A recent issue of Science includes an important study about hydrothermal ore deposits, that concludes "ore formation is linked to influx of anomalously metal-rich fluids into systems dominated by barren fluids for much of their life."
This contrasts with the interpretation that many standard crustal fluids can become viable ore fluids under "the right perturbations in physiochemical conditions" and produce ore deposits.
Reference: , Jamie J. Wilkinson, Barry Stoffell, Clara C. Wilkinson, Teresa E. Jeffries, and Martin S. Appold, Science 6 February 2009: Vol. 323. no. 5915, pp. 764 - 767, DOI: 10.1126/science.1164436