Mathematics
Scientific paper
Nov 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3987l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 11, pp.3987-4013
Mathematics
Scientific paper
A unified description of supergene enrichment of a porphyry copper protore is presented based on the principles of metasomatic zoning. Mathematically this formulation represents the supergene weathering process as a moving boundary problem. A first principles approach is used to describe quantitatively the transport of oxygenated water through a porous host rock consisting of a Cu-bearing protore. Mineral reaction rates are represented by kinetic rate expressions for both precipitation and dissolution. Reaction zones containing mineral alteration products making up a leached zone, oxide zone, and enrichment blanket propagate downward in the direction of fluid transport. The time evolution of the system is completely determined by the governing transport equations once initial and boundary conditions are specified, giving the composition of the unaltered host rock and infiltrating fluid entering at the surface of the weathering column. The description provides for both fully saturated conditions and undersaturated conditions in the presence of a water table at depth. An order of magnitude estimate of the velocity of propagation of a redox front demonstrates that metasomatic effects become increasingly important as the O content of the infiltrating water increases. Regions of higher permeability, resulting in higher fluid flow velocities, lead to increased enrichment compared to regions of lower permeability, with reduced flow velocities, in agreement with field observations. Results of numerical calculations suggest that the enrichment blanket may be subdivided into three distinct zones: 1. (1) an enriched zone at the top of the blanket 2. (2) the middle blanket in which chemical reactions of Cu-bearing minerals are absent; and 3. (3) the nascent blanket forming the interface between the protore and middle blanket. Both the nascent and middle blanket contain the same Cu grade as the protore. These results are in agreement with field observations of a high Cu grade within a relatively narrow zone in the upper portion of the blanket. Results of numerical calculations for three different scenarios are presented. The first example considers a simplified representation of the enrichment process with chalcopyrite, covellite, and chalcocite as the major Cu minerals. In this case chalcocite forms an enriched zone at the top of the blanket which migrates downward increasing in width. The second and third examples differ by the presence of a water table at depth, and include brochantite, tenorite, native Cu, and cuprite as secondary Cu minerals in addition to the minerals used in the first example. Surprisingly, in these two examples the enriched blanket zone does not grow indefinitely with time, but reaches a steady-state profile that is displaced rigidly in the direction of fluid transport without changing shape. In the presence of a water table at depth, the major enrichment occurs in the oxide zone lying above the blanket.
Biino Giuseppe G.
Lichtner Peter C.
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