In March 2010, Sumitomo Metal Mining Pogo LLC (Sumitomo) commissioned SRK Consulting Canada to undertake a structural geology interpretation of the Pogo district in central Alaska, to assist ongoing exploration targeting. The interpretation was focused on compiling a structural geology interpretation of recently acquired geophysical data, combined with staged site visits, to examine the structural geology of the Pogo deposit and collaborate with the Sumitomo exploration team.
The Pogo deposit is Alaska’s largest producing gold mine. It is underlain by high-grade gneisses of the Late Proterozoic to mid-Paleozoic Yukon-Tanana terrane, which have been intruded by granitoid bodies, mostly belonging to a voluminous mid-Cretaceous suite. The Yukon-Tanana terrane and Cretaceous intrusions extend from west of Fairbanks through the central Yukon, for a distance of over 700 kilometres.
Gold mineralisation at Pogo is hosted within the structurally controlled Liese vein system. Overall, the Liese vein system dips shallowly northwest and is a stacked system of three laminated veins (L1, L2, and L3). SRK and Sumitomo recognised that the lateral extent of the Liese vein system is large, with the L1 vein currently known to have a down-dip extent of greater than 1700 metres. The northeast margin of the Liese vein system is truncated by a post-mineralisation diorite (about 95 million years old). The crosscutting nature of the diorite, combined with the lateral extent of the vein system, indicated to SRK and Sumitomo that the vein system may extend beyond the northeast margin of the diorite, representing a significant exploration target in the shadow of the existing mill.
In 2011, Sumitomo began underground and surface drilling examining the potential for vein systems northeast of the diorite and have since outlined a significant new gold zone, the East Deep zone. The East Deep zone represents the lateral continuation of the Liese vein system beyond the diorite and will represent a significant contribution to the resource inventory of the Pogo deposit. Sumitomo is currently planning a new portal and decline ramp from the surface that will link in with two exploration drifts that have been driven over to this zone.
In addition, a significant outcome for Sumitomo’s collaboration with SRK was the ability to start 4-dimensional reconstructions. A great example of this was the realisation that the Liese Creek diorite existed north of the 4021 prospect (figure above). In addition, when overlain with the interpreted structural events, this offset is seen to be from a northeast-trending sinistral fault. By reconstructing this movement, Sumitomo will have a better understanding of targeting gold mineralisation between the Liese and 4021 zones. This has led to the construction of an updated “Liese Zone” model of gold mineralisation.
