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A n account is given of the e a r l y g e o ~ o g i e ~ l exploration and
mining: history of the Co~?~~mndlel GoZdfield, The wriltepts interprets-
tion of the basemat and igneous geologg, and of the meta3..logmesis of gold-~rilver anti Wse metal o r e s has resulted i n r a d i c a l changes.
The o r i g i n a l threefold sub-&loision of the J u r a s s i c sedimentarg basement rocks ( ~ a n a i a H i l l Qroup) has been condensed t o two fornations
( ~ o k a t e a H i l l an8 Nmaia ~ ~ 1 1 ) on the b a s i s of' t h e i r d e t r i t n l content and structure; standara seetions a r e describe& The o l d e r (Ffirun~Aa R i l l orm mat ion) i s characterised by l i t h l c m l c m f c greym~lcke- and
sulbgreywacke-type sedimentary rocks, that were derived f r o m a landmass comprising an assemblage of calc-alkaline oa lcanlc and plutonic racks, w i t h mtmr sedimentary rockrs, The deposit m s f'bnneti by t t v b l d i t y currents an a continental slope bordering a geoqyncline, Seconeary minerals dlalplostic o f l o w grade metamorphism of the ZeoZlte an8
P r e h n i t z ? ' ~ e l l . y I t e Facies occur 1ocalZy. I n the Manaja H Z 1 1 Formcnction, These not only include n o ~ m a l p r e h n i b , laurnanti te-leonhar? i t e ,
malcime* anS! a c t l n o l i t e , bat dLm prehnfte with momalous7~ low ref%active indices.
b c o n t r a s t , the rocko of the younger fomn~ltion ( ~ o k a t e a H i l l ) Rre f elaspath1 c greywaclce s a h a st devoid of valctulic d e t r i t u s excm t n e w the formation boundmy with Manaia Hlll Formatian. They hnve a s e r i c i t e - r i c h , a u t h i r ~ e n i c matrix, but no zeolftea or prehnlte. T h e sec'Zncnts mere derived f r o m a l~admaoe of lorn r e l i e f composed of 'grnnodiorite- d l o r i t e plutonic rocke a d minor sedimentary rocks. The lower m n r t s o f t h e fosmation a r e slope t u r 5 i d i t e n while t?e w p e r p a r t s P-re the product of s t a b l e depositton i n deeper water,
The strucLure o f the Ellamia N i l 1 Group is r e l a t i v e l y simple over most of t h e Coromndel. arm, The beds have a north-west to north- north-v~est stpike and are f o l d e d about axes with a ~ l r n i l a r trend;
j o i n t edrectians corroborate *is conclusion, A north-e~.st trena has been fmposeB on the rocks o f the Iloehau region by t h e i n t r u s i o n of ParStu Quar%z-diari te.
Conformity between the two formations ie shown by ~Plxdational d e t r i t a 2 content and by s t r u c t u r a l continuity.
I n older descriptions, the volcanic rocks of CoromanCiel were subdivided i n t o cur older, pol&-bearing, and a younger, barren s e r i e s of m d e s i t e s v & t h minor d a c i t e s nnd rhyol i t e s , By ignorlrq post- eruptional properties ( i , e , hydrothermal a l t e r a t !
m)
a 8if'ferentview o f the volcanic stratigraphy is demonstrated.
The extrmsive rocks a r e now subdivided Into nine formations
wlm
no mgor regtonal break but with dtscon%mnity or uncanfarmlty 'between them caused by l o c a l quiescence, l V i t h the addition of two intrusive
f a m t i o m with volcanic afflnf t i e s (Castle Rock Daclte and Xai-it%
~ o r p h y r i t e s ) a l l the volcanic racks are inclw3ed together a s the
Coromandel Group of Miocene age, The new effusive f o m t l o n s from oldest t o youngest are: Part Charles Ande~ite; Omaho Formation
(rhysli t i c ) ; Kokwnata h c i ta; Te Xaraka h d e s i t e ; Cousin Jack Andesite; Paraketa, Fbrrmatlon (daci te-andesita) g Rauporoa Basaltie- nndesi te; Tuateam Andesite; and Beesons Islana Volcanlcs (andesite-
&cite). The l a s t formatlon has been considerably redefined since it is the sole surviving name fmm e a r l i e k published descriptions of' the
volcanic geology, Two zeolites (phlllipsitcs an8 'levyne) are &errcribed.
Hydra thermal dl, teration and mineralf s a t l a n c u t across fbmnatlon baundrnrf es and are contraiXecl by F~iu3.t~ and ahear zones, The writer
,
unlike e a r l i e r workers, recognises f a u l t s ae impartant i n the dmelap-
ment of the geology. The fracture p a t t e r n i n the basement pocks not on2y controls the regional fault patterm but also the fracture pattern i n the volcanlc racks Imnediately a ~ e r l y i n c the basement.
S t r e m patterns have been synthesised from structural a n h v s e s of f a u l t s , shears, veins, and polddbenring fodes, which s u ~ c e s t that since the FEesozoic, a major compressional s t r e s s has been directed approximately north-east/sou%h-west. A l o c a l secontfam- s t r e s s (north- no~th-~est/sautla.-8outh~east) is iMicate8 during the ad-Tertf ary
.
MR jar h t e Tertlavy n o m l fault movement is explained by postulating
1
horlzontsl tension r e l i e f by gravity followingmf
f e r e n t i a l v e r t i c a lI movement along pre-existing f a u l t s , a s a r e m l t of dondng induced by
lntruslon and eruption of magma,
1 A regional gravity survey hasl been t i e d tn me known gea10,gy and stmcture. Models of posrslble mass dlstri'butian a r e computed lor
lndivltlual Bougue~ anomalies. Major ge02oglcal features a r c cmfi.l*me&
but stations a r e too spread to relate mlneralisatian with the gravity pattern.
T h e results of a stream oediment reconnaiss~~nce r;eochera%cal mrroerg of cold extmctable anB t o t a l cappep, lead, end arsenic, m e appralaed In r e l a t i o n to known geological 8tructmal end hislorica3. features,
The e f f e c t s of pH, flocculants, and contamlnaldan a r e considered and the anmly t i c a l methods reviewed, The standards of anomaJous concen tration l e v e l s are s t a t l s t i cally calculated, and related t o d i f f erent possible
oousces of metal m i thin a e v e n terrain, Recommendations f o r flw t h e ~ exploration and analytical work are made for those areas where poasible
economfc n d n e r a U ~ a t i o n has been detected. In particular, the Floehau region near ParfLu Quartz-d&orite shows s t r o n g signs of capper enrich- ment,
No examples of ope from any of the o l d mines were available$ the study of t h e sulphide mineralisation has been canflneti t o outcrops of veins, and samples from tlp-heads, Althoupa many af' the samples show
indefinite d n e r a l relationships, the sZm2hrity of paragenetic
sequence f o r a11 areas suggests that the deductions fox the Thole f i e l d axw valfd. Several minerals have not been previously recor8e8. f r o m
Corammrdel, notably wo Wrami t e , molfbdenlle, p y r r h o t i t e and tetsahetiri te.
Tha structtlral conCm1 of mineralisation by the f r a c t u r e pa t t e m i a emphaslacd, and also the absence of' any d l r e c t genetic rela tionship t o t h e volcanic rocks of the Coromandel G r o u p , Xnstead, there i s a zonal genetic rela tionship of the are minerals of the Noebu region about Pfbritu Quartz-diori tie, The silmilarlty of? the paragene t i c aeqnenca of other deposits t o t h a t of the Ir'toehau regton " q g e s t 8
simKlar sources, I. e. hydro themnal exhalatione f r o m consoletiating
eubjacemt granodlorltrtype plutons. W l l s t future economic pmspects f o r Commwdel i t s e l f may dep& on deeper workings approaching emch bodies, the Parlw O u a r t z 4 i o r i t c pluton could contain a more accessible porphyry copper d e p o s i t , w i t h associated tungsten and mly'bdenum
minerals.
