Summary:
The mineralized zones in the Las Minas district are Au-Cu-bearing skarn deposits.
Copper and gold mineralization have been recognized in three settings within the Las Minas property: proximal skarn, distal skarn and quartz veins. Proximal-type skarn is the dominant skarn alteration observed within the Las Minas resource zones (El Dorado and Santa Cruz) while distal and gold-bearing quartz veins occur in the exploration targets to the east and north of the Las Minas resources.
Proximal skarn developed along marble-diorite contacts, both as exoskarn developed within the sedimentary rock, and as endoskarn developed within the intrusion. The skarn alteration has a typical zoning of marble-exoskarn-endoskarn-diorite. The distinction between exoskarn and endoskarn can be very difficult because the skarn alteration (especially garnet replacement) can be texturally destructive.
Proximal skarn alteration is dominantly garnet-rich with lesser amounts of pyroxene, and locally garnet appears to have replaced pyroxene. The skarn contains variable amounts of magnetite and lesser sulfide minerals.
Distal skarn is exposed within the canyon walls at elevations from ~1,700 m to 2,100 m. The distal skarn is highly variable in character. Distal skarn consists of massive sulphides and/or massive magnetite in stockworks, pipes, and replacement pods within the variably marbleized limestone.
Mineralization associated with various distal skarn mineral types is seen within the Changarro – La Perdida area at various locales such as Mina Changarro which appears to have exploited a near- vertical pipe of massive magnetite plus sulphides that has a width of 50 m. to 100 m.
Gold-bearing veins are found primarily at Pueblo Nuevo which is located about 4 km north of the Las Minas village. The veins are narrow, with widths ranging from 0.2 m to 0.8 m, strike generally within 10° of north-south, and have near-vertical dips. Veins with significant gold values persist over >1.5 km of strike and >650 m of elevation within a 600 m wide deformation zone, though the veining and deformation may be much more extensive. Vein gangue material is almost exclusively quartz, showing repeated or zoned open-space filling textures. Gold is associated with pyrite that forms < 5% of the vein fill, and with trace to locally 10% iron-rich sphalerite, galena and minor copper and silver minerals. These veins are prominent in both undeformed dioritic rocks which are believed to be the same intrusion that underlies the main skarn deposits at Las Minas, and in the overlying clastic meta sedimentary rocks and sheared limestone, as observed at the La Miqueta mine. The lower levels of the veins within the meta-clastics tend to show higher gold grades than in the higher level, limestone-hosted portions of the veins. Wallrock alteration is generally minimal in all country rock types.
El Dorado skarn occurs primarily at the contact between diorite and overlying marble. Being visually distinctive from the surrounding marble and intrusion, the skarn is seen in outcrop as dark-colored, with significant iron- and copper-staining. The occurrence of mineralized skarn was obvious to early explorers and was the locus of historical activity at the Juan Bran mine area on the west side of the canyon and El Dorado mine on the east side of the canyon. The El Dorado zone is broken into two horizons that are separated by a barren, north-northwest trending, 100 m to 150 m wide diorite dike that extends up from the lower diorite intrusion.
Modeling indicates that the El Dorado skarn mineral zone on the west side of the diorite dike has an 800 m northwest strike length, extends up to 450 m to the southwest away from the diorite dike, is on average 15 to 20 m thick, and can reach over 50 m in thickness along the northweststriking contact with the diorite dike. In contrast, the El Dorado zone on the east side of the dike has a strike length of 250 m northwest, extends up to 200 m to the northeast from the diorite dike, and is 5 to 10 m in thickness.
Highest metal grades within both zones are often within the skarn just below the marble-skarn contact. This contact can be very sharp with often a <0.1 m transition from weakly mineralized marble to high- grade skarn. The lower mineral contact is more gradational as the skarn alteration decreases into the weakly altered intrusive. The intrusive contact is sub-parallel to the observed remnant bedding in the overlying marble, which indicates that the intrusive contact might represent the upper contact of a thick sill.
The Santa Cruz zone lies about 0.5 km south of the Las Minas pueblo and is well exposed on a west-facing canyon wall just above a tributary of the Rio Las Minas. Skarn within the Santa Cruz zone lies along the west side of the dike, immediately to the south of and stratigraphically higher than the El Dorado and Juan Bran zones. The primarily east-dipping mineralization at Santa Cruz is more complex and discontinuous than observed at El Dorado due to the more variable intrusive-marble contact orientations (both near- vertical dike and east-dipping sills). In contrast to the El Dorado zone, magnesian skarn is common as indicated by the presence of olivine and serpentinite along the intrusive contacts. The increased magnesian skarn indicates a more dolomitic protolith.
Due to the steep west-dipping topography, drilling has been primarily east-directed, which is subparallel to the bedding/sill/mineralization orientation, making geologic interpretations somewhat uncertain. The Santa Cruz zone has a 200 m northwest strike length, extends up to 200 m downdip to the northeast from the topographic surface, and can be up to 50 m thick, though thickness is highly variable and often consists of stacked sequences of mineralized skarn. Occasional skarn intervals >100 m in thickness have been intersected, though this would include some intervals of dike and marble. In fact, quite commonly skarn (both exoskarn and endoskarn) are interlayered with thin (generally <5 m intervals of intrusion or marble.
The El Dorado deposit is separated into east and west mineralized zones by a roughly 150 m wide diorite dyke. The sub-horizontal skarn on the west side of the dyke has an 800 m northwest strike length, extends roughly 400 m in width and is 10 to 15 m in thickness on average. Towards the dyke, this zone forms an east-dipping keel extending up to 100 m with an average dip between 55 to 60 degrees. The portion of the El Dorado deposit on the east side of the dyke is sub-horizontal and has a strike length of 250 m, a width of 200 m, and is 5 to 10 in thickness.
The Santa Cruz deposit lies near surface, roughly 0.5 km south of the Las Minas pueblo. The deposit is near vertical, has a 200 m northwest strike length and extends up to 200 m down dip.