Three stratigraphic assemblages have been mapped in the Quartz Rise area: (1) Upper Palaeozoic sediments and volcanics of the Stikine Assemblage, (2) clastic sediments of the Triassic Stuhini Group, and (3) Lower Jurassic sediments and andesitic volcanics of the Hazelton Group which underlies most of the immediate Johnny Mountain area and hosts the Quartz Rise prospect.
Four plutonic suites intrude the above; (1) Late Triassic calc-alkaline intrusions coeval with Stuhini Group stratigraphy, (2) Jurassic Copper Mtn, (3) Jurassic Texas Creek and (4) Tertiary Coast Plutonic suite. Metallogenically, Texas Creek potassium feldspar megacrystic porphyries are of primary interest as they are spatially associated with several gold deposits and showings in the area.
Mapping by Seabridge in 2017 showed a shallow northeast-dipping sequence of Jurassic felsic volcanic rocks deposited unconformably on Triassic marine sedimentary rocks. The volcanic stratigraphy established from surface mapping and drill holes shows a series of coarse lithic tuff flows intercalated with densely welded flows, porphyritic sills and tuffaceous sedimentary rocks. This is reported to be the geological time break that has proved to be productive elsewhere in the region.
Drilling by Seabridge, found evidence of a gold-bearing intermediate sulphidation epithermal system beneath the Quartz Rise lithocap as was anticipated by Seabridge. Intercepts included 1.5 metres grading 8.26 grams per tonne gold in QR-17-01 and 1.5 metres grading 74.1 grams per tonne gold in QR-17-07. Sampling of a cliff face north of Quartz Rise returned very high grades ranging from 1.49 to 125.3 grams per tonne gold (Seabridge Gold Inc., News Release, November 20, 2017).
A subsequent 3D induced polarization (IP) geophysical survey together with detailed geological mapping and sampling of available rock faces indicated that the 2017 drilling may not have been in the correct orientation.
Drilling in 2018 was designed to follow gold-bearing veins in the coarse lithic tuff away from a cliff face into the chargeability anomaly. Results from the 2700 metre drill program confirmed that much of the Quartz Rise lithocap had been eroded, leaving little opportunity for a sizeable high-grade epithermal occurrence in this area. However, drilling was reported by Seabridge Gold to have encountered the hallmarks of a large copper-gold porphyry system such as: a robust hydrothermal breccia (diatreme) containing abundant clasts of chalcopyrite-pyrite-magnetite in veined intrusive rocks and stockworks positioned over a large IP chargeability anomaly coincident with a magnetic high.
Hole QR-18-14 was designed to test a broad, steeply dipping IP anomaly. This hole discovered a diatreme containing clasts of veined diorite porphyry with well-preserved porphyry copper-gold mineralization in a matrix of fine-grained diorite and milled wallrock with textures indicative of hydrothermal fluid flow. Diorite breccia clasts showed multiple cross-cutting relationships in stockwork veins containing chalcopyrite-pyrite-magnetite. Textures and mineralogy of the diorite breccia clasts and veins are characteristic of potassically-altered porphyry copper-gold systems. QR-18-17 was drilled to off-set the intersection in QR-18-14 and found a narrower interval of the diatreme.
The 2019 work by Seabridge identified a large intrusive system at relatively shallow depth that was likely responsible for the Quartz Rise lithocap and elevated gold and copper concentrations. Detailed work on the surface expression of the diatreme found that it plunges to the south toward the highest intensity chargeability anomaly and geochemical signature. The geophysical footprint was therefore expanded to the south and southwest into an area where glacial erosion had exposed the system vertically to a depth of over 800 meters. Surface mapping and sampling of this vertical exposure found extensive gold and copper anomalies within favorable thermally altered wall rock. Several intrusions have been identified in association with this diatreme and these features have been dated to about 186 million years.
WORK HISTORY.
The Iskut Project was acquired by Seabridge Gold Inc. from SnipGold Corp. in June 2016. In 2016, Seabridge conducted a multipronged exploration program on its 100-per-cent-owned Iskut project identifying a prospective new porphyry copper-gold system with a potentially intact epithermal precious metals zone at its top – the Quartz Rise prospect.
Seabridge completed a limited amount of drilling in 2016 at the old high-grade Johnny Mountain gold mine to test detailed ore control concepts on a known deposit while proceeding with the comprehensive program. The nearby, higher elevation Quartz Rise was thought to have resisted glaciation, remaining largely intact in part due to its higher silica content. Visible gold was observed at Quartz Rise in 2016.
In 2017, Seabridge Gold Inc. completed ten core holes totalling 4,459 metres. This year's drilling was designed to test a graben feature discovered during surface work on the Quartz Rise lithocap. The graben was thought to have constrained the most intense hydrothermal alteration in the area of the lithocap. The graben-forming faults were suspected to be significant precious-metal fluid pathways; they were traceable over several kilometres and they projected into the historical Johnny Mountain mine where they are recognized as controlling gold distribution within the mine.
In 2018, Seabridge drilled a total of 2,700 meters of core in at least 9 holes on the Quartz Rise.
In 2019 Seabridge completed 1.4 square kilometers of 3D IP surveys, 260-line kilometers of high-resolution magnetic surveys, collected 184 surface geochemical and samples and conducted detailed mapping.
Seabridge Gold Inc.'s 2020 program was designed to test geophysical anomalies below the quartz rise lithocap that is host to a diatreme pipe containing gold-copper mineralized vein fragments. A total of 8,961 metres of core drilling were completed in 11 holes. Results were consistent with the alteration halo from a large porphyry system. Mineralized intervals of up to 158 metres grading 0.16 gram per tonne gold and 0.16 per cent copper were intersected, indicating that drilling to date is in the outer portions of a gold-copper porphyry (Seabridge Gold News Release November 11, 2020).
See Johnny Mountain (104B 107) for further geological details and subsequent work history in the Johnny Mountain area.
