Ore Textures

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ORE TEXTURES VOLUM E 3 O VERPRINTING TEXTURES 23

PLATE ]0

STRLCfURAI. SLPERI"II'OSlTI01\'VEI1\'S, PARALlll. RIoACfIVATlON. Un~no ... n too mi"", E"'an K~ng~'oo Itill~ hnficld, nouh Qu..-.""'l ......

Thi~ ~p.'Cin"'n h~. Mn ",1,'Ct,'" to ,Uu~t,~t~ tllt, dirlicJlty in establishing o,~'p,ill, i llg

, .. l .. !ionship, in situ~tioos ",he'e infill ;< "ot .ccom pa -,i"d by signific."t ah"',1tion . 11 al . .. 11I",trot",. llIueh o,"rt""k",d ,t ua t ion, lIt t lta t Iddspa thic .1 11 ... alio". a,e I,~q,,,'ntly aomp ."I,,

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ORE TEXTURES VOLU ME 3 OVERPRINTING TEXTURES 25

PLATE 11

STRUcruRAL SUPERlMPOSITION-VEIN, BRECCIA Mt Isa copper-lead-zinc-silver mine, Mt Isa, no rth Queensland, Australia.

ThiS specimen has bren selected lo iIlustrate a very clea r cut case of overprinting relationships in b reccia formato

The mast strik ing feature is the comprehensive ly shattered crackled white material (dolomite) which is associated w ith a network of m ic rove ins con ta ining cha lcopy rite (ye llow), and dark material(?) . The microvein network Iinks to more substa nti ve blotches dominated by chalcopyr ite and mi nor pyri te (paJer yellow).

It is very difficult to determine w helher or not the cha1copyrite represents nHU or alte ration . This is a cammon problem with chalcopyrite which very rarely forms c1early recogn isab le crysta ls. The substra te dolomite (white) also gives very Hule help in lerms oE providing clearly recognisable crystals projecting inlo potential chalcopyrite filJed vugs. Sorne oE the ragged edged doLomite and smalJ fragments? (mid to lower right) suggest that dolomite may have been dissolved prior lo chalcopyrite deposition or simply altered lo chalcopyrite? However, witho ut furlher data it is impossible to be certain of the proportions o infill versus altera tion, concerning chalcopyrite. The writer would suspect both are present.

The fracture system controlling chalcopyrite also cuts Ihe dark (si liceous) fragments, and although no l

26 ORE TEXTURES VOLUME 3 OVERPRINTlNG TEXTURES

lotally clear may link w ith some d irty grey dolomite mic rovei ns (bo ttom r ight) associa led wit h grey spotting (alteration) within the fragment.

The relationship between the dark siliceous fragments and the white dolomite is especially interesting. Taken at face va lue there is a lmost no evidence lo suggest thal the fragments are being altered. They have sharp edges, and there are several examples of fragments which are microveined by dolomite. In these cases the sha rp wa ll s ca n be ma lched, and /o r parlially matched , leav ing Ihe overa ll impression o f h igh pressure flu id jacki ng w ilh subsequent coa rse dolomite infill. The current concepts at Mt Isa which regard mos t oE thi s do lo mite as a lteration (replacement) certainly merit re-examination.

The siliceous na tu re oE the fragmen ts is a lso ve ry suggestive o alteration prior lo fragmentation.

All of this suggests a th ree stage paragenesis oE:

Early dark silica a lteration (channelways uncertain?)

Major fractu ring and teclonic brecciation involving a high pressure flu id w hich preci pita tes dolomi te (moslly infill).

Major fracturing and brecciation/crackling linked lo the introduction of the copper rieh fluids. (proportions of infill v alteration uncertain).

This specimen is typica l of much oE the cop per o rebod y. Obviou s ly so rn e good petrog raph ic observation is required lO sharpen this perception.

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ORE TEXTURES VOLUME 3 OVERPRINTING TEXTURES Ti

6 FIRST ORDER CRITERIA - STIW CTURAL SUPERIMrosIT1QN. 811 CCClA (f IlAGi\IENTS OF EAIIL Y ST A GI, i\1 1N EIIAUSATI0N COl\'TA1N EO WITl-I IN LATER ST ACES).

TEXTURES AN U 1'1I08 LEMS PL\.TES 12-1 5

... K ... "I'.'''on ..r '" "p,,""nS ,,"h,'ns 1 ... ""S'""'', o .. Hu,d, 1I) ~'oO ......... 1 1m"0 I"'''p .... ) .. io h.~'. I" tI .. II,' fon .... " .. " .. '" oup) , .... 1'>O .......... 11 ..,.-10.. ,""" "'" "" ~I\,, ... )",,~ ... ... I- . ... _ ... oou .on.d , .... """""", .... < ....

Ih ........ pp li.~ ." Ih .... ., .. bo~. e ............ po-.. .... o.,.,,., Un .... h"J ""'1" "'''''1 .. "Iu" (" oppo>it, p ldt ... belo", 'Nding the dibly uuscd by bL>, ling ,""ti,,,)'.)

n... mldd ", ,t .. ge is besl obsen'od b)' IrolCki"g Ih-< ' '''''' yell" ... bull m.\le,ial> .... I,;ch a ... "ar)'ins rombll1.uiQtlS oi St'r;'::il,,.,1(! PY'''' TIll'SC actually alllink lgdher in ,1 cumple, "d ... o rk . dt bo lh a eo,,,,,,, ."d fine !O:., I ~ 01 Ir.1 ch"i"lI . -rhe dil/e,e nri ation belw .... " i" l i l] ~"d oher.>li"" ",ilhin lhis , 'a,!;" is difficu l' . and n,,! ' e., II )' poSSib k-. Intuili\'ely ji Sl'(>mS Ihat pyrilt' is in""lwd JS mllll. b UI it .1 .... p roba b l)' "'-'curs os an .lIe,alio" prod u

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ORE TEXTURES VOLUME 3 OVERPRINTING TEXTURES 29

: PLATE 13

STRUCfURAL SUI"ERJM J'OSJTION (ANO MJNERA LSUPERJ.\1 POSTTION) BRECCJA R.Wen5wood goId mining district. north Qurensbnd. Au~tralia (A",a 2 vrin Carpentaria GoId).

ThiS sfecimcn has booen included as an example 01 complex inlill overprinling and brl-ccialion whkh refl~-cts long history 01 deposition within " "ein structu ....

\'Jith sit .... tions 01 this style it is always ,."')' diffocult to know whe re to slar l. The working baekwa.d~ principie (from Ihe [atest lO earliesl) is nOl always .pprop .... te. It i5 oftCf\ mo ... eff~""ive to settle 00 onc: small ~""",t tha t i. el .... , and then selec! anothe . Thi. can ohen I>(! done by working lTOm dear crystal sh"pes to s~.., the n~xt mine. al to be dcpositcd, or allernati,'e ly I.om a laye. which ;s coa l ing eilhe . CT)'sIals O" bmken material.

A mOSI Ilriking lealu", i~ he s ltongly brecdated material (lowe. Idl hnd edge). This ;, pn.'dominalcly fi""ly comminuted sphale.ite (block) and .rs.nopyrite (silver) within a black (siHca?) malrix . The f. agmen lS a re sharply angular lO wel1 .ound.>d, and give a vcry good idea 01 how the two dilfe.en t sulphides break "p. The breccia brea.., thmugh p.-..--existing sulphidcs, "nd Iragments ha,'e .. I~~rly h ..... n ffltcf'f ll lly tra nsported. Sorne sty le 01 "xplo.i,.,/intrusi,'., b.ecdat ion (milling) un be inl"rrcd u a bte s tage e-'''''I.

In Ihe Idl hand 10p CO rner a con l inuou . rim 01 .rsenopyrile (silver) with semi lo good crysl.l lorm COOI. 0\'.".. large sphalcril" (dark). """ I"tter secms lo "xhibit D cryst. l shape, although it d

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ORE TEXTURES VOLUME 3 OVERPRINTlNG TEXTURES 31

PLATE 14

snwcrul< A L SUPlo/U \11'OSITON (AND Id IN EIC~S" .

J! Of;:1 rt:XTURESVOt.LMf }OHRr~l"n;';G TI:xruRfS

Thr-' frasme"I, a,~ OO'ioo'ly mix~xI, and el",", lo ""m .. nf Ihci, o,igi n> a ... ~"'m in Ihe tp .iShl 'rgion. liere 1 .... mot lled bufl gr'ff

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ORE TEXTURES VOLUME 3 OVERPRI NTING TEXTURES 33

r LATE 15

STRIJCTURAL SUI'F.RJ~II'OSITl0N - VfINS. BR[CCIA U"~,,o .... n ti" mine. E,,"~n. K.ng.""" Hill< ti"lldd, norl" Qu('t'Il.I,,,>J. Australia . 5p0""(' o/ Ihe f'ust ralions ;,,,.ol,,,d ",,;th I"w ans,", CTO!II> cutting sili.dll .. ltn.. romb qUlnl ,do,.oo un 0111)' be t,.oce(! "';Ihin it. 1)0 1 .... )' "'f"">""ll lh .. e""", clIllinS sl.se in 1"" boltom right hard romn?

11 lltiro .1.SS

Th .. pro bl .. ms bn:ome e'- .. n mo .... ' ulr ,,-h"n the ,-.. i0

E.-e" tlls da.k m~lr Ti.1 on cloS

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7 SECON D O RDER eRITERlA; SUSPIClON AROUSING

TEXTURES AN D I~ROBLEMS ASSOClATED WITH VEIN RE-OPEN INC, INCLU D INC-QVERPRI NTINC I~ARALLEL INFILL, ALTERATIQN-INFILl M ISMATCHINC, INCONSI STENT ALTERATION SELVACES ANO AL TERATION QF ALTERATI ON.

PLATES 16-22

7.1 The following section of illuslralion s has becn st>lectcd lo lIus tra le slI spicion aroll sing or second order ,ritera.

The majorily of Ihese occur where prc-ex i s ting c h ilnnelway slruclures (usually \ 'eiI15) have been reilcti\'illed . The reacti \'ation o( planar structures, naturall}' focuSl's Ihe failure SUrfilC('s in il pilfalleJ, o r St'lllipiU.ll1el, nli1nner lO the original fracture. Mosl common!y faiJ u re occurs along \' ('in rnargins, or directl y up Ihe ce nt ral zo ne, and ranges froln non di s rup li \'c s imple frilclllT(' propilSillion lo di s rupl ive brecciillion. In mos l Cilse s, Ihe

rCilc l ivil lion will il lso resu[1 in new frilcture s urface s within Ihe sur rollndin g hos t rock s. The se art' especiillly co ncent rated afollnd Ihe fr inge Lones of tht' original fraclure. Mo s l of Ihe new fea lures are bes l approached b y loo kin g for , and colhx ling spcrimens whilsl in Ihe fie ld, and this should be Ihe first appf()'lch al normal visual sc.1Ie. Thus al outcrop or drill core seale Ihe first aclion should be lo move away from Ihe cenlre of minerali sa lion, and seck isolated microveins within llniform hos! rocks. The objec t ive is always lO Iry and d ecide which infi ll assemblages belong lO which alleralion a sscm b lScs.

Forlunlely inf ill assc mbl age s .1fe usually les!; affcclt'd by changes in hosl rock Ihan .lIteration asscmblagcs, and in thl' rilrc cases of multiple hosl rocks it qui c kl y beco mes apparen! !1M! \ 'arial io n s in aheration do in r.1et belong lO onc mineralis ing ~ tagc and s imp ly rcflee l Ihe chcm is l ry of differcnl hosl rocks.

Ir Ihe obsen'cr only has an individual s lab/specimcn of rock, and is unablc lo mo"c Ihere are Olany helpful s lIspicion arous ing circ llOl sl ances (set' sl'c tion 2.3).

PLATE 16

SUSPICION AROUSI NG COMP LEX OVERPRI NT ING, PARA LL EL REACTlVATlON, STRUCTURAL SUPERIMPOSITION-VEINS. Ravenswood gold mining dis trict, north Queensland, Australia (SYC Pit? Carpentaria Gold ).

Th is s pecimen ha s been included lo illustrate the common s iluation of re pea le d re-opening, and infilling of Ihe s.lme fracture system. 1I also ilJustrales just ho\\' difficult this ca n be lO dec ipher. The problem is compounded when each s tage con tains simila r componen ts.

The narro\\' ve in sys tem tra ve rses a s li g hl ly 'wea thered ' granitic rock, composed of quartz (grey-white) and fe ldspar (pink-orange iron stained) \\'ith minor biotile? (rusly brown). The fe ldspa rs are slightly argillised and minor acid leaching has occurred v ia so me of Ihe crack syslems. These Irave rse Ihe vein horizontall y and link lo lo\\' angle fractures (lefl hand side). No clea r alteration halo borders Ihe vein system. A close inspection of the vei n indicales Ihat it conlains al leasl three separate components.

The upper-left vein SI.."C tion is noticeable, as it contains very we ll developed wh ite quarlz crysta[s growing irregulady from Ihe vein wall, succeeded by pyrite (pale yellow). Thi s region is close ly associalcd wilh

36 OI~E TEXTURES VOLUME 30VERPRINTING TEXTUI{ES

small elongate brown components (mild ly oxidised marcasite?). Thi s asse mbla ges can be Iraced a long mos l of Ihe lefl hand s ide of Ihe v('in , and Ihen beco mes vague, bul re-appears in the bottom righ! hand edge. The botlolll r ighl hand occurrence is clearly c ut off, by an assemb lage of slightl y g reyer silica associated w ith arsenopyrite (sil ver). This ca n be traced a long mosl of Ihe righl hand side of Ihe vein, and is visi ble again lo Ihe righl of the fir s l s tage assemblage allhe IOp of Ihe plale.

The arsenopyrile-grey q ua rtz slage is in lurn cul by a noth er vcin, w hi ch is le ss \\,e[1 defined, and com posed of occa sional dark brown (s phalerile) cryslals, associated w ilh paje buff (siderile?) malerial, and probably pa le yellow pyrite and quartz.

The relalionships between Ihese s tages are oflen very confll sed and difficult to deci pher. Indeed Ihe alllhor is by 110 means positive Ihal Ihe above in terprelalion is to ta ll y corree!. Thi s serves lo illus trate jus i ho\\' effecti vely vei ns of si milar composil iol1 can ' hide' as Ihey o ve rprint along the sa me controJling slruclural weakness. Once again inspection of more exa rnples is required lo confirm initial suspicions. This exam ple also illustrales Ihe need for very carC'ful observa lion. The vein is gold bearing, an d it is nol cerlain which of Ihe slages is gold relaled!

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ORE TEXTURES VOLUME 3 OVERI'R INTING TEXTURES 37

PLATE 17

SUSPICION AROUSING - COMPLEX OVERPRINTING, PARALLEL REACTlVATION. STRUCTURAL SUPERIMPOSITIONVEINS. Ravenswood gold mining dis tricl, north Queensland, Austra lia (Buck Reef - SYC Pil, Carpen taria Gold).

ThiS plale [lustrales good second arder suspicion arousing criteria which Icad to observer lo suspecl t\Vo s tages of mineral isation.

The firsl reaclion lO the rack is lo note Ihe sulphide rich infill channel, and the aHendan! dark alteralan zonc. The hosl rack is tonalite, and Ihe alteration is biotite. Biotite alteration is conventionall y associaled w ith minute veins in porphyry copper systems, bul is actuatl y quite w idesp read in many other magma tic associaled fluid syslems (especially skarns) and can assume major propor tions. The suspic ion arousing ci rcumstances relate lo lhe micro*lhin ve ins, which exlend al low angles nlo Ihe host rock. These are composed of py ri te s i li ca? (inf i ll), bul ha ve no alteration halo. Given the intensity of Ihe black biotile a lteration this is an obvious anomaly.

38 ORE TEXTURES VOLUME 3 OVERPRINT1NG TEXTURES

C lose inspeclion of the infill channe l reveal s !wo d istinclly different malerials. Coarse gra ined pyrile crystals form one associalio n with hinls of black triangu la r textures (biolite?) occurr ing around Ihe centre lef! regon. A second association conta ins grey wh ile maleria l (silica) wilh fi ner elongale s trin gy pyrite. A small vein cross clltting Ihe biotile alteration appears lo link lO Ihis (bottom righl).

The re lationsh ip between Ihe t\'Vo infi lling slages is difficult lo discern. Intuiti vely, the aulhor suspects Ihe biotite relalcd material lo be ea rly util ising Ihe ind irecl criterion concern ing temperature of deposition. Biotile a lteration/infi ll implies high temperature ( >400~C). This plale is another good example of refra ctu ring along a pre*existing c hannel\'Va y, and again emphasizes Ihe need for good underslanding of Ihe infiJl /a lteration linking. Infill textures at Ihis sca le a re freqllenl[y overlooked.

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PLATE 18

SUSI'ICION AKOUSINC CO.\ll 'LEX OVERI'RINTINC PARALLEL REACTIVAT1QN, AI.T[RATlON ~II SMATCl I STIccond ly 1!>e ..... 're lIC," ~.al hinl~ thal ~rse""i,)".il .. may bt in tin)' .... in. Iha' ~""OSs ~,,' /.ngl~ ac~ ,!>e sph..o1o>rit... The spkal..il~ """"111 in Ih~ ')pi~ .. 1 eo.'s~ .ounded a)"SI.1 (o.m . n d illd" id ... 1 gr.,,,,, 1oo~ lo b.- brokn """' in , ..... urP'" ....,1,,,,, of th~ righl h..Ind ,~in. Th .. no is no! ~no~gh

..:1 ORL lU;l\JRES \(M.L\IE J O\ERI'1I.INTINC nxn.~

evid,~~ lo be o .t in. bul f.onl otl".. 'pt' 11", 10p I .. fl ,1 sph.lle.iI~ i"fill is in di."", ro"'",,' ".ilh d~.k ".dll roc~ and 1""'" is no )'dlo,," .. lIeraIiO"_ This ahe.ation mi sma tch i, .gain suspkion a rousing. Th .. )" .. 110"" (se.idle ) alt.,.-a tio" cou ld jus, be motchNI lo Ih .. a ...... nop)"rit~ or mo.e li kdy is ",I .. ,.d 10 the mkroth,n ~10> (Pl'ril"') "";ns. N .. ithe. a no pa.'icubrl)' cOI",i"";ns! s..,' .. ra l :>oin ls c .. n be obsen....t ............ II!.- d ~ g ........ minnol in II!.-h"", ro'" is slo",l) e""'e"'''S into tM dull .... .,.,y. ), .. 110,," g . .... " .... k il ... E' .. n Ih .. o . igi nal .ock i5 nol ",hal il se

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ORE TEXTURES VOLUME 3 OVERPRINTINC TEXTURES .J I

PLATE 19

SUSI'ICION AROUSING - INCONSISTENT AllUlA'110N - COl\lI'tEX OVERI'RINT'lNG. I'ARAI I.J, I. Rf.ACTIVATIONSTRUCfURAL SUII:RI~II'OSlnON VEIN5. $o'fMons tin min~. M"unt .\towbr,l )' di s lrkr. In i"ibly ~UMI7. (1"1' lefl).

A p,ubl"m imm,'

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PLATE 20

SUSPIClON AKOUSING - INCONSISTENT ALTERA TlON, CO"II'LEX OVERI'RI r-.'TING, ALTERArlO,\ OF AL TERATION. srRUCTURAL SUP ERI M PQSITlON-VEINS. "Ibite " I t~ r ~t; "n. Malk-e Gap C~k. Clon gi,.,s Ih,> elue lo ,,'p.Hating an ,'pp",\-nl l)' single ,tase into [",o.

The ",al! rod, (bOllO"'. ,ent r .. ) is composcd 01 fi n.-g,. ned ., Ibite (grey) wlh ol,

Th~ o"c'p,inling m.'k,i., ls (whik / pink ) dea rly exi,t as ~ltcr"l i on, ",ith the ",hile, (al bilc) ocen,ring as a" . Ikrat io" "r tlw h~t ","c k. At firs! sigh t Ih., "hile

44 O~ F.I EX1'U Io: I:S I'OlUME 3 OI'ERI'RINTII>:G TEXTURf.s

(albile) "",noS lo .,"'-u, as" fringe lo Ihe pi .. k na tNi.1 a nd a n i"itia l inl"'p"'lation ",onld a"ess this as a zoned Jltera!ion fro"t, Ho,,"c\'(" ir Ihe bscn' N ca,d"lIy I,aeh all aronnd the f,onl. I h~,c is point (

o 1cm ORE TEXTURES VOLUME 3 OVERPRINTING TEXTURES 45 ~I __________________ ~ ________________ __JI

PLATE21

SUSI'IClON A~OUSING INCONSISTENT Al URATlON. ALTERATION OF ALTEKATIOI\ . STRUCI'URAL SU I'ERI1>II"051T10N-Vl:I NS. I'tgmont, Ic.;odnc p~. Cloneu'f)' nogion, nonh ~~od. A'OSIT~li~_ 5pim .... don . tOO by Dr. 1'.1. Williams.

Th~ 1,lole h. , bt...,n ..,I"" , .. d lo lIo ,t t," su bU ... o''''p,;,,';ng t "~tur"s. with good e ~a "'pl.s of

;nn"'S;Sh> 111 .1t"'Ati"n halos. and alt"ra'iOfl of a lter.llion . It ",jll b~ of special "leTu I lo In~"y ,,,.,,lers. as il is pdrt "f ,he stockwor~ >y.t'.'CI.

The 1>0,,1 !"OC"(s) ..., fi,\( grair.ed meta""'mentM in t ......... oo lo >illY-silnd. si, .. Tang .... 1-.- ..... ron'posro 01 '.1')''''8 rombi .... ,ions 01 mi.,."..I" p.1t.. grttrl g~' (f"ldspa,?j pale grey (quartzJ dark black-grcen (chlorit ... biolit .. ?) and gJ..ssy red (goo..-nt1?).

Th .. m~t b,-;o", chan" .. '",.}' is r .. pr~nrl.'d by d.,k infill -n .. in s tr uel u.e (cent'e . ;ght). Thr d~ r k m.I ... i.b (g"-')' 'l"Mlz and bl.ock 10". 0"'li .... ) "",m lo rel ale lo ~ ,ery iHegular alte ra llo" h~lo ",hl;h is mosll y while, wilh some irregubrly d i>lribu l~>d Mle )'e llo\\" co"'po"e"I$. The aeh", 1 ,'cj" is im".,rSi,lent. a"d Ilwr~ ~rc b lolch~'S 01 darl bufl ",ateridl ~ssoda l~'d wi th do.;e~" al,on il .ppt'''''' lo be .. p lace

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PLATE 22

SUSI' IClO,'\' A ROUSINC - Al TEKATlo,,\' 01' ALHRATlON, LA YER cm.n ROl. Tkk 11,11 80ld mi",-, C lo,,'u,.)' ,egioo. no r lh Ou.os Mn ;"",,dfd to muSlra,,- re,a',,''''Y 00"':;",,' o,-e'p"nting whi,h is ess..-ntially laye. (Onlroll~d_ 11 a lso s ho,,", a subll ... ua mpl~ of alter.lhon alkring ,.Ite,.uion.

Th,- s~in1l'n ",igina'''. """ Tick IIilI gold mine. ~nd lhe pin l< rock . Ihe gold b".ring unit. It is , t>mp is al", an Q"c'p,inl, ""d OCCIl"", an " It"r.,tion of bolh Ihe pi,,1. and sr.",,,. A ,m. 1I SS cuning miero,'ein ;s ... ell d"pla yed (le/! hand _ide' cenl ... ) aS in cross cul~ Ih. srcen rock and 31t~", 1"'-' darl? and / or ... m,,,,nt al!>tc (pink) ron'p

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ORE TEXTURES VOLUME 3 OVERrRINTING TEXTURES 49

8 A C KNOWLEDGM ENTS

This Ihird "olum~ h~s laken more li",,, lo produce Ihan oolh 01 Ihe prcdt'C .... sors. Ot,,"ioo.ly the ""thors d"S!nl into m itd demensia h,1S r

9 : REFERENCES

Dong. c., MQTri:l()r"l, C.IV. and J~ircth. S. 1995. Qu~rU t.,~tures in .,pid....,. ..... 1 , .,ins. ~and: ct.ssiflcation. origin and impliNtion. F.ronomicG .. ()logy. V. 90. p. 1841-1856.

Taylor, Ite. 1m. Ore TeXlull."S Volume 1 lnfi ll. Economi("(>1lsland. SSp.

ORE TEXHJRES VOUJME 30VERl'RlNTlNG TF.Xl1JRES 51

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