Lithogeochemistry of the Collahuasi porphyry Cu–Mo and ...

Lithogeochemistry of the Collahuasi porphyry Cu–Mo and epithermalCu–Ag (–Au) cluster, northern Chile: Pearce element ratio vectors to oreEsteban Urqueta 1,2 , T. Kurt Kyser 1,* , Alan H. Clark 1 , Clifford R. Stanley 3 &Christopher J. Oates 41 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, K7L 3N6,Canada2 Present address: Exploration Division, Anglo American Chile, Santiago, Chile3 Department of Geology, Acadia University, Wolfville, Nova Scotia, B4P 2R6, Canada4 Anglo-American plc, 20 Carlton Terrace, London, SW1Y 5AN, UK*Corresponding author (e-mail: kyser@geol.queensu.ca)Abstract: Lithogeochemical vectors to ore-bodies are obscured by the variety oflithological units and the alteration events that have affected them. In themagmatic-hydrothermal context, Pearce element ratio (PER) analysis can be acost-effective lithogeochemical technique that identifies and eliminates nonhydrothermalsources of variation, thus permitting the definition of material transferrelated only to hydrothermal alteration. Dedicated PER diagrams for mafic and felsiclithologies in the world-class porphyry Cu–Mo and epithermal Cu–Ag (–Au) clusterof the Collahuasi district, I Región, northern Chile, effectively model backgroundvariability and discriminate between fresh or propylitically altered rocks and thosewith a hydrolytic alteration overprint. Furthermore, PER diagrams allow for thedefinition of an alteration index (AI) that quantifies the degree of metasomaticexchange during hydrolytic alteration of a particular rock. Plots of different PERvalues against the AI show that potassium enrichment during hydrolytic alterationwas followed by potassium, calcium and sodium depletion as a result of thedestruction of feldspars during sericitic and later argillic alteration. Although PERplots record no mass transfer processes involving major elements during propyliticalteration, the integrated AI for both mafic and felsic units spatially defines the majoralteration centres within the district.KEYWORDS: lithogeochemistry, Pearce element ratio analysis, Collahuasi, porphyry Cu–Mo,hydrothermal alteration, metasomatismINTRODUCTIONLithogeochemical techniques have been widely applied in theexploration for hydrothermal ore deposits, particularly to determinethe effects of metasomatic alteration within and aroundmineralized centres. The basic exploration principle that supportsthis methodology as a cost-effective exploration tool isthat alteration haloes constitute larger targets than the depositsthemselves. This is particularly true for porphyry copperdeposits, where alteration halos are known to extend for severalkilometres (Gustafson & Hunt 1975; Padilla et al. 2004).Nonetheless, the analysis of a district-scale lithogeochemicaldataset represents a major challenge because the wide variety ofhost rocks and the complex interrelation between differentalteration events commonly obscure the metasomatic eventsthat directly accompanied the mineralization. The present studyuses the Pearce element ratio (PER) lithogeochemical approach(Pearce 1968; Stanley & Madeisky 1996) to examine the natureand extent of the alteration haloes in the world-class porphyryCu–Mo and epithermal Cu–Ag (–Au) cluster of the Collahuasidistrict, I Región, northern Chile. The PER lithogeochemicalGeochemistry: Exploration, Environment Analysis, Vol. 9 2009, pp. 9–17DOI 10.1144/1467-7873/07-169technique has elsewhere been effective in eliminating nonhydrothermalsources of variation, thus permitting the definitionof material transfers related strictly to the alteration directlyassociated with ore formation (Madeisky 1995; Whitbread &Moore 2004).GEOLOGICAL RELATIONSHIPSThe Collahuasi district, located in northern Chile on the westernslope of the Central Andean Cordillera Occidental, covers anarea of c. 200 km 2 centred on the world-class Rosario porphyryCu–Mo deposit (2058'S; 6841.2'W), at altitudes ranging from4300 and 5200 m a.s.l. Representing the northernmost identifiedcluster of known large deposits associated with the ‘WestFissure’ or Domeyko fault system, the district comprises thecurrently or recently mined Quebrada Blanca, Ujina and Rosarioporphyry Cu–Mo deposits, several epithermal deposits includingthe high-sulphidation La Grande Cu–Ag (–Au) and RosarioCu–Ag and the Moctezuma low-sulphidation Ag–Au veins, allof which have been historically mined, as well as the Huinquintipaexotic copper deposit (Fig. 1).1467-7873/09/$15.00 2009 AAG/Geological Society of London

Lithogeochemistry of Collahuasi deposits 11Fig. 2. Simplified geological map of the Collahuasi District and location of surface samples (solid quadrangles) and sampled drill holes (openquadrangles) (modified from Moore & Masterman 2002). Box shows area of mineralized systems.volcanic host rocks (Masterman et al. 2004). The orthoclase–biotite alteration core is overprinted by sericite–chlorite alterationand a chlorite–epidote halo (Bisso et al. 1998).Several epithermal vein systems have been described in thedistrict. The most important occur in the vicinity of the Rosariomineralized system (Fig. 1). High-sulphidation systems dominate,the most important being the La Grande vein system. Allsuch veins exhibit advanced argillic envelopes grading outwardsto epidote–chlorite alteration (Masterman et al. 2005). Recentgeochronological data for the La Grande and Rosario veinsshow that they formed simultaneously 1.8 Ma after porphyrystylemineralization at Rosario (Masterman et al. 2004). Themost significant low-sulphidation veins in the district occur inthe Moctezuma area.METHODOLOGYApproximately 300, c. 4–5 kg surface rock chip samples werecollected over a 1000 km 2 area surrounding the major hydrothermalcentres of the Collahuasi district (Fig. 2). The averagesampling spacing was one per km 2 within a radius of 15 km orless from a known mineralized centre, and 1 per 2 km 2 outsidethis. In addition, 400 drill-core samples were collected from 21drill-holes in and around the Rosario porphyry deposit. Fifteen ofthese define three NE–SW representative sections of the Rosariosystem, and six are located at distances greater than 1 km from itscore. The latter holes show no significant mineralization, butmoderate to intense propylitic alteration. Sampling of the drillholeswas carried out at 30-m intervals to include all the alterationand lithological domains in the mineralized system.Core and surface rock chip samples were prepared andanalysed at Acme Analytical Laboratories Ltd. Samples werecrushed and pulverized to a minimum of 95% passing 150mesh using a ceramic rod pulverizer to avoid contamination.Total abundances of the major oxides and several minorelements are reported on a 0.2-g sample analysed by ICPemissionspectrometry, following a lithium metaborate (LiBO 2 )fusion and dilute nitric digestion. Rare earth and refractoryelements were determined by ICP-mass spectrometry alsofollowing LiBO 2 fusion and nitric acid digestion of a 0.2-gsample. In addition, a separate 0.5-g split was digested inaqua regia and analysed by ICP-mass spectrometry for theprecious and base metals. Loss on ignition (LOI) was determinedby weight difference after ignition at 1000C. Totalcarbon and sulphur concentrations were determined with aLECO elemental analyser. Appropriate standards and sample

12E. Urqueta et al.Fig. 3. (a) PER diagram of Zr versus TiO 2 and (b) Zr/Ti versusNb/Y classification diagram (Winchester & Floyd 1977) for samplesin this study.analytical duplicates were introduced in each sample batch at arate of 5%.To quantify the intensity of hydrothermal alteration aroundthe major mineralized zones, it is necessary to use a techniquethat isolates the metasomatic impact of the mineralizing fluids,a prerequisite for which is the definition of pre-existinggeochemical heterogeneities in the host rocks. It is also necessaryto discriminate between the effects of hydrothermalactivity and unrelated events such as weathering or metamorphism.PER analysis uses molar element concentrations ratioedto an element, the molar concentration of which has remainedunchanged during mass transfer processes, i.e. a ‘conserved’element (Stanley & Madeisky 1995, 1996). This approachovercomes the effects of closure (Rollinson 1993; Stanley &Madeisky 1996). Furthermore, because PERs are strictly relatedto the stoichiometry of the rock-forming minerals, it is possible,once the mineralogy of the protolith is known or predicted, tocreate a bivariate plot of linear combination of PERs, on whichthe position of an unaltered rock is known. Any deviation fromthis expected position in the plot must be related to a laterprocess. Such a plot can also be used to quantify the intensity ofa hydrolytic alteration process, because the graphic distancebetween where an altered rock plots and its expected position ifunaltered is directly proportional to the degree of metasomaticexchange that the rock has undergone.RESULTSConserved elementsThe identification of conserved elements is a prerequisite formass balance calculations for PER analysis. Figure 3a presentsFig. 4. PER diagrams of selected elements ratioed to the conservativeelement Zr to describe the changes during hydrolytic alterationfor (a) felsic lithological units and (b) andesites. All elementsexpressed as molar quantities except Zr. Line controlling the slope ofthe ratios by various minerals are also indicated.a plot of Zr versus TiO 2 for the main lithological units in thedistrict. For each of the more silicic units (i.e. CollahuasiFormation rhyolites and dacites and the mineralized porphyries),both Zr and TiO 2 are essentially conserved, and describelinear trends that can be projected through the origin of theplot (see Stanley & Madeisky 1996). This is supported by thefact that only very minor amounts of mineral phases that wouldcontrol the distribution of these two elements are observed inthese rocks. Zircon is very scarce and titanium-bearing mineralsare absent except at the core of the deposit where magnetite ispart of the paragenesis of pre-ore magnetite–albite alterationstage, and rutile is an accessory mineral of the intermediatehydrothermal alteration stage. Predictably, the intensely alteredand mineralized Rosario porphyry is the felsic unit that showsthe most scatter in Figure 3a.Andesites exhibit considerable scatter on this diagram,mainly because Ti is not a conserved element. All the studiedandesites exhibit accessory magmatic magnetite and titanite andminor ilmenite, which would partition Ti during crystallization.Hydrothermal magnetite and rutile are also present, and titaniteis an accessory mineral of the propylitic assemblage. Nevertheless,plots such as Zr versus Nb, Zr versus Y, and Y versus Nbshow that Zr is the most conserved element. Zirconium hastherefore been adopted as the basis for the PER analysis of themain host rocks.Using the relationship between TiO 2 and Zr, four volcanicunits are delimited within the Collahuasi Formation, informallynamed Rhyolite 1, Dacite 1, Dacite 2 and Andesite. The felsicunits correspond to rhyolites and dacites according to theZr/Ti versus Nb/Y classification diagram (Fig. 3b) and most

Lithogeochemistry of Collahuasi deposits 13Fig. 5. Spatial relationship of the modified alteration index in the Collahuasi district. Mineralized centres are indicated by black solid areas (seeFig. 1). The major alteration systems within the district (Rosario, Quebrada Blanca, Ujina and La Grande) are well defined by degree of alterationexpressed by the modified alteration index.mafic rocks are basaltic andesites and andesites. Some plot inthe dacite field, but are classified as andesites on the basis oftheir mineralogy. The assumption of immobility of the traceelements used for the classification may be invalidated by thepresence of alteration minerals that preferentially partition thesetrace elements, including rutile, titanite and allanite.PER analysisSeparate PER plots were generated for felsic and mafic rocks,and were tailored to investigate the compositional control of theinitial mineralogy in unaltered samples and that of the hydrothermalalteration product. The relationship between (2Ca + Na+ K)/Zr and Al/Zr (Fig. 4a) discriminates between backgroundand hydrolytically altered felsic samples. This diagram has beenconstructed so that unaltered felsic samples, dominated byK-feldspar and plagioclase, plot along a line through the originwith a slope of 1. Completely sericiticized samples would plotalong a line through the origin with a slope of 1/3, and rocks inwhich all alkali-bearing minerals have been altered to kaolinite/pyrophyllite or chlorite group minerals would plot along thex-axis. Rocks affected by potassic alteration, whether biotitic orK-feldspathic, are not discriminated from unaltered rocks onthis diagram. The development of epidote-group alterationminerals would be represented by a line with a slope slightlyhigher than 1 for allanite, of 4/3 for zoisite, and up to 16/3 forepidote.Figure 4a describes the degree and nature of the hydrolyticalteration in the Collahuasi district samples. The least alteredsamples, i.e. those further away from known hydrothermalcentres, predictably plot closer to the feldspar control line.Samples that have undergone intense quartz–sericite alterationplot around a line with the slope of 1/3, feldspars having beencompletely altered to muscovite. Such is the case of the moreintensely altered samples from the intermediate hydrothermalstage at Rosario. Samples that plot at or near the x-axis

14E. Urqueta et al.propylitically altered rocks from rocks that have undergoneeither hydrolytic hydrothermal alteration or weathering. Hence,samples further away from known hydrothermal systems plotcloser to the plagioclase and hornblende control lines.Detailed petrographic and X-ray powder diffraction analysesof propylitic rocks indicate that the main assemblage ischlorite–epidote–albite–quartz, with subordinate calcite, magnetite,pyrite, titanite and allanite. The aggregate effect of thismineral assemblage on each of the PER diagrams is nil,probably because propylitic alteration did not involve significantmetasomatism. Thus, PER analysis records no materialtransfer during propylitic alteration. In support of this is thebetter development of propylitic alteration in andesites than infelsic volcanics, which contain only minor ferromagnesiansilicates.The only setting where propylitic alteration involves materialtransfer is that contiguous with the mineralized centres. Here,void filling by an epidote–chlorite–calcite assemblage in veinletsand amygdules as well as the replacement of primary ferromagnesiansilicate minerals constitute the main alteration style.Thus, the only samples that plot above the line with the slopeof one on the PER diagrams have significant amounts ofepidote–chlorite–calcite veinlets and amygdule-fill. The developmentof epidote is therefore ascribed to weak Ca, Fe and Mgmetasomatism.Fig. 6. Ca/Zr (molar/ppm) versus alteration index (x-axis; molar/molar ratio) for (a) felsic rocks and (b) andesites. Also shown are thealteration index locations on these plots of various minerals.correspond to those close to the Rosario vein system or withinthe supergene enrichment or leached zones of the Rosarioporphyry system. In the former case, samples show variableamounts of pyrophyllite as determined by a portable infraredmineral analyser (PIMA), whereas kaolinite is the major alterationmineral in supergene samples.The PER plot of (18Ca + 13Na)/Zr versus (2Si + 7Al +4(Fe + Mg))/Zr (Fig. 4b) discriminates between essentiallyunaltered andesites and their hydrolytic alteration products. Inthis diagram, fresh andesites would plot along a line with aslope of 1. The end-member products of sericitic (muscovite),potassic (biotite and potassium feldspar) and argillic (pyrophyllitegroup minerals) alteration would plot on this diagramalong the x-axis. The control line for epidote, a majorproduct of propylitic alteration, would have a slope of 8/3(true epidote). In creating this diagram, the initial mineralogicalcomposition of an unaltered andesite has been assumed tobe dominated by plagioclase and hornblende, as is evidentpetrographically, although minor amounts of primary biotitecan be present.Even though it is possible to create a PER plot thatconflates plagioclase, hornblende and biotite along the sameline, it is more useful to consider biotite as an alteration mineral,given the intensity of biotitization of the andesites at the core ofthe Rosario porphyry. Thus, biotite has been omitted from the‘primary mineralogy’ control line. As a result, unaltered samplesmay plot along a line with a slope of 1 that intercepts the x-axisat a value of 0.6.Figure 4b is less selective than that developed for the felsicunits because it does not discriminate between sericitic andargillic alteration. Nevertheless, it differentiates unaltered orDISCUSSIONAlteration index (AI)The PER diagrams developed for felsic units and andesites canbe used to determine the degree of hydrolytic alteration that aparticular rock has undergone. Unaltered and potassicallyaltered (in the case of felsic rocks) samples plot along a linewith a slope of 1, and hydrolytic alteration results in displacementtowards the x-axis. Consequently, the slope of a lineconnecting a sample point and the origin, or through point (0.6,0) for the andesites, provides a quantitative measure of thedegree of hydrolytic alteration. An ‘alteration index’ (AI) isdefined by dividing the y-coordinate (ordinate) by thex-coordinate (abscissa), such that unaltered samples have an AIvalue of 1. Hence, a plot of the PER values against the AIdocuments the mass transfer processes during increasinghydrolytic alteration.To plot the AI spatially, it is first necessary to integrate theindices for the felsic and mafic lithologies. The main differencebetween the index value for each lithological group is thatsericitic alteration for the felsic units has an AI value of 1/3 andargillic alteration has a value of 0, whereas for the andesitesboth alteration end-members have a value of 0. Thus, a simpleway of integrating both indexes is setting 1/3 as the minimumvalue for the felsic AI. Felsic samples with an AI lower than1/3 are displaced to this minimum, thus accommodating theargillically and sericitically altered samples. It is now possible torescale this conflated felsic AI from 1 for fresh, propylitic orpotassically altered samples, to 0 for totally sericitized orargillized samples, paralleling the AI for the andesites.To reflect the percentage of alteration spatially (Fig. 5), theAI values were rescaled so that a value of 1, representing anunaltered sample, has a modified AI of 0% and a totally alteredvalue has a modified AI of 100%. These percentages ofalteration, or modified AI, were plotted using bubble plots, withthe ranges of the size of the bubbles determined using thenatural breaks in the cumulative population distribution ofthe modified AI. To confirm the validity of this technique as auseful exploration tool, no mineralized core samples wereincluded in this analysis. In spite of the wide sample spacing, all

Lithogeochemistry of Collahuasi deposits 15Fig. 7. Na/Zr (molar/ppm) versus alteration index (x-axis; molar/molar ratio) for (a) felsic rocks and (b) andesites. Also shown are thealteration index locations on these plots of various minerals.major porphyry copper deposits are clearly identified (Fig. 5).The modified AI defines the sericitic halo surrounding theUjina deposit with anomalous modified AI values up to 4.5 kmaway from the core of the hydrothermal system. The QuebradaBlanca deposit is reflected by an AI anomaly that extends for atleast 3 km to the NE of the centre of the mineralized system.A lack of samples precludes delimitation of the sericitic halo ofthis system to the south and west of the pit. Anomalous AIvalues define a 25-km 2 alteration area to the SW of the Rosariodeposit, in which rocks show different degrees of hydrolyticalteration, represented by AI values ranging from 9 to 73%.The hydrothermal event to which these anomalous AI valuesrelate is not obvious. The wide variety of hydrothermal systemsthat crop out or sub-crop out in this area, as well as the widesample spacing, precludes assignment of each punctual anomalyto a particular deposit, but the gradients towards the mineralizedcentres are clearly defined. Furthermore, alteration eventshave widely overprinted each other. Thus, the sericitic halo ofthe Rosario deposit is overprinted by the Rosario highsulphidationvein system, and it is likely that other epithermalsystems coincide spatially with its outer portion. No anomalousAI values occur to the NE of the Rosario deposit. This areacorresponds to the hanging wall of the Rosario fault system andshows only propylitic alteration, which is evidence for thedownward displacement of this block (Fig. 2).Mass transfer processesThe AI can be used to study elemental behaviour as hydrothermalalteration progresses. Mass transfer processes can therebyFig. 8. Si/Zr (molar/ppm) versus alteration index (x-axis; molar/molar ratio) for (a) felsic rocks and (b) andesites. Also shown are thelocations on these plots of various minerals.be quantified for each relevant element during increasingalteration. A strong depletion in Ca with an increasing degree ofhydrolytic alteration can be observed when Ca/Zr is plottedagainst the AI for each lithological suite (Fig. 6). Similar trendsare shown by Na/Zr (Fig. 7) and Sr/Zr. The destruction offeldspars and the generation of muscovite, biotite, chlorite orkaolin-group minerals explain the depletion in Ca, Na and Sr.Although some intensely altered samples have up to 85%SiO 2 (Fig. 8), this was attained by depletion of other componentsduring hydrolytic alteration rather than by the addition ofSi. Aluminium exhibits the same behaviour, with no significantmass gains or losses during alteration.To illustrate the behaviour of K during different alterationevents, K was ratioed to Al rather than to Zr because Al occursin the same minerals as K (Fig. 9). Aluminium, like Zr and Ti,behaves as a conserved element, and K/Al can therefore beconsidered as a PER. These plots spatially define the locationsof the main alteration minerals by discriminating betweenpotassic, sericitic and argillic alteration for both felsic andandesitic lithologies. A strong enrichment in K as potassicalteration progresses is identified for the main lithological units.This is better depicted by the PER diagram of K/Al versusalteration index for andesites (Fig. 9b). In this diagram, the leastaltered rocks plot closer to (1, 0). Potassic alteration minerals(K-feldspar and biotite) plot towards the (0, 1) point in thediagram, muscovite plots at (0, 1/3) and kaolin group mineralsand chlorite at (0, 0). On this basis, there is strong Kenrichment in progressing from the least altered rocks towards

16E. Urqueta et al.Fig. 9. K/Al versus alteration index (x-axis) for (a) felsic rocks and (b) andesites, and bubble plots for (c) felsic rocks and (d) andesites. Thesize of the bubbles is inversely proportional to the distance from a known mineralized centre. Also shown are the location on these plots ofvarious minerals.the potassically altered samples (Fig. 9c and d), the latter beingcloser to a known mineralized centre. This is followed by Kdepletion as quartz–sericite becomes more important, and evenfurther depletion during later argillic alteration. The K depletiondue to quartz–sericitic and argillic alteration is better documentedby the PER diagram of K/Al versus AI for felsicvolcanics (Fig. 9a). In this diagram the least altered rocks plotcloser to (1, 0). The potassic alteration minerals (K-feldspar andbiotite) plot towards the (1, 1) point in the diagram. Muscovitewould plot at (1/3, 1/3) and kaolin group minerals and chloriteat (0, 0). Sericitic and argillic alteration produces a depletion ofK due to the destruction of K-feldspar and biotite.CONCLUSIONSThe use of dedicated PER diagrams for each major lithologicalunit in the Collahuasi district defines material transfers strictlyrelated to hydrothermal alteration. These plots model thebackground variability for different lithologies, and discriminatebetween fresh or propylitically altered rocks and their hydrolyticalteration products. Further, it is possible from these plots todefine an alteration index (AI) that quantifies the degree ofhydrolytic alteration that a rock has experienced.In spite of the widely spaced regional sampling of 1 sampleper km 2 , the spatial representation of the AI defines and vectorstowards the main mineralized hydrothermal centres within thedistrict. This is due to the gradual intensification of hydrolyticalteration towards the cores of the major hydrothermal cells.The elemental variability during each alteration stage isdocumented by plotting the molar ratio of each relevantelement against AI. Potassium enrichment during the potassicalteration event, followed by Ca and Na depletion during thesericitic and later argillic alteration stages, are the most importantmass transfer processes. PER analysis documents no masstransfer during propylitic alteration besides a minimal increasein the carbon content. Vectoring on this basis within thepropylitic halo by means of PER analysis is therefore notpossible.This study is a component of the senior author’s PhD research andconstitutes a contribution to the Queen’s University Central AndeanMetallogenic Project (Q CAMP). Field studies were facilitated by thegeology staff at Rosario. This project has received generous financialand logistic support from Anglo-American plc, and was furthersupported by the Natural Sciences and Engineering Research Councilof Canada (NSERC), the Canada Foundation for Innovation (CFI)and the Ontario Innovation Trust (OIT). Reviews by D. Gabouryand an anonymous reviewer greatly improved the manuscript.REFERENCESBISSO, C.B., DURÁN, M.&GONZALES, A.A. 1998. Geology of the Ujina andRosario copper porphyry deposits Collahuasi district, Chile. In: PORTER,T.M. (eds) Porphyry and Hydrothermal Copper and Gold Deposits: A GlobalPerspective. PGC Publishing, Adelaide, 217–232.CLARK, A.H., ARCHIBALD, D.A., LEE, A.W., FARRAR, E.&HODGSON, C.J.1998. Laser probe 40 Ar/ 39 Ar ages of early- and late-stage alterationassemblages, Rosario porphyry copper–molybdenum deposit, Collahuasidistrict, I Region, Chile. Economic Geology, 93, 326–337.

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