THE GEOLOGY AND GEOCHEMISTRY OF THE

GUNUNG PANI GOLD PROSPECT, NORTH SULAWESI,

INDONESIA

by

Imants Kavalieris

A thesis submitted as the requirement for admission to the

Degree of Master of Science at the Australian National University

October 1984

-89-

IV. GEOCHEMISTRY OF THE PANI VOLCANIC COMPLEX AND RELATED ROCKS

4.1 Introduction

The importance of whole rock geochemistry in

this study lies in its possible application to the

following problems:

(1) classification and comparison of the Pani Volcanics

to similar geochemical and lithological suites;

(2) interpretation of tectonic environment, in terms

of island-arc versus continental margin; and

(3) nature of the alteration system.

4.2 Samples, methods and limitations of data

A summary of rock samples analysed by x-ray fluorescence

spectroscopy (XRF) is given in Table 2. In total there

are six samples from Pani ridge, twelve samples from Gunung

Baganite, one from the Tabulo ring-dyke Complex, and two

from the basement granitoids adjacent to the Pani Volcanic

Complex. The samples from Pani ridge and Gunung Baganite

are from lOOgm splits of 2m drill core intervals, crushed

for Au assay at the P.T. Tropic Endeavour laboratory,

Gorontalo.

Sample preparation and laboratory techniques for

the Gorontalo laboratory are described by Carlile and

Hisamuddin (1983) ; it suffices to mention that size

reduction and final milling utilized tool steel equipment.

As a result minor Fe contamination in major elements, and

significant Mn, Cr, V contamination in trace elements is

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Table 2 Summary of geochemical

LOCATION DRILLHOLE DEPTH LITHOLOGY ALTERATION

Pani ridge.

Pani ridge

Pani ridge

Pani ridge

Pani ridge

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Gunung Baganite

Pani ridge

Sungai Batudulanga

Tabulo

Transit

(m) s Silicified 0 Oxidised

(Above WT)

GPO l 42-44 Rhyodacite s, 0, Qtz-Alb-Kf Chl-Py-An

GPO l 212-214 Rhyodacite s, Qtz-Alb-Kf Chl-Ser-Py-An

GPO 2 16-18 Rhyodacite s, 0, Qtz-Alb-Chl-py-An

GPO 2 180-182 Rhyodacitic Chl-Ser-Cal lapilli tuff (Little altered)

GPO 3 134-136 Rhyodacite Chl-Ser-Cal

GPO 4 48-50 Rhyodacitic s, 0, Qtz-Alb-Kf lapilli tuff Chl-Ser-Py

GPO 4 122-124 Rhyodacite s, Qtz-Alb-Chl-Py-An

GPO 4 160-162 Rhyodacite s, Qtz-Alb-Kf Chl-Py-An

GPO 4 200-202 Rhyodacite Qtz-Alb-Kf Chl-Py-An

GPO 4 278-280 Rhyodacite Alb-Kf Chl-Py-An

GPO 4 326-328 Rhyodacite Kf-Alb-Chl-Ser-Fe/Mn Carb-Py-An

GPO 4 398-400 Rhyodacite Kf-Alb-Chl-Ser-Fe/Mn Carb-Py-An

GPO 5 102-104 Rhyodacite Alb-Chl-Ser-Py-An

GPO 5 234-236 Rhyodacite Alb-Chl-Ser-Py-An

GPO 6 52-54 Rhyodacite Alb-Chl-Ser-Py-An

GPO 6 118-120 Rhyodacite Alb-Chl-Ser-Py-An

GPO 6 189-190 Rhyodacite Alb-Chl-Ser-Py-An

GPO 7 114-116 Rhyodacite s, 0, Qtz-Alb-Kf Chl-Ser-Py-An

Foliated Minor Chl microgran-odiorite

Hb/Bi Minor Chl, Sph microgran-odiorite

Granodiorite Qtz-Chl-Ep-Py

Symbols used:

Qtz : quartz; Alb : albite; Kf : K-feldspar; Chl : chlorite; Ser : sericite; Py : pyrite; An anatase; Sph sphene; Ep : epidote; WT : Water table

AU ASSAY

(ppm)

0.11

0.94

0.20

0.19

0.68

0.31

0.13

0.18

0.99

0.25

0.86

0.52

0.16

0.17

5.84

samples

SAMPLE UTAH NUMBER SAMPLE NO.

IKS l 40070

IKS 19 40154

IKS 7 44358

IKS 4 44440

IKS 18 44605

IKS 2 44174

IKS 10 44211

IKS ll 44230

IKS 12 44250

IKS 6 44289

IKS 13 44483

IKS 14 44524

IKS 15 44710

IKS 5 44776

IKS 16 44808

IKS 17 44835

IKS 3 44875

IKS 20 44957

IKS 8

IKS 9

IKS 21

-91-

possible. This is apparent for Cr which must be

disregarded for samples IKS 1-7, IKS 10-20. All other

samples were size-reduced and milled to -2~m at the

Geology Department, Australian National University (AND) ,

utilising tungsten carbide equipment.

Major elements, including Na, were determined

in duplicate by XRF methods (Norrish and Hutton, 1969)

on lithium tetraborate glass discs, and trace elements were

determined in duplicate on pressed powder discs (Norrish

and Chappell, 1977), with corrections utilising direct

measurement of mass absorptions of Rb and Sr Kcr (methods

by Chappell, AND, unpublished). FeO was measured in

duplicate by titration against K2Cr207, and H20+ + C02 ·

by collection in microabsorption tubes.

In addition, two samples (S266, Una-Una) were

analysed for major elements by energy-dispersive TPD

electron probe at the Research School of Earth Sciences,

AND (see Ware, 1980 for computer programs and calibration),

on glass beads prepared by fusion (similar methods to

Nicholls, 19 7 4) •

The samples from the basement and the Pani

Volcanic Complex, with the exception of S266, are strongly

modified by hydrothermal alteration and weathering. The

freshest sample excluding Una-Una is IKS 9, a hornblende/

biotite microgranodiorite from the Tabule ring-dyke Complex.

In general, since it is not possible to compare

altered rocks with completely fresh rocks in the Pani

Volcanic Complex, and due to the limited number of samples

representing possible related volcanics, or basement, the

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primary geochemical characteristics of the Pani Volcanics

are difficult to separate from the effects of alteration.

4.3 Major and trace-element data (Table 3)

4.3.1 Pani Volcanics

Within the limited Si02 range (69-75%) available

for the Pani Volcanics a wide scatter of points likely

due to alteration, largely precludes meaningful evaluation

of primary trends of major eleraents (Figs. 8, 9) .

Major elements can be interpreted to exhibit

typical trends for acid calc-alkaline rocks, i.e. Al203,

MgO, CaO, Na20, P205, Ti02, MnO and FeO decrease with

increasing Si02, while K20 shows a slight increase.

These trends are very poorly demonstrated by the variation

diagrams.

The dataare most consistent for Ti02 which

averages 0.2% at 72% Si02. The Tabule ring-dyke (IKS 9)

initially assumed consanguineouson petrographic grounds

to the Pani Volcanic Complex plots within the range of the

data for most elements, and could belong to the same

differentiation trend. Of the basement granitoids IKS 21

is clearly a separate geochemical suite to the Pani

Volcanics.

A comparison of silicified with non-silicified rhyodacites

and hornblende/biotite microgranodiorite (Tabule) is shown

in Table 4 • This table also shows data for Gunung

Bulu (unaltered Tpii lithology) and the Una-Una volcano,

a volcanic island 60km SW of Marisa in the Gulf of Tomini.

Table 3 : Major and trace-element data

SaMplet IKS1 IKS2 IKS3 IKS4 IKSS IKS6 IKS7 IKSB IKS9 IKS10

Si02 73.30 70.90 70.90 70.30 70.80 ?5.20 73' 00 66.50 70.00 73.30 Ti02 0.18 0.21 0.21 0.20 0,19 0 '16 0.20 0.44 0.29 0.18 Al203 13.27 14.91 14.60 13.95 13.70 12.05 13.57 16.25 15.18 12.54 Fe203 0.19 0.31 0.81 1.72 0.27 1.40 O.bO 2.12 i. 08 FeO 1.27 i. 80 1 • .22 1.57 1.74 i. 30 i. 73 0.98 i.6b i. 51 HnO 0.02 0.02 0.03 0.04 0.07 0,04 0.01 0.04 0.05 0.05 HgO 0.04 0.12 0.35 0.52 0.44 0.19 0.14 1.40 0.81 0.12 C<>O 0.10 0.12 0.19 0.18 1.92 0 '11 0.17 3.98 2.33 0' 10 Na20 3. 04 2.73 3.66 3.63 3. 01 2.70 3.64 3.77 4.45 i. 84 K20 6.41 6.57 5.58 6.39 3.68 4.72 5.69 2.88 J, 14 7' 10 P20S 0.04 0.02 0.04 o.os 0.03 0.02 O.Ob 0.08 0.03 0.03 s 0.01 0.01 0.17 0.31 0' 20 0.15 0.01 0.01 H20+ 1.24 1.09 0.89 1.35 0.41 0.81 o.ss 0.92 0.52 0.78 H20-· 0.29 0.25 0.45 0.21 0.15 0' 11 0.20 O.Sb 0.22 C02 0.15 0.15 0 '61 0,34 0.10 0.47 0.90 0.22 0.76 0' 12 rest 0.20 0.18 0.20 0.16 0' 19 0' 19 0' 19 0.19 0' 18 0.19

99.71 101.16 96,96 100.71 O=S 0.08 0.15 0.10 0.07 t otol 99.75 99.14 99.63 101.01 96.86 99.51 100.64 99.97 99.97 99.17

Trace eleMents

Ba 1>75 570 690 b3S 640 625 585 750 675 640 Rb 312 216 233 137 221 241 255 82 76 299 I Sr 384 293 375 277 440 400 41>6 462 458 346

1.0 Pb 35 54 57 46 55 57 34 29 14 43 Th w u I Zr 111 91 110 87 97 100 99 111 121 86 Nb o.s 7.0 9.0 7.0 B. 0 8.0 7.5 b.S 7. 0 7. 0 y 12 12 15 12 14 15 11 12 13 13 La 18 17 17 18 1b 1b 14 21 17 14 Ce 35 34 38 39 35 37 33 44 39 29 Sc v ib 12 20 20 13 12 8 27 48 ib Cr 49 75 35 61 54 32 44 6 19 54 Hn 125 440 285 605 120 390 iSS sos 375 435 Co Ni 3 4 5 3 5 5 3 3 6 3 Cu 5 8 10 4 10 19 7 3 s 13 Zn 1>3 159 BS 41> 40 51 80 43 28 61 Ga

CIP~l norMS

Q ;10, 0 28.2 26.9 23.4 33.3 40.4 28.6 23.2 24.7 34.3 c 1.2 3.1 2.2 0.8 1.3 2.3 1 '2 0.2 1.1> or 37.9 38.8 33.0 37.8 21 .a 27.9 33.6 17.0 1B.b 42.0 ab 25.7 23.1 31.0 30 '7 25.5 22.9 30.8 31.9 37.7 15.6 an 0 .s o. 7 0.9 O.B 9.1> 0.7 0.7 18.9 11 '7 0.5 di 0 .5 hy 2.0 3. 0 1.8 1.9 3.5 1.5 2.4 3.3 4.7 2.0 Mt 0.3 0.5 1. 2 2.5 0,4 2.0 0.9 2.0 1.1> i1 0 '3 0.4 0.4 0.4 0.4 0.3 0.4 0 .B 0.1> 0.3 hM 0.7 ap 0' 1 0' 1 0' 1 0' 1 0' 1 0' 1" 0.1 0.2 0' 1 0.1 pr o.o o.o 0.3 O.b 0.4 0.3 0.0 0.0 total 97.9 97.8 97.8 98.8 9b' 1 97.9 99.0 98.5 98.0 97.9

Silicate analyses by Geological Department, ANU.

Table 3 (cont.)

5oMplet IK511 IK512 IK513 IK514 IK515 IK516 IK517 IK518 IK519 IKS20 IK521

Si02 71.00 71.60 69.60 72.20 71.70 70 .so 70.80 72.80 73.70 73.90 73.10 Ti02 0.20 0.20 0.21 0.20 0.20 0.21 0.23 0.20 0.20 0.19 0,32 Al203 13,37 13.42 14.50 14.54 13.97 14.40 15.88 14.05 13.83 13.32 13.10 Fe203 0.86 0.67 0.40 0.50 1.04 1.15 1.15 0.53 0.67 0.52 1.19 FeD 3,02 2.01 2.40 1.79 1. 01 0.87 1. 03 1.48 1. 01 1.28 0,90 MnO o. 06 0.07 0,04 0.07 0.03 0.04 o. 04 0.01 0.02 0.02 0.04 MgO 0. 31 0.37 0.50 0.42 0.27 0.25 0.40 0.27 0.31 0.22 0.85 CoO 0.18 0.21 0,55 0.32 0.13 0.11 0.15 0.15 0.22 0.13 2.27 No20 3,33 2.81 3.40 4.00 2.86 2.33 2.86 2.96 1.92 2.27 4.'12 K20 5.63 5.95 5,18 3,68 6.7'1 6.05 5.30 4.62 4.97 5.51 0,86 P205 0.01 0.12 0.04 0.02 0.02 0.04 0.02 0.01 5 o. 04 0.29 0.16 0.16 0.1'1 0.28 0.10 0.09 H20+ 0.71 0.55 1.02 1. 06 0.85 1.51 1. 51 1.10 1.12 1. 08 1.17 H20- 0.13 0,27 0.21 0.19 0.08 0.31 0.40 0.27 0.20 0.20 0.27 C02 0.34 0.59 0,63 0.65 0.63 0,27 0.12 0.12 0.52 0. 61 0.75 rest 0.20 0.19 0.20 0.18 0.21 0.20 0.20 0.18 0.17 0.18 0.08

99.38 99.21 99.12 100.00 98.91 99.18 99.55 99.72 0=5 0.02 0.14 0.08 0.08 0.08 0.14 0.05 0.04 total 9?.36 'i''i'.07 9'i'. 04 9'i'.92 'i''i'.77 'i'8.52 100.07 98.83 9'i'. 04 99.50 'i'9,68

Trace e:lettents

Bo 650 600 750 535 730 735 '120 590 565 5'i'O 130 I Rb 236 258 230 166 2'i'6 283 251 231 250 266 31.0

1.0 Sr 417 362 411 329 397 318 301 2'12 228 319 216 Pb 52 51 51 43 53 56 49 53 28 36 20 ,j:::.

Th I u Zr 9'i' 9'i' 10'i' 110 102 113 122 110 112 114 116 Nb 8.0 8.5 'i'.5 8. 0 9,0 8.5 10.0 'i'. 0 8.5 7.5 1.0 y 14 14 15 14 13 15 14 15 12 12 35 La 16 16 17 16 16 18 1'i' 1'1 15 1'1 7 Ce 33 35 39 34 33 40 38 36 34 36 17 Sc v 16 17 20 18 18 16 19 19 18 1'1 27

Cr 75 4? 43 48 83 43 36 40 17 4'i' Mn 560 630 375 650 255 365 330 105 140 160 350 Co Ni 4 4 4 4 3 3 4 5 3 2 Cu 15 15 6 8 6 6 3 10 3 5 !:i

Zn 87 85 43 147 55 95 123 87 125 6'1 2'i'

Go

CIPW norMs

Q 27.1 30.3 26.7 32.5 28,1 33.3 32.6 36.4 42.4 38.3 36.2

c 1.4 1.'i' 2.5 3.4 1.7 3.8 5,1 3.8 4.'i' 3.4 0.3

or 33,3 35.2 30.6 21.8 40.0 35.8 31.3 27.3 29.4 32.6 5.1

ob 28.2 23.8 28.8 33.9 24.2 19.7 24;2 25.1 16.3 19.2 39 .. 9

on 1.2 1.2 2.2 1.5 0.8 0.7 1.0 1.0 1.0 0. 7 11.3

dl. 2.2 hy 5.3 3.3 4.7 3.4 1.4 1.0 1.6 2.3 1.2 2.0 Mt 1.3 1.0 0.6 0.7 1.5 1.7 1.7 0.8 1.0 0.8 1.7

l.l 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.6

h .. o.o op o.o 0.3 0.1 0.1 0.1 0 .1 0.1

0,2 0 .1 0.5 0.3 0.3 0.3 0.5 0.2 pr 97.5

1 o tal 98.0 97.6 9'1.1 97.9 98.1 96.3 97.9 97.2 97.1 97.5

Silicate analyses by Geological Department, ANU.

_____________ ,.,. . ---.-~ .... -.~~·-·-·--~------"-. --·-----~·

Sample

IKS 3-6 11-17

IKS k,7, 10,19,20

IKS 9

IKS 8

IKS 21

Fig. 8

-95-

Sample code and symbols used for variation diagrams (Fig. 8 and 9 )

Symbol Lithology Unit

0 Unsilicified Pani Volcanic rhyodacite Complex (Tpi)

E9 Silicified Pani Volcanic

® rhyodacitics Complex (Tpi) and pyroclastics

Hornblende/ Tabulo ring-biotite micro- dyke Complex granodiorite (possibly Tpii)

0 Fine-grained Basement foliated micro-granodiorite

Medium-grained Basement granodiorite (low K-granitoid)

1) Regression line only for the Pani Volcanic Complex and Tabulo ring-dyke Complex.

2) The regression line has little meaning for 8c,d,e,g, but is shown for uniformity of presentation.

3. The wide scatter of data is generally attributed to alteration.

-96-

Fig. 8 Variation diagrams

-97-

Fig. 9

1) Regression line only for the Pani Volcanic

Complex and Tabule ring-dyke Complex.

2) The wide scatter of data precludes meaningful

interpretation of trends in most cases, especially

9 a,b,f. Regression line is shown for uniformity

of presentation.

3) 9 g ,h show the data in terms of discrimination

diagrams after Winchester and Floyd (1976).

Refer to Fig .. 10.

Alkaline and subalkaline fields are shown and the

dividing line at 72% Si02 divides the data into

rhyolite and rhyodacite. For convenience

rhyodacite is used throughout the text.

6

0

~4

2

700

600

500

dl 400

300

200

100

0. 03

..D

~ 0.02 :::.::

0. 01

0.08

2l 0.06 -~

1-, ~ 0.04

0.02

Fig. 9

A

66 68

c

2

E

2

G

66 68

-98-

Variation diagrams

e o • 0 0 6> _@_ .. ---

-cr-.,-- @

0 0

A

70 72

A

0

X Si02

4 X K20

4 X K20

70 72 X Si02

0 0

74 76

6

•

6

74 76

B

0. 12 0

Ln

~ 0.08 a..

--~ 0.04 ct- _o @@

0. 0 - .... _

• 0 0---o_

0 * 66 68 70 72 74 76

X Si02

D

300

0

• 100

2 4 6 X K20

F

300

100 200 300 400 Sr ppm

0.7~H _____ ~_~ __ LI_NE ______ 0 ~0~1~ 0 i ·---0. 6 - - .-,--o-tlo -0 -

A ~ 0 0 E 0. 5 i a.. i

:- 0· 4 SUQALKALINE l '- I

:@ 0.3 j" ::~~l~~--L-~~~-L~~~--L-~

66 68 70 72 74 76 % Si02

-99-

Table 4 Geochemistry of Pani Volcanics and related rocks

LOCATION Pani prospect Tabule Gunung ring-dyke Bulu

SAMPLE IKS3 - IXSl. 7, IKS 4 IKS 2 S726 S226 6,11,18 10,19,20 (IKS9) (2)

CLASSIF-'l'pi 'l'pii 'l'pii

I CATION

LITHOLOGY Rhyodacite Silicified · · Rhyodacitic Silicified Hb/Bi Black Bi rhyodacite pyroclastic rhyodacitic mgd. rhyodacite

pyroclastic

5102 73.58 75.13 71.33 72.43 71.47 73.39 Ti02 0.20 0.19 0.20 0.21 0.29 0.17 Al2o3 14.43 13.61 14.16 15.23 15.50 15.64 Fe2o 3 0.82 0.62 1. 75 0.32 FeO 1.66 1.39 1.59 1.84 1.69(1) 1.17(1) MnO 0.05 0.02 0.04 0.02 0.05 0.05 MgO Q.35 0.17 0.53 0.12 0.83 0.81 cao 0.38 0.14 0.18 0.12 2.38 2.33 Na2o 3.17 2.60 3.68 2. 79 4.54 4.45 K20 5.35 6.07 6.48 6. 71 3.21 3.14 Pao5 0.03 0.04 0.05 0.02 0.03 0.03

K 44 416 50 393 53 797 55 706 26 649 8a 660 611 635 570 675 Rb 241 276 137 216 76 Sr 366 349 227 293 458 Pb . 52 35 46 54 14 Zr 106 104 87 91 121 Nb 7.8 7.8 7.0 7.0 7.0 y 14 12 12 12 13 La 17 16 18 17 17 Ce 36 33 39 34 39 v 17 15 20 12 48 Ni 4 3 3 6 cu 10 7 4 8 5 Zn 82 79 48 159 28

K/Rb 184 183 393 258 351 Rb/Sr 0.66 o. 79 0.60 o. 73 0.17 K/Ba 67 82 85 98 39 Sr/Ba 0.55 0.57 0.36 0.51 0.68

NO. OF 11 SAMPLES

Major element analyses recalculated on an anhydrous basis to total 100\.

(1) FeO total iron (2} 5266 Average from 12 probe analyses on a fused sample ( 3) Una-Una Average from 10 probe analyses on a fused sample {4) Analysis by l<oomans, 1934

una-una Volcano

una(J) 423(4)

.llb/8i Hb/Bi andesite andesite

64.44 63.05 0.25 0.38

17.75 15.49 3.66

3. 36(1) 1.25 0.09

2.24 2.12 3.42 4. 70 5. 38 5. 75 2.92 3.27 0.21 0.22

-100-

Silicification correlates with an increase in

K20 (and Rb) and depletion in FeO, MnO, MgO, cao and

Na20. A similar trend is apparent for the pyroclastics,

but the data is insufficient to be meaningful.

Of the major elements, Ti02 and possibly P205

are the least affected, while CaO is the most affected

(depleted) by secondary processes.

Table 5 compares oxidised (above the watertable)

and non-oxidised silicified rhyodacites. It suggests

Fe0/Fe203 ratios are unaffected, and in general the effects

of weathering may not be important.

The most strongly altered rhyodacites are

obviously silicified and Na-alkali metasomatised (section

5.2.2), but it is likely all rocks analysed from the prospect

exhibit this alteration to varying degrees. As is apparent

from the Si02 variation diagrams alteration and differentiation

trends are not readily separable and may in fact lead to

similar changes.

This may be consistent with the argument (section

~2) that silicification and accompanying alkali metasomatism

is deuteric, and an extension of late magmatic processes,

rather than due to later superimposed hydrothermal

alteration.

Despite problems of alteration, in general it is

thought that the Pani prospect rhyodacites (Tpi} have a

restricted Si02 (69-75%) and are highly potassic (even if

K2o was added later) and have a high total alkali content,

corresponding with possibly low primary (as well as

Table 5

Si02 Ti02 Al203 Fe203 FeO MnO MgO CaO Na20 K20 P205

-101-

Comparison of major element geochemistry of oxidised and non-oxidised silicified rhyodacites

Non-Oxidised Oxidised

75.47 74.70 0.19 0.19

13.54 13.59 0.90 0.95 1.29 1.45 0.03 0.02 0.23 0.13 0.16 0.13 1.93 3.02 6.20 5.96 0.04 0.04

No. of samples 2 3

Major element analyses recalculated on an anhydrous basis to total 100%.

Note: The small number of samples are individually variable and the value of this comparison is therefore limited.

-102-

secondary) cao and MgO. Upon the alkali content

(K20 + Na20) of 8-10%, the composition is strongly

peraluminous. However the normative corundum shown for

most samples is probably due to loss of CaO. Where cao

has not been lost (IKS 9) corundum is not significant.

It can be argued from a subsequent section

(5.8.1) on the chemistry and alteration of the Baganite

rhyodacite dome that a high K20/Na20 ratio (>1) may be a

primary feature. Irrespective of the Pani Volcanic

Complex data, the K20 content from the unaltered sample

IKS 9 from the Tabule ring-dyke Complex still allows these

rocks generally to be classified as high K-rhyolites

(Ewart, 1979).

In addition the Tabule Complex rocks (Tpii) contain

hornblende, and therefore higher cao, whereas the Tpi

rhyodacites in the prospect show no evidence from their

crystallisation history (section 3.5.1.6)to have precipitated

either early or late hornblende. Instead the main

ferromagnesian mineral is biotite, with sphene (Ca bearing)

as a late accessory phase in the groundmass. Consequently

Tpi volcanics are likely to be more potassic, than Tpii.

Finally, petrographic studies of alteration

(section 5.2) suggests that alkali metasomatism mainly

involves albitisation.

Therefore an addition of a few percent K20

during alteration is unlikely, and although there is good

evidence for a loss of CaO (lack of Ca bearing minerals,

primary or secondary) a high K20 would probably correlate

with low primary CaO in Tpi volcanics.

-103-

In general, the 'incompatible', large ion trace

elements Ba, Rb, Sr, Pb of 'potassic-type' are particularly

important in understanding the geochemistry of felsic

rocks. Unfortunately they may be susceptible to secondary

alteration, particularly alkali metasomatism of the

feldspar phenocrysts. Noble et al. (1979) suggest Sr is

added rather than depleted during near surface alteration

of silicic ash-flow tuffs.

Sr (av 403 ppm) seems high for the quartz­

biotite-sanidine rhyodacites, and may have been affected

by Na alkali metasomatism, but it is also high in the

Tabule ring-dyke Complex (Sr 458 ppm) which does not show

appreciable albitisation.

Rb (av 243 ppm) is also high and corresponds to

high K20 in Tpi (K/Rb<200), while at Tabule, Rb is

relatively lower (K/Rb 351) .

Pb (av 47 ppm) (as well as Cu and Zn) are part

of the sulphide alteration mineralogy at the Pani prospect

and higher levels are not surprising. Ba abundance

(av 640 ppm) in silicic rocks follows K, being held in

K-feldspar, but also in biotite, which complicates its

behaviour at high Si02 compositions (Jake~ and White, 1972)

and final differentiation trends for silicic rocks often

show a decrease in Ba. Tabule with lower K2o has a

relatively higher Ba content (675 ppm) . Whether the

Ba content of the Pani Complex is due to differentiation

or depletion due to alkali metasomatism of plagioclase

and chloritisation of biotite is not clear.

-104-

In summary, the usefulness of Ba, Rb, Sr, Pb

is complicated by possible mobility due to alteration;

nevertheless by comparison with Tabulo it is thought

high Sr, and a low K/Rb ratio (<400) may be a primary

characteristic of the Tpi rhyodacites.

The 'immobile' trace elements La, Ce, Y, Nb, Zr

show a strong coherence for all the Pani Volcanics and

suggest very strongly that the Tabulo ring-dyke Complex

is consanguineous.

The absolute abundance of the light rare earth

elements (LREE), (La and Ce) is low, particularly when

compared to silicic rocks formed by crystal fractionation

(e.g. Noble et al., 1979).

With respect to chrondritic REE abundances

(Hanson, 1980) enrichment of La is by a factor of 54, and

Ce by 43.

Y can be correlated with the behaviour of the

HREE (Taylor, 1966, p.l61) and with Zr and Nb. The low

abundances of these elements reflect low levels of the

HREE's as well. With respect to chrondritic abundances

(Taylor, 1980) Y shows an enrichment of about 7.

Therefore, with the exception of Eu for which no

data are available it is concluded that the Pani Volcanics

are characterised by low and possibly relatively

unfractionated REE abundances.

The content of Zr (av 104 ppm) is close to Zr

saturation in peraluminous melts (Watson, 1979).

-105-

In contrast to the behaviour of the REE, and

the high field strength elements (HFSE) (Zr, Nb, Ti, P),

it seems the Pani Volcanics are strongly enriched in the

large ion lithophile elements (LILE) (K, Rb, Ba, Sr),

possibly including U and Th. This conclusion is based on

data from the unaltered sample from the Tabulo ring-dyke

Complex and accepting arguments that high K20 is a

primary feature of the Pani Volcanic Complex. Hence the

volcanics have a high LILE/HFSE ratio, perhaps unusually

high. Tarney and Saunders (1979) suggest a high LILE/HFSE

ratio may be a fundamental characteristic of subduction

related magmas.

Ni, Ti and V are low (Tables 3, 4. in the

Pani Complex and comparatively higher in the Tabulo ring-

dyke, which has a correspondingly higher content of

ferromagnesian minerals, particularly hornblende.

On discriminant diagrams based on Y/Nb and

Zr/Ti02 ratios (Winchester and Floyd, 1976) the Pani

Volcanics plot in the subalkaline field, as expected

(Fig. 9g, h, Fig. 10).

Usefulness of AFM diagrams or FeO total/MgO

versus Si02 plots (Miyashiro, 1974) are limited due to

alteration. They illustrate that MgO is abnormally low.

4.4 Comparison of the Pani Volcanics with silicic igneous rocks from island arcs and continental margins

Mineralogically the Pani Volcanics in the

prospect area (Tpi) are characterised by phenocryst-rich

-106-

Fig. 10 Y/Nb and Zr/Ti02-sio2 discriminant diagrams

'16 Pan." RHYOLITE ' 12. I

I COHENOITE I

6'd R.H'r'OOKITE I

I PANTELLERITE

64 TRACHYTE

~ &0 0 v; ANDESITE ~ 0 56 PHONOLITE

St S'U8- ALK.l.LINE

1+1 B A.SI\LT

44 NEPI-\ELINITE

40 0·01

Zr-/TaO'J. 0·10 1·00

1(, PaM

RHYOLITE

11 COMENDITE

PI\NTELLER\TE f,~ RHYODACITE

w

f, ~ z

TRili.CH'<TE

C'l '0 .... < 0

~

ANDESITE eo ~ :::> ...I

"' <( TRAC.HY-

ll>.NOESITE

S1 ALKALI

48 SUG -ALKALINE BASALT

eASALT

44

ItO 0·01 o-10 Nb/Y

1·00

Chemtcal d1scnmmatton d1a~rams aHer Winchester <9- Flo~ d. ,(I '116).

PHONOLITE

......... , BMINITE

NEPHELINIT£

10·0

-107-

assemblages comprising plagioclase, quartz, biotite,

sanidine, in rhyodacites and rhyolites; and plagioclase,

quartz, hornblende, biotite, in microgranodiorites; with

accessory phases of magnetite, allanite, sphene and

apatite, as well as zircon.

This mineralogy is compatible in general with

orogenic acid calc-alkaline volcanics and in particular

to high K types (Ewart, 1979, p.l4). It is also

consistent with classifying these rocks as derived from

I-type magmas (Chappell and White, 1974).

Tpii volcanics, including the Tabule ring-dyke

Complex, share textural and mineralogical characteristics

with Tpi, but differ in that they are more intermediate in

composition. Total ferromagnesian phenocryst amount to

10% and hornblende can exceed biotite, while quartz

phenocrysts are generally less than 5%. The presence

of hornblende generally characterises Tpii volcanics.

In addition Tpii eruptives have characteristically

smaller phenocrysts than Tpi. Common characteristics with

Tpi include rare sanidine megacrysts or phenocrysts and

bipyramidal quartz. Biotite-hornblende andesites from

the Una-Una volcano may correlate to Tpii.

The ferromagnesian minerals from the Pani

Volcanics and related rocks are relatively Fe and Ti-rich

(Table 5, Appendix 6).

In island-arcs, calc-alkaline rocks with Si02>63%

are rare, and volumetrically insignificant (Jakes and

-108-

White, 1972) in contrast to continental margins where

silicic igneous rocks often predominate. Useful

comparative examples to the Pani Volcanics of sub-alkaline

silic rocks from island arcs are lacking. Table 6

compares the Pani Volcanics with rocks of broadly the

same composition.

The comparison highlights the apparent high

K20/Na2o ratio exhibited by Tpi, with concurrent low

cao. Ti02 and P205 are lower in the Pani

Volcanics than in most other highly silicic rocks, with

the exception of the most differentiated Bishop Tuff.

Possibly Tio2 can be used as a differentiation index at

Gunung Pani.

The LREE's, Y, Zr, Nb are very low in the Pani

Volcanics; lower than in the SW USA, Western South America,

N Queensland and Lachlan continental tectonic settings and

lower than in New Zealand at Taupo, the Banda-Arc in

Indonesia and the SW Pacific island arcs.

The remarkable trace element gradients

(Bandelier Tuff; Smith, 1979) (Bishop Tuff, Hildreth, 1979)

shown by rhyolites in caldera-type silicic magmatism are

clearly absent at Gunung Pani.

The disparate behaviour between the REE's, Y, Zr,

Nb, Ti, P (HFSE) and K, Rb, Sr, Ba, or LILE's contrasts

with somewhat more coherent behaviour in examples of

fractional crystallisation (Tarney and Saunders, 1979).

This is also consistent with the behaviour of REE and HFSE

trace element patterns of the Lachlan granitoids, where

.....

Table 6 . Comparison of the Pani Volcanics with silicic igneous rocks from island . arcs and continental margin

Pani Volcanics

Pani Prospect

Non- Western USA Western Bishop Tuff Tabulo

SiliCified All South

rhyodacite Data America West Belt East Belt Early Late (1) (2) (3)

Si02 71.47 73.58 73.8 71.11 71.60 70.45 77.40 75.5 Ti02 0.29 0.20 0.20 0.40 0.35 '0.46 0.07 0.21 Al203 15.50 14.43 14.23 15.06 15.02 15.43 12.30 13.0 Fe2o 3 - 0.82 0.79 1. 75 1.46 1. 76 - -FeO 1.69 1.66 1.59 0.65 0.89 0.98 0.7t l.lt MnO 0.05 0.05 0.(\4 0.07 0.06 0.05 0.04 0.02 MgO 0.83 0.35 0.30 0.64 0.50 0.84 0.01 0.25 CaO 2.38 0.38 0,28 1.83 1.96 2.27 0.45 0.95 Na20 4.54 3.17 3.01 3.82 3. 73 3. 72 3.9 3.35 K20 3.21 5.35 5.68 4.58 4.34 3.90 4.8 5.55 P2o5 0.03 0.03 0.03 0.11 0.09 0.14 0.01 0.06

K 26 649 44 416 47 155 38 023 36 030 32 377 39 850 46 076 Ba 675 660 640 1 330 1 324 570 < 10 465 Rb 76 241 243 163 140 173 190 95 Sr 458 366 403 339 455 214 < 10 110 Pb 14 52 47 30 28 Zr 121 106 104 248 176 79 85 140 Rb 7.0 7.8 8.3 18 17 > 25 < 5 y 13 14 13 29 29 25 12 La 17 17 17 77 95 19 61 Ce 39 36 35 174 184 45 98 Yb 2.5 3.8 2.6 1.1 v 48 15 16 26 27 40 Cr 4 15 3.0 2.5 Mn 270 175 Ni 6 4 4 5 8 13 < 3 < 3 cu 5 10 9 13 12 16 Zn 28 82 83 43 90 38 40

K/Rb 351 184 194 233 257 187 210 485

Rb/Sr 0.17 0.66 0.60 0.48 0.31 0.81 0.86

K/Ba 39 67 74 28 27 57 99

Sr/Ba 0.68 0.55 0.63 0.25 0.34 0.37 0.24

Major element analyses recalculated on an anhydrous basis to total 100'.

'"

tectonic settings

Upper Palaeozoic Lachlan rvld Belt Volcanics Granites SW Georqetown Average Averaqe Indonesia Pacific Taupo Antarctic Inner Nth Qld I-type s-type Rhyolite Peninsula

N.ZI8) Granite( 9 ) (4) (5} (6) (7}

73.5 69.03 70.33 70.91 71.28 74.91 76.34

0.31 0.45 0.56 0.35 0.42 0.26 0.16

13.59 14.71 14.56 15.10 14,49 13.53 12.75

1.13 1.29 0.74 1.68 1.43 1.04 1.09t

1.42 2.61 3.29 1.30 1. 75 0.80

0.06 0.08 0.06 0.05 0.08 0.05 0.03

0.50 1. 78- 1.85 0.77 0.65 0.29 0.12

1.56 3.84 2.54 2.58 2.44 1.58 0.57

3.47 2.99 2.24 3.53 4.57 4.07 4.34

4.39 3.10 3.70 3.67 2.81 3.38 4.55

0.06 0.03 0.13 0.07 0.09 0.05 0.04

36 445 25 736 30 717 30 468 23 328 28 060 37 774

605 520 480 762 870 167

212 132 180 154 100 108 173

105 253 139 155 152 125 24

24 16 27 25 14 18 22

201 143 170 351 160 92

9 11 35 11

49 27 32 35 51 25 19

45 29 31 48 26 19

86 63 69 76 43.5 49 7. 7

74 72 13 13 1.7

4 17 3 - <1

9 12 7 6 -83 64 49 62 - 21

169 195 171 250 218

2.01 0.52 1.29 0.86 7.2

59 49 64 32.3 226

0.17 0.48 0.29 0.14 0.14

Feat : Total iron as FeO

Sources of data : (1), (2), (6), (7) Ewart (1979); (3) Hildreth (1979); {4\ Shen:t+on Qnd. L.abonne (l~ll) (5) White and Chappell (1983); (8) Ewart .. nd Stipp (1968); (9) Tarney and Saunders (1979).

I I-' 0 1.0 I

-110-

fractional crystallisation is not considered to be of

p~imary importance (Chappell, 1984) and where these

trace elements may reflect the nature of the source.

The occurrence of silicic, high K, igneous

rocks in a young island-arc is highly unusual,

particularly since not far, along the arc to the east,

the active Sangihe arc is clearly on oceanic crust.

4.5 Basement granitoids

4.5.1 Foliated hornblende-biotite microgranodiorite

The major element chemistry of this lithology

is consistent with acid calc-alkaline trends (Fig. 11).

On this diagram and in terms of mineralogy these rocks

can also be reasonably compared to the Tabulo ring-dyke.

The trace element data (Table 3) suggest these

rocks may be geochemically coherent to the Pani Volcanics

generally if compared to data from the Tabulo ring-dyke

(IKS 8). Values for Ba, Rb and Sr are similar.

However Rb from the quartz-biotite-rhyodacites (Tpi)

in the prospect is three times higher than for Tabulo

and the microgranodiorite and may not be primary.

La and Ce are slightly higher than for the

Pani Volcanics while Y is lower, suggesting the REE

pa·ttern may be relatively more fractionated. Zr, Ti,

P are probably comparable as the Sio2 content is lower.

Although the Pani Volcanic Complex clearly

intrudes the foliated microgranodiorite the age difference

Fig. 11

-111-

Alkalinity diagram.

I

, , I

lO 6 +--1

I I

,o

60 I I I

I

PE RALKALINE

I I

I I

1 4

Ah03 + (G\0 + K7.0+ No.,O

A h. 03-t CllO- (Kt0+ No.1o)

6

ALKALINITY

DIAG-RAM.

(after Wru:~hi 1 l%9)

0 PanL CompleX' rnl1o dacite

+---?- Trend.. ciu.e to poss1ble (QQ d<eplehon

6. T abu\o Rm9 d.y ke

0

I I

I

Fme foliated.. Hb /BL m1croqranod.1onte basement

C ale alka \rne trend.

-112-

is not known. The above arguments suggest the foliated

microgranite may be genetically related to the Pani

Volcanics.

4.5.2 Low K-granitoid- amphibolite basement

Medium-grained equigranular or foliated

hornblende/biotite granodiorites from the basement

petrographically and chemically may have affinity to

low K-granitoids of oceanic crustal environments.

Unfortunately the specimen analysed (IKS 21) is strongly

altered; apart from epidote replacing the ferromagnesian

minerals, silicification and loss of K20 cannot be

excluded.

Ba (130 ppm) and Rb (31 ppm) are consistent

with the low K20 (0.86%), but together with Sr (216 ppm)

cannot be reliably used due to possible mobility.

The LREE's La (7 ppm) and Ce (17 ppm) are

low, and in terms of chrondritic abundances (Hanson,

1980) correspond to enrichment factors of 22 and 21

respectively. The Y value (35 ppm) at least suggests

the HREE are not abnormally low in the sample analysed.

The mineralogy, La, Ce and to a lesser extent Y

abundances are comparable to low K-granitoids of the

Tholo group in Viti Levu, Fiji, which may have been

derived by parital fusion of an amphibolite source

(Gill and Stork, 1979).

-113-

Their occurrence would be consistent with

the associated amphibolites and geological setting of

the Marisa hinterland as it is currently understood.

However, this interpretation must be considered as

tentative only.

4.6 Tpii rhyodacites, Pinogu Volcanics and Una-Una

Sample 266 is typical of Tpii lithology,

which represents the latest phase of volcanic activity

in the Pani Volcanic Complex, and is characteristically

very fresh with vitreous black biotite phenocrysts.

Comparison petrographically and chemically with

Tabule (Table 4) suggests close similarity, with the

microgranodiorites representing a deeper intrusive level.

Una-Una (Table 4) is a compositionally unusual

volcanic island and may be related to Tpii volcanicity.

The major element data alone does not substantiate this

hypothesis. However the composition as well as

optical properties of hornblende and biotite from Una­

Una (Appendix 6, Table 10) is comparable to that in

Tpii units (Gunung Bulu, 8266) and Tabulo (8276).

The Pinogu Volcanics outcropping near Narisa

are mainly basaltic rocks, but elsewhere, such as east of

Gorontalo, they have remarkable resemblance to Tpii

volcanics. This is compatible if the Pinogu Volcanics

are a bimodal acid-basic volcanic suite, with acid

lithologies correlating to Tpii.

-114-

4.7 Relationships between Tpi and Tpii and their magma source

Mineralogy and texture, as well as field

relationships, strongly indicate Tpi and Tpii volcanics

are genetically related and originate from a common

magmatic source. This is supported by the trace

element geochemistry and helps establish the primary

major element characteristics of Tpi, by comparison

with unaltered Tpii. Tpi lithologies are the most

differentiated, silica-rich and potassic and

mineralogically characterised by quartz, sanidine, biotite

and sphene. While Tpii lithologies share this

mineralogy, hornblende is typically the predominant

ferromagnesian phase.

Composite and glomeroporphyritic aggregates

of phenocrysts of relatively large size suggests a

relatively long rest time for Tpi magma in the upper part

of a high level crustal magma chamber. It is inferred

from the mineralogy and eruptive history that the magma

chamber was compositionally zoned with Tpii magma below

Tpi. The absence of early hornblende in Tpi and

lack of evidence from trace elements abundance patterns

for differentiation by extensive crystal fractionation

tends to support liquid state compositional zoning in

the magma chamber (e.g. Hildreth, 1979).

4.8 Speculations on tectonic environment and petrogenesis

The Marisa hinterland is interpreted to be

at least in part composed of a deeply eroded terrain,

-115-

representing the root zone of a former Oceanic island

arc (low K-granitoid-amphibolite basement), possibly

of pre-Tertiary age, intruded by younger foliated

calc-alkaline granitoids. The volumetric significance

of low K-granitoids is poorly known but they are unlikely

to exceed basic and metabasic rocks. This older

basement underlies or more likely is faulted against

the Eocene Tinornbo Fm. The Pani Volcanic Complex and

other related volcanic structures are due to the latest

igneous events and are most likely controlled by

extensional tectonics.

A major problem then, in terms of crust of

predominantly oceanic derivation, is the generation of

potassic and silicic magmas. Despite the alteration,

the geochemistry of the Pani Volcanics suggests a

continental affinity.

The occurrence of peraluminous K-rhyolites is

restricted mainly to continental margin settings, and

seems to require thickened continental crust (e.g. Coulon

and Thorpe, 1981).

In continental margin settings such as Western

USA there is a close association between high K-rhyolites

and bimodal rhyolites-basalts (Christiansen and Lipman,

1972; Ewart, 1979).

Despite lack of evidence for extensive

fractional crystallisation in the differentiation of

the Pani Volcanics, the high K-rhyolites of Western USA

(Ewart, 1979) provide a reasonable comparison to the

Pani Volcanics in terms of overall chemistry and mineralogy.

-116-

In this analogy the Pinogu Volcanics are

consistent with an associated bimodal suite.

It is believed that the geochemistry for

the Pani Volcanics is consistent with partial melting

of relatively unfractionated source rocks.

The crystallisation sequence for the quartz­

biotite rhyodacites (section 3.5.1.6) shows that

titanium-rich biotite was an early phase, rather than

hornblende, which may reflect a high silica activity

and a relatively hydrous early melt (Wones, 1981).

Experimental work (Clemens and Wall, 1981) shows that

at temperatures up to 800°C, for a wide range of melt

water content titanium-rich biotite is the main

ferromagnesian phase that crystallises from moderately

peraluminous magmas.

Hydrous melts are low temperature and can be

'generated at crustal levels by anatexis, but have

limited intrusive capacity (Cann, 1970). Their

composition is strongly controlled by their source.

The foliated microgranodiorite is chemically

compatible with the Pani Volcanics (section 4.5.1) and is

interpreted as an early deep level intrusion.

A special tectonic environment for the

generation of the Pani Volcanics based on the

tectonic evolution of Sulawesi by Hamilton (197.9) and

bthers is speculated upon.

The deformation of Sulawesi, involving collision

of the microcontinent (Sula spur) and large scale

-117-

obduction of oceanic crust is shown in Fig. 3

(Section 2.4).

In this model the Marisa hinterland is

involved in the collision zone between the arc and

the continental fragment. A consequence of such a

scenario may be underplating by continental crust,

and therefore sialic source rocks for magma genesis,

but irrespective of this, it would produce a rapid

thickening of the crust, and possibly metamorphism

and partial melting. In addition magma genesis can

be related to subduction in the North Sulawesi Trench.

Subsequent high level silicic magmatism may be controlled

by rifting, following the compressional collision.

Generation of S-type magmas at continent­

continent collisions zones, by anatexis of marginal

basin and crust, has been advocated for the Malaysian

tin belt (Beckinsdale, 1979; Mitchell, 1977). The

nature of magmas that could be produced in a continent­

island arc collision would depend on the mechanism and

nature of the materials incorporated.

An alternate hypothesis to generating magmas

with a continental aspect would be to consider that

older cratonised crust extends east from Palu to the

Marisa area (Fig. 4) and this influences the composition

of subduction related magmatism.

Finally, irrespective of whether or not

anatectic melting due to collision played a role in

-118-

generation of parental magma, the I-type character­

istics of the Pani Volcanics suggest subduction

related processes must also be involved.