Overview

Extracted from the Geological Report on the Santa Teresa Ag-Pb-Zn-(Cu) Carbonate Replacement Prospect, Feb 29, 2009 prepared by R.H. McMillan Ph.D. P.Geo. The full report complete with graphics can be accessed by following this link. This information appears on pages 19 through 21.

The Northern Mexico CRD province is characterized by high-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits hosted by Jurassic-Cretaceous sedimentary sequences floored by Paleozoic or older crust (Megaw et al, 1988). The important mines are shown on Figure 1.

The mineral districts occur within or at the margins of major fold/thrust zones. Altered and/or mineralized volcanic and intrusive rocks occur associated with all but three of the seventeen CRD districts reviewed by Megaw et al (1988). The deposits are highly irregular with strong structural controls.

Three principal types of morphology are common: mantos, chimneys and pods -- and are composed of massive sulphides and/or calc-silicate skarn. Sulphide mantos and chimneys are enclosed within carbonate rocks and are remote from intrusive bodies. Sulphide mantos commonly graded down into sulphide chimneys, which in turn grade downward into skarn chimneys coredby dykes.

Structural controls include faults, fold axes, fractures, fissures, open-space cavern zones and intrusive contacts. Eight of the seventeen CRD districts reviewed by Megaw et al (1988) had a total resource (production plus reserve) of > 10 million tonnes of ore other nine districts each contained more than a half million tonnes of ore at similar grade. Figure 1 (after Simmons et al, 2005) shows the location of the main CRD and precious base metal deposits in northern Mexico. Most of the CRD orebodies are composed exclusively of sulphide minerals with minor carbonate or quartz gangue. Skarn mineralization may be present where the topographically higher manto and chimney massive sulphide mineralization grades downward into skarn mineralization associated with intrusive bodies (Megaw et al, 1988).

According to Megaw et al (1988) massive sulphide orebodies contain > 65% sulphide minerals with a gangue composed of carbonate minerals, fluorite and quartz +/- barite. Skarn mineralization consists of Ca-Fe-Mg-Mn silicate minerals with variable amounts of sulphide minerals. The upper portions of many orebodies are deeply oxidized with the original mineralogy only recognizable by interpreting relict primary pseudomorphs.

The Northern Mexico CRD district deposits are hosted within a thick Mesozoic sedimentary section reflecting a transgressive sequence deposited over pre-Middle Triassic basement by a Jurassic-Cretaceous sea (Megaw et al, 1988). A typical depositional succession section consists of basal continental red beds with evaporates followed by a series of shale and carbonate facies shelf-sedimentary strata (Megaw et al, 1988). The carbonate section commonly exceeds 3000 metres and consists of thick sections of platformal and basinal limestones and minor dolomites.

Mineralization is hosted within the carbonate strata. Mineralization is believed (Megaw et al, 1988) to have been emplaced in mid-Tertiary time after the Laramide-aged folding and thrusting and perhaps contemporaneous with the Tertiary Sierra Madre ignimbritic volcanism.

Massive sulphide ores commonly contain galena, sphalerite, pyrite, pyrrhotite, marcasite, chalcopyrite and arsenopyrite (Megaw et al,1988). Tetrahedrite-tennantite, chalcopyrite, and arsenopyrite occur most commonly at deeper levels, near skarns and related intrusive bodies. Acanthite, cinnabar, stibnite, realgar and silver sulphosalts are most common at the margins of mineral districts. Gangue minerals include carbonates, quartz, fluorite, barite, anhydrite and rarely adularia and palygorsite. Galena is generally more abundant than sphalerite in sulphide mantos, whereas the reverse is true in sulphide chimneys.

Silver grades are generally highest in mantos, lower in chimneys and lowest in the skarns. Chalcopyrite content generally increases with depth, as does pyrrhotite if present. Skarn deposits, if present, contain the same ore minerals, but also contain bornite, covellite, molybdenite, scheelite, powellite and cassiterite (Megaw et al, 1988).

Gangue minerals in the skarns include garnet, pyroxene, wollastonite, vesuvianite and tremolite-actinolite, epidote, chlorite, serpentine and quartz. Magnetite and hematite are found in many of the skarns. Alteration in the form of limestone re-crystallization can extend for considerable distances from related intrusive body or be restricted to a thin halo (Megaw et al, 1988). Re-crystallization of limestone around mantos is considerably less extensive than around mineralization in deeper zones -- in many mantos, chimneys and dyke and sill contact skarn bodies alteration is commonly confined to narrow selvages of re-crystallization and bleaching. Manganese oxide staining is evident as peripheral alteration features in some CRD orebodies, as is silicification and jasperoid development in a few (Megaw et al, 1988).

The largest Mexican CRD deposit is the Santa Eulalia Mine in Chihuahua. The mine has been in production since its discovery in 1591 (Maldonado E., D., 1991), and continues in production today. Although the recent figures are not available to the author, Megaw et al (1988) reported a total resource (production and reserves) of 50 million tonnes of ore with production of silver, lead, zinc, copper and gold as well as tin and tungsten from mantos, chimneys and skarns. The district produces from two centres 5 km. apart, 15 km. east of the city of Chihuahua - the eastern district from the San Antonio Mine and the west district from the Buena Tierra and Potosi Mines (Maldonado E., D., 1991). Hostrocks are Lower Cretaceous carbonate rocks. Felsic intrusive rocks are closely associated with the deeper, skarn-type orebodies. Basal redbeds and evaporates occur in the regional stratigraphic section (Megaw et al, 1988).