The Saindak Porphyry Type Cu (Au) Deposit Of Chagai, SW. Pakistan: Alteration, Mineralization Characteristics And Its Genesis

Saindak copper and gold deposits is located in Saindak, Chagai Arc of northwestern Baluchistan province, Pakistan, and associated with a succession of calc-alkaline to shoshonite, monzonitic to dioritic subvolcanic porphyry stocks emplaced within cogenetic volcanic rocks. Arc magmatism in the area resulted in porphyry Cu/Au deposit formation, has been studied in this paper, including its alteration, mineralization characteristics and its genesis. The present research is based on extensive field work and detailed field investigation of various intrusive bodies by conducting a field traverses across the Saindak area in Chagai arc terrene. A total number of 100 rock samples were collected. Intensive laboratory studies have been carried out on selected samples, which included petrography and whole-rock major, trace and rare earth elements geochemistry. More than 332 polished slabs and thin sections were prepared from the intrusive rock samples collected from surface and bore holes of the South Ore Block of Saindak Copper Mine for microscopic study.19 doubly polished waters (150μm thick) prepared for fluid inclusion studies were examined petrographically. And 13 least altered samples were selected for bulk rock analysis. Major, trace and rare earth elements (REE) were analyzed using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA- ICP- MS) at the China Academy of Geosciences in Beijing, China. Magmatic zircon dating using LA- ICP- MS was carried out at the isotopic dating laboratory of University of Science and Technology of China in Hefei.Chagai arc has been formed due to collision of Indian-Asian continents in the Cenozoic era, contains porphyry Cu (-Au) deposits of Miocene to Oligocene age at Saindak. The copper (-gold) mineralization at Saindak has been proved as porphyry type deposit formed within an EW-trending arc of 150 km width and 500 km length. The porphyry copper deposit is comprises closely spaced, low- grade ore bodies. Each is centered on tonalite porphyry stock emplaced during the lower Miocene into a late Cretaceous- Oligocene volcano. Sedimentary succession dominated in the immediate vicinity of the ore bodies are Oligocene siltstone. The magmatism started from Late Cretaceous and continued intermittently up to Pleistocene.No pre-Cretaceous rocks are known in the district. Cretaceous deposition of thick flysch-type deposits of shale, volcanic sandstone and thin, un-fossiliferous limestone. Cretaceous intrusion is also consists of very large andesite porphyry sills. At the end of Cretaceous, there will be folding and rocks are uplifted and erosion started. Submergence and deposition of shale and siltstone of Juzzak formation started in Plaeocene. Volcanic activity was strong and large volumes of coarse conglomeratic debris were formed. Eocene volcanic activity of intensity continued and the lower part is composed of these materials at Saindak. During the latter part of Eocene, the volcanic activity ceased and soft gray shale, siltstone and sandstone formed interbeded with some limestone. The epicontinental red-bed sequence of the Amalaf formation formed in Oligocene. During Miocene arc uplifted due to stresses and folding occurs within the rocks. Due to volcanism occuerd in Miocene and formed some intrusive bodies with porphyry type copper and gold mineralization. Uplifting of rocks tend to erosion. Pliocene volcanism and erosion continued. Pleistocene to present erosion of rocks continued due to stresses and upliftment and some playa lake deposits formed.Porphyry copper gold mineralization is hosted in the quartz diorite, diorite and andesitic intrusions, and is associated with hydrothermal alteration. Mineralization occurs as disseminated to stockwork styles, and as minor hydrothermal breccias. Some mineralization occurs in fault zones as quartz-sulfide veins. The main ore minerals are chalcopyrite and pyrite, with minor molybedenite and magnetite, and the average Cu (0.41%), Au (0.5g/t) and Mo (0.002%) grades are closely associated with potassic alteration zones.A typical porphyry deposit was formed in response to a physico-geochemical continum from low temperature magmatic to conventional hydrothermal conditions. This process in case of a homogenous host-rock produces a symmetrical distribution of hydrothermal mineral assemblages from the interior of the ore body to outward. The potasic alteration zone (quartz+ potash feldspar+ biotite assemblege) lies the central position. The phyllic alteration zone (quartz+ sericite+ pyrite assemblege), the argilic alteration zone (quartz+ kaoline+ chlorite assemblege) and the propylitic alteration zone (chlorite+ epidote+ carbonate+ adularia+ albite assemblege) sequentially follow it. The Saindak porphyry deposits contain three main types of Cu (-Au) deposits, including North, South and Central ore block. The characteristics like mineralization style, alteration zoning and metal association of Saindak porphyry Cu (-Au) deposits are similar in many aspects with arc settings. Mineralization controlling factors mostly based on magmatic rocks, country rocks, alteration characteristics and zonation and structure control.The petrogenetic studies based on major, trace, rare earth elements suggests that volcanism at Saindak South ore block is calc- alkaline and fractionated from a magma derived from an enriched sub- arc mantle source by 15 to 25% partial melting. The volcanics show typical continental margin type arc characters. The arc collided with the Afghan block during the latest Paleocene, which converted it into a continental margin. The arc volcanics shows higher ratio of LILE/HFSE and LREE/HREE. This suggests that during Oligocene-Miocene period, comparatively more LILE and LREE enriched fluids from the sub-ducting oceanic crust were added to the sub- arc mantle source. It is further suggested that this high rate of addition of LILE and LREE may be related to the rapid and shallow subduction of the Arabian oceanic plate below the Zagros- Makran convergence zone, that might have been developed due to the opening and rapid spreading of the Red Sea during that period.Laser- ablation U- Pb zircon indicate the Miocene age (22～19Ma) magmatic events. Four samples containing ore were analyzed for dating from porphyry diorite, conducted LA-ICP- MS U- Pb dating method. The formation age of the samples is from (22.02±0.16) Ma to (22.50±0.26) Ma, where the active magmatism time is more concentrated (crystallization time □0.5Ma). From the tested data, part of the zircon Pb. loss, although there are obvious, but the next intersection isotope Pb zircon age is more consistent, and thus does not affect the accuracy of age. Meanwhile, Pb isotopic composition of magmatic zircons basically have more consistent evolution curve, is an ancient shale formations lead product evolution, reflecting the area porphyry ore source regions with distinct characteristics of the shell source. The north section of the ore mine, ore block most of the southern extension of the known ore body has not yet control the border, deep orebody tendency is still continue, to the deep Cu grade rich taste and trends have changed, and the vast majority of the lot yet to discover silicified mineralized better potassic zone orebody is not over yet, and therefore, this mine still has a large potential for prospecting. And at Sor Baroot and other area, there aree also has a large potential.