Experiments on convection and their relevance to the genesis of massive sulphide deposits

Three configurations have been proposed for heaters thought to be responsible for fluid circulation that produces volcanic‐hosted massive sulphide deposits. One of these, the horizontal sill of felsic volcanic terrains, lends itself to experimentation using Hele‐Shaw cells. Experiments simulating hydraulic continuity between the permeable rocks and overlying (sea) water show that the resulting convection cells may have very high aspect ratios. Experiments in which only part of the permeable medium is heated show that upwelling plumes are confined to the heater, and that the number of cells developed depends on the ratio of the heater length to the height of the medium, and whether or not there is hydraulic continuity with an overlying fluid reservoir. The scale of natural systems is controlled by the availability of metals in the permeable medium and the heater, and the thermal energy required. Although magmatic contributions of metals and alkalies seem likely in the massive sulphide systems, there is only limited evidence of their presence. Calculations ignoring magmatic input indicate catchment areas of about 100 km² for major deposits, values consistent with the observed spacings between large deposits. Initiation of convection in a permeable medium involves development of larger and larger cells in a series of growth steps until a steady pattern is established. Scaling calculations suggest that these steps occupy sufficient time to produce massive sulphide deposits. They should be of increasing size and spaced at increasing intervals. The Iberian pyritite province and the Mt Read volcanic belt have deposits with size‐spacing criteria supporting this thesis. Experiments show that in the cooling cycle the cells do not decrease progressively in size but retain their steady, large‐scale flow pattern until flow ceases. This hysteresis effect helps explain the late stage hydrothermal alteration commonly observed in the hanging wall of massive sulphide deposits.

Key words:

• convection
• Hele‐Shaw cell
• ore genesis
• volcanic‐hosted massive sulphides