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Los Filos: The Right Angle

by Virginia Heffernan on May 29, 2013 discovery

David Jones

David Jones reviews exploration plans for a Minaurum Gold project in the Guerrero Gold Belt (GGB).

Sometimes exploration success comes down to a decision as routine as choosing the angle of a borehole. That’s what David Jones learned when he drilled the discovery hole at Goldcorp’s Los Filos, now one of the largest gold mines in Mexico, with annual production of more than 300,000 oz.

Although the discovery was made almost two decades ago, it remains instructive for those exploring other projects along the Guerrero gold belt, currently one of the world’s hottest exploration plays -- but also one of the most frustrating because, although outcrop is plentiful, the gold rarely expresses itself at surface.

Jones, who continues to explore the belt in southern Mexico on behalf of juniors such as Minaurum Gold, Acapulco Gold and Citation Resources, says the first step to finding the elusive gold is to pinpoint the Tertiary granodiorite porphyries that characterize the 80-km-long belt and are most commonly associated with gold mineralization. While magnetics can sometimes detect them, detailed mapping of structure and alteration is Jones’s preferred method of tracking down these intrusive rocks and the ska rn alteration that surrounds them.

That advice comes after years of experience exploring the belt. In the 1990s, when Jones was mapping in the mountains just north of Acapulco, not far from the small Nukay gold mine that had operated on and off since 1947, there wasn’t a lot to see. Even though the host rocks outcropped, they were barren at surface. What did make Jones stop and think “I should drill this!” was some float with iron oxide veining in an area of endoskarn.

“It was puzzling because it was different from anything I’d seen before,” says Jones.

So he and his crew built a crude switchback into the area to serve as an access road for a drill rig. Rather than drill directly into the altered intrusive with a vertical hole, Jones decided to angle the hole to see if he could intersect the carbonate wall rocks where mineralization was more likely to have been deposited by cooling hydrothermal fluids.

It was the right decision. The hole hit 12 metres grading 11 grams gold per tonne, the first in a series of rich intersections at Los Filos for joint-venture partners Miranda Mining and Teck. When Jones revisited the site 600 holes later to see what would have happened had he chosen a vertical orientation, the corresponding intersections proved to be nearly barren.

Shown above is a mineralized intrusion sample from Goldcorp’s Los Filos mine.

The Los Filos find, combined with an earlier discovery nearby (Bermejal) and a rising gold price, accelerated exploration along the belt and resulted in the discovery of almost 21 million ounces of gold. Resources are being proved up at a rate of about 600,000 oz. per year, though this is bound to slow as juniors struggle to raise cash.

There are two main deposits at Los Filos, both of which are iron-gold skarns with minor amounts of copper and silver at the intrusive-limestone contact. The original Los Filos occurs in a diorite sill that acted like a “giant sponge,” soaking up the mineralization as it was dumped out of the fluids. Bermejal, brought into the Los Filos fold by Goldcorp in 1995,occurs both within the granodiorite stock and at the limestone contact. Together the two deposits contain remaining proven and probable reserves of about 7.4 million ounces, according to Goldcorp.

Bermejal, discovered in 1987 as a result of sampling jasperoid, is obvious at surface. Los Filos has no surface expression. Yet of the two ore bodies, Los Filos is the richer. For Jones, that discrepancy is one of the most important lessons for explorers along the Guerrero belt: the most subtle indicators often lead to the best deposits.

Since the continental crust under the belt is relatively thin, magmas tend to rise and and cool quickly. The resulting alteration halos – though intense and typically dominated byquartz, sericite, clays, hypogene iron oxide, and sulphides-- tend to be narrow and difficult to find without precise mapping practices.

Although many of the granodiorite stocks do not have magnetic signatures, geophysics has been successful in finding some deposits as well as detecting the Los Filos deposit retroactively. For instance, the main deposits at Torex Gold’s 4-million-ounce Morelos project--El Limon and Los Guajes-- have only a weak magnetic signature. But Torex is currently intersecting skarn that carries gold, silver and copper at the contact between the intrusive and sedimentary rocks by following up magnetic highs on the property.

Several companies exploring along the belt are using VTEM to detect the clay alteration that is often associated with the intrusives. At Minaurum’s promising Vuelcos del Destino project, for instance, helicopter-borne VTEM/magnetic surveys have outlined three targets which coincide with a 2-km-diameter granodiorite porphyry stock that has alteration and structural features similar to those of the Los Filos deposit. The geophysical targets are coincident with stream-sediment anomalies.

Meanwhile, Cayden Resources is using a 3-D magnetic inversion model to guide drilling at its Magnetita target. The property contains a magnetic stock that coincides with gold-in-soil and gold-in-rock anomalies. A 5,000-metre drill program is underway.

And at Citation’s Biricu project, a 1,400-line-km VTEM survey has identified 19 potential conductive and/or magnetic target zones. Six of these zones are aligned along a 5-km northeast-trending corridor. Mapping is underway to prioritize areas for drill testing.

If these juniors are able raise enough cash to follow through with their drilling plans, expect gold resources along the rich Guerrero belt to continue growing at a rapid clip this year.

The Eureka Drill Hole That Started It All: From the drill hole that hit right down in the center of Nukay (now Los Filos) to what is now an 80km trend of mineralized skarn gold discovery deposits. Watch the video posted by Minaurum Gold.

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