METASOMATIC MASKING OF POTENTIAL HYDROTHERMAL TIN DEPOSITS

I-Type granitic rocks are characterized by late stage sulfide mineralization whereas S-type granites are typically low in sulfides but may possess non-sulfide economic minerals in greisen zones or in pegmatites. Events may occur, however, that can confuse this ideal picture, resulting in exploration efforts not being directed toward some igneous rock bodies because of the presumed absence of certain types of minerals. A classic example is the presumption that tin, tantalum, beryl, etc. should not be expected in trondhjemitic-adamellite plutons.

Granitic plutons intruded into amphibolite-facies pelitic schists and feldspathic metagraywackes in the Eastern Blue Ridge belt (southern U. S.) possess distinctive S-type affinities whose major and trace elements and oxygen and carbon isotopes indicate an origin by partial melting of metasediments in an Andean-type convergent margin. Fractional crystallization produced late-stage hydrothermal fluids and Q+Ksp+Ms+Bi pegmatites with accessories phases including beryl, cassiterite, tourmaline, tantalum/columbite, and apatite. These rocks are unusual because while fluorine is typically a significant component of the fluid phase producing pegmatites, only a weak correlation exists between F and tin contents of these granites and F is very low when compared with tin-bearing granites found elsewhere. As such, it is clear that fluorine was not a significant fluid component and that other complexing agents exerted influence on the ore fluids. A prograde Acadian metamorphic event generated S-type granite magmas that in turn produced hydrothermal pneumatolytic fluids via fractional crystallization. These fluids scavenged LIL's and other trace elements from the magma host and surrounding pelitic rocks to form pegmatite bodies with an S-type geochemical signature. Subsequent generation of retrograde metamorphic fluids produced a Na-metasomatic event that was superimposed on the geochemistry of the plutons, masking the original S-type chemistry. Hence, contrary to conventional models of trondhjemite petrogenesis, terranes affected by Na-metasomatism do have the potential to host significant tin-bearing deposits and to be also associated with trondhjemite plutons that possess composite S- and I-type geochemical signatures.