- Property underlain by Triassic-aged Stuhini Group sediments, and Jurassic-aged sedimentary / volcaniclastic units of the Hazelton Group.
- Sequence has been folded along the Bitter Creek anticline, which is the focus of the Red Mountain ore deposits.
- Represents a rapid marine transgression forming relatively deep seaway between the ancestral North American continent, and advancing volcanic island chains
- Increasing volcanic influence moving up through the stratigraphic sequence
The property’s oldest rocks are the Middle to Upper Triassic Stuhini Group. These sediments grade upward into Lower Jurassic Hazelton Group clastic and volcanoclastic sequence, which outcrop in the northeastern portion of the property. The rocks are generally folded along an axis that plunges towards 345° and has a steep southwestern dip. The trend of this structure is followed by an approximate contact between the two groups and occurs along the projected trace of the Bitter Creek anticline which has been mapped in the northwest of the property.
The stratigraphy of the Red Mountain property indicates a rapid marine transgression into a relatively deep seaway or embayment that was proximal to the ancient North American land mass and offshore volcanic island chains. The closure of this seaway (occupying a tectonic back-arc basin) led to the formation of the igneous intrusive material, responsible for the mineralisation and ore bodies.
The following diagram illustrates the property specific stratigraphy (Randell, 2016).
Stratigraphic log of major lithologies and there outcrop locales at Red Mountain (Randell, 2016)
Structural and Stratigraphic Setting of Gold Mineralization at Red Mountain (Greig et al., 1994) (Banks Island Gold technical report, 2013)
Jurassic Porphyry Units
- Two phases of the Early Jurassic intrusions are exposed on the property; the Hillside and Goldslide Porphyries
- The two phases are differentiated by mineralogy, texture, and geochemistry and likely represent fractionation and depletion of a shared magmatic source over time.
- The Hillside Porphyry is a fine to medium grained hornblende – plagioclase intrusive which outcrops mainly around the summit of Red Mountain and along the southeasterly ridge.
- The Goldslide Porphyry is a medium to coarse grained hornblende, biotite ± quartz intrusive, primarily exposed along the Goldslide Creek, and extending east-west across Red Mountain.
The porphyritic units of at Red Mountain intrude the sedimentary sequences of the Stuhini and Hazelton groups, with the Goldslide apparently penetrating the entire pile. The Hillside Porphyry sits lower down in the sequence and was emplaced at a time when the sedimentary stack was still wet, forming complex pepperitic margins and regular phreatic events evidenced in pebble dykes. The intermixing of the Hillside Porphyry with the more carbonaceous units of the Hazelton group appears significant in providing the correct chemical environment to allow the emplacement of gold within massive sulphides, something that is absent in the higher volcaniclastic units.
The Hillside Porphyry has been described in several ways during historic logging, although recent work shows that the textural evolution of the unit is a smooth ‘continuum’, whereby wall rock clasts are mixed with the intrusive material, altered under pressure to form dissolution seams, and eventually becoming completely assimilated into the magma. Clasts of mudstones and other carbonaceous rocks are the most conspicuous and form a ‘crackle breccia’ as they are assimilated, whereas tuffaceous units tend to become absorbed into the igneous material with little chemical influence.
Both the Hillside and Goldslide units at Red Mountain appear to be derived from the same magma source that underwent chemical changes through fractionation and depletion. Elevated sodium levels and the occurrence of quartz suggests that Goldslide fractionated into a more acidic (felsic) intrusive from the original melt.
Lost Valley Monzonite
- Later intrusion, likely very latest Cretaceous to Early Eocene, exposed in the southern portion of the property
- Medium to coarse-grained biotite-bearing quartz monzonite
The Lost Valley Monzonite is found on the southern portion of the property at McAdam point and Lost Valley. It is a medium to coarse grained biotite bearing quartz monzonite that has large K-spar megacrysts. This extensive body was totally covered by ice until very recently and so work is ongoing to understand its emplacement. Initial observations show a highly variable range of textures which have likely been formed partly due to ‘nested’ structures from different phases of intrusion. The rocks are generally classed as monzonites, although some units are almost granitic in nature.
Mineralisation within the pluton is currently being investigated, as much of the massive sulphide veining appears to be related to an even later structural event that fractured the ground, creating fluid conduits. The pluton was likely not fully cooled at the time, providing the heat to convect and enrich fluids. There are however examples of molybdenum mineralisation that appear to be concurrent with emplacement, especially within the carapace of the pluton.
Lost Valley, Red Mountain Property
K-Feldspar Megacryst in Lost Valley Monzonite Outcrop
Lost Valley Handsamples
Lost Valley Handsamples
- Deformation at property scale is consistent with regional levels
- Triassic-Jurassic successions are folded, and post-Cretaceous faulting affects all the rock units at Red Mountain
Structural deformation at the property scale is consistent with the observations at the regional and tectonic levels. Folds occur in the entire Triassic-Jurassic succession with north to northwest plunging axes and generally steeply dipping limbs. Timing of these folds suggests they are related to the Cretaceous Skeena fold belt deformation event, a time of stress exerted on the crust as the offshore island arc was pushed into ancestral North America.
Post-Cretaceous brittle deformation affects all rock units at Red Mountain. This includes northeast striking, steeply northwesterly dipping faults that offset the mineralized zone (ex. Rick Fault) and gouge-rich, north to northwest trending faults that have broader alteration envelopes.
Observations during the 2016 field season in the Lost Valley area indicates a series of north-south trending strike-slip faults that conspicuously offset and shear a late set of lamprophyre dykes. Although the offset is relatively small (under 100m), these faults have been traced running the length of the property. They are also part of an array of strike-slip faults that run parallel to each other, spaced 100m to 300m apart. Many of the mineralising structures in Lost Valley can be attributed to Reidel Shears and reverse faults propagating from these regional breaks, which follow a classic ‘strain ellipse’ model. This can also be used to explain the occurrence of structures such as the Rick Fault which is accommodated by this model, thus representing an important breakthrough in understanding that could lead to new targets in the future. Much of this information was not available in the past due to the extent of ice covering the property until the late 1990’s.