Tombstone Gold Belt

The Tombstone Gold Belt (TGB) is a 600 km long district of gold occurrences and deposits that extends across central Yukon and Alaska and which is associated with reduced, low magnetic signature plutons (Hart and Lewis, 2006 and Hart, 2006).

Since the discovery of the intrusion-related Fort Knox gold deposit in Alaska in the early 1990s, gold exploration in Yukon (particularly within the TGB) has increasingly focused on plutons of Mid-to Late Cretaceous age (Hart and Lewis, 2006). The strong association between gold mineralization and intrusions of the Tombstone Plutonic Suite within the TGB has provided the foundation for a reduced intrusion-related gold systems (RIRGS) model that has gained widespread use and acceptance (Hart and Lewis, 2006).

The best known RIRGS deposits in the TGB are the Eagle Gold Mine (Victoria Gold Corp.) Fort Knox Mine (Kinross Gold Corp.), the former Brewery Creek Mine (Sabre Gold Mines Corp.), and the AurMac (Airstrip, Powerline, Aurex Hill deposits - Banyan Gold Corp.), the RC (Blackjack and Elger deposits - Sitka Gold Corp.), and the Florin (St. James Gold Corp.) deposits. The recently discovered Valley Zone (Snowline Gold Corp.’s Rogue property), and the Yellow Giant Trend (Trailbreaker Resources Plateau property) are both great examples of recent RIRGS discoveries in the district and show how underexplored and fertile the district truly is. Listen here to gain a better understanding as to why intrusion-related gold deposits are so desirable and why Crescat has invested in several of these companies within this belt.

Geological Model for RIRGS Gold Deposits

RIRGS gold deposits are the product of local-scale fluids derived from cooling of a proximal granitoid intrusion. Figure 1 illustrates several of the geochemical, lithological, structural and mineralogical features of RIRGS gold deposits. These deposits share several characteristics with orogenic deposits, which are considered to result from crustal-scale fluids derived through metamorphic dehydration (Hart, 2005). Features that are common to both deposit types include:

1) metaluminous, subalkalic intrusions or intermediate to felsic compositions that lie near the boundary between ilmenite and magnetite series;

2) carbonic hydrothermal fluids;

3) a metal assemblage that variably combines gold with elevated bismuth, tungsten, arsenic, molybdenum, tellurium and/or antimony and low concentrations of base metals; and

4) a low sulphide mineral content, with a reduced ore mineral assemblage that typically comprises arsenopyrite, pyrrhotite and pyrite and lacks magnetite and hematite (Hart, 2005).

Other features exist that exclusively differentiate RIRGS deposits (Hart, 2005), including:

1) ideal tectonic setting is ancient continental margin behind accretionary or collisional orogens and subduction-related magmatic arcs;

2) typically formed late in orogeny;

3) preferred host strata include basinal miogeoclinal sedimentary or metasedimentary rocks;

4) concentric zoning due to steep thermal gradients surrounding cooling plutons – zones can develop outward from pluton margin for up to a few kilometres and gold mineralization is associated with pluton-proximal mineralization (bismuth, tellurium and tungsten), aureole-hosted mineralization (arsenic or antimony) and distal mineralization (silver, lead and zinc);

5) several different styles of mineralization due to rapid cooling of mineralizing fluids – styles include variably intrusion and country-rock hosted skarns, replacements, disseminations, stockworks and veins;

6) sheeted arrays of parallel, low-sulphide, single-stage quartz veins found over tens to hundreds or metres and preferentially located in the pluton’s cupola are the most distinctive style of gold mineralization;

7) plutons should exhibit physical evidence and geochemical support for high volatile contents, fluid exsolution, rapid fractionation, zonation, porphyry textures, presence of aplite and pegmatite dykes, quartz and tourmaline veins, greisen alteration, mariolitic cavities and/or unidirectional-solidification textures;

8) felsic, ilmenite-series plutons that lack magnetite, have low magnetic susceptibilities and aeromagnetic response, and have low ferric:ferrous ratios; and

9) coeval (± 2 million years) with their associated, causative pluton.

The abundance of RIRGS deposits correlates inversely with the surface exposure of the related intrusion because stocks and batholiths with considerable erosion are generally less prospective (Lefebure and Hart, 2005)

Figure 1: Geochemical, lithological, structural and mineralogical features of RIRGS gold deposits.

References

Victoria Gold Corp. (https://vgcx.com/production/eagle-gold-mine/2019-technical-report-executive-summary/)

Kinross Gold Corp. (https://s2.q4cdn.com/496390694/files/doc_downloads/2018/Fort-Knox-June-2018-Technical-Report.pdf)

Sabre Gold Mines Corp. (https://www.sabre.gold/projects/brewery-creek/)

Banyan Gold Corp. (https://banyangold.com/)

Sitka Gold Corp. (https://www.sitkagoldcorp.com/)

St. James Gold Corp. (https://stjamesgold.com/st-james-gold-corp-is-pleased-to-announce-updated-resource-on-the-florin-gold-project-yukon-with-an-inferred-resource-of-2474000-oz-of-gold/)

Hart, C.J.R.

2005 Classifying, distinguishing and exploring for intrusion-related gold systems; in The Gangue, October 2005, Issue 87; GAC Mineral Deposits Division and CIM Geological Society.

2006 Intrusion-related gold – Tombstone Gold Belt; Yukon Geological Survey Brochure 2006-6. Available at: http://ygsftp.gov.yk.ca/publications/brochures/ygs/ygs_brochure_2006_6_tombs tone_gold_belt.pdf 

Hart, C.J.R. and Lewis, L.L.

2006 Gold mineralization in the upper Hyland River area: A non-magmatic origin; in Yukon Exploration and Geology 2005, D.S. Emon, G.D. Bradshaw, L.L. Lewis and L.H. Weston (eds.), Yukon Geological Survey, p. 109-125.

Lefebure, D.V. and Hart, C.

2005 Plutonic-relted Au quartz veins & veinlets (L02); in Mineral Deposit Profiles; YGS and BSGS. Available at: http://www.geology.gov.yk.ca/pdf/l02_plutonic_related_au_quartz_veins_and_ veinlets.pdf