La Colosa is a large porphyry gold deposit owned by AngloGold Ashanti near Cajamarca, Tolima, Colombia.

Estimated to contain 28.33 million ounces (Moz) of gold, it is considered to be the biggest gold deposit in Colombia.

AngloGold Ashanti started extensive exploration drilling in 2007 after discovering the La Colosa property in 2006. It also completed a conceptual study to develop an open-pit mine in 2008.

The £3.1bn ($5bn) project was expected to produce one million ounces (Moz) of gold annually over an estimated mine life of 25 years, starting from 2020 onwards.

However, the project has been put on hold since the exploration works were suspended in April 2017 due to resistance from the local inhabitants.

Suspension of La Colosa gold project

The La Colosa gold mining concession covers 600km2 of land, which is designated as forest reserve under the Colombian law.

The La Colosa gold project witnessed persistent campaigns by local farmers and activist groups over the alleged economic, environmental and social impacts of the proposed large-scale open-pit mine, ever since it was launched in 2007.

Finally, in a referendum conducted in April 2017, 98.8% of the residents of Cajamarca voted against the mine construction, which led to the immediate suspension of project activities.

The anti-mining vote was made possible by a Constitutional Court decision that allowed the provincial governments and local municipalities in Colombia to challenge the exploration permits issued by the national government.

AngloGold Ashanti had invested approximately £680m ($900m) on La Colosa before the referendum verdict stalled the project in 2017.

La Colosa gold project location, geology, and mineralisation

The La Colosa gold project is situated approximately 150km to the west of Bogota, the capital city of Colombia, while the Ibague town lies approximately 30km east to the project site.

La Colosa is a multiphase diorite porphyry gold complex that intrudes into Paleozoic metasedimentary rocks.

The gold mineralisation at the La Colosa deposit is associated with early porphyry intrusions with potassic and sodic-calcic alteration. The high-grade gold grains are found disseminated in sulphide veinlets and silicates.

The La Colosa porphyry ore body is elliptical in shape. The known mineralization zone extends 1.2km along the north-south axis.

Exploration and drilling at La Colosa gold property

A total of 148,062m of exploration drill holes was completed before the suspension of activities in 2017.

It also included the completion of three compliance drill holes, as well as one geotechnical-hydrogeology drill hole in 2017.

A total of 153 drill holes for a total length of 43,529m were scanned using Terracore’s sisuMobi geospectral imaging solution.

Gold reserves at La Colosa

The La Colosa gold mine is estimated to hold 1,051 million tonnes (Mt) of indicated and inferred resources grading 0.84 g/t gold.

The contained gold resources of the mine are estimated to be 28.33Moz or 881tonnes (t).

Mining and ore processing

La Colosa was proposed to be developed as an open-pit mine involving drill-blast-load-haul operations.

A 50 million tonnes per annum (Mtpa) gold processing plant as well as a 1,700Mt tailing storage facility was also proposed to be built near the mine. Froth flotation technique was considered for gold recovery from the sulphide ore.

The water requirement for the La Colosa mine operation was proposed to be fulfilled through rainwater harvesting as well as water supply from the Coello River that flows below the project site.