Summary:
A striking feature of the Galena Complex mineralization is that two entirely distinct types of utterly different ore mineralogy occur within the mineralized envelope. These are: 1) silver-copper veins and disseminations, dominated by tetrahedrite, and 2) silver-lead veins and disseminations, dominated by galena.
Mineralization at the Galena Mine is typified by structurally controlled veins that can extend for a few thousand feet of depth and hundreds of feet of strike. The general strike is N 50 W, with steep dips to the south, but there are many local variations. Due to the complex threedimensional arrangement of the numerous veins, which include NW, NE and N-S trends, a two-dimensional graphic presentation is difficult.
The veins principally contain silver, lead, copper and zinc in relatively simple mineralogy. Silver is the primary economic metal at the Galena Mine. Historically, the “silver-copper” veins, containing argentiferous tetrahedrite have been the focus of production at the Galena. The silver-copper ratio averages 30 to 35 ounces per percent copper. Typically, the silver-lead ratio of silver-lead ore at the Galena Mine is about 0.9 opt silver per 1.0 percent of lead.
The mineralized veins at the Galena Mine occur along four major fracture systems and three major faults (South Argentine, Argentine, and Polaris). The veins generally strike east-west to northeast-southwest, and dip vertically to steeply to the south. Thickness ranges from a few inches to over fifteen feet.
The vein fillings are a gangue of siderite with variable amounts of pyrite and quartz as blebs and stringers. The ore bearing sulfides are predominately galena, tetrahedrite and chalcopyrite, with pyrite.
Wall rocks for the vein mineralization are not significantly mineralized except, where cut by narrow unnamed veins or stringers.
The Silver Vein at the Galena Mine is one of the widest and most productive veins in the history of the mine. The vein cuts through quartzites and siltites of the Revett Formation for over 3,600 vertical feet with an average strike length of 1,000 feet. It can be as narrow as 1.0 foot and as wide as 15.0 feet but typically averages about 4 feet wide. The Silver Vein consists of massive siderite with pods of quartz and chalcopyrite and scattered blebs and stringers of tetrahedrite.
The 185 Vein in the Galena Mine strikes east-west within a hard quartzite unit and consists of zones strongly enriched in silver bearing galena. Common accessory minerals are pyrite, ankerite, barite, and quartz. Only trace amounts of tetrahedrite are typically found within the vein.
The fault-bounded veins are found within three major structures that cross through the Galena Mine. These veins include the 31, 72, 133, and 164 Veins hosted by the Polaris fault, the 117 Vein hosted by the South Argentine fault, and the 123 Vein hosted by the Argentine fault. The wall rocks encompassing these fault-controlled “veins” vary throughout the mine from soft siltite argillites to very hard quartzite.
The Polaris fault is a major fault and strikes west-northwest to east-southeast and cuts through the middle of the Galena Mine, and south of the Coeur Mine. The Argentine and South Argentine faults are smaller east-west striking faults that lie north of the Polaris fault.
The 72 Vein is a silver-enriched narrow vein bounded by the Polaris fault. It strikes at approximately N 75 degrees W and dips to the SW at -70 degrees. The genesis of the 72 Vein ore body is hypothesized to be the result of left lateral oblique movement along the Polaris fault that produced dilation zones in brittle quartzite units creating openings for silver concentrations from metal rich metamorphic fluids. The ore-bearing quartzites are bounded by more ductile siltite units that according to drill data cut off the mineralization along strike and up-dip. At the current drill limits the 72 Vein exhibits an ore grade strike length of 1,200 feet and a down-dip length of 900 feet. However, drill data between the 5500 and 5800 elevations reveals that the 72 Vein ore body is open down dip, showing increasing Ag grades within a thickening quartzite package.
Mineralogically the 72 Vein is composed primarily of massive siderite veins that contain variable amounts of tetrahedrite, chalcopyrite, pyrite, and trace galena. These siderite veins and stringers are commonly found as brecciated angular to sub-rounded clasts set in a fault gouge matrix. Tetrahedrite is not always visible and can appear as black powder.
The 117 Vein is hosted by the South Argentine fault. It is a strong siderite vein with small tetrahedrite blebs and parallel stringers. Hard quartzite in the hanging wall and siltite and argillite in the footwall bound the vein. Silver bearing tetrahedrite mineralization appears to rake flatly up dip to the northwest at about 40 degrees. The vein varies in width from 2 to 12 feet. In certain areas of the mine the vein will pinch out and only a black mineralized fault is present.
The 123 Vein is a narrow siderite vein with small parallel tetrahedrite stringers. The vein is found within the Argentine fault, north of the Polaris fault and the 117 Vein. The hanging wall of the 123 Vein is a barren soft argillite while the footwall is a mineralized siltite-quartzite. The vein averages 2 to 4 feet wide and during mining the soft argillites in the hanging wall are found to be not competent which can cause dilution.
The Coeur Mine, northwest of the Galena Mine, contains a Mineral Resource which is dominantly within the 425 Vein, the 400 Vein and the 356 Vein with Measured, Indicated and Inferred Resources on 12 other veins. The Coeur Mine had been idle since 1997 until Americas Silver rehabilitated the mine and resumed production in the second half of 2012 in the 425 Vein. Production ceased in 2014 due to low prices and the focus changed to silverlead mineralization.
The Coeur Mine mineralization generally resembles the Galena Mine, in containing a complex vein set controlled largely by the pattern of faults, and the geology of the Revett quarzitic and argillitic units. Pre-1997 workings at Coeur attained a depth of 4,225 feet below surface (3900 Level), and were mostly in the 356, 400, and 483 Veins.
It has been noted that the tetrahedrite of the Coeur Mine has a lower silver/copper ratio than that in the Galena Mine, about 15-20 ounces of silver per percent of copper.
The disseminated mineralization consists of small stringers and disseminations of galena in wall rock sediments. This style of mineralization is locally termed "blue rock".
Disseminated galena mineralization mainly occurs in thicker bedded, relatively more siliceous stratigraphic intervals. The disseminated mineralization ranges from narrow, weak zones less than a foot thick, to strongly mineralized zones which are several tens of feet thick. Disseminations vary from mottling and streaking to completely pervasive. Disseminated galena commonly follows lamination, where present, in siliceous rock. Galena grains in “blue rock” are usually quite fine, but variable. Blebs and pods of galena up to about ½ inch comprise a minor part of the disseminated mineralization. Pyrite is often present in minor amounts.
Disseminated mineralization is usually strong (greater than 6% Pb) around major lead veins. Major lead veins are mainly on the southwest (stratigraphically lower), side of the Caladay Zone. These veins, typically about 2 feet thick, may carry more than 40% Pb and relatively high pyrite (greater than 20%). The stronger intervals of “blue rock” in general contain galena-bearing stringers and pods. Stringers up to about 0.2 foot or 0.3 foot in width average roughly one per two to four feet in the strongest “blue rock”. These stringers consist of quartz, galena, pyrite, and siderite. Very high-grade stringers carrying more than 40% Pb are present.