Jan 9, 2010 - for prospecting for hydrothermal deposits, but, in low-grade ...... Clark, A.H., Archibald, D.A., Lee, A.W., Farrar, E., and Hodgson, C.J.,1998,.
©2012 Society of Economic Geologists, Inc. Economic Geology, v. 107, pp. 1457–1478
Recognizing Propylitic Alteration Associated with Porphyry Cu-Mo Deposits in Lower Greenschist Facies Metamorphic Terrain of the Collahuasi District, Northern Chile—Implications of Petrographic and Carbon Isotope Relationships Merline L. Djouka-Fonkwé,1,* Kurt Kyser,1,† Alan H. Clark,1 Esteban Urqueta,1 Christopher J. Oates,2 and Christian Ihlenfeld3 1 Department 2 El
of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Refugio, 49 School Lane, Chalfont St. Peter, Gerrards Cross, Buckinghamshire SL9 9AZ, United Kingdom
3 Geochemistry
Division, Anglo American plc, 20 Carlton House Terrace, London SW1Y 5AN, United Kingdom
Abstract The Collahuasi district is a porphyry Cu-Mo subprovince located on the Andean high plateau of northern Chile. Late Eocene-early Oligocene mineralization in the district involved hydrothermal alteration of the upper Paleozoic to Lower Triassic Collahuasi Group granodiorites, diorites, andesites, dacites, and rhyolites, which had previously experienced regional lower greenschist facies metamorphism as well as uneconomic porphyrystyle mineralization and associated sericitic alteration. Alteration mapping represents an effective technique for prospecting for hydrothermal deposits, but, in low-grade metamorphic terrains, recognition of propylitic assemblages that represent the boundary of the mineralizing system is hampered by their resemblance to metamorphic mineral assemblages. Our results from Collahuasi indicate that petrographic criteria can distinguish propylitic alteration from the effects of lower greenschist facies metamorphism, but distinctions vary with rock type. Mafic, intermediate, and felsic rocks with varying degrees of propylitic alteration can be distinguished from background metamorphism mainly by the presence of calcite. However, propylitic alteration can also be distinguished by high whole-rock d13C values and carbon contents (–19.1 to –1.4‰ and up to 0.79 wt %, respectively) that result from the incorporation of magmatic CO2 during the mineralizing events. Lower d13C values (–32.7 to –13.1‰) and carbon contents ( propylitic Sericitic > propylitic Propylitic Propylitic Background RGM Argillic Propylitic Background RGM Sericitic Sericitic Argillic Argillic > propylitic Sericitic Sericitic Propylitic Propylitic Sericitic Propylitic Background RGM Background RGM Propylitic Background RGM Background RGM Propylitic > sericitic Sericitic Sericitic > propylitic Propylitic > sericitic Sericitic
Degree of alteration 4 3 1 3 2 1 4 1 3 2 4 1 4 2 2 3 3 3 3 1 2 1 3 2 2 2 1 2 2 3 3 2 1 3 1 1 2 3 1 2 4 3 2 3 4 1 3 2 2 2 4 3 3 2 2 2 2 2 2 2 1 4 4 2 1 3
d13C (‰)
Total C (%)
–23.9 –7.7 –23.0 –22.6 –21.1 –23.8 –23.1 –23.1 –24.6 –7.0 –22.3 –24.6 –20.7 –5.5 –5.2 –4.3 –6.4 –18.5 –19.1 –23.8 –21.2 –24.3 –25.4 –3.7 –24.2 –5.2 –12.6 –17.8 –7.4 –3.6 –21.6 –6.6 –24.3 –24.3 –24.0 –7.9 –15.8 –24.5 –17.0 –18.7 –4.6 –5.6 –19.0 –22.8 –5.6 –24.1 –23.6 –22.5 –21.5 –24.5 –20.1 –27.7 –10.8 –6.4 –20.2 –1.4 –19.1 –19.5 –19.0 –21.6 –20.7 –13.7 –24.1 –20.6 –5.9 –22.7
0.02 0.04 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.08 0.01 0.01 0.02 0.26 0.15 0.47 0.20 0.01 0.02 0.01 0.01 0.01 0.02 0.70 0.01 0.09 0.02 0.02 0.12 0.57 0.01 0.23 0.01 0.01 0.01 0.12 0.03 0.03 0.01 0.01 2.80 0.03 0.01 0.01 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.79 0.14 0.01 0.10 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.29 0.01
Carb. d13C (‰)
Carb. d18O (‰)
–5.1 –3.3
19.1 14.1
–5.8
19.1
–3.6
12.5
–3.3 6.7 –4.2
18.9
–3.5
23.0
–4.5
12.3
–3.4
22.7
–5.6
15.2
1478
Djouka-Fonkwé ET AL. Table A4. Mineral Assemblages and Carbon and Oxygen Isotope Compositions of Rhyolites from the Collahuasi District
Sample no. East North Mineral assemblage Alteration type MAA21501 534160 7664180 Ser-Clay Background RGM MAA21502 532070 7664150 Ser-Ms-Hem-Clay Background RGM MAA21504 527800 7664070 Ms-Cal Sericitic > propylitic MAA21510 530330 7666240 Cal-Ser-Clay-Hem Propylitic MAA21512 533740 7665430 Ser-Clay-Hem Background RGM MAA21513 535250 7668350 Ser-Clay Sericitic MAA21522 528140 7668250 Ser-Clay-Hem Background RGM MAA21541 524000 7672210 Ser-Clay Background RGM MAA21542 521980 7672040 Ser-Rt Sericitic MAA21544 520200 7673860 Cal-Ep-Chl-Ms Propylitic MAA21546 522270 7673940 Ser Background RGM MAA21547 522950 7673900 Ser-Clay Background RGM MAA21552 523000 7675180 Cal-Ser Propylitic MAA21553 524070 7675280 Ser-Qtz Background RGM MAA21554 525040 7675293 Ser Sericitic MAA21557 524020 7676460 Ser-Clay Sericitic MAA21558 524890 7676010 Ser Background RGM MAA21569 523010 7676380 Ser-Clay Sericitic MAA21575 523700 7678330 Ser-Hem-Clay-(Ep) Sericitic MAA21577 526130 7677970 Cal-Ser-Clay Propylitic > sericitic MAA21580 529990 7678190 Ser-Hem-Clay Background RGM MAA21592 523760 7679440 Cal-Ms Propylitic > sericitic MAA21596 528970 7681280 Clay-Ser Argillic MAA21601 525260 7679810 Clay-Ser Argillic MAA21605 528310 7677440 Clay-Ser Argillic MAA21607 523730 7680000 Ser-Qtz Background RGM MAA21619 526050 7682410 Clay Background RGM MAA21624 532280 7678060 Ser-(Clay) Background RGM MAA21625 531020 7678100 Ser-Clay Background RGM MAA21626 531110 7677210 Ser-Qtz-Hem Sericitic MAA21631 532980 7676810 Cal-Ser-(Clay) Propylitic MAA21632 533030 7675060 Ser-Qtz Sericitic MAA21637 534120 7679970 Cal Propylitic MAA21639 533120 7679890 Ser-Clay Sericitic MAA21642 533950 7678090 Cal-Hem-(Ser)-(Clay) Propylitic MAA21643 535380 7678280 Ser-Hem-Qtz Sericitic MAA21644 534080 7678980 Ser Sericitic MAA21645 535090 7679330 Ms-Clay Sericitic MAA21646 533010 7676230 Ser-Hem-Clay Sericitic MAA21650 536130 7675010 Cal-(Ser) Propylitic MAA21654 537980 7675900 Cal-(Ser) Propylitic MAA21660 521350 7683110 Ser-(Clay_ Sericitic MAA21662 523040 7683070 Ser-(Clay) Sericitic MAA21663 524200 7683120 Ser Sericitic MAA21664 525020 7683160 Clay-(Ser) Argillic MAA21665 525840 7682991 Clay-Ser-Rt Argillic MAA21670 527020 7683980 Ser-(Clay) Sericitic MAA21671 527130 7684860 Ser Sericitic MAA21674 526080 7684050 Ser-(Clay) Sericitic MAA21678 534130 7681300 Cal Propylitic MAA21679 534990 7681160 Cal-Clay-Hem Propylitic MAA21681 537150 7681100 Cal-Ser-Hem Propylitic MAA21682 537900 7681180 Clay-Cal Argillic > propylitic MAA21683 537960 7680390 Clay Argillic MAA21685 536070 7680220 Ser Sericitic MAA21695 529980 7685010 Clay-Ser Argillic > sericitic MAA21697 531980 7684110 (Ser)-(Clay) Background RGM MAA21699 532830 7683010 Ser-Cal Sericitic > propylitic MAA21701 534050 7682070 Ser-Hem-Clay Background RGM MAA21702 535070 7682020 Cal-Ser-Clay Propylitic MAA21703 535920 7681880 Cal-Ser-Clay Propylitic MAA21709 537880 7682000 Ser-Rt-Cal Sericitic > propylitic MAA21710 524990 7684000 Ser Sericitic MAA21711 523100 7684290 Ms-Rt Sericitic MAA21740 528080 7686030 Ser Sericitic MAA21744 530880 7686040 (Ser) Background RGM MAA21746 531580 7687950 Cal-Ser-Hem Propylitic > sericitic MAA21748 537490 7691850 Ser-Qtz-Clay Sericitic MAA24140 528500 7677870 (Ser) Background RGM MAA24141 529130 7678360 (Ser) Background RGM MAA24142 529430 7678530 Ser-Qtz Background RGM MAA24144 531560 7678990 Ms-Chl-Clay Sericitic > propylitic MAA24146 533520 7679430 (Ser) Background RGM Notes and abbreviations as in Table A1
Degree of alteration 2 2 2 2 1 2 1 1 3 2 1 1 3 1 1 3 1 2 4 3 1 3 4 3 2 2 1 1 1 3 1 2 2 2 3 4 1 4 1 3 2 2 3 2 3 3 2 3 2 1 2 3 1 1 4 2 1 1 1 3 3 3 3 2 1 1 1 3 1 1 1 3 2
d13C (‰)
Total C (%)
–15.8 –17.9 –13.8 –8.8 –19.2 –18.7 –18.7 –23.4 –23.0 –8.5 –22.5 –23.6 –6.8 –22.2 –19.1 –23.6 –20.6 –17.1 –18.6 –6.6 –21.0 –5.7 –22.6 –23.9 –18.9 –20.5 –23.4 –17.3 –18.5 –17.1 –7.6 –29.4 –6.5 –25.3 –3.8 –22.3 –23.5 –22.1 –22.6 –6.8 –8.0 –25.9 –21.0 –22.4 –23.1 –26.4 –22.0 –23.2 –19.7 –8.1 –6.5 –4.9 –14.3 –24.9 –25.2 –23.6 –21.7 –13.1 –23.1 –4.9 –4.4 –18.8 –25.0 –22.6 –22.7 –24.2 –10.8 –19.0 –18.4 –20.2 –24.3 –21.1 –16.8
Carb. d13C (‰)
Carb. d18O (‰)
0.01 0.01 0.01 0.04 0.01 0.01 0.01 –4.0 15.0 0.01 0.01 0.06 0.01 0.01 0.33 0.01 0.01 0.01 0.01 0.01 0.01 0.33 –6.2 10.1 0.01 0.33 0.02 0.01 0.01 0.04 0.01 0.01 0.01 0.01 0.03 0.01 –4.5 19.5 0.09 0.01 0.06 –1.5 21.2 0.01 0.01 –3.6 11.3 0.01 0.01 0.05 0.04 0.01 –3.7 17.7 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.24 –5.4 10.6 0.18 0.03 –4.2 11.4 0.01 –5.0 15.5 0.01 0.01 0.01 0.02 0.01 –6.0 14.4 0.20 –3.9 11.8 0.19 –3.8 9.9 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.02 –4.1 14.0
