The hydrothermal alteration and mineralization zonation in the Haftcheshmeh PCD provides the similarity as the other major porphyry deposit (e.g., Lowell and Guilbert, 1970; Sillitoe, 2000; Seedorff et al., 2005; Sillitoe, 2010) and are broadly developed as a result of the hydrothermal ore-forming activités synchronous with the structurally controlled emplacement and intrusion of the Oligo-Miocene Haftcheshmeh porphyries adjacent to the intersection of the NE and NW faults, caused the abundant sulfide-bearing quartz vein-veinlets.
On the basis of the detailed geological mapping, field data and petrographic observations of the drill core and outcrop samples of the Haftcheshmeh PCD, can be identified the characteristics and spatial distribution of five widespread hypogene hydrothermal porphyry-style alteration types from center to outward, consisting dominant early potassic and peripheral propylitic alterations, successively followed by sericitic alteration and locally argillic accompanied by Silicic alterations.
In the Haftcheshmeh PCD area, the pale green to grayish propylitic alteration is the most abundant alteration type, which peripherally occurs in the all rock types of the Southern and North-West parts of the Haftcheshmeh PCD especially dioritic to gabbro-diorite intrusive rocks including mainly vein-veinlets of chlorite, epidote; broadly coincides and surrounded by the distribution of the potassic alteration zone with a relatively sharp boundary in the deeper parts. In propylitic alteration, the sulfide content is low and mainly consists of relatively scarce disseminated and veinlet of pyrite.
Extensive East-West Potassic alteration related to early hydrothermal activity developed pervasively contemporaneous with propylitic alteration, spatially associated with intruding of the Oligo-Miocene granodiorite into the micro diorite/gabbro-diorite stocks in the central part of the Haftcheshmeh PCD. The potassic alteration consists of quartz, k-feldspar and primary biotite phenocrysts which are replaced by K-feldspar and secondary biotite, respectively. The potassic alteration is associated with abundant disseminated chalcopyrite, pyrite, and minor molybdenite and bornite mineralization (Fig-).
Fields observations and petrographic studies indicate that low intense scattered intermediate argillic alteration usually controlled by fractures and fault zones in the southern and NE parts of the study area and is characterized by partial to complete replacement of plagioclase and secondary biotite phenocrysts by clays minerals with accessory sericite, epidote.
A volumetrically limited quartz-sericite-pyrite (QSP) alteration in the Haftcheshmeh deposit is developed in North, NE and central parts of the Haftcheshmeh district especially complies with the intrusion of the dioritic to gabbro-diorite bodies. The (QSP) alteration occurs usually either as the almost complete replacement of original magmatic minerals such as plagioclase and K-feldspar and biotite by fine- to coarse-grained sericite, or as dissemination and veins and veinlets of pyrite, chalcopyrite and minor bornite, muscovite and quartz in the groundmass.
The extensive stockworks of silicification consisting of quartz, Fe oxide-hydroxides along with disseminated pyrite, particularly synchronous with potassic and sericitic alteration zones, is structurally controlled by NE-SW to N-S trending faults and well exposed in some limited area including the northwestern part of the study area.
The Hypogene ore mineralization as dissemination and stockworks veins-veinlets in the Haftcheshmeh porphyries stocks have occurred during potassic and potassic-sericitic altération zones and included the abundant chalcopyrite, pyrite, and minor bornite and molybdenite as the sulfide-ore minerals accompanied by quartz, K-feldspar, sericite, chlorite, biotite, muscovite, and epidote as the gangue minerals.
Core logging and petrographic observations on more than 100 polish and thin sections have revealed that porphyry Cu-Mo mineralization in Haftcheshmeh controlled by stock worked quartz-sulfide veins around the diorite to gabbro-diorite and granodiorite porphyry stocks have been subjected to cross-cutting sulfide-bearing quartz vein-veinlets which spatially and temporally recognized within the potassic, sericitic and propylitic alteration zones. Considering mineralogical assemblage of the different alteration types and crosscutting relationships between the various vein-veinlets, the mineralization processes responsible for the hypogene ore formation in the Haftcheshmeh PCD can be roughly divided into the following four main stages of veining, consists of EB, A, B, and D type veins, from early to late, respectively (Gustafson and Hunt, 1975; Gustafson, 1995; Landtwing, 2004; Redmond and Einaudi, 2010; Sillitoe, 2010).
Stage I: The Pre-ore forming stage in the deep zone includes the early biotitic, similar to EB veins presented by Gustafson, 1995, which are formed during the presence of potassic alteration. The quartz-poor EB veins are characterized by the typical alteration assemblage of biotite + K-feldspar. Quartz occasionally has occurred in the centre line accompanied by faint concentrations of pyrite and chalcopyrite. There is not usually distinct alteration halo in this type, but commonly a pale k-feldspar halo may be present (Gustafson, 1995).
Stage II: The early ore-forming stage, similar to the A-vein of Gustafson and Hunt, 1975 and Gustafson, 1995, is typified by an early granular quartz–K-feldspar veins, surrounding by the clusters of secondary biotite, commonly lack sulfides but disseminated and separate grains of chalcopyrite-pyrite assemblages have occurred, which are a typical texture, on deep levels (Gustafson and Hunt, 1975) (Fig. 4c). The altered diorite to gabbro-diorite intrusive rock, especially mineralized granodiorite stock, are cut by granular to straight-walled coarse-grained quartz vein containing biotite, trace chalcopyrite, and/or pyrite surrounded by biotite-K-feldspar alteration haloes (Fig. 4) during the gradual reduction of the potassic alteration. (Fig. 4a, 4c). The granular quartz– K-feldspar- biotite of the A-veins are the earliest vein types in the Haftcheshmeh deposit and cut by stage III and IV veinlets (Fig. 4d 4f).
Stage III: The middle ore-forming stage is the main mineralization stage mainly in the potassic and sericitic alteration zones and represented by the widespread formation of a relatively coarse- to medium-grained quartz centrally in-filled by later sulphides mainly disseminated chalcopyrite, magnetite and pyrite Fig. 5a, b, c) with minor molybdenite+bornite (Fig. 5d, e), which have similar characteristics to B-veins (Sillitoe, 2010; Gustafson and Hunt, 1975; Gustafson, 1995). The weak to occasionally sporadic of specific alteration halo around the coarse-grained quartz of the B-veins is the distinctive features of B-vein with coarse-grained quartz A-Veins. Sulfides are generally concentrated either within or along the margins of this stage generally have inclusion texture, which is a type of chalcopyrite-pyrite-bornite assemblages on deep levels (Gustafson and Hunt, 1975) (Fig. 5d, e). The occurrence of the “B” veins and the abundant “A” veins developed in the deep samples of the Haftcheshmeh PCD are consistent with other porphyry Cu-Mo deposits in the world (Sillitoe, 2010).