Geology
The Pick and Winston Lake deposits are defined as VMS deposits. Modern analogues include “black smoker” polymetallic base-precious metal sulphide deposits seen on the ocean floor. Within this family of deposits, the common denominator is that they are associated with sub-marine volcanism. Copper-rich deposits are generally associated with more mafic, deeper, “hotter” systems, while zinc-rich deposits are associated with more felsic, shallower and “cooler” systems. VMS deposits differ in terms of size, metal ratios, metal grades, metal zonation, morphology, and style of mineralisation. An idealised VMS deposit is shown in image 1.
The Pick and Winston Lake deposits are characterised as being very rich in zinc and deficient in other metals, aside from minor copper and gold. They are also associated with pyrrhotite, not commonly seen with sphalerite but fortuitous in this case as it has a very pronounced geophysical signature which led to the discovery of both deposits.
VMS deposits are never as simple as depicted in Image 1 above. Hydrothermal fluids circulating through the volcanic sequence alter the mineralogy of the host rocks, often in a recognizable, zoned fashion, but not always, and subsequent structural events can deform the sequence to an almost unrecognizable extent. This is the case at the Pick Lake deposit where typical “footwall alteration” was documented stratigraphically above, not below, the deposit; this is less the case at Winston Lake where alteration was documented in its normal position. Later metamorphism can exacerbate this effect even further by changing the already altered original mineral assemblages for a second time. A simplified series of sections across the Project area depicting the respective locations of the various known deposits and the associated hydrothermal alteration is highlighted below.
As neither the Pick nor Winston Lake deposits were exposed at surface, direct observation of the features summarised above was not possible. Identification of these deposits and potential new deposits must therefore rely upon indirect methods that are based upon recognising the geochemical and geophysical expression of these features.
Image 1 – US Geological Survey – Volcanogenic Massive Sulphide Occurrence Model, chapter 8, page 141
Image 2 – Source: Geodynamic Reconstruction of the Winston Lake Greenstone Belt and VMS Deposits: New Trace Element Geochemistry and U-Pb Geochronology, Journal of Economic Geology, volume 109, pages 1291–1313
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