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Helvecia |
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Background Located at elevations of 2,000m in the southwest sector of La Rioja province and accessible all year round, Helvecia is one of Yamiri’s most promising targets for sediment-hosted lead-zinc-silver, uranium, and gold occurrences. The property has a history of mining lead-zinc-silver stratabound “mantos”, and uranium from shallow, high-grade sources. A molybdenum-rich skarn is an additional target on the property. Covering some 188 square kms, the Helvecia property lies immediately to the north of the Gualcamayo gold property of Yamana Gold Corp. (located in the province of San Juan to the south and now at an advanced stage of mine development) and is underlain by the same favourable suite of sedimentary rocks intruded by younger sills, dykes, and felsic intrusives. The property lies in the heart of one of the major uranium districts of Argentina, the Guandacol district. In the 1960s and 1970s, the Argentine Nuclear Commission (CNEA) identified several airborne radiometric anomalies, some of which remain to be followed up on the ground. A 1977 report by CNEA estimated that one area (Urcal) hosts a resource of 392,000 tonnes grading 0.035% U3O8, and another (Urcuschún) hosts a resource of 413,000t grading 0.115% U3O8 (both are historical resources not yet compliant with NI 43-101 rules). Large areas of prospective ground for sandstone-hosted uranium hosts small, high-grade copper-silver-uranium veins some of which have been mined on a small-scale. Historical Highlights Lenses and mantos of lead-zinc-silver-barite were mined intermittently from the early 1900s until the late-1900s. These historical underground workings were re-visited in the 1990s when Plata Mining Ltd. completed a work program to extend the mineralization along strike and down dip around a core zone of 2-3 square kilometres; this was generally not successful probably in part to a very wide drill spacing not consistent with the size of the manto targets along the unconformity surface. In the 1960s, the state nuclear commission CNEA began exploration for uranium that continued until the late 1980s. Several surface and underground targets were investigated, principally at Urcal and Urcuschún (the latter also drilled by the private company Uranco). As part of a NI 43-101 Technical Report, Hatch Ltd. completed a favourable investigation of the property including reconnaissance sampling for gold that returned anomalous values of 50-100ppb from limestone-sandstone contacts. Update of Exploration Activities, June 2007 Yamiri Gold and Energy Inc. holds 100% title to the Helvecia property covering some 18,800 hectares except for a small claim over an abandoned portion of the historical underground lead-zinc workings. Yamiri Gold’s exploration team has now completed a Phase One work program focused on uranium, lead-zinc-silver, and molybdenum, while also assessing the regional setting for sediment-hosted gold deposits on the property. In late 2006, an integrated program of 100m Induced Polarisation (IP) and Magnetics of approximately 10 line kilometers was completed by Quantec Geoscience Argentina S.A. over the Urcuschún ridge zone targeting uranium, skarn, and manto lead-zinc-silver mineralisation. A brief follow-up program of more detailed 50m Pole-Dipole IP on 3 lines was initiated in May 2007, although only two lines could be completed owing to the weather conditions. At Urcuschún, Carboniferous siliceous meta-sedimentary rocks overly Ordovician limestones along a regionally-prominent, shallow-dipping unconformity surface. A collapse-breccia unit sitting on this unconformity surface is a known host for high-grade “manto” lead-zinc-silver mineralization on the property, for example at the historic Helvecia mine approximately 3km to the north of Urcuschún, where grades of 30-35% combined lead-zinc and 100-200g/t silver over 3-4m widths were mined from underground workings in the 1900s. The late-2006 geophysics program identified a broad zone of anomalous chargeability values (>7.5mV/V) along the Urcuschún ridge with a surface footprint of approximately 1,000m north-south by 300m east-west, and a thickness of typically 50m starting at a depth of approximately 100m (please see our website for a level map and cross-section). Within this broader anomaly, two higher chargeability zones (15-20mV/V) can be identified. On Line 2, follow-up IP at a 50m electrode spacing has now confirmed an anomalous 300m by 200m footprint of high chargeability coincident on section with a sharp resistivity contrast clearly marking the limestone unconformity surface. Further to the north, a 400m by 200m high-chargeability zone straddles Lines 5 and 6, displays the same coincident resistivity features as seen on Line 2, and remains open to the north Although IP chargeability responses can be generated by either a sulphide or non-sulphide source, the chargeability values at Urcuschún are consistent with sphalerite-galena mineralisation similar to other Argentine and Mexican (carbonate-hosted) mineral deposits. Furthermore, soil sampling of the creeks immediately down-dip west and south from Urcuschún demonstrate anomalous zinc values of 90-534ppm. The Urcuschún exploration results suggest a model of lead-zinc-silver mineralization occurring as pods and mantos in the limestone breccia zone that marks the contact between lower limestones and the overlying package of silicified metasediments. Successful discovery of lead-zinc-silver mineralization at Urcuschún would open up a significant strike length of favourable ground further north along the unconformity contact, offering shallow targets that could be easily accessed for underground mining. The following plan and section illustrate the extent and characteristics of the Urcuschún geophysical anomalies.
Plan Map on 2450m Elevation, 100m Dipole Spacing Shows two high-chargeability anomalies within 7.5mV/V halo
Section on Line 2 Anomaly Shows Resistivity and Chargeability, 50m Dipole Spacing Geological Setting The Helvecia property is underlain by the same series of limestones, shales, sandstones, conglomerates, and interbedded units of chert of the Lower Palaeozoic that host Yamana Gold’s gold-silver Gualcamayo project in the Province of San Juan to the south. At a regional scale, thick sedimentary sequences were deposited in an elongated basin stretching SW-NE for several hundred kilometres from Mendoza Province (SW) into the Bolivian and Peruvian basins (NE). Several thousands of metres of sediments were deposited over the crystalline basement of the Sierras Pampeanas with some of the thickest sections of Lower Palaeozic in South America being found in the provinces of La Rioja, San Juan, and Mendoza. Unconformities have been recognised both within the Ordovician formations and at the contact between the Ordovician marine sequences and the overlying Carboniferous continental sandstones where the lead-zinc-silver mantos of Helvecia have developed. The unconformity contact may also have seen a period of re-mobilisation along thrust structures related to subsequent tectonics. The stratigraphy has been folded along N-S axes into a series of broad monoclinal structures dipping gently to the west. Tensional relief along NE axes has allowed late-stage swarms of dacitic and andesitic dykes to occupy zones of weakness, probably fed by late, silica rich intrusive stocks identified to the west and east of this broad monoclinal structure. Lead-Zinc-Silver At a local scale around the old mine workings at Helvecia, historic lead-zinc-barite production has come from lenses and mantos intermittently located along the Ordovician-Carboniferous unconformity (sandstone-limestone unconformity). Other showings of manto-style lead-zinc-silver have been identified at several points over several square kilometres around the old mine workings. In addition, skarn and hydrothermal base metal occurrences, and uranium mineralisation in the Carboniferous sandstones was developed intermittently on a small scale in the 1960s thru’ the 1980s. The following schematic section illustrates the lead-zinc-silver occurrences at the Helvecia mine and the district-scale geological setting of these carbonate replacement deposits:
Some thicker, near-surface lenses have been worked from pits and limited underground workings to depths of a few tens of metres while drilling and sampling has traced some narrow occurrences for up to 100m down dip and along strike. In addition to the principal lead-zinc minerals, occasional chalcopyrite, tetrahedrite, and pitchblende have been reported within a gangue of quartz, calcite, and barite. Secondary zinc minerals are common, principally the calamine suite of smithsonite and hemimorphite. Patches of yellow alteration product, probably after pitchblende and/or arsenic, can also be seen at several locations on the west-facing slopes of the canyon.
Galena-sphalerite-barite mineralisation, underground workings Uranium Several diverse styles of uranium mineralisation are found on the property including palaeo-channel deposits in sandstones, and vein/breccia zones in limestone.
More than 20 Uranium Occurrences identified at Helvecia At Urcal, brecciated zones in limestone host an historic resource of 392,000t grading 0.035% U3O8 (see Mineral Resources section). Small bulk sampling tonnages of high grade ore (up to 1.67% U3O8 (37lbs/t)are reported from 1980s reports. The depth extent of the mineralized structures remains open below the deepest mine working at 40m below the shaft collar. Limited drilling of 800m in 30 holes only tested near-surface mineralisation but encountered true thicknesses up to 23.5m (average 3.5m).
Further north, a string of fluvial sandstone-hosted showings over a 5-10km strike length (Sonia-Martita Trend) and adjacent untested geophysical anomalies will be a priority target for Yamiri’s 2006 exploration program. Uranium values reported in the range of 0.05-1.00% U3O8 occur within discrete stratigraphically-controlled lenses that have been investigated on surface and in several shallow underground workings. Significant copper-silver values accompany the uranium in places (>1% Cu and >3 ozs/t respectively). The uranium mineralisation is related to palaeo-channel structures and content of carbonaceous material.
Favourable cross-bedded Carboniferous sandstones, Sonia-Martita trend
Green U-Cu minerals around carbonaceous detritus in coarse To the south east at Urcuschún, secondary uranium mineralization is found in a fractured series of quartzites within a 50-100m package of metasedimentary rocks overlying a limestone basement along an unconformity contact. The anomalous uranium zone can be identified over at least 350m north-south, and remains open to the north:
North-south Longitudinal Section of Urcuschún Ridge Zone, looking east In mid-2006, Yamiri completed re-sampling of trenches on the east and west flanks of the mineralized zone. Results from the southern section returned average values of 0.16% U3O8. The secondary uranium mineralization appears to occur in at least two thin manto-style zones, each of about 2-3m.
Urcuschún Zone, Uranium values in trenches
Skarn mineralisation along a 750m long by 150m wide zone adjacent to a prominent intrusive quartz monzo-diorite carries values in molybdenum, copper, lead, zinc, silver, and gold (recent reconnaissance sampling by Yamiri returned grades up to 0.40% Mo, 0.06g/t Au, and Cu-Pb-Zn values all >1%). Gold Occasional references to sedimentary “Carlin style” gold occurrences can be found in some of the historical literature. Scattered sediment-hosted gold values at low levels have been reported from past work on the property, and may represent a regional-scale target for future exploration. A new gold zone associated with calcareous breccia units has been identified approximately 2kms north-east of the Helvecia mine workings with grades of 0.3-0.4g/t Au associated with barite, and lead-zinc-silver values. A recent independent consulting report has identified some general similarities with Yamana Gold’s Gualcamayo project such as the extent of silicification and decarbonation of host breccia units. Further basic mapping and sampling will be carried out to further investigate the potential for sediment-hosted gold at this and other locations on the property. |
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