Summary:
The La Colorada deposit is an atypical gold–silver deposit located in the centre of Sonora. It is not similar to the typical epithermal systems of the Sierra Madre Occidental. The deposit type is not well constrained and a number of deposit types have been suggested.
According to Herdrick (2007), the La Colorada Project area contains at least three parallel vein trends that were mined by underground and open pit methods. Drill targeting is guided by structural analysis and recognized vertical zoning, supported by fluid inclusion and alteration studies indicating gold in the pits formed from boiling in an epithermal system. Upper boiling-zone levels are typically barren and overlie potentially gold-bearing vein segments at depth; any such barren levels at La Colorada were likely removed by erosion, leaving mineralization well exposed at surface.
Fluid inclusion studies by Albinson (1997) indicated mineralizing temperatures mostly in the high-200°C to mid-300°C range, consistent with a relatively hot epithermal environment. Albinson classified the veining as deep epithermal or high-level magmatic-mesothermal, likely related to shallow plutons emplaced above a copper porphyry system. Two mineralization styles were identified:
• a structurally controlled primary hydrothermal quartz vein system with Au-Ag and base metals;
• a later supergene gold enrichment hosted by cross-cutting fractures with iron oxides, interpreted as gold leached from adjacent primary veins.
Perez (2017) supported this interpretation and also identified two phases of gold mineralization, including late supergene enrichment.
Zawalda et al. (2001) noted that at El Crestón, the vein–intrusive relationship, relatively deep formation conditions, low to moderately high salinities, and association of gold with base metals and quartz-carbonate veins suggest affinity with quartz-carbonate–base metal–gold systems. These can occur distally above porphyry Cu-Au deposits, placing El Crestón between potential deeper porphyry mineralization and a near-surface epithermal environment.
Further evidence for a porphyry association includes molybdenum veinlets in the intermediate La Colorada/Gran Central zone hosted by diorite and rhyolitic porphyry, locally with K-feldspar and fine secondary biotite halos (Navarro, 2016). Early porphyry-style alteration is overprinted by structurally controlled Au-Ag veins with base metals, occurring within the gold system and in later, more distal structures. Overall, the post-intrusive, post-skarn La Colorada vein system may be described as deep epithermal or high-level magmatic-mesothermal developed above a copper porphyry system.
The El Crestón, La Colorada/Gran Central, and Veta Madre deposits have been mined using open pit methods.
El Crestón
The El Crestón veins constitute the largest vein system on the La Colorada Project.
Deposit Dimensions
Mineralization is approximately 1,000 m along strike, 250 m wide, and 250 m in vertical extent. Mineralization remains open along strike of the veins and to the north and south and at depth.
Structure
Structurally, the Colorada Sur Fault is the main controlling feature. It has a variable easterly strike which has an average azimuth of 060° and dips vertically to steeply north. Steeply-dipping east– west and north–south-oriented faults are also important mineralization controlling structures. Low angle faults are visible in the pit walls and interpreted in geological modeling.
The veins generally strike east to east–northeast, dipping an average of 75° N (Lewis, 1995). The veins have well-defined contacts and below the 100 m level are simple with few spurs and parallel veins. Typically, the best metal grades are found where the veins are thickest. Although the veins represent distinct mineralized zones, they coalesce and bifurcate in a subparallel series of principal veins and associated stockworks. Pre-mineral faulting created open fractures that were later filled by the vein systems. Minor post-mineral fault offsets of a few metres are common.
Mineralization
Mineralization is hosted by a system of veins and hydrothermal breccias having grey to green microcrystalline quartz, white crystalline, or banded drusy quartz. Occasionally barite and magnetite are identified. Sulphides identified include galena, sphalerite, pyrite, and traces of chalcopyrite. Gold occurs in quartz as electrum containing 70–75% gold and 25–30% silver. Silver-rich mineralization is associated with polymetallic veinlets of galena, sphalerite, argentite, and occasionally traces of chalcopyrite.
La Colorada/Gran Central
Mineralization is approximately 450 m along strike, 200 m wide, and 150 m in vertical extent. Mineralization remains open to the west at depth.
Structure
The east–west-trending Gran Central Fault is the main controlling structure and has a northerly dip, averaging 50°. The Gran Central Fault consists of several sub-parallel splays, where quartz veins, stockworks and breccia zones are often associated with clay–chlorite gouge.
Mineralization
Fine native gold is present in the deposit. Sulphide minerals range between 1% and 3% by volume and are distributed within the unoxidized portions of the deposit. Sulphides include galena, sphalerite, lesser chalcopyrite, minor tetrahedrite, and traces of chalcocite, covellite and molybdenite.
Gold-bearing quartz veins and stockworks within the La Colorada vein system are hosted in an east-west-striking fault zone with a north dip averaging 45°. Veining is hosted by rhyolite porphyry and diorite rocks and is within the same dioritic stock which hosts the Gran Central deposit. The zone averages about 20 m thick.
Lewis (1995) states that according to historical records, mineralization is terminated at a depth of approximately 200 m by a flat fault, below which non-mineralized granite is present.
Veta Madre
Veta Madre is located 1.5 km east of the El Crestón open pit.
Deposit Dimensions
Mineralization is approximately 500 m along strike, 200 m wide, and 200 m in vertical extent. Argonaut drill programs have determined that a down-faulted, and generally higher-grade, portion of the vein system continues a significant distance to the west. Mineralization remains open to the west down-plunge at depth and to the east near the surface.
Structure
Veta Madre is a massive, mineralized body that differs from the El Crestón and La Colorada/Gran Central deposits, where the mineralization occurs in three main veins. Veta Madre is a vein-fill system with a preferred strike of N65E and a steep dip to the northwest. The thickness of the Veta Madre ore body varies; in the eastern and shallower zones, it measures only 10–15 m wide, while at greater depths and to the west, it can exceed 100 m in width.
Mineralization
Veta Madre, like La Colorada/Gran Central and El Crestón, is part of a single mineralizing system. This system was formed as a vein-fill, located within a regional structure oriented approximately east–west and dislocated by multiple normal faulting events. This geological activity is associated with the extension of the Basin and Range and low-angle listric faulting that dislocates the main orebodies.
Veta Madre is characterized by the presence of breccias and multi-stage quartz veins, developing crustiform textures, banding, and blade textures. Veta Madre is mostly in an advanced state of oxidation, with occasional fresh pyrite and traces of galena and sphalerite observed. The mineralization is primarily associated with quartz veining that occurs in multiple stages, followed by the formation of hematite after the oxidation of sulphides. Occasionally, barite and magnetite are identified.
Gold occurs in quartz as electrum containing 70–75% gold and 25–30% silver. Silver-rich mineralization is associated with polymetallic veinlets of galena, sphalerite, argentite, and occasionally traces of chalcopyrite.