Epithermal-skarn transition at the La Colorada deposit, Chalchihuites district, Zacatecas, Mexico, The

Abstract

This study examines whether an intermediate-sulfidation epithermal (IS) deposit hosted in volcanic rocks can be genetically linked to an underlying, carbonate-hosted, skarn-CRD (± porphyry) deposit. If the two styles of mineralization are related, this study also seeks to define vectors that can be employed in the shallow epithermal environment to find the related skarn-CRD mineralization at depth. The La Colorada deposit in the Chalchihuites District of Zacatecas, Mexico features IS, carbonate replacement (CRD), skarn, and porphyry styles of mineralization, and will be used as the case study for this purpose. The three major IS veins at La Colorada (NC2-HW, Amolillo, and Recompensa) are hosted in ca. 67 Ma (zircon U-Pb) volcanic rocks that overlie Cretaceous limestone-shale, with combined measured and indicated resources of 5.3 Mt containing 33.8 Moz Ag, 26.2 koz Au, 0.04 Mt Pb, and 0.08 Mt Zn as of June 30th, 2022. Deep drilling since 2018 has revealed a significant Zn-Pb-Ag skarn-CRD with porphyry Cu-Mo mineralization in limestone-shale below the eastern extent of the longest IS vein (NC2-HW). With 243.7 Mt of indicated and inferred resources averaging 29 g/t Ag, 2.48% Zn, and 1.23% Pb, and containing 229 Moz Ag, 6.1 Mt Zn, and 2.7 Mt Pb, the skarn is among the top five largest Zn-Pb skarns in the world. The main causative intrusion for the skarn-CRD-porphyry is a 62.5±1.0 Ma (2σ; zircon U-Pb) rhyodacite porphyry. Molybdenite from porphyry-style mineralization in the intrusion is dated at 62.0±0.2 Ma (2σ; Re-Os), and garnet from the surrounding exoskarn is dated at 61.1±2.4 Ma (2σ; U-Pb), all statistically identical to the ca. 62 Ma age of the intrusion.

Field observations reveal that IS veins and breccias in the volcanic rocks transition downwards to CRD veins with the same quartz-calcite-sulfide (mainly pyrite, sphalerite and galena) assemblage, but with more calcite. Towards depth, the CRD veins transition to massive CRD, and then skarn above the rhyodacite porphyry intrusion, which contains porphyry-style mineralization and red-garnet endoskarn veins. Sphalerite color changes from white in the shallow IS veins to red by ~600 m depth in the skarn, likely because the Fe in sphalerite came from the intrusion, which is depleted by shallow levels in the deposit. In the skarn, the garnet/pyroxene ratio increases towards the rhyodacite porphyry, the garnet color darkens, and the pyroxene color changes from brown to green, indicating a decrease in the Mn-content of pyroxene. Rhodonite and Mn-calcite occur in the most distal skarns. The 0.2-1.0 µm fraction of pure illite from the alteration halo of the NC2-HW vein is dated at 54.8±3.4 Ma (2σ). The date is interpreted as the minimum age for the IS veins, due to the low closure temperature of fine-grained illite (<317 °C). Therefore, the age of the IS veins is constrained between ~55 and 67 Ma, making it permissible that the IS veins formed at the same time as the CRD-skarn-porphyry.

Temperature trends in the shallow environment (< 700m depth) are investigated using illite crystallinity (IC) and fluid inclusion microthermometry. The IC values of altered dacitic volcanic rocks < 1 m from IS veins range from 1.9 to 0.6. Maximum average homogenization temperatures (Th) of fluid inclusion assemblages in sphalerite and quartz from volcanic and carbonate-hosted veins range from 325 °C to 234 °C. The IC and Th results spatially overlap, and together indicate two hot centers below the NC2-HW vein. The eastern hot center is next to the surface projection of the rhyodacite porphyry and related skarns, which verifies the temperature trend. The second hot center is in the middle of the NC2-HW vein, and is predicted to be related to another intrusive center. A third hot center to the NW, along the Recompensa vein, is predicted to be related to yet another intrusive center.

The results of this study suggest a transitional relationship between IS and skarn-CRD mineralization at La Colorada. In districts where a volcanic cover overlies carbonates, IS veins in the volcanic rocks may lead to skarn-CRD mineralization at depth in the carbonates. This encourages exploring for the related skarn-CRD by drilling through the IS veins, following vectors in the shallow epithermal environment such as illite crystallinity, which can be rapidly collected and interpreted in the field. Where white mica alteration does not occur, measuring Th in fluid inclusions from veins may extend any temperature trends indicated by IC. Once a distal skarn is intercepted, skarn zoning patterns may be used to vector towards the skarn-porphyry mineralization center. This exploration strategy applies to districts globally where carbonates underlie volcanic-hosted IS veins.