand its GAIP survey in 2007.
This year, Sherwood surveyed about 86 miles, a four-fold-plus increase over the previous year. The program had two primary targets. The first was to the south of the main Minto deposit, expanding coverage to areas of known prospectivity not covered in 2006. The second area was to the north of the Minto mine, where multiple coincident copper-in-soil and magnetic anomalies were detected but very little drilling had been completed. After positive drill results were obtained from drilling on the Airstrip-Copper Keel prospects in 2007, the GAIP survey was expanded to encompass and evaluate these areas more fully.
New areas located
A particularly exciting discovery is in the southwest Airstrip-Copper Keel area, Mercer said. This area was previously thought to be of low prospectivity, but recent drill results show a new zone of high grade copper-gold mineralization.
These results are located on the edge of large geophysical anomalies discovered by the 2007 GAIP survey. Initially, this prospect was thought to comprise two separate targets but now appears to be one system that is cut by late, post-mineral fault(s). Sherwood has drilled 10 holes on this prospect and assay results have returned on three of them. The results show characteristics typical of the Minto Project, but a new style of copper sulphide also has been identified.
“The recent geophysical survey results from Minto are encouraging and indicate exploration potential in areas previously thought to have limited prospectivity,” said Sherwood President and CEO Stephen Quin. “These geophysical results compliment the recently announced discovery of a new style of copper-gold mineralization in the Airstrip-Copper Keel area, coincident with the edge of the largest of the newly defined geophysical anomalies, and dominated by chalcocite as opposed to the more typical bornite-chalcopyrite copper mineralization found in the main Minto and Area 2 deposits.
“Our exploration team is confident that GAIP will prove to be another valuable technique in our exploration toolbox, improving the success rate of our exploration drilling and offering potential for more discoveries beyond those already announced in 2007,” Quin said
Looking north
Encouraged by the relationship between drill results and the anomalies found with the GAIP survey, Sherwood plans future drill testing of the new anomalies located during the 2007 survey.
Two areas are already being scrutinized for future drilling as a result of this year’s geophysical work. One is a very large horseshoe-shaped area just to the northeast of the Minto pit. The other area is a strong east-west linear feature located about 600 meters north of the pit.
“Both of these chargeability anomalies are associated with strong magnetic anomalies and elevated copper-in-soil geochemistry, and are, therefore, high priority drill targets for future exploration programs,” Sherwood said in a release.
Minto reaches commercial production
Sherwood announced that the Minto Mine achieved full production as of Oct. 1.
“Achieving commercial production at our high grade Minto copper-gold mine is a major milestone for Sherwood Copper,” said Quin.
The company purchased the Minto property in 2005 and within two years has re-drilled the property, completed a feasibility study, and converted Minto into a commercially producing mine.
“Credit for this outstanding achievement belongs to the management and employees at the Minto Mine who have worked diligently through the commissioning process, Quin added.
Ore produced at Minto is being shipped by truck about 156 miles to the Skagway Ore Terminal in Alaska where it will be loaded on ships destined for smelters in Asia.
Skagway terminal ships first ore in 10 years
After stockpiling copper-gold concentrates from the Minto Mine in the Yukon Territory since July, the Skagway Ore Terminal shipped out its first load of ore in 10 years in late October.
The terminal stores ore concentrates for Minto’s owner, Sherwood Copper Corp., until it is loaded onto ships bound for Asia under terms of an agreement with the Alaska Industrial Development and Export Authority
“The refurbishment of the Skagway Ore Terminal was the most cost-effective option for the marine shipment of Minto’s high-grade copper-gold concentrates,” said Brad Kopp, Sherwood’s corporate development manager.
Kopp told Mining News that Sherwood hopes to ship an average of 5,000 tons of ore a month. Canadian Lynden Transport Co. is hauling the ore to the terminal.
Sherwood subsidiary, Minto Exploration, has agreed to pay nearly $10 million over the next seven years to AIDEA as reimbursement for the cost of refurbishing the Skagway terminal. Minto Exploration also will pay for loading the concentrates at the terminal. If other companies use the terminal during the term of the contract, Minto Exploration’s portion of the costs could be reduced pro rata, according to Karl Reiche, projects development manager for AIDEA.
Other mining companies, primarily Canadian, also have shown interest in using the terminal since it reopened. With nearly 100,000 square feet of storage space and movable interior barriers that can be configured to accommodate several companies’ needs, Reiche said the terminal is ready and willing to store and ship more concentrate.
