DESCRIPTIVE MODEL OF REPLACEMENT Sn

MODEL 14c

By Bruce L. Reed

APPROXIMATE SYNONYM Exhalative Sn (Plimer, 1980; Hutchinson, 1979).

DESCRIPTION Stratabound cassiterite-sulfide (chiefly pyrrhotite) replacement of carbonate rocks and associated fissure lodes related to underlying granitoid complexes (see fig. 34).

GENERAL REFERENCE Patterson and others (1981).

GEOLOGICAL ENVIRONMENT

Rock Types Carbonate rocks (limestone or dolomite); granite, monzogranite, quartz porphyry dikes generally present; quartz-tourmaline rock; chert, pelitic and iron-rich sediments, and volcanic rocks may be present.

Textures Plutonic (equigranular, seriate, porphyritic).

Age Range Paleozoic and Mesozoic most common; other ages possible.

Depositional Environment Epizonal granitic complexes in terranes containing carbonate rocks. Note: the epigenetic replacement classification for these deposits has been questioned and an alternative exhalative synsedimentary origin followed by postdepositional metamorphic reworking hypothesis proposed (Hutchinson, 1979, 1982; Plimer, 1980; Lehmann and Schneider, 1981).

Tectonic Setting(s) Late orogenic to postorogenic passive emplacement of high-level granitoids in foldbelts containing carbonate rocks; alternatively, tin and associated metals were derived from submarine exhalative processes with subsequent reequilibration of sulfide and silicate minerals.

Associated Deposit Types Greisen-style mineralization, quartz-tourmaline-cassiterite veins, Sn-W-Mo stockworks, Sn-W skarn deposits close to intrusions.

DEPOSIT DESCRIPTION

Mineralogy Pyrrhotite + arsenopyrite + cassiterite + chalcopyrite (may be major) + ilmenite + fluorite; minor: pyrite, sphalerite, galena, stannite, tetrahdedrite, magnetite; late veins: sphalerite + galena + chalcopyrite + pyrite + fluorite.

Texture/Structure Vein stockwork ores, and massive ores with laminations following bedding in host rock, locally cut by stockwork veins, pyrrhotite may be recrystallized.

Alteration Greisenization (± cassiterite) near granite margins; sideritic alteration of dolomite near sulfide bodies; tourmalization of clastic sediments; proximity to intrusions may produce contact aureoles in host rocks.

Ore Controls Replacement of favorable carbonate units; fault-controlled fissure lodes common. Isolated replacement orebodies may lie above granitoid cupolas; faults provide channels for mineralizing fluids.

Geochemical Signature Sn, As, Cu, B, W, F, Li, Pb, Zn, Rb.

EXAMPLES

Renison Bell, AUTS (Patterson and others, 1981)

Cleveland, AUTS (Collins, 1981)

Mt. Bischoff, AUTS (Groves and others, 1972)

Changpo-Tongkeng, CINA (Liang and others, 1984)