Subscribe to the Magazine

Get new articles sent directly to
your mailbox.


Join the network

Twitter Updates Group Forum

Earth Explorer is an online source of news, expertise and applied knowledge for resource explorers and earth scientists.
Sponsored by Geosoft.

Featured Explorer

Community News & Views

News Archive

April 2, 2014

TGDG Mini-Symposium on Mexico Exploration

On April 9, the TGDG is presenting a mini-symposium on Mexico Exploration: Finding the Treasure of the Sierra Madre and more. A portion of this event is being broadcast online free of charge...

March 31, 2014

KEGS presentation on Neural Network Targeting

Featured KEGS presentations are now open for free online registration. The next presentation, scheduled for April 8, is on Supervised Neural Network Targeting and Classification Analysis for Mineral Exploration...

March 30, 2014

Top 10 mining events in northern Ontario history

A review of northern Ontario’s rich and vibrant mining history by Stan Sudal...

March 29, 2014

Predicting the condition of the ocean

New statistical models could lead to better predictions of ocean patterns and the impacts on weather, climate and ecosystem, MU scientist finds...

March 28, 2014

Top 5 Inversion Best Practices: Web Series

What are some of the most common, impactful things you can do to improve your 3D geophysical inversion models?...

March 26, 2014

Geosoft UXO Marine update delivers new functionality and improved workflow

Geosoft has released a software update for UXO Marine, adding new functionality and tools for conducting underwater site investigations with geophysics...

March 24, 2014

African Geology meetings in London, Houston - and Africa

Over the past twelve years, the Petroleum Exploration Society of Great Britain and the Houston Geological Society have jointly established the premier global meeting devoted to Africa's petroleum geology...

March 13, 2014

Recent investigations of geophysics abilities in aquifer mapping and resource evaluation

During the last several years Geological Survey of Canada has been investigating geophysical methods potential, particularly airborne electromagntics, for mapping and resource evaluation of buried valleys...

December 12, 2013

Heading south for the winter

Thirteen earth science students from Carleton University will spend this winter break in an unusual classroom: the waters and shores of Antarctica...

Where there is one VMS, there is usually another

by Virginia Heffernan on June 24, 2013 Applied

Panoramic view of the El Roble project site in Columbia. Atico Mining’s exploration program for El Roble is focused on discovering several million tonnes of high grade copper – gold ore.

In the high-stakes game of mineral exploration, companies that can test their model at an operating mine before venturing into the surrounding district are at a distinct advantage. It’s an approach that is paying off for Vancouver-based Atico Mining as the junior searches for volcanogenic massive sulphide (VMS) deposits near the El Roble copper-gold mine in Colombia.

“Having access to the mine workings helps us understand the geological and structural controls on mineralization,” says Atico CEO Fernando Ganoza, “and that in turn is helping us find additional resources.”

Atico uses the data collected from ongoing brownfields exploration combined with historical data to create 3D models using Geosoft’s Target and Geochemistry for ArcGIS. The models help guide exploration below the current workings and within the prospective host horizon that extends along strike from the mine.

“We input all of the geophysical and geochemical data into ArcGIS and then, based on the understanding of the geology, set parameters in the software,” explains Senior Exploration Manager Joseph Salas. “That’s how we generate targets.”

In early 2011, Atico signed a deal to earn a 90% interest in El Roble in return for paying its Colombian owners US$2.25 million in cash over two years plus a lump-sum payment of US$14 million due in January 2014.  Over a production history of roughly 22 years, the underground mine has turned out 1.5 million tonnes grading an average of 2.5% copper and 2.5 grams gold per tonne.

Field geology discussion.

“Having access to the mine workings helps us understand the geological and structural controls on mineralization and that in turn is helping us find additional resources.”
- Fernando Ganoza

Although most of the historical resource, consisting of several VMS lenses, has been mined, Atico has found more ore at depth. These new targets were established by mapping the geology both at surface (1:5,000 and 1:10,000 scale) and underground (1:250 scale), following structural controls and mineral zoning patterns typical of VMS deposits.

Geophysics and Geochemistry Zero in on Targets
On of the most significant conclusions from the surface mapping program is that the mineralized contact between basaltic volcanic rocks and black and grey cherts extends for 10 km across the El Roble concessions. Follow-up prospecting has confirmed that resistivity lows and chargeability highs can be used as indicators of the favourable chert horizon.

However, additional geophysical tools were needed to distinguish between conductive graphite in the chert and VMS mineralization. So Atico launched a ground magnetic orientation survey around the mine and reprocessed about 40 line-km of ground magnetic data collected in the mid-1980s by Nittetsu Mining, a former joint-venture partner on the property. The reprocessed magnetic map shows a weak magnetic anomaly over the El Roble mine and four stronger magnetic anomalies to the north.

In the air, Atico ran a 500-km versatile time-domain electromagnetic survey to cover the trend with lines every 100 m and a spacing of 50 m between stations. (Time-domain EM surveys are often effective in pinpointing VMS mineralization above a depth of 200 m.) A follow-up ground gravity survey further refined drill targets by identifying several anomalies that could represent pockets of VMS mineralization.

[Click to enlarge]

El Roble – Geology Section 20 South.

Although drilling along the trend has yielded little of interest so far, underground drilling is showing that a gold-silver halo in the black chert is a significant geochemical indicator that can be used for both brownfield and greenfield exploration. Results from the northern end of the operating mine include 119 m of 6.9% copper and 6.3 grams gold per tonne.

The host rocks at El Roble are part of the Cretaceous Cañasgordas Group, which can be traced for more than 800 km along the western cordillera of Colombia. Locally, mafic volcanic rocks include pillow basalts, tuffs, hyaloclastites, and agglomerates, while pelagic sedimentary rocks consist of chert, siltstone, and minor limestone.

The VMS mineralization is dominantly pyrite and chalcopyrite, mostly fine-gained and showing little internal structure or banding. Underground drilling between the 2,000- and 1,980-metre levels has identified stockwork mineralization consisting of chalcopyrite with subordinate pyrite in massive veins and patches in a gangue of stockwork quartz and chlorite veins.

Since VMS deposits tend to occur in clusters and the mined-out zones at El Roble represent only about half of what an average VMS cluster would yield, there’s a good chance Atico will find more ore along the 10-km trend as it uses information from the mine to seek out economic mineralization.

 


View all articles »