Striking silver - mining in Mexico
Intensive mining has made it harder to find high-grade deposits, yet the silver belt in Mexico is still full of potential. Michael Forrest reports
Over past decades there has been an inexorable decline in the metal grades that support today's mines – a reflection of 200 years of exploration, and the ability to economically mine low-grade mineralisation. For example Mount Isa, Australia, discovered in 1923 grades 2.2% copper; Palabora, South Africa, 1.6% copper; Escondida, Chile, 1.5% copper; and Pebble deposit, Alaska, 0.4% copper.
Low-grade mines require large mineral resources to support the investment, that can run into billions of US dollars, and most are open cast, however, large-scale underground mining, such as block caving, can support low-grade deposits. This is not always true and there are still high-grade mines to be found and exploited. These are often found in established mining districts where the right geological conditions come together – a source of metals, a process to concentrate mineralising fluids, a way to transport these fluids and finally an environment where the metal in the fluids can be deposited and retained.
One of these areas is the central Mexican silver belt, which has been mined for centuries. This belt extends for approximately 800km along the Sierra Madre Occidental mountains and is the world’s most productive silver mining district. Silver is found in this region in a combination of epithermal vein and carbonate replacement deposits with production grades ranging from five to 30oz per tonne.
Canadian company Excellon Resources Inc is mining in this belt, with properties at Platosa and Miguel Auza. Company Chairman Peter Crossgrove explains that his company acquired the historic Platosa mine property in Durango State in 1996. By 2001, in a joint venture with Apex Silver Mines Ltd, the adjacent Saltillera concessions were incorporated, and in 2005 Excellon began test mining at Platosa and selling crushed ore to a subsidiary of Mexican mining giant Penoles.
In early 2009, the tolling agreement ended and the company began trucking ore to the flotation mill of Silver Eagle Mines Inc, at Miguel Auza in the Zacatecas State, 220km to the south of Platosa and it began producing separate silver-lead and silver-zinc concentrates. By the end of 2010 recoveries averaged 87% total silver, 71% lead and 72% zinc. In June 2009 Excellon acquired Silver Eagle and all its assets, including the mill and 41,000ha of exploration lands.
‘The historic production and the high grades established during exploration are what attracted Excellon to the Platosa property. In an era of worldwide declining ore grades, these deposits and their extensions represented an excellent opportunity for the company,’ states Crossgrove. ‘The mineralisation at Platosa is defined as manto sulphides, which is sub-horizontal sedimentaryhosted silver, lead and zinc deposits. At Platosa we have the highest mined silver grades in Mexico.’
The origin of these deposits is most likely linked to felsic intrusives within the shelf and slope facies rocks of Mesozoic age that lie atop the Coahuila platform, a fault-bounded uplifted basement block. In general, the local stratigraphy constrains their distribution in porous sediments in a structural trap. They are not shear related and there may be an association with organic material, such as hydrocarbons, although this and the intrusive link may not be apparent.
‘Our exploration has identified a number of high-grade mantos. They are carbonate replacement deposits (CRD) comprising a series of pods and lenses of massive and semi-massive silver-lead-zinc sulphides. The mineralisation, associated alteration, and complexity seen at Platosa, are similar to larger-scale Mexican CRD systems that contain from five to 50Mt of ore,’ says Crossgrove.
The mineralisation at Platosa is typical of the irregular, high-grade (distal) fringes of replacement systems, at the centre of which are found largerscale (proximal) deposits in the La Faja de Plata (the Spanish name for the silver belt). The sulphide mineralisation is massive, banded, disseminated and fracture-filling, fine-to coarse- grained galena and sphalerite, with accessory pyrite.
The primary silver mineral is acanthite, which occurs as coarse blebs and fine-grained intergrowths with galena. Native silver and proustite occur locally. Lead, zinc, and silver grades are often quite high, especially in the more massive sections. Silver grades in the thousands of grammes per tonne are not uncommon. Lead and zinc sulphides can comprise over 80% of the rock mass in core samples and this is reflected in the assays.
Drilling has also intersected anomalous copper and gold values, but not with sufficient thickness or continuity to be considered part of the mineral resource. Gangue minerals include fine-grained calcite, coarse gypsum, quartz and fluorescent purple fluorite.
In late 2009, the company received an updated and significantly increased mineral resource estimate. Taking into account the 79,000t mined between 3 February 2008 and 31 October 2009, the indicated mineral resource increased 66% to 579,000t grading 909g/t (26.5 oz/t) silver, 9.095% lead and 10.51% zinc.
Accounting for the majority of the increases was the 2008 discovery of the NE-1 Manto, located northeast of the Rodilla and Guadalupe mantos, and, in mid-2009, the discovery of the 623 Manto close to existing underground infrastructure. The 623 Manto hosts an indicated mineral resource of 62,000t grading 1,183g/t (34.5 oz/t) Ag, 10.27% Pb, and 8.52% Zn. The primary exploration tool is the drill bit with the objective of discovering larger volume mineralisation, proximal CRD mineralisation and additional highgrade distal mantos.
Exploration during 2010 resulted in the addition of significant sulphide mineralisation in the 6A-6B Manto, extending it over 150m beyond the existing resources, again with high grades of 1,032g/t (30 oz/t) Ag, 7.63% Pb, and 17.89% Zinc. And later in the year there was the discovery of the Pierna Manto located between the Rodilla and NE-1 mantos.
Future targeting will be aided by 3D induced polarisation (3D IP) ground geophysical surveying and sophisticated airborne geophysical surveying, carried out during 2010.
The Platosa Mine is accessed by a 4x4m, -15° ramp and employs a modified room and pillar mining method. Development headings are 3x3m. The company operates a small fleet of modern scoop trams and haulage trucks, and has two single-boom jumbo drills primarily used for development headings. Production mining is done with jack-legs, and is carried out on a nominal three shift per day, seven days per week schedule and, in recent months, production has averaged 150-200t per day.
Most of the mining takes place at around 135m vertical below surface. The mantos can be either level or inclined, and in certain areas roof and wall support with rock bolts is required.
During 2010, the company added to its mobile fleet, increased pumping capacity, completed the construction of a modern office building and warehouse, upgraded its standby power generating capacity and completed a new 4.5km long plastic-lined ditch to carry mine water to a surface dispersal site north of the mine. In addition, modifications were made to the crushing plant, resulting in increased hourly throughput, decreased utilisation time, and significant savings in power costs.
Underground water remains a concern at Platosa and the company continues to develop grouting solutions within a hydrological model for the mine and surrounding area. Seven monitoring wells were drilled near the mantos during 2009, and, in early 2010, the company drilled a test dewatering well. Emergency water-control doors have been installed in two locations underground and two more are planned. Also, a project has begun, drilling a series of 20-60m long sub-horizontal grout cover holes ahead of critical development and production faces in order to provide continuous protection from sudden water inflows.
The acquisition of Silver Eagle Mines Inc in March 2009 included a mine developed by the company on an epithermal silver, lead, zinc vein system similar to others in the Mexican silver belt. Although the mine was placed on care and maintenance in December 2008, the surrounding area holds significant exploration potential. Excellon carried out an initial exploration programme, including 12 drill holes between late 2009 and late 2010.
‘The Miguel Auza flotation plant has transformed the economics of Platosa. In addition, it has the necessary capacity as we work towards expanding production in this exceptional mineralised trend,’ added Crossgrove.