"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.
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 metallic red colour, visible in the illustration.
Silver veins were discovered in 1546 at Fresnillo, some 60 km northwest of the state capital, Zacatecas. 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).
GARCIA M,E, QUEROL S,F and LOWTHER,GK (1991) Geology of the Fresnillo mining district, Zacatecas. In `Economic Geology, Mexico' (Salas,GP editor), GSA DNAG volume P-3, 438pp., 383-394.
GASPARRINI,C (1984) The mineralogy of silver and its significance in metal extraction. CIM Bull. 77 no.864, 99-110.
GEMMELL,JB, SIMMONS,SF and ZANTOP,H (1988) The Santo Niño silver-lead-zinc vein, Fresnillo district, Zacatecas, Mexico: Part I. Structure, vein stratigraphy, and mineralogy. Econ.Geol. 83, 1597-1618.
GEMMELL,JB, ZANTOP,H and BIRNIE,RW (1989) Silver sulfosalts of the Santo Niño vein, Fresnillo district, Zacatecas, Mexico. Can.Mineral. 27, 401-418.
PANCZNER,WD (1983) Notes from Mexico. Mineral.Record 14, 385-386.
PANCZNER,WD (1987) Minerals of Mexico. Van Nostrand Reinhold Co., 459pp.
RUVALCABA-RUIZ,DC and THOMPSON,TB (1988) Ore deposits at the Fresnillo Mine, Zacatecas, Mexico. Econ.Geol. 83, 1583-1596.
WALLACE,TC, BARTON,M and WILSON,WE (1994) Silver and silver-bearing minerals. Rocks and Minerals 69 no.1, 16-38.
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