First Quantum Minerals Ltd. has agreed to invest US$2 million on a drill program aimed at further investigating the potential of Millrock Resources Inc.’s highly-prospective copper-gold project in western Alaska. The roughly 500,000-acre property extends about 75 miles from Stepovak Bay near the southwestern end of the Alaska Peninsula to a few miles north of Chignik Bay, one of the primary ports in the area.
Millrock optioned the property in 2012 from Bristol Bay Native Corp., the Alaska Native regional corporation that owns the subsurface mineral rights. Surface rights are held by Alaska Native village corporations.
Millrock President and CEO Greg Beischer told Mining News that he feels fortunate that BBNC at large and particularly the shareholders that live in the nearby villages have granted his company permission to explore this highly prospective region of their lands.
Since cutting a deal with the corporation, Millrock has advanced exploration in measured strides that includes a US$600,000 reconnaissance program funded by First Quantum in 2014. In exchange for funding the exploration of some most intriguing targets on the property, Millrock granted First Quantum an exclusive right to enter an option to earn up to an 80 percent joint venture interest in the property if it liked what it saw on the first-pass exploration.
Aleutian Arc
The Alaska Peninsula project covers a stretch of the Aleutian Arc, a Pacific Ring of Fire island arc being formed as the Pacific Ocean plate dives under the North American plate. This ongoing tectonic activity is witnessed in the geologically young mineralization – 3 million to 21 million years – being targeted by Millrock and First Quantum’s exploration.
The geologically active region has received limited modern exploration and is prospective for epithermal gold as well as intrusive-related deposits such as skarns and porphyry copper-gold mineralizing systems.
While systematic mineral exploration of this region is limited, at least 34 known mineral occurrences have been identified on the Alaska Peninsula property.
To narrow the search, the partners started off the 2014 season with 1,140 line-kilometers of high-resolution airborne magnetic and resistivity surveys flown over three of the highest quality porphyry copper-gold occurrences – Kawisgag, MDB and Dry Creek.
Millrock and First Quantum geologists followed up the geophysical work with a mapping and sampling program.
Millrock reported, “The results were very encouraging. Classic alteration patterns for porphyry copper deposits were defined at each prospect from rock sample analysis. Zoned metal anomalies from soil sampling match well with the alteration patterns. Drill testing is warranted at each of the three prospects.”
Results from this initial work, intrigued First Quantum enough to take up the option and fund a 2,400-meter drill program aimed at investigating what lies below the surface at two the most promising prospects – Dry Creek, formerly known as Bee Creek, and MDB, short for Mallard Duck Bay.
Classic porphyry signature
Located near the northern end of the Alaska Peninsula land package optioned by Millrock, Dry Creek is the only prospect that has been previously drilled.
Dated at roughly 3.6 million years, the porphyry mineralization found at this prospect is hosted in hornfelsed sediments intruded by multi-phase diorite intrusive rocks containing mineralized veins and disseminated chalcopyrite, molybdenite and pyrite.
The prospect was initially explored by Bear Creek Mining, which drilled five holes in 1976. One hole cut 160 meters averaging 0.25 percent copper, 0.011 percent molybdenum, including 20 meters of 0.35 percent copper and 0.018 percent molybdenum, according to a Bear Creek Mining report.
Similar results were encountered in two holes successfully completed by Metallica Resources Inc. and Full Metal Minerals Ltd. in 2006, including one intercept of 118 meters averaging 0.32 percent copper and 0.21 g/t gold.
Soil sampling completed by Full Metal and Metallica outlined a 0.5 percent copper anomaly covering roughly 250 acres and a more localized 0.05 percent molybdenum anomaly and discontinuous 0.5 parts-per-million gold anomaly.
This historical work along with sampling completed last year outlines a core zone rich in copper, gold and molybdenum surrounded by a halo of anomalous zinc, lead and manganese. All told, the zoned mineralization covers roughly 3,000 acres.
“We found this beautifully zoned alteration system, and we found this beautifully zoned metallic anomaly in soil,” Beischer said. “It is just an absolute classic porphyry signature.”
Results of an airborne magnetic survey flown over the area suggest that mineralization identified on the surface at Dry Creek may continue below a ridge to the southwest.
A promising induced polarization chargeability anomaly that has eluded past explorers will be the target of one of the holes drilled this year. Bear Creek drilled above the anomaly in 1976, but without the benefit of knowing what lies below and below the depth of an open-pittable deposit, stopped the hole short.
“Those guys knew what they were doing,” Beischer reflected on the historical drilling.
Full Metal, which completed the IP survey, recognized the promising target but was unable to reach it due to losing the hole to poor drilling conditions.
Millrock is sending a drill bit down to reach this elusive IP anomaly in drilling now underway.
Although several holes have been drilled at Dry Creek, Millrock and First Quantum have plenty of untested geochemical and geophysical anomalies at this prospect also waiting to be drilled. One such anomaly within 500 meters of the first hole will be the target of a second hole at Dry Creek.
“There is another good anomaly within the same alteration system that we will drill,” Beischer explained.
Huge alteration system
Situated roughly 20 miles south of Dry Creek and about three miles south of the local community of Chignik Lagoon, MDB hosts a large alteration zone that was first recognized by prospectors at the turn of the 20th Century.
“MDB is … is a huge alteration system but there doesn’t appear to be a lot of base metals at surface,” Beischer told Mining News.
Two additional waves of exploration investigated MDB, including mapping and sampling by Bear Creek Mining and Resource Associates in the 1970s and 1980s; and polarization and ground magnetic surveys completed during Full Metals’ time in the region.
The 2005 geophysical work identified two chargeability anomalies, one coincident with exposed mineralization in a potassic alteration zone that has been dated at about 21 million years.
Beischer said the enormous size of the alteration system coupled with the lack of metals at surface made MDB a challenging prospect for previous explorers to target with drilling.
Millrock, however, is up to the challenge of tackling this huge alteration system that could be the surface expression of a porphyry ore body waiting to be discovered, he said.
In 2014, Millrock and First Quantum carried out systematic exploration that included airborne magnetic surveys, mapping and rock sampling. This program has helped the partners refine the geological interpretation of the large and largely unexplored prospect.
This year, the partners will test their ideas by drilling the best of a number of strongly anomalous zones their work has identified at MDB.
