Abstract:
The formation of the Sar Cheshmeh porphyry Cu-Mo
deposit is related to the culmination of calc-alkaline igneous
activity in the Kerman region.
The deposit comprises a suite of Late Cenozoic
intrusive sub-volcanic and extrusive rocks emplaced into a
folded series of Eocene andesitic lavas and pyroclastic
sediments. The earliest stage of magmatism was emplacement of
a large granodiorite stock about 29 m.y.b.p. This was followed
by intrusion of two separate porphyritic bodies at 15 (Sar
Cheshrneh porphyry) and 12 m.y.b.p. (Late porphyry) and a
series of sub-volcanic dikes between 12 and 9 m.y.b.p.
Magmatic activity terminated with multi-phase extrusion of
a Pelean dacitic dome complex between 10 and 2.8 m.y.b.p.
The country rocks and the earlier porphyritic
intrusions are pervasively altered to biotite-rich potassium
silicate (metasomatic and hydrothermal) sericite-clay, phyllic
and chlorite-clay, argillic assemblages. These grade outwards
to an extensive propylitic zone. Within the ore body, the
later intra-. and post-mineral dikes only reach the propylitic
grade. At least three different sets of quartz veins are
present, including a sericite-chlorite-quartz set which
locally retrogrades pervasive secondary biotite to sericite.
In the hypogene zone, metasomatic and hydrothermal
alteration is related to all stages of magmatism but copper
mineralization and veining are restricted to a period of 15
to 9 m.y.b.p.related to the early intrusive phases.
The copper mineralization and silicate alteration do
not fit a simple annular ring model but have been greatly
modified by, 1. The existence of an ititial, outer ring,
of metasomatic alteration overprinted by an inner.ring of
hydrothermal alteration and, 2. later extensive dilating
effects of intra- and post-mineral dikes.
The hydrothermal clay mineral assemblage in the hypogene
zone is illite-chlorite-kaolinite-smectite (beidellite).
Preliminary studies indicate that the amount of each of these
clays varies vertically and that hydrothermal zonation of
clay minerals is possible. However, these minerals alter to
illite-kaolinite assemblages in the supergene sulfide zone
and to more kaolinite-rich assemblages in the supergene leached
zone. Hydrothermal biotite breaks down readily in the supergene
zone and is not well preserved in surface outcrops.
The distribution of copper minerals in the supergene
sulfide enrichment zone is only partly related to rock type
being more dependent on topography and the availability of
fractures.
