The Nelson Island Skarn is situated on a north-facing slope south of an unnamed creek linking West Lake and Mackechnie Lake located in the centre of Nelson Island. The area of interest is a heavily tree covered (second growth) prominent ridge along which recent logging road construction uncovered a previously unknown north-trending band of altered limestone with skarn-type mineralization. There is no record of exploration for metallic minerals on the island.
Nelson Island is underlain by intrusive rocks of the Coast Plutonic Complex (GSC Open File 611). The western two-thirds of the island are predominantly underlain by quartz diorite; exposures of granodiorite occur on the south end of the island at Quarry Bay and at the mouth of Blind Bay at the island's western extremity. The northeast end of the island is underlain by diorite. A belt of Upper Triassic Karmutsen Formation volcanic rocks trends northwest across the middle of the island.
Limestone (Upper Triassic Quatsino Formation?) forms a northwest-trending band across the island. At the "road-cut" exposure the limestone band is about 100 metres wide but it may widen along strike or with depth. It is cut by numerous diorite dikes. Most of the limestone has been metasomatically altered to a combination of marble and skarn. The marble displays wildly contorted banding defined by irregularly alternating plain white and black carbonaceous, pyritic layers. Boundaries between limestone and marble are gradational over tens of centimetres.
Numerous narrow (for example, 0.5 to 3.0 metres in width) bands of skarn cut the outcrop. There are three varieties: massive, pale brown garnet skarn; spotted brown garnet and pale green diopside skarn, and pistachio-green spotted (retrograde) epidote skarn. The more intense garnet-bearing skarn varieties are commonly associated with massive sulphide pods up to 0.75 metres across. Magnetite- actinolite zones occur outside the limestone band and within hornfelsed diorite that is exposed at the western limit of the limestone. Diorite is dark grey-green, fine-grained and commonly contains two to three per cent disseminated pyrite. A network of epidote-coated fractures cross-cuts the diorite. Medium to coarse-grained equigranular quartz diorite forms the eastern margin of the limestone. The contact zone displays apparent granophyric texture over a width of less than a metre. Sulphide mineralization consists of disseminated and fracture-controlled pyrite and chalcopyrite.
This new discovery is evidence of the potential of this area to host skarn mineralization. Similar occurrences may exist in under- explored, densely forested areas in the locality.
