MINERAL EXPLORATION — CASE STUDIES — Porphyry Mountain discovery a solid technical success

In industry jargon, they are “technical successes” — remarkable discoveries accomplished under challenging conditions and which, at first glance, appear to have fallen short of the economic threshold needed to justify development.

The description is applicable to the Porphyry Mountain copper-molybdenum deposit at Murdochville, Que. However, under the right conditions, this relatively new discovery might one day cross the all-important finish line to extend the life of the historic Gaspe mining district, which has been producing since 1955.

Details of the discovery and subsequent exploration work were made available by Noranda (NOR-T) and its subsidiary, Brunswick Mining & Smelting, at the convention of the Prospectors & Developers Association of Canada, held in Toronto earlier this year.

Geologists Jeff Hussey and Pierre Bernard provided a fascinating summary of the project, which began in earnest in 1994, when four exploration holes discovered a buried porphyry/skarn copper-moly deposit between 1,000 and 1,700 metres below the top of Porphyry Mountain. A few years later, a total of 19,500 metres of drilling in 12 holes had outlined 200 million tonnes grading 0.73% copper and 0.08% molybdenum.

The discovery followed an extensive compilation and re-interpretation of the geology of the existing mines at Murdochville, and included brain-storming sessions with porphyry-skarn experts.

The project is in the Gaspe Peninsula, which is part of the Appalachian Mountain belt and essentially composed of Early Paleozoic sediments.

Regional-scale and tensional faults acted as controlling conduits for mineralizing quartz monzonitic intrusions known as the Copper Mountain and Porphyry Mountain intrusives. These intrusive stocks are the two known apophyses of a large batholith at depth.

In 1921, after tracing mineralized boulders over 8 km, prospector Alfred Millar and his brothers staked claims on the western flank of Copper Mountain. Noranda came into the picture 17 years later, when it optioned the property and barged a drill to the property to test the find. The porphyry/skarn copper orebodies at Gaspe Mines were discovered in 1938, though various challenges meant production did not start until more than two decades later.

Subsequent mining has focused on the Copper Mountain and Needle Mountain open-pit mines, as well as several underground skarn deposits. More than 150 million tonnes averaging 0.9% copper have been mined. Proven reserves at the end of 1997 totalled 1.2 million tonnes at 3.3% copper, which will sustain mining only for a year or two. The nearby custom smelter will continue to operate after the mine is closed.

The skarn and manto deposits are hosted by the Forillon and Shiphead formations, which consist of mudstones with varying degrees of calcareous content that are interbedded with two limestone units, known as L1 and L2, and a more siliceous limestone unit that was metasomatized to a wollastonite-rich horizon called W1. Geologists are reported to have mapped more than 100 tuffaceous marker horizons, which were then used to interpret fault displacements and assist in determining target depth in the stratigraphy.

Regional deformation caused magmatic processes to generate a hypabyssal intrusion at depth (outlined by an airborne magnetic anomaly). This intrusive stock generated hydrothermal fluids that altered and mineralized Gaspe Mines stratigraphy sediments. The copper-moly mineralization is strongly controlled by the stratigraphy, and consists of four types: * cupriferous manto — massive to disseminated sulphide replacement of marble and calcareous porcellanite;

* skarn front — stratiform disseminated and semi-massive sulphides in garnet-clinopyroxene skarns that replaced marble in the L1, L2 and W1 units; * porphyry copper — low-grade, weakly disseminated stockwork mineralization in porcellanite and skarn at the L1 and L2 units; and

* porphyry copper — late retrograde sulphide veins predominantly found in porcellanite.

Property-scale zonation maps of the L1 and L2 horizons were compiled using information from more than 3,000 drillhole intercepts along these horizons.

The intercepts were classified as: unaltered limestone, marble, skarn, marble-skarn, marble/sulphides and massive sulphides. This enabled the geologists to outline the skarn front, the extent of the marble aureole, and the most important target areas.

Exploration then focused on higher-grade, skarn front targets and manto deposits in the marble aureole. In 1994, while exploring the deep northeastern edge of the skarn front (1,300 metres), lower-grade porphyry-style copper-moly mineralization was intersected over vertical thicknesses of up to 575 metres. An aggressive drilling program was carried out in 1995 and 1996, which outlined the existing resource.

Most of the metalliferous fluids have drained off from the porphyry stock into the very receptive L1 and L2 skarn horizons, thereby preventing the formation of the large fluid/temperature convective cells that result in a typical porphyry. Also, the L1 and L2 skarn horizons are commonly replaced by monzonite adjacent to the stock. For these reasons, Noranda geologists are generally of the view that, notwithstanding its name, Porphyry Mountain is not a classic porphyry deposit.

About 40% of the deposit is found in the intrusive rocks at the core of the deposit, while the remaining 60% is in the adjacent porcellanite and skarn.

Within the intrusion, mineralization appears predominantly as tightly healed veins of quartz, chalcopyrite and molybdenite. In the L1 and L2 skarn horizons, chalcopyrite occurs as veinlets, interstitial to silicates, and locally as massive sulphide replacements.

Molybdenite occurs mostly along quartz veinlet margins that may also contain chalcopyrite. About 20% of the moly is fine-grained and interlocked in quartz, which poses challenges in the mill. However, metallurgical tests have shown 92% recoveries for copper and 80% for moly.

After the deposit was outlined, a multi-disciplinary team was put to work to evaluate the deep, low-grade deposit through derivatives of block caving and sub-level caving mining methods.

The anticipated operating costs for block caving (not including mill costs) are in the order of $5 per tonne, significantly less than the nearest competitive bulk-mining method. Noranda has focused on proving the viability of block-caving, as it will have the highest impact on overall economics.

But there will be challenges other than the depth of the deposit and its relative low grades. The rock mass is competent, with a large joint spacing, which gives the deposit a much higher mining rock mass rating than most of the existing deposits being mined by block-caving methods.

Ongoing research and innovative thinking could lead to important breakthroughs that will bring about a new lease on life for one of this country’s most important, but often ignored, mining camps. However, development and production may also have to await a substantial and sustainable improvement in metal prices.