Summary:
The Shafter silver deposit is considered an example of a polymetallic replacement deposit. Because of their irregular, but sharp contact with the enclosing carbonate host rocks, deposits of this type have been categorized as high-temperature, carbonate-hosted deposits.
Polymetallic deposits consist of massive lenses and (or) pipes, known as mantos or replacement orebodies, and veins of iron, lead, zinc, and copper sulfide minerals that are hosted by and replace limestone, dolomite, or other sedimentary rocks; most massive deposits contains more than 50 percent sulfide minerals. Sediment-hosted deposits commonly are intimately associated with igneous intrusions in the sedimentary rocks. Emplacement of these intrusions triggered mineral formation and they host polymetallic veins and disseminations that contain iron, lead, zinc, and copper sulfide minerals. Some polymetallic replacement deposits are associated with skarn deposits in which host carbonate rocks are replaced by calc-silicate±iron oxide mineral assemblages. Most polymetallic vein and replacement deposits are zoned such that copper-gold mineralization is proximal to intrusions, whereas lead-zinc-silver mineralization is laterally and vertically distal to intrusions.
There is little evidence in the Shafter district to indicate the source of the mineralizing solutions. No evidence of contact metamorphism has been noted, and this may indicate that the mineralizing solutions had traveled some distance, either horizontally or vertically through the stratigraphy.
The Shafter deposit is hosted within the gently dipping beds of the Permian Mina Grande Formation, just below their contact with Cretaceous rocks. The reef-derived dolomite and limestone of the Mina Grande Formation were susceptible to differential weathering and karst activity at the upper level of the formation, and passageways for mineralizing solutions formed along facies contacts and bedding planes.
The deposit is parallel to the bedding, has a tabular form, and is called a manto deposit, following colonial Spanish terminology for a blanket-like or tabular mineralized body. The deposit has some irregularities in its shape but dips generally east. Veins containing the same minerals as the manto are common in the eastern part of the Shafter district. Many of these veins are fissure fillings and have brecciated zones. Rozelle (2001) stated that the mineralization took place after the intrusion of dikes and sills of Tertiary age, and Ross (1943) reported that dikes in the Presidio mine are somewhat mineralized.
Mineral deposition took place in four main phases: (1) a limited amount of dolomitization; (2) silicification; (3) deposition of calcite and metallic minerals including galena, sphalerite, and acanthite; and (4) supergene alteration. Aurcana identified two separate stages of metal mineralization on the Shafter property – an initial lead stage potentially associated with the north-trending Mina Grande fault, followed by a second stage consisting of silver and anomalous lead and zinc, thought to be associated with the Herculano fault system and multiple east-trending faults that served as distal feeder systems (Lambeck, 2012). Contacts of the mineralized zones with unaltered wall rocks are generally sharp.
Silver mineralization located to the east of the Presidio mine historical workings (designated Block Groups I and II in the historical reports and re-named the Shafter area for use in this report) appears to be continuous within the manto deposit, which extends over 6,000ft of strike length along a zone trending roughly N60°E and lies between 700 and 900ft below the surface. The entire Presidio/Shafter deposit is up to 1,500ft wide in a north-south direction and extends at least 2.5 miles on an east-west trend (Balfour Holdings, Inc., 2000). There appears to be a high-grade core within the broader mineralized zone located just below the Cretaceous-Permian unconformity. The high-grade core is very continuous east of the Presidio mine workings in the Shafter area and in the upper workings of the Presidio mine (Balfour Holdings, Inc., 2000).
About 5,000ft northeast of the eastern limit of stoping in the Presidio mine, silver values decrease markedly. About 1,000ft further east, the favorable Basal and Pseudobreccia units of the Mina Grande Formation were removed by pre-Cretaceous erosion or dolomitization (Kastelic, 1983). West of the Presidio mine, dolomitization has also destroyed much of the favorable host rock for the Shafter-type mineralization (Kastelic, 1983).
The mineralized material consists of a massive aggregate of medium-grained, vuggy silica stained with varying amounts of iron and manganese oxides. Mineralogy is fairly consistent within the district. The mineralization originally consisted of sulfide minerals, which are now almost thoroughly oxidized. Secondary minerals include iron and manganese oxides, acanthite, hemimorphite, descloizite, embolite, plumbojarosite, cerargyrite, native silver, cerussite, anglesite, and small amounts of covellite, chrysocolla, and possibly other copper minerals. Primary minerals include dolomite, calcite, quartz, pyrite, sphalerite, galena, argentite, chalcopyrite, covellite, molybdenite, and tetrahedrite. Silver occurs predominately as oxidized acanthite in fine-grained aggregates of quartz, calcite, and goethite, with lesser dolomite, hemimorphite, willemite, anglesite, galena, smithsonite, and sphalerite. Lead and perhaps zinc appeared to be more plentiful relative to silver in the outlying mines of the district than in the Presidio mine, although the outlying mines are scattered and were poorly developed so generalizations are difficult (Ross, 1943).
Mineralization occurs over a 13,000 ft east-northeast strike length, is up to 1,200 ft across, and is generally 10 to 20 ft thick. The resource is at a depth of less than 100 ft in the west-central portion of the deposit and then gradually deepens to a depth of more than 1,000 ft within the eastern end of the deposit following the general stratigraphic dip. Manto thickness and silver grades can be highly variable, often related to near-vertical structures that served as fluid conduits and/or structural traps.
Although silver mineralization is generally continuous along the 13,000 ft length of the deposit, the resource is fragmentary in the vicinity of the historic Presidio mine due to the removal of mined-out material, as well of west of the historic Presidio mine in the area more recently mined by Aurcana.