Notes on the Mount Lyell mine

194

NOTES

ON THE

MOUNT

LYELL

MINE.

By

E. D. Peters, Jun.,

M.E.,

M.D.

The

westernccaatof

Tasmania

possessesbut

few

indentations thatcan

serve as reasonable harbours. Of these the

most

important one,

com-merciallyspeaking, is

Macquarie

Harbour, on

which

is situatedtheport

of Strahan.

The

country north of Strahan

was

subject to

enormous

fluctuations of levelduringearly Silurian days,and deepseasalternated

with wide, butshallow, lagoons,

which

in time rose into rugged hills, only againtodi-appearbeneaththewaters

and

losetheiridentity

under

the

mud

and

pebblesthatfilled

up

the depressions

and

levelled the sea

flooraswitha

smoothing

iron.

The

mud

and

siltbrought

down

by

the

ancientrivers,hardened into clay, slate,

and

schist, the pebbles

were

cemented

into the ubiquitous conglomerate; the blanketed seafloor, unableto loseitsheat

by

radiation, sank deeperanddeeper,causing the crumpling

and

upheavingthat ledtothelastcycleofmountain-building,

and

the general configurationo' thecountry

became

perhaps something

as

we now

seeit,though no doubt

much

lowered

and

scored, as well as

filled up,byerosion

and

glacialaction, Let us for a

moment

go

back

toaperiod just before this last upheaval,

and

imaginea shoal

pond

or

series of

ponds

nearlyfilledwiththepebblesbroughtintoit

by

the

foam-ing riversof that period,

and

undergoing, in

common

with the region surroundingit,aslowsubsidence,say somethinglikethe coastof

Norway

at present,

which

Ibelieve

amounts

to aconsiderable

number

ofinches in a century.

Owing

to causes that

we

have no timetoconsider,the roar-ing

mountain

torrents were diverted orsuppressed,

and

were replaced

by more

sluggish and feebler streams, that flowed through extensive

bands

of a schistrockbefore enteringour chain of ponds. Thisbeltof schist contained then, as it dees still, specksof sulphidesof iron

and

copper

—

pyrites

—

scattered through it,

and

although the

amount

of these sulphidesina singlecubic yard of the rock

was

very small, yet

their aggregate in even a single quarter of a mile of the belt

waa

enormous, as can easily be determined at the present day. Pyrites,

under

these conditions, decomposes very rapidly,

and

forms soluble

sulphates of iron

and

copper, whilst a considerable proportion of

any

silverthat

may

be presentisalso dissolved bythewatersofthestream.

Even

gold will go into solution toa minute extent, especially if the watersof the stream contain a little chlorine. This extremely dilute

*'_{mineral water}" _{enters our pond, or ponds, through} m,any diff'erent littletricklingrills,

and

we

caneasilyimaginethe flow to be so slow,

and

theevaporationfromtheextensive surfaceof shoal water to be so great, that the solutionisperceptiblyconcentratedinitssluggishpassage

toward

the outletofthechainoflagoons.

The

evaporation

might

even equalthe supply, in

which

case

we

shouldLavea"greatsaltlake,"with a decided admixtureof themetalsreferred to above.

But

there are

no

evidencesto

warrant any

suchconclusion,whilst there are strong

grounds

for believing that the

amount

cf salt in the water

was

not only very

small,butthat the lakes

had

anoutlet.

But

a

new

element

must

now

be introduced, without

which

Ifear thatthewealth

now

locked

up

safely

inthe

Mount

Lyell

mine would

have quietly flowed out of the

ponda

exactly asit

went

intothem,

and

eventually have gone to

augment

the

metallic contentsofthe oceans. In

most

parts of the world,

though

apparemtly very rarely in Australia or Tasmania, nearly

any

local

geologistcouldpointoutto

you

a

swamp

orpeat

bog

into

which

streams dischargethathavepercolatedthroughslateor schistose rockscarrying

pyrites. If

you watch

carefully atthepoint

where

the sluggish, acrid

BY

E. D. PETEES, JUN., M.E., M.D. 195

the bog,

you

will find fresh bright crystals or nodules of iron pyrites. Selectabladeof sirassorstemof a plant that has alittle

bunch

ofthese crystals hanging; to it,

and

after counting

them and

drawing an exact sketch of

them

in your notebook, return in a

month

and investigate again. Ifthe conditions are favourable,

you

willnotonly find

many new

crj^stals,buta greataugmentationin sizeof the original ones. This is without question the

key

to the formation of

Mount

Lyell

and

all

similar pyrites deposits,

and

it isalmostuniversallyacceptedassuch

by

tho3e

who

havegiven the subject attention. Imyself feel no doubt in

thematter. Organic, peaty acids have this

power

of reducingsoluble sulphates of certain metals to insoluble sulphides, and precipitating

them

in situ. If it is done very slowly, the metals will be

thrown

down

ina

more

or lesscrystalline form. If rapidly, in an

amorphous

and

massive form.

The

subsidence of the land, and the consequent filling

up

oftheponds withsulphides,continued through

many

hundreds

orthousandsofcenturies.

But

at last

came

atime

when

the elevation

of a ridge, or

some

change in the configuration of the country, the supply of metallic waters

was

cut ofi"from our pond-holes, or,

more

probably, lost in the inundation of

muddy

waters that blanketedall thisregionwitha thick layer of hydro-mica-schist,

and

thesulphides

disappeared from view, to undergo,under its

heavy

covering of

mud

rocks, the changes

and metamorphoses

that time andheat are sureto

bring about. Successive layers of pebbles,of sand

and

of other debris

buried our hydro-micabelt,tillat last

came

the final act in the great

drama, andthestage

was

set forthelast act,the periodwhich

we

now

see. Thislastshiftingwas, perhaps, themost striking

and

dramaticof

any

of the various scenes that I have endeavoured to portray,

and

furnishes uswith

many

interesting and obviousdetails, thatif Icould only dwell

upon would

tend greatly to prove the correctnessof the

theory here advanced, as well as to explain

many

interesting,

and

apparentlydifl&cult, details that

we

now

notice in the deposit. This

was

the last periodof

mountain

buildinsr, in

which

the present great elevations were reared,

and

the strata

between

them

were dislocated

and

setonedge.

Wherever

abreak

happened

to

come

acrossoneofour bog-holedepositsof sulphides

we

areenabledto find

and

utilise them.

But

thechancesareinfinitelyagainstsucha pieceofgood fortune,

and

no doubt dozens of such pyritesmasses are buried unsuspected under therocks that

we

daily

walk

overinthe vicinityofLyell,

some

of

them

hundredsof thousandsof feetfrom daylight,andothers,quite possibly,

within afew feet ofourunsuspiciousbootsoles. Ican only

compare

it

to aflatcake, containing avery few

plums

scatteredthroughitsinterior.

Ijetusbreakthiscakeacross in

two

or three places,

and

setthe

frag-ments up

on edgeinaplate.

Out

of thedozen or

more plums

thatour

supposititiouscake

may

contain,

we

may,

perhaps,bring one or

two

to

light,

and

in the

same

way

natureandaccidenthave opened to us such

deposits as those under consideration.

Mary

other deposits, equally

goodorbetter,

may

exist in close proximity to the s'ngle one or

two

that

happen

tobebroughttothe surface

by

thebreaking

and

tilting of

thestrata,but nothingbetrays their presence,

and

unless

we

find

them

by

deepboring, or by running prospecting tunnels at a venture, their

wealthwill remain wasted to us. Let us assume, therefo'e, that the

stratahave been brokenacrossinsucha

manner

that theline offracture

comes

across one end of our bog-hole,

now

filled with pyrites,

and

deeply covered

by

other strata of rocks. Let us further

assume

that

the fractured strata are next tilted so that the exposed end of the sulphide

body

comes to the surface,

and

the whole maesof pyrites, insteadof lyinginits original horizontal position, is

now

standing on

edge, so that

what was

originally its depth is

now

its thickness,

and

what was

once its lateral extentis

now

itsdepth. Thisisthepresent

196

NOTES

ON

THE

MOUNT

LYELL

MINE.

possible toform

some

slight opinion asto its probable extent in depth.

Take any

ordinarylagoon, orshallow swamp-hole,

and

we

may

sayina general

way

thatits length or breadth is

many

times its depth.

An

ordinary shoal

pond

of this kind

might

be perhaps 10ft. deep,

and

certainly500ft. to 5,000ft. in length.

And

if it

were

100ft. deep

we

should expectitslength

and

breadthtobe very considerable.

But

the

Mount

Lyell

mine had

anoriginaldepth

_—

present

width—

ofover300ft.,

so that

we

may

reasonably expectthatitwillcontinuein

depth—

former

horizontalextent

—

to a distance far

beyond

our

powers

of penetration.

The

experienceof suchminesallovertheworldbears out these theories to perfection,

and

I cahnot, at present, recollect a single large

body

of pyrites that has

"played out"

in depth.

The

200ft. tunnel in the

Mount

Lyell

mine shows

that at this depth the depositisthicker than

at the surface

—

aplain indication that

we

are asyet

working

at or near

toitsupperedge, forit islikely tugoonincreasing gradually in thick-ness until the centreoftheformer

swamp-hole

isreached, after

which

it

will gradually

grow

thinner, as the

pond

shoaled towards its farther

bank.

The

fillingof an ordinary

pond

will usually give us a

more

or

lesslens-shaped body,

and

it isanotablefactthatthis is so

commonly

the shapeofthese deposits thatthey are often called "pyri'ic lenses."

While

thisgreat

body

ofsulphides

was

deeply

entombed

under

hundreds

of feet of

more

recently

formed

facts,it isprobablethatbut few changes occurred withinitself, asair

and

water, the

two

greatagentsof

decom-position,werepracticallyabsent.

But

assoonasoneedgeofthedeposit

was

raisedtothe surface, theunceasingactionof thesepowerful agents began, neverto entirely cease until every particle of sulphides in the

entire deposithave been modified

up

to their highest possible pitch of

oxidation

and

hydration. In the caseofiron pyrites thisfinalstageisa hydrated oxide of iron,or frequently a hematite.

The

copper of the pyrites,beingeasily

changed

intoa soluble sulphate, is soon dissolved

and

carried

away

by

water.

With

thesefacts inmind,let us

examine

the presentupturned edgeofthe deposit at

some

point

where

ithasnot been covered

by

graveland debris rolling

upon

it from the hill above

it,

and

see

how

actual resultscorrespond with our theories as to

what

oughttotakeplace.

Here

we

find,just ua

we

shouldexpect,an

immense

mass

of oxide of iron, partly hydrated

and

partly non-hydrated. It

contains no copper

—

copper as 1 have explained being so extremely

soluble that it quickly disappears in a

wet

climate

—

the silverveins,

is,onthewhole, increased.

The

latter

phenomenon

may

seem

peculiar

atfirstsight,butitisstrictlyas

we

should expect from our

knowledge

of the propertiesof thismetal.

Gold

isalmostinsoluble,whilstcopper isextremely,

and

iron slightly, soluble, as sulphates,

and

the sulphur itself all disappears

by

oxidation or solution.

Hence

from the

mere

change

of the sulphides intoanoxide,

we

haveadecided diminution in

weight,

and

a consequent concentration of the gold.

But

astill

more

potent agentassiststheenrichmentof this surfaceore,or gossan,ingold, especially very close to the surface. This is theremoval of oxide of iron mechanically in

minute

particles, either as a dry dust,

by

the wind, or

more

deeply

by

the trickling of drops of water through the

easily permeable gossan,

and

the mechanical removal of particles of

oxideofiron as finescum,to bedeposited, perhaps, a few feet further on, asabedofiron-echre oriron-sinter,entirely freefromgold. Plenty of such material is found near the surfaceof this

and

similar deposits.

Thus

certainportionsofthegossanlosealltheircopper

and

sulphur,

and

much

of theiriron,

and

having lost23rds to 5-8ths, or perhaps even

19-20thsofthese elements,

become

correspondinglyricheringold.

The

accuracyof thisexplanationisverycuriously

and

beautifully attested

by

thepresence of a substance that I have not yet mentioned, but that

BY

E. D.

PETERS,

JUN., M.E.., M.D. 197

existinevery

pound

ofsulphides in the entire deposit. Thisisthe

well-known

mineral

heavy

spar, or sulphate of baryta, a

common

enough

gai]gue-roekin^

many

regions,

and

onethatisalways dreaded from its stubborn

and

infusible character. In the average ore of the

Mount

Lyell

mine

thereis toolittleheavyspar to have

auy

perceptible effect

upon

the metallurgicaltreatmentoftheore. This mineralisabsolutely insolubleand undecomposable under

any

ordinaryconditions,

and

thus during the oxidation

and

removal of the sulphur, copper,

and

part of the iron,theheavysparremains behindtogetherwiththe gold

and

the

rest oftheiron. If

you

will take the trouble to

examine

an average sampleofthe

Mount

Lyell gossan,

you

willfind that insteadof

contain-ing,asdoestheore,about2 per cent, of

heavy

spar, this mineral has

risento 30, 40, or even 50 per cent., whilst the iron has diminished

accordingly,

and

thegoldhas increasedfrom 3dwt. to 20dwt., 30dwt., 40dwt. ormore. Surely no prettier

harmony between

theory

acd

fact

could be askedfor. Thereislittle

more

to besaidaboutthe restofthe

deposit.

As

alreadj^explained,it is agreat lens-shaped mass of

mixed

iron

and

copper pyrites set slantingly on edge, with its upper edge

slightly

decomposed

onthe extreme surfaces. Its averaaie width

may

be regarded as 300ft., while it has beenfollowed forabout1,000ft. in length,and about30Oft, indepth, atwhichpointit isiraduailywidening.

Aftergetting afewfeetfromthe wall-rock,

and

fairly into the deposit, the pyrites is to pure

and

massive, that in fourmonths'residence at themineInever

saw

afragment of

gangue

rock as large as a cherry.

The

averagecompositionoftheunalteredoreisaboutas follows:

—

Ironpyrites 83 percent.

Copper

pyrites , 14 _„

Heavy

spar 2 ,,

Silica 1

„

Total 100

And

theaveragecontentsofthe oreinthe valuable metals, throwing off"

enough

tocoverordinarylosses,is

Copper

4|to 5percent.

Silver 3 to 4oz.

Gold

2ito3dwt.

Although

the oreissimply anideal sulphur oreforthemanufacture of sulphuric acid, not only containing 50 per cent, of sulphur, but also

beingan almostperfectburningoremechanicallyspeaking, the

company

will be obliged to forego this advantage for the present. For the

new

processfor

making

sodahas so injured the acid

market

that pyrites ores aresimplyadrug,evenif it

would pay

to transport the Lyell ores to

thecoast,

and

thencetoEngland.

To

make

acid on the spot can of coursenot evenbeconsidered,as thereisno

market

foritathome,

and

the costoftheverylowest freighttoa

market

would

beabouttwice

and

onehalfthe valueofthe bestacid. Ineed hardly say that the

treat-ment must

necessarilybe the amplified form of the old blast-furnace

method,i.e., roastingin stallsundercover,smelting the roasted ore for

anatte containing the copper, silver

and

gold,blowingthisnatte

up

to a96percent, pig-copperinBessemerconverters,shippingthepig-copper

toEngland,

where

the gold

and

silver willbeseparated

by

electrolysis, whichconsistsindissolvingthe pigsof copperin

weak

sulphuiic acid at

onepoleof the battery, whilst chemically pure copper is re-deposited at the otherpole,andthegold

and

silverfallto thebottom of the tank

198

NOTES

ON

THE

MOUNT

LYELL

MINE.

of the deposit,

and

the

magnitude

of the entire scheme,1,000 tons of ore dailyshould betreated, yielding,say

—

50tons copper,

worth

£44

perton £2,200

3,000oz.silver,

worth

2s. 6d. peroz. 375

3,000dwt. gold,

worth

4s. per

dwt

600

Making

a dailyoutputof £3,175

This paperisnotintendedtotouch atall

upon

thecommercialaspect of

the mine,

and

1only mention these

few

facts, that have already been

made

publicin

my

report, for the satisfactionof those

who

cannot under-stand

how

such low gradeorescanbe

made

to pay. I cannot possibly

gointo detailsof costortreatmentof theore, but to furnish a clue to those

who may

desire to

make

their

own

calculations, I will state that

the one imporiant item of cost in the entire treatment is the cost of

smeltingtheore.

The

mining will be done

by

open quarrying for 20

years.

Ihe

stall-roasting of similar ore has not cost

me

Is. per ton

any

timeinthepast 16 years

—

reduced to

Taamanian wages and

con-ditions.

The

bessemerising

and

remainingoperations

come

solelyon the matter,so that theyare reduced to a very small

sum

for each ton of ore,butthesmelting

comes

onthe original roasted ore,

and

is always the

heavy

expense.

A

modern

waterjacket willput through 100 tons of oredaily,per 24hours, will require five

men

pershift,

and

1 tonof

coketoeach 7 or 8 tons of ore.

As

the

company

possesses a water

power

far

beyond

the needs of all its machinery, very little need be

added

torblast.

From

theserough statementsitwillbeseen that even

the smelting cost need not be very alarming,

when

executed on this large scale.

To

return to our proper subject. Thereisyet one point

more

thatIhavetospeakof,

and

it isin thispoint that the Lyell

m:ne

isunique. Inallpoints

—

exceptits unusual richness

and

width—

it is almostpreciselylikescores

and

hundredsof similar pyrites deposits in otherpartsoftheworld. Allhavethe

same decomposed

gossanontop,

except

where

glacialor other causes have scored

them

clean. All are richeringoldabovethan in pyrites below,

and

all that contain gold

invariablyruninto pyritesindepth. All,ornearlyall, have the

same

sinter

and

iron-ochre

amongst

the gossan in places that

makes

inex-perienced people regard

them

asgeysers orhot springs,

when

in reality this

phenomenon

issimplyasecondarymetamorphosisof thegossan. I

have even been told that

some

geologists have pronounced these

characteiisticallyaqueous deposits as volcanic,

and

the

heavy

spar a resultof sublimation.

But

I

am

not willing to believe this, for surely

every geologist

knows

thatheavysparisoneofthe

few

substancesthat are absolutelynon-sublimable,

and

canonly beformed

by

aqueous solu-ti-ns,whilst iron pyritesisbi-sulphide; thatevenif there were no air presentto roastit,

would

atonceloseone

atom

ofsulphur onthe slightest

approachof heat,

and become

a mono-sulphid magnetic-pyrites, whilst

the sublimed sulphur

would

be deposited in the crevices of the rocks.

But

it isas iileatthe present

day

toarguein favour of the deposition

of these deposits

by

the

means

I have described, as to waste time in trying toprovethat the earth

moves

around the sun.

Everybody

who

isfamiliarwiththese deposits--and theyare

amongst

themcst

common

in

many

countries

—

is agreed as to their

method

of formation,

and

insteadof their being unique, as I have heard one or

two

people call them,

Mount

Lyell

and

Mount

Morgan,

and

similar deposits are the

commonest and

best

known

class ofminesthat

we

have.

The

infinitesi-mal

extrapercentageof goldthatthey

happen

tocontain gives

them

an

enormous

commercial interest, but, geologically-speaking, they are

almosttoo

common

and

their

mode

of formationtoowellunderstood to

BY

E. D.

PETERS,

JUN., M.E., M.D.

199

sure to result greatbedsof kaolin from the action of the decomposing

sulphides

upon

thesame. Ihave never seen or read a description of

Mount

Morgan,

butafterhearingof the rich goldinhematite

and

beds

ofkaolinI

am

ascertainofthe existenceof pyrites below as though I

had

seenituncovered. Eitherof these

two

points prove the existence of sulphides below.

The

single unique feature that

Mount

Lyell can

show

isthe existence of a very extensive mass of very solid oxide of iron,containingaboutas

much

goldasthe pyrites, but free from silver

or copper.

As

it isevidentthat this ferruginous

mass

was

derived in

some manner

fromthe pyritesitbecomesinteresting to learn

what

can have

become

of the large

amounts

of copper

and

silver that belong to thismassof half-a-million tons or

more

of iron. Recentexplorations at

a depthof 250ft. seemtohavesolvedthisquestion. For at this point,

on gettmg

unaer onecornerofthe

mass

ofiron-stoneonthe foot-wall of thedeposit,aseries of shootsof argentiferouscopperpyriteshave been discovered

and

worked, thatare, Ithink, unparalleled for theirrichness in the historyof theselow grade ores.

The

massive copper pyrites is sometimes6ft.or

more

in width, and not only carries

some

600oz. to 800oz,silverper ton,but is filled with grains, nodules,

and

mas-ses of

pure sulphide of silver

—

87 per cent, silver

—

sometimes as large as a cocoanut,

and

increasing the balk value of the ore to something like 2,O0Ojz.perton.

Over

100tonsof thisorehave already been

mined and

shipped,

and

we

have every reasontobelieve that

we

are only on the edgeof the deposit. Imentionthispeculiar featureofthe

Mount

Lyell

mine more

to explain

how

it is that a deposit consisting of such

low

gradeore asIhave been describing can ship such extraordinarily rich material as the

Mount

Lyellis

known

tobe

now

doing,than becdUtseit has

any

bearing on the

"

genesis" of the deposit itself.

As you

see,

this rich ore isquite a secondary affair,

and

has no bearing on

my

subject. This completes all that I have to say in this brief paper. I

must

apologise for its fragmentary

and

unsatisfactoryform.

But

I have beenforcedtowrite at sea,

and

in the intervals of

more

serious labours. Ican hardlyimaginea

more

interesting subjectforan enthu-siastic student of the natural sciences to investigate than this

very-Mount

Lyell mine, for there are

many

minor, though extremely