Elsevier

Ore Geology Reviews

Volume 135, August 2021, 104222
Ore Geology Reviews

Fluid source, element mobility and physicochemical conditions of porphyry-style hydrothermal alteration-mineralization at Mirkhani, Southern Chitral, Pakistan

https://doi.org/10.1016/j.oregeorev.2021.104222Get rights and content

Highlights

  • Pyrite trace element contents imply felsic source(s) of the ore-forming fluids.

  • The sulfur isotope of pyrite shows a typical magmatic-hydrothermal origin.

  • Low As and high Co contents of pyrite imply medium to high-temperature conditions.

  • The ore-forming fluids were enriched in Cu, As, Au, Bi, REE, HFSE, and LILE.

  • Cl and F complexes were responsible for REE and HFSE mobilization, respectively.

Abstract

The Cretaceous calc-alkaline granodioritic rocks in north-western Kohistan island arc host porphyry-type mineralization at Mirkhani in the Khyber Pakhtunkhwa province of Pakistan. In this paper, we present whole-rock geochemistry, mass-change calculations, and type and degree of element mobility regarding the hydrothermal mineralization-alteration processes. There are two distinct types of pyrite mineralization: (1) euhedral to subhedral pyrite (Py1) and cataclastic pyrite (Py2). To comprehend the source of hydrothermal fluids and their physicochemical conditions, trace element and S (sulfur) isotope compositions of the two types of pyrite were determined. Besides, the pyrite and silicate minerals were investigated through electron probe microanalysis (EPMA).

The average low Ni (46), Zn (5), Cd (below detection level), and Bi (1) contents (in ppm), high Co/Ni ratio (>10) of both Py1 and Py2, and the range of sulfur isotope values for Py1 (−0.84‰ to 1.49‰) and Py2 (−0.93‰ to 1.84‰) indicate affiliation of the mineralization with felsic magma-derived hydrothermal fluids. The average low and high contents (ppm) of Se (0.7), As (2.6) and Zn (2), and Co (1089), respectively, in Py1 than Py2 (Se = 1.3, As = 4, Zn = 6 and Co = 747) indicates that former pyrite grains crystallized at a higher temperature than the latter, however, both types of pyrites were formed in medium to high-temperature conditions.

The observed strong mobility of REE (rare earth elements) and HFSE (high field strength elements), high La/Yb ratios (avg. 23), and low (<1) Nb/La and Th/La ratios in the altered rocks reflect mobilization by slightly acidic solutions dominated by chloride and fluoride-rich complexes (±SO4 and OH). The gain-loss calculations reveal that SiO2, CaO, and TiO2 were leached out, whereas Al2O3, Fe2O3, MgO, P2O5, K2O, Cu, As, Mo, Ag, Au, Bi, and REE, HFSE, and LILE (large-ion lithophile elements) were variably added to the rocks during the hydrothermal process.

Temperature calculation for chlorite formation and the type of gangue and ore mineral assemblage (epidote + sericite + albite + pyrite) implies a mineralization temperature range between ~ 215 °C and ~ 322 °C. The porphyry style of hydrothermal alteration-mineralization, trace element and S isotope composition of pyrite, and elemental gain and loss indicates that the mineralization at Mirkhani could be a part of a major porphyry Cu-Au system.

Introduction

Investigation of hydrothermal alteration zones, which are developed by geochemical reactions between circulating fluids and wall rocks, provides useful evidence regarding paleo-environmental conditions (e.g., Pirajno and Van Kranendonk, 2005, Pirajno, 2009, Anschutz, 2015) and the source of metallic elements in ore deposits (e.g., Touray et al., 1989, Yang and Scott, 2006, Richards, 2011, Ding et al., 2018). For example, the magnitude of the mobility of some key elements in altered rocks may furnish important clues about the source of ore metals (e.g., Straub and Layne, 2003, Shanks and Thurston, 2012, Vigouroux et al., 2012, Smith et al., 2017).

Pyrite is one of the most commonly and abundantly occurring ore minerals in hydrothermal ore deposits. Previous studies have shown that significant amounts of Cu, Zn, As, Co, Ni, Sb, Se, Ag, Au, Te, Hg, Pb, Tl, and Bi, occasionally reaching up to weight percent levels, may occur in pyrite (Reich et al., 2006, Reich et al., 2013, Deditius et al., 2014, Morishita et al., 2018). The trace element composition of pyrite is temperature sensitive and thus has been used to interpret the paleo-environment. For example, high contents of Co, Ni, and Se indicate higher temperature, whereas high concentrations of As, Mo, Ag, Sb, Au, and Tl show the low temperature of pyrite formation (e.g., Sykora et al., 2018). In addition, the trace elements in pyrite can also provide information about the pyrite precipitating fluids, the chemistry of which depends on their source and is influenced by reworking via wall rock/ fluid interaction (Zhao et al., 2011). For example, high Ni content shows precipitation of pyrite from fluids derived from mafic-ultramafic melts of mantle origin (Palme and O'Neill, 2003), whereas low Ni contents indicate pyrite solidification from felsic magma (crustal melts)-derived fluids (Rudnick and Gao, 2003, Zhao et al., 2015, Hu et al., 2019). In addition, the sulfur isotope composition of pyrite is widely used to identify the possible source of fluids (e.g., Bajwah et al., 1987, Deditius et al., 2011).

Element mobility is a commonly reported phenomenon occurring in hydrothermal alteration-mineralization. Studies have found that RE and HFS elements are mobile in hydrothermal systems under particular conditions (e.g., Winchester and Floyd, 1977, Nesbitt and Young, 1982, Rubin et al., 1993, Fulignati et al., 1999, Jiang, 2000, Salvi et al., 2000, Jiang et al., 2005, Alvarado et al., 2007, Hikov, 2011, Paoli et al., 2019). The behavior of RE and HFS elements has been used to better understand the physicochemical characteristics of ore-forming hydrothermal fluids such as in Asarel porphyry copper deposit, Noranda District, Quebec (Hikov, 2011) and Axi epithermal Au deposit, western Tianshan, China (Liu et al., 2018a).

The northern regions of Pakistan have complex and diverse geological settings due to the development of intra-oceanic Kohistan Island Arc, followed by different geological events that make them favorable for hosting a variety of economic mineral deposits including arc and back-arc epithermal precious metal and arc-related porphyry Cu, Au and Mo deposits (Sweatman et al., 1995, PMDC, 2001). From the early to late 1990 s, an extensive stream sediment survey (~2,000 samples) was conducted by an Australian Aid Program, Pakistan Mineral Development Corporation (PMDC), and Sarhad Development Authority (SDA) covering an area of approximately 80,000 km2 in northern Pakistan (Halfpenny and Mazzucchelli, 1999). Later, Ali et al. (2014) analyzed additional 269 stream sediment samples of different sizes and used modern statistical and geological mapping programs to trace possible lode Cu-Au mineralization in northern Pakistan. Results from these studies reveal anomalously high Cu, Au concentrations and hence potentials for the occurrence of economically viable Cu-Au gold deposits in the study (i.e. Mirkhani) area (Siddiqi and Nawaz, 2014).

According to the available petrographic details, the rocks hosting ore deposit at Mirkhani contain epidote, chlorite, sericite, and pyrite (e.g., Heuberger et al., 2007, Farhan et al., 2021). But very little is currently known about their whole-rock geochemistry and the gains and losses of chemical components that may have accompanied the process of alteration-mineralization. More importantly, details on the source of fluids and physicochemical conditions of the ore-forming processes are currently lacking. Here, we report newly generated petrographic and whole-rock geochemical data, details regarding mass-changes and element mobility, and trace element and sulfur isotope composition of pyrite to identify the possible source of fluids and understand the physicochemical conditions of the porphyry Cu-Au system.

Pangea, a supercontinent formed during Paleozoic by the combination of all continents, broke apart into Gondwana and Laurasia at ~ 180 Ma which were isolated by the Paleo-Tethys Sea (Rogers, 1996, Golonka, 2004, Kazmi and Abbasi, 2008). The separation of the Karakoram block from Gondwana during the late Permian-Triassic time led to the formation of Paleo and Neo-Tethys. The Karakoram block traveled across the Paleo-Tethys and got accreted to the southern boundary of the Eurasian plate (East Hindukush-Wakhan) along Tirich Mir Fault during 184–189 Ma (Mesozoic) (Gaetani et al., 1996, Zanchi et al., 2000, Golonka, 2004). The collision between Eurasia and Karakoram caused the closure of Paleo-Tethys and the opening of Neo-Tethys towards the north of the Indian plate. The detachment of the Indian plate from Gondwana resulted in the initiation of narrowing of the Neo-Tethys at ~ 130 Ma. The northward movement of the Indian plate led to intra-oceanic subduction and the resultant formation of the Kohistan-Ladakh Arc (K-LA) (Bignold et al., 2006). The Kohistan-Ladakh Arc collided with the southern margin (Karakoram block) of the Eurasian plate along Main Karakoram Thrust (MKT) or Karakoram-Kohistan Suture Zone (KKSZ) during mid-Cretaceous forming an Andean type continental margin in the region (Petterson and Windley, 1985, Searle et al., 1999, Danishwar et al., 2001, Khan et al., 2009). Suturing along the MKT has brought high-grade metamorphic onto low-grade metamorphic rocks (e.g., Searle et al., 1999).

Towards north and northwest, the MKT separates Karakoram terrane from the Kohistan part of K-LA. Eastward, it becomes Shyok Suture which demarcates Karakoram terrane from the Ladakh portion of K-LA (Honegger et al., 1982). The K-LA was formed during the Mesozoic time above a subduction zone within the Tethys Ocean (e.g., Tahirkheli, 1979, Bard, 1980, Petterson, 2010) and is now sandwiched between the Indian and Eurasian continental plates (Fig. 1).

The study area (Mirkhani/ Ashiret; Fig. 2) is located ~ 10 km SW of Drosh town in district Chitral. Geologically, the area falls in the northern part of Kohistan Island Arc (KIA) and lies in the vicinity of MKT (Fig. 2). The focus of the present study is the Cretaceous diorite-granodiorite pluton, referred to as the Mirkhani/ Ashiret granodiorite, and the associated mineralization. The granodiorites are intruded into arc-related Albian-Aptian (125–112 Ma) Drosh Formation and Gawuch Formation (Chalt Volcanic Group rocks) (Tahirkheli et al., 2005, Heuberger et al., 2007). Red shales of the Purit Formation (120 ± 31 Ma; Zeitler, 1985) unconformably overlie the granodiorite, and the unconformity zone is overprinted by a narrow fault zone (~10 m wide). The fault zone contains lenses of serpentinite and talc schists and separates the Purit red clastics from the metamorphosed volcanic (greenschists) and sedimentary rocks (marbles) of the Gawuch Formation to the south. At Mirkhani, the granodiorite intruded into Gawuch volcanics in the form of sills. The Mirkhani granodiorites contain K-feldspar, plagioclase, quartz, hornblende ± zircon ± titanite as primary (magmatic) minerals, whereas biotite, epidote, and chlorite represent products of alteration (Heuberger et al., 2007, Farhan et al., 2021). The Mirkhani granodiorite is dated at 111.52 ± 0.40 Ma, which is consistent with the older, i.e. Aptian (125–112 Ma) age of the intruded volcanic rocks and limestones. In addition, the Hf isotopic composition (εHf = +13 to + 15) of Mirkhani granodiorite is also very close to the MORB-type mantle (εHf > +15) indicating that the intrusion is part of the initial island arc magmatism above an intra-oceanic subduction zone (Heuberger et al., 2007).

Section snippets

Samples and analytical methods

To collect representative samples, the geological overview of the area was done, and borders between two different rock types were thoroughly marked. Detailed compositional and textural studies to understand the nature of the host rock were also carried out. Two types of representative samples namely bulk (10–15 kgs) and grab (~1 kg) samples were collected during the field work. Nine representative bulk and grab samples each were collected from the least altered granodiorites and their

Petrography

The relationship between least altered (granodiorite) and altered (chlorite-epidote-sericite) rocks were studied in the field (Fig. 3A−C). The alteration/ mineralization is observed along fractures and joints in the form of fracture fillings, selvages and pervasive alteration products of granodiorite (Fig. 3B, G). Later, the collected samples were studied under the polarizing microscope (Fig. 3H−O; 4A−H) followed by a detailed investigation using SEM. Back-scattered electron (BSE) images were

Tectonic affinity

Among the incompatible trace elements, the negative and positive anomalies of Nb and Ta, and Pb, respectively, and the high LILE/HFSE ratios are generally considered as characteristic peculiarities of subduction-related igneous rocks (Grove and Kinzler, 1986; Hawkesworth et al., 1993; Pearce and Peate, 1995). These features are often described by mixing of hydrous fluids liberated from subducting oceanic lithosphere, which are enriched in certain fluid-mobile and LIL elements (e.g., Pb, U, with

Conclusions

  • 1.

    The Mirkhani granodiorites are calc-alkaline and display typical oceanic island arc signatures.

  • 2.

    The crystallization temperature (~215˚C to ~ 322˚C), estimated on the basis of chlorite chemistry, and the co-existence of epidote, sericite, albite, and pyrite suggest that the Mirkhani rocks experienced porphyry-style of alteration.

  • 3.

    Volume change accompanying the alteration is found to be insignificant and thus consistent with the estimated weak to intermediate weathering profile.

  • 4.

    Results from

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to thank Ulrike Westernstroeer, a chemical engineer at Kiel University (Germany), for the help during the LA-ICP-MS analysis. We thank Dr. Naveed Anjum for his valuable suggestions and helpful discussion. The authors would like to thank two anonymous reviewers for their comments that improved the quality of the manuscript. This study was partly funded by the National Natural Science Foundation of China (Grant nos. 41761134051, 91858213, 41776057).

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