Seabridge Gold

KSM (Kerr-Sulphurets-Mitchell): GEOLOGY


KSM is in the “Stikinia Terrane”, a series of late-Triassic and early-Jurassic volcanic arcs that collided with the Paleozoic basement of the North American continental margin in middle-Jurassic time. Stikinia is the largest of several fault bounded, foreign terranes on the western margin of North America that make up the Intermontane Belt. The Intermontane Belt represents a collection of preserved Mesozoic oceanic crust and island arcs that occupies the zone between Tertiary intrusions of the Coast belt and the continental margin sedimentary prisms of the Foreland (Rocky Mountain) Belt. Colliding terranes into a continental margin, building intrusions on the oceanward side and sedimentary rocks on the continent side, is the normal process of expanding a continent and gives Western North America its current form.

In the KSM area, Stikinia rocks are variably deformed oceanic island arc complexes of the Triassic Stuhini Group and Jurassic Hazelton Group. These groups are represented by a diversity of rocks that includes a range of volcanic rocks, shallow water sedimentary rocks, deep water sedimentary rocks and an assortment of intrusive rocks. Folding, thrusting and faulting have affected all these units, affirming the tremendous forces that pushed against western North America. Geologists have sorted out many of the variations within the Stikinia Terrane and that work continues to identify promising sections where significant mineral deposits are located.

Early Jurassic intrusive complexes responsible for some of the volcanic rocks are common in the Stikinia Terrane and are host to several well-known precious- and base-metal-rich mineral systems. These include gold-copper porphyry deposits and clusters such as Galore Creek, Red Chris, Kemess, Mt. Milligan, and KSM. Intimately associated with these mineralized porphyry intrusive complexes are polymetallic skarn and epithermal deposits like the Premier Mine and Brucejack deposits. In the surrounding sedimentary and volcanic piles there are strataform, semi-massive sulfide deposits like the Eskay Creek Mine and Granduc Mine. The Stikinia Terrane is the heart of BC’s Golden Triangle and exploration investment focused in the area will yield more discoveries.


The most prominent feature in the Sulphurets District, where the KSM deposits are located, is the Sulphurets Thrust Fault (STF). This feature is a north-trending west-dipping low-angle fault, appearing from under ice south west of the Kerr deposit and traced over more than 10 kilometers to Iron Cap where again it is hidden by an ice field. Above the STF are a series of sedimentary rocks with localized intrusions and below the STF are a series of sedimentary and volcanic rocks where intrusive rocks are common. The STF has a number of smaller accompanying thrusts, like the Mitchell Thrust Fault (MTF). Whereas the displacement along these complementary faults is in the order of one kilometer, the throw on the STF is uncertain.

Erosion through the STF has exposed a cluster of porphyry gold-copper systems that are the KSM deposits. Fundamental to the KSM deposits is a collection of diorite, monzodiorite and syenite dikes, sills and stocks that have been age dated as early-Jurassic (roughly 198 to 190 Ma) and locally referred to as the Mitchell Intrusions. These intrusions, although deformed and faulted by subsequent tectonic forces, still show the classic mineralizing characteristics of porphyry systems and are host to most of the gold and copper at KSM. The erosional window through the STF has provided access to this remarkable cluster of porphyry gold-copper deposits that contain Measured and Indicated Resources of 2.92B tonnes with 49.2M ounces gold and 13.7B pounds copper along with Inferred Resources of 3.96B tonnes with 48.9M ounces gold and 28.1B pounds copper (see resource table).

Kerr Deposit

The Kerr deposit is centered on a north-south trending, steep westerly dipping, tabular intrusive complex that drilling demonstrates has a horizontal extent of 2,400 meter and a vertical extent of at least 2,200 meters. The complex includes a 300-meter-thick east and 500-meter-thick west limb that likely coalesce at depth. Radiometric age dating of the intrusions indicates an age of approximately 190 million years in the Kerr zone. Succeeding tectonic events have imparted a north plunge to the deposit. At the southern end of Kerr, high gold-copper grades, typical of the central core of porphyry systems, are found near the surface at an elevation of about 1,600 meters. At the northern end, at lower elevation, intrusions are cut by veins with clay-alteration assemblages characteristic of much shallower parts of a porphyry system.

There are several distinct intrusive phases in Kerr, the earliest of which are fine grained porphyritic diorite containing 5% to 60% quartz-sulfide stockwork veining that contributes the majority of the contained metals. Later intrusions envelope and sometimes invade the earlier phase, and are characterized by coarser textures, less veining, and lower metal contents. An intrusion breccia and wall rock complex above the west limb are moderately to intensely veined with gold and copper grades proportional to veining. The intrusions are introduced into an Early Jurassic sequence of bedded siltstones, sandstones, and conglomerates that have been altered adjacent to the intrusions. In general, the sedimentary rocks contain marginal metal grades except when in direct contact with an early mineralized intrusion.

Hydrothermal alteration at Kerr is characteristic of porphyry gold-copper deposits, and much of the sequence expected in these systems is preserved. In the central core zone of the system associated with the early stockwork-veined diorite and highest grades, potassic alteration is dominant with potassium feldspar (orthoclase), biotite and abundant magnetite. Outboard of that, in younger intrusions and some of the sedimentary wall rock, is moderate grade quartz-sericite-pyrite alteration (phyllic alteration). On the fringes of the system where grade is poor, propylitic and argillic alteration are present, consisting of chlorite, carbonate, epidote, kaolinite, quartz and pyrite. Primarily in the east limb of Kerr is a zone of intense anhydrite-quartz flooding which seems to overprint other alteration events.

The dominant copper mineral is chalcopyrite, which typically is as isolated grains about 0.2 to 2 mm across, disseminated and clustered in quartz veins, fractures, and surrounding haloes. Bornite is present almost exclusively in the north half of the east leg, within a phase containing over 50% crackled quartz veins, accompanied by coarse grained chalcopyrite and minor tennantite. Tennantite is rare, but widely distributed in late quartz-carbonate veins, mostly in wall rocks with minor sphalerite, rare galena, and arsenopyrite. Molybdenite is a minor constituent of Kerr, distributed widely within the early diorite and intrusion breccia units and closely associated with copper. Visible gold has not been observed in the deposit except under microscopic examination, where it is in inclusions within chalcopyrite and pyrite or at sulfide grain boundaries.

Sulphurets Deposit

The Sulphurets Deposit, smallest deposit in the KSM porphyry cluster, consists of two distinct zones—Raewyn and Breccia Gold. These two zones are spatially related but differ in form. This form, composition of intrusive rocks, thermal metamorphism and simple ore mineralogy, have led us to conclude that Sulphurets represent a peripheral mineral occurrence to the large porphyry systems in the KSM cluster. As a consequence of proximity, the Raewyn and Breccia Gold zones are treated as a single deposit for exploitation, but because each has a unique form the resource estimation treats them separately. The Sulphurets deposit (Raewyn-style) is exposed on the cliff face above Sulphurets Lake, in contact with and below the STF. Much of the deposit has an apparent north-northeast trend, dipping about 45o to the northwest, and is dismembered by several sub-parallel faults to the STF. Drill defined length of the deposit is 1,000 meters, and open to the northeast and southwest. Thickness is 250 meters and it has been traced down dip 550 meter, however it remains open down dip along most of the strike.

Raewyn represents the most extensive style for the Sulphurets Deposit. It is a disseminated chalcopyrite and pyrite body, associated with stockwork veins in altered sedimentary and volcanic rocks. These sedimentary and volcanic rocks are intensely hydrothermally altered to chlorite-biotite-sericite-magnetite, with extensive and intense thermal effects, producing a tabular altered hornfels mineralized zone. The thermal effects and in-part the alteration assemblages are directly related to numerous narrow dikes and sills of monzodiorite and granodiorite that intrude these rocks. Ore mineralogy is simple, consisting of chalcopyrite, auriferous pyrite and minor molybdenite.

The Breccia Gold Zone is a discreet, roughly circular, breccia body that crosscuts the volcanic and sedimentary stratigraphy and apparently the Raewyn Zone. Clasts in the breccia are dominantly altered intrusive rocks that seem to be like the Raewyn dike and sill intrusions. Matrix to the breccia is massive to semi-massive gold-bearing pyrite, chalcopyrite and minor sulfosalts mixed with potassium feldspar (orthoclase), quartz, and tourmaline. An intense structurally controlled sericite-quartz-pyrite alteration overprint parts of the Breccia Gold Zone, obscuring lithologies and earlier alteration, but preserving the sulfide minerals. The zone has only been identified in drill holes; its full vertical extents remain open.

Above the STF and the Sulphurets Deposit is a porphyritic monzonite with pervasive potassic hydrothermal alteration. The thermally altered sedimentary rocks and small blocks of copper-magnetite skarn surrounding this intrusion contain widespread low-grade disseminated copper-gold mineralization. This area is referred to as Main Copper and although it is not an important part of the KSM mineral system, it is the only occurrence identified above the STF.

Mitchell Deposit

The Mitchell Deposit crops out in Mitchell Valley, through an erosional window in one of the major splays of the STF, the Mitchell Thrust Fault (MTF). Recent glacial melt back has provided exceptional surface exposure of a relatively fresh gold-copper porphyry system. Mitchell is roughly cylindrical and bound between the MTF and a basal shear zone or failed thrust fault, giving it dimensions of about 1,600 meters across and 1,500 meters down plunge. This location between thrust faults has created a moderate to intense deformation in the intrusive rocks and the stockwork veining. Deformation ranges from clear east-southeast foliation and folding to mylonite zones. Variations in deformation intensity have been shown to be a function of the hydrothermal alteration variation in the Mitchell deposit. The deposit is zoned from a central core of gold + copper outward to gold + copper > molybdenum and then gold + molybdenum > copper.

Mitchell is genetically related to multiple fine-grained diorite intrusions, of roughly similar age, that host most of the mineralization and intruding sedimentary and volcanic rocks. The earliest porphyritic diorite intrusions are intensely veined by both stockwork veins and oriented sheeted veins and these rocks contain most of the gold-copper mineralization. A second series of porphyritic diorite intrusions produced disseminated gold-copper-molybdenum mineralization peripheral to the earlier veined intrusions. The next event was an explosive eruption of porphyritic diorite that produced breccias and little mineralization.

Alteration associated with the earliest intrusions and main mineralizing event is intensely potassic. This potassic alteration is characterized by abundant biotite (now chlorite), potassium feldspar (orthoclase) and magnetite. Although overprinting of younger intrusive events and hydrothermal alteration along with the intense deformation that post-date ore formation have obscured much of the original rock textures, vein morphology, vein mineralogy and clasts of this rock within other intrusions demonstrate that it is the first mineralizing event with characteristic vein styles of the heart of porphyry systems. It was pervasively altered to potassium-bearing minerals and introduced abundant chalcopyrite and magnetite. The second intrusive event that produced the disseminated mineralization contains a mixture of localized potassic alteration and phyllic alteration, consisting of sericite-quartz-pyrite and anhydrite. The youngest events produced intense quartz-sericite and clay alteration.

The Mitchell Deposit, like the rest of the KSM camp, was deformed during development of the Skeena fold and thrust belt (mid-Cretaceous time). During that period of deformation, the less competent sericite-rich part of the deposit surrendered under the stress creating the thrust planes of the MTF and the basal shear zone under the deposit. Dislocation of the upper Mitchell Deposit, above the MTF, now lies about 1,000 meters to the east-southeast, and is known as the Snowfield Deposit of Pretium Resources Inc. This reconstruction provides a complete picture of a cylindrical or stock-like porphyry gold-copper system, and unlike the Kerr and Sulphurets deposits where grades diminish rapidly away from the mineralized zones, in Mitchell gold and copper grade remain constant over large distances.

Iron Cap Deposit

The Iron Cap deposit is a separate but related mineralized system within the KSM cluster. It is structurally above the Mitchell Deposit and the MTF, and again sitting just below the STF. This deposit is a north-northeast trending zone plunging north-northwest, strike extension is at least 1,200 meters and dip projection is now 1,300 meters. The highest-grade zones of Iron Cap remain open both down plunge and to the north west, consequently an entire picture of this deposit has not yet been completed. Iron Cap has the same intrusive suite as Mitchell and Kerr that were intruded into and hydrothermally altered a sequence of sedimentary rocks. Margins of Iron Cap contain diverse sulfide mineralogy suggesting that much of the upper and outer parts of the deposit may be preserved.

Similar to the Mitchell and Kerr, Iron Cap shows multiple porphyritic diorite intrusive events along with other intrusive compositions. The deposit is bounded on the top and bottom by the STF and MTF, respectively, and to the south by the Iron Cap Fault. Mineralization is hosted in the intrusive rocks but also within the sedimentary wall rocks and breccia. A series of porphyritic diorite intrusions in the central part of the Iron Cap shows intense potassic alteration, weak to moderate stockwork veining and high-grade gold-copper; however, large blocks of sedimentary rocks and breccia adjacent to these intrusions contain these features as well. Several additional intrusive events have been recognized at Iron Cap indicating this was a very active area for magmatic intrusions and volcanic eruptions.

Alteration at Iron Cap is complex possibly indicating that rapid lateral and vertical changes were taking place in this shallow intrusive complex. On the margins of the deposit intense silica alteration has obliterated original textures in the rock. This alteration is accompanied by disseminated chalcopyrite-pyrite; the original rocks are interpreted as both sedimentary rocks and intrusions with few identifiable contacts. Approaching the central part of Iron Cap, and at depth, potassic alteration with well-developed gold-copper-bearing stockwork veining is evident, comparable to the higher-grade zones at both Mitchell and Kerr. An intense sericite-quartz -clay alteration is widespread at shallow levels but becomes focused by steep structures in the deeper potassic alteration. This alteration is accompanied by some discrete high-grade gold zones and invades the intrusive rocks and breccia zones. The overprint of sericite-quartz-clay is accompanied by pyrite, chalcopyrite, bornite and traces of sulfosalts indicative of the upper levels of a porphyry mineral system. Local increases in sulfosalt content correlate with depletion of copper and gold. The source of sericite-quartz-clay alteration that overprints the central core potassic alteration must await further drilling to establish the lateral and depth limits of Iron Cap.