Hydrothermal Alteration in Porphyry Cu-Mo-Au Mineralizations of theChagai Arc, Balochistan, Pakistan

Abstract

Subduction-relatedeast-west trending Chagai Arclocated in the western part of Pakistan. This arc is the western extension of Tethyan Magmatic Belt which is about 5000 km long and extends from central Europe, Turkey, Iran,andPakistan. During the last four decades, several porphyry copper settings were reported from the Chagai Arc. Seven more famous occurrences are selected for the present study owing to more detailed work carried out on them. From west to east these deposits include Saindak, Reko Diq (formerly known as Koh-e-Dalil), Humai, Missi,Durbanchah, Dasht-e-KainandZiarat Pir Sultan. Hydrothermal alterations in the porphyry copper settingof Chagai Arc are mostlyassociated with tonalite porphyry stocks, except Durbanchah and Humaiprospects which are hosted in dacite porphyry stocks, whereas Missi Prospect occurs in a granodiorite batholith. The alteration is normallydeveloped in a concentric zonal pattern as observed in the majorityof the world deposits, except that absence of a regular argillic and peripheral zone. In most of the occurrences the potassium silicate alteration zone (K-alteration zone) occurs usually within the intrusive porphyry stock, but in Saindak and Reko Diq deposit, some of the adjacent wall-rock sediments, and in Durbanchah setting the microdioriticcountry rock, has also undergone the K-alteration. Quartz sericiticor phyllicalterations zones are developed in all the occurrences as continuous or discontinuous haloes around the K-alteration zone except in Durbanchah prospect. In Humaiprospect an advanced argillic alteration zone is developed around the K-alteration zone. Propylitic alteration has also developed in almost all settingsand generally encircles the quartz sericiticalteration.The oxide mineralization is generally represented by goethiticjarositic, hematitic limonites and malachite, with minor pitch limonite, chrysochola, neotocite, brochanthiteandmolybdite. Hypogene mineralization is represented by pyrite, chalcopyrite andminor bornite. A regular supergene sulfide enrichment is only reported from the Reko Dik deposit and represented by chalcositeand covellite in other deposits restricted incipient type supergene sulfide enrichment is encountered. In terms of ore reservesthe Riko Dik deposit is on the top with 2,200 million tons of copper followed by Saindak with 412 million tons and Dasht-e-Kain 350 million tons. Grade wise Reko Diq Deposit is again on the top with 0.53% Cu, 0.01% Mo and 0.3g/t Au, followed by Saindak with 0.4% Cu, 0.015% Mo and 0.35 g/t Au. Dasht-e-Kain deposit contains 0.35% Cu and 0.015% Mo. Gold has not been reported from the Dasht-e-Kaindeposit. From other deposits of the Chagai Arc ore reserves and grade are not reported so far