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Sinclair, W.D.. (1995): Porphyry Sn, in
Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics
and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry
of Energy of Employment and Investment, Open File 1995-20, pages 97-100.
IDENTIFICATION
SYNONYM: Subvolcanic Sn
COMMODITIES (BYPRODUCTS): Sn
(Ag, W)
EXAMPLES (British Columbia - Canada/International):
Mount Pleasant (New Brunswick, Canada), East Kemptville (Nova Scotia,
Canada), Catavi, Chorolque and Cerro Rico stock (Bolivia), Ardlethan and
Taronga (Australia), Kingan (Russia), Yinyan (China), Altenberg (Germany).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Fine-grained
cassiterite in veinlet and fracture stockwork zones, breccia zones, and
disseminated in porphyritic felsic intrusive rocks and associated country
rocks.
TECTONIC SETTING: Zones of weak to
moderate extension in cratons, particularly post orogenic zones underlain
by thick crust, possibly cut by shallow-dipping subduction zones.
GEOLOGICAL SETTING: High-level to
subvolcanic felsic intrusive centres in cratons; multiple stages of
intrusion may be present.
AGE OF MINERALIZATION: Paleozoic to
Tertiary.
HOST/ASSOCIATED ROCK TYPES:
Predominantly genetically related intrusive rocks and associated breccias,
but may also include related or unrelated sedimentary, volcanic, igneous
and metamorphic rocks. Genetically related felsic intrusive rocks are F
and/or B enriched and are commonly porphyritic. Tuffs or other extrusive
volcanic rocks may be associated with deposits related to subvolcanic
intrusions.
DEPOSIT FORM: Deposits vary in shape
from inverted cone, to roughly cylindrical, to highly irregular. They are
typically large, generally hundreds of metres across and ranging from tens
to hundreds of metres in vertical extent.
TEXTURE/STRUCTURE: Ore is
predominantly structurally controlled in stockworks of crosscutting
fractures and quartz veinlets, or disseminated in hydrothermal breccia
zones. Veins, vein sets, replacement zones may also be present.
ORE MINERALOGY (Principal and
subordinate): Cassiterite; stannite, chalcopyrite, sphalerite
and galena. Complex tin- and silver-bearing sulphosalts occur in late
veins and replacement zones.
GANGUE MINERALOGY: Pyrite,
arsenopyrite, löllingite, topaz, fluorite, tourmaline, muscovite,
zinnwaldite and lepidolite.
ALTERATION MINERALOGY: In the
Bolivian porphyry Sn deposits, sericite + pyrite ± tourmaline alteration
is pervasive; in some deposits it surrounds a central zone of quartz +
tourmaline. Sericitic alteration is typically bordered by weak propylitic
alteration. In other deposits (e.g. , Ardlethan, Yinyan), central zones
are characterized by greisen alteration consisting of quartz + topaz +
sericite; these zones grade outward to quartz + sericite + chlorite
alteration.
WEATHERING: Oxidation of pyrite
produces limonitic gossans. Deep weathering and erosion can result in
residual concentrations of cassiterite in situ or in placer deposits
downslope or downstream.
ORE CONTROLS: Ore minerals occur in
fracture stockworks, hydrothermal breccias and replacement zones centred
on 1-2 km2, genetically related felsic intrusions.
GENETIC MODEL:
Magmatic-hydrothermal. Large volumes of magmatic, highly saline aqueous
fluids under pressure strip Sn and other ore metals from temporally and
genetically related magma. Multiple stages of brecciation related to
explosive fluid pressure release from the upper parts of small intrusions
result in deposition of ore and gangue minerals in crosscutting fractures,
veinlets and breccias in the outer carapace of the intrusions and
associated country rocks. Mixing of magmatic with meteoric water during
waning stages of the magmatic- hydrothermal system may result in
deposition of some Sn and other metals, particularly in late-stage veins.
ASSOCIATED DEPOSIT TYPES: Sn veins
(I13), Sn-polymetallic veins (H07).
COMMENTS: Some of the deposits
listed (e.g. , Taronga, East Kemptville) are not "subvolcanic" but they
are similar to some porphyry Cu deposits with regard to their large size,
low grade, relationship to felsic intrusive rocks and dominant structural
control (ie., mineralized veins, fractures and breccias).
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Sn, Ag, W,
Cu, Zn, As, Pb, Rb, Li, F, B may be anomalously high in hostrocks close to
mineralized zones and in secondary dispersion halos in overburden.
Anomalously high contents of Sn, W, F, Cu, Pb and Zn may occur in stream
sediments and Sn, W, F (topaz) and B (tourmaline) may be present in heavy
mineral concentrates.
GEOPHYSICAL SIGNATURE: Genetically
related intrusions may be magnetic lows (ilmenite- rather than
magnetite-dominant); contact aureole may be magnetic high if pyrrhotite or
magnetite are present in associated skarn or hornfels zones. Radiometric
surveys may be used to outline anomalous U, Th or K in genetically related
intrusive rocks or in associated altered and mineralized zones.
OTHER EXPLORATION GUIDES: Sn (-Ag)
deposits may be zoned relative to base metals at both regional (district)
and local (deposit) scales.
ECONOMIC FACTORS
GRADE AND TONNAGE: Tens of millions
of tonnes at grades of 0.2 to 0.5% Sn. Mount Pleasant (New Brunswick): 5.1
Mt @ 0.79% Sn; East Kemptville (Nova Scotia): 56 Mt @ 0.165% Sn; Catavi
(Bolivia): 80 Mt @ 0.3% Sn; Cerro Rico stock, Bolivia: averages 0.3% Sn;
Ardlethan (Australia): 9 Mt @ 0.5% Sn; Taronga (Australia): 46.8 Mt @
0.145% Sn; Altenberg, (Germany): 60 Mt @ 0.3% Sn; Yinyan (China): "large"
(50 - 100 Mt?) @ 0.46% Sn
ECONOMIC LIMITATIONS: Low grades
require high volumes of production which may not be justified by demand.
IMPORTANCE: A minor source of tin on
a world scale; when it was in production, East Kemptville was the major
producer of tin in North America.
REFERENCES
Grant, J.N., Halls, C., Avila, W. and
Avila, G., (1977): Igneous Systems and Evolution of Hydrothermal
Systems in some Sub-volcanic Tin Deposits of Bolivia; in Volcanic Process
in Orogenesis; Geological Society of London, Special Paper
Publication 7, pages 117-126.
Grant, J.N., Halls, C., Sheppard, S.M.F.
and Avila, W., (1980): Evolution of the Porphyry Tin Deposits of
Bolivia; in Granitic Magmatism and Related Mineralization, Ishihara, S.
and Takenouchi, S., Editors; The Society of Mining Geologists of Japan,
Mining Geology Special Issue, No. 8, pages 151-173.
Guan, X., Shou, Y., Xiao, J., Liang, S.
and Li, J., (1988): A New Type of Tin Deposit - Yinyan Porphyry Tin
Deposit; in Geology of Tin Deposits in Asia and the Pacific, Hutchison,
C.S., Editor, Springer-Verlag, Berlin, pages 487-494.
Lin, G., (1988): Geological
Characteristics of the Ignimbrite-related Xiling Tin Deposit in Guangdong
Province; in Geology of Tin Deposits in Asia and the Pacific, Hutchison,
C.S., Editor, Springer-Verlag, Berlin, pages 494-506.
Reed, B.L., (1986): Descriptive
Model of Porphyry Sn; in Mineral Deposit Models, Cox, D.P. and Singer,
D.A. Editors; U.S. Geological Survey, Bulletin 1693, pages 108.
Sillitoe, R.H., Halls, C. and Grant,
J.N., (1975): Porphyry Tin Deposits in Bolivia; Economic Geology,
Volume 70, pages 913-927.
Taylor, R.G. and Pollard, P.J., (1986):
Recent Advances in Exploration Modelling for Tin Deposits and their
application to the Southeastern Asian Environment; GEOSEA V Proceedings,
Volume 1, Geological Society of Malaysia, Bulletin 19, pages
327-347. |
