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Regional multi-element drainage geochemistry in the Himalayan Mountains, northern Pakistan
ELSEVIER
Journal of Geochemical Exploration 67 (1999) 223–233 www.elsevier.com/locate/jgeoexp
Regional multi-element drainage geochemistry in the Himalayan Mountains, northern Pakistan Rohan Halfpenny a,1 , Richard H. Mazzucchelli b,* a
Minorco Services, BV Budapest, Hungary b Searchtech Pty Ltd., Perth, Australia Accepted 2 September 1999
Abstract Drainage reconnaissance sampling was initiated in northern Pakistan in 1992 as part of an Australian aid project and later extended by Pakistani agencies, achieving a coverage of some 100,000 square kilometres by 1997. Panned concentrate and <200 µm fraction (minus 80 mesh) samples were taken at varying sampling densities from some 4260 sites in the Northwest Frontier Province and Northern Areas, and analysed for Au, Bi, Co, Cu, Ni, Pb and Zn by atomic absorption spectrophotometry. In 1997, Minorco Services BV re-analysed the accumulated samples for a more comprehensive element suite, using methods with enhanced sensitivity for the previously analysed elements. This paper presents an interpretation of the new results in relation to regional geology and mineralisation. The survey encompasses the collision zone between the Eurasian and the Indian plates, and an intervening Cretaceous–Tertiary island arc system (the Kohistan and Ladakh arcs). These three main tectonic crustal units are separated by two major thrust faults, the Northern Suture Zone (NSZ) and the Main Mantle Thrust (MMT), which are zones of shearing up to 4 km wide that incorporate oceanic ophiolitic volcanics and alpine serpentinite intrusions. The area is characterised by extreme topographic relief, with peak elevations over 8000 m. Despite the semiarid climate, much of the higher ground has permanent snow cover and active glaciers, the meltwaters from which are the source of the extensive Indus River drainage system. Compression associated with the northward movement of the Indian Plate has resulted in 20 to 40 km of uplift since the Eocene (55 Ma), and erosion levels are deep. The remnants of former erosional episodes are retained in the form of various types of terrace deposits within river valleys. Aspects of the regional geology are well reflected in the multi-element geochemistry. The Eurasian and Indian plates are both characterised by high background values for Ba, Pb, Sn, Tl and the ratios K=Na and Rb=Sr. The Kohistan–Ladakh arcs are highlighted by elevated values for Co, Cu, Fe, Mn and V. A series of elongate patterns of enrichment for Cr, Ni and the ratio Mg=Ca delineate the NSZ and MMT. These relationships are well summarised by the results of principal component analysis, the first two factors derived clearly relating to the three regional geological units and the structures which separate them, respectively. Other factors can be interpreted to represent different styles of mineralisation, although no significant occurrences have been located to date. The results for some elements are influenced by variations in catchment area. Median concentrations for Ba, Co, Cu, Mo, Ni and Zn decrease with increasing catchment size, which is the normal pattern for downstream dilution. Arsenic, Au, Cr, Nb, Pd, Pt, Sb, Sn, V and W show the reverse tendency, i.e. enrichment in larger catchments. This behaviour is typical for elements which are dispersed in heavy mineral phases and is magnified by the sampling of trap sites, which are the loci for hydraulic concentration of heavy minerals. In the case of Au in panned concentrates, the extreme concentrations encountered (up to 480 ppm) are considered to represent 1
Current address: Anglo American plc, London, U.K. author. Fax: C61-8-9257-2334; E-mail: [email protected]
Ł Corresponding
0375-6742/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 6 7 4 2 ( 9 9 ) 0 0 0 6 9 - 2
224
R. Halfpenny, R.H. Mazzucchelli / Journal of Geochemical Exploration 67 (1999) 223–233
patterns of concentration, rather than dispersion, of Au, some of which may be derived from bedrock sources which have been completely eroded. A brief orientation study suggests that more meaningful dispersion patterns could be delineated by sampling and analysis of the <75 µm fraction of low-energy sediments, rather than trap sites. 1999 Elsevier Science B.V. All rights reserved. Keywords: geochemical exploration; heavy mineral; stream sediments
1. Introduction A regional drainage reconnaissance survey was initiated in the Karakoram region of northern Pakistan in 1992, as part of an Australian aid programme (Sweatman et al., 1995) and later extended by teams from the Pakistan Mineral Development Corporation (PMDC) and the Sarhad Development Authority (SDA). During the course of this programme, designated ‘GEMAP’, drainage sediments were sampled over an area of some 100,000 square kilometres in northern Pakistan (Fig. 1) and both the <200 µm ( 80 mesh) fraction and panned concentrates analysed by the Sarhad Development Authority Mineral Testing Laboratory in Peshawar for the following elements (with detection limits in ppm shown in parentheses): Au (0.05), Cu (5), Pb (5), Zn (5), Ag (0.5), Co (5), Bi (10), and Ni (5). In 1997, Minorco Services BV entered into exploration agreements with the Pakistani authorities, the terms of which included the re-analysis of stored duplicate samples
Fig. 1. Location plan — northern Pakistan drainage survey.
for a more comprehensive multi-element suite and to lower detection limits where appropriate, and an interpretation of the results. Accordingly, some 4120 panned concentrate samples were analysed for Au, Pd and Pt to a 1 ppb detection limit and a similar number of <200 µm samples were analysed for Au to a 1 ppb detection limit and a further 31 major and trace elements by Ultra Trace Laboratories in Perth, Western Australia. The results are interpreted in terms of the geology and mineralisation in the area and observations made with regard to the sampling techniques applicable in regional drainage surveys in mountainous terrain.
2. Geology and metallogeny On a regional tectonic scale north Pakistan straddles the collision zone between the northern Eurasian and southern Indian continental plates (Fig. 2). The Kohistan island arc, bounded north by the Northern Suture Zone (NSZ) and south along the Main Mantle Thrust (MMT), is sandwiched between the plates. Following the Late Jurassic breakup of Gondwanaland and rapid northwards drift of the rifted Indo–Pakistan plate, two north-dipping subduction zones developed. A volcano-plutonic Andean-type arc formed in the Eurasian Plate within Palaeozoic siliciclastic and carbonate platform sediments and submarine volcanics in the Early Cretaceous (115 Ma). Today the roots of the arc outcrop north of the NSZ as the elongate Karakoram Batholith. In the ancient Tethys Ocean (south of Eurasia) the Kohistan island arc developed and the Kamila Amphibolite and Jaglot Group sediments represent parts of this arc. Between 80 and 100 Ma the evolved island arc accreted to Eurasia along the NSZ and the tectonic setting switched to an Andean-type margin. Ongoing subduction resulted in the Kohistan– Ladakh Batholith and calc-alkaline Dir Volcanics,
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Fig. 2. Regional geology, north Pakistan.
deep- and shallow-level coeval igneous phases, respectively (Searle, 1991). The Indian Plate collided with the Eurasian Plate along the MMT in the Eocene (55 Ma) and subduction ceased. However, ongoing compression to the present ‘underplated’ the former north beneath the latter (‘a-subduction’) for up to 500 km. Associated uplift is in the order of 20–40 km in the area north of the MMT and the outcropping rocks are of greenschist to lower amphibolite metamorphic grade with pervasive ductile deformation fabrics. Uplift is greatest around the Nanga Parbat syntaxis; uplift is less on the flanks of the province towards the Afghanistan and Indian borders. Anatectic garnetand muscovite-bearing S-type leucogranites (e.g. Tirch Mir Batholith) and pegmatites formed from partial melting of the underplated Indian Plate. In the upper crust, decoupling and southwards obduction is concentrated along major thrust faults, e.g. Main Boundary Thrust, (MBT) and older rocks commonly overlie younger rocks (Treloar et al., 1989). The present-day landsurface is a product of widespread mechanical weathering and glaciation and erosion levels are deep, in the order of 20–40 km. The major control on the metallogeny of north
Pakistan is the late collision-related uplift tectonic event. The dominant and widespread alteration and mineralisation event is associated with upper greenschist metamorphism. Tensional dilation zones on major reactivated thrust faults, e.g. NSZ and Reshun Fault (RF), are the loci for metamorphic fluids and Pb–Sb–Cu–(Au) deposits, e.g. Kaldam Gol, Awerith Gol–Sewakht. The deposits typically contain white comb quartz C chlorite C epidote C ankerite C sericite C calcite. Chalcopyrite, galena, stibnite, and sphalerite are common ore minerals with rare tetrahedrite and boulangerite. These structurally controlled metamorphogenic deposits account for up to 70% of the mineralisation in north Pakistan but have minimal tonnage and grade potential (Sillitoe, 1979). Intrusive rocks are common throughout the area but are typically equigranular and medium- to coarse-grained plutonic and foliated. Minor porphyry dyke phases are present at Dommel Nissar and Shagri Bala, south adjacent to the NSZ on the flanks of the province (where uplift and erosion are lower), with weak sporadic diopside–magnetite–andradite (prograde) and epidote–actinolite (retrograde) skarn phases. In the Eurasian Plate in the north, narrow orpiment and realgar vein occurrences (with fluorite)
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are scattered along the Tirch Mir Fault (TMF) but associated Carlin-style silicification, decalcification, carbonisation, brecciation and sulphides are lacking. Ophiolite slivers derived from Tethyan oceanic crust are localised along the MMT and NSZ and have affinities with Alpine-type ultramafics. Narrow associated podiform chromite lenses to 70 m length contain weakly anomalous platinum group elements (PGE) and Ni–Cr at Malakand and Manidara. The Chilas Complex, a large 300 km long ð 40 km wide gabbro-diorite intrusive, hosts several small layered anorthosite-mafic bodies at Chilas. The origin of the complex is unresolved; it has variably been interpreted as forming the ‘root’ zone to the Kohistan Island Arc or as a later rift-related intrusive in a forearc setting. Worldwide, large layered mafic– ultramafic intrusions, e.g. Bushveld and Stillwater, host large PGE deposits in an intracratonic rift setting; in contrast, the Chilas dunite bodies are small and intracontinental rifting is absent. Sulphide-rich feeder zones to intrusive bodies are an exploration target and talus sampling at Thurly Nala has confirmed geochemically anomalous PGE coincident with minor Cu mineralisation.
3. Geomorphology The area is characterised by extreme topographic relief, extending from the Karakoram Mountain Range, which includes peak elevations over 8000 m, to the Indus River valley, which ranges upwards from elevations of about 800 m in the project area. Although the area has a semiarid climate, much of the higher ground has permanent snow cover and active glaciers, the meltwaters from which are the source of an extensive drainage system flowing in a southwesterly direction to the Arabian Sea. Geochemical dispersion patterns are potentially complicated by the reworking of earlier sediments which form extensive terraces on the river valley margins. This material includes deposits of fluviatile, lacustrine, morainic, glaciofluvial and fanglomeratic origin and ranges in age from pre-Pleistocene, through three separate periods of Pleistocene glaciation (Searle, 1991), extending to Recent valley fill sediments.
4. Analysis and interpretation The re-analysis was conducted by Ultra Trace Analytical Laboratories of Perth, Western Australia, using fire assay=ICP–OES for Au (1 ppb), Pd and Pt (5 ppb) on the panned concentrates and a combination of ICP–OES and ICP–MS for the <200 µm fraction samples. Elements determined were Ag (0.2), As (0.5), Au (1 ppb), Ba (1), Bi (0.1), Ca (10), Co (1), Cr (5), Cu (1), Fe (10), Hg (10 ppb), K (20), Mg (10), Mn (1), Mo (0.2), Na (10), Nb (0.5), Ni (1), P (10), Pb (1), Rb (0.02), Sb (0.2), Se (1), Sn (1), Sr (0.1), Te (0.2), Table 1 Summary statistics — northern Pakistan drainage samples; results for panned concentrates designated ‘p.c.’ Element
Minimum
Maximum
Median
Ag, ppm As, ppm Au, ppb Au (p.c.), ppb Ba, ppm Bi, ppm Ca, % Cd, ppm Co, ppm Cr, ppm Cu, ppm Fe, % Hg, ppb K, % Mg, % Mn, ppm Mo, ppm Na, % Nb, ppm Ni, ppm P, ppm Pb, ppm Pd (p.c.), ppb Pt (p.c.), ppb Rb, ppm Sb, ppm Se, ppm Sn, ppm Sr, ppm Te, ppm Ti, % Tl, ppm V, ppm W, ppm Zn, ppm
<0.2 <0.5 <1 <1 20 <0.1 0.17 <0.2 2 <5 <1 0.18 <10 0.1 0.15 50 <0.2 <0.1 0.5 <1 20 <1 <5 <5 1.7 <0.2 <1 <1 32 <0.2 0.02 <0.2 4 <0.5 8
5.8 683 2,759 480,000 7,200 75 32.1 2 140 3,700 1,900 36.7 14,000 4.37 19.2 3,800 35 4.38 350 2,500 21,000 1,000 118 2,315 450 523 11 33 4,700 8.2 2.37 2.6 1,000 297 440
<0.2 8.5 1 7 320 0.3 3.96 <0.2 21 65 38 4.91 <10 1.6 1.78 950 1 1.71 10 36 890 19 <5 <5 84 0.4 1 2 240 <0.2 0.49 0.4 140 1.5 82
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Ti (10), Tl (0.1), V (2), W (0.5) and Zn (1), all from a hydrochloric–nitric–perchloric–hydrofluoric acid digestion, except for Au and Hg, for which an aqua regia digestion was used. The analytical detection limits, given in brackets, are expressed in ppm unless shown otherwise. Summary statistics for the results obtained are given in Table 1. Results obtained for a suite of standards submitted with the <200 µm fraction samples indicated excellent precision (within 10% at the 95% confidence level) for As, Ba, Cr, K, Mg, Mn, Mo, Na, Nb, Pb, Ti and V. Good precision (10–30%) was recorded for Au, Ca, Co, Cu, Fe, Ni, P, Rb, Sn, Sr, W and Zn. Precision ratings for Hg, Sb and Se were of a lower order but still acceptable, given that the concentration levels for these elements (and Ag, Bi, Cd, Te and Tl) in the standard samples were close to, or below, the analytical detection limit. The analytical results were merged with sample location coordinates supplied by the SDA and
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PMDC for statistical analysis and plotting. The regional distribution of all elements was compared with geology by means of a series of image plots, prepared after extrapolation of the data onto a 10 by 10 km grid. Results below the detection limit were arbitrarily assigned values of 40% of the limit. Potassium, Na, Mg, Ca, Rb and Sr were considered as the ratios K=Na, Mg=Ca and Rb=Sr for statistical and plotting purposes, on the basis that these elements have an antipathetic relationship in most geological situations, and the ratios serve to eliminate fluctuations due to variable weathering intensity.
5. Geochemical dispersion patterns The dispersion patterns for a number of elements closely reflect aspects of the regional geology. The Eurasian and Indian plates are both characterised by
Fig. 3. Distribution of Pb in <200 µm sediments in relation to major tectonic features, northern Pakistan.
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Fig. 4. Distribution of V in <200 µm sediments in relation to major tectonic features, northern Pakistan.
high background values for Ba, Pb (Fig. 3), Sn, Tl and the ratios K=Na and Rb=Sr. In addition, As is strongly enriched in the Eurasian Plate. The Kohistan and Ladakh arcs are highlighted by elevated values for Co, Cu, Fe, Mn and V (Fig. 4). A series of elongate patterns of enrichment for Cr, Ni (Fig. 5) and the ratio Mg=Ca delineate the NSZ and the MMT. The mafic–ultramafic Chilas Complex and Kamila Amphibolite are also apparent as a broad zone of elevated Ni immediately to the north of the western MMT in Fig. 5. These relationships are well summarised by the results of factor analysis. The first two factors derived clearly relate to the three regional geological units and the structures which separate them, respectively. Factor 1 is dominated by negative loadings for the elements enriched in the island arc environment (Fe–V–Zn–Mn–Ti), so the Kohistan–Ladakh arc appears as an area of low scores for Factor 1. Similarly, Factor 2 has high
negative loadings for Ni–Cr–Mg=Ca–Co and negative anomalies coincide with ophiolite slivers derived from oceanic crust which trace the main thrust faults. Factor analysis highlighted a further eight factors, at least five of which are construed as representing mineralisation. These include Factor 3 (W–Bi–As– Mo–Ag) which highlights known skarn mineralisation associated with porphyry dykes near Skardu, Factor 5 (Cu–Mo–Te–Pd–Cd) for which high scores again occur near Skardu, with porphyry dykes, magnetite and retrograde skarn at Dommel Nissar and in a zone to the north of Gilgit, and Factor 6 (Sb–Pb–Ag–Cu) with peaks near Chitral and Abbottabad. In the Chitral area, small fault-controlled mesothermal Pb–Sb–Au–Ag deposits are the source. Factor 9 combines pathfinder elements typical of the sediment-hosted gold association (Hg–Cd–Zn–As– K=Na–Ag), with high scores correlating with orpiment occurrences along the Tirch Mir Fault and its
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Fig. 5. Distribution of Ni in <200 µm sediments in relation to major tectonic features, northern Pakistan.
apparent eastern extension in the Chapursan Valley. In most cases, mineralisation has been confirmed in the anomalous areas, but economic discoveries have so far proved elusive. Elevated values for Au are widespread, particularly in panned concentrate samples, which range from less than 1 ppb to a maximum concentration of 480 ppm (Fig. 6). Despite such high concentration levels for Au, there is little correlation, either spatial or statistical, with pathfinder response (As, Sb, Hg, Hg etc.) and evidence for the presence of significant primary gold mineralisation in the area is lacking. Factor 8 has high loadings for Au, both in panned concentrates and the <200 µm fraction, but is associated predominantly with other elements of detrital habit (Pt and Pd) rather than those commonly found in primary gold deposits. Initially, samples were collected to represent catchments of 10 to 50 sq. km in area. However,
lack of access dictated that larger catchments were sampled in some cases, while subsequent detailed follow-up has led to an extreme variation in the areas of catchments represented in the database. These range from 0.2 to over 1500 sq. km. Statistical studies have demonstrated that background abundances for many elements are strongly affected by variations in catchment area. Barium, Co, Cu, Mo, Ni and Zn concentrations decrease with increasing catchment size, which is the normal pattern of downstream dilution. Fig. 7 shows the frequency distribution of Cu in subsets based on catchment area. The distributions become more bunched with increasing catchment size and notional threshold values decrease from approximately 90 ppm Cu in the smallest catchments, to about 50 ppm Cu in the major drainage basins. Arsenic, Au, Cr, Nb, Pd, Pt, Sb, Sn, V and W show the reverse tendency, with enrichment in larger
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Fig. 6. Distribution of Au in panned concentrates in relation to major tectonic features, northern Pakistan.
catchments. This behaviour is typical for elements which are dispersed in heavy mineral phases and was magnified in this survey by the sampling of trap sites, which are loci for the hydraulic concentration of heavy minerals. The distribution for Au in panned concentrates is an extreme case (Fig. 8). The observed distributions represent patterns of concentration, rather than dispersion patterns. This effect is also shown by Au in the <200 µm fraction, although to a lesser extent, the modal value remaining <1 ppb Au throughout all four catchment subsets (Fig. 9).
6. Discussion The regional survey has generated a valuable database for mineral exploration in northern Pakistan, particularly now that data are available for a wide
range of elements at levels of concentration which enable characterisation of background and anomalous populations. The strong correlation between both univariate and multivariate geochemical data with regional geology provides confidence in the relevance of the geochemical data to mineral exploration. Dispersion patterns for many ore-related and pathfinder elements are related to known mineralisation while others delineate potential targets for future exploration. Some doubt exists as to the significance of the strongly anomalous results for Au, particularly as measured in panned concentrate samples. The high Au abundances recorded for such a large proportion of the panned concentrates collected from major drainage channels are inconsistent with the known or suspected endowment for primary gold mineralisation. In particular, the distinct populations of samples
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Fig. 8. Frequency histograms for Au (panned concentrates) in subsets based on catchment area (logarithmic values). Fig. 7. Frequency histograms for Cu in subsets based on catchment area.
from catchments greater than 100 square kilometres in an area with Au concentrations of 500 to 100,000 ppb (Fig. 8), suggest patterns of concentration rather than dispersion patterns related to primary gold deposits. The sampling of panned concentrates from trap sites was no doubt intended to enhance the geochemical expression of mineralisation for elements which are dispersed as heavy minerals. However, for the precious metals, which are dominated by native metal grains of extreme density, the collection of panned concentrates from trap sites has highlighted placer concentrations of Au from widely dispersed sources, both in a spatial and temporal context. Tak-
ing into account the recycling of erosional products through terrace deposits dating back to the Tertiary, it is quite likely that many of the Au anomalies recorded in this survey relate to primary deposits which were eroded in the early stages of the 20–40 km of uplift to which the area has been subjected. Apart from the doubt which exists about the relationship between panned concentrate samples from trap sites and primary gold deposits, Au results in panned concentrates are subject to extreme sampling error. This is due in large measure to the loss of fine gold particles during panning, resulting in a bias towards fewer and larger particles (>50 µm) of native gold in the sample taken. This leads to increased
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error. A brief orientation study at Dommel Nissar has confirmed that high contrast anomalies for Au and Cu associated with porphyry skarn Cu–Au mineralisation can be detected by analysing the <75 µm fraction from low-energy sediments, without the need to sample trap sites (Fig. 10).
7. Conclusions
Fig. 9. Frequency histograms for Au (<200 µm fraction) in subsets based on catchment area (logarithmic values).
sampling error both at the sample site and in subsequent sub-sampling for chemical analysis. The latter effect is clearly evident in replicate analyses. Fletcher (1997) has demonstrated the technical merits of sampling the fine fraction of stream sediments in drainage geochemical surveys for Au. Such techniques retain the very fine gold particles (down to 1 µm), whose hydraulic behaviour is more closely related to the dominant silicate minerals in drainage systems. Sampling of fine fraction sediments not only results in homogeneous and extensive dispersion patterns, similar to those for elements such as Cu, which are dispersed with silt and clay fractions, but greatly reduces the problem with sampling
The results from this survey demonstrate the close relationship between geochemical dispersion patterns and regional geological features. In some cases the links override simple lithological relationships and embrace the larger tectonic units present in the area. In mountainous terrain like the Himalayas, as in other environments, geochemical dispersion patterns can be expected to provide information relevant to geology as well as mineralisation. The collection of panned concentrate samples from heavy mineral trap sites may enhance the signature of distant mineralisation, dispersed as heavy minerals, but has the undesirable effect of highlighting patterns of alluvial concentration for precious metals like Au, which may bear no relationship to actively eroding primary gold mineralisation. The sampling of ultrafine (<75 µm) low-energy sediments is an effective alternative to panned concentrates in geochemical drainage surveys, and provides more valid anomalies for the precious metals as well as being a suitable medium for the measurement of all other elements of interest.
Acknowledgements The authors acknowledge Minorco Services BV for encouragement and permission to publish the results of this study. The staff of the Pakistan Mineral Development Corporation and the Sarhad Development Authority made a substantial contribution to this investigation by their diligence and persistence in sampling, often under harsh conditions. Their role is gratefully acknowledged and in particular we wish to express our appreciation to Shakirullah, of the Sarhad Development Authority, for helpful discussions and guidance during field work.
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Fig. 10. Size fraction distribution for Au and Cu in six orientation samples, compared to GEMAP project results (<200 µm fraction), plotted in composite histogram format.
References Fletcher, W.K., 1997. Stream sediment geochemistry in today’s exploration world. In: Gubins, A.G. (Ed.), Proceedings of Exploration ’97: 4th Decennial International Conference on Mineral Exploration, pp. 249–260. Searle, M.P., 1991. Geology and Tectonics of the Karakoram Mountains. Wiley, Chichester, 358 pp. Sillitoe, R.H., 1979. Speculations on Himalayan metallogeny based on evidence from Pakistan. In: Farah, A., De Jong,
K.A. (Eds.), Geodynamics of Pakistan. Geological Survey of Pakistan, Quetta, pp. 167–180. Sweatman, T.R., Clavarino, J.G., Dawney, R.L., 1995. Drainage geochemical exploration and mineral potential of Northern Pakistan. Third Report. Report by Rex Sweatman and Associates for Australian Agency for International Development. Treloar, P.J., Williams, M.P., Coward, M.P., 1989. Metamorphism and structural stacking in the North Indian Plate, North Pakistan. Tectonophysics 165, 167–184.
Journal of Geochemical Exploration 67 (1999) 223–233 www.elsevier.com/locate/jgeoexp
Regional multi-element drainage geochemistry in the Himalayan Mountains, northern Pakistan Rohan Halfpenny a,1 , Richard H. Mazzucchelli b,* a
Minorco Services, BV Budapest, Hungary b Searchtech Pty Ltd., Perth, Australia Accepted 2 September 1999
Abstract Drainage reconnaissance sampling was initiated in northern Pakistan in 1992 as part of an Australian aid project and later extended by Pakistani agencies, achieving a coverage of some 100,000 square kilometres by 1997. Panned concentrate and <200 µm fraction (minus 80 mesh) samples were taken at varying sampling densities from some 4260 sites in the Northwest Frontier Province and Northern Areas, and analysed for Au, Bi, Co, Cu, Ni, Pb and Zn by atomic absorption spectrophotometry. In 1997, Minorco Services BV re-analysed the accumulated samples for a more comprehensive element suite, using methods with enhanced sensitivity for the previously analysed elements. This paper presents an interpretation of the new results in relation to regional geology and mineralisation. The survey encompasses the collision zone between the Eurasian and the Indian plates, and an intervening Cretaceous–Tertiary island arc system (the Kohistan and Ladakh arcs). These three main tectonic crustal units are separated by two major thrust faults, the Northern Suture Zone (NSZ) and the Main Mantle Thrust (MMT), which are zones of shearing up to 4 km wide that incorporate oceanic ophiolitic volcanics and alpine serpentinite intrusions. The area is characterised by extreme topographic relief, with peak elevations over 8000 m. Despite the semiarid climate, much of the higher ground has permanent snow cover and active glaciers, the meltwaters from which are the source of the extensive Indus River drainage system. Compression associated with the northward movement of the Indian Plate has resulted in 20 to 40 km of uplift since the Eocene (55 Ma), and erosion levels are deep. The remnants of former erosional episodes are retained in the form of various types of terrace deposits within river valleys. Aspects of the regional geology are well reflected in the multi-element geochemistry. The Eurasian and Indian plates are both characterised by high background values for Ba, Pb, Sn, Tl and the ratios K=Na and Rb=Sr. The Kohistan–Ladakh arcs are highlighted by elevated values for Co, Cu, Fe, Mn and V. A series of elongate patterns of enrichment for Cr, Ni and the ratio Mg=Ca delineate the NSZ and MMT. These relationships are well summarised by the results of principal component analysis, the first two factors derived clearly relating to the three regional geological units and the structures which separate them, respectively. Other factors can be interpreted to represent different styles of mineralisation, although no significant occurrences have been located to date. The results for some elements are influenced by variations in catchment area. Median concentrations for Ba, Co, Cu, Mo, Ni and Zn decrease with increasing catchment size, which is the normal pattern for downstream dilution. Arsenic, Au, Cr, Nb, Pd, Pt, Sb, Sn, V and W show the reverse tendency, i.e. enrichment in larger catchments. This behaviour is typical for elements which are dispersed in heavy mineral phases and is magnified by the sampling of trap sites, which are the loci for hydraulic concentration of heavy minerals. In the case of Au in panned concentrates, the extreme concentrations encountered (up to 480 ppm) are considered to represent 1
Current address: Anglo American plc, London, U.K. author. Fax: C61-8-9257-2334; E-mail: [email protected]
Ł Corresponding
0375-6742/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 6 7 4 2 ( 9 9 ) 0 0 0 6 9 - 2
224
R. Halfpenny, R.H. Mazzucchelli / Journal of Geochemical Exploration 67 (1999) 223–233
patterns of concentration, rather than dispersion, of Au, some of which may be derived from bedrock sources which have been completely eroded. A brief orientation study suggests that more meaningful dispersion patterns could be delineated by sampling and analysis of the <75 µm fraction of low-energy sediments, rather than trap sites. 1999 Elsevier Science B.V. All rights reserved. Keywords: geochemical exploration; heavy mineral; stream sediments
1. Introduction A regional drainage reconnaissance survey was initiated in the Karakoram region of northern Pakistan in 1992, as part of an Australian aid programme (Sweatman et al., 1995) and later extended by teams from the Pakistan Mineral Development Corporation (PMDC) and the Sarhad Development Authority (SDA). During the course of this programme, designated ‘GEMAP’, drainage sediments were sampled over an area of some 100,000 square kilometres in northern Pakistan (Fig. 1) and both the <200 µm ( 80 mesh) fraction and panned concentrates analysed by the Sarhad Development Authority Mineral Testing Laboratory in Peshawar for the following elements (with detection limits in ppm shown in parentheses): Au (0.05), Cu (5), Pb (5), Zn (5), Ag (0.5), Co (5), Bi (10), and Ni (5). In 1997, Minorco Services BV entered into exploration agreements with the Pakistani authorities, the terms of which included the re-analysis of stored duplicate samples
Fig. 1. Location plan — northern Pakistan drainage survey.
for a more comprehensive multi-element suite and to lower detection limits where appropriate, and an interpretation of the results. Accordingly, some 4120 panned concentrate samples were analysed for Au, Pd and Pt to a 1 ppb detection limit and a similar number of <200 µm samples were analysed for Au to a 1 ppb detection limit and a further 31 major and trace elements by Ultra Trace Laboratories in Perth, Western Australia. The results are interpreted in terms of the geology and mineralisation in the area and observations made with regard to the sampling techniques applicable in regional drainage surveys in mountainous terrain.
2. Geology and metallogeny On a regional tectonic scale north Pakistan straddles the collision zone between the northern Eurasian and southern Indian continental plates (Fig. 2). The Kohistan island arc, bounded north by the Northern Suture Zone (NSZ) and south along the Main Mantle Thrust (MMT), is sandwiched between the plates. Following the Late Jurassic breakup of Gondwanaland and rapid northwards drift of the rifted Indo–Pakistan plate, two north-dipping subduction zones developed. A volcano-plutonic Andean-type arc formed in the Eurasian Plate within Palaeozoic siliciclastic and carbonate platform sediments and submarine volcanics in the Early Cretaceous (115 Ma). Today the roots of the arc outcrop north of the NSZ as the elongate Karakoram Batholith. In the ancient Tethys Ocean (south of Eurasia) the Kohistan island arc developed and the Kamila Amphibolite and Jaglot Group sediments represent parts of this arc. Between 80 and 100 Ma the evolved island arc accreted to Eurasia along the NSZ and the tectonic setting switched to an Andean-type margin. Ongoing subduction resulted in the Kohistan– Ladakh Batholith and calc-alkaline Dir Volcanics,
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Fig. 2. Regional geology, north Pakistan.
deep- and shallow-level coeval igneous phases, respectively (Searle, 1991). The Indian Plate collided with the Eurasian Plate along the MMT in the Eocene (55 Ma) and subduction ceased. However, ongoing compression to the present ‘underplated’ the former north beneath the latter (‘a-subduction’) for up to 500 km. Associated uplift is in the order of 20–40 km in the area north of the MMT and the outcropping rocks are of greenschist to lower amphibolite metamorphic grade with pervasive ductile deformation fabrics. Uplift is greatest around the Nanga Parbat syntaxis; uplift is less on the flanks of the province towards the Afghanistan and Indian borders. Anatectic garnetand muscovite-bearing S-type leucogranites (e.g. Tirch Mir Batholith) and pegmatites formed from partial melting of the underplated Indian Plate. In the upper crust, decoupling and southwards obduction is concentrated along major thrust faults, e.g. Main Boundary Thrust, (MBT) and older rocks commonly overlie younger rocks (Treloar et al., 1989). The present-day landsurface is a product of widespread mechanical weathering and glaciation and erosion levels are deep, in the order of 20–40 km. The major control on the metallogeny of north
Pakistan is the late collision-related uplift tectonic event. The dominant and widespread alteration and mineralisation event is associated with upper greenschist metamorphism. Tensional dilation zones on major reactivated thrust faults, e.g. NSZ and Reshun Fault (RF), are the loci for metamorphic fluids and Pb–Sb–Cu–(Au) deposits, e.g. Kaldam Gol, Awerith Gol–Sewakht. The deposits typically contain white comb quartz C chlorite C epidote C ankerite C sericite C calcite. Chalcopyrite, galena, stibnite, and sphalerite are common ore minerals with rare tetrahedrite and boulangerite. These structurally controlled metamorphogenic deposits account for up to 70% of the mineralisation in north Pakistan but have minimal tonnage and grade potential (Sillitoe, 1979). Intrusive rocks are common throughout the area but are typically equigranular and medium- to coarse-grained plutonic and foliated. Minor porphyry dyke phases are present at Dommel Nissar and Shagri Bala, south adjacent to the NSZ on the flanks of the province (where uplift and erosion are lower), with weak sporadic diopside–magnetite–andradite (prograde) and epidote–actinolite (retrograde) skarn phases. In the Eurasian Plate in the north, narrow orpiment and realgar vein occurrences (with fluorite)
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are scattered along the Tirch Mir Fault (TMF) but associated Carlin-style silicification, decalcification, carbonisation, brecciation and sulphides are lacking. Ophiolite slivers derived from Tethyan oceanic crust are localised along the MMT and NSZ and have affinities with Alpine-type ultramafics. Narrow associated podiform chromite lenses to 70 m length contain weakly anomalous platinum group elements (PGE) and Ni–Cr at Malakand and Manidara. The Chilas Complex, a large 300 km long ð 40 km wide gabbro-diorite intrusive, hosts several small layered anorthosite-mafic bodies at Chilas. The origin of the complex is unresolved; it has variably been interpreted as forming the ‘root’ zone to the Kohistan Island Arc or as a later rift-related intrusive in a forearc setting. Worldwide, large layered mafic– ultramafic intrusions, e.g. Bushveld and Stillwater, host large PGE deposits in an intracratonic rift setting; in contrast, the Chilas dunite bodies are small and intracontinental rifting is absent. Sulphide-rich feeder zones to intrusive bodies are an exploration target and talus sampling at Thurly Nala has confirmed geochemically anomalous PGE coincident with minor Cu mineralisation.
3. Geomorphology The area is characterised by extreme topographic relief, extending from the Karakoram Mountain Range, which includes peak elevations over 8000 m, to the Indus River valley, which ranges upwards from elevations of about 800 m in the project area. Although the area has a semiarid climate, much of the higher ground has permanent snow cover and active glaciers, the meltwaters from which are the source of an extensive drainage system flowing in a southwesterly direction to the Arabian Sea. Geochemical dispersion patterns are potentially complicated by the reworking of earlier sediments which form extensive terraces on the river valley margins. This material includes deposits of fluviatile, lacustrine, morainic, glaciofluvial and fanglomeratic origin and ranges in age from pre-Pleistocene, through three separate periods of Pleistocene glaciation (Searle, 1991), extending to Recent valley fill sediments.
4. Analysis and interpretation The re-analysis was conducted by Ultra Trace Analytical Laboratories of Perth, Western Australia, using fire assay=ICP–OES for Au (1 ppb), Pd and Pt (5 ppb) on the panned concentrates and a combination of ICP–OES and ICP–MS for the <200 µm fraction samples. Elements determined were Ag (0.2), As (0.5), Au (1 ppb), Ba (1), Bi (0.1), Ca (10), Co (1), Cr (5), Cu (1), Fe (10), Hg (10 ppb), K (20), Mg (10), Mn (1), Mo (0.2), Na (10), Nb (0.5), Ni (1), P (10), Pb (1), Rb (0.02), Sb (0.2), Se (1), Sn (1), Sr (0.1), Te (0.2), Table 1 Summary statistics — northern Pakistan drainage samples; results for panned concentrates designated ‘p.c.’ Element
Minimum
Maximum
Median
Ag, ppm As, ppm Au, ppb Au (p.c.), ppb Ba, ppm Bi, ppm Ca, % Cd, ppm Co, ppm Cr, ppm Cu, ppm Fe, % Hg, ppb K, % Mg, % Mn, ppm Mo, ppm Na, % Nb, ppm Ni, ppm P, ppm Pb, ppm Pd (p.c.), ppb Pt (p.c.), ppb Rb, ppm Sb, ppm Se, ppm Sn, ppm Sr, ppm Te, ppm Ti, % Tl, ppm V, ppm W, ppm Zn, ppm
<0.2 <0.5 <1 <1 20 <0.1 0.17 <0.2 2 <5 <1 0.18 <10 0.1 0.15 50 <0.2 <0.1 0.5 <1 20 <1 <5 <5 1.7 <0.2 <1 <1 32 <0.2 0.02 <0.2 4 <0.5 8
5.8 683 2,759 480,000 7,200 75 32.1 2 140 3,700 1,900 36.7 14,000 4.37 19.2 3,800 35 4.38 350 2,500 21,000 1,000 118 2,315 450 523 11 33 4,700 8.2 2.37 2.6 1,000 297 440
<0.2 8.5 1 7 320 0.3 3.96 <0.2 21 65 38 4.91 <10 1.6 1.78 950 1 1.71 10 36 890 19 <5 <5 84 0.4 1 2 240 <0.2 0.49 0.4 140 1.5 82
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Ti (10), Tl (0.1), V (2), W (0.5) and Zn (1), all from a hydrochloric–nitric–perchloric–hydrofluoric acid digestion, except for Au and Hg, for which an aqua regia digestion was used. The analytical detection limits, given in brackets, are expressed in ppm unless shown otherwise. Summary statistics for the results obtained are given in Table 1. Results obtained for a suite of standards submitted with the <200 µm fraction samples indicated excellent precision (within 10% at the 95% confidence level) for As, Ba, Cr, K, Mg, Mn, Mo, Na, Nb, Pb, Ti and V. Good precision (10–30%) was recorded for Au, Ca, Co, Cu, Fe, Ni, P, Rb, Sn, Sr, W and Zn. Precision ratings for Hg, Sb and Se were of a lower order but still acceptable, given that the concentration levels for these elements (and Ag, Bi, Cd, Te and Tl) in the standard samples were close to, or below, the analytical detection limit. The analytical results were merged with sample location coordinates supplied by the SDA and
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PMDC for statistical analysis and plotting. The regional distribution of all elements was compared with geology by means of a series of image plots, prepared after extrapolation of the data onto a 10 by 10 km grid. Results below the detection limit were arbitrarily assigned values of 40% of the limit. Potassium, Na, Mg, Ca, Rb and Sr were considered as the ratios K=Na, Mg=Ca and Rb=Sr for statistical and plotting purposes, on the basis that these elements have an antipathetic relationship in most geological situations, and the ratios serve to eliminate fluctuations due to variable weathering intensity.
5. Geochemical dispersion patterns The dispersion patterns for a number of elements closely reflect aspects of the regional geology. The Eurasian and Indian plates are both characterised by
Fig. 3. Distribution of Pb in <200 µm sediments in relation to major tectonic features, northern Pakistan.
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Fig. 4. Distribution of V in <200 µm sediments in relation to major tectonic features, northern Pakistan.
high background values for Ba, Pb (Fig. 3), Sn, Tl and the ratios K=Na and Rb=Sr. In addition, As is strongly enriched in the Eurasian Plate. The Kohistan and Ladakh arcs are highlighted by elevated values for Co, Cu, Fe, Mn and V (Fig. 4). A series of elongate patterns of enrichment for Cr, Ni (Fig. 5) and the ratio Mg=Ca delineate the NSZ and the MMT. The mafic–ultramafic Chilas Complex and Kamila Amphibolite are also apparent as a broad zone of elevated Ni immediately to the north of the western MMT in Fig. 5. These relationships are well summarised by the results of factor analysis. The first two factors derived clearly relate to the three regional geological units and the structures which separate them, respectively. Factor 1 is dominated by negative loadings for the elements enriched in the island arc environment (Fe–V–Zn–Mn–Ti), so the Kohistan–Ladakh arc appears as an area of low scores for Factor 1. Similarly, Factor 2 has high
negative loadings for Ni–Cr–Mg=Ca–Co and negative anomalies coincide with ophiolite slivers derived from oceanic crust which trace the main thrust faults. Factor analysis highlighted a further eight factors, at least five of which are construed as representing mineralisation. These include Factor 3 (W–Bi–As– Mo–Ag) which highlights known skarn mineralisation associated with porphyry dykes near Skardu, Factor 5 (Cu–Mo–Te–Pd–Cd) for which high scores again occur near Skardu, with porphyry dykes, magnetite and retrograde skarn at Dommel Nissar and in a zone to the north of Gilgit, and Factor 6 (Sb–Pb–Ag–Cu) with peaks near Chitral and Abbottabad. In the Chitral area, small fault-controlled mesothermal Pb–Sb–Au–Ag deposits are the source. Factor 9 combines pathfinder elements typical of the sediment-hosted gold association (Hg–Cd–Zn–As– K=Na–Ag), with high scores correlating with orpiment occurrences along the Tirch Mir Fault and its
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Fig. 5. Distribution of Ni in <200 µm sediments in relation to major tectonic features, northern Pakistan.
apparent eastern extension in the Chapursan Valley. In most cases, mineralisation has been confirmed in the anomalous areas, but economic discoveries have so far proved elusive. Elevated values for Au are widespread, particularly in panned concentrate samples, which range from less than 1 ppb to a maximum concentration of 480 ppm (Fig. 6). Despite such high concentration levels for Au, there is little correlation, either spatial or statistical, with pathfinder response (As, Sb, Hg, Hg etc.) and evidence for the presence of significant primary gold mineralisation in the area is lacking. Factor 8 has high loadings for Au, both in panned concentrates and the <200 µm fraction, but is associated predominantly with other elements of detrital habit (Pt and Pd) rather than those commonly found in primary gold deposits. Initially, samples were collected to represent catchments of 10 to 50 sq. km in area. However,
lack of access dictated that larger catchments were sampled in some cases, while subsequent detailed follow-up has led to an extreme variation in the areas of catchments represented in the database. These range from 0.2 to over 1500 sq. km. Statistical studies have demonstrated that background abundances for many elements are strongly affected by variations in catchment area. Barium, Co, Cu, Mo, Ni and Zn concentrations decrease with increasing catchment size, which is the normal pattern of downstream dilution. Fig. 7 shows the frequency distribution of Cu in subsets based on catchment area. The distributions become more bunched with increasing catchment size and notional threshold values decrease from approximately 90 ppm Cu in the smallest catchments, to about 50 ppm Cu in the major drainage basins. Arsenic, Au, Cr, Nb, Pd, Pt, Sb, Sn, V and W show the reverse tendency, with enrichment in larger
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Fig. 6. Distribution of Au in panned concentrates in relation to major tectonic features, northern Pakistan.
catchments. This behaviour is typical for elements which are dispersed in heavy mineral phases and was magnified in this survey by the sampling of trap sites, which are loci for the hydraulic concentration of heavy minerals. The distribution for Au in panned concentrates is an extreme case (Fig. 8). The observed distributions represent patterns of concentration, rather than dispersion patterns. This effect is also shown by Au in the <200 µm fraction, although to a lesser extent, the modal value remaining <1 ppb Au throughout all four catchment subsets (Fig. 9).
6. Discussion The regional survey has generated a valuable database for mineral exploration in northern Pakistan, particularly now that data are available for a wide
range of elements at levels of concentration which enable characterisation of background and anomalous populations. The strong correlation between both univariate and multivariate geochemical data with regional geology provides confidence in the relevance of the geochemical data to mineral exploration. Dispersion patterns for many ore-related and pathfinder elements are related to known mineralisation while others delineate potential targets for future exploration. Some doubt exists as to the significance of the strongly anomalous results for Au, particularly as measured in panned concentrate samples. The high Au abundances recorded for such a large proportion of the panned concentrates collected from major drainage channels are inconsistent with the known or suspected endowment for primary gold mineralisation. In particular, the distinct populations of samples
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Fig. 8. Frequency histograms for Au (panned concentrates) in subsets based on catchment area (logarithmic values). Fig. 7. Frequency histograms for Cu in subsets based on catchment area.
from catchments greater than 100 square kilometres in an area with Au concentrations of 500 to 100,000 ppb (Fig. 8), suggest patterns of concentration rather than dispersion patterns related to primary gold deposits. The sampling of panned concentrates from trap sites was no doubt intended to enhance the geochemical expression of mineralisation for elements which are dispersed as heavy minerals. However, for the precious metals, which are dominated by native metal grains of extreme density, the collection of panned concentrates from trap sites has highlighted placer concentrations of Au from widely dispersed sources, both in a spatial and temporal context. Tak-
ing into account the recycling of erosional products through terrace deposits dating back to the Tertiary, it is quite likely that many of the Au anomalies recorded in this survey relate to primary deposits which were eroded in the early stages of the 20–40 km of uplift to which the area has been subjected. Apart from the doubt which exists about the relationship between panned concentrate samples from trap sites and primary gold deposits, Au results in panned concentrates are subject to extreme sampling error. This is due in large measure to the loss of fine gold particles during panning, resulting in a bias towards fewer and larger particles (>50 µm) of native gold in the sample taken. This leads to increased
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error. A brief orientation study at Dommel Nissar has confirmed that high contrast anomalies for Au and Cu associated with porphyry skarn Cu–Au mineralisation can be detected by analysing the <75 µm fraction from low-energy sediments, without the need to sample trap sites (Fig. 10).
7. Conclusions
Fig. 9. Frequency histograms for Au (<200 µm fraction) in subsets based on catchment area (logarithmic values).
sampling error both at the sample site and in subsequent sub-sampling for chemical analysis. The latter effect is clearly evident in replicate analyses. Fletcher (1997) has demonstrated the technical merits of sampling the fine fraction of stream sediments in drainage geochemical surveys for Au. Such techniques retain the very fine gold particles (down to 1 µm), whose hydraulic behaviour is more closely related to the dominant silicate minerals in drainage systems. Sampling of fine fraction sediments not only results in homogeneous and extensive dispersion patterns, similar to those for elements such as Cu, which are dispersed with silt and clay fractions, but greatly reduces the problem with sampling
The results from this survey demonstrate the close relationship between geochemical dispersion patterns and regional geological features. In some cases the links override simple lithological relationships and embrace the larger tectonic units present in the area. In mountainous terrain like the Himalayas, as in other environments, geochemical dispersion patterns can be expected to provide information relevant to geology as well as mineralisation. The collection of panned concentrate samples from heavy mineral trap sites may enhance the signature of distant mineralisation, dispersed as heavy minerals, but has the undesirable effect of highlighting patterns of alluvial concentration for precious metals like Au, which may bear no relationship to actively eroding primary gold mineralisation. The sampling of ultrafine (<75 µm) low-energy sediments is an effective alternative to panned concentrates in geochemical drainage surveys, and provides more valid anomalies for the precious metals as well as being a suitable medium for the measurement of all other elements of interest.
Acknowledgements The authors acknowledge Minorco Services BV for encouragement and permission to publish the results of this study. The staff of the Pakistan Mineral Development Corporation and the Sarhad Development Authority made a substantial contribution to this investigation by their diligence and persistence in sampling, often under harsh conditions. Their role is gratefully acknowledged and in particular we wish to express our appreciation to Shakirullah, of the Sarhad Development Authority, for helpful discussions and guidance during field work.
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Fig. 10. Size fraction distribution for Au and Cu in six orientation samples, compared to GEMAP project results (<200 µm fraction), plotted in composite histogram format.
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K.A. (Eds.), Geodynamics of Pakistan. Geological Survey of Pakistan, Quetta, pp. 167–180. Sweatman, T.R., Clavarino, J.G., Dawney, R.L., 1995. Drainage geochemical exploration and mineral potential of Northern Pakistan. Third Report. Report by Rex Sweatman and Associates for Australian Agency for International Development. Treloar, P.J., Williams, M.P., Coward, M.P., 1989. Metamorphism and structural stacking in the North Indian Plate, North Pakistan. Tectonophysics 165, 167–184.