MODEL 15b
By Bruce L. Reed
APPROXIMATE SYNONYM Cornish type lodes.
DESCRIPTION Simple to complex quartz-cassiterite ± wolframite and base-metal sulfide fissure fillings or replacement lodes in ore near felsic plutonic rocks (see fig. 34).
GENERAL REFERENCE Hosking (1974), Taylor (1979).
GEOLOGICAL ENVIRONMENT
Rock Types Close spatial relation to multiphase granitoids; specialized biotite and(or) muscovite leucogranite common; pelitic sediments generally present.
Textures Common plutonic textures.
Age Range Paleozoic and Mesozoic most common; may be any age.
Depositional Environment Mesozonal to hypabyssal plutons; extrusive rocks generally absent; dikes and dike swarms common.
Tectonic Setting(s) Foldbelts and accreted margins with late orogenic to postorogenic granitoids which may, in part, be anatectic; regional fractures common.
Associated Deposit Types Sn greisen, Sn skarn, and replacement Sn deposits.
DEPOSIT DESCRIPTION
Mineralogy Extremely varied; cassiterite ± wolframite, arsenopyrite, molybdenite, hematite, scheelite, beryl, galena, chalcopyrite, sphalerite, stannite, bismuthinite; although variations and overlaps are ubiquitous, many deposits show an inner zone of cassiterite ± wolframite fringed with Pb, Zn, Cu, and Ag sulfide minerals.
Texture/Structure Variable; brecciated bands, filled fissures, replacement, open cavities.
Alteration Sericitization (greisen development) ± tourmalization common adjacent to veins and granite contacts; silicification, chloritization, hematization. An idealized zonal relation might consist of quartz-tourmaline-topaz, quartz-tourmaline-sericite, quartz-sericite-chlorite, quartz-chlorite, chlorite.
Ore Controls Economic concentrations of tin tend to occur within or above the apices of granitic cusps and ridges; localized controls include variations in vein structure, lithologic and structural changes, vein intersections, dikes, and cross-faults.
Weathering Cassiterite in stream gravels, placer tin deposits.
Geochemical Signature Sn, As, W, B are good pathfinder elements; elements characteristic of specialized granites (F, Rb, Be, Nb, Cs, U, Mo, REE, see model 14b).
EXAMPLES
Cornwall, GRBR (Hosking, 1969)
Herberton, AUQL (Blake, 1972)