Plata Verde Project- Mexico
- High-grade silver mineralization up to 1070 g/t Ag.
- Large scale historic underground bulk mining operation from 1800's.
- Newly re-discovered by Radius, never explored
- In July 2022 Radius entered into an agreement with Fresnillo plc. ("Fresnillo") to drill and explore the project.
The Plata Verde project located in Chihuahua, Mexico, north of Radius Gold's Amalia Gold-Silver project and east of the historic Batopilas silver mining district (1708 to 1920) which reputedly produced over 300 million ounces of silver from high grade veins and structures (Figure 1). A new road has been constructed to enable access to the property.
Radius geologists in January 2020 re-discovered an undocumented large scale underground bulk mining operation that is estimated to have occurred in the late 1800's. Historic miners had hand excavated an extensive series of anastomosing caverns (Image 1) and produced silver bars at an associated smelter operation on the river margins, 1 km away from the Plata Verde Mines, Silver "bars" at 0.5kg each were also found by local residents within the project area as well as the remains of a crushing circuit.
The project appears to be un-explored since the miners ceased their operations, and no references to the project have been found within the Mexican Geological Survey. The few local residents have no knowledge of exploration companies working in the area and there are no indications of prior exploration within the mines or surrounding outcrops. There was a pre-existing mineral concession covering the area, but the owners of the concession had never visited the property and were happy to option the property to Radius.
Geology and Mineralization
At Plata Verde, silver mineralization occurs as massive to crustiform banded barite calcite with silver chlorides, sulphosalt minerals and native silver infilling and cementing fractures and breccia within a basaltic/andesite extrusive volcanic (Image 5 and 6). The green silver chloride oxides is after which the project was named. The basaltic/andesite host occurs at the base of the Upper Rhyolitic Volcanics of the Northern Sierra Madre. In general the host volcanics and associated silver mineralization is covered by the overlying rhyolitic ignimbrites and is only exposed along the length of a small creek.
On the eastern side of the creek a number of small entrances, hardly visible, lead to an extensive network of anastomosing mining tunnels and cavities. The mining and excavation was done by hand with hammer, chisel and black powder explosives, leaving behind large underground cavities up to 50 m x 50 m x 30 m high. Throughout the mines roman numerals painted on the walls record up to 58 mining areas. The mining excavations define a strike extension of over 500m and up to 100m wide. The host volcanic unit is at least 60 m thick. The southern extension is abruptly cut by a fault which forms a large cliff face. To the north, east and west the host volcanic flows and silver mineralization is covered by the ignimbrites, but remains open to unknown extent in these directions. On the opposite western side of the creek exposures of the mineralized unit host good silver grades.
Geological model and silver mineralization
Radius's geological team have completed several weeks of detailed underground mapping and sampling of the historic Mina Real and Mina Mojonera. Three distinct mineralization styles have been defined within the basaltic andesite volcanic host rock:
- Multiple large scale breccia zones with chimney type structures up to 75 m diameter and sampled on multiple mine levels. The breccias are cemented by massive to crustiform banded barite calcite with silver chlorides, sulphosalt minerals and native silver.
- Fracture fill and stockwork silver mineralization occurs as massive to crustiform banded barite calcite with silver chlorides, sulphosalt minerals and native silver.
- Disseminated style mineralization with fine silver sulphosats disseminated within the volcanic host with little to no brecciation, veining or fracture fill.
All three mineralization styles host significant silver grades, although the highest grades are related to intense brecciation and fracturing.
In total, 73 new 2 x 2 m panel samples were collected from the historic Mina Real and Mina Mojonera. Each mine covers a shallow dipping anastomosing sequence of mining areas on at least 3 levels with Mina Real covering approximately 200x200 m and Mina Mojonera 150x150 m (Figure 2). The latest results reported between 2 and 815 g/t Ag and averaging 185 g/t Ag. Samples were collected to represent all rock types and mineralization styles.
Historic Mine Rock chip samples Average all rocks
Mojonera 133 168 57 262 Real 122 143 17 244 Total 255 156 74 258
Table 1. Summary of underground rock chip sampling. Majority are 2 x 2 m rock panel samples.
The sampling completed within the historic mines shows that the mineralization is open to expansion in all directions.
See Figure 3 for Mina Mojonera geology summary and rock chip sampling with silver assays in g/t Ag.
Mina Real, located 150 m south of Mina Mojonera, is an extensive network of anastomosing mining tunnels and cavities covering roughly 150 m x 140m. The height of the excavation varies from 3 to 20m. Radius has collected a total of 75 prospecting rock chip samples from the walls and remaining pillars with silver grades ranging between 8 and 730 g/t Ag and averaged 276 g/t Ag. See Figure 4 for Mina Real geology summary and rock chip sampling with silver assays in g/t Ag.
See Figure 5 for Plate Verde satellite imagery and rock chip samples with silver assays in g/t Ag
Plata Verde Geophysics
Radius completed geophysical programs at Plata Verde, consisting of 7.5 line km magnetic survey and 4.5 line km IP/Resistivity survey conducted by consultants, Geofisica TMC. The program was designed to locate potential feeder systems below the historic silver mines and successfully identified compelling drill targets below the known mines.
Initial IP/Resistivity section through the historic Mina Real (Figure 6).
Initial IP/Resistivity section through the historic Mina Mojonera (Figure 7).
Metallurgical test work
- Work index for grinding (Wi) averaged 8.67 kWh/t, classifying the samples as “soft” for ball milling.
- Cyanide leach test work reported average recoveries of 93% for silver.
- Flotation studies generate Pb/Cu concentrates with a high grade of silver and good values of lead and copper with recovery of around 85% for silver, 52% for lead and 64% for copper.
- Combining flotation + tailings cyanidation results in average overall silver recovery of 97%.
- Future work should consider separation of Pb and Cu concentrates to generate commercial concentrates.
Head grade analysis
*average value obtained by direct assay, by fractions and metallic screen
Table 1. Summary of underground rock chip sampling. Majority are 2 x 2 m rock panel samples.
In general the silver mineralization is covered by the overlying rhyolitic volcanics and is only exposed within the historic mines and at surface in a few areas along the length of a small creek.
The barite/silver chloride mineralization appears to be a late-stage low temperature mineralizing event with the source and feeder systems an attractive exploration target. Barite and silver chloride are often part of the upper levels or supergene zone around large silver deposits. The solubility of barite and silver chlorides is low, and hence the source zone is likely to be close by. Extensions of the known mineralization below the ignimbrite cover to the north, east and west are open. Potential feeder structures have been clearly defined by the geophysics. Radius has completed an environmental study in support of drill permits which have been recently filed. The Company looks forward to drill testing the compelling targets once drill permits are granted.
A hidden mining camp
The historic mines at Plata Verde were large scale underground bulk mining operation where in the late 1800's, historic miners hand excavated an extensive series of anastomosing caverns, covering a strike length of at least 500m, and width known to date of +200m. Despite the large scale of excavations, the project has remained undocumented and unknown for over 100 years due to some unique geological features. Firstly, there is low sulphur in the system and the dominant silver minerals are silver chlorides. This means no scandalous alteration, no red iron oxides in the creeks. The oxides of silver are green and camouflaged. The smelters and processing facilities were located at the waters edge of a large river, and with floods over the last 100 years most traces of the buildings and all of the tailings dumps were washed away. Lastly the mineralized basaltic andesites are largely covered by post mineral ignimbrite, all assisting to hide the project until Radius' recent re-discovery (Figure 8 and 9).
Quality Assurance and Quality Control
Surface rock samples are acquired by Radius Gold's geologists and trained sampling teams under the supervision of the geologists. Majority of the rock samples consist of chip channels ranging from 0.5 to 3m. These samples are forwarded to an ISO/IEC accredited ALS Geochemistry laboratory for analysis of Au, Ag and other elements using ALS codes Au-ICP21 and ME-ICP61. Au-ICP21 is a fire assay method with an ICP-AES finish for Au. ME-ICP21 is a 33 multi-element (includes Ag) analysis that uses a four acid-digestion with an ICP-AES finish. For over-limits of Au or Ag, analysis using a fire assay method with gravimetric finish is automatically conducted. Quality control samples (blankets, standards etc.) are included with the field samples at regular intervals. The QA samples' results are monitored to ensure the returned results are within the acceptable range.