Intrusion Related Gold Systems
Intrusive Related Gold Systems (“IRGS”) are today’s “last frontier” for gold exploration. They are the most recent of the major types of gold system to be discovered and defined, and thus appear to have the greatest discovery potential.
IRGS deposits are one of three major types of gold deposit, as defined here. There is still some debate as to what defines IRGS but nonetheless they do have a recognisable combination of geological characteristics (metal assemblage, metal zoning, type of hydrothermal fluids and zonation, tectonic setting, etc.) that warrant separate classification.
IRGS deposits formed at the same time as the associated intrusive body, which is usually of granodioritic to granitic composition (that is, coarse-grained and with quartz as the dominant mineral). The deposits occur in three settings: hosted within the related intrusion itself; in the immediate vicinity of the intrusion; or in nearby overlying sediments.
Gold (Au) is the main metal in these deposits and it is usually accompanied by silver (Ag), along with bismuth (Bi), arsenic (As) tin (Sn) tungsten (W) and molybdenum (Mo). The signature metals, those that can be used as an exploration pathfinder, are typically tin, tungsten and molybdenum.
The deposits generally occur as sheeted veins and veinlets with a characteristic alteration assemblage that is a useful pathfinder for exploration. They are typically low in sulphur, the main sulphide minerals are the iron sulphides, pyrrhotite and pyrite. IRGS deposits don’t have associated oxidised iron minerals such as magnetite and hematite and are therefore referred to as “reduced”.
The key economic features of IRGS deposits are that they are usually low to modest grade, 0.5 to 2.0g/t gold, but can be >10.0g/t; can be very high tonnage, up to 500 million tonnes or more; and are amenable to low cost, bulk mining methods and the gold is free milling (see below for definition of “free milling”).
Where Are They
Large IRGS deposits occur in specific tectonic belts around the world. The main belts are: the Paleo-Tethys Closure (fabulous name) that stretches from Portugal in the west through Kazakhstan to China in the east; the well-studied Tintina Belt that stretches through Alaska and western Canada; the Bolivian Polymetallic belt in Bolivia; and several belts in eastern Australia that are poorly understood in terms of IRGS potential.
The biggest and best known IRGS deposit is the undeveloped Donlin Creek deposit in Alaska. It is jointly owned by Barrick Gold US Inc and Novagold Resources Inc. It hosts total reserves of 528 million tonnes at an excellent grade of 2.1g/t gold for 34 million ounces of gold.
Fort Knox in Alaska, which commenced production in 1996, has remaining reserves of 314 million tonnes at a grade of 0.43g/t gold, for 4.3 million ounces of gold and has produced 4.6 million ounces to date.
In Queensland, Australia, Kidston, now closed, produced 4.5 million ounces of gold and Mt Leyshon, also closed produced around 3.0 million ounces of gold.
Australian IRGS Explorers
I have compiled a preliminary list of Australian-based companies with IRGS projects as a primary or secondary focus of exploration. This is not a complete list and inclusion of any company on the list is in no way an endorsement of the quality of the company or its assets. I will review some of these companies over the next month or so.
Company name ASX Code Marketcap ($M) at 31/01/13
Anchor Resources Limited AHR 9.9
Hudson Resources Limited HRS 30.7
Renaissance Minerals Limited RNS 39.4
Signature Gold Limited Pre IPO N/A
Sovereign Gold Company Limited SOC 32.0
Strategic Elements Limited SOR 2.4
Thomson Resources Limited TMZ 2.9
Torque Mining Limited IPO N/A
IRGS deposits are a worthy exploration target. They are amenable to several geophysical techniques, have a distinctive alteration halo and geochemical signature, and make for large drill targets.
Can be very large and with good economic characteristics, thus providing, with enough geological evidence, a good joint venture opportunity for junior explorers.
The relatively recent recognition of this type of gold deposit means geological evidence may not have been recognised. For example, numerous tin tungsten occurrence in the New England Fold Belt of Eastern Australia, and associated enigmatic gold shows, have not been systematically explored for IRGS gold.
A gold ore is said to be “free milling” when the majority of the gold can be extracted by a standard cyanide circuit without previous chemical preparation (such as roasting or bacterial leach), unduly fine grinding, or excessive reagent consumption. In sum, free milling ore is cheaper to process than other ore types.