Summary:
The La Colorada area is locally covered by mid-Cambrian to lower Ordovician quartzites, marbles, carboniferous limestones, and sandstones. In addition, the district hosts Triassic oligomictic conglomerate, limestones, shales, and a turbiditic sedimentary sequence of the Porfuna Basin formed by an alternation of shales, carbonaceous shales, flint horizons and an upper body of massive quartzites several metres thick.
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, with a marked northwest control and lesser northeast-trending structures. The deposit type is not well constrained and a number of deposit types have been suggested.
Epithermal-type gold–silver deposits in the Pacific Rim and in Eurasia were the source of much of the world’s gold supply. This has resulted in an improved understanding of epithermal-type precious metal deposits and has allowed for construction of models which could be very useful in future exploration of the La Colorada Project.
Epithermal deposits are found in the shallow parts of subaerial high-temperature hydrothermal systems and are very important in Tertiary to Recent calc-alkaline and alkaline volcanic rocks. Host rocks are variable and include volcanic and sedimentary rocks, diatremes, and domes. Structural controls include dilatant zones related to extensional faulting and favourable lithologies in permeable or brecciated host strata in the near-surface environment. Although some mineralization can be disseminated, most commonly mineralization is hosted by steeply-dipping vein systems.
Mineral textures include banded, crustiform–colliform and lattice textures composed of platey calcite sometimes pseudomorphed by quartz. An important feature of epithermal deposits is a pronounced vertical zonation, with quartz veins carrying base metal sulphide mineralization at depth, becoming silver-rich higher in the system and finally gold-rich near the top.
Low-sulfidation deposits typically range from veins, through stockworks and breccias to disseminated zones. Mineralized bodies in low-sulfidation systems are commonly associated with quartz and adularia, with carbonate minerals or sericite as the major gangue minerals. Major metallic minerals can include pyrite/marcasite, pyrrhotite, arsenopyrite and high-iron sphalerite. Less abundant metallic minerals include native gold and electrum, cinnabar, stibnite, gold-silver selenides, sulfosalts, galena, chalcopyrite and tetrahedrite/tennantite.
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.