Summary:
The El Pilar copper deposit occurs within unconsolidated, poorly sorted, poorly bedded, proximal facies alluvial wash deposits that are overlain by dissected younger alluvial fan deposits. At the northern boundary of the deposit, these basin-fill sediments are juxtaposed against unmineralized Precambrian granitic rocks by an east-west to northwest-trending, south dipping zone of faulting and hydrothermal brecciation. The faulting is of unknown displacement. The breccia zone comprises a multi-stage, highly silicified, copper mineralized hydrothermal breccia that is up to 100 m wide and 600 m long. The El Pilar copper deposit is interpreted to have been formed by erosion of this breccia over time into the range-front topographic depression.
Mineralization predominantly consists of the copper oxide mineral chrysocolla, which occurs as coatings on clasts of highly silicified breccia and as grains in the sedimentary gravel matrix. The main gravel sequence that hosts copper mineralization consists of poorly consolidated angular to sub-rounded fragments of breccia, intrusive rocks and minor volcanic fragments cemented in a sandy matrix. These productive gravels, referred to as Quaternary Alluvial Wash Deposits Upper (Qwu) and Quaternary Alluvial Wash Deposits Transitional (Qwt), range from 30 m to 180 m in thickness. The main zone of copper mineralization occurs within a southwest/south trending channel that extends for more than 2 km.
The El Pilar property hosts an unusual gravel hosted or transported copper resource that is atypical for the area. Copper at El Pilar is hosted by range front alluvial wash deposits with clasts of intrusive, porphyry and highly silicified rock derived from a proximal, exposed hydrothermal breccia zone. The mineralization is interpreted to have been derived at least in part from that. Reconstruction of events suggests that the breccia was mechanically weathered and eroded, transported and deposited in a channel and alluvial fan sequence that overlies a lower more indurated alluvial wash unit (Qwl).
MINERALIZATION
Cu Mineralization
The El Pilar Cu deposit is about 2,300 m long N33°E, and approximately 750 m wide, NW-SE with a mean thickness of 110 m (range 5 m to 220 m) and plunges 28° to the southeast. Approximately 98% of the Cu mineralization at El Pilar is hosted by Quaternary alluvial flow wash deposits proximal to bedrock and the remainder is within the E-Wtrending mineralized breccia.
Prior to erosion, the breccia is interpreted to have been much larger than its present extent. It appears that proximal erosion of this mineralized, but now oxidized breccia, into the nearby range front depression during weathering was the primary source of the El Pilar Cu deposit. Although this erosional/depositional event was largely dilutive of Cu mineralization, because many of the larger clasts in the sedimentary material are comprised of unmineralized older intrusive and possibly younger volcanic rocks, total Cu grades are very consistent throughout the sedimentary sequence.
Total Cu grades are very consistent throughout the core of the mineralized alluvial body at El Pilar. Once the 0.30% total Cu envelope is encountered in drilling, essentially all the material within that envelope grades above 0.30% total Cu and the grades consistently fall within a narrow range of approximately 0.30% to 0.50% total Cu. As a precursor to a discussion of other Cu assay grades variations at El Pilar, it is first important to understand some relative Cu grade terminology. Very importantly, whereas some of the Cu at El Pilar is tied up in minerals that are soluble under laboratory acid soluble assay tests, some of the Cu at El Pilar is not easily soluble under the same acid laboratory procedures. Cu that is assay soluble is referred to in this report as acid soluble Cu (soluble Cu). The less assay soluble Cu is referred to herein as residual Cu. The combination of both soluble Cu and residual Cu is referred to as total Cu. The other important Cu grade designation at El Pilar is the ratio of soluble Cu to total Cu, or the Soluble Cu Ratio. This ratio is referred to as the Ratio or percent soluble Cu. The formula for calculating the Ratio is soluble Cu percent divi ded by total Cu percent.
At El Pilar the relative amounts of soluble Cu and residual Cu, and as a consequence the Soluble Cu Ratio, vary noticeably from the surface of the deposit downwards. Mean soluble Cu grades decrease with depth, whereas residual Cu bench average grades increase. Due to the additive effects of changes in both the soluble Cu and residual Cu grades, the Ratio or percentage of soluble Cu declines from an average of about 64% soluble Cu near the top of the deposit to 26% soluble Cu at the bottom. These grade changes by bench translate to a similar drop in the percentage of soluble Cu encountered over the mine life, as progressively deeper benches are mined.
As mentioned previously, total Cu grade is very consistent throughout the El Pilar deposit and average bench total Cu grades do not change significantly with depth. This is because as soluble Cu grades decline downward residual Cu grades increase by about the same amount.