DESCRIPTIVE MODEL OF Sn SKARN DEPOSITS
MODEL 14b
By Bruce L. Reed and Dennis P. Cox
DESCRIPTION Tin, tungsten, beryllium minerals in skarns, veins, stockworks and greisens near granite-limestone contacts (see fig. 34).
Figure 34. Cartoon cross section showing between Sn skarn, replacement Sn and Sn vein deposits, and granite intrusions.
GENERAL REFERENCE Einaudi and Burt (1982), Einaudi and others (1981), Scherba (1970).
GEOLOGICAL ENVIRONMENT
Rock Types Leucocratic biotite and(or) muscovite granite, specialized phase or end members common, felsic dikes, carbonate rocks.
Textures Plutonic textures most common (granitic, seriate, fine-grained granitic). Also porphyritic-aphanitic; skarn is granoblastic to hornfelsic, banded skarn common.
Age Range Mainly Mesozoic, but may be any age.
Depositional Environment Epizonal(?) intrusive complexes in carbonate terrane.
Tectonic Setting(s) Granite emplacement generally late (post orogenic).
Associated Deposit Types W skarn, Sn greisen, and quartz-cassiterite-sulfide veins; at increasing distances from intrusive-carbonate contact Sn replacement and fissure lodes may develop (as at Renison Bell).
DEPOSIT DESCRIPTION
Mineralogy Cassiterite ± minor scheelite ± sphalerite + chalcopyrite ± pyrrhotite ± magnetite ± pyrite ± arsenopyrite ± fluorite in skarn. Much Sn may be in silicate minerals and be metallurgically unavailable.
Texture/Structure Granoblastic skarn, wrigglite [chaotic laminar pattern of alternating light (fluorite) and dark (magnetite) lamellae], stockworks, breccia.
Alteration Greisenization (quartz-muscovite-topaz ± tourmaline, fluorite, cassiterite, sulfides) near granite margins and in cusps. Topaz tourmaline greisens. Idocrase + Mn-grossular-andradite ± Sn-andradite ± malayaite in skarn. Late-stage amphibole + mica + chlorite and mica + tourmaline + fluorite.
Ore Controls Mineralized skarns may or may not develop at intrusive contact with carbonate rocks; major skarn development up to 300 m from intrusion controlled by intrusion-related fractures; cross-cutting veins and felsic dikes.
Weathering Erosion of lodes may lead to deposition of tin placer deposits.
Geochemical Signature Sn, W, F, Be, Zn, Pb, Cu, Ag, Li, Rb, Cs, Re, B. Specialized granites characteristically have SiO2 > 73 percent, K2O > 4 percent and are depleted in CaO, TiO2, MgO, and total Fe. They are enriched in Sn, F, Rb, Li, Be, W, Mo, Pb, B, Nb, Cs, U, Th, Hf, Ta, and most REE. They are depleted in Ni, Cu, Cr, Co, V, Sc, Sr, La, and Ba.
EXAMPLES
Lost River, USAK (Dobson, 1982)
Moina, AUTS (Kwak and Askin, 1981)