A Model the Origin of Dam-type DeposIts

-~~-- -- ~--~--- -~~---- ~---.

Sciences:; Dartmouth NH

and

3017 Dumbarwn St. N.W., Washington, DC 20007, U.SA

Ores.'zes. N. and lY1. 1993. 1/, Smclair, WD, Thorpe, R.L and Du,l<e,

of Canada, Special Paper 40, p. 615-633.

Abstract ',{l(}f-m!JUl_' body of hematite and economic and unusual

in Kirkham,

copper-uranium-gold-silvcr-rare earth element deposlt~ South Australia~ hc{/e led to rOR,,,nPT'n

modeL Recent work various Gutr...ors "'0'''''''< af-p(YSllS characterized by

exrensioTJ ..... a.i tectonic Sweden,o the soutroeast Missouri iron Great Lahe distri£i, northwest Canada; the ooyan Obo di-strict, and the Redban.'z district, Nortr-em Austmlia. We refer to this class of deposits Proterozoic iron-oxide

enULTonlnent. (l) the

the presence of major unusual or van;,CLua

Dam-type" In addition to

Resume hemallt£!, ['importance economique air~i que les caracteristiques

dargent et d 3elements des terres rares de facteurs lelaboration d'un modele

caracterisee par ses

615

ORESKES A!\D HITZ?vlAN

visual and textural similarities between Dam breccias and the uraniferous breccias Wernecke and the Mount Painter

616

lov,/'-ti­mid-Proterozoic

of the

the range of ore textures observed in Kiruna district. in

recent years, the debate over Kiruna has

the same

a class of

that Dam surface end member in a group of

a range of shallow propose

a ncar­formed

DA1\I-TYPE DEPOSITS

as heat sources flow and may have

1

ORESKES AND HITZMAN

arc the features and associated the presence of have

mappmgand unlnin£ral1zed rachs that ouerhe the top the

618

x'"

6r~<:ci<lt;i;i <:lIld IrQcluflid .. "",illl

Faul!, '''hiff!ld

Oik!l

at the 350 Tn based on Not shown the 300 m

the cross-sections as drawn at

The

intrudes older Proterozoic and Archean basement of the Stuart Shelf Reeve et al~ ~ extends over

consists of a 1 numoer hematitic brec-

cia bodies within host

toward

, ana

OLYivIPIC DAM-TYPE ITS

is very increases from the

heterolithic of

most common \vithin heterolithic and hematite brec-cias. Within heterolithic foliation

and sheared lenses of seri­beterolithic and hema­

foliation also takes the form of alternat-bands of red. black and brown hema-

tite~ In cases, tends to be to the strike of the breccia bodies. Within

r,ematite iJ"''"uuv

mI-xtures Hematite

6~a Iv

ORESKES AND HITZMAN

of ments within the

rimmed cut hematite veinlets. Within brecciated hema-tite alteration is more abundant and Within the heterolithic and hematite breccia there is evidence of intense iron metasomatism: hem-

relict

within hematite breccia have been lron metasomatized. The result is a rock with a

to

exposures reveal that the siltstone and laminated barite

are blocks and that occur within hetero-

These in the upper

that

system mentioned above. the tuffaceous sediments and vol­canic are restricted to a fault-bounded

centre of the

intense sericitic alteration.

Mineralization Dam consists of

620

disseminated and within breccia matrices. The are

bornite-rich mineralization et ai., The zona-

bornite-chalcocite heterolithic breccias. occur within hematite

may be more there :lre local areas than average

enrichment of trast, Oreskes

in a faulted zone et ai.,

mineralization and the tex­and hematite

con­breccias

tain very chalcocite zones as the a conclusion and Cross

hematite- and sericite-rich intense silicification. Within 1 km

at Darn,

hematitization becomes more abundant and appears to mentioned

----~--~ .. + .... - .. ~ .... ~ ,.- .. ~ ~ .... ----~~"" .. -",,,""~,,""'''

.... .. ........ -- .,. .,. .,. .... .,. "" .,. "" .. ~ ~ .... .,. ........ .,. .... .,. <> .. '" .... .. - ..... ---- .... ~ .. -~ .. --.~ ~----"" .. - ... -" .. -.... .,. ... -.. .. .. .. .. .. .. ~ .. .,. .. ~ .. .,. .............. .., .. .,. .,.. "" ..... "" "" .. ,.. .,. .. .,... .."'''' ... ~ .... ~ .. ,.. .. ~ ~ -.... " ..... .,.. ... .,. .... - .. .,. "'.. .. ~ ~ .. .,. ...... -- .... ~ .. ~ .,. .. .,. '" .. q .,. .,. .,. .,. ..... .,. ... .-

.. "'" • __ .. ~_~ .. __ .. ~ .. ~. __ ~ ___ .+ .. _'T"' ......... + .. _~ ..... .,._ ~_~ ..... ~_~.o __ .. _ .. ,.._ .. _ .... _ +0/_'-

.. +- ~ .. - '" .,. .,. .... ;- .,. .. .,. .,. ... .,. ............... ,. .. ,. .... '" .............. ... ... ... .. ... .. .. .. .,. .. .,. .... .,. - .,..,..,. ~ .. .,. ~ ~ .,. .... .,... ~ "" "" .... .,. ...... ..

r __ ..... _.,. ... • .. .,. .. ____ .... - .................... .,. ..

.,. .,. .,. .. ~ .. .,. ..... .,. .... .,. .,. .... .,. .,. .,. .,. .,. .. .,. .. .,. .. .,. "" .. ~ '" .,. .. .,. ..... """ .. .,. ........ .,. ........ -+ ... --- ............. '" .. " .... "' ........ .. .. .. .. ~ ..... .,. ...... .,. .,. .,. .. ~ .... r ....... ~ ...... .,. .. .,. ................. .,. ..

.,. .,... ,. .,. ...... .,. ~ .. .,. .. .,. ........ ~ .,. .,. .,. .. '" .,. .,. + ~ ~ .... .,. ...... .. .,. .. ~ ... " ~ .... .,. .,. .... ~ " .,. .... "" ~ .. .,. .,. .............. .,. .... .,. ... ..

a _::::_:::::::::-::::"':::::::=:::~~ .... ~~ ..

i ~ •• ~ ••• ;. ~~ •. '(? ••••••••••••••••

c

central column entrain':i sediments in tll£ centre the breccia

(s ercJded to level marked 300 Tn below the

OLYMPIC nL\J\1-TYPE DEPOSITS

alteration ment

the faulted zone of the eastern side of the intense or over-

associated with the zone of

al. 'TD,YH"'" sequence of The

consists of the eastern

which are associated chlo-rite-carbonate aiteration weak brecciation of tbe

tinued hematite formation in the central of massive hematite zones.

the hematite breccias. Late barite veins cut the hematite breccias in the core of the

The

the breccias formed and iron metasomatism Granite. The host

fissures and vugs precursor minerals. In

many cases, reiict precursor mineral textures are evident within hematitite

em-

of

researchers has established that the overall mor-

ORESKES AND HITZMAN

reflect fabrics waH rock or fluidization textures

formation of the breccias. Dam is not a sediment-hosted

do a near-surface environment of formation. Chief

among these the presence of hematitic siltstones as v,rjthin the breccias. It is unclear whether

these siltstones formed as

supergene event.

to

The occurrence of the within the Downs Granite led several workers to suppose a direct relation between rock and the

et

was therefore the age of the uranium-lead of zircons as 1588 ± 4 Ma

Oreskes and Einaudi with this conclusion on three counts:

1. The REE content of fresh and altered IS

with REE derivation from the host suggests that fluids de-

rived from an as earth element source at 2. The textural features of the

Downs Granite are inconsistent with

622

formation for the ore of the

near-surface environment erosion and

the onset of the

Downs Granite. The uraniliII1-lead zircons in felsic breccias and associated hematitic sediments

of 1593 ± 7 Ma and 1584 ± 20 that the breccia

tics confirm that the source of flEEs in the hematite breccias: breccias are characterized average values of

whereas the value for the Downs Granite range from -3 to -5, with the latter value associated with the least-altered and

The contrast between these results and earlier studies remains unresolved but

that later or supergene events may have remobilized uranium and disturbed Lubidiu.!Il-strontilL!Il some time between 1.5 Ga and 1.3 Ga, Creaser rubidium-strontium dates Stuart Shelf are younger than uranium­lead zircons dates for the same and 5180

H"~v'.v0 for

on consistent rubidium-strontium ages frem within the

Downs whole-rock r',.lbidium-stron-

7

tium from the

a major event occurred 1500 Ma

80-100 m.y. after formation of the

hematite stockwork which occur within a deformed and chlorite-

altered The host rock is similar to other Stuart which

at 1.8 Ga. At

OTHER IRON DEPOSITS

numerous workers

of the \"lernecke Mountain district the

Bear Lake

OLYl\fPIC DA.'VI-TYPE DEPOSITS

similarities that have been are as follows: L have been within mid-Proterozoic host rocks (1.1-1.8 Phanerozoic have been described Avnik but whereas the Proterozoic of mineralized be much smaller. 2. Tectomc

'-'~.~LH!-n"" appear to

that were cratonic or "VUv.liltOH'Cal

ments the districts share a

silicic-alkalic vol­southeast )\'1i5-

shallow-shelf sedi-

on the relevant factors. In the remainder of this paper, we summarize some of the common features we consider most relevant to

of this class and modeL

Hitzman et al.

iron-rich bodies occur as bodies. Two other

Wernecke Mountain and the Redbank

are also characterized iron-rich breccias.

Wernecke Mountain breccias. In the Wernecke l'vlountains of the Yukon copper- and urani­

breccia bodies cut the Wer­succession of reeks of mid-Protero-

metres to more than bulbous bodies from 100

623

ORESKES AND HITZMAN

diameter~ vast of m to >3 km breccia appear to have formed faults and west,

anticlinal axes oriented either north-north­faults in the Richardson

fault zone that

structures~

rrhe breccias in in texture, but pearance to

Wernecke district are variable m ap-

of

and alteration minerals that form the breccia matrix

individual breccia bodies in the Wernecke Mountains are of the order of several hundred breccia bodies are found

of

rich breccia is more abundant at sic alteration with hematite-rich breccia IS marc abundant at shallower levels and in distal

and

mineralization appears to be a event, and several of copper occurrence have been observed~ In breccias the lower of the

been identified to date~ association of breccia bodies

between weak The

IS

ate to mafic volcanic rocks and mInor and

that the Wernecke

Redbank trict of the Northern copper­iron mineralization occurs within hemati-tic breccias that the Middle Proterozoic

mixed sequence of volcanic and continental

The district is located faults that cut the

veined the breccias are

of volcanic and

metasomatism, In to have been brecciated

the veins in the host rocks are

zoned outward from to an-kerite-siderite-hematite et ai., The age between these various alteration-mineralization are unknown,

Mineralization \vithin the breccia consists of disseminated

as interstitial microvein!ets

cias also are enriched in traled in and in matrix to some late breccias

The brec-which are concen-

that forms a

As mentioned the breccias et al..

show a structures,

Dam and in the Wernecke district, there

are source, there 1S

to

rocks that have been source, Further-

f

more, these authors out that the indicate most of the carbonate and

the district are

The and breccia the Wer­necke Mountains and Redbank districts are strik-

maSSive contain breccias as a mmor or

mode of occurrence. For tain

The hematitic breccias were silicification and sericitization of the and calcite, mmor

In contrast \vith the e~~aJnr)je's Proterozoic iron-oxide cordant with their host

Marikos et ai., al., The

many are tabular and con-

is the famous Kirunavaara

avera vertical cross -section, its also occur at

have chosen not to discuss this The interested reader is

albite form line veinlets within the

At shallower come more abundant.

The concordant nature of the Kinmavaara its has contributed in measure to the

whereas discordant as

OL't'1'vIPIC DAM-Ty'FE DEPOSITS

intrusions the purposes of discussion, we focus on elements appear to be common to both the discordant and the concordant ore and that we consider essential to

textures of teration and u,",U1LWC>U

debate: contact wall-rock a1-

Contact relations. the concordant bodies contacts

semi nations toward abundance and

toward the centre of

facies,

den facies that form bulbous masses or stockwork have some~

"ore breccias" because of wall a magne-

matrix. "Ore breccias" occur beneath to tabular-concordant Loussavaara

and are the dominant at the Tuolluvaara

ore breccia \L,UIIUU~:l In the Great bodies of massive

stcckwork zones link to similar textures seen in discordant orebodies>

Textures of mineralization. In common with the breccias at cord ant of

foliated or stratified iron ore, In many cases, wall ~rock textures are

within the ores, that of the ore may have HH-rnpn

of the host rocks. Such been most demonstrated

which evidence of intensive iron metasomatism. The lower

consisted of massive ore a zone breccia and overlain

stockwork and disseminated ore, The upper which is at the consists of

625

ORESKES AND HITZMAN

lam inated hematitic siltstones that uch as

.ill Panno and

Panna and Hood between iron-oxide mineralization and the

host-rock and observed relict

volcanic rock iron oxide. The textures were inherited from precursor

focussed Panno and Hood's concluded that similar

of the upper Kiruna

iron oxides,

iron meta-somatism and evidence of described

exhibit a wide range of wall-rock that In the

rock alteration shows a alteration associated with

to

references cited In the Great Bear Lake

monzonite to diorite

associated with

Hildebrand alteration associated with iron­oxide mineralization may extend more than 1 km from the Toward the roof of the

albite and

1n

rocks. In the

contain albitized ments in an albite matrix. Outward from the albitized host rock contains disseminated and zones and massive

that are similar

626

to the "ore breccias" of the Kiruna of the

consists The siltstones are interstratified with coarse con­

but 10-clasts of hematite and hematite-al­

rocks in a hematite

of these rocks as of selective matrix silts and of porous volcanic clasts

tite in

whereas Anderson and Nold that mineralized before

environment. Whatever their hematitic siltstones at Pilot Knob are

found as

1983: Oreskes and from blocks of siltstone

Pea and their occurrence to a

surficial or near-surface environn18nt of formation. facies have been described in

of the volcanic

and concordant evidence of near-surface envirop_ments of formation for both

that may be essential and to the of similar

model for

vironment of a

environment of formation. both dis-

indicated the presence of units. some of which may exhalative facies. This a ,",V0D'V":f

role for meteoric waters in the formation or modifica-tion of these we also note that in the Wernecke Mountains and the Kinma district mineralization extends over a vertical

to lie

No

6kmand that

be more abundant in the economic

other than as iron ores. seems shallow end members.

such as

Weak association with many cases,

association between mineralization and contemporaneous.

seems to be

In rocks of similar age to the

but a connection to to be demonstrated. The dis-

tricts where evidence may be are

Great Bear Lake. have reckoned the ores and the host the massive orebodies are

thus and mineralization within an inter-

m.y. The authors however, that

OLYMPIC DAM-TYPE DEPOSITS

coeval with mineralization 0Jlilton,

evidence for such a claim is that these iron iron-rich

or

extensive

district. the best evidence of a direct link between

mineralization and comes from the Great Bear Lake district. Hildebrand (

to conclude that tism and mineralization were coeval at L87 and showed that mineralization and associated al­teration are zoned around the The host rocks consist of monzonites, monzodio-rites and at of 2-3 rather than unusual iron-rich or alkalic rocks.

evidence of structural controL In con~

poraneous the show evidence of structural case of the discordant ores and tion was fracture

control. in the mineraliza­

faults or fold

These structures are evident as In the case of concordant ore­

structural control COflSists of mineralization focussed welded hori-zons within tuff units. common occurrence of

fabrics within the ores appears to be a func-tion of this structural control: fabrics may be inherited from precursor zones, In textures in volcanic host roo..k5.

Intense alteration and

shear or flow

show some evidence of iron-

627

ORESKES AND

oxide and some show evidence of iron metasomatism. In some meta soma-tism appears eralization.

forms of

dominant mechanism of min­also exhibit more

trend from sodic alteratien at shallow

and silicification at very shal­In many cases, sadie or

alteration is intense to result in sodic or IJ'"'C0''''''''-' metasomatism of the host rock.

Alteration related to is confined to sodic and po­

alteration zones, whereas hematite nrp,-j""n

natcs in level sericitic alteration zones and in breccia bodies. Both

hematite are found in the '-")C<2",,,,,llo zone; nAme,UD,,.

appears that much of hematite in this zone may be altered

Paleosurface

the sequence. This late age of copper-uranium mineralization may its concentra-tion in upper alteration zones and in hematite-rich breccia bodies. Rare-earth element mineraliza­tion is fOlL.'ld

concentration of associated with

and sericitic alteration or with hematite-rich breccia bodies. The distribution mineralization in these systems not well appears to be concentrated in late zones of silifica-tion that may the final and

stockwork

5. in Proterozoic iron-oxide From a1.

628

ation. textures of and sedimen-tary rocks have been demonstrated in several

these alteration and textural of a

more evidence in the latter districts to be

consistent with our In contrast, dence for direct appears to be that have the most

served elsewhere. ciated alteration effects reqUlre a

few direct

host

and textures ob­textures and asso-

we note that in the district where the association of mineralization with coeval

rocks has been most demonstrated Hildebrand

OLYMPIC DAlvr:fYPE DEPOSITS

scnted

mechanism of

po­sys-

members and a continuum of processes with associated with anorthosites.

insofar as REEs may into the iron-rich frac-tions of immiscible melts. But in be more abundant the

presence of differentiated igneous basement rocks may be crit­ical not so much as a source of immiscible melts but as source of \..V.L1.Hn<::

as with the Mount

located on the Stuart Shelf.

we note that Mount Gunson is a strata-bound copper

ilarities to other stratabound "Arm,o¥'

Stuart Shelf

Dam. Given that one of the distinctive features of the

seems to answered,

is the relative

The very low combined with the low

the

629

ORESKES AND HITZMAN

absence of concentrations in most of the Proterozoic

lack of rather than solution.

One characteristic of the relevant to any

at an elevated involved in formation.

volumes fluid couid have been gener-ated from exsolution of a volatile-rich In very

..

extensive magma bodies amounts of meteoric

source would be the necessary heaL The

in continental areas

OLY'vlPIC DA:\1

VlEP~'.JECKE

REDBANK

in:m-ox:de

with

SAYAN OB()

crustal extension may circulation and of

amounts of meteoric water. In either case, the association of

structural features of such structures in from to shallow crustal levels.

whom we have discussed our work over many years.

KIRUNA S.E. MISSOURI

GRE·\T BEAR

tectonic continental

Proterozoic iron-oxide iron-oxide

the

TrJ! settmg. From IlLtzman et

630

i

ORESKES AND HITZ.\tA.N

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and Middle Pro-South ~t\us­

Melbourne, Aus-434 p.

RE., Nuelle, L.M .. Marikos, :,Lk and New data on the origin of Pea Ridge

southeastern iYli;souri Implications Olympic Dam-type [abstr.]: Geologic~l Society

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OLYMPIC DAM-TYPE DEPOS

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E'1 Cooper, H.L., Paterson, l-LL,~ Cross, K .. and 191'1', Zircon

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ORESKES AND HITZMAN

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99·103,

Note Added in Proof

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