| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Journal of Petrology Volume 41 Number 9 Pages 1439-1453 2000
© Oxford University Press 2000
Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England
1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, QUEEN’S ROAD, BRISTOL BS8 1RJ, UK
2DEPARTMENT OF MINERALOGY, THE NATURAL HISTORY MUSEUM, CROMWELL ROAD, LONDON SW7 5BD, UK
3DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK
Hydrothermal fluid evolution north of the St Austell granite, southwest England, has been studied through geochemical analysis of tourmaline from a fault breccia of <2 cm width within massive quartz–tourmaline rocks at Roche. Brecciated tourmaline grains have overgrowths of <400 µm width [Fe/(Fe + Mg) = 0·31–0·99] with four chemically distinct zones (1–4, towards the margins). Variations in overgrowth composition were caused by episodic mixing between Mg-, Al-rich magmatic hydrothermal fluids (dominant in zone 1), with an increasing component of more oxidizing, Fe-rich formation waters (zones 2 and 4). More oxidizing conditions are supported by high Sn contents in zone 2 (<0·35 wt %), with Sn probably present as Sn4+ rather than Sn2+, the usual form in hydrothermal fluids. From X-ray maps, zones 1 and 3 occur exclusively as overgrowths on pre-existing grains, indicating that overgrowth formation was kinetically favoured over tourmaline nucleation. In zones 2 and 4, nucleation and growth occurred, possibly as a result of supersaturation with respect to tourmaline during increased mixing with formation waters. Tourmaline is associated with the main episode of mineralization in many important mineral deposits, often unaffected by alteration. This method of studying hydrothermal fluid evolution may therefore have uses in exploration, particularly for tourmaline-breccia-hosted ores in Cu-porphyry deposits.
KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  S. Demirel, M. C. Goncuoglu, G. Topuz, and V. Isik GEOLOGY AND CHEMICAL VARIATIONS IN TOURMALINE FROM THE QUARTZ-TOURMALINE BRECCIAS WITHIN THE KERKENEZ GRANITE-MONZONITE MASSIF, CENTRAL ANATOLIAN CRYSTALLINE COMPLEX, TURKEY Can Mineral, August 1, 2009; 47(4): 787 - 799. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 V. J. van Hinsberg and J. C. Schumacher The geothermobarometric potential of tourmaline, based on experimental and natural data American Mineralogist, May 1, 2009; 94(5-6): 761 - 770. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. Wagner, M. S. J. Mlynarczyk, A. E. Williams-Jones, and A. J. Boyce Stable Isotope Constraints on Ore Formation at the San Rafael Tin-Copper Deposit, Southeast Peru Economic Geology, March 1, 2009; 104(2): 223 - 248. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. S.J. Mlynarczyk and A. E. Williams-Jones ZONED TOURMALINE ASSOCIATED WITH CASSITERITE: IMPLICATIONS FOR FLUID EVOLUTION AND TIN MINERALIZATION IN THE SAN RAFAEL SN CU DEPOSIT, SOUTHEASTERN PERU Can Mineral, April 1, 2006; 44(2): 347 - 365. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Muller, B. J. Williamson, and M. Smith Origin of quartz cores in tourmaline from Roche Rock, SW England Mineralogical Magazine, August 1, 2005; 69(4): 381 - 401. [Abstract] [Full Text] [PDF]
|
|