Knowing that porphyry deposits like Pebble are seldom an isolated occurrence, explorers are wondering if another enormous copper-gold deposit of this type lies hidden under the glacial gravels that cover much of Southwest Alaska. That was a question tackled by Pebble Partnership Chief Geologist Jim Lang at the 2011 Minerals Exploration Roundup in Vancouver, B.C.
The geologist told the crowd that Pebble may not have an unknown identical twin, but by looking for similar traits, explorers may yet find a large sibling nearby.
“The discovery potential is very high for porphyry deposits generally in this arc environment, but I would also say that Pebble-look-alikes are probably not likely. However, this does not preclude the potential to find other large porphyry deposits with their own unique sets of characteristics. So, we should not over-focus on the Pebble model, but we should focus more on the favorable environment that Pebble represents,” Lang told the audience at the annual exploration conference.
Porphyry deposits – formed when oceanic plates plunge under landmasses – account for nearly 70 percent of the copper and more than 90 percent of the molybdenum currently being produced.
With its 80.6 billion pounds of copper, 107.4 million ounces of gold and 5.6 billion pounds of molybdenum, the Pebble deposit ranks amongst the world’s largest for any of these metals – combined it is extraordinary.
“When we look at the amount of contained gold in porphyry copper deposits, Pebble ranks right at the top, but if we look at all of the other metals it contains as well, several of them also indicate, by any standard, it is a world-class deposit – vast amounts of copper, molybdenum, rhenium, several hundred million ounces of silver, even anomalous palladium in some parts of the deposit,” Lang said.
The Pebble modelThe geologist said a number of factors have contributed to the enormous amount of copper, gold, molybdenum and other metals deposited at Pebble. One of the key dynamics is the deposit’s location.
“It marks an approximate boundary between the Peninsular Terrane to the southeast and the Kahiltna Terrane, dominated by flysch, to the northwest – so it is a major structural corridor.”
Flysch is sedimentary rock formed in the ocean and then pushed up on land. The flysch that dominates the Kahiltna Terrane was bulldozed up as the Peninsular and Wrangellia island arcs slammed into the mainland as the Kulas Plate carrying the isles dove under Alaska some 90 million years ago.
Lang said when you take a closer look at Pebble’s location you find several other factors that contributed to Pebble’s size.
“The area has been magmatically and hydrothermally very busy for a very long time. We often see that as an indication in other porphyry districts around the world,” he said.
An area referred to by Lang as a brittle ductile deformation zone, or an area of rocks broken up by fault movement, seems to have served as a conduit to channel these metals-bearing fluids into the deposit area.
“The deposit has this rather unusual flat tabular geometry. The east and the west zones are two centers of one hydrothermal system, but the east zone is a much better mineralized part of that system – it has better structural features and it has also been subjected to multiple stages of hydrothermal activity and consequent mineralization. The brittle-ductile deformation zone itself has been a major conduit for channeling these mineralizing fluids over the history of the deposit,” he explained.
The Pebble geologist said copper-gold-molybdenum-rich fluids were channeled into the deposit at least twice. Unable to permeate the flysch that covers the richer Pebble East deposit, these fluids spread westward.
“The brittle-ductile deformation zone focused fluids at different times during the life of this system and allowed different stages of alteration and mineralization to overprint each other and augment the grades over time. There is an outward flaring that we see both in the early quartz veins and in the later advanced alkalic alteration that suggest there might have been something that is impeding the vertical flow of fluids and imposing a strong lateral component to fluid-flow in the system,” Lang explained.
“Don’t over-focus on Pebble, keep an open mind. There’s lots of variety (in) porphyry-type deposits in the environment we have here in Southwest Alaska,” he advised.
“If we look at the specific individual features of Pebble, there is nothing at Pebble that you cannot find at another porphyry deposit somewhere. However, the sum of its parts may be more unique. The combination of structural, magmatic, host rock and hydrothermal characteristics that we see is quite specific and is probably the factor in determining what we have at Pebble today,” he said.
Although the overall Pebble-model may not be repeated, some of the deposit’s dynamics may serve to guide geologists seeking porphyry deposits in this part of the world.
Geophysical survey could be usefulThe Pebble geologist said geophysical surveys are a vital tool in narrowing the search for Pebble-like deposits in Southwest Alaska.
“The existing databases are not very detailed, but they are sufficient to get some idea of what is going on,” he explained. “Here we will be using these regional datasets of different types to identify major structural intersections – we are talking about major deep-seeded crustal-scale structures – their intersections and their environment of intersections where we have the potential to form deep-seeded long-lived magmatic complexes of various compositions.”
The United States Geological Survey has been working with the Pebble Partnership to study the characteristics of the Pebble deposit in order to refine assessment techniques for concealed mineral deposits in Southwest Alaska. According to the agency, aeromagnetic surveys could prove useful in vectoring in on porphyry deposits in the region.
In a presentation titled “Geophysical and Geochemical Assessment Studies at the Concealed Pebble Porphyry Cu-Au-Mo Deposit, Southwest Alaska,” Eric Anderson of the USGS said data collected from five aeromagnetic surveys over the region “show several clusters of northeast-trending magnetic anomaly highs. The Pebble deposit is located on the southeastern edge of one such cluster that occurs over the Late Cretaceous Kaskanak batholith, which is genetically related to mineralization. The dimensions of the clustered anomalies are 25-40 km long (15.5-25 miles) and 20-30 km (12.5-18.5 miles) wide.”
According to the USGS, the spacing of the northeast trending magnetic highs “is similar to patterns shown by the numerous porphyry deposits in northern Chile and suggests favorability for additional discoveries in southwestern Alaska.”
Referring to Anderson’s work, Lang said, “Certainly in northern Chile the role of the intersection of cross faults with the West Fissure Fault is a well-accepted control on the localization of porphyry deposits, and I would suggest that there is a high-probability we have a similar scenario in Southwest Alaska.”
The 375-mile-, or 600-kilometer-, long West Fissure fault is associated with most of the major Chilean porphyry deposits.
Geophysical data may help vector the search, but the glacial history of Southwest Alaska may limit the usefulness of conventional mapping and geochemistry in further refining exploration targets.
“Geology is always important, but in this area, there is an awful lot of glacial and tertiary cover, and traditional geological mapping is fairly limited in application except on a very local scale. Geochemistry is very useful but is hindered by the complex glacial history. This is particularly the case for soil geochemistry. We may have to consider using other methods like hydro-geochemistry or porphyry-copper indicator minerals in tills, things like that,” Lang suggested.
He said the search for another world-class porphyry deposit in Southwest Alaska will not likely be easy or cheap.
“Critically, of course, you are going to have to spend a lot of time and a fair bit of money doing successful exploration in this part of the world. You are going to have to accept the fact that you are going to have to drill a lot of holes, and you are going to have to bite the bullet and drill a few of them very deep. I think that if you do – it’s a terrible challenge – it might well be worth the effort,” Lang concluded.