Hydrodynamic Constraints on the Formation of Kuroko Deposits

Simple calculations are presented to illustrate the kinds of constraints hydrodynamic considerations can place on genetic hypotheses. It is tentatively shown that the rhyolite plugs commonly associated with Kuroko deposits are too small to be the cause of Kuroko mineralization. A larger intrusive heat (or magmatic fluid) source at depth is needed. Convective velocities in rocks of reasonable permeability are so much smaller than minimum ocean bottom current velocities that it is very unlikely Kuroko mineralization could have been precipitated from solutions above the sea-sediment (or rock) interface. Hydrothermal solutions would be swept away as soon as they left the protective sediment or rock cover. Finally hydrothermal convection may be expected to promote ore slumping if the permeability of the precipitated ore is low. The "slumped" nature of some Kuroko ores may thus find a natural genetic explanation. Some of the significant geological and geochemical aspects of Kuroko deposits have been described in the articles accompanying this one. From these descriptions and others it is clear mineralization was deposited from upward convecting hydrothermal solutions. The origin of the solutions and the manner of mineral precipitation is not so clear, however, although isotopes and deposit studies can potentially impose some useful constraints. The purpose of this article is to illustrate the kinds of constraints hydrodynamics can impose on the genesis of Kuroko deposits. I would like to emphasize that it is the intention of this paper to simply illustrate and explore the kinds of calculations and considerations that may prove useful. Suggestions made should be regarded as working hypotheses, unproven, at this stage by rigorous comparison with available geochemical and geological data. One of the most useful kinds of computation that can be made asks the question: given an intrusive of a certain size and initial temperature in host rock of some permeability, how fast will the intrusive cool, and how much water will circulate through the intrusive in the course of cooling? The methods of making this kind of computation in a way which includes the true properties of water (including the effects of boiling and condensation) are described in CATHLES (1977). Figure 1 shows the cooling and convection history of a Kuroko deposit assuming the heat which drives the convection is supplied only by the small rhyolite plug often (but not always) found associated with Kuroko deposits. The initial temperature of the rhyolite plug is 700•Ž as shown in Figure la, and the permeability of the plug and rock intruded is assumed to be a uniform 0.25 millidarcies. This value of permeability is a little less than the 0.45 millidarcies estimated for oceanic crust by RIBANDO et al. (1976) or the value of 0.62 millidarcies found for the average permeability of the oceanic plate by FEHN and CATHLES (1978) in their preferred model. The sea depth is assumed in the model of Figure 1 to be only 100 m, so the pressure at the sea sediment interface is only 10 bars. The rhyolite plug is small (200 m thick and 300 m wide) and is intruded so its top surface is 200 m below the sea-sediment interface. Figure 1 shows the cooling of the rhyolite plug is very rapid. The circulating fluids boil as they enter the base of the heat anomaly of the plug (e.g., Figs. 1b and c) and condense as they enter cooler rock overlying the plug. The heat anomaly of the plug migrates upward * Recived April 7,1978 . ** Ledgemont Laboratory, Kennecott Copper Corporation, 128 Spring St., Lexington, Mass. 02173 U.S.A.