"Rock of the Month # 63, posted for September 2006" ---

Jasperoid is a venerable term, associated in particular with diverse mining camps of the American West. Limestone or other wall rocks may be replaced by fine-grained silica. This kind of hydrothermal alteration is most often described in gold deposits, but also in limestone-hosted lead-zinc deposits and other geological settings.

This sample comes from the Marlboro Canyon Pit of the Jerritt Canyon (Enfield Bell) mine. Collected in situ by A. James Macdonald on 01 November 1984, from an outcrop about 5 metres from a carbon-blackened fault zone in the Roberts Formation. Parts of both this formation and the underlying Hanson Creek Formation are silicified, and both formations can constitute gold ore. Jasperoid is abundant, 35-40% of the mine rocks. A minor part of the ore (<10%) is hosted by jasperoid.

The sample is a dark silicified limestone. The sediment has been decalcified, and so does not effervesce in dilute hydrochloric acid. It is brecciated and cut by quartz veinlets, and hosts open vuggy cavities. In thin section (see photo) the former limestone is now approximately 90% fine-grained quartz, grain size 5-20 microns. Most of the remainder of the rock is thin veinlets of somewhat coarser quartz. These may have carbonaceous selvages, as shown in the illustration. The quartz veinlets vary widely in grain size, 20-200 microns, and may enclose flakes of the wall rock that have spalled off into the veins Besides silica, the rock contains 1% pyrite, 1% carbonaceous matter (in part as laminae that predate the quartz veinlets), and 1% antimony minerals (mostly secondary valentinite, and a trace of the sulphide stibnite). There is a trace of secondary iron oxide, goethite. Absolutely no carbonate grains were identified. It appears most likely that some of the carbonaceous matter predated the deformation and alteration, and that some was then remobilized along the quartz veinlets.

Gold is mined from both carbonaceous and oxidized ores, within a window in the Roberts Mountain Thrust (Birak and Hawkins, 1985; Birak, 1986). The golds is usually <2 microns in size. The main stage of silicification forming jasperoids is also thought to be pre-gold. According to Birak et al. (1987) the jasperoids are most abundant in the chert-rich rocks of the Hanson Creek Formation. Brittle chert beds were preferentially fractured and repeatedly silicified. Mineralized jasperoids are coarser, and early textural features have been obliterated. Many of the Carlin-type, sediment-hosted gold deposits in this region of the Great Basin have Cenozoic dates, circa 42 to 30 Ma (Hofstra et al., 1999).

The term jasperoid is most commonly applied to finely altered, silica-flooded rocks associated with metallic mineralization (Emmons, 1917; Locke, 1926). Multi-generation, commonly fine-grained silica replacement and quartz veinlets are often associated with gold-silver mineralization. Alunite and electrum may occur with granular and comb-textured quartz, as in the Scossa district of Nevada (Jones et al., 1931). Limestone and dolostone, crackle breccias and jasperoid also feature in base-metal and silver districts of Utah and New Mexico (e.g., Heyl, 1963, pp.78-80; Young and Lovering, 1966). Jasperoids in the U.S.A. were reviewed by Lovering (1972). Jasperoid-associated mineralization often contains native metals and alloys and oxide species, such as native silver and electrum, Mn oxides, Sb oxides, barite and jarosite. Chalcedony, carbonaceous matter, native sulphur and sulphides (realgar, stibnite, cinnabar, etc) may also be present (Lovering, 1972; Wilson and Rucklidge, 1987).

References

Birak,DJ (1986) Exploration and geologic development of the Jerritt Canyon gold deposits, Elko County, Nevada, U.S.A. Proc. `Gold '86' Symposium, Toronto (Macdonald,AJ editor), 517pp., 488-496.

Birak,DJ and Hawkins,RB (1985) The geology of the Enfield Bell Mine and the Jerritt Canyon district, Elko County, Nevada. In `Geologic Characteristics of Sediment- and Volcanic-Hosted Disseminated Gold Deposits - Search for an Occurrence Model' (Tooker,EW, editor), USGS Bull. 1646, 150pp., 95-105.

Birak,DJ, Mancuso,TK and Dahl,AR (1987) Sediment-hosted disseminated gold mineralization at Jerritt Canyon, Nevada. I - jasperoids as the key to the time-space framework. GSA Abs.w.Progs 19 no.7 (Annual Meeting, Phoenix), 589.

Emmons,WH (1917) The Enrichment of Ore Deposits. USGS Bull. 625, 530pp.

Heyl,AV (1963) Oxidized Zinc Deposits of the United States. Part 2. Utah. USGS Bull. 1135-B, 104pp. plus 9 maps and plates.

Hofstra,AH, Snee,LW, Rye,RO, Folger,HW, Phinisey,JD, Loranger,RJ, Dahl,AR, Naeser,CW, Stein,HJ and Lewchuk,M (1999) Age constraints on Jerritt Canyon and other Carlin-type gold deposits in the western United States - relationship to mid-Tertiary extension and magmatism. Econ.Geol.94, 769-802.

Jones,JC, Smith,AM and Stoddard,C (1931) The preliminary survey of the Scossa mining district, Pershing County, Nevada. University of Nevada Bull. 25 no.4, 14pp.

Locke,A (1926) Leached Outcrops as Guides to Copper Ore. Bailliere, Tindall and Cox, Covent Garden, London, 175pp. plus 24 plates.

Lovering,TG (1972) Jasperoid in the United States - its characteristics, origin, and economic significance. USGS Prof.Pap. 710, 164pp.

Wilson,GC and Rucklidge,JC (1987) Geology, geochemistry, and economic significance of carbonaceous host rocks in gold deposits of the Timmins area. In `Geoscience Research Grant Program, Summary of Research 1986-1987', OGS Misc.Pap. 136, 241pp., 66-76.

Young,EJ and Lovering,TG (1966) Jasperoids of the Lake Valley mining district, New Mexico. USGS Bull. 1222-D, 27pp.