Mansfield, Antares pumped on Rio Grande potential

Salta province, Argentina — Based on the sheer size of the mineralized system, joint-venture partners Mansfield Minerals (MDR-V, MFMNF-O) and Antares Minerals (ANM-V, ANMFF-O) are keen on the potential of their Rio Grande copper-gold project, in western Salta province’s Puna district, in northwestern Argentina.

Rio Grande, just 12 km northwest of Mansfield’s Lindero gold porphyry deposit, first caught the company’s attention in 1999 after regional surface reconnaissance exploration turned up broad zones of chalcopyrite and magnetite mineralization within intensely potassic altered volcanic rocks.

Under a previous financing agreement, Teck Cominco (TCK.B-T, TCK-N) had right of first refusal on the project; liking what it saw, the major exercised its option to earn up to 65% of Rio Grande.

Initial geochemical analysis and petrographic studies of mineralization at the property pointed towards an iron oxide copper-gold (IOCG) model for the system, and a 3-trench program in mid-2000 returned intervals of up to 96 metres of 0.58% copper and 0.98 gram gold per tonne.

Geophysics delivered a strong induced-polarization (IP) anomaly coincident with soil geochemistry and trench results, triggering Teck to begin an 11-hole, 3,000-metre drill program in early 2001.

Several holes returned significant zones of copper and gold mineralization, averaging about 0.36% copper and 0.43 gram gold over an average interval of 37 metres.

Despite the positive drill results, trench results that ranged from 0.3-0.5% copper and 0.2-1 gram gold over widths of 20-132 metres, and indications that some of the holes bottomed in significant copper (up to 1%), Teck Cominco dropped its option in early 2002 in the midst of a weak metals market.

After regaining full title to Rio Grande, Mansfield signed a brief option agreement with Rio Tinto (RTP-N, RIO-L) in mid-2002, but the major relinquished its option after only two months.

By early 2003, Mansfield had Tenke Mining, now part of Lundin Mining (LUN-T, LMC-X), interested in the project, but the pair were unable to hammer out a formal earn-in agreement and no deal materialized.

With metal prices beginning a rally, Mansfield continued work on Rio Grande, all the while looking to line up another joint-venture partner.

In mid-2004, the company optioned the project to Antares Minerals’ predecessor company Planet Ventures. Recent revisions to the agreement will see the junior earn a 50% interest in Rio Grande by spending US$3.4 million and issuing 300,000 shares to Mansfield by Sept. 30, 2007.

Recent exploration

Trenching, sampling and mapping by Antares in late 2004, followed by a 6-hole, 1,700-metre drill program in 2005, and a 10-hole, 3,300-metre campaign in 2006, have given the partners a clearer picture of the project geology.

“The system is starting to show that it has the potential to develop some very good grades,” says Antares’ Rio Grande project geologist Javier Robeto, explaining that holes 24 and 26 from last year’s program were especially good.

“After two-and-a-half years of work, it’s taking shape,” Robeto continues. “We know have more knowledge of the property and we feel very confident we can now be much more aggressive and test the potential.”

Antares recently began a third phase of drilling, using three rigs, that is expected to total about 15,000 metres.

Geology

Rio Grande sits in a porphyry environment, emplaced into a “red-bed” sequence in an Andean-cycle back-arc basin, and appears associated with a pair of regional-scale structural transverse zones.

It is bound on the north by the northwest-trending Calama-Olacapato-El Toro transverse zone and on the south by the parallel trending Archibarca transverse zone that continues through into Chile, where it intersects other structures and is coincident with the Escondida and Zaldivar mines.

The Rio Grande volcanic complex consists of a pair of overlapping dacitic-andesitic centres flanked by extensive shallow intrusive plugs, dykes and sills in a volcaniclastic suite, including breccias, agglomerates, and lahars.

Two styles of mineralization are observed on the project: an early stage IOCG-style hydrothermal system in an oval-shaped ring zone roughly 2.2 by 1.9 km; and typical copper-gold porphyry alteration assemblages found in an intense calcium-sodium-iron metasomatic overprint and rimmed by strong potassic alteration.

Vein-controlled potassium feldspar alteration is also present with quartz-limonite veinlets cutting argillic altered rocks.

Mineralization styles in the IOCG zone includes a pervasive blue-green “stain,” and fracture-filling copper oxides (mainly chrysocolla, malachite and possibly azurite) with associated gold in potassic-altered rocks. The assemblage is thought to be a result of oxidation of fine-grained chalcopyrite.

Coarser clots, disseminations and stringers of chalcopyrite (with gold) and an oxidized “copper limonite,” usually associated with scapolite-diopside-magnetite alteration, are also evident. Some blue-green copper oxide (possibly chrysocolla) rims the chalcopyrite.

Other mineralization in the IOCG zone includes finely disseminated chalcopyrite (with gold) and copper oxides present in potassic altered volcanics and volcaniclastic rocks; chalcopyrite blebs with some copper oxides on fractures in late-stage, weakly altered dykes; and copper oxides or rare chalcopyrite (with gold) associated with late, cross-cutting magnetite-hematite veins.

Most of the copper-gold mineralization occurs in potassic altered zones and is controlled by north-northwest, east-northeast and west-northwest-trending structures that likely reflect ring structures of the volcanic centre.

Porphyry-style copper-gold mineralization primarily occurs as chalcopyrite in stockworks and sheeted quartz-magnetite-sulphide veinlets in zones of strong clay-iron-oxide and potassic alteration.

During a recent visit by The Northern Miner, the size of the mineralizing system was apparent, as was abundant copper oxide mineralization, near surface and in extensive trenching.

Exploration indicates the Sofia zone (forming the southeastern portion of a ring structure) is contiguous with the Discovery zone located to its north, forming a 1.5-km-long zone averaging 130 metres wide.

Based on 10 drill holes and about 20 trenches, the combined zones have returned an average grade of 0.33% copper and 0.35 gram gold and remain open along strike and at depth.

“At the beginning of this year, we finished a 5,500-metre trenching program,” Robeto explains. “It has doubled the prospective ground; and we’ve increased (the mineralized zone) by more than one kilometre on strike to be explored.”

The other target that has Mansfield and Antares excited is the deeper central porphyry intrusive. The duo is optimistic the system has potential to host a large copper-gold system similar to the Bajo de Alumbrera mine, located in neighbouring Catamarca province.

Drilling to date has only really tested about 30% of the ring structure and none of the central porphyry. Antares’ current drill effort will infill previous drilling on the eastern portion of the ring structure and will fence back into the central zone to test for the porphyry intrusive.

With its deep weathering horizon (more than 200 metres in sections), Rio Grande offers potential for a significant copper oxide system amenable to solvent extraction-electrowinning recovery.

Another target that has undergone some surface trenching is the Northeastern zone, a separate diorite-hosted gold porphyry similar to the Lindero deposit.Mansfield recently completed a $5.25-million financing in Antares to hold about 9.8% of its JV partner.

Rio Grande enjoys similar potential access to infrastructure as the Lindero project. The several-hour drive westward from Salta is along a paved highway that parallels high-voltage power lines and a natural gas pipeline that supply the major copper mines of northern Chile.