baldwin and pearce-1982

Economic Geologt Vol. 77, 1982, pp. 664-674

Discrimination of Productive and Nonproductive Porphyritic Intrusions in the Chilean Andes

J. A. BALDWIN AND J. A. PEARCE Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, England

Abstract

A geochemical comparison between productive intrusions (those associated with mineralization) and nonproductive intrusions (those unrelated to mineralization) has been made on the basis of over 100 fresh granitoid rocks from the Chilean porphyry copper belt. The study was carried out on the scale of a mining district (El Salvador-Potrerillos), then on the scale of a single traverse (El Salvador-Chilean coast), and finally on the scale of the Andes itself. The productive intrusions are best characterized by large negative anomalies in Y, Mn, Th, and the heavy rare earth elements. A discriminant diagram of Y against MnO is recommended for predicting whether an apparently fresh intrusion has associated peripheral mineralization.

The low values of Y and Mn in productive intrusions can be partly explained by the early crystallization of Y- and Mn-rich hydrous phases such as hornblende and by the loss of Mn- rich fluids from the magma. The same elements make useful discriminants in other porphyry copper provinces, although the exact position of the discriminant boundary varies from island arcs through continental margin to continental settings.

Introduction

THE Andes of central and northern Chile contain numerous plutonic rocks of intermediate, calc-alkaline character, but only a tiny proportion of these are associated with porphyry copper deposits. In such arid terrain, stream sediment and soil geochemistry have a limited application in exploring for these deposits (Coope, 1975). On the other hand, the good exposure means that lithogeochemical prospecting may prove fruitful, provided suitable techniques can be developed. This paper investigates one possibility: the use of whole-rock geochemistry to evaluate the exploration potential of an intrusion or intrusive complex in the absence of obvious hydrothermal alteration or mineralization.

Several attempts have been made to do this in other porphyry belts, with mixed success. Halogens and water, the obvious discriminants between productive (ore-forming) and nonproductive intrusions, have been shown by Kesler et al. (1975a, b) to be generally unsuccessful, for both biotite and whole-rock analyses. The most comprehensive study based on other elements is from the Papua-New Guinea-Solomon Is- lands belt, where Mason and McDonald (1978) have compared the compositions of productive and nonproductive intrusions from island-arc, continental margin, and continental settings. They found no single element capable of discriminating the two groups but noted that, when different rock types are present in a district, the porphyry copper deposits are associated with late, low potassium intrusions. An inter- esting extension of this work was the suggestion by

Mason and Feiss (1979) that productive intrusions were characterized by low ratios of A1203/(K20 q- Na20 q- CaO). There is, nonetheless, a significant overlap in this ratio between known productive and nonproductive intrusions. Other major elements and selected trace elements (mainly Rb, Sr, and the transition metals) (Olade and Fletcher, 1975, 1976; Oyar- zun, 1975; Armbrus et al., 1977; Chaffee, 1976) have provided useful means of defining orebodies on a mine scale, but their applicability on a regional scale has not yet been tested.

This particular study is based on analyses for over $0 major and trace elements in about 100 productive and nonproductive intrusions. The discrimination is examined on three scales (Fig. 1): (1) on the scale of a single mining district, that of E1 Salvador-Potrerillos in central and northern Chile; (2) on the scale of a single east-west transect from E1 Salvador to the Chil- ean coast; and ($) on the scale of a single tectonic province, the Chilean Andes. The techniques developed are then applied to some other porphyry copper provinces.

The El Salvador-Potrerillos District

Selection of samples The porphyry copper mines of E1 Salvador and

Potrerillos are located in the Atacama desert of northern Chile some 800 km north of Santiago (Fig. '1). The geology of the E1 Salvador mine area has been described by Gustarson and Hunt (1975) and the geology of the district by Tobar (1978). The main volcanic sequences are the Upper Cretaceous Cerillos For-

0561-0128/82/29/664-1152.50 664

PORPHYRITIC INTRUSIONS IN CHILEAN ANDES 665

mation, which is predominantly andesitic, and the lower Tertiary Hornitos and La Peineta Formations, which largely comprise silicic tuffs and ignimbrites. The intrusions can be divided into an older monzonite-rhyolite suite (approximately 70-50 m.y.) and a younger granodiorite suite (approximately 45-$0 m.y.) (Fig. 2).

The E1 Salvador porphyry copper deposit itself cen- ters on a cluster of porphyritic intrusions that have been dated at about 42 m.y. Two of these, the X porphyry and the K porphyry, are obviously mineralized and show strong hydrothermal alteration (Gustarson and Hunt, 1975). These altered rocks have not been used in this work. The L porphyry, which forms the core of the intrusive complex, is slightly mineralized with a weak K silicate alteration assem- blage in places but elsewhere is a barren granodiorite containing hornblende, biotite, and ilmenite as the main mafic minerals. It contains internal intrusive contacts and varies in texture from almost equigranular to strongly porphyritic with a fine-grained aplitic groundmass. Samples of this L porphyry were chosen 'as examples of a productive intrusion; they are associated with mineralization but are not usually them- selves visibly mineralized, and are fresh enough to give a valid comparison with nonproductive intrusions.

The Potrerillos porphyry copper deposit, which lies 25 km southeast of E1 Salvador, has been dated at $8 to $4 m.y. (Zentilli, 1974). Recent mapping by Co- delco geologists showed that, like E1 Salvador, it is associated with an intrusive complex, known as the Cobre porphyry, the core of which consists of an apparently fresh hornblende diorite (L-type) porphyry. Samples were taken of this hornblende diorite porphyry, and of a very weakly mineralized satellite, the North porphyry. These samples were also included in the productive intrusion group.

The distribution of nonproductive intrusions from this district is shown in Figure 2; those that are num- bered are referred to in the text or have their analyses recorded in Table 1. The sampling program covered both the monzonite and the granodiorite suites and included rock types ranging from diorites to dacites. The samples were otherwise limited to those whose age, mineralogy, and texture were comparable with the productive intrusions. Thus their age range is lower Tertiary (55-$4 m.y.), their mineralogy is dom- inated by feldspar, hornblende, and biotite phenocrysts, and their texture is porphyritic.

Also included in the samples analyzed were A porphyries and latite dikes, two minor intrusives from E1 Salvador. The A porphyries are characterized by an abundance of mafic phenocrysts and were in- truded during or after crystallization of the L porphyry. The latites contain plagioclase and hornblende

i I i

ARICA

I _

El Abra \

, .huquicamata

-2' EIJ I_s -- F,g E' S{],vador Potrerillos [L :,9 7 t,/r

! /

! I

/' 0 100km - 30S ) -

Anda11o I

I 'lnient e (

- 20S

Fie. 1. Location map of the Chilean porphyry copper deposits mentioned in the text. Areas of more detailed study are denoted as insets.

phenocrysts and postdate the main intrusive complex by up to 2 m.y. (Gustarson and Hunt, 1975). Neither classifies clearly into the productive or nonproductive categories. The A porphyry is itself weakly mineralized and the genetic relationship between the latites and the productive L porphyry is uncertain. Choice of geochemical discriminants

Our attempts to discriminate between the productive and nonproductive intrusions using whole-rock abundances of the transition metals typically associated with porphyry copper deposits met with limited success. Figure $ summarizes the results.

Of the elements, copper, gold, and molybdenum, which are concentrated in the primary central zones of porphyry copper deposits (Jerome, 1966; Levinson, 1974), gold (Fig. $) showed no detectable difference between productive and nonproductive intrusions,

666 J. A. BALDWIN AND J. A. PEARCE

4km I

R29

EL SALVADOR town

R10

Tertiary Intrusives, ':.'.:...:i Tertiary Lavas Gravels and Cretaceous Lavas

Granodicrite samples Monzonite samples -I- Rhyolite samples

.Mine

POTRERILLOS


TABLE 1. Representative Geochemical Analyses for Some Productive and Nonproductive Intrusions from the E1 Salvador District

Sample no.

Productive Nonproductive

ES $$$8 IL 6BL 24L Pot9B R 10 R29 R 17 R 11 R 13 RY19

L porphyries (Gd) Gd DP Gd Gd QEP QMP RYP SiO2 62.5:3 62.$4 60.94 62.08 62.79 55.77 60.95 6:3.05 67.86 65.09 75.60 TiO2 0.70 0.67 0.64 0.74 0.61 0.74 0.64 0.55 0.23 0.63 0.17 AI2Oa 15.99 16.65 16.81 18.26 15.71 14.84 15.95 15.99 16.07 15.98 12.90 Fe2Oa $.56 $.69 $.74 $.85 $. 15 7.$$ 5.54 4.$9 2.$2 $.75 1.14 MnO 0.052 0.025 0.057 0.019 0.024 0.151 0.091 0.071 0.065 0.065 0.15 MgO 2.25 1.91 2.08 1.50 2.15 5.12 $.41 2.68 0.85 0.87 0.14 CaO 4.41 4.14 4.54 $.55 $.68 5.99 5.24 $.93 2.51 2.01 0.28 NazO 5.00 4.77 5.25 5.41 5.45 3.46 $.95 4.49 4.51 5.$0 5.00 K20 2.70 2.17 2.54 1.95 2.19 2.64 2.46 $.39 $. 17 4.82 4.37 PO5 0.25 0.21 0.25 0.28 0.18 0.28 0.16 0.18 0.01 0.17 0.01 L.O.I. 1.77 2.65 1.55 1.17 1.98 $.66 1.14 1.27 2.79 1.91 0.71

Subtotal 99.17 99.25 98.$6 98.75 97.87 99.96 99.55 99.99 100.$8 100.59 100.45

XRF

Rb 74 56 58 $4 $6 65 67 122 68 118 161 Ba 565 541 589 598 688 652 752 813 450 1286 153 Sr 696 602 676 690 784 558 565 605 257 $67 280 Th

668 j. A. BALDWIN AND J. A. PEARCE

I It- '-I Hi{' .-{ Non-produchve AM t' '{ Productive 0.0 lppm 0 1

FI4 -t'. t-N -- ,.t-- -N- -- -- ,,- I,-,,. ,,= -- N, -- -- ,-,-{ Non-produchve 1 Cu Produchve I I I 10ppm 100 500 1000

I-- H.- 'H 1-{ --I- -++ }-' '1 Non-producbve Zn ::;: ; :: : ',',', Y,;', ',', ',', Produchve

__

10ppm 1 O0 5 O0 1000

1 pp m I-ltll-I- N4- 4- .4- -- -t Non-produchv i

10 50 100

FIG. 8. Bar graphs of the abundances (ppm) of Zn, Cu, Pb, and Au in productive and nonproductive intrusions in the El Salvador- Potrerillos district. The significant overlap in all eases makes these elements unsuitable discriminants. (Zn and Pb analyzed by XRF at Birmingham University, and Cu and Au by atomic absorption at the El Salvador mine, Chile).

1966). We might therefore expect a productive intrusion to be depleted in Pb or Zn relative to a nonproductive intrusion. Zn shows no discrimination (Fig. 8), but Pb shows, on average, lower values in productive compared to nonproductive intrusions (Fig. 8). Again, however, there is too much overlap to provide a completely reliable discriminant. The halogen elements, fluorine and chlorine, were also plotted in this way, but these proved poor discriminants, as Kesler et al. (1975a) showed for other areas.

The likely values of other elements in carrying out the discrimination between productive and nonproductive intrusions can be deduced by constructing geochemical patterns such as those shown in Figure 4. The patterns are constructed from ratios of the element abundances in the rock to the element abundances in a typical barren granodiorite. Any barren rock could have been used as the normalizing factor; R29 (see Table 2) was in fact chosen because it su- perficially resembled most of the productive samples in its granodiorite composition and intermediate SiO2 (61%) and because its main component minerals (plagioclase, hornblende, biotite, and quartz) were apparently unaffected by weathering or alteration. The elements in the pattern include most trace elements that are readily analyzed by X-ray fluorescence and instrumental neutron activation analysis techniques. The major elements K and Ca have also been included. The elements are approximately arranged

from left to right as large ion lithophile elements, high field strength elements, and transition metals but with some modification to highlight anomalies.

Patterns characteristic of nonproductive intrusions are the fiat pattern of the barren granodiorite (for the granodiorite suite) and the pattern of R18 (for the monzonite suite); the monzonite exhibits an enrich- ment in K, Rb, Ba, Th, Zr, Hf, Ta, and the light rare earth elements and a depletion in Ca and Sr relative to the granodiorites. Two L porphyries, chosen as characteristic of productive intrusions, have also been plotted. Some of the hornblende in sample 1L has been altered to biotite, but 8888 shows negligible signs of alteration.

The principal features of the productive intrusions are the strong negative anomalies in the elements, Mn, Y, Yb, and Th. Sr shows a small positive anomaly, but other elements appear to be of little use by them- selves in carrying out the discrimination. It should also be noted that K, Rb, and Ba, which proved ef- fective in Pacific arcs (Mason and McDonald, 1978), are ineffective in this setting. These observations can now be used to construct a discriminant diagram based on all available data.

Discrimination diagram for productive and nonproductive intrusions

Of the four elements that showed the largest anomalies in productive intrusions, Mn and Y are in great- est absolute abundance and can easily be analyzed to the required level of precision by X-ray fluorescence. Yb (which behaves in a similar way to Y) and Th can both be analyzed accurately by neutron activation analysis, but this is not a practical method for most exploration programs. Accordingly, a covariation diagram of MnO against Y has been drawn

ROCK

// x, /% R13 /. .,__ / \ Nonroducbve

_/i. G (R29) IL 3338 Productwe

Ca Sr K Rb Ba La Ce Zr HI Ta Y Yb Ti Mn Zn Co Sc

FIC. 4. Geochemical patterns for productive and nonproductive intrusions from the El Salvador-Potrerillos district. Data normalized to typical barren granodiorite (T. B. G., R29). The arrows highlight important negative anomalies in the productive intrusions.


as Figure 5 to highlight the combined effect of these anomalies, although the limited available data indicate that diagrams such as MnO against Th or Yb, and Th against Yb, are likely to be equally successful. The points plotted in this diagram have been restricted to samples that are fresh or exhibit only in- cipient alteration, although the same features are apparent when all samples from the area are plotted. On the basis of the relatively fresh samples, a single line could be drawn which discriminates between unambiguous productive and nonproductive intrusions with a classification success of about 92 percent. A problem with any diagram of this type is the possibility that the discrimination may be masked by the effects of fractional crystallization. In calc-alkaline intrusions MnO usually decreases from diorites to granites whereas Y can increase slightly, decrease slightly, or remain constant. In Figure 5 the SiO2 values of the samples used can be seen to vary from 55 to 76 percent. There is, however, no obvious correla- tion between SiO2 and either MnO or Y, indicating that fractional crystallization variations are too small to mask the large negative anomalies of these elements in productive intrusions.

It should be noted, in addition, that the boundary between productive and nonproductive intrusions is not fixed but will depend on what size and grade of deposit can be mined at any given time. It was thus thought necessary also to define a field of subproductive intrusions, which are not associated with cur-

Yppm

10

Non-productive

1 ,

,o.N e 750 750 855 ,"' /t. 76

64 e59' o60 D58 60 60 62

62 4 61 ' e60

e64 .

_Productve El Salvador 'L'-porphyry Potrerdlos Cobre porphyry

Sub-productive e El Salvador 'A'-porphyry e Potrerlllos 'North'-porphyry

Non-productive o El Salvador latltes D El Salvador-Potreriltos

traverse

, i , , , , , , , , , , , , , , ,

005 01 05 1

MnO%

FIG. 5. Covariation diagram of Y against MnO, limited to the freshest and most porphyritic intrusions in the E1 Salvador-Potre- rilles district. SiO2 values for each sample are shown. A discriminant line has been drawn which separates most productive from most nonproductive intrusions.

711 .

? El Salvador

-- Calde, .

I

Siena El Jardin

? 26 - 50kin I INTRUSIONS Basement X Jurassic

-F Cretaceous

o Tertiary

0 27-

FIG. 6. Location of samples and intrusions in the Copiapo-E1 Salvador transect (much of coastal data from J. Tarney).

rently minable deposits but which are associated with significant mineralization. For example, the A porphyries at E1 Salvador and the North porphyry at Potrerillos, both shown in Figure 5, can contain up to 0.8 percent Cu. It might have been thought that the latite dikes at E1 Salvador could be classified as productive intrusions in the same way as L porphyries. However, they plot in the nonproductive and subproductive fields in Figure 5. Moreover, micro- probe analyses of amphiboles in the latites are very different from those in L porphyries, and plagioclases are significantly more calcic (An47_44) than L porphyry plagioclases (Ana_4). These features, and the 2-m.y. age difference between the 89-m.y. latite and the 41-m.y. L porphyry (Gustarson and Hunt, 1975), suggest that the latites may be part of an independent intrusive event. If true, this implies that it may not be justified to use late dikes as fresh samples representative of a porphyry suite.

Only one anomalous barren sample, R6, plots in the productive field. Its age is 86 m.y. (Tobar, 1977), and its chemistry closely resembles that of a typical L porphyry except for higher Ba and a higher initial SrS7/SrS6 ratio It appears to be unrelated to mineralization, although it could be the root of an eroded, poorly developed porphyry copper deposit.

El Salvador-Copiapo Transect Selection of samples

The region from E1 Salvador and Potrerillos to the coast is characterized by a general inland migration of igneous activity from Jurassic to Recent (Farrar eta!., 1970) (Fig. 6) and an increase in erosion level toward the coast. Two sets of data are used: one from samples collected in our own traverse and one frtm a study of coastal intrusions made by J. Tarney and others (pers. commun., 1978). Both sets of data, making 170 analyses in all, were obtained on the same


Yppm

10

El Salvador Transec!

Non-produchve r

/

Sub- / productsveX /

. . . / .'/% s,,dor

,.' * ' .' * '. " Potrerdlos 'Cobre' porphyry 5 ' . Sub-productive

o., s,;:dor ' El Salvador-Potrerdlos traverse

El Salvador-coast traverse

I I i I I I 1011 I I I 015 I I I I I 001 0 5 1 MnO%

Flg. 7. Covariation diagram of Y against MnO for productive, subproductive, and nonproductive intrusions in the Copiapo-E1 Salvador transect. The samples shown are from porphyritic and nonporphyritic, fresh and slightly altered intrusions. Fields have been drawn for each intrusive type and show productive intrusions to the lower left (solid line), nonproductive intrusions to the upper right (fine-dashed line), and subproductive intrusions in the center. The field for nonproductive intrusions has been extended to include data from 30 andesite samples from the El Salvador district (not plotted in this diagram).

XRF spectrometer at Birmingham University using the method of Tarhey et al. (1978).

No restriction was made on the texture and composition of the samples; both equigranular and porphyritic samples having an SiO2 range from 48 to 75 percent were included. The vast majority of these samples were barren and had no obvious association with porphyry copper mineralization, although some could have underlain now-eroded deposits. The only intrusions obviously associated with mineralization are from Sierra E1 Jardin, a subeconomic porphyry copper prospect marked in Figure 1 (Oyarzun, 1973; Neumann, 1972). Samples from Copper Hill, a satellite deposit of E1 Salvador and not plotted earlier because of slight alteration, have also been included. Geochemical discrimination

Figure 7 shows the covariation diagram of Y against MnO for all available fresh to moderately fresh rocks from the El Salvador traverse, including those plotted in Figure 5. It is apparent that the boundary between productive and nonproductive intrusions, as defined in Figure 5, applies to the whole traverse as well as to the E1 Salvador district; in fact the classification success rate for productive and nonproductive intrusions is greater (95%). In general the

most porphyritic rocks in the nonproductive field have lowest Y and MnO values, but they do nonetheless plot outside the area occupied by the productive intrusions. R6, discussed earlier, remains the only really anomalous point. The data from the submin- eralized intrusions at Copper Hill and Sierra E1 Jardin provide further confirmation that it is also possible to define a subproductive field. Compositional fields, for productive, subproductive, and nonproductive intrusions have been drawn by eye onto this diagram. It is also worth noting that, although only intrusions are considered in Figure 7, some 30 andesites sampled from the transect would also plot within the nonproductive field.

The Chilean Andes

Sample collection In addition to the deposits of E1 Salvador and Po-

trerillos and the subeconomic prospect at Sierra E1 Jardin, a limited number of samples were analyzed from the other porphyry copper deposits shown in Figure 1, namely, Chuquicamata, El Abra, Ando- collo, and E1 Teniente. Y and MnO data for these intrusions are plotted in Figure 8.

Chuquicamata: One of the world's largest copper mines, it is located along a major structure known as the West fissure and related to a complex of granodiorite porphyries. Mineralization is thought to have been caused by the Chuquicamata porphyries, which have been dated at between 33.6 and 30.3 m.y. and which have been subdivided texturally into the East, West, and Banco porphyries (Ambrus, 1979). These

Other Chilean Porphyry Copper Deposits

\ \

I Non-productive i \

I I i I /,/

5 Produchve

o

1

I I | I I I I I011 I I I I ! I I I I 01 005 . 05 1 MnO%

FIG. 8. Covariation diagram of Y against MnO for some other Chilean porphyry copper deposits. The fields for productive subproductive and nonproductive intrusions are taken from the Co- piapo-E1 Salvador transect (Fig. 7).

Yppm

10 Relabvely fresh sample with Amphlbole

Productive El Abra not fresh

El Temente samples Chuqulcamata

Sub-produchve Andacollo porphyries Fortuna granodlorlte

Non-productive El Temente late lamprophyre Andacollo Tablalume batholith Daclte El mineral

Problemahc

El Abra dorlte

PORPHYRITIC IN TR USINS IN CHILEAN ANDES 671

rocks are exposed in the central area of the open pit. Samples of East and West porphyries were collected although the whole zone had suffered pervasive wall- rock alteration. A further granodiorite intrusion, thought by Ambrus (1979) to be of similar age and related to the same complex, the Elena granodiorite, was also sampled from the east side of the pit, although this, too, had been strongly altered. An ad- ditional nonproductive Tertiary intrusion from the area, E1 Mineral, was also sampled.

El Abra: This deposit lies 42 km north of Chuqui- camata, is of similar age ($5.4-$$.5 m.y.) and is also close to the West fissure (Ambrus, 1977). Samples were taken from each of the intrusions making up the complex: the Southern granodiorite, the E1 Abra diorite, the quartz monzonite, and the dacite porphyry. All these intrusions are hosts for the mineralization, but the two most important are the diorite and dacite. Most samples collected were, therefore, strongly altered, an important exception being the diorite, which is exposed outside the zone of mineralization.

El Teniente: In central Chile this is the youngest of the large Tertiary porphyries, dated at 4.$ m.y. (Camus, 1975). The important intrusions associated with the orebody are a quartz diorite and a dacite porphyry. The final intrusions are lamprophyre dikes which have a similar relationship to the orebody at E1 Teniente as the latite dikes have to the orebody at E1 Salvador. Samples were collected from all three rock types, but only the lamprophyre was unaltered.

Andacollo: This deposit is much nearer the coast and is thought to be older than the other porphyry copper deposits, probably Upper Cretaceous (Llau- met et al., 1975; Ambrus, 1979). A representative sample was taken from a nonproductive intrusive, the Tablalaluma batholith, in the northeast of the district. Three samples were taken from a porphyritic tonalitc stock and its related dikes and sills, collectively known as the Andacollo porphyries, with which the mineralization is genetically related. Geochemical discrimination

At E1 Abra, the fresh E1 Abra diorite (EA4) from south of the E1 Abra alteration zone (Ambrus, 1977) has high Y and Mn and consequently plots as nonproductive (Fig. 8). It is possible that this intrusion does not conform with the discrimination presented. However, there are alternative possibilities: the diorite is less fractionated (58% SiO2) than the rocks used in the original discrimination and should contain more MnO and Y than a granodiorite and it is not certain at present whether this intrusion caused mineralization or was merely the passive host to the dacite porphyry. Further studies should distinguish between these possibilities.

The positions of the data points from Andocollo are also problematical. The Tablalaluma batholith plots correctly as nonproductive, but the three samples of porphyries plot as subproductive. Andocollo is not, however, typical of other Chilean porphyries, being of Upper Cretaceous age and near the coast, and having characteristics of manto, as well as porphyry copper, deposits. It is possible that the precise position of the discriminant boundary is different for intrusions of this age or for intrusions in this segment of the Andes. However, further work is needed in the Andocollo area before such hypotheses can be sub- stantiated.

In summary, the Y-MnO discriminant diagram gives meaningful results for plutonic rocks related to the various Chilean porphyry copper deposits, although it is much more successful when limited to a single transect. Each transect may have a particular background Y and MnO value that needs to be cat- egorized. Comparisons with Porphyry Copper Deposits in

the Pacific Area

Using data from Mason and McDonald (1978) and Mason and Feiss (1979), Y-MnO diagrams have been plotted for nonproductive and productive samples in the Pacific arcs and the New Guinea mobile belt. The results are shown in Figure 9A and B. Discrimination in each case is good, but the discriminant boundaries are different from each other and from that deter- mined for the E1 Salvador transect. Mason and McDonald (1978) recognized that Y values in their plutonic rocks increased (at a given SiO2 content) from island-arc through continental margin to continental settings. This shift in the position of the discriminant axis is therefore to be expected. Of course, variable factors such as the thickness of crust and the depth of the underlying Benioff zone, have also caused variations within the E1 Salvador-coast traverse (Zentilli, 1974). In this case however, the effect on the discrimination was relatively small.

This difference between Chile and the southwest Pacific emphasizes that the ideal unit for considera- tion is a single tectonic province and that the exact details of discrimination between nonproductive and productive samples (especially for Y) will vary ac- cording to the tectonic province in question. Explanations for the Y-MnO discrimination of productive and nonproductive intrusions

Depletion of Y and MnO could theoretically have taken place at various stages in the history of the intrusion. The four main possibilities are:

1. During partial melting: the productive intrusions could have been derived from a source region depleted in these elements or phases that accomodate

672 j. A. BALDWIN AND J. A. PEARCE lOO

Pacihc Arcs

50

[ Non-produchve I \ I

t a / //

ppm / /

o ///// 1 I I I I I I I IoI 001 005

Productive

Non-produchve

I I I I I I I I I 1

100

50

Y ppm

10

5

1

Pacific Continental Margins (New Guinea Mobile Belt)

I []

I [] / . I ///

/

' "... x / "".. ////

Productive

E Non-productive

I I g5 I I I I I I I I I I I I I I 0 01 1 MnO%

FI(;. 9. Covariation diagrams of Y against MnO for productive and nonproductive intrusions in Pacific arcs (A) and continental margins (B) (data from Mason and McDonald, 1978, and Mason and Feiss, 1979). "A" gives the discriminant boundary for Pacific arcs (heavy line) and the fields for the Chilean transect (from Fig. 7). "B" shows how discriminant boundaries for Pacific continental margins (heavy line), Pacific arcs (heavy dashed line), and the Chilean transect (dotted line) are displaced to progressively lower Y and MnO values.

these elements could have been left in the melt residue.

2. During fractional crystallization: phases crystallizing from productive intrusions could have

greater distribution coefficients for Y and Mn than phases crystallizing from nonproductive intrusions.

8. During expulsion of a volatile phase: a CO-rich phase expelled at depth (Gustarson, 1978, 1979) or an HO-rich phase expelled at high levels in the crust could have removed Y and MnO from the magma.

4. During hydrothermal alteration: passage of hot fluids of magmatic or meteoric derivation could have reacted with the productive intrusions after crystallization, removing Y and Mn in solution.

The effects of hydrothermal alteration are easiest to assess. None of the samples plotted have undergone sericitic, argillic, or intense K silicate alteration. How- ever, the samples do show a variable degree of sub- solidus replacement of hornblende, sphene, and ilmenite by biotite and rutile. The changes in MnO and Y that result from these reactions can be assessed by comparing Figure 5 (which was restricted to samples with negligible to slight replacement of hornblende) with Figure 7 (which also contained samples showing significant to total replacement). When the more altered samples were included, the field of productive intrusions expanded to lower MnO, but similar Y, values. Thus, it is apparent that replacement of hornblende causes MnO to decrease but does not significantly affect Y. On the Y-MnO diagram, therefore, the alteration vector is subhorizontal, and it is thus impossible to move from the nonproductive to productive fields solely as a result of alteration. The fact also remains that some of the intrusive rocks in the productive field were almost fresh and that nonproductive intrusions with comparable degrees of weak alteration have higher concentrations of both MnO and Y. We therefore have to look to precrystallization processes for the causes of the Y anomaly and at least part of the MnO anomaly.

Of the precrystallization processes, loss of a volatile phase may be the most important in the case of manganese, since most hydrothermal fluids contain significant concentrations of manganese, and since most porphyry copper deposits are known to be surrounded by manganese-rich halos. However, hornblende and ilmenite can contain significant concentrations of manganese and, if these phases crystallized from the magma or if these minerals were concentrated in the residue from partial melting, the resulting magma could be depleted in MnO without the need to invoke volatile transfer. The depletion in Y and heavy rare earth elements is less easy to explain. These elements are not strongly partitioned into volatile phases (Flynn and Burnham, 1978). They are, however, partitioned into hornblende, and into minor phases such as sphene and apatite.

Detailed petrogenetic modeling, to be published elsewhere, suggests that the manganese depletion is


best explained by loss of Mn in the volatile phase whereas Y depletion is best explained by crystallization of phases (hornblende and sphene) that are stable under hydrous conditions or by the presence of such phases in a melt residue. It appears therefore that the two axes of the Y-MnO covariation diagram complement each other. Low Y in a magma may indicate the involvement of hydrous phases during the earlier history of the magma, whereas low MnO may indicate extensive loss of magmatic fluids from the magma and thus be directly related to the mineralization event.

Exploration Application Exploration at the reconnaissance stage for por-

phyry copper deposits in arid or glaciated terrains would be improved by any technique of lithogeochemical sampling that could fingerprint apparently fresh intrusions which were associated with hidden copper mineralization. In this study of the E1 Sal- vador-Copiapo transect, Y and MnO were found to be more useful than other elements, low values of both being typical of productive suites. In a less well- explored metallogenic province or transect, a similar program of whole-rock sampling of the accessible intrusions, say 200 to 300 samples, and XRF analysis for Y and MnO, would be relatively cheap. A more detailed follow-up investigation or extra sampling could be carried out around intrusions with unusually low Y and MnO.

As a cautionary note the following limitations ought to be taken into account:

1. The possible SiO2 range of samples is around 48 to 75 percent but preferably 55 to 70 percent (quartz diorites, granodiorites, and quartz monzonites). The fresh productive rocks considered in this study contain significant proportions of modal amphibole and ilmenite but are still depleted in Y and MnO. Nev- ertheless, it should be noted that it is theoretically possible for rocks containing abundant cumulate ilmenite to have anomalously high MnO and rocks containing abundant cumulate hornblende to have anomalously high MnO and Y.

2. Sampling should be restricted to fresh and slightly altered or slightly weathered samples. In- tensely altered samples may have lost both Y and MnO and may therefore give misleading results on the discrimination diagram.

3. Sampling should be limited to a particular tectonic province or transect, as background Y values vary with the tectonic province in question.

4. The Y and MnO discrimination may only be useful in tectonic provinces situated at Phanerozoic convergent plate boundaries, which, anyway, is where the main porphyry copper provinces are located (Sil-

litoe, 1972). It would not be practical to use Y as a discriminant in Archcan terrains where intrusions are already strikingly depleted in Y and heavy rare earth elements, perhaps reflecting a different petrogenetic history.

Conclusions

Trace element data on whole-rock samples can be useful in identifying porphyry copper productive suites. No single element is an adequate discriminant, but productive and nonproductive samples are well separated on Y and MnO covariation diagrams, productive samples being anomalously low in both elements. The discrimination works best when limited to a particular transect (e.g., E1 Salvador-Copiapo) or tectonic belt (e.g., a Pacific arc), because Y abundances may vary from province to province.

Low Y may reflect the distinctive magmatic processes necessary for porphyry copper genesis whereas low MnO may reflect the size and strength of de- velopment of a hydrothermal system. Y and MnO discrimination could be a useful tool at the reconnaissance stage of exploration and could give evidence of which intrusive suites crystallized from magma that generated porphyry copper mineralization.

Acknowledgments We are grateful to John P. Hunt and Alvaro Tobar,

and to Enrique Tidy, Jorge Quiroga, and other Cod- elco geologists, for their support and help in this project. We would also like to thank Peter Francis, Chris Hawkesworth, Richard Holt, and Andy Tindie for helpful discussions, John Tarney and Nick Marsh for permission to use their Y and MnO data, and Juan Carlos Marquardt for providing samples from An- docollo. The authors were both supported in their work by the Natural Environment Research Council of the United Kingdom. July 30, November 20, 1981

REFERENCES

Ambrus, J., 1977, Geology of the El Abra porphyry copper deposit, Chile: ECON. GEOL., v. 72, p. 1062-1085.

-- 1979, Emplazamiento y mineralizacion de los porfidos cupri- feros de Chile: Unpub. Ph.D. thesis, Univ. Salamanca, 313 p.

Armbrus, G. A., Oyarzum, J., and Arias, J., 1977, Rubidium as a guide to ore in Chilean porphyry copper deposits: ECON. GEOL., v, 72, p. 1086-1100.

Camus, F., 1975, Geology of the E1Teniente orebody with emphasis on wall-rock alteration: ECON. GEOL., v, 70, p. 1341-1372.

Chaffee, M. A., 1976, The zonal distribution of selected elements above the Kalamazoo porphyry copper deposit, San Manuel district, Pinal County, Arizona: Jour. Geochem Explor., v 5, p. 145-165.

Coope, J. A., 1973, Geochemical prospecting for porphyry copper- type mineralization--a review: Jour. Geochem Explor., v. 2, p. 81-102.

Fartar, E., Clark, A. H., Haynes, S. J., Quirt, G. S., Conn, H., and


Zentilli, M., 1970, K-Ar evidence for post-Paleozoic migration of granitic intrusion loci in the Andes of northern Chile: Earth Planet. Sci. Letters, v. 10, p. 60-66.

Flynn, R. T., and Burnham, C. W., 1978, An experimenthi deter- mination of rare earth partition coefficients between a chloride containing vapour phase and silicate melts, in Drake, M. J., and Holloway, J. R., eds., Experimental trace element geochemistry: Proceedings of an International Conference, held in Sedona, Arizona, 12-16 September, 1977: Oxford, Pergamon, v. $, p. 685- 701.

Gustarson, L. B., 1978, Some major factors of porphyry copper genesis: Ecoa. GEOL., v. 75, p. 600-607.

-- 1979, Porphyry copper deposits and calc-alkaline volcanism, in McElhinny, M. W., ed., Earth: its origin, structure and evolution: New York, Academic Press, p. 427-468.

Gustarson, L. B., and Hunt, J.P., 1975, The porphyry copper deposit at El Salvador, Chile: Ecoa. GEOL., v, 70, p. 857-912.

Jerome, S. E., 1966, Some features pertinent in exploration of porphyry copper deposits, in Titley, S. R., and Hicks, C. L., eds., Geology of the porphyry copper deposits, southwestern North America: Tucson, Univ. Arizona Press, p. 75-85.

Kesler, S. E., Issigonis, M. J., and Van Loon, J. C., 1975a, An eval- uation of the use of halogen and water abundances in efforts to distinguish mineralized and barren intrusive rocks: Jour. Geo- chem. Explor., v. 4, p. 255-245.

Kesler, S. E., Issigonis, M. J., Brownlow, A. H., Damon, P. E., Moore, W. J., Northcote, K. E., and Preto, V. A., 1975b, Geo- chemistry of biotites from mineralized and barren intrusive sys- tems: Ecoa. GEOL., v. 70, p. 559-567.

Levinson, A. A., 1974, Introduction to exploration geochemistry: Wilmette, Illinois, Applied Pub., p. 614.

Llaumet, C., Olcay, L., Marin, C., Marquardt, J. C., and Reyes, E., 1975, E1 yacimiento cobre porfidico "Andacollo", Provincia de Coquimbo, Chile: Rev. Geol. Chile, v. 2, p. 56-66.

Mason, D. R., and Feiss, P. G., 1979, On the relationship between whole rock chemistry and porphyry copper mineralization: Eco. GEOL., v. 74, p. 1506-1510.

Mason, D. R., and McDonald, J. A., 1978, Intrusive rocks and porphyry copper occurrences of the Papua New Guinea-Solomon Islands region: A reconnaissance study: ECON. GEOL., v. 75, p. 857-877.

Neumann, H. J., 1972, Sierra Del Jardin, Provincia de Atacama, geologia y alteracion hidrotermal: Santiago, Chile, Instituto de Investigaciones Geologicas, unpub. company rept., 19 p.

Olade, M. A., and Fletcher, W. K., 1975, Primary dispersion of rubidium and strontium around porphyry copper deposits, High- land Valley, British Columbia: Ecoa. GEOL., v. 70, p. 15-21.

-- 1976, Trace element geochemistry of the Highland Valley and Guichon Creek batholith in relation to porphyry copper mineralization: Ecoa. GEOL., v. 71, p. 755-748.

Oyarzun, J., 1975, Distribucion de Cobre, molibdeno, plomo y rub- idio en el area de Sierra Jardin, Provincia de Atacama: Santiago, Chile, Instituto de Investigaciones Geologicas, unpub. company rept., 15 p.

Sillitoe, R. H., 1972, A plate tectonic model for the origin of porphyry copper deposits: Ecoa. GEOL., v. 5, p. 184-197.

Tobar, A. B., 1977, Stratigraphy and structure of the El Salvador- Potrerillos region, Atacama, Chile: Unpub. Ph.D. thesis, Berkeley Univ., 117 p.

Tarney, J., Saunders, A.D., Weaver, S. D., Donnellan, N. C. B., and Hendry, G. L., 1978, Minor element chemistry of basalt from Leg 49, North Atlantic Ocean: Deep Sea Drilling Project, Initial repts., v. 49, p. 567-691.

Zentilli, M., 1974, Geological evolution and metallogenic relation- ships in the Andes of northern Chile between 26 and 29 south: Unpub. Ph.D. thesis, Queen's Univ., Kingston, 410 p.

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