Los Cuarentas Gold Project

Over 100 samples between 0.8 g/t Au and 29.4 g/t Au (historical samples).

Low Sulfidation Epithermal Au system.

Ready-to-drill, Available for Option.

Location: Sonora, Mexico

Size: 807 has


The Los Cuarentas Gold Project is a low sulfidation epithermal vein system deposited along a northwest trending structural corridor similar to the Premier’s Gold Mercedes Mine, which produced 89,699 oz Au and 321,814 oz Ag in 2018[1]. Mineralization is controlled by both structures and lithology and is primarily hosted in volcanic sequences overlaying the Bisbee sedimentary group. Gold mineralization occurs at surface as quartz veins for a cumulative distance of 0.8 km and extends across two ready-to-drill targets: Santa Rosalia and Santa Rosalia Sur. Historical assays returned lateral continuity of high-grade up to 29.4 g/t Au. In addition, the pervasive alteration across the Los Cuarentas Project has led Riverside geologists to identify the third target, El Sombrero, which provides further exploration and discovery potential outside of the primary targets.

[1] https://www.premiergoldmines.com/2018-4th-quarter–year-end

[2] http://www.silvercrestmetals.com/_resources/704-ENG.VMIN03016-05-Las-Chispas-PEA_a190719_SEDAR.pdf

Figure 1: Left Picture: Hanging wall of Santa Rosalia Mine and old working; Right Picture: Riverside’s geologists standing on a massive hydrothermal breccia dipping 50 degrees to the SW


Los Cuarentas project is located 170 km northeast of Hermosillo and accessible from Arizpe from a 15-km-long well-maintained dirt-road. The project is located near highway 89 with direct access from the nearest town, Arizpe if necessary (see Figure 2 below).

The project is within the vicinity of multiple mine sites under production such as the SilverCrest Metals’ Las Chispas PEA (Preliminary Economic Assessment), which estimates annual production of 9,559,000 oz AgEq[2] and the Premier’s Gold mine.

Figure 2: Location of the Los Cuarentas Project

Project History

Los Cuarentas Project reportedly processed in excess of 100,000Mt of ore in the early 1950s. In 2010, Paget undertook an exploration program within and outside of Riverside’s current claim boundaries. Starting in 2014, Millrock Resources took over the project and performed additional reconnaissance including detail mapping, sampling (sediments, channel, soil, rock), and road building for drilling. An Induced Polarization (IP) survey was also completed by Centerra Gold in 2016 while under option from Millrock. Results identified a strong resistive body that appears to coincide with the surface expression of the Santa Rosalia Sur vein systems.

All combined since 2010, a total of close to 5,000 samples were analyzed including rock, stream, soil, and channel sampling, and another 864 Terraspec analyses. Those results extend beyond the current Riverside’s claim block.

Some of the best results at Los Cuarentas returned numerous multi-gram assays with recurrent lateral continuity. It includes over 80 samples along the Santa Rosalia trend with values higher than 0.5 g/t, and one up to 17.5 g/t Au over 1 m. The highest result sampled by Millrock returned 29.4 g/t and is located along a parallel structural feature to Santa Rosalia Mine, called El Zorro (see press release dated September 11, 2019).

Riverside acquired Los Cuarentas in mid-2019 and took over all the historical data from Paget and Millrock. The company has compiled the load of historical data and investigated the field to confirm previous results and perform additional test sampling.

Disclaimer: Assay results from previous operators work on the Los Cuarentas Project is historical. The reliability of historical data is uncertain but is considered to be relevant by Company management; however, there is a risk that the Company’s confirmation work or future drill testing may produce results that substantially differ from the historical results.

Geology and Mineralization

The historical Santa Rosalia area comprises a series of mineralized (< 1 m to 2.5 m wide) parallel, low sulfidation epithermal veins of banded crystalline to cryptocrystalline quartz and calcite. Mineralization shows gold, silver, galena, argentite, acanthite, sulfosalts, minor chalcopyrite and pyrite. Textures observed are typical of a low sulfidation epithermal system and include the presence of incipient banded quartz, quartz breccia, silica after bladed calcite and quartz veinlet arrays inside fault breccia and gouge (Figure 3). Vein flexures and cross intersections provide some of the higher-grade bonanza shoots, which remain to be further explored over the strike length.

Figure 3: Recent Riverside’s sampling and example of veining at Santa Rosalia and Santa Rosalia Sur targets

The historical mine workings, which have been abandoned since the 1950s, consist of a vertical stope open to surface with horizontal extensions reaching out possibly 250 meters. There are several tens of thousands of tons of material based upon Millrock and Paget’s calculations near the abandoned mill structure. While the extent of the workings is not fully known, there are indications that they may extend vertically down to possibly 300 feet.

Veins are hosted primarily in andesitic, volcanic rocks that show strong alteration minerals including sericite and high acid conditions clay minerals. The stratigraphy is similar to that of Mercedes and Las Chispas mines where sedimentary units are overlain by andesitic flows and flow breccias. Mineralization seems to be somewhat related to certain strongly silicified rhyolitic dikes, near which metric-sized quartz vein or extensive argillic alteration can be found. Alteration varies spatially from low sulfidation to the north and higher temperature to the south as suggested by Millrock’s Terraspec analysis highlighting alteration products such as dickite and pyrophyllite.

Riverside Exploration Work

Riverside Resources has identified three primary targets, including Santa Rosalia, Santa Rosalia Sur, El Sombrero, all of which are located along principal splays from a northwestern transform corridor (See Figure 4 below; see press release dated September 11, 2019).

Figure 4: Geological map of Los Cuarentas Project and historical results

Santa Rosalia:

The historical mine represents over 20 old shafts and adits over a 1.5 km long area and a near 500-m-long stope following the principal mineralized zone at about 300-ft deep. The mineralized structural corridor follows an east-west trend, along which the vein zone varies from 1 m to 2.5 m including vein, breccia, stockwork. Sampling highlighted a continuity of high gold grade increasing near fault intersections and structural flexure. Historical results returned multiple high-grade values 3.8 g/t Au, 16 g/t Au, 17 g/t Au repeatedly along 1 kilometer.

Santa Rosalia Sur:

This target is identified by an 800-m long structural zone showing extensive alteration and veining along the northern end. The mineralization is characterized by both low and high angle veins showing evidence of boiling texture such as quartz-calcite replacement within hydrothermal multistage breccia, stockwork, and sheeted veins. Millrock’s rock chip and channel sampling highlight gold values up to 19.3 g/t Au and 26 samples between 1 g/t Au and 14.8 g/t Au.

El Sombrero:

At the El Sombrero target, the alteration is very extensive showing silica caps indicative of high-level boiling zones within an epithermal system. The El Sombrero target is located E-SE of Santa Rosalia Sur and comprises altered andesites, dacites and agglomerates. This alteration in conjunction with Centerra Gold’s geochemical soil survey and selected rock chip samples has delineated this region as a third priority target.



Low Sulfidation Epithermal Au System

Type of Deposit

Sonora, Mexico


807 Ha.

Surface Area