"Rock of the Month # 20, posted February 2003" A specimen of rich ("bonanza-grade") silver ore from Fresnillo in Zacatecas state, north-central Mexico. Sample 061, collected from an active stope on the steeply-dipping, strongly-banded ore of the Santo Niño vein, 10 m above the 425 level of the Fresnillo mine, on 29 September 1981, a highlight of a field trip organized by A. James Macdonald. The sample includes this 666.58-gram shard, 15x7x4.5 cm, and six smaller sawn pieces, total 260.26 grams. Other samples show similar features, including 060 (1144.01 grams: this is a twin of sample No.6 presented to the University of Toronto Ore Deposits Workshop). Samples from the stope contain carbonate-silica-pyrite gangue as well as sulphosalt ore minerals. The ore is dense and compact, with only minor vugs. The rock is not appreciably magnetic, magnetic susceptibility 0.005 to 0.05x10^-3 SI units.

In contrast to gold (which is generally found as the native element, alloyed with silver to form electrum, or in various tellurides and a few rarer minerals), silver forms a broad range of widely-occurring ore minerals. Pyrargyrite, rhombohedral Ag₃SbS₃, is a good example.

This sample is composed largely of quartz, calcitic carbonate and pyrargyrite, with accessory pyrite, galena, proustite (the arsenic equivalent of pyrargyrite, Ag₃AsS₃) and chalcopyrite. The pyrargyrite-proustite sulphosalts are a red colour, with submetallic lustre, visible in the photographs. They appear to comprise roughly 5 volume percent of the ore, and infill wavy layers no more than a few mm thick. Some of the layers appear (in two dimensions) to form thin shells, enclosing and in turn hosted by the carbonate-rich gangue. At the time of sampling in 1981, veins such as the Santo Niño and Santa Elena offered fresh pickings in the southern part of the mining district, with silver grades averaging 2 kilograms per tonne (0.2 wt.% - pretty rich for a precious metal!). Rich indeed, though it is said that the original great glory hole in the area operated at about 40 kg/tonne silver!

Silver veins were discovered in 1546 at Fresnillo, some 60 km northwest of the state capital, Zacatecas. Mining has proceeded since 1553. A quartz monzonite porphyry stock intrudes Mesozoic calcareous, carbonaceous marine sediments, overlain by terrestrial rhyolites (Macdonald et al., 1986). The mining town has long been one of Mexico's premier mineral localities (Panczner, 1987). The regional geology includes early Cretaceous sediments, and Tertiary sediments, volcanic rocks and intrusive quartz monzonite porphyry. Mineralization includes replacement chimney and manto bodies, disseminated sulphides and vein deposits. So-called manto orebodies (named after a Spanish word for a cloak or blanket) normally develop in breccia or stockwork zones, at vein walls, and along favourable calcareous beds in the Cretaceous strata. Alteration in the wall rocks has produced axinite (formed preferentially in shales), hedenbergite and epidote (replacing greywacke).

The veins may contain euhedral crystals of adularia, a potassium feldspar. Replacement bodies carry a variety of sulphides and sulphosalts, and either predate or accompany base-metal veins of similar mineralogy. Younger Ag-rich veins are mostly pyrargyrite with lesser acanthite, proustite, miargyrite, polybasite, tetrahedrite, galena, sphalerite and chalcopyrite (Ruvalcaba-Ruiz and Thompson, 1988). The Ag-bearing sulphosalts in the Santo Niño vein show complex chemical and textural variations. Thus there are spectacular symplectites of galena and pyrargyrite enveloped by polybasite, and of tetrahedrite and galena surrounded by pyrargyrite (Gemmell et al., 1989). The inferred age of the mineralization is mid-Oligocene, age dated at 30-29 Ma (Garcia M. et al., 1991).

The Santo Niño silver-lead-zinc vein is the largest in the Fresnillo district, with a strike length exceeding 2.5 km and a vertical extent more than 500 m, averaging 2.5 m in thickness (Gemmell et al., 1988). The vein has produced some outstanding pyrargyrite crystals (e.g., Panczner, 1983).

Silver minerals (Wallace et al., 1994) are extremely diverse, and a number are common enough to be favourites with collectors, notably native silver, the sulphosalt pyrargyrite and the sulphide acanthite. The diversity of silver mineralogy, with some 200 discrete mineral species, is the subject of an elegant review by Gasparrini (1984).