Favorability of Dolomites as Host Rocks for Ore Deposition

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PAVORABILITy OP DOLOMITES AS HOST ROCKS FOR ORE DEPOSITION

■ library COLORADO SCHOOL OF M I N m GOLD EK , COLORADO

By R. N. D av id so n

ProQuest N um ber: 10781403

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5 3 80 9

A t h e s i s s u b m itte d to th e F a c u lty and th e B oard o f T r u s te e s o f th e C o lo rad o S ch o o l o f M ines i n p a r t i a l f u l f i l l m e n t o f th e re q u ire m e n ts f o r th e d e g re e o f M a s te r o f S c ie n c e .

S ig n ed :

xvOdotv

Ne Davidson

G o ld en , C o lo rad o D a te ;

Mon/

, 1950 CO

/

o CO

A pproved: y

^

2^ . t

P.' M."T(K'T iy l'----

Truman H.

V

CONTENTS A b s tr a c t . . . . ....................................................................

. .

I n t r o d u c t i o n ............................ ....................................................... R easons f o r F a v o r a b i l i t y o f D olom ite ................................ O r ig in o f D o l o m i t e ................ . ............................ . . . . D e f i n i t i o n •........................................................................ T h e o rie s o f O r ig in ....................................................... D o lo m itiz a tio n by I|ydro th e rm a l A c tio n

. . .

P h y s ic a l P r o p e r t i e s o f D o lo m ites Conducive t o F a v o r a b i l i t y ......................................... .... D i f f e r e n t i a l P r e p a r a tio n . ..................................... P la s tic ity

..............................................

Competency • • • • ................................ .... P o r o s ity .

. . .

S t r u c t u r a l C o n tro l . Summary

* ...................

. . . . . .

..................................................................................

The L e a d v llle F o r m a t i o n ....................... .... ............................... Aspen

............................................................................................

Alma

....................... .... ...............................................................

L e a d v llle

. . . . . .

............................ . . . . . .

G i l m a n ......................................... .................................................. D yer F o r m a t i o n ..................................................

. . .

G ilm an S an d sto n e ............................ . . . . . . . L e a d v llle F o rm a tio n

. . . . . .

E f f e c t o f P o r o s ity i n R eplacem ent

....................... ...................

11

C hem ical and P h y s ic a l T e s ts and I n t e r p r e t a t i o n s

• .

FW» 61

P o r o s ity D e t e r m i n a t i o n s .................................................

62

T h in S e c t i o n s ....................................

65

C hem ical A n a l y s e s ................................................................

63

G raph C o n s t r u c t i o n ...............................................................

64

G raph S e t 1

............................................................... . .

65

G raph S e t 2

.....................................................................

66

G raph S e t 3

......................................................

67

G raph 4

....................................................................

68

C o n c l u s i o n s .......................................................................................

70

B ib lio g ra p h y .......................................................................................

76

#

ill

ILLUSTRATIONS P la te

Page

1

In d ex H a p ..................................................

. . . .

.

22

2

C o m p arativ e S t r a t i g r a p h i c C o l u m n s .........................25

3

C o m p arativ e T a b le o f S t r a t i g r a p h i c T erm inology • * ..................................................

24

4

P orphyry s i l l s c a p p in g L e a d v llle d o lo m ite

.

28

5

P h o to m ic ro g rap h s o f D yer d o l o m i t e ......................... 29

6

P h o to m ic ro g rap h s o f L e a d v ille d o lo m ite . . .

30

7

P h o to m ic ro g rap h s o f L e a d v ille lim e s to n e

. .

37

L e a d v llle lim e s to n e from n e a r M in tu rn , and p h o to m ic ro g ra p h o f h ig h ly c r y s t a l l i n e L e a d v ille lim e s to n e . . . . . ............................

38

Dyer d o lo m ite o u tc ro p from M in tu rn , and p h o to m ic ro g ra p h s o f D yer d o lo m ite . . . . .

39

8

9 10

P h o to m ic ro g rap h s o f Waxy b e d , z e b ra r o c k , and " sa n d e d ” z e b ra ro c k ; p o lis h e d s u r f a c e o f L e a d v i l l e d o lo m ite ......................................43

11

L e a d v ille d o lo m ite c l i f f in E a g le R iv e r V a l l e y ........................................................................................44

12

L e a d v llle d o lo m ite o u tc ro p i n E ag le R iv e r V a l l e y ........................................................................................45

13

P h o to m ic ro g ra p h s and p o lis h e d s u r f a c e o f L e a d v llle d o lo m ite . . . . .

46

Z eb ra ro c k from G ilm an , as se e n i n o u tc r o p , p o lis h e d s u r f a c e , and p h o to m ic ro g ra p h . . .

50

14 15

P h o to m ic ro g rap h s and p o lis h e d s u r f a c e s o f z e b ra r o c k ......................................................................51

16

O re -w aste c o n t a c t I n th e L e a d v llle d o lo ­ m ite a t G i l m a n .........................................

17

54

P h o to m ic ro g rap h s o f L e a d v llle d o lo m ite a t G i l m a n ...............................................................................55

IV

Page 18

P y r i t i c z in c ore r e p la c in g L e a d v ille d o lo m ite i n Yak T unnel a t L e a d v i l l e , as s e e n i n p o lis h e d s u r f a c e and p h o to ­ m ic ro g ra p h s ....................... . ..............................................57

19

B e d d in g -p la n e c o n t r o l i n o re d e p o s itio n i n th e L e a d v ille d o lo m ite a t O ilm an , w ith p o lis h e d s e o tio n . . . . . . . . . . . . . .

20

58

P h o to m ic ro g rap h s o f L e a d v ille d o lo m ite fro m O i l m a n .......................................................................... 59

G raph S e ts 1 , 2* and 3

....................... ....

G raph N o * 4 « « * » * # » . . . . . . . * . « . . »

74 75

ARTHUR CAKES LIBRARY COLORADO SCHOOL OF MINES GOtOEM, CO 8 0401

ABSTRACT

The q u e s tio n o f f a v o r a b i l i t y i n r e l a t i o n to m i n e r a l i ­ z a t i o n i s as o ld as th e p r o f e s s io n o f m in in g g e o lo g y . The f a v o r a b i l i t y o f d o lo m ite s e s p e c i a l l y h a s l e d t o much d i s ­ c u s s io n . In t h i s p a p e r th e q u e s tio n i s t r e a t e d i n th r e e p h a s e s . P a r t 1 i s a g e n e r a l stu d y o f d o lo m ite f a v o r a b i l i ­ t y ; p a r t 2 i s a d e t a i l e d stu d y o f th e p ro b lem ; and p a r t 3 i s an a n a ly s is o f some c h a r a c t e r i s t i c s fo u n d i n th e d e ­ t a i l e d s tu d y . Each o f th e s e p a r t s i s c o n s id e r e d more s p e c i f i c a l l y b e lo w . P a r t 1 t r e a t s o f th e l i t e r a t u r e on v a r io u s r e p l a c e ­ ment o re d e p o s its form ed i n o r a s s o c ia te d w ith d o lo m ite s . I t i s co n o lu d ed t h a t th e g e n e r a l f a v o r a b i l i t y o f d o lo m ite s i s d u e , I n a num ber o f c a s e s , to d i f f e r e n t i a l p r e p a r a t i o n o f th e d o lo m ite th ro u g h b r e c c i a t i o n . I n o t h e r c a s e s th e f a v o r a b i l i t y o f th e d o lo m ite i s due t o a c o in c id in g c o n t r o l ( u s u a lly s t r u c t u r a l ) o v e r h y d ro th e rm a l d o l o m l t i z a t l o n and o re d e p o s i t i o n . P a r t 2 d e a ls w ith th e l i t e r a t u r e o n , and a f i e l d stu d y by th e a u th o r o f, th e L e a d v ille ( M is s is s lp p ia n ) f o r m a tio n i n f o u r c e n t r a l C o lo rad o m in in g d i s t r i c t s . I t i s c o n c lu d e d th a t: D i f f e r e n t i a l p r e p a r a t i o n ( s e l e o t i v e b r e c c i a t i o n and s i m i l a r p r e p a r a t o r y e f f e c t s ) in f lu e n c e d f a v o r a b i l i t y a t Alma; t h a t th e pred o m in an ce o f c a p p in g s i l l s i n th e L ead­ v i l l e i s th e m ain r e a s o n f o r th e seem ing f a v o r a b i l i t y o f th e L e a d v ille f o r m a tio n o v e r th e o th e r s a t L e a d v ille ; and t h a t c o in c id in g s t r u c t u r a l c o n t r o l o f d o lo m l tiz a tlo n and o re d e p o s i t i o n i s th e r e a l ca u se o f f a v o r a b i l i t y o f th e L e a d v ille d o lo m ite a t O ilm an w ith in c r e a s e d p o r o s i t y due t o d o l o m l t i z a t l o n , r e c r y s t a l l i z a t i o n , and " s a n d in g ” e x e r t ­ in g se c o n d a ry c o n t r o l and th u s f u r t h e r i n g f a v o r a b i l i t y o f th e L e a d v i l l e . P a r t 3 i s a d e s c r i p t i o n o f th e v a r io u s c h e m ic al and p h y s ic a l t e s t s made upon sam ples o f th e L e a d v ille and D yer f o r m a tio n s . The r e s u l t s a r e p l o t t e d on g ra p h s a g a i n s t t h e i r r e l a t i o n s h i p t o m i n e r a l i z a t i o n and th e g ra p h s a r e i n t e r p r e t e d . I t was found t h a t th e p o r o s i t y and p e r c e n t­ age o f I m p u r i tie s a r e in v e r s e ly p r o p o r t i o n a l and th e CaCOaAjgCO** r a t i o d i r e c t l y p r o p o r t i o n a l t o th e d is t a n o e from m e t a l l i z a t i o n . The p o s s i b l e use o f th e s e f a c t o r s as g u id e s t o o re i s s u g g e s te d .

2

INTRODUCTION

On© o f th e many p ro b lem s I n th e s tu d y o f th e f o rm a tio n o f o re d e p o s its i s th e s o - c a l l e d 11f a v o r a b i l i t y ” o f c e r t a i n b e d s o r fo rm a tio n s o v e r o t h e r s as a h o s t ro c k f o r o r e . P ro b a b ly th e m ost d is c u s s e d ro c k ty p e a s a f a v o r a b le h o s t , I n se d im e n ta ry ro c k s a t l e a s t , I s d o lo m ite .

T h is i s p a r t i ­

c u l a r l y t r u e i n th e c a se o f l e a d - z ln c d e p o s i t s .

O th e rs a re

th e a f f i n i t y g o ld shows f o r q u a r t z i t e and ca rb o n a c e o u s s h a le s and le a d f o r c r i n o i d a l lim e s to n e s . m ig h t b e m e n tio n e d .

S t i l l o th e r s ,

One o f th e m ost c o n s i s t e n t ro c k ty p e s

i n show ing f a v o r a b le c h a r a c t e r i s t i c s , h o w ev er, i s d o lo m ite o r d o lo m itic lim e s to n e . T here a r e many a r e a s w here d o lo m ite was more f a v o r a b le f o r o re d e p o s i t i o n th a n th e a d ja c e n t b e d s .

F o r exam ple,

th e L e a d v llle lim e s to n e ( M l s s i s s l p p l a n ) , w h ich i s w h o lly o r p a r t i a l l y d o lo m itiz e d a t L e a d v llle ( 1 3 ) , G ilm an ( 1 2 ) , Alma ( 3 7 ) , and Aspen ( 4 2 ) , C o lo ra d o , i s a v e iy f a v o r a b le o re h o r iz o n .

At K e n n e c o tt, A lask a ( 3 ) , a l a r g e p a r t o f th e o re

I s found i n t h e d o lo m itiz e d p o r t i o n of th e C h i tis t o n e lim e ­ s to n e .

At E a s t T i n t i c , U tah ( 3 3 ) , and a t J e f f e r s o n , Tenne­

s s e e (2 3 ), o re d e p o s i t s a r e fo u n d p re d o m in a te ly i n d o lo ­ m ite s .

S i m i l a r l y , a t G o o d sp rin g s, Nevada ( 2 3 ) , i n th e

H orsesho e and S acram en to d i s t r i c t s , C o lo rad o ( 1 1 ) , I n th e **Refers to number' o f r e f e r e n c e s I n b ib lio g r a p h y a t end o f t h i s p a p e r.

3

W isc o n sin lo a d and z in c d e p o s its (4 )# th e M e ta lin e d i s t r i c t , W ashingto n ( 3 6 ) , A u s t i n v i l l e , V i r g i n i a ( 8 ) , P e n o le s , S i e r r a M ojada, M exico ( 4 0 ) , H lg u e ra s d i s t r i c t , M exico ( 2 2 ) , M ines V ie ja s and Nuevo L eon, M exico (2 2 ), m ost o r a l l o f th e o re i s fo u n d i n d o lo m ite s .

The m e t a l l i z a t i o n fa v o r e d t h e d o lo ­

m ite s o v e r lim e s to n e s , s h a l e s , q u a r t z i t e s , and s e v e r a l o t h e r ro c k ty p e s ; h o w ev er, i t i s no i s o l a t e d o c c u rre n c e when a d o lo m ite i s fo u n d t o be a f a v o r a b le h o r iz o n . I n th e H ig u e ra s d i s t r i c t i n n o r th e r n Mexico ( 2 2 ) , th e o re form s p ip e s i n th e lim e s to n e b u t s p r e a d s o u t t o form l a r g e re p la c e m e n t b o d ie s w here th e s e p ip e s t r a n s e c t d o lo ­ m it e s .

At K en n e co tt (35) th e o re s o l u t i o n s t r a v e l e d

th ro u g h a b a s a l lim e s to n e member to fo rm o re d e p o s its I n th e d o lo m itic b ed s ab o v e.

At P e n o le s , S i e r r a M ojada, Mex­

ic o ( 2 2 ) , th e Manto d o lo m ite i s h e a v ily m in e r a liz e d w h ile th e u n d e r ly in g S u iz a lim e s to n e i s p r a c t i c a l l y b a r r e n .

In

n o r th e r n M exico ( 2 2 ) , th e r e i s a 5 ,0 0 0 - f o o t lim e s to n e s e c ­ t i o n o f w hich 500 f e e t I s d o lo m ite .

A ll ecohom ic m i n e r a l i ­

z a t i o n , a c c o rd in g to Hayward and T r i p l e t t , i s i n th e d o lo ­ m ite b e d s . From th e s e ex a m p le s, i t I s o b v io u s t h a t d o lo m ite form ­ a t i o n s a r e commonly f a v o r a b le h o s ts f o r o r e .

The re a s o n

f o r f a v o r a b i l i t y h a s b e e n th e to p ic o f much d is c u s s io n .

In

t h i s p a p e r , th e p roblem i s ap p ro ach ed i n th e f o llo w in g man­ n e r:

P a r t 1 d e a ls w ith a g e n e r a l rev ie w o f th e l i t e r a t u r e

on t h e s u b j e c t .

P a r t 2 i s a d e t a i l e d stu d y o f th e L ead-

4

v ill© lim e s to n e i n s e v e r a l c e n t r a l C o lo rad o m in in g d i s ­ tric ts .

P a r t 3 d e a ls w ith a s e r i e s o f c h e m ic al and p h y s i­

c a l t e s t s made on sam ples o f b o th f a v o r a b le and u n fa v o ra b le f o rm a tio n s and w ith th e i n t e r p r e t a t i o n o f the r e s u l t s o f th e s e t e s t s . The p u rp o se o f th e p a p e r i s to b r in g o u t s e v e r a l b ro a d g e n e r a l i z a t i o n s as t o why d o lo m ite i s so o f t e n f a v o r a b le and to show t h e p o s s i b l e a p p l i c a t i o n o f th e s e g e n e r a l i z a ­ t i o n s to th e s e a r c h f o r o r e .

I t h a s b e e n s u g g e s te d t h a t ,

w ith m o d i f i c a t i o n s , th e p o r o s i t y s t u d i e s i n p a r t 2 m ig h t be a p p l ic a b le t o r e s e r v o i r problem s i n p e tro le u m g e o lo g y . T h is p a p e r i s th e r e s u l t o f r e s e a r c h co n d u c ted a t th e C o lo rad o S chool o f M in es.

The w r i t e r i s in d e b te d t o th e

S h e ll O il Company f o r th e f i n a n c i a l a s s i s t a n c e th ro u g h th e S h e ll F e llo w s h ip i n g e o lo g y ; to th e New J e r s e y Z inc Company f o r a c c e s s to t h e i r G ilm an, C o lo ra d o , p r o p e r ty ; t o F . M. Van T u y l, Truman H. Kuhn, b o th o f th e C o lo ra d o S ch o o l o f M in es, and R. E . R adabaugh, C h ie f G e o lo g is t f o r New J e r s e y Z inc a t G ilm an, f o r t h e i r c o u n s e l; and to W illia m Jo h n so n , New J e r s e y Z in c Company g e o l o g i s t a t G ilm an, f o r h i s a id i n c o l l e c t i n g an d l o c a t i n g sa m p les.

5

PART I REASONS FOR FAVORABILITY OF DOLOMITE O r ig in o f D olom ite D e fin itio n D o lo m ite , a s d e f in e d by P e t t i j o h n (3 8 , p . 3 1 8 ) , i a t h a t v a r i e t y o f lim e s to n e c o n ta in in g more th a n 50 p e r c e n t c a rb o n a te o f w hich more th a n h a l f i s d o lo m ite*

The te rm

" d o lo m ite , n a s u se d i n th e d e f i n i t i o n , r e f e r s t o th e m in­ e r a l d o lo m ite w hich h a s th e c o m p o s itio n o f 54*35 p e r c e n t CaC0 3 1111(1 4 5 .6 5 p e r c e n t MgCOj when p u r e .

S h ro c k , a s n o te d

by P e t t i j o h n (3 8 , p . 3 1 2 ), w ould c a l l th e r o c k ty p e " d o lo s to n e " to d i f f e r e n t i a t e i t from th e m in e ra l d o lo m ite , b u t s in c e t h i s te rm i s n o t i n common u se i t w i l l n o t b e u sed h e r e and ’VlolomlteF w i l l b e u se d to r e f e r to e i t h e r th e m in­ e r a l o r th e r o c k . T h e o rie s o f O rig in P. M. Van T uyl (47) g iv e s th r e e m ain t h e o r i e s o r g ro u p s o f t h e o r i e s o f th e o r i g i n o f d o lo m ite . 1.

T hese a r e :

P rim ary d e p o s i t i o n o f th e d o lo m ite , e i t h e r c l a s t i c o r c h e m ic a l.

2.

S e l e c t i v e le a c h in g o f c a l c i t e from an o r i g i n a l m ix tu re o f c a l c i t e and d o lo m ite .

3.

R eplacem ent ( p a r t i a l ) o f an o r i g i n a l c a l c i t i c lim e s to n e w ith MgCC>3 .

6

P e t t i j o h n (3 8 , p . 314) s t a t e s "E v id en ce t o s u p p o rt th e f i r s t two c o n c e p ts i s m eager w h ereas th e e v id e n c e f o r th e re p la c e m e n t i s o v e rw h e lm in g .”

A lth o u g h th e c o n s id e r a ti o n

o f th e o r i g i n o f d o lo m ite i s n o t th e p rim a ry p u rp o se o f t h i s p a p e r , one p h ase of th e a l t e r a t i o n th e o r y , h y d r o th e r ­ mal d o l o m i t i z a t i o n , i s b ro u g h t o u t i n many d e s c r i p t i o n s o f o re d e p o s its i n d o lo m ite ; so a s h o r t d i s c u s s i o n o f o r i g i n i s in c lu d e d h e r e . I f th e d o lo m ite i s p r im a r y , th e o r i g i n i s u n im p o rta n t so f a r as o re d e p o s i t i o n i s c o n c e rn e d .

The same a p p lie s t o

d o lo m ite s form ed th ro u g h th e in c r e a s e i n th e MgCC>3 c o n te n t by s e l e c t i v e le a c h i n g .

H ow ever, i f a d o lo m ite i s form ed by

a l t e r a t i o n , and e s p e c i a l l y i f th e a l t e r a t i o n i s h y d r o th e r ­ m a l, t h e im p o rta n c e o f g e n e tic s tu d y i s o b v io u s .

N ote t h a t

o n ly th e o r i g i n i s u n im p o rta n t i n th e p rim a ry and le a c h e d d o lo m ite s , and th e y a r e s t i l l p o t e n t i a l l y f a v o r a b le h o s t ro c k s d ep en d in g upon o t h e r p r o p e r t i e s w hich th e y may o r may not p o ssess.

T hese p r o p e r t i e s w i l l b e d is c u s s e d a t l e n g th

la te r. D o lo m ltiz a tlo n by H y d ro th erm al A c tio n E x p e rim e n ta l e v id e n c e ( 4 7 , p . 314) i n d i c a t e s t h a t d o l o m i t i z a t i o n i s b e s t ac co m p lish ed a t e l e v a t e d p r e s s u r e s w h ic h , o f c o u r s e , w ould n e a r ly alw ays be p r e s e n t i n any h o r iz o n w here h y d ro th e rm a l a c t i o n i s im p o r ta n t.

T here a r e

many d e p o s its w here a s h e l l o f d o lo m ite I s fo u n d around th e o re d e p o s i t s .

Such a s h e l l o c c u rs i n some lim e s to n e r e p la c e -

7

m ent b o d ie s a t A spen, C olorado*

H ew ett (23) n o te s th e

o c c u rre n c e o f an en v e lo p e o f d o lo m ite around th e o re b o d ie s a t J e f f e r s o n , T e n n e s s e e , and th e r e i s an i n t r i c a t e a s s o c ia ­ t i o n o f d o lo m ite and m in e r a liz e d a r e a s i n th e M is s is s i p p i V a lle y r e g io n ( 2 3 ) .

C. E. S ie b e n th a l (41) n o te s "g ra y

s p a r ” (d o lo m ite ) a d ja c e n t t o th e m in e r a liz e d a r e a s i n th e J o p l i n reg io n *

D olom ite i s fo u n d as a gangue m in e ra l i n

th e M ia m i-P ic h e r z i n c - l e a d d i s t r i c t ( 4 3 ) .

I n th e M e ta lln e

d i s t r i c t , W a sh in g to n , m ost o re o c c u rs i n a d o lo m ite , b u t w here i t i s fo u n d i n a lim e s to n e i t i s su rro u n d e d by a s h e l l o f c o a rs e c r y s t a l l i n e d o lo m ite ( 3 6 ).

D o lo m ite o c c u rs

as a h y d ro th e rm a l gangue m in e ra l i n many d e p o s its i n a l l ty p e s o f r o c k s .

I f d o lo m ite can o c c u r as a v e in gangue

m a t e r i a l , o r fo rm a s h e l l around an o r e d e p o s i t , i t sh o u ld b e p o s s i b l e f o r an e n t i r e d i s t r i c t t o b e d o lo m itiz e d i f th e volume o f th e s o l u t i o n s i s g r e a t enough and s tr o n g enough and h a s a c te d lo n g enough*

L o v erin g (£2) h a s advanced t h i s

th e o ry f o r th e mode o f d o lo m i tiz a tio n o f th e L e a d v ille lim e ­ s to n e a t G ilm an , C o lo ra d o , w hich w i l l b e d is c u s s e d a t le n g th l a t e r .

He a l s o b e l i e v e s h y d ro th e rm a l a l t e r a t i o n to

b e th e cau se o f extrem e d o lo m i tiz a tio n a t E a s t T i n t i c , U tah (3 3 ).

The d o l o m i t i z a t i o n o f th e lim e s to n e a t E a s t T i n t i c

i s a s e p a r a te and i n d i v i d u a l , v e ry w id e s p re a d , p r e - m e t a l l i ­ z a t i o n ph ase o f a l t e r a t i o n and i s u n d o u b te d ly h y d ro th e rm a l. L o v e rin g n o te s t h a t th e f o o tw a lls o f lo w -a n g le f a u l t s a re th e m ost e x t e n s iv e ly d o lo m itiz e d a r e a s , a n d , a lth o u g h n o t alw ays th e c a s e , th e d o lo m ite o f t e n s to p s a b r u p tly a t th e ARTHUR LAKES LIBRARY rnLORADO SCHOOL OF MINES

GOLDEN, CO 80401 ^

8

f a u l t w ith u n a l te r e d lim e s to n e on th e h an g in g w a ll .

The

d o lo m i t i z a t i o n shows a b ro a d r e l a t i o n t o c e n t e r s o f ig n e o u s a c t i v i t y , a n d th e volume o f d o lo m itiz e d ro c k ex ceed s th e m in e r a liz e d ro c k volume by s e v e r a l th o u sa n d tim e s .

L o v er­

in g a s c r i b e s th e d o l o m i t i z a t i o n to h o t , d i l u t e s o l u t i o n s o f m ixed c h l o r i d e s w ith io n i c s u b s t i t u t i o n o f magnesium f o r c a lc iu m a l t e r i n g th e lim e s to n e to d o lo m ite .

I t is in te r­

e s t i n g to n o te h e re t h a t c h l o r id e in c l u s i o n s a r e fo u n d i n th e d o lo m ite a t L e a d v ille ( 1 3 ) . H ew ett s u g g e s ts t h a t th e l e a d , z i n c , and m ag n esia w ere added s im u lta n e o u s ly i n th e M i s s is s i p p i V a lle y r e g io n , a l ­ th o u g h Van Tuyl ( p e r s o n a l com m unication) s t a t e s t h a t t h i s does n o t h o ld i n th e e n t i r e r e g io n .

H ew ett (23) a ls o n o te s

l a r g e d o lo m itiz e d a r e a s a t G o o d sp rin g s, N evada, w i t h 73 o f th e 75 p ro d u c in g m ines i n d o lo m ite .

H ew ett a l s o c i t e s

K e n n e c o tt as an exam ple o f p r e - m e t a l l i z a t i o n d o lo m i tiz a tio n o f a la rg e a re a .

I n a d i s c u s s i o n o f th e M onarch and K ick­

in g H orse o re d e p o s its o f B r i t i s h C olum bia, G oranson (16) l i s t s t h r e e s ta g e s o f d o l o m i t i z a t i o n b e g in n in g w ith a w id e­ s p r e a d , co m p lete d o lo m i tiz a tio n o f th e u p p e r lim e s to n e b e d s fo llo w e d by a p a r t i a l re p la c e m e n t o f th e d o lo m ite b t th e f i r s t s ta g e by a t o t a l l y d i f f e r e n t d o lo m ite and f i n a l l y a c ra c k -a n d v u g - f i l l i n g s ta g e w hich was contem poraneous w ith o re d e p o s i t i o n . T hese exam ples sh o u ld b e s u f f i c i e n t to e s t a b l i s h th e f a c t t h a t th e h y d ro th e rm a l d o l o m i t i z a t i o n o f a l a r g e a r e a i s n o t o n ly p o s s i b le b u t i s p ro b a b ly more common th a n i s

9

u s u a lly r e a l i z e d .

The two exam ples c i t e d d e s c r ib e d i n

p a p e rs by L o v e rin g a re e s p e c i a l l y c o n v in c in g ( 3 2 , 3 3 ). T h is w ould i n d i c a t e t h a t , i n some c a s e s , th e d o lo m ite was n o t n e c e s s a r i l y a f a v o r a b le h o r iz o n b u t r a t h e r t h a t th e d o lo m i tiz a tio n was c o n t r o l l e d by th e same s t r u c t u r a l f e a ­ t u r e s as was th e l a t e r o re d e p o s i t i o n .

The d o lo m i tiz a tio n

may b e v e r y w id e s p re a d and h en ce c o n s id e r e d e n t i r e l y in d e ­ p e n d e n t o f th e o r e d e p o s i t s , b u t L o v e rin g (33) h as shown t h a t th e d o lo m i tiz a tio n a ls o may be an e a r l y w id e s p re a d a l t e r a t i o n f e a t u r e w ith th e o re d e p o s itio n f o llo w in g on a more r e s t r i c t e d s c a le *

T h is makes p o s s i b le an e x p la n a tio n

b a s e d on common g e n e tic and s t r u c t u r a l c o n t r o l o f th e s o c a l l e d f a v o r a b i l i t y o f d o lo m ite i n a number o f d e p o s i t s .

P h y s ic a l P r o p e r t i e s o f D o lo m ites C onducive to F a v o r a b i l i t y D i f f e r e n t i a l P r e p a r a tio n I n s p i t e o f th e r e s u l t s o f Rove ( 4 0 ) , w hich m a in ly n e g a tiv e , a re v ie w o f a number o f p a p e rs on re p la c e m e n t o re b o d ie s ( p r i n c i p a l l y le a d - z in o )

i n d o lo m ite* h as shown t h a t

th e r e a re c e r t a i n p h y s ic a l c h a r a c t e r i s t i c s o f many d o lo ­ m ite s w hich te n d to make them more f a v o r a b le h o s t ro c k s f o r o re d e p o s itio n th a n o t h e r s .

The m ain one o f th e s e i s th e

te n d e n c y o f d o lo m ite to s h a t t e r and b r e c c i a t e when d efo rm ed , w h ile o t h e r ro c k s flo w o r d efo rm p l a s t i c a l l y .

Newhouse

(35) n o te s t h a t many o f th e ro c k s a r e f a v o r a b le b e c a u se

10

th e y a re b r i t t l e

and f r a c t u r e b e f o r e d efo rm in g p l a s t i c a l l y *

I t i s p o s s i b l e t h a t b r e c c i a t i o n was p r e - d o l o m i t i z a t i o n , b u t a h e a lin g o f f r a c t u r e s and c r a c k s w ould be e x p e c te d i n t h i s c a s e and t h i s was n o t m en tio n ed i n any o f th e p a p e rs and p ro b a b ly w ould have b e e n o b v io u s had i t e x i s t e d .

To p ro v e

th e p o in t f u r t h e r , t h e f o llo w in g q u o ta tio n i s ta k e n fro m a d i s c u s s i o n by V a n d e rw ilt (4 6 ):

" . . . t h e b r i t t l e n e s s o f d o lo

m ite and th e to u g h n e ss o f lim e s to n e a r e w e ll know n." P la s tic ity A ll se d im e n ta ry r o c k s , w ith th e p o s s i b le e x c e p tio n o f q u a r t z i t i c s a n d s to n e , w i l l flo w p l a s t i c a l l y i f th e c o n f in ­ in g p r e s s u r e i s g r e a t enough ( 1 8 ).

However, a d e p th ap­

p ro a c h in g 20 m ile s i s n e c e s s a r y t o make a p u re lim e s to n e flo w i n th e ab sen ce o f a f l u i d , b u t an im pure lim e s to n e o r s h a ly lim e w i l l flo w w ith much l e s s c o n f in in g p r e s s u r e and some s h a le s w i l l deform by p l a s t i c c re e p a t th e s u r f a c e (1 9 ).

T h is i s c i t e d t o i n d i c a t e th e d i f f e r e n t m anners by

w h ich v a r io u s ro c k s w i l l r e a c t to a d i f f e r e n t i a l s t r e s s b e ­ c a u se t h i s i s a p p a r e n tly one o f th e s e c r e t s o f t r u e f a v o r a ­ b i l i t y o f d o lo m ite .

I n o t h e r w o rd s, m ost d o lo m ite s a c t as

c o m p ete n t r o c k s . Com petency "C o m p ete n t," as d e f in e d by M eK instry ( 2 9 ) , " r e f e r s to r o c k s t h a t a r e r e l a t i v e l y s tr o n g b u t , when th e y do b r e a k , th e y b r e a k a s th o u g h th e y w ere b r i t t l e m a t e r i a l .

♦Incompe­

t e n t f r e f e r s t o ro c k s w hich a re weak and te n d to deform

p l a s t i c a l l y , o r by f l o w .”

Rove (4 0 , p . 5 1 ) , i n h ia c o n c lu ­

s i o n s , s t a t e s t h a t th e m ost f a v o r a b le h o r iz o n s a r e th o s e w hich f a i l by m ixed te n s io n and s h e a r and t h a t i n an i n t e r ­ bedded lim e s to n e and d o lo m ite , th e lim e s to n e w i l l deform by r e c r y s t a l l i z a t i o n and flo w w h ile th e d o lo m ite w i l l f r a c t u r e b e f o r e flo w a g e can o c c u r.

I f a h y d ro th e rm a l s o l u t i o n , r e ­

g a r d l e s s o f I t s s o lv e n t a c t i o n , c a n e n t e r a b r e c c i a t e d zone i n a h o r iz o n w h ich i s c h e m ic a lly more u n f a v o r a b le th a n ano­ t h e r w hich i s u r ib r e c c ia te d , th e o re w i l l b e d e p o s ite d i n th e fo rm e r and re p la c e m e n t w i l l be e a s i e r b e c a u s e o f th e in c r e a s e d s u r f a c e .

F o r an ex trem e ex am p le, r e f e r e n c e may

b e made t o Sheep C re e k , B r i t i s h C olum bia ( 3 5 ) , w here th e o re i s fo u n d I n q u a r t z i t e s and h a r d , a r g i l l a c e o u s q u a r t z i t e s w h ile a lim e s to n e s c h i s t i s b a r r e n .

At th e Combina­

t i o n m ine (35) th e o re was d e p o s ite d i n p u re lim e s to n e b e d s and n o t i n th e s h a le and s h a ly lim e s to n e .

At B isb e e (40)

th e o r e I s fo u n d I n th e c r y s t a l l i n e u p p e r o n e - t h i r d o f th e M a rtin and lo w e r, th ic k - b e d d e d , e a s i l y s h a t t e r e d o n e - t h i r d o f th e E s c a b ro s a lim e s to n e , w h ile th e lo w e r s h a ly M a rtin lim e s to n e and th e u p p e r th in - b e d d e d E s c a b ro s a lim e s to n e a re more o r l e s s b a r r e n .

At Alma, C o lo ra d o ( 3 7 ) , th e o r e , i n

many c a s e s , p a s s e s up b o th lim e s to n e and d o lo m ite to be d e ­ p o s it e d I n a p o rp h y ry and i n a q u a r t z i t e .

T h is i s a t t r i ­

b u te d t o th e open f i s s u r e s and s h a t t e r zo n es i n t h e more b r i t t l e m a t e r i a l w hich d ie o u t I n t h e d o lo m ite .

T h u s, I t

may b e s e e n t h a t com petency i s a r e l a t i v e th in g and f a v o r a ­ b ility

i s d i r e e t l y p r o p o r t i o n a l to th e d e g re e o f com petency

12

i n a r e a s w here p r e p a r a t i o n was by d e fo rm a tio n *

I n a com­

p a r is o n o f th e L e a d v ille and D yer f o r m a tio n s , b o th o f w hich a r e d o lo m ite s i n th e H o rsesh o e and S acram ento d i s t r i c t s i n C o lo ra d o , B u t le r and S in g ew ald (11) n o te t h a t th e L e a d v ille shows more s h a t t e r i n g th a n th e Dyer and th e r e I s a c c o rd in g ­ ly a g r e a t e r amount o f o re i n th e L e a d v ille *

At A spen,

w here th e o re o c c u rs i n e i t h e r o r b o th th e lim e s to n e o r th e d o lo m ite s e c t i o n s o f th e L e a d v i l l e , V a n d e rw llt (46) n o te s t h a t b r e c c i a t i o n i s more oommon i n t h e d o lo m ite th a n i n th e lim e s to n e , and i t was fo u n d t h a t a g r e a t e r p o r t i o n o f th e o re i s fo u n d i n th e d o lo m ite w here b r e c c i a t i o n h as o cc u rred * H ere a g a in i t m ust b e assum ed t h a t th e b r e c c i a t i o n o c c u rre d a f t e r d o l o m i t i z a t i o n , a lth o u g h t h i s was n o t s ta te d *

I n th e

W isc o n sin le a d and z in c d e p o s i t s ( 1 7 ) , m ost o f th e o re I s i n th e G a le n a , w h ic h , w ith t h e e x c e p tio n o f two l o c a l i t i e s , I s a r e l a t i v e l y p u re d o lo m ite*

B ehre (5) a c c o u n ts f o r t h e

predom inan ce o f o re i n t h e G alen a by d i f f e r e n t i a l b r e c c i a ­ t i o n f a v o r in g th e G ale n a o v e r th e b a r r e n , s l i g h t l y d o lo m it l c P l a t t v i l l e lim e s to n e .

I n a d e s c r i p t i o n o f th e J e f f e r s o n

C ity a r e a , T e n n e s s e e , Brokaw and Jo n e s (7) n o te d th e f o l ­ low ing*

The d o lo m ite b e d s , i n th e c r e s t s and tr o u g h s o f \

a n t i c l i n e s and s y n c l i n e s , r e s p e c t i v e l y , a re b r e o c i a t e d and t h i s b r e c c i a was a f a v o r a b le h o s t to th e e x t e n t t h a t m ost o f th e o re o c c u rs i n th e b r e o c i a t e d c r e s t s and tro u g h s* The lim e s to n e , w hioh i s th in - b e d d e d and s h a ly , i n c o n t r a s t w ith th e b r i t t l e , m a s s iv e , c r y s t a l l i n e d o lo m ite , te n d e d t o bend and flo w when deform ed w ith th e t e n s i o n a l o p en in g s

13

c l o s in g by p l a s t i c flo w o f th e s h a le and lim e sto n e *

To t h e

a u th o r , t h i s seems t o b e c l e a r l y a n o th e r c a s e w here b e t t e r p r e p a r a t i o n o f th e d o lo m ite due to I t s co m p eten t c h a r a c t e r made i t more f a v o r a b le th a n th e r e l a t i v e l y In co m p e ten t lim e s to n e .

I n t h e l e a d - z i n c d e p o s its a t P in e P o in t, G re a t

S la v e Lake ( 6 ) , th e o re i s fo u n d p re d o m in a n tly i n th e F r e s q u 'i l e f o r m a tio n .

T h is i s a c r y s t a l l i n e d o lo m ite and

i s u n d e r la in by a b a r r e n , th in - b e d d e d , sandy d o lo m ite and i s o v e r l a i n by a b a r r e n , s h a ly lim e s to n e .

I n G o ra n so n ’s

d e s c r i p t i o n (16) o f th e M onarch and K ic k in g H orse o re dep o ­ s i t s I n B r i t i s h C olum bia, th e f o llo w in g seq u en ce was ob­ se rv e d .

A f t e r d o l o m l t i z a t l o n th e f o l d i n g p ro d u ced an a n t i ­

c lin a l s tru c tu re .

I n t h i s a n t i c l i n e , th e m a ssiv e o o m petent

d o lo m ite was b r e o c i a t e d w h ile th e s l i g h t l y d o lo m itic lim e ­ s to n e u n d e rn e a th deform ed p l a s t i c a l l y and d id n o t f r a c t u r e . The o r e - f o m l n g s o l u t i o n s , w hich fo llo w e d f o l d i n g , b y -p a s s e d th e u n f r a c tu r e d lim e s to n e and e n te r e d th e b r e c o l a t e d d o lo ­ m i t e , r e p la c in g i t w ith o r e .

I n th e M e ta lin e d i s t r i c t ,

W ash in g to n ( 3 6 ) , th e o re b o d ie s w ere form ed I n a b r e o c i a t e d d o lo m ite u n d e r an u n b re c c i& te d s l a t e c a p .

I n t h e c o n c lu ­

s io n s o f a d i s c u s s i o n o f l e a d - z i n c o r e s i n n o r th e r n M exico, Hay ward an d T r i p l e t t (22) b r in g o u t th e fo llo w in g !

In a l te r ­

n a t i n g d o lo m ite and lim e s to n e b e d s , th e d o lo m ite i s u s u a lly more o o a r a e ly c r y s t a l l i n e and i s , t h e r e f o r e , more e a s i l y b r e o c i a t e d and h en ce more f a v o r a b le f o r h y d ro th e rm a l r e ­ p la c e m e n t th a n t h e lim e s to n e . T here c a n , o f c o u r s e , b e no r u l e w ith o u t an e x c e p tio n ,

14

and s e v e r a l o f th e s e e x c e p tio n s have b e e n n o te d h e r e .

How­

e v e r , th e s e many exam ples d e s c r ib e d by d i f f e r e n t a u t h o r i ­ t i e s show a d e f i n i t e t r e n d .

Prom t h i s t r e n d , th e f o llo w in g

g e n e r a l i z a t i o n i s d e r iv e d . When a s t r a t i g r a p h i c s e c t i o n composed o f b e d s o r fo rm a tio n s o f v a ry in g r e l a t i v e com petency i s d e fo rm e d , th e m a s s iv e , b r i t t l e , c r y s t a l l i n e ro o k s w i l l form more f a v o r a ­ b le h o s ts f o r o re d e p o s i t i o n th ro u g h d i f f e r e n t i a l s e l e c t i v e p re p a ra tio n .

A ls o , i n l i n e w ith th e p r e s e n t s tu d y , When

d o lo m ite i s in v o lv e d , u n le s s i t i s v e ry im p u re , i t w i l l form a more f a v o r a b le h o s t th a n s h a l e s , s h a ly lim e s to n e s , lim e s t o n e s , o r s l i g h t l y d o lo m itic lim e s to n e s .

T h is a p p l i e s

t o d o lo m ite s o f a l l o r i g i n s , h y d r o th e r m a l, p r im a r y , m a rin e a l t e r a t i o n , and a l l o th e r ty p e s a l i k e . P o r o s ity I n h i s e x p e rim e n ta l w o rk , Hove (40) c o n c lu d e d t h a t , w ith two p o s s i b l e e x c e p ti o n s , p e r m e a b ility was u n im p o rta n t i n d e te rm in in g th e f a v o r a b i l i t y o f a f o r m a tio n .

He d id

n o t , how ev er, make a s tu d y o f p o r o s i t y , and th e a u th o r b e ­ l i e v e s t h a t i f s u f f i c i e n t p o r o s i t y i s a v a i l a b l e , ev e n th o u g h n o t i n t e r c o n n e c t i n g , i t may b e im p o r ta n t, as any s o l u t i o n c a p a b le o f re p la c e m e n t w i l l f i n d no d i f f i c u l t y i n d i s s o l v i n g p o re w a l l s .

S in c e p o r o s i t y w i l l b e d is c u s s e d

i n d e t a i l l a t e r , a s h o r t d i s c u s s i o n o f th e o r i g i n o f p o ro ­ s i t y i n d o lo m ite s w i l l be In c lu d e d h e r e . I n a p rim a ry d o lo m ite , t h a t i s , one w hich was d e p o s i t -

15

ed d i r e c t l y a s a d o lo m ite , th e p rim a ry o p en in g s sh o u ld b e th e same as th o s e fo u n d i n a lim e sto n e #

The f o llo w in g i s

m o d ifie d from a c l a s s i f i c a t i o n by H o lt (2 4 , p . 9 ) . P rim ary P o r o s ity 1#

V oids b e tw e e n th e i n d i v i d u a l c o n s t i t u e n t p a r t i c l e s o f d e t r l t a l c a rb o n a te rocks#

2.

V oids b etw e en i n d i v i d u a l c r y s t a l s and alo n g i n d i ­ v id u a l c le a v a g e p la n e s o f c r y s t a l s #

3#

V oids a lo n g b e d d in g p la n e s due t o d i f f e r e n c e s i n m a t e r i a l d e p o s ite d and d i f f e r e n c e s o f c r y s t a l s i z e and arran g em en t#

4#

V oids w ith in th e s k e l e t a l s t r u c t u r e o f i n v e r t e ­ b r a t e s and w ith in th e t i s s u e s o f c a lc a r e o u s a lg a e #

P rim ary p o r o s i t y i s u s u a l l y v e ry low and i s unim por­ t a n t e x c e p t f o r th e e f f e c t i t m ig h t have on d o lo m i tiz a tio n o f a p r e - e x i s t e n t lim e s to n e o r on th e d ev elo p m en t o f se c o n ­ d a ry p o r o s i t y i n a d o lo m ite .

S eco n d ary p o r o s i t y , h o w ev er,

i s v e ry Im p o rta n t i n some d e p o s its # S eco n d ary P o r o s ity P o r o s ity th ro u g h D o lo m ltiz a tlo n #—D o lo m ltiz a tlo n o f a lim e s to n e may ta k e p la c e i n tw o w a y s, v i z :

b y m o le c u la r

re p la c e m e n t o r by v o lu m e tr ic r e p la c e m e n t.

I n th e f o rm e r,

a d e c r e a s e i n volum e o f b e tw e e n 12 and 13 p e r c e n t sh o u ld ta k e p la c e (4 7 , p . 2 8 6 ),

H ow ever, t h i s m ethod o f r e p l a c e ­

ment i s n o t w id e ly re g a rd e d as o f common o c c u rre n c e i n n a tu r e (30)#

Volume s h r in k a g e may o c c u r, ev en th e s o - c a l l e d

16 v o lu m e -fo r-v o lu m e re p la c e m e n t by an e x c e s s o f s o l u t i o n e v e r p r e c i p i t a t i o n , a lth o u g h I n t h a t e a s e I t l a n o t t r u e volum efo r-v o lu m e re p la c e m e n t*

T h is I s p ro p o se d b y L andes (28)

w ith ground o r m e te o ric w a te r a s th e d o l o a i t i z i n g ag en t* However, t h i s m ethod o f p o r o s i t y fo rm a tio n s h o u ld h o ld e q u a lly w e ll i n d o l o a i t l z a t i e n by h y d ro th e rm a l a c tio n * H ew ett (23) h a s c o n c lu d e d t h a t th e t e x t u r e i s u s u a l l y c o a r s e r an d p o r o s i t y h ig h e r i n d o lo m ite th a n i n th e o r i g i n ­ a l lim e s to n e *

Hayward and T r i p l e t t (22) a l s o n o te d t h a t

th e d o lo m ite s w ere g e n e r a l l y more p o ro u s*

P o r o s i ty may

a l s o o o ae from s t y l o l l t l c seams ( 4 4 ) , J o i n t s , and c r a c k s ; and b e d d in g p la n e s a r e alw ay s im p o rta n t*

I n c r e a s e d p o ro ­

s i t y th ro u g h b r e c c i a t i o n i s v e r y im p o rta n t i n & g r e a t number o f c a s e s , a s was shown p r e v io u s ly *

The l a s t , r e -

o r y s t a l l l z a t l o n , how ever, i s a f a c t o r i n th e in c r e a s e o f p o r o s i t y w hich i s n o t f u l l y a p p r e c ia te d * R e c r y s t a l l i z a t i o n *— One o f th e t h e o r i e s on p o r o s i t y i n c r e a s e th ro u g h d o lo m l tiz a tlo n n o te d by L andes (28) i s th e p o r o s i t y b etw een i n t e r l o c k i n g o r y s t a l s fo rm ed d u r in g r e e r y s ta lllz a tlo n *

At L e a d v l l l e , C o lo rad o ( 1 4 ) ; G ilm an, C olo­

ra d o (3 2 , p« 8 0 ) ; and i n t h e H o rsesh o e an d S acram ento d i s ­ t r i c t s , C o lo rad o ( 1 1 ) , r e o r y s t a l l i z a t l o n h a s p ro d u ced " z e b r a ro c k " fro m th e d o lo m ite*

T h is r e c r y s t a l l i z a t i o n p ro d u c e s

l a r g e w h ite d o lo m ite c r y s t a l s a t th e ex p en se o f th e s m a ll c r y s t a l s o f d a rk g ra y o r b la c k d o lo m ite*

S eco n d ary p o ro ­

s i t y i s d e v e lo p e d b etw een in d i v i d u a l c r y s t a l b o u n d a r ie s , a lo n g c le a v a g e p la n e s i n i n d i v i d u a l o r y s t a l s , and i n vugs

17

and c h a n n e ls form ed by th e r e c r y s t a l l l z l n g s o l u t i o n s . a g a in we have e x c e ss o f s o l u t i o n o v e r d e p o s itio n #

Here

T hese

c h a n n e ls a r e n e a r ly p a r a l l e l o r p a r a l l e l t o t h e b e d d in g w hich in c r e a s e s p o r o s i t y and p e r m e a b ility i n t h i s d i r e c t i o n much more th a n p e r p e n d ic u la r to th e bed d in g #

T h is was a ls o

n o te d by Rove (40) i n r o c k w hich had b e e n d o lo m itiz e d b u t d id n o t show z e b ra s t r u c t u r e .

He a t t r i b u t e d th e g r e a t e r

p e r m e a b ili ty p a r a l l e l to th e b e d d in g t o s t y l o l i t i c seams w h ich a r e common i n th e u n r e o r y s t a l i i zed d o lo m ite b u t w hich a re d e s tro y e d by z e b ra fo rm a tio n #

S t a t i s t i c a l p ro o f

o f in c r e a s e d p o r o s i t y due to d o lo m i tiz a tio n w i l l b e g iv e n la te r# " S a n d in g # " -- F u r t h e r I n c r e a s e i n p o r o s ity i s fo u n d a t G ilm an , C o lo rad o (3 2 , p# 8 5 ) , a t L e a d v i l l e , C o lo rad o ( 1 4 ) , and a t o th e r p la c e s due t o " s a n d in g " o f th e d o lo m ite .

T h is

" s a n d in g " c o n s i s t s o f lo o s e n in g th e d o lo m ite g r a i n s u n t i l , a lth o u g h th e o r i g i n a l t e x t u r e may s t i l l b e s e e n i n p l a c e s , th e g r a i n s w i l l flo w l i k e b e a c h sand when d is t u r b e d s l i g h t ­ ly#

I t s fo rm a tio n h a s b e e n a t t r i b u t e d to th e a t t a c k o f

a c id w a te rs ( 1 4 , p* 36) a lo n g th e c r y s t a l b o u n d a rie s w ith a s l i g h t ro u n d in g o f th e c o r n e r s and edges r e s u lt in g # u n lo c k s th e c r y s t a l s #

T h is

L o v e rin g (3 2 , p . 85) a l s o a t t r i b u t e s

" s a n d in g " t o a p e n e t r a t i n g p r o c e s s o f s o l u t i o n a lo n g I n t e r g r a n u l a r fa c e s #

A f u r t h e r d i s c u s s i o n o f " s a n d in g " a t G i l ­

man I s In c lu d e d i n p a r t 2# I t I s e v i d e n t , th e n , t h a t d o lo m i tiz a tio n o f a lim e ­ s to n e r e s u l t s i n in c r e a s e d p o r o s i t y , r e c r y s t a l l i z a t i o n r e ­

4

18

s u i t s I n f u r t h e r in c r e a s e d p o r o s i t y , and "s a n d in g " s t i l l f u r t h e r in c r e a s e s p o r o s ity *

S econdary p o r o s i t y th ro u g h

d e f o r r a a tio n a l b r e c c i a t i o n o r w e a th e rin g and alo n g c r a c k s , j o i n t s , and s t y l o l i t i c seams may a l s o b e p r e s e n t and im por­ ta n t.

The f i n a l r e s u l t o f th e in c r e a s e d p o r o s i t y i s t h a t ,

i f a l l o t h e r th in g s a r e e q u a l ( s t r u c t u r a l c o n t r o l , a v a i l a ­ b i l i t y , e t c . ) , th e o re s o l u t i o n s w i l l s e l e o t th e more p o r­ ous o f th e f o r m a tio n s .

T h is , i n m ost o a s e s , w i l l f u r t h e r

th e f a v o r a b i l i t y o f d o lo m ite s due to th e u s u a l g r e a t e r p o r o s i t y i n th e d o lo m ite .

T h is a g a in i s n o t a r u l e , b u t

r a t h e r a g e n e r a l i z a t i o n i n w hich a l a r g e p e r c e n ta g e o f th e exam ples c i t e d i n th e l i t e r a t u r e f i t . S t r u c t u r a l C o n tro l The p re s e n c e o f a f a v o r a b le b ed o r f o r m a tio n , o f c o u r s e , i s no g u a r a n te e t h a t o re w i l l be d e p o s ite d ev en i f th e m in e r a liz in g s o l u t i o n s f i n d t h e i r way to th e b e d .

A

b r e c c l a t e d se n s m ig h t o n ly s e r v e as an e x i t t o th e s u r f a c e f o r th e h y d ro th e rm a l s o l u t i o n s i f th e p r o p e r s t r u c t u r a l c o n t r o l s w ere n o t p r e s e n t .

T h is c o n t r o l m ust be p r e s e n t

to t r a p o r impound th e s o l u t i o n s lo n g enough t o b r i n g ab o u t d e p o s itio n by a change i n p h y s ic a l a n d /o r c h e m ic al c o n d i­ tio n s .

F o r ex am p le, a t L e a d v ille (14) th e o re was d e p o s ite d

i n th e L e a d v ille d o lo m ite when th e m in e r a liz in g s o l u t i o n s w ere im pounded by a p o rp h y ry c a p .

Where th e s e p o rp h y ry

s i l l s in tr u d e d th e r e l a t i v e l y u n f a v o r a b le Y u le (M anitou) f o r m a tio n , o re d e p o s i t s w ere o f t e n fo rm e d .

At O ilm an,

C olo rad o ( 3 2 ) , a c a p p in g p o rp h y ry i s a l s o p r e s e n t .

S im i­

19

l a r l y , a t Alma t h e r e i s a r e l a t i v e l y im p e rv io u s cap o v e r some o f th e d e p o s i t s .

The te rm " im p e rv io u s n when u sed i n

c o n n e c tio n w ith a cap im p lie s r e l a t i v e l y co m p lete irap e rv io u s n e s s o n ly t o th e m e t a l l i c io n s a s th e s p e n t s o l u t i o n s m ust b e a llo w ed to e scap e p e r m it tin g th e e n tr a n c e o f a f r e s h s u p p ly .

The s p e n t s o l u t i o n s may, o f c o u rs e , e s c a p e u p d ip

u n d e r t h e cap and do n o t n e c e s s a r i l y go th ro u g h th e c a p . At Aspen ( 3 9 ) , w here o re i s fo u n d i n b o th lim e s to n e and d o lo m ite , b o th a r e b a r r e n e x c e p t a t th e i n t e r s e c t i o n o f e i t h e r l a r g e , lo w -a n g le f a u l t s o r s t r u c t u r e s form ed by un­ c o n f o r m itie s and a s te e p c r o s s f a u l t .

A f a u l t , o r some

o th e r ty p e o f f e e d e r , i s a n e c e s s i t y i n a l l c a s e s .

I t may

b e a l a r g e m appable f a u l t o r a s e r i e s o f s m a ll o b sc u re c ra c k s o r j o i n t s .

A f a v o r a b le h o r iz o n , th e n , i s o n ly one „

o f a number o f f a c t o r s w h ich m ust fo rm a c o r r e c t com bina­ t i o n t o prom ote th e f o r m a tio n o f an o re d e p o s i t . T hese s t r u c t u r a l c o n t r o l s have b e e n e x c e p t i o n a l l y w e ll d e s c r ib e d and d is c u s s e d i n a number o f p a p e r s .

Some o f

th e s e c o n t r o l s su c h as c a p p in g s i l l s , f a u l t i n t e r s e c t i o n , and j o i n t p a t t e r n s a r e o f t e n o b v io u s , e a s i l y re c o g n iz e d f e a t u r e s w h ich may b e m apped, p r o j e c t e d , and i n t e r p r e t e d . F a v o r a b i l i t y , h o w ev er, i s much more o f an a b s t r a c t t h i n g . I n th e fo llo w in g summary o f th e g e n e r a l i z a t i o n s made In p a r t 1 o f t h i s p a p e r , i t i s hoped t h a t some o f th e s o - c a l l e d f a v o r a b le f e a t u r e s o f d o lo m ite w i l l become a l i t t l e

le s s

a b s t r a c t and c l o s e r to b e in g u s a b le t o o l s i n th e s e a r c h f o r m e ta lliz a tio n .

20

Summary 1.

T here a r e num erous e a s e s w here a d o lo m ite b ed h a s

p ro v e d more f a v o r a b le as a h o s t ro c k th a n th e a s s o c ia te d r o c k ty p e s . 2.

D i f f e r e n t i a l p r e p a r a t i o n i n many c a s e s , h as

f a v o r e d d o lo m ite o v e r th e o t h e r f o r a a t l o n s due t o th e t e n ­ dency o f d o lo m ite t o s h a t t e r and b r e c c i a t e u n d e r a deform ­ in g s t r e s s , w h ile o t h e r ro c k ty p e s flo w and d efo rm p l a s t i ­ c a lly .

Competency i s a r e l a t i v e th in g and f a v o r a b i l i t y i s

d i r e o t l y p r o p o r t i o n a l t o i t I n a r e a s w here p r e p a r a t i o n was by d e f o r m a tio n . 3.

P o r o s ity u s u a lly in c r e a s e s d u rin g d o lo m l tiz a tio n

o f a lim e s to n e i n a number o f w ay s, and t h i s i s e s p e c i a l l y t r u e i f d o lo m l tiz a tio n i s due t o h y d ro th e rm a l a c t i o n . 4.

F u r t h e r in c r e a s e s i n p o r o s i t y i n d o lo m ite may r e ­

s u l t from r e c r y s t a l l i z a t i o n ( a ls o h y d ro th e rm a l i n many c a s e s ) and from " s a n d in g * w G r e a te r p o r o s i t i e s i n d o lo m ite s th a n i n th e u n d o lo m itiz e d , u n r e c r y s t a l l i z e d , u n -"san d e d * ro c k s f u r t h e r s th e f a v o r a b i l i t y o f th e d o lo m ite . 5.

I n many c a s e s , th e " f a v o r a b i l i t y " o f th e d o lo m ite

i s p a r t i a l l y o r w h o lly due t o th e c o in c id in g c o n t r o l o f d o l o m l t i z a t i o n (h y d ro th e rm a l) and o f o re d e p o s i t i o n .

In

o t h e r w o rd s, th e o re s o l u t i o n s d id n o t a c t u a l l y f a v o r th e d o lo m ite , b u t r a t h e r th e d o lo m l tiz a tio n i s m e re ly a w id e­ s p re a d p r e - m e t a l l i z a t i o n a l t e r a t i o n f e a t u r e form ed u n d e r th e same c o n t r o l s as th e su b s e q u e n t o re d e p o s i t s .

21

PART I I THE LEADVILLE FORMATION

The L e a d v ille fo rm a tio n i s p r e s e n t i n a num ber o f cen ­ t r a l C o lo ra d o m in in g d i s t r i c t s .

At L e a d v i l l e , G ilm an, Alma,

/

and A spen, th e L e a d v ille i s a f a v o r a b le h o r iz o n ,

A stu d y

was made o f some o f th e ch e m ic a l and p h y s ic a l p r o p e r t i e s o f th e L e a d v ille f o r m a tio n from sam ples o b ta in e d p r i n c i p a l l y a t L e a d v ille and G ilm an.

A p a r a l l e l s tu d y was made on th e

D yer f o r m a tio n f o r co m p ariso n , and th e p u rp o se o f p a r t 2 o f t h i s p a p e r i s to r e l a t e th e c h a r a c t e r i s t i c s o f th e s e i n d i ­ v id u a l d e p o s i t s and f o r m a tio n s t o th e g e n e r a l i z a t i o n s r e ­ g a r d in g f a v o r a b i l i t y made i n p a r t 1* The L e a d v ille and D yer fo rm a tio n s a t G ilm an and Lead­ v i l l e and th e L e a d v ille fo rm a tio n a t Alma w ere s tu d ie d from th e s ta n d p o in t o f p o r p s i t y and c h e m ic a l c o m p o s itio n I n r e ­ l a t i o n t o th e p r o x im ity o f th e sam ples t o o r e , th e p o s i t i o n i n s e c t i o n , and th e d e p th from th e s u r f a c e .

No sam ples

w ere ta k e n from A spen; h e n c e , no c o n c lu s io n s c a n be a p p lie d t o t h a t d i s t r i c t , b u t t o make th e b ack g ro u n d m ore c o m p le te , a s h o r t resum e o f th e d e s c r i p t i o n o f th e L e a d v ille and D yer w i l l b e ta k e n fro m th e w orks o f S p u rr ( 4 2 ) , V a n d e rw ilt ( 4 6 ) , and R o h lfin g (39)#

T his w i l l be fo llo w e d by a d e s c r i p t i o n

o f th e w ork done by th e w r i t e r a t Alma, L e a d v i l l e , and f i n a l l y th e s t u d i e s a t G ilm an.

MAP

INDEX

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9.

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a g o SCHOOL OF

umm

Iowa G eo l.