EXPLORATION ’95 — Plata to focus on Argentine gold and zinc

Gold and copper are not the only commodities being pursued in Argentina. Newly listed Plata Mining (ASE) intends to focus on the historic Hualilan gold property in San Juan province. But the junior also has its eye on a zinc property in Rioja province, where mineralization appears to be of the Mississippi Valley type.

Plata director David Wahl says an exploration program is already under way on the Hualilan property, consisting of detailed geological mapping and ground geophysics, to be followed by about 1,000 metres of diamond drilling. Plata can acquire the property by paying $2.4 million over four years. The property includes all of the deposits and past-producing mines in the district, where gold was first mined in the mid-1500s.

Early records are skimpy on information, but, based on the extent of underground development and the grade of mineralization in remnant stockpiles, Plata estimates that between 250,000 and 500,000 oz. could have been produced from material averaging 19.8 grams gold per tonne (0.7 oz. per ton).

The Sierra de las Minas (Mountain Range of the Mines) hosts the mineral deposits in the Hualilan district. This range consists of the San Juan formation, a thick sequence of Ordovician limestone which strikes northerly and dips to the west at 25 to 70. This unit is overlain to the west by the younger Silurian Tuncunuco formation, consisting of sandstone-pebble conglomerate and finely bedded shales. The entire sequence has been intruded by Tertiary dacite porphyry stocks, dykes and sills.

The most significant regional structural feature is the northerly trending Hualilan thrust fault, while the most significant local features are high-angle faults which transect the ridge. These faults have acted as channel systems for gold-bearing hydrothermal fluids, and have been the loci of vein deposition. They have also generated extensive zones of hydrothermal brecciation.

There are two distinct styles of mineralization on the property, the high-temperature skarn regime and the low-temperature distal hydrothermal regime. Historic mining focused on the skarn deposits at Cerro Sur and the manto deposits at Cerro Norte. A previous operator identified an inferred mineral resource at Cerro Sur and Cerro Norte of 245,000 tonnes averaging 8.95 grams gold and 46.28 grams silver per tonne.

Base metals are the target of interest on the Helvecia property, described as having a resource potential of up to 18 million tonnes averaging 10-12% zinc, based on the nature and extent of known mineralization.

Plata can acquire a 90% undivided interest in this property by spending US$1.3 million over three years.

The geological setting is a geosynclinal sedimentary basin within the Precordillera range. The mineralization occurs at the unconformity between the Ordovician limestone and Carboniferous sandstones. The primary minerals are sphalerite, tetrahedrite and pitchblende — all associated with calcite and quartz. The secondary oxide zinc minerals — smithsonite and hemimorphite — have been the most important zinc minerals mined to date. The past-producing mines on the property contained secondary oxide mineralization averaging 40-49% zinc and 0.2-0.4% lead, plus 40-70 grams silver, over a thickness of 0.5-1.2 metres.

Primary sulphide mineralization intersected underground returned 37.1% zinc and 2.2% lead, plus 135 grams silver, over a thickness of 3.5 metres.