Intrusion-Related Gold Systems

Proof of Concept

Many systems overseas typically contain greater than 3Moz (Geoscience Australia, GA7241. Intrusion Related Gold Deposits, Phillip L. Blevin, PetroChem Consultants Pty Ltd)

North American IRGS examples, similar to Sovereign Gold’s:

Donlin Creek - 323Mt @ 2.7g/t for 32 Moz
Fort Knox - 1000m x 600m intrusion at 0.9 g/t for 7Moz
Brewery Creek - 10 ore bodies in 12km 'corridor'
Dublin Gulch - Eagle 2.3 Moz, 3 other orebodies

IRGS refers to Intrusion-Related Gold Systems, a new proven gold deposit type only defined in 1999. IRGS was first identified in the Tintina Gold Province (TGP), Alaska-Yukon.

According to the US Geological Survey, more than 50 million ounces of lode gold resources have been defined in the last 15 years through this new class of IRGS gold deposits.

Given IRGS has previously been overlooked in Australia, there is a potential for a “New Gold Rush” in World Class discoveries in Australia. The closest IRGS analogue is the Donlin Creek Gold Deposit, 32Moz @ 2.91g/t. The Rocky River-Uralla Goldfield and Mt Adrah Hobbs Deposit IRGS exhibits all the hallmark diagnostic features of the Donlin Creek Gold Deposit.

In these, gold mineralisation is associated with bismuth, tungsten, arsenic, molybdenum, tellurium and antimony, and the deposits typically have low base metal content. The most common type of Intrusion-Related Gold deposit is hosted in altered igneous rocks ranging in composition from quartz diorite to granite (Lang et al., 2000), associated with ilmenite-series (low magnetic signature) plutons.  The style of mineralisation has in the past led to some Intrusion-Related Gold deposits being described as "gold porphyry deposits" (Hopwood, 2012).

In simple terms IRGS deposits rely on gold existing in the fluids associated with the magma (White, 2001), and the inevitable discharge of these hydrothermal fluids into the wall-rocks (Lowenstern, 2001).

In an IRGS model, gold mineralization can occur in multiple parallel veins and stock-works that have a high likelihood of continuing at depth. In the project area there are old gold workings at or near the surface and which are untested at depth. Under the IRGS model, drilling is likely to intersect gold mineralization at depth below these prospects.

General characteristics of the model include:

  • A metal assemblage variably combining Au with Bi, Te, W, Mo, As, Sb and an ore mineral assemblage typically comprising arsenopyrite, pyrrhotite and pyrite and lacking magnetite or hematite.
  • Metal zoning centred on a central mineralising intrusion.

Many systems overseas typically contain greater than 3 Moz. (Geoscience Australia, GA7241. Intrusion Related Gold Deposits, Phillip L. Blevin, PetroChem Consultants Pty Ltd)
 

Conceptual location of cross-section A - B of present erosional level of Rocky River Intrusion-Related Gold System. High grade vein dykes up to 8m wide and bulk open-cuttable areas of narrow sheeted veins. Preferred sites of intrusion-hosted Au mineralization are above the cupola, where exsolved fluids will accumulate, and mineralized fractures developed in the pluton's apex and shoulders.

 

General plan model of RIRGS from the Tintina Gold Province

 

Note the wide range of mineralization styles and geochemical variations that vary predictably outward from a central pluton (modified from Hart et al., 2005).

Exploration Potential
Soil and stream sediment geochemical surveys have located parallel gold anomalies that are coincident with sheeted quartz vein arrays in hornfelsed metasediments (e.g. above plunging shoulder of Khatoun Tonalite) and the Uralla Granodiorite. Some anomalies are up to 10s of metres wide and 100s of metres long – situated in carapace of Uralla Granodiorite.

Many gold occurrences can be linked to highly fractionated minor plutons and quartz-exsolved felsic dykes. Collectively the data at Rocky River is a positive indication for the existence of a substantial IRGS mineralising event not previously recognised. It is associated with a magnetic linear structure and pluton-metasediment contacts that could host shallow bulk mineable stockworks or multiple vein systems and deeper hypogene gold.
 

 

 

 

 

 

 

 

 

 

last updated 30/05/2016

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