The use of dempster-shafer model and GIS in integration of geoscientific data for porphyry copper potential mapping, north of Shahr-e-Babak, Iran
Each map to be used as evidence to evaluate a proposition (e.g. “this cell contains a porphyry copper deposit”) is associated with a pair of belief functions, the support function and the plausibility function. In practice, these functions are usually held in map attribute tables, where each class on the map is associated with a support value and a plausibility value. Suppose we have map A, we will simply denote the value of the support due to A, as SupA, and the plausibility due to A, as PlsA . Because functions vary with the value (map class) of A, they can therefore be mapped in their own right by lookup operations from map A. For a given value of map A, the uncertainty is denoted as UncA, calculated as PlsA -SupA, and the disbelief, DisA, is 1-PlsA. Thus the sum SupA+ UncA+DisA = 1. The disbelief is the belief that the proposition is false, i.e., that a cell does not contain a porphyry copper deposit. Note that plausibility is greater than or equal to support. Where plausibility and support are equal, the uncertainty is zero, and Sup+Dis = 1, as in the probability approach. The relationship between these functions is well illustrated in Wright and Bonham-Carter 1996, Figure 33.
For each map used as evidence, two independent functions must be estimated, usually either the support and disbelief, or the support and plausibility, but in some cases the uncertainty may be calculated with one of the other functions values. An et al. (1994a) and Chung and Fabbri (1993) discuss the estimation procedure.
Given two maps A and B, with the support and disbelief functions for each, Dempster’s rule of combination for estimating the combined support, disbelief and uncertainty are shown in Equations (1) to (3) (Wright and Bonham-Carter 1996).
Geology and Mineralization
The study area (30°19˘ - 30°30˘ N, and 55° 05˘ - 55° 30˘ E) is located north of Shahr-e-Babak town, in SW Kerman province (Fig.1). It is situated within the southern part of the central Iranian volcano-sedimentary complex. The geological evolution of the area can be simplified as formation and folding of early Tertiary volcano-sedimentary rocks, and emplacement of late Tertiary granodiorite, diorite, monzonite and tonalite in the volcano-sedimentary complex. A more detailed description of the geology of an area between Rafsanjan Belt in NE to Sirjan Belt in SW, is given by Dimitrijevic (1973).
Eocene volcano-sedimentary rocks consist of alkalibasalt-andesite flows and tuffs, and volcaniclastic sediments. These are intersected with Pliocene and Eocene sandstones, marls, sandy calcarenites and conglomerates. The oldest and youngest exposures are Upper Cretaceous rocks and the Quaternary alluvial deposits and gravel fans, respectively. Some well known copper and lead- zinc deposits and occurrences are shown in Figure 1.
Hydrothermal alterations, mainly in the form of chloritization, biotitization, sericitization, epidotization, carbonization, and silicification, developed in the intrusive and the volcanic rocks, are widespread over the area. The zonal pattern of alteration at Meiduk is concentric and almost symmetrically arranged around a Tertiary porphyry intrusive. Potassic, phyllic, argillic, and propylitic alterations and silicification have been recognized at Meiduk (Amraie, 1991). Drill core observations indicate a relatively large core of potassic alteration surrounded by a thick shell of phyllic alteration. Later argillic alteration has been locally superimposed on the two earlier alteration zones. The superimposition has occurred in places where the surface water is driven down in contact with suitable minerals. Argillization is more prominent and extensive at higher elevations, at surface and especially in tunnels where acidic waters circulate more freely. Propylitic alteration seems to be
entirely hosted by andesitic wall rocks and extends outward for
hundreds of meters around phyllic alteration zone.
Several deposits and numerous important mineral occurrences exist
in the Kerman region, among which the copper deposits and
occurrences are of prime importance. Two main types of
mineralization are identified, porphyry-type and the vein-type mineralization. Porphyry-type mineralization is more important and is located in the vicinity of Post-Eocene intrusive bodies in the Eocene volcanic-sedimentary complex. The vein-type mineralizations are controlled by faults of different trends and cut both the intrusives and extrusives. Meiduk and Sara are the most important porphyry copper deposits in the region, while the Chah-Mesi is a polymetallic deposit.