Discovering the Giants
Everyone wants one and very few find them. World-class and giant deposits are identified as belonging to the top 10% of their deposit group by their metal endowment. At present, the classification for gold is defined by having a pre-production resource of >100 tonnes (>3 million ounces) of gold for world-class deposits and >250 tonnes (>7.5 million ounces) of gold (or gold + copper equivalent for iron oxide copper-gold) for giant deposits. Super giants are anomalies within the classification exceeding 250 tonnes of gold: Golden Mile Mines in Western Australia with >1,457 tonnes of gold and the Hollinger-McIntyre Mine in Eastern Canada with 995 tonnes of gold.
How are giant gold deposits formed?
Giant deposits tend to be well-studied resulting in enormous amounts of data. In addition, the data collected are from many different specialties including geochemistry, geophysical properties, structures, rock types, ages, temperatures, pressures, and the list goes on. Further complications arise as some deposits are thought to be a result of the superposition of two or more mineralizing systems and their geological environments may consist of super-imposed environments making it difficult to determine which geological features are coincidental and which are directly related.
Fortunately, Groves, Goldfarb and Santosh (2016) took at look at the giant gold deposits around the world and came up with a set of parameters to provoke thought into the discovery of giant gold deposits in non-arc environments:
Large alteration halos are a major factor in the early discovery of new mineral provinces.
First order tectonic controls occur in the form of deep, mantle-tapping, fault or shear zones and structural corridors, as well as terrane-bounding sutures.
Spatial as well as contemporaneous to subsequent relationships of gold mineralization occur with lamprophyric intrusions derived from the mantle along deep fault zones indicating longevity of these structures.
Structural traps, such as large anticlinal hinges with minor folds superimposed, or major thrust faults are present to focus sufficient fluid.
Lithology: Orogenic Gold Regional contacts between underlying volcanic rocks and overlying sedimentary sequences where the sedimentary rocks act as a relatively impermeable cap Carlin Gold Reactive carbonate-rich host rocks capped by thrusted, relatively impermeable marine shales Iron-Oxide Copper-Gold (IOGC) Megabreccias formed at high crustal levels due to the deep derivation of anomalously volatile-rich magmas
Complex geometries, from belt to district scale, as a result of mis-aligned jogs in first-order faults, re-working of earlier structures by later deformation events, anomalously large sizes of reactive host units and the occurrence of competent bodies (e.g., small intrusions in shear zones).
With fewer new deposits being discovered each year it may be time to look outside the box of traversing the ground searching for surface mineralization or following up that one anomalous regional sample. There is now a wealth of regional information, and numerous advanced techniques for data collection and processing, that could be effectively utilized to generate unique targets that would not have been considered, or thought of, in the past.
About the Author:
Diana has over 20 years of experience working in the mineral exploration industry searching for diamonds and metals in a range of roles: from heavy minerals lab technician to till sampler, rig geologist, project manager and business owner. Following a Master of Science degree in diamond indicator mineral geochemistry, Diana has conducted field work in BC, NWT, YT, ON (Canada) and in Greenland. She has also been involved, remotely through a BC-based office, on mineral exploration projects located in South America, Africa, Eurasia, and the Middle East. Diana finished a Ph.D. at UNBC in 2017 researching geochemical multivariate statistical analysis and interpretation. Currently, Diana is the owner of Takom Exploration Ltd., a small geological and environmental consulting firm focused on metal exploration in BC and the Yukon.