The use of dempster-shafer model and GIS in integration of geoscientific data for porphyry copper potential mapping, north of Shahr-e-Babak, Iran
The Meiduk porphyry copper deposit, situated near Meiduk village (Fig. 1) is hosted by Eocene andesitic and basaltic rocks. The main intrusive body responsible for mineralization-alteration at Meiduk is an epizonal dioritic porphyry stock of Oligocene to Pliocene age (Bazin and Hubner 1969). This deposit was mined in ancient times and is currently mined for its porphyry copper. The copper ore grade in the supergene zone varies between 1% and 3%, the average being 1.52% Cu. The supergene plus hypogene ore reserves amount to 20 million tones (Geological Survey of Iran 1973).
The Sara copper occurrence is located northwest of Meiduk deposit (Fig. 1). The geology and mineralization is very similar to that of Meiduk. Altered dioritic porphyrite, is exposed over an area of about 1.5 km2 and surrounded by weakly altered Eocene volcanites and pyroclastites. Geochemical measurements indicate copper concentrations of 120-500 ppm. The exploratory drilling in the altered intrusive confirmed the presence of porphyry-tye copper mineralization. The ore grade was low, on average about 0.15% Cu, and no supergene enrichment was detected (Geological Survey of Iran 1973).
The Chah-Mesi polymetallic deposit is located at the east of the Meiduk village (Fig. 1).The rocks of this area are comprised of Eocene volcanic-sedimentary complex, subvolcanic intrusive bodies of intermediate composition, and Quaternary deposits. Both volcanites and porphyrite intrusives are highly faulted and fractured. The richest mineralization occurs along a diagonal fault between Eocene volcanites and diorite-porphyrite intrusives. According to Omaljev (1972), average ore grade include 1.27% copper, 1.01% lead and 2.12% zinc with minor amounts of silver (10-150 ppm) and gold (up to 7 ppm).
Figure 1: Geological map of the study area in the north of Shahr-e-Babak, Kerman, Iran.
Data input and pre-processing
The most time-consuming step in most GIS projects is building the database. According to variations in data models, data structures, and data formats, the input layers may have many sizes and shapes. A digital geoscientific data to be used in porphyry copper mineralization mapping in the study area was established.
A Geological map of Shahr-e-Babak, compiled by Geological Survey of Iran at a scale of 1:100000 was used as a source of lithologic and lineaments information. The bedrock geology was digitized as a series of polygons. In order to link each polygon to a lookup table containing attributes, a unique identifying number was given to it. There were 176 unique polygons that were regrouped into 24 geological categories so that each unit could be separately evaluated within the modelling process. A generalized version of the geological map is shown in Figure 1.
Remotely sensed data including aeromagnetic and Landsat Thematic Mapper (TM) were used as other sources of information on magnetic anomalies, part of structural features, and hydrothermal alteration haloes.
Stream sediment geochemical data consisting of 335 individual sample points were also used in the analysis. For each sample, data on geographic coordinates, geological sheet number, and concentration of 14 elements were provided. Concentrations of 6 elements, (Cu, Pb, Zn, Mo, As, and Ag), were processed and used in this study as pathfinders for porphyry copper deposits. The raw data were run by SPSS (Standard Package for Social Sciences) software and basic statistics for the samples were computed. The anomalous and background values of each element were then calculated. Geochemical anomaly patterns for the elements noted above were generated using the Surfer software. The output files were then converted into DXF format of AutoCad and imported into PC Arc/Info to be edited and clipped for the study area. Database for the anomalies of elements were managed using Foxpro software and the final coverages were then converted to a raster format usable in GIS.
Fig. 2: Support map for the porphyry copper deposits at the north of Shahr-e-Babak, Iran. The ordered legend is on the basis of increasing support. The locations of Meiduk and Sara porphyry copper deposits are also shown in this figure and the followings.
Linear structural features interpreted from aeromagnetic data were combined with lineaments available in geological map to generate a lineament map. The interpreted lineaments were digitized and input into GIS as a vector file. The lines provided in this layer were classified and coded into four main classes according to their direction. A geologic expert has considered the E-W direction as a criterion for porphyry copper occurrence in the Kerman region (Dimitrijevic 1973). In this form, weighting of lineament information by direction and distance to potential copper mineralization was possible. The E-W lineaments were selected, combined with lineaments interpreted from aeromagnetic data with same direction and finally converted to a raster format.
Porphyry copper deposits and occurrences have a direct spatial association with granitic to intermediate plutonic rocks (Guilbert and Park 1986). These plutonic rocks may act as a source for the metallic elements and / or the hydrothermal fluids responsible for mobilizing metals. For these reasons, contacts of appropriate intrusive bodies were considered as an important input parameter.
To enhance the alteration haloes around intrusive bodies,
the Crosta technique for principal component analysis
(Crosta and Moore 1989) was applied on reflective bands of Landsat
TM (Tangestani and Moore 2001).