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The relationship of dolomite and ore, with special reference to the Jackpot Property, Ymir, B.C. Heddle, Duncan Walker 1951

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THE RELATIONSHIP OF DOLOMITE AND ORE WITH SPECIAL REFERENCE TO THE JACKPOT PROPERTY, YMLR, B.C. by DUNCAN WALKER HEDDLE A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of Geology and Geography We accept t h i s thesis as conforming to the standard required from candidates f o r the degree of MASTER OF^APPLIED SCIENCE Members of the Department of Geology and Geography THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1951 A p r i l 15, 1951. Dr. H.C. Gunning, Head, Department of Geology and Geography, Un i v e r s i t y of B r i t i s h Columbia, Vancouver, B.C. Dear S i r , I t gives me pleasure to submit the following t h e s i s , "The Relationship of Dolomite and Ore with Special Reference to the Jackpot Property, Ymir, B.C.", i n p a r t i a l f u l f i l l m e n t of the require-ments of the course leading to the degree of Master of Applied Science i n Geology a t the U n i v e r s i t y of B r i t i s h Columbia. Yours t r u l y , D.W. Heddle. TABLE OF CONTENTS Page Introduction 1 Acknowledgements 3 Discussion and d e f i n i t i o n of some terms involved h A general comparison of the properties of dolomite and limestone 7 The o r i g i n of dolomites i n general 7 The a s s o c i a t i o n of dolomitized limestone and lead-zinc ore 9 General theories on the process of dolomitization which may-be applied to lead-zinc deposits i n limestone 13 (1) The d i f f e r e n t i a l leaching theory 13 (2) The a l t e r a t i o n theory... 16 Possible reasons f o r the as s o c i a t i o n of dolomite and ore ...21 (1) The association of dolomite and ore i s e n t i r e l y fortuitous. . 2 2 (2) Dolomite i s a pr e f e r r e d host rock ..23 (3) Dolomitizing and ore-bearing solutions have an i d e n t i c a l source 29 (U) Dolomitizing and ore-bearing solutions have been l o c a l i z e d by the same s t r u c t u r a l controls but are not g e n e t i c a l l y r e l a t e d 30 The d e r i v a t i o n of a magnesia-rich s o l u t i o n as proposed by Faust 361 Possible evidence supporting Faust's proposals 39 The Jackpot Property Introduction General Geology The orebodies Method of study Descriptions of specimens studied Discussion Conclusions Suggestions f o r future study Appendix Staining technique I l l u s t r a t i o n s Bibliography ILLUSTRATIONS Page P l a t e I . A . D o l o m i t e i n t h i n s e c t i o n 77 B. T h i n s e c t i o n s h o w i n g c a l c i t e t o r e p l a c e d o l o m i t e 77 I I . A . T h i n s e c t i o n s h o w i n g c a l c i t e t o r e p l a c e d o l o m i t e 78 B . The a s s o c i a t i o n o f c a l c i t e a n d d o l o m i t e i n " c a l c i f i e d " d o l o m i t e . • 78 I I I . A . S t a i n e d p o l i s h e d s u r f a c e o f J a c k p o t s u r f a c e s p e c i m e n • 79 B . A n e n l a r g e d s e c t i o n o f P l a t e I I I . A . s h o w i n g t h e c a l c i t e - d o l o m i t e r e l a t i o n s h i p . • • 79 I V . A v e i n o f d o l o m i t e c u t t i n g o l d e r d o l o m i t e , c a l c i t e and s p h a l e r i t e 80 V . A . S t a i n e d p o l i s h e d s u r f a c e o f a m i n e r a l i z e d s e c t i o n - s p e c i m e n Jl5-156' 81 B . A n e n l a r g e d s e c t i o n o f P l a t e V . A . s h o w i n g t h e r e l a t i o n s h i p o f c a l c i t e , d o l o m i t e a n d s u l p h i d e s • • • • • • 8 1 V I . A . L e n s e s o f s e r p e n t i n e a r e shown t o be e n v e l o p e d b y c a l c i t e w i t h i n d o l o m i t e - s p e c i m e n J15-327' 82 B . A n e n l a r g e d s e c t i o n o f P l a t e V I . A . s h o w i n g a s e r p e n t i n e l e n s e n v e l o p e d b y c a l c i t e 82 V I I . A . The a s s o c i a t i o n o f s u l p h i d e s a n d d o l o m i t e i n c a l c i t e - s p e c i m e n J15-171' • '^3 B . The a s s o c i a t i o n o f s p h a l e r i t e w i t h s m a l l masses o f s e r p e n t i n e - s p e c i m e n J33-55U1 3^ F i g u r e 1. E q u i l i b r i u m a s s e m b l a g e s o f m i n e r a l p h a s e s i n d e r i v -a t i v e s o f s i l i c a - b e a r i n g d o l o m i t i c l i m e s t o n e s ; G.Q:2''.oa i n e x c e s s . . A f t e r T u r n e r (19^9, p . 73) 59 ABSTRACT The close a s s o c i a t i o n of lead-zinc deposits with dolom-i t e and dolomitic limestone and the common occurrence of these de-pos i t s w i t h i n dolomitic envelopes i n limestone present an i n t e r e s t -ing and challenging problem to economic g e o l o g i s t s . A review has been made of the manners i n which dolomite and lead-zinc ore are found to be associated. Possible reasons f o r the ass o c i a t i o n of dolomite and ore and processes by which lime-stones i n the v i c i n i t y of lead-zinc deposits may be a l t e r e d to dolo-mite have been considered. Dolomites i n general are believed to be very favorable host rocks f o r lead-zinc ore but whereas t h i s f a v o r a b i l i t y may f a c i l i t a t e the l o c a l i z a t i o n of ore i n primary dolo-mites, i t can hardly be regarded as the primary l o c a l i z i n g f a c t o r i n those deposits which occur within dolomitized zones i n limestone. The a l t e r a t i o n of limestone i n the v i c i n i t y of lead-zinc deposits may be best a t t r i b u t e d to magnesium-rich hydrothermal s o l -utions, the magnesium of which i s not genetically r e l a t e d to the par-ent magma. The mechanism proposed by Faust, whereby magnsium-rich solutions are derived from dolomites which have been thermally d i s -sociated i n the v i c i n i t y of an intruding magma, adequately f u l f i l s the requirements of a dolomitizing agent. I f we can assume that about ore-bearing solutions have a r i s e n from the same magma that brought A the d i s s o c i a t i o n of a pre-e x i s t i n g dolomite, then dolomite and ore may n e c e s s a r i l y be c l o s e l y associated by reason of dolomitizing and ore-bearing solutions having been l o c a l i z e d by the same s t r u c -t u r a l c o n t r o l s . A study of specimens from the Jackpot property, Ymir, B.C., a deposit occurring i n dolomitized limestone, has revealed l i t t l e information with respect to the process of dolomitization other than i n d i c a t i n g that dolomitization preceded sulphide mineral-i z a t i o n . Most of the sulphide m i n e r a l i z a t i o n at the Jackpot pro-perty occurs w i t h i n c a l c i t i c zones i n dolomite. I t i s believed that dolomite has been replaced by the sulphide minerals and has been l a t e r p a r t i a l l y replaced by c a l c i t e i n the zones of sulphide min-e r a l i z a t i o n . I f F aust 1s proposal that magnesium-rich solutions may be derived from a thermally d i s s o c i a t e d dolomite i n the v i c i n i t y of an intruding magma i s v a l i d , one may conclude that at a l a t e stage i n the hydrothermal a c t i v i t y , when magnesium has been l a r g e l y r e -moved from the thermally d i s s o c i a t e d dolomite, the solutions may become r e l a t i v e l y r i c h i n the l e s s soluble calcium carbonate. These l a t e r calcium carbonate-rich solutions may be responsible f o r the replacement of dolomite by c a l c i t e i n the mineralized zones. THE RELATIONSHIP OF DOLOMITE AND ORE WITH SPECIAL REFERENCE TO THE JACKPOT PROPERTY, YMIR, B.C. Introduction The problem of the o r i g i n of dolomites or dolomitic lime-stones i s one which has engaged the i n t e r e s t of geologists f o r a great many years. The many theories proposed have r e s u l t e d i n much controversey. Some theories appear very convincing f o r c e r t a i n occur-rences but no one theory seems applicable to a l l occurrences. The broad problem of the o r i g i n of dolomites and dolomitic limestones of great a r e a l extent, so often r e f e r r e d to as "primary" dolomites, however, w i l l not be discussed i n any d e t a i l by the w r i t e r . The close a s s o c i a t i o n of lead-zinc deposits with dolomites and dolomitic limestones and the common occurrence of these deposits w i t h i n dolomitic envelopes i n limestones has been noted by geologists f o r a hundred years or more. This a s s o c i a t i o n has become of extreme i n t e r e s t to economic geologists w i t h i n the present century. The great importance of lead and zinc i n world economy during recent years has accentuated the study of dolomitization as a feature which may lead 1. to the discovery of new ore bodies. The o r i g i n of the dolomitic envelopes so commonly developed around lead-zinc replacement deposits i n limestone i s believed to be a s p e c i a l i z e d case of d o l o m i t i z a t i o n . While there may be a degree of p a r a l l e l i s m i n the o r i g i n of the dolomitic envelopes associated with lead-zinc deposits and some of the widespread "primary" dolomites and dolomitic limestones, the o v e r a l l processes of t h e i r development are believed to be d i s s i m i l a r . Owing to the widespread occurrence of "primary" dolomites, some lead-zinc deposits might n e c e s s a r i l y occur i n these. The occur-rence of lead-zinc deposits i n dolomites or dolomitic limestones, however, seems too common to be f o r t u i t o u s and suggests the p o s s i b i l -i t y of them being exceptionally favorable host rocks. The common occurrence of dolomitic envelopes surrounding lead-zinc deposits i n limestone suggests the p o s s i b i l i t y of there being some r e l a t i o n s h i p between the processes.of dolomitization and ore deposition. The w r i t e r has made a study of the possible r e l a t i o n s h i p s between these two.processes. The more general material f o r t h i s t hesis was obtained l a r g e l y from a review of the l i t e r a t u r e . Studies were made of s p e c i -mens from the Jackpot Property, Ymir, B.C. which i s believed to be a f a i r l y t y p i c a l example of a lead-zinc deposit occurring i n dolom-i t i z e d limestone. The specimens examined are believed to be repre-sentative of the r e l a t i o n s h i p between dolomitized limestone and ore. The random manner i n which the samples were taken and the inadequacy of the number of samples w i l l probably allow no s t a t i s t i c a l conclu-sions to be a r r i v e d at from t h i s study. I t i s hoped, however, that some of the r e l a t i o n s h i p s observed between the m i n e r a l i z a t i o n and the host rock w i l l permit the formulation of conclusions of a general nature. Acknowledgements The w r i t e r g r a t e f u l l y acknowledges the h e l p f u l suggestions and cooperation of members of the Department of Geology and Geography. Spe c i a l acknowledgement i s given to Dr. H;C. Gunning and Dr. W.H. White who guided the w r i t e r and f a c i l i t a t e d the a c q u i s i t i o n of rock specimens and data. The w r i t e r also wishes to acknowledge the kindness of the management of New Jersey Zinc Explorations Limited i n making data a v a i l a b l e and permitting samples to be taken. In add i t i o n the w r i t e r wishes to thank Mr. J.A. Donnan f o r assistance and advice given i n the preparation of polished specimens and t h i n sections. Discussion and D e f i n i t i o n of some Terms Involved The term dolomite i s commonly used by geologists as being equivalent to magnesian limestone. Properly the term should be r e -s t r i c t e d to the d e f i n i t e double carbonate (CaCO^. MgCO^) which occurs both as a w e l l . c r y s t a l l i z e d mineral and as a massive rock co n s i s t i n g of 5k,35 percent CaCO^ and U5.65 percent MgCO^. , Most modern geologists d i f f e r e n t i a t e between dolomites com-posed c h i e f l y of the mineral dolomite and dolomitic limestone i n which CaCO^ i s present i n excess of MgCO^. Limestones containing only small amounts of magnesium are commonly r e f e r r e d to as magnesian limestones. P e t t i j o h n (19U8, pp. 312-313) defines dolomites as follows: "Dolomites are those v a r i e t i e s of limestone containing more than f i f t y percent carbonate of which more than h a l f i s ; dolomitic". He goes f u r t h e r to break the rock types i n t o three groups5 dolomitic limestone, c a l c i t i c dolomite, and dolomite on the basis of the proportion of c a l c i t e to dolomite i n the rock. Because of a loose usage by writers of the various terms involved, much of the value of the l i t e r a t u r e on the topic i s l o s t . The carbonates of calcium and magnesium do not form a com-p l e t e isomorphous s e r i e s . According to Steidtmann (1917, p. 1*33) the magnesia content of c a l c i t e r a r e l y exceeds two percent. The CaO content of dolomite i s nearly constant but MgO i s commonly replaced by small amounts of FeO or of FeO and MnO. The maximum ferrous i r o n content i s probably about 10 percent. Andrews (1950, p. 91) reports that MnCO^ seems to form a complete seri e s of s o l i d solutions with c a l c i t e . He a t t r i b u t e s t h i s tor. the f a c t that the i o n i c diameter of manganese i s nearer to that of calcium than magnesium or i r o n . Both the elements magnesium and i r o n form a double carbonate i n which the ions of calcium on the one hand and magnesium or i r o n on the other occupy, i n an ordered arrange-ment, the l a t t i c e s i t e s which i n c a l c i t e s o l i d solutions would be occupied i n a random manner. Andrews suggests that complete s o l i d solutions are favored by i o n i c diameters which are not g r e a t l y d i f f e r e n t , w h i l s t the dolomite type of compound i s preferred i f the diff e r e n c e becomes marked. Ford (1917, p. 2 l l ) has suggested that manganese carbonate would be expected to dissolve i n calcium carbon-ate to the l a r g e s t extent, i r o n carbonate next and magnesium carbonate the l e a s t , and that s o l i d s olutions would probably be r e s t r i c t e d . No extensive s o l i d solutions of FeCO^ and MgCO^ i n c a l c i t e have been reported. From a study of dolomitization, Bar (192U, p. 116) 1 found that c a l c i t e could e x i s t next to dolomite i n stable equilibrium but that the as s o c i a t i o n of c a l c i t e with magnesite was unstable and would produce dolomite. Furthermore^ magnesite and dolomite could e x i s t C i t e d from Faust and Callaghan (19^8). as an assemblage of minerals with stable relationships, whereas mag-nesite with a small amount of calcite would be unstable and dolomite would form as a reaction product. In rare cases the magnesia content i n dolomite exceeds the proportion required i n normal dolomite and in these cases i t must be assumed that this excess i s due either to the existence of MgCO^  in isomorphous mixture i n the dolomite or to the presence of free crys-tals of magnesite. Ferruginous dolomite or ankerite i s not rare. A l l the iron of the carbonate rock, however, i s not necessarily a part of the carbonate. I t may be present as hydroxide or i n clay-like impurities, and these possibilities must be taken into account in any interpretation of the dolomites. Dolomitization i s defined by Rice (19h8, p. 110) as a gen-eral term for the processes whereby dolomite takes the place of ca l -cium carbonate i n limestones, the latter thus becoming dolomitic limestones or dolomites. Dedolomitization i s defined as a process whereby a dolomite, during metamorphism, loses i t s content of magnes-ium carbonate, the magnesium remaining as oxide or hydroxide ar s i l -icate. Rice does not believe i t i s desirable to extend the term to include the mechanical removal of dolomite. A General Comparison, of the Properties of Dolomite and Limestone The colors, textures, and other physical properties gener-a l l y ascribed to limestone also apply to dolomite. Dolomites have a tendency to weather to a buff or tan color, due i n many cases to their iron content. Many dolomites are quite porous but this property i s by no means universal. Dolomite i s somewhat harder than limestone. It i s also somewhat heavier than limestone, i t s specific gravity being 2.87 as compared to 2.71. If a carbonate rock i s pure dolomite, i t effervesces i n cold dilute hydrochloric acid either not at a l l or only feebly; but i f i t i s fi n e l y pulverized, i t effervesces vigorously even in cold acid. The use of stains (described i n appendix) to distinguish be-tween calcite nad dolomite i s based on the difference i n chemical properties between the two minerals. The weathered surfaces of dolomitic limestones are often quite diagnostic. The dolomite grains are l e f t standing i n r e l i e f against the more rapidly weathering calcite. The Origin of Dolomites i n General The origin of dolomites i s a matter of much interest and controversey. Evidence points to the fact that dolomites have been formed i n two general ways: (1) as syngenetic dolomite which i s a dolomite formed either by direct precipitation from sea water or by the alteration of limestone by sea water before the overlying stratum was l a i d down. (2) as secondary dolomite, by alteration of limestone to dolomite long after the limestone was formed. The most extensive dolomites are believed to have been pro-duced by direct precipitation or a process of penecontemporaneous dolomitization. Many geologists regard the common occurrence of i n -terbedded, sharply defined beds of dolomite and limestone as proof of direct precipitation. There are a great many other points which favor the formation of dolomite by direct precipitation but there are also a great many points which show this view to be untenable. Dolo-mite has been produced a r t i f i c i a l l y as a direct precipitate, but ex-periments have failed to indicate the conditions under which dolomite can be precipitated directly at ordinary temperatures and pressures. Most geologists favor a process of penecontemporaneous dolomitization under shallow seas for the origin of extensive dolomite beds. That dolomite or dolomitic limestone can be formed by the alteration of limestone long after the formation of the limestone there seems l i t t l e doubt. These "secondary" dolomites or dolomitic limestones generally occur i n smaller masses than the "primary" dolomites but i n some di s t r i c t s thick uniform masses appear to have been formed by the wholesale transformation of limestone. The dolomitic envelopes so commonly associated with lead-zinc deposits in limestone are generally considered to be of secondary or epigen-etic origin. The Association of Dolomitized Limestone and Lead-Zinc Ore Of a l l lead-zinc deposits, those occurring as replacements i n limestone probably form the most important group. A great many of these deposits, indeed many of the more important ones, show the limestones i n the v i c i n i t y of the orebodies to be dolomitized. I t seems l i k e l y that many of the large deposits reported to occur i n unaltered limestone do exhibit some degree of alteration but this alteration has been overlooked or considered to be of l i t t l e s i g -nificance by writers. The lead-zinc deposits occurring i n dolomitized limestone could be subdivided into many types. In general however, most of these deposits might be broadly subdivided on the basis of temperature of formation. The deposits form a gradational series from the well known low temperature telethermal or Mississippi type to the less common higher temperature contact metamorphic types. The Mississipjf type of deposit i s characterized by extremely simple mineralization. Galena and sphalerite are the most important ore minerals but some deposits contain silve r and others chalcopyrite. Characteristic gangue minerals are pyrite, marcasite, calcite, jas-peroid and quartz. Barite and fluorite occur as gangue minerals i n some deposits of this type. Visible intrusive rocks other than small post-ore intrusives are almost to t a l l y absent. The contact metamorphic type of deposit i s well exemplified by the lead-zinc deposits at Fahlun i n central Sweden. Where the sulphides occur here i n limestone, this rock i s altered more or less completely to dolomite. The dolomite i s more or less altered to ophicalcite and to skarn consisting of such minerals as tremolite, actinolite, diopside, anthophyllite, cummingtonite, humite minerals, forsterite, serpentine and t a l c . Usually sphalerite and galena appear i n the skarn and dolomite. From an economic point of view, the lower temperature type of deposit forms the more important group. Between the two extreme types mentioned there are types which show development of some s i l i -cate minerals which are predominantly magnesian. Higher temperature minerals such as pyrrhotite also become conspicuous. In these lead-zinc deposits from the one extreme to the other, i t i s interesting to note the prominance of minerals of high magnesium content. Dol-omite i s generally the only magnesium mineral present i n the Miss-issippi type of deposit while magnesium silicates i n addition to dol-omite are commonly present i n the contact metamorphic type. In either type of deposit, the dolomitization of limestone appears to be a sig -nificant type of alteration. The form of the dolomitized zone associated with lead-zinc deposits varies a great deal. In some cases the deposits appear to be uniformly enveloped by the zone of alteration. In many of these cases the boundaries of the deposits coincide very well with the boundaries of the dolomitic aureoles. A good example of this phen-omenon i s shown at the Federal Gordon Mine, Picher Field i n the Miss-issippi Valley (35, p» 57)• In some deposits, however, large areas of dolomite occur without the presence of ore and ore sometimes l i e s against unaltered limestone. The width of the dolomitic zone about ore deposits i s usually variable, varying from a very narrow zone to hundreds of feet. In most cases the dolomitic alteration has progress-ed both l a t e r a l l y and v e r t i c a l l y throughout the limestone beds. The boundaries of dolomitized zones are i n many cases poorly defined and i n other cases may be very abrupt. Of the East Tennessee Field, Brokaw (35, p. 75) says: The change from limestone to dolomite may be abrupt, par-t i c u l a r l y at a zone of fracturing, but just as commonly the change i s gradational, from a completely dolomitized block to an adjacent block containing just a few dolomite scatter-ed here and there i n the limestone. The dolomitization i n many cases can be associated with fractures i n the limestone. Of the Gold H i l l Mining D i s t r i c t , Utah, Nolan (1935, p. 9k) says: The marked distinction i n color between the unaltered lime-stone and the dolomite makes i t possible to prove dependence of the alteration upon lines of fracturing i n the beds. I t seems probable that the dolomitization of limestone beds i s related to zones of weakness i n most cases but these zones have been obscured by post dolomitization deformation or ore deposition. In most instances the process of dolomitization is consider-ed to have preceded ore deposition. In some cases the process is considered to have occurred during ore deposition. Of the Raibl Mine, Italy, Radcliffe (1936, p. 73) says: The dolomite of the country rock was formed at the same time as the metallic sulphides, as is evidenced by the fact that the distance of outward dolomitization varies with the strength of the orebody. The validity of such a deduction, however, seems very debatable. Commonly several ages of dolomite are apparent, the later dolomite occurring in the form of a rather coarse grained pure gangue mineral. This gangue dolomite commonly fills fractures or breccia openings in the previously dolomitized limestone and is often associat-ed with sulphide minerals. Some geologists believe this gangue dol-omite results from the introduction of late magnesian solutions, others believe it results from the recrystallization of earlier dolomite along fractures and breccia openings. In some instances sphalerite is more closely associated with dolomite than is galena. In the Picher Field, Mississippi Valley, galena tends to avoid the massive dolomite areas and to be associated characteristically with jasperoid and calcite gangue while the richest sphalerite deposits occur as a replacement of massive dolomite. Vertical and horizontal zoning are recognizable in the Tri-State District, U.S.A. Dolomite and silica increase with depth while galena predominates at the higher horizons. The apparent association of sphalerite and dolomite here may be a function of temperature zoning rather than preferential replacement. General Theories on the Process of Dolomitization which may be  applied to Lead-Zinc Deposits i n Limestone Several theories have been proposed to account for the dolomitization of limestone i n general, but a discussion w i l l be made only of those which might have been effective i n connection with the dolomitization associated with lead-zinc deposits i n limestone. The tiro broad theories which might be applicable ares (1) The differential leaching theory (2) The alteration theory The Differential Leaching Theory When a dolomitic limestone i s subjected to solvent action, calcite i s taken into solution much more rapidly than dolomite, thus giving rise to concentration of the latter con-stituent. Many geologists have been of the opinion that by this process, a dolomite might i n time result. The power of carbonated waters to remove calcite more rapidly than dolomite from dolomitic limestones under certain conditions, has been demonstrated. Van Tuyl (l?l6) has advanced the following points in support of the differential leaching theory: (1) The development of dolomite along lines of weakness such as fractures in limestone (2) The apparent tendency of dolomite to be developed in limestone at those points where CO2 and humus acids are abundant (3) The porous character of some dolomites (k) The existence of stalactitic and stalagmitic deposits of nearly pure CaCO^  in caverns in dolomitic limestone (5>) The increase of magnesium content of weakly dolomitic limestone as an accompaniment of weathering Some of these points however, support equally well the altera-tion theory. Murray (1930, p. U62) in discussing dolomites as o i l reservoirs, states that these rocks generally owe their porosities to differential leaching. He says: Experiments on calcite and dolomite have shown that cal-cite is many times more soluble in weak acids than dol-omite, and analyses of ground water and of streams which have their source in areas underlain by dolomitic lime-stone show that magnesium is being retained in the rocks whereas calcium is being removed. The residues of the rocks are therefore much higher in magnesium and lower in calcium than the original rocks and more porous, many having porosities greater than could possibly have been rendered by dolomitization. Since organic acids and car-bonic acid operate most efficiently on the land surface where circulation has been established, i t is concluded that the porosity of a dolomitic limestone is largely produced by leaching action while they are at or near the surface of the land mass. While there appears to be much field evidence bearing on the fact that calcium carbonate is more soluble than dolomite under near surface conditions, evidence in the literature points out that under certain conditions dolomitic limestones might provide magnesia-rich solutions. This somewhat controversial issue w i l l be discussed further i n a l a t e r section of t h i s report. The process of enrichment i n dolomite by d i f f e r e n t i a l solution can be r e a d i l y attacked on account of the very extensive solution and removal of CaCO^ that i s required before the percent-age of MgCO^ i n the residual rock i s appreciably raised. Van Tuyl (1916) points out that i f only CaCO^ i s assumed to be removed, i n an o r i g i n a l limestone containing one percent MgCO^, 93 percent of the o r i g i n a l rock would have to be removed by solution before the MgCO^ content i n the remainder would reach 16 percent. Such an extensive removal of CaCO^ would p r a c t i c a l l y o b l i t e r a t e a l l primary rock structures. The dolomitized zones associated with lead-zinc deposits commonly show l i t t l e or no o b l i t e r a t i o n of primary structure. Sardeson (19U0, p. 192) has suggested that the leaching of calcium carbonate i n the Galena Limestone of the Upper Miss-is s i p p i Valley may have been responsible f o r the concentrations of dolomite, galena and sphalerite. He of course takes the view, which i s now considered rather r a d i c a l , that galena and sphalerite were syngenetic constituents of a magnesian limestone. The solu-t i o n of calcareous beds has been noted frequently i n connection with lead-zinc deposits i n limestone, but i n no case (excepting the suggestion of Sardeson) has the writer found reference to dolomitization being directly related to the leaching of calcium carbonate from beds i n which dolomitization occurs. The effect of solution action on calcareous beds has been noted particularly i n the lead-zinc deposits of the Upper Mississipi Valley. The effect has been noted by Behre and Heyl (35, p. 65) at the Graham-Ginte Mine. They say: A feature particularly well developed i n this mine i s the thinning by solution action of the calcareous beds, appar-ently during the period of ore deposition. This solution action resulted i n p a r t i a l or complete removal of the lime i n the more calcareous layers, leaving only a shaly residue, in rare cases representing as l i t t l e as a fourth of the original thickness of the bed. Such effects are concen-trated i n areas under the greatest tectonic pressure! thus, solution had the effect of accentuating the folding* No mention, however, i s made here of an enrichment i n dolomite. Although leaching may have been responsible i n part for the dolomitization associated with lead-zinc deposits i n limestone i n isolated cases, there seems l i t t l e evidence i n favor of i t being of general occurrence. The Alteration Theory With respect to the dolomite associated with lead-zinc deposits i n limestone, the most generally accepted belief today i s that i t was formed by the alteration of limestone. The con-version of limestone to dolomite by alteration requires the add-i t i o n of large quantities of magnesia and carbon-dioxide and the removal of large quantities of lime. This conversion i n most i n s t a n c e s i s a p p a r e n t l y b r o u g h t a b o u t b y s o l u t i o n s c a r r y i n g mag-n e s i u m i n some f o r m w h i c h p a r t i a l l y r e p l a c e s t h e c a l c i u m i n t h e l i m e s t o n e . I f as i s commonly c o n c e d e d , one o u t o f e v e r y two e q u i -v a l e n t s o f GaCO^ i s r e p l a c e d b y MgCO^, a l o s s i n v o l u m e o f 12.3 p e r c e n t w o u l d r e s u l t , p r o v i d i n g a l l o f t h e e x c e s s CaCO^ i s r e m o v e d . A l t h o u g h d o l o m i t e s a r e commonly more p o r o u s t h a n l i m e s t o n e s , p o r -o s i t i e s a s h i g h as 12.3 p e r c e n t a r e r a r e . Many d o l o m i t e s a p p a r -e n t l y f o r m e d b y t h e a l t e r a t i o n o f l i m e s t o n e s a r e i n d e e d v e r y d e n s e . M u r r a y (1930) b e l i e v e s t h a t d o l o m i t i z a t i o n i s n o t a m o l e c u l a r r e p l a c e m e n t b u t a v o l u m e r e p l a c e m e n t . He i s o f t h e o p i n i o n t h a t i f t h e r e p l a c e m e n t r e s u l t e d i n a l o s s o f v o l u m e , f r a c t u r i n g a n d p e r h a p s a d e c r e a s e i n t h i c k n e s s w o u l d b e d e v e l o p e d , b u t n o t p o r o s i t y . He p o i n t s o u t t h a t i g n e o u s r o c k s , o n l o s s o f v o l u m e due t o c o o l i n g , do n o t d e v e l o p p o r o s i t y b u t j o i n t s a n d c r a c k s ; t h e same i s t r u e o f s e d i m e n t s o n d e h y d r a t i o n a n d o f mud o n d r y i n g . To M u r r a y , t h e p r e s e n c e o f l a r g e numbers o f p o r o u s beds b e t w e e n d e n s e beds o f d o l o m i t e a n d dense m o t t l i n g s o f d o l o m i t e i n l i m e s t o n e s i n d i c a t e t h a t d o l o m i t i z a t i o n i s n o t a m o l e c u l a r r e p l a c e m e n t b u t u s u a l l y a v o l u m e r e p l a c e m e n t . C a v i t i e s w h i c h c u t a c r o s s l a m i n a t e d d o l o m i t e w i t h o u t d e f o r m a t i o n show t h a t t h e c a v i t i e s w e r e n o t p r o d u c e d b y s h r i n k a g e . L i n d g r e n (1933, p . l i t ? ) b e l i e v e s t h a t c o n t r a c t i o n j o i n t s f o r m e d b y s h r i n k a g e when l i m e s t o n e i s c h a n g e d t o d o l o m i t e may p r o v i d e c h a n n e l -ways f o r m i n e r a l b e a r i n g s o l u t i o n s b u t makes n o m e n t i o n o f a n i n c r e a s e i n p o r o s i t y . O t h e r s , i n c l u d i n g P e t t i j o h n (191*8, p . 317) a n d Bateman (19U6, p . 172), b e l i e v e t h a t t h e p r o c e s s i n v o l v e s a v o l u m e f o r v o l u m e exchange so that no increase i n porosity w i l l r e s u l t . In connection with the a l t e r a t i o n theory, the mediums by which the dolomitization was carried out must be considered. Two apparent alternatives are: (1) Circulating groundwaters (2) Solutions related to magmatic a c t i v i t y The preceding two points w i l l be considered separately. (1) Circulating Groundwaters While groundwaters may be capable of accomplishing l o c a l dolomitization by the a l t e r a t i o n of limestone, there has been a tend-ency on the part of some writers to believe t h i s method of a l t e r a t i o n i s of f a r reaching sign i f i c a n c e . Most writers who have supported t h i s view have emphasized the importance of the MgCO-j of groundwater as the dolomitizing agent. The l o c a l dolomitization effects i n the Leadville Limestones of the Aspen d i s t r i c t , Colorado, are attributed by Spurr (I898, p. 206) to the action of the magnesia of groundwater. Analyses of groundwaters i n both igneous and sedimentary rocks and analyses of waters from spring deposits show calcium to be most commonly present greatly i n excess of magnesium. Analyses of surface waters given by Leroy and Crain (19U9, pp. 188-210) likewise show an excess "of magnesium over calcium to be very uncommon. An analysis given by Lindgren (1933, p. U3) of groundwaters i n the Jop-l i n d i s t r i c t of Missouri where dolomitization i s a common phenomenon, shows them to contain more than twice as much calcium as magnesium. I t i s rather d i f f i c u l t to understand how such waters can dolomitize limestone when they already contain CaCO^ g r e a t l y i n excess of MgCO-j. Although dolomites are more abundant i n older rocks, pure limestones are quite common i n the G r e n v i l l e rocks. This f a c t suggests that groundwaters do not play an important r o l e i n d o l o m i t i z a t i o n . F i e l d examples which show that dolomitic limestones have r e s u l t e d from the action of magnesium-bearing solutions on calcareous rocks seem to be r e l a t e d to pre-heated waters of some type or other. In the case of mineral springs i t i s conceivable that magnesia might be present i n some cases i n s u f f i c i e n t proportions to accomplish l o c a l d o l o m i t i z a t i o n . I f the dolomitic a l t e r a t i o n associated with mineral deposits were accomplished by groundwaters, f r a c t u r e s or other s t r u c t u r a l con-t r o l s which l o c a l i z e d the ore deposits must have e x i s t e d long before the egress of the mineralizing s o l u t i o n s . The p o s s i b i l i t y that ground-waters have been responsible f o r the dolomitic a l t e r a t i o n associated with ore deposits except i n l o c a l instances seems to be questionable. (2) Solutions Related to Magmatic A c t i v i t y The form of the dolomitic aureoles about many ore deposits suggests the p o s s i b i l i t y of there being some r e l a t i o n s h i p between dolomitization and ore deposition. The f a c t that the distance of out-ward dolomitization from the orebody i n some cases varies with the strength of the orebody i s also suggestive of a r e l a t i o n s h i p . The common association of dolomite, galena and sphalerite may suggest a genetic relationship between these three minerals but such need not be the case. There are a number of possible explana-tions for the relationship between dolomitization and magmatic activ-ity without proposing a genetic relationship. Hayward and Triplett (1931) from a study of mineralized dolomite in northern Mexico, concluded that dolomitization was the result of regional metamorphism which preceded ore deposition. The ore bearing solutions added no magnesium but merely moved and con-centrated i t . These field observations agree closely with those of Vanderwilt (1935) in Colorado and Park (1938) in the Metaline district of Washington. The emplacement of dolomite as a gangue mineral during per-iods of sulphide mineralization, as is evidenced in many of the de-posits of the Mississippi Valley district, indicates that hydrothermal solutions can be called upon to transport magnesium. The close assoc-iation of zones of dolomitic alteration with lead-zinc ores, both in space and in some cases time of formation, would favor hydrothermal solutions as the dolomitizing agent. Heated solutions, possibly con-taining permeating volatiles common to early hydrothermal solutions, might be best called upon to account for the broad zones of dolomitic alteration which envelope many lead-zinc deposits. The form of the dolomitic envelopes surrounding many of the lead-zinc deposits almost requires that the dolomitizing and ore-bearing solutions were l o c a l i z e d by the same s t r u c t u r a l c o n t r o l s . I f hydro-thermal solutions can be c a l l e d upon to account f o r the dolomitic a l t -e r a tion, there need be no f o r t u i t y i n the s p a c i a l association of zones of dolomitic a l t e r a t i o n and lead-zinc ore; the ore-bearing and dolo-m i t i z i n g solutions were l o c a l i z e d by the same s t r u c t u r a l c o n t r o l s . The l o c a l i z i n g s t r u c t u r a l controls may have been formed by deformation accompanying the emplacement of the magma from which the dolomitizing and ore-bearing solutions o r i g i n a t e d . The following conclusions may be drawn with respect to the process of dolomitization: (1) An enormous t r a n s f e r of material i s required (2) Dolomitization i s probably an a l t e r a t i o n process which proceeds on a volume f o r volume basis (3) The process of dolomitization, as associated with ore de-p o s i t s , i s probably i n some way r e l a t e d to magmatic a c t -i v i t y . r Possible Reasons f o r the Association of Dolomite and Ore Many reasons have been advanced to account f o r the assoc-i a t i o n of dolomite and ore; four points which w i l l be considered are: (1) The asso c i a t i o n of dolomite and ore might be e n t i r e l y f o r t -u itous. Palaeozoic dolomites are widespread and hence n e c e s s a r i l y frequently the host rock f o r ore occurring i n Palaeozoic areas. (2) The asso c i a t i o n of dolomite and ore might i n d i c a t e that dolomite i s a preferred host rock f o r p h y s i c a l or chemical reasons. (3) The as s o c i a t i o n of dolomite and ore i s due to the f a c t that the" dolomitizing and ore-bearing solutions have an i d e n t i c a l source. (U) The asso c i a t i o n of dolomite and ore might be due to the f a c t that the dolomitizing and ore-bearing solutions have been l o c a l i z e d by the same s t r u c t u r a l controls but need have no d i r e c t genetic r e l a t i o n s h i p s . In t h i s case the dolomitizing solutions may be c i r c u l a t i n g groundwaters or hydrothermal solutions r e l a t e d to igneous a c t i v i t y . The preceding four points w i l l be considered separately. (1). The Association of. Dolomite and Ore i s E n t i r e l y Fortuitous A very large proportion of lead-zinc deposits occur i n d o l -omites and dolomitic limestones of Palaeozoic age. These rocks are much more common i n Palaeozoic formations than i n more recent form-ations. Since Palaeozoic dolomites and dolomitic limestones are wide-spread, they might n e c e s s a r i l y frequently be the host rocks f o r lead-zinc deposits occurring i n Palaeozoic areas. Behre (19k0), from a great many observations, concludes that i n v i r t u a l l y a l l the lead-zinc mines i n Europe, French Worth A f r i c a and the United States, wherever the s t r a t i g r a p h i c sequence includes both limestone and dolomite, the ore seeks out the dolomitic layers preferentially or occurs i n parts of the beds which have been dolo-mitized. Hayward and Triplett (1931) conclude that several mining di s t r i c t s i n northern Mexico show the same feature. In the Good-springs d i s t r i c t of Nevada, Hewett (1928) found that of the ore bod-ies of seventy-five mines examined i n the d i s t r i c t , only two were found which were not enveloped i n dolomite and these were on the outer edge of the d i s t r i c t . The widespread association of dolomite and lead-zinc ore seems too general to be entirely fortuitous. The lead-zinc deposits associated with dolomite and dolomitic limestone might be subdivided into two groupsj those occurring i n primary dolomites, and those occurring i n dolomitized zones within limestone. While there may be a degree of fortuity i n the association of ore and primary dolomites in formations where these rocks are prevalent, the occurrence of ore within dolomitized aureoles i n limestones strongly suggests the poss-i b i l i t y of there being some non-fortuitous relationship between the processes of dolomitization and ore deposition. (2) Dolomite i s a Preferred Host Rock Dolomites or dolomitic limestones are believed by most economic geologists to be very favorable host rocks for ore. The chief characteristics of dolomites which have been considered to render them favorable to ore deposition are the following: (a) The chemical nature of the rock (b) The physical nature of the rock (i ) Porosity ( i i } Competency These characteristics w i l l be considered individually, (a) The Chemical Mature of the Rock Many geologists have attributed the favorability of dolo-mite as a host rock for ore to i t s chemical nature, but few i f any modern geologists attempt to name definitely the chemical character-i s t i c s that make i t more favorable to replacement than limestone, Behre (19k0, p, 22) makes the following statements Some mines i n the outlying parts of the Leadville d i s t r i c t , Colorado, show mineralization i n the higher parts only of the Leadville dolomitic limestonej analyses of carefully selected, representative samples extending from top to bottom of the formation show no chemical difference adequate to account for the localization of ore. The chemical nature of dolomitic limestones, however, may indirectly f a c i l i t a t e the acquisition of favorable physical character-i s t i c s . The chenical nature of some dolomitic limestones may be such that favorable porosities are developed by differential leaching, (b) The Physical Nature of the Rock (i) Porosity A great deal has been written with respect to the relative porosities of dolomite and limestone. Hewett (1928), from a study of dolomites and limestones from the Goodsprings area, Nevada, obtain-ed the following results: Range i n porosity i n 6 limestones 0.36 to l.k9 % Range i n porosity i n 6 dolomites 1.69 to 3.20 % He found that the dolomites contained drusy cavities filled with secondary calcite. Hewett felt that i f this calcite were removed, the porosity would range from about 3 to 6 percent, probably an average for most secondary dolomites. Of Mexico's lead-silver manto deposits, Fletcher (1929) stresses the importance of favorable beds. In the Mexican replace-ment deposits, a number of the best orebodies are found in dolomitic limestones, but i t is Fletcher's belief that magnesian content is in no way an essential feature of a favorable horizon, and that the pro-perty of these beds that lent itself to magnesian replacement was the same one which later made the* amenable to replacement by the mineral-izer. It is the texture and not the magnesian element of their com-position which was responsible for the mineralizers' invasion. Bain (1936) made a quantitative evaluation of porosity with respect to replaceability in limestones. He concluded that crystal-line limestones are replaced more often than very porous limestones and that cavities in cavernous beds and openings in breccias are usually filled, replacement being negligible. By experimental means, Bain found that rocks with openings 32xlO~7 Cm. wide were most sus-ceptible to metasomatlc replacement. These rocks also had the great-est amount of linear contact between grains per square cm. and the greatest area of contact between grains per cubic cm. Of the South-East Missouri District, U.S.A., Behre (35, p.59) believes the coarser grained dolomite to be more favorable to re-placement, but the finer grained types also contain disseminated ore and only the beds of dense dolomite are unfavorable, (ii) Competency From a review of the literature, the conclusion may be reached that the most important characteristic of dolomites or dolo-mitic limestones that makes them preferred host rocks is their manner of fracturing. The difference in competency between limestones and dolomites is such that limestone has the capacity to recrystallize or flow instead of fracturing under stress. Hewett reached this con-clusion with respect to mineral deposits at Goodsprings, Nevada. Hayward and Triplett also reached this conclusion with respect to the deposits in northern Mexico. Behre (19U0, p. 22) believes, however, that the favorability of dolomitic beds is modified by other factors such as the thickness of the beds. Behre concludes that within two chemically identical beds, the thicker is the more favorable because it tends to break in long continuous fractures rather than shattering in short irregular fractures. The fracturing characteristics of dolomites alone, however, are inadequate to explain the widespread association of lead-zinc deposits and dolomites. The localization of ore in primary dolomites or in dolomitized limestones fractured subsequent to dolomitization may in many cases be attributed to fracturing characteristics. In such cases open space filling commonly occurs accompanied by some replacement. In the case of lead-zinc deposits occurring within 2 dolomitic aureoles i n limestone, b r e c c i a t i o n does not appear to be the prtttohaaartt l o c a l i z i n g f a c t o r although b r e c c i a ores are quite com-mon here a l s o . The r e l a t i v e f a v o r a b i l i t i e s of the metamorphosed equiva-l e n t s of pure limestones as compared to dolomitic limestones might also be discussed here since they appear to be l a r g e l y dependent upon p h y s i c a l c h a r a c t e r i s t i c s . The o r i g i n a l composition of these rocks would seem to determine to a large extent t h e i r r e l a t i v e f a v o r a b i l -i t i e s to the deposition of ore. Brown (1°U0) has formulated the following r u l e s with respect to the f a v o r a b i l i t y as host rocks of metamorphosed carbonate rocks: (1) Bands or masses of pure limestone are very r e s i s t a n t to replacement by ore s o l u t i o n s , and are l i k e l y e i t h e r to contain no ore or to r e s i s t replacement u n t i l near the end of m i n e r a l i z a t i o n . The reason f o r t h i s i s apparently p h y s i c a l , not chemical, since microscopic data i n d i c a t e that when ore i s deposited by f a r the greater part of i t substitutes f o r carbonate. The c h i e f f a c t o r i s believed to be lack o f p o r o s i t y i n massive limestone that has under-gone flowage and r e c r y s t a l l i z a t i o n . (2) Masses of i n t e n s e l y s i l i c e o u s or s i l i c a t e d rock are l i k e -wise unfavorable ore receptacles. This seems to be account-ed f o r by two f a c t o r s ; (a) chemical u n s u i t a b i l i t y or r e -sistance to replacement, and (b) p h y s i c a l r e s i s t a n c e to small-scale b r e c c i a t i o n and opening of e f f e c t i v e channel-ways f o r s o l u t i o n s . Some low grade ore may occur, however, and also sporadic f r a c t u r e f i l l i n g s . (3 ) The optimum condition f o r ore reception appears to be found i n bands or masses containing carbonate and s i l i -cates (or quartz) i n t i m a t e l y mingled i n something l i k e equal proportions. In t h i s s i t u a t i o n movement i s l i k e l y to r e s u l t i n mixed flowage and f r a c t u r e , producing a f i n e -textured p o r o s i t y that admits solutions f r e e l y and pro-vides access to an abundance of replaceable carbonate. Cleavage cracks i n some minerals such as diopside are believed to f a c i l i t a t e the penetration of s o l u t i o n s . (k) Certain minerals or assemblages are regarded as favorable impurities i n a limestone and others as unfavorable. Original quartz and subsequently developed diopside, and perhaps garnet, are typical premineral gangue matter i n general, rather equidimensional, b r i t t l e minerals. Tre-molite, b i o t i t e , and probably wollastonite, are typical unfavorable premineral gangue—inrgeneral, highly elong-ated and somewhat flexible minerals l i k e l y to develop under strong shearing stresses. (5) Tentative opinion i s that the ratio of lime and magnesia in a limestone i s of l i t t l e or no importance. From Brown's observations i t might be said i n general that the mineral assemblage produced by the metamorphism of a siliceous or silicated dolomite or dolomitic limestone would be more amenable to replacement than that of a siliceous or sil i c a t e d limestone. Harker (1950, p. Qh) states that i n the thermal metamorphism of im-pure (siliceous) magnesian limestone, s i l i c a reacts with the magne-sium i n preference to the calcic carbonate. Unless disposable s i l i c a i s present i n an amount sufficient for complete decarbonation of the rock, one incident of the metamorphism i s dedolomitlzation. The f i r s t mineral to form, and with a limited supply of s i l i c a , the only mineral, i s the magnesian forsterite. I f the rock contained more s i l i c a than would suffice to convert a l l the magnesia to forsterite, diopside makes i t s appearance accompanying or replacing forsterite. The min-eral assemblage thus formed by the dedolomitlzation of a dolomitic limestone might be very amenable to replacement by ore-bearing solu-tions . (3) Dolomitizing and Ore-Bearing Solutions have an Identical  Source If dolomitizing and ore-bearing solutions have an identical source, dolomitization or some process involving the development of magnesian minerals might be expected to occur i n a l l lead-zinc de-posits regardless of the type of host rock. The host rock of the lead-zinc ores i n the Coer d'Alene d i s t r i c t of Idaho are schists and slates rather than limestones and the veins are s i d e r i t i c rather than dolo-mitic. The well known lead-zinc orebody of the Sullivan Mine at Kim-berly, B.C. occurs i n a r g i l l i t e s and siltstonesj magnesian minerals are not developed here i n quantities that would suggest magnesia was introduced. Hewett (1928) i n reviewing the di s t r i c t s i n which lime-stones have been dolomitized i n the v i c i n i t y of metalliferous deposits, found the outcrops of pre-mineral intrusive rocks to be either sparse and small or to t a l l y absent. The rock types range from siliceous to basic, the average having a composition near that of quartz-monzonite. I f magnesia i s a constituent of hydrothermal solutions related to magmatic processes, i t would be expected that genetically related, undifferentiated igneous rocks would be rather basic (Magnesia-rich) i n composition. In conclusion, there i s l i t t l e or no evidence to suggest that dolomitizing and ore-bearing solutions are genetically related. 3« (k) Dolomitizing and Ore-Bearing Solutions have been Localized by  the same Structural Controls but are not Genetically Related The localized development of dolomite associated with ore deposits cannot readily be attributed to primary deposition or alter-ation by sea waters. I f the dolomite or dolomitic limestone i s assumed to have been formed by the differential leaching of calcium carbonate from syngenetic magnesian limestone, a consideration of the source of magnesia i s unnecessary. I f the dolomitizing solutions cannot be related genetically to hydrothermal solutions of magmatic origin, the source of the magnesia must be considered:, this of course does not rule out the p o s s i b i l i t y that solutions related to magmatic activity might be the agents of dolomitization. Two remaining possible sources of magnesia are: (a) Bodies of intrusive rock (b) Sedimentary and metamorphic rocks The merits of these possible sources of magnesia w i l l now be con-sidered. (a) Bodies of Intrusive Rock While igneous rocks are potential sources of appreciable quantities of magnesium, some mechanism for i t s release i s required. Hewett (1928) noted that any intrusive rocks i n areas where dolomitiz-ation has taken place are generally sericitized, although i n many cases where dolomitization i s apparent, the intrusive rocks are not s e r i c i t -ized. The seric l t i z a t i o n of mafic minerals i s capable of releasing 3: both lime and magnesia. Comparative p a r t i a l analyses of fresh and sericitized samples of two types of igneous rocks i n which s e r i c i t i z -ation has been the predominant type of alteration are shown below. Qtz. Monzonite x Andesite Fresh Sericitized Fresh Sericitized S i 0 2 67.53$ 69.61$ 62.16$ 71.1k$ A1203 15.U6 15.12 I6 .k5 15.2k F e 2 ° 3 2.18 • — 3.27 1.77 FeO 2.k2 .37 2.71 .26 MgO .16 trace 2.20 .16 CaO 3.2k .05 k.13 .09 NagO 3.2k •k2 k.07 .2k K 2 0 3.86 k .5k 3.k5 6.31 The above analyses show that both lime and magnesia can be released by the seric i t i z a t i o n of these rock types. Hewett considered that this process of seri c i t i z a t i o n would occur at considerable depths and the released magnesia might be trans-ported by ascending groundwaters to f a i r l y shallow depths where dolo-mitization would take place. 1 Ransome, F.L. (1917): Geology and ore deposits of Breckenridge d i s t -r i c t , Colorado; U.S.Geol.Surv., Prof. Paper 2 75, pp. 95-101 Spurr, J.E. (1905): Geology of the Tonapah mining d i s t r i c t , Nevada; U.S.Geol.Surv., Prof. Paper k2, pp. 207-252 Since the intrusive rocks most commonly found near dolo-mitized limestones are quite acid, enormous quantities of the rock must be sericitized to release an appreciable quantity of magnesia. While small amounts of magnesia may undoubtedly be supplied from this source, i t can hardly be considered to be of general significance, (b) Sedimentary and Metamorphic Rocks Considerable quantities of magnesium are present i n many sedimentary and metamorphic rocks but in those other than dolomites or dolomitic limestones, the magnesium i s present largely i n the s i l -icate form. The problem of the release and transportation of magnes-ium seems to be one of great controversey. That groundwater i s cap-able of transporting magnesium there i s np doubt. Magnesium can be transported i n large quantities i n either the sulphate or chloride form. Metamorphic rocks i n general are probably not of great importance as potential sources of magnesium. While many metamorphic rocks such as chlorite schists and serpentines contain large amounts of magnesium, no mechanism for i t s release i n large quantities i s evident. Sediments other than dolomites or dolomitic limestones may likewise be regarded as poor potential sources of magnesium. Sedi-mentary rocks such as graywacKes can contain large amounts of magnes-ium, but again i t s release i n large quantities would require a complex mechanism. Dolomites and dolomitic limestones, both from the point of view of their magnesium content and their amenability to the re-lease of magnesium, appear to provide an ideal source of magnesium. The widespread occurrence of these rocks also favor them as potential sources of magnesium for the process of dolomitization. Laughlin and Behre (19310 have suggested that epigenetic dolomite i n the Leadville d i s t r i c t , Colorado, was derived from the country rock, especially as the lime-magnesia ratio of the country rock nearly coincides with that of the pure dolomite. Hayward and Triplett (1931) and Park and Cannon ( 19 l i3) , in speaking of the ore deposits of northern Mexico and the Metaline d i s t r i c t of Washington respectively, believe that no magnesium was introduced during the mineralization of dolomites. These writers are of the opinion that magnesium derived from the country rock was pa r t i a l l y redeposited during the replacement of the country rock. I f however, the original immediate country rock contains no magnesium, one must assume i t was introduced from a source some distance away. In such a case a mech-anism for the release and transportation of magnesium must be considered. The two obvious mediums of transportation for the magnesium are ground-waters and hydrothermal solutions. The derivation of magnesium-rich solutions through the leaching of dolomite involves the relative s o l u b i l i t i e s of the car-bonates of magnesium and calcium which make up the dolomite molecule. T y r r e l ' s unmodified statement (I9J48, p. 228) that calcium carbonate i s more soluble than magnesium carbonate i s i n d i r e c t con-, t r a s t with Twenhofel's statement (1939, p. 33k) that magnesium car-bonate i s more soluble than calcium carbonate. Lindgren (1933, p. 823) gives the s o l u b i l i t y of magnesium carbonate i n pure water at 18° Cent-igrade as 0.100 grams per hundred grams of water whereas the s o l u b i l -i t y of calcium carbonate i s given as only 0.0013 grams per hundred grams of water. Twenhofel (1939, p. 353) summarizes much of the a v a i l a b l e data on the problem; i n most instances magnesium carbonate i s shown to be'the more soluble of the two but the e f f e c t of the s a l i n i t y of the medium i s not f u l l y known. Irving (1926, p. kkk) shows that an increase i n a l k a l i n i t y favors the p r e c i p i t a t i o n of mag-nesium, e s p e c i a l l y a f t e r the pH r i s e s above 10. The preceding obser-vations, however, showing that magnesium carbonate i s generally more soluble than calcium carbonate, may not be applicable to these com-ponents when they are incorporated i n the mineral dolomite. Balconi and F e r r a r i o (1939, pp. 397-kOk) 1 subjected dolomite to the a c t i o n of water saturated at atmospheric pressure and room temperature with C0 2, under the following conditions: (1) keeping the dolomite with a d e f i n i t e unrenewed quantity . of the l i q u i d (2) t r e a t i n g i t with flowing C0 2-saturated water Cite d i n Chemical Abstracts, American Chemical Society, V o l . 3k, 6899. In the f i r s t case analysis of the l i q u i d showed that the quan t i t i e s of d i s s o l v e d CaCO^ and MgCO^ were i n i t i a l l y i n the r a t i o of l . U : 1 and t h i s r a t i o increased with time. In the second case the r a t i o of CaCO^ to MgCO^ dis s o l v e d remained at about l.k s 1. The r e s u l t s of t h i s experiment suggest that under these conditions a magnesia-rich s o l u t i o n cannot be derived from dolomite rock. A consideration of the d e r i v a t i o n of magnesium-rich s o l u -tions through the leaching of dolomitic limestones, however, involves the r e l a t i v e s o l u b i l i t y of c a l c i t e with respect to dolomite i n add-i t i o n to the r e l a t i v e s o l u b i l i t i e s of magnesium and calcium carbonates i n the dolomite molecule. A considerable number of f i e l d examples i n d i c a t e that, i n the case of dolomitic limestones, c a l c i t e i s more soluble than dolomite. Weathered surfaces of dolomitic limestones commonly show dolomite grains standing i n r e l i e f against more r e a d i l y soluble c a l c i t e . The occurrence of s t a l a c t i t e s and s t a l a g a i t e s of pure c a l c i t e i n caverns i n dolomitic limestones would i n d i c a t e that c a l c i t e i s more soluble than dolomite. There i s l i t t l e evidence to in d i c a t e that the leaching of a dolomitic limestone, under near sur-face conditions at l e a s t , can provide a magnesium-rich s o l u t i o n . While l i t t l e evidence can be found i n the l i t e r a t u r e and from f i e l d examples to in d i c a t e that a magnesium-rich s o l u t i o n can be derived by the leaching of dolomites and dolomitic limestones, the p o s s i b i l i t y cannot be overlooked. I t seems quite probable that the r e l a t i v e s o l u b i l i t i e s of the two carbonates involved are depend-ent on the nature of the environment. Hence the e f f e c t s of a l k a l i n i t y , s a l i n i t y , pressure and temperature may be of considerable importance. Faust (19U9, p. 789) has found that i f dolomite i s p a r t i a l l y d i s s o c i a t e d to a mixture of CaCO^ and MgO and t h i s i s reacted with a s o l u t i o n of water charged with carbon-dioxide, the MgO w i l l be con-verted to MgCO^ and remain i n the so l u t i o n , whereas the CaCO^ i s only s l i g h t l y dissolved and a r e l a t i v e l y small amount enters i n t o the s o l -u t i o n . The mechanism f o r the de r i v a t i o n of a magnesia-rich s o l u t i o n as ou t l i n e d by Faust w i l l be discussed i n the next s e c t i o n . The Derivation of a Magnesia-Rich Solution as Proposed by Faust Faust suggests that w i t h i n the aureole of metamorphism r e -s u l t i n g from the i n t r u s i o n of a magma int o dolomitic rocks, dedolom-i t l z a t i o n takes place i n those beds c l o s e s t to the magma. The f i r s t stage i n the dedolomitization of dolomitic beds i s the thermal d i s -s o c i a t i o n of the dolomite. The completeness of the decarbonatization i s dependent on the temperature. I f the temperature i s s u f f i c i e n t l y high, the reac t i o n i s as follows: CaCOyMgCO^ + A »CaCO^+1 MgO + CaO c o 2 c o 2 At lower temperatures the d i s s o c i a t i o n i s incomplete: t CaCO,.MgCO- + A CaCO MgO CO, Faust places the temperature f o r complete decarbonatization a t 926 °C« and the incomplete decarbonatization a t 792° C. The following f a c t o r s however, a f f e c t the d i s s o c i a t i o n temperature of dolomite: g r a i n s i z e , rate of heating, pressure, closure of the system and associated min-e r a l s . I f the system i s closed so that C 0 2 cannot escape, then the e f f e c t s of thermal metamorphism w i l l c o n sist l a r g e l y of r e c r y s t a l -l i z a t i o n of the dolomite to dolomite marble. I f the system i s open, allowing CCX, to escape, p e r i c l a s e can form. I f the d i s s o c i a t i o n r e -ac t i o n goes to completion i t i s accompanied by a volume decrease of 38 percent. I f the d i s s o c i a t i o n i s only p a r t i a l l y complete, that i s , CaCO^ does not d i s s o c i a t e , the reaction i s accompanied by a volume decrease of 22 percent. As both of these reactions r e s u l t i n a diminution i n volume, hydrostatic pressure w i l l favor the thermal d i s s o c i a t i o n i f COg can escape. I f the temperature i s s u f f i c i e n t l y high to d i s s o c i a t e dolomite completely, the r e s u l t i n g minerals would be lime (CaO) and p e r i c l a s e (MgO). No geologic occurrence of a com-p l e t e l y d i s s o c i a t e d dolomite which contains lime i s known, whereas the p a r t l y c a l c i n e d dolomites c a l l e d predazzite and pencatite have been observed i n many places. Faust assumes that the heat necessary f o r the thermal met-amorphism i s supplied by a magma. The m o b i l i t y of the emanations from a magma permits the gases and solutions to move out i n t o the country rock, and accordingly i s an important f a c t o r i n the t r a n s f e r of heat from the magma to the country rock. I t i s generally conceded that the p r i n c i p a l constituents of emanations are H 2 0 and CO2, the former being more abundant. The thermal metamorphism of dolomite a l s o releases a d d i t i o n a l CCv, at high temperatures which contributes much heat to the system. In addition, small amounts of B, C l , S e t c . are present. As long as dolomite i s being dis s o c i a t e d , there w i l l be COg gas a v a i l a b l e f o r the thermal metamorphic process. According to Faust, the three f a c t o r s which make magmatic emanations important i n contact metamorphism are; t h e i r m o b i l i t y , which transports heat q u i c k l y to the country rock; t h e i r greater s p e c i f i c heat; t h e i r a b i l i t y to enter i n t o r e a c t i o n with the country rock while at elevated temperatures• From physico-chemical data concerning the system MgO-CaO - COg - HgO, Faust has drawn the following conclusions: (1) c a l c i t e i s much l e s s soluble than magnesium carbonate i n water containing carbon dioxide (2) MgO forms a h i g h l y soluble carbonate when i t reacts with a s o l u t i o n r i c h i n carbon-dioxide. As a r e s u l t of these r e l a t i o n s h i p s , a dedolomitized dolomite, c o n s i s t i n g of MgO (p e r i c l a s e ) and GaGO^ ( c a l c i t e ) w i l l react with a s o l u t i o n of water r i c h i n carbon-dioxide with the trans f e r of most of the magnesium but with only a minimum amount of calcium i n the l i q u i d phase. I f the hydrothermal s o l u t i o n migrating from the cooling magma i s poor i n carbon-dioxide, i t w i l l r e act with the e a r l i e r formed p e r i c l a s e to produce b r u c i t e . The hydrothermal solutions which form b r u c i t e do not contain much C 0 2 ; brucite i s unstable i n contact with C 0 2-rich s o l u t i o n s . I f the hydrothermal s o l u t i o n migrating from the cooling magma i s r i c h i n carbon-dioxide, the e a r l i e r formed p e r i c l a s e w i l l be taken into s o l u t i o n . As t h i s hydrothermal s o l u t i o n , r i c h i n mag-nesium and carbon-dioxide migrates away from the zone of higher temp-eratures i n t o the cooler country rock, some of the carbon-dioxide may be l o s t . Such a condition may bring about the p r e c i p i t a t i o n of magnesite. I f the hydrothermal s o l u t i o n r i c h i n magnesium and carbon-dioxide migrates i n t o limestone areas, dolomitization takes place. Channelways or f r a c t u r e systems i n limestone may be armoured by dolo-mite, and thus may:iyield channelways along which the hydrothermal solutions can migrate without f u r t h e r r e a c t i o n with the limestone. Magnesite may be deposited i n these armoured channelways. Faust points out an i n t e r e s t i n g r e l a t i o n s h i p which e x i s t s between the occurrence of the metamorphic rocks, predazzite and pen-c a t i t e , and the type of i n t r u s i v e rocks which are responsible f o r t h e i r metamorphism. These rocks are predominantly the s i l i c i o u s types such as granites and g r a n d i o r i t e s . Their close a s s o c i a t i o n with the de-dolomitized rocks suggests that the composition of the i n t r u s i v e rock i s a f a c t o r i n determining the, degree of metamorphism of the dolomite. Possible Evidence Supporting Faust's Proposals In most cases dolomitization i s s a i d to precede sulphide min e r a l i z a t i o n ; the mechanism as proposed by Faust f o r the d e r i v a t i o n of a magnesia-rich s o l u t i o n would f u l f i l l t h i s requirement. Fluorine-bearing minerals are f a i r l y common constituents of dolomites and dolomitic limestones. The occurrence of the mineral f l u o r i t e i s common i n many of the lead-zinc deposits i n dolomitized limestone i n the M i s s i s s i p p i V a l l e y , U.S.A. and i n the Pennine D i s t -r i c t of England. The fluorine-bearing minerals phlogopite and chon-drodite commonly occur i n metamorphosed dolomitic limestones. This a s s o c i a t i o n of flu o r i n e - b e a r i n g minerals with dolomites and dolomitic limestones might suggest that the process of dolomitization i s r e l a t e d to the introduction of e a r l y fluorine-bearing hydrothermal s o l u t i o n s . Griggs (19W-, p. 527) has had moderate success i n the a r t i f i c i a l syn-thesis of dolomite from c a l c i t e . He believes that a promising mode of attack on the synthesis of dolomite i s experimentation with the common permeating v o l a t i l e s which leave t h e i r traces widespread i n nature; namely, boron and f l u o r i n e . Berg (19U3)''" has found that the presence of chlorine lowers the d i s s o c i a t i o n temperature of dolomite. On heating some samples of dolomite,Berg found the f i r s t exothermal e f f e c t to appear at 730° C. accompanied by superheating; i n other samples d i s s o c i a t i o n began at a considerably lower temperature without any superheating. Those of the l a t t e r kind were found to contain c h l o r i n e , and when t h i s was washed out, t h e i r behavior was normal. By the addition of small amounts of NaCl or other a l k a l i s a l t s the abnormal heating e f f e c t can be r e -produced, the d i s s o c i a t i o n point f a l l i n g to k90-520° c . Since chlorine C i t e d i n Abstracts, Miner. Mag., 2 7 ( 1 8 ? ) , 1 9 L U . also i s g e n e r a l l y considered to be a constituent of e a r l y hydrothermal solutions, i t s presence may w e l l a i d i n the process of dedolomiti-zation as proposed by Faust. Faust's r e l a t i o n s h i p between the occurrence of dedolomiti-zed rocks and s i l i c i o u s types of i n t r u s i v e rocks may be corr e l a t e d with Hewett's observations of the r e l a t i o n s h i p between the occurr-ence of dolomitized limestone and s i l i c i o u s types of i n t r u s i v e rocks. Since dolomitized limestones and dedolomitized dolomites apparently, occur under s i m i l a r conditions, the conclusion might be drawn that a r e l a t i o n s h i p e x i s t s between these two types of rocks. The d e r i v a t i o n of a magnesia-rich so l u t i o n as proposed by Faust presupposes that dolomitic rocks must occur w i t h i n the teaches higher temperature Aof an intruding magma. In some formations, es-p e c i a l l y those of Palaeozoic age, i n which limestones occur i t seems almost inevitable that an intruding magma w i l l contact dolomitic rocks at depth. The occurrence of lead-zinc deposits i n unaltered limestones may suggest that the parent magma has not contacted dolo-m i t i c rocks. The Jackpot Property Introduction The Jackpot property i s i n the Salmo d i s t r i c t of B r i t i s h Columbia about 1? miles south of Nelson and 3ir miles east of Ymir. The claims are situated on the c r e s t and northern slope separating Porcupine and Hidden Creeks. New Jersey Explorations Limited hold an option on the property. A l l work done on the property to date has been of an ex-p l o r a t o r y nature. Exploration by means of diamond d r i l l i n g and open-cutting has been confined to several of the more favorable areas of the property. General Geology"*" The rocks of the area c o n s i s t of r e c r y s t a l l i z e d and met-amorphosed limestones and argillaceous to q u a r t z i t i c sediments which have been complexly f o l d e d and f a u l t e d . These metamorphosed rocks are intruded by s i l l - l i k e g r a n i t i c and a p l i t i c masses which are probably r e l a t e d to a g r a n i t i c stock exposed a short distance to the south. The sediments are considered to be of E a r l y Palaeozoic age. By personal communication with Dr. H.C. Gunning, Department of Geology and Geography, U n i v e r s i t y of B r i t i s h Columbia and from B r i t i s h Columbia Dept. of Mines, Annual Report of the M i n i s t e r of Mines; 19k9, A I 6 9 . The northern part of the property i s underlain by an assemb-lage of s c h i s t , p h y l l i t e and qu a r t z i t e interbedded with impure lime-stone and i n j e c t e d and replaced by granite and a p l i t e . This com-plex i s considered to be the footwall of the ore-bearing ground. Overlying t h i s complex i s a bed of c r y s t a l l i n e limestone having a maximum thickness of about 35>0 f e e t . This lower limestone member i s succeeded upward by an assemblage of argillaceous and q u a r t z i t i c sediments having a thickness of something l e s s than 100 f e e t . The qu a r t z i t e i s i n places i n j e c t e d and replaced by g r a n i t i c rocks. The q u a r t z i t i c and a r g i l l a c e o u s members are o v e r l a i n by a second lime-stone stratum which has a minimum thickness of about 350 f e e t . The s t r u c t u r a l p i c t u r e i s very complex. In general the s t r a t a l i e i n broad flexures which dip at low angles southward. L o c a l l y the structure i s complicated by close f o l d s which plunge f o r the most part south-southwestward. The known mineral deposits occur as disseminated sulphide replacements i n dolomitized limestone. Both limestone members are extensively dolomitized, the dolomitic zones being generally par-a l l e l to the bedding. The dolomitic zones are characterized i n surface exposures by granular dolomitic masses which stand out i n r e l i e f against c a l c i t e . Serpentine and other s i l i c a t e minerals are developed i n parts of the dolomitic zone. The ch i e f sulphide minerals are sph a l e r i t e and p y r i t e . P y r r h o t i t e i s abundant l o c a l l y and galena occurs e r r a t i c a l l y i n the hangingwall or footwall of the zinc bodies. The heaviest sulphide m i n e r a l i z a t i o n appears to be somewhat l e n t i c u l a r i n form and i s l o c a l i z e d i n calcareous zones with i n large dolomite envelopes. In general the areas i n which serpentine and other s i l i c a t e minerals are developed appear to be unfavorable to sulphide m i n e r a l i z a t i o n . The Orebodies 1 The orebody known as the Main Showing occurs on the Jack-pot mineral claim and l i e s near the base of the lower limestone. This ore zone has the form of a southerly plunging s y n c l i n a l trough. The showing has been explored by means of numerous open cuts and some twenty diamond d r i l l holes. Several l e n t i c u l a r bodies of d i s s -eminated sulphides occur within dolomitized limestone i n t h i s area. The West Showing i s on the Jamesonite mineral claim some 75>0 f e e t southwest of the Main Showing. This showing appears to be i n the upper limestone member. M i n e r a l i z a t i o n exposed by open cuts was of s u f f i c i e n t i n t e r e s t to j u s t i f y diamond d r i l l i n g . Sub-s t a n t i a l widths of rather low grade p y r i t i c zinc m i n e r a l i z a t i o n were in t e r s e c t e d by diamond d r i l l holes. The Lerwick Showing i s on the Ink Spot mineral claim about lLOO fe e t southeast of the Main Showing. This showing probably occurs i n the lower limestone member. Sparse m i n e r a l i z a t i o n with 'By personal, communication with Dr. H.C. Gunning, Department of Geology and Geography, U n i v e r s i t y of B r i t i s h Columbia and from B r i t i s h Columbia Dept. of Mines, Annual Report of the M i n i s t e r of Minesj 19U9, A I 6 9 much p y r r h o t i t e has been found i n two zones w i t h i n dolomitized lime-stone . The East Showing i s s i t u a t e d some 2l|00 f e e t northeast of the Lerwick Showing. The s t r a t a i n t h i s area dip f a i r l y steeply eastward. M i n e r a l i z a t i o n has been exposed i n several open cuts which may represent one or more mineralized zones i n the lower lime-stone member. This showing has been explored by several diamond d r i l l holes, some of which have i n t e r s e c t e d encouraging mineralized zones. The m i n e r a l i z a t i o n here a l s o appears to be confined to the more dolomitic zones. Zones of white marble do not contain s i g n i f -i c a n t amounts of sulphide m i n e r a l i z a t i o n . Some zones i n which s e r -pentine and other s i l i c a t e s are developed i n small aggregates i n the carbonate rock, however, do contain appreciable amounts of sulphide minerals. Method of Study Specimens selected from two general areas of the Jackpot property were studied i n some d e t a i l . These two areas are the Main Showing and the East Showing. The specimens from the Main Showing were selected from d r i l l hole J-l£. The sections of core selected are b e l i e v e d to be f a i r l y representative of the mineralized zones, the dolomite, the dolomitic limestone, and zones where serpentine and other s i l i c a t e s are developed. From a study of the d r i l l hole log , an attempt was made to determine the r e l a t i o n s h i p of the spe-cimens to the d r i l l hole s e c t i o n . The specimens from the East Showing vrere selected from diamond d r i l l holes J - 3 3 , J - 3 U , and J - 3 5 . The diamond d r i l l core from these three holes was examined roughly by the writer and some general relationships were determined. The specimens selected were believed to be representative of the various zones of the d r i l l hole sections. One surface specimen of mineralized dolo-rdtic limestone was examined. In order to d i f f e r e n t i a t e c a l c i t e from dolomite, the se-lected specimens of d r i l l core were s p l i t by means of a diamond saw and polished and stained by the method described i n the appendix. Thin sections were made of those parts of the core which showed interesting relationships on the stained polished surfaces. The specimens from diamond d r i l l hole J-15 were studied i n the greatest d e t a i l . Thirty power binoculars were used to study the stained polished surfaces. Descriptions of Specimens Studied Dolomite Sections of diamond d r i l l core which consist almost en t i r e -l y of dolomite were studied i n t h i n section. The rock i s now re-c r y s t a l l i z e d dolomite marble made up of anhedral grains varying i n size from 0 .5 to 1.5 mm. The grain boundaries are commonly very ragged and in t e r l o c k i n g . Most of the dolomite grains are very heavily twinned. In addition to having the normal interlocking texture of r e c r y s t a l l i z e d carbonate rocks, some of the grains have what might be termed a "graphic" texture. Islands of dolomite which exting-uish together and have p a r a l l e l twinning are i s o l a t e d w i t h i n dolomite grains of d i f f e r e n t o p t i c a l o r i e n t a t i o n (Plate I A ) . This "graphic" texture might suggest that the dolomitization of the o r i g i n a l lime-stone took place i n several stages. The dolomitization of a lime-stone i s i n most cases s a i d to be accompanied by an increase i n grain s i z e . Interference during c r y s t a l growth gives r i s e to t h i s i n t e r l o c k i n g texture. No dolomite c r y s t a l s of rhombohedral ou t l i n e were observed. Some of the nearly pure dolomite rock contains small amounts of high magnesium olivene^which w i l l be re f e r e d to as f o r s t e r i t e . F o r s t e r i t e occurs i n small subhedral grains which are u s u a l l y i n t e r -s t i t i a l to dolomite grains but i n some cases are completely w i t h i n dolomite grains.. A few l a t h s of a micaceous mineral which i s believed to be phlogopite occur i n the dolomite sections. Dolomitic Limestone Two specimens which might be considered to be p a r t i a l l y dolomitized limestone were studied i n stained t h i n s e c t i o n . One specimen, Jl5-l0!?J occurs i n the d r i l l hole section 100 f e e t or more from a mineralized zone of any s i g n i f i c a n c e . The other spe-cimen was taken from a surface outcrop, the weathered surface of 'Determined as F o r . 9 ~ Fay.^ by Mr. G.A. Wilson by means of the Universal Stage Microscope. which shows dolomite grains to be standing i n r e l i e f against c a l -r c i t e . The dolomite grains form mosaic-like bands i n the c a l c i t e which suggests an alignment with the o r i g i n a l bedding of the rock. Some sulphides occur i n the c a l c i t i c areas of the l a t t e r specimen (Plate I I I A). The impression gained from the study of the stained t h i n sections of both these specimens i s that they do not represent par-t i a l l y dolomitized limestone, but rather p a r t i a l l y " c a l c i f i e d " dolo-mite. Ve i n l e t s of c a l c i t e penetrate dolomite g r a i n boundaries and i n some cases c a l c i t e cuts and.embays portions of dolomite grains which have o p t i c a l c ontinuity (Plates I B, I I A and I I I B). Specimen Jl^-lOf? shows rounded f o r s t e r i t e grains to occur f o r the most p a r t within the' c a l c i t i c areas, although a few grains of t h i s mineral were observed to be completely within dolomite g r a i n s . Some f o r s t e r i t e grains are p a r t i a l l y a l t e r e d to serpentine. Lath-l i k e grains of phlogopite are f a i r l y common i n t h i s rock and several small subhedral grains of apatite were observed. P y r i t e occurs sparingly throughout the c a l c i t i c and dolomitic portions of the rock. In several cases o p t i c a l l y continuous islandw of f o r s t e r -i t e were observed to be completely embayed by c a l c i t e . I f the f o r -s t e r i t e were formed by the thermal metamorphism of a s i l i c e o u s or s i l i c a t e d dolomite, the presence of some c a l c i t e i n the v i c i n i t y of f o r s t e r i t e would be expected. The amount of c a l c i t e present, however, appears to be i n excess of that which would be expected by the r e a c t i o n alone. A d d i t i o n a l c a l c i t e was probably introduced a f t e r the thermal metamorphism of the dolomite. The introduced c a l c i t e has replaced dolomite, and to some extent f o r s t e r i t e , l a r g e l y i n the areas where c a l c i t e had been formed by the e a r l i e r metamor-phic a c t i o n . Serpentine and Other S i l i c a t e Rock Serpentine occurs t y p i c a l l y i n some of the more calcareous sections of the diamond d r i l l core. Stained surfaces of serpentine rock, however, indi c a t e that the carbonate i s present l a r g e l y as l o c a l i z e d c a l c i t e halos which envelope and transect serpentine lenses. The stained polished surface of specimen J15-3271 (Plates VI A and B) c l e a r l y i l l u s t r a t e s one r e l a t i o n s h i p which e x i s t s between c a l c i t e and serpentine. The serpentine, varying i n c o l o r from pale green to greenish-black, occurs i n narrow i r r e g u l a r lenses which appear to be generally p a r a l l e l to the o r i g i n a l bedding of the rock. In t h i n section the serpentine lenses are seen to be transected by minute v e i n l e t s of cloudy c a l c i t e . The c a l c i t e envelopes are most commonly enclosed by dolomite but i n several cases masses of fibrous diopside which are p a r t i a l l y a l t e r e d to serpentine occur adjacent to the outer c a l c i t e rim. F o r s t e r i t e grains, some of which are p a r t i a l l y a l t e r e d to serpentine, occur i n the dolomitic portions of the rock. While the t h i n sections reveal that the serpentine occurs as an a l t e r a t i o n product of both f o r s t e r i t e and diopside, there i s l i t t l e evidence i n the l a r g e r serpentine lenses to i n d i c a t e from which mineral they were formed. In some cases i d e n t i c a l l y oriented diopside remnants appear to be replaced by c a l c i t e and no serpentine a l t e r a t i o n i s apparent. The serpentine bands contain a few scattered 1 grains of p y r i t e . The greenish-black bands of serpent-ine owe t h e i r c o l o r to the presence of an opaque disseminated mineral which i s probably graphite. The most commonly observed s i l i c a t e mineral to be developed, other than f o r s t e r i t e and i t s a l t e r a t i o n product serpentine, i s w o l l a s t o n i t e . Some sections of core are composed almost e n t i r e l y of t h i s mineral. Specimen Jl$-k3^, shows an assemblage of wollas-toni t e and diopside, wollastonite being present i n the greater amount. The diopside occurs i n lenses which have an alignment suggestive of o r i g i n a l bedding. The l a r g e r wollastonite grains are rather ragged and appear i n p a r t to be replaced by diopside. Mineralized Sections - Main Showing The w e l l mineralized sections, when stained, show i n gen-e r a l that the sulphide m i n e r a l i z a t i o n i s concentrated w i t h i n w e l l defined areas of c a l c i t e which are enveloped by almost pure dolomite (Plate V A). Some narrow bands and disseminated grains of sulphides, however, do occur i n the dolomite, unaccompanied by " c a l c i t e . Specimens from two mineralized zones of the Main Showing were studied, namely; J15-156' and J15-171'. The sulphide minerals observed i n the mineralized sections of d r i l l hole Jl5 are p y r i t e , s p h a l e r i t e and p y r r h o t i t e . The s u l -phide grains appear to be completely anhedral with the exception of a few cubic c r y s t a l s of p y r i t e which occur within blebs of p y r r h o t i t e . The sulphide bands wi t h i n any one mineralized section are e s s e n t i a l l y p a r a l l e l and are probably i n alignment with the bedding planes of the o r i g i n a l carbonate rock. The contacts between bands of mineral-i z e d c a l c i t e and surrounding dolomite are abrupt, there being no apparent gradation of c a l c i t e i n t o dolomite. In one case the bound-ary between c a l c i t e and dolomite i s marked by a narrow somewhat i n t e r -mittent v e i n l e t of sulphide minerals. Blebs and small i r r e g u l a r masses of dolomite occur within the areas of mineralized c a l c i t e . These i r r e g u l a r masses of dolomite are commonly c l o s e l y associated with sulphide masses (Plate V B). In some cases blebs of dolomite are completely embayed by sulphide minerals, i n other cases sulphide masses are p a r t i a l l y rimmed by blebs of dolomite. Small masses of dolomite, however, occur unassoc-i a t e d with sulphide minerals within areas of c a l c i t e . There i s no apparent diffe r e n c e i n g r a i n s i z e between the c a l c i t e and the dolomite, the average being about .7 mm. C a l c i t e i s d e f i n i t e l y more cloudy i n t h i n section than i s dolomite. Most of the dolomite grains are broadly twinned while c a l c i t e grains show l i t t l e twinning. The c a l c i t e grains, however, show much better cleavage than do the dolomite g r a i n s . Thin section J15-171' reveals the f a c t that very few s u l -phide grains occur i n c a l c i t e without the a s s o c i a t i o n of dolomite. Nearly a l l sulphide grains are at l e a s t p a r t i a l l y rimmed by dolomite (Plate VII A). The sulphides are c l e a r l y younger than the dolo-mite as i s evidenced by t h e i r c u t t i n g o p t i c a l l y continuous grains of dolomite. In several cases v e i n l e t s of sulphide minerals were observed to f i n g e r out along dolomite grain boundaries from sulphide masses, p o s s i b l y i n d i c a t i n g the manner i n which the replacement was i n i t i a t e d . C a l c i t e also appears to be younger than dolomite since t h i s mineral i n some cases cuts o p t i c a l l y continuous grains of dolo-mite. No r e l i a b l e evidence was seen, however, to d e f i n i t e l y determine the age r e l a t i o n s h i p between the c a l c i t e and the sulphides. I f the c a l c i t e was introduced p r i o r to the sulphides, then the dolomite has d e f i n i t e l y been replaced by the sulphides i n preference to the c a l c i t e . The f a c t , however, that i n one case c a l c i t e was observed f i l l i n g a fra c t u r e i n p y r i t e suggests that c a l c i t e i s at l e a s t young-er than t h i s mineral. Since s p h a l e r i t e f i l l s f r a ctures i n p y r i t e and p y r r h o t i t e , i t i s d e f i n i t e l y younger than both these minerals. The occurrence of cubes of p y r i t e within masses of p y r r h o t i t e sug-gests that the p y r r h o t i t e i s the younger mineral of the two. The paragenesis of the sulphide minerals from older to younger than i s p y r i t e , p y r r h o t i t e and s p h a l e r i t e . In one instance both c a l c i t e and dolomite are cut by a narrow v e i n l e t of dolomite (Plate IV). This may be taken to i n d i c a t e that at l e a s t some of the dolomite was formed or emplaced at a l a t e stage. Mineralized Sections - East Showing While the study of mineralized sections from the Main Showing i n d i c a t e s that zones in'which s i l i c a t e minerals have been developed are generally unfavorable to sulphide m i n e r a l i z a t i o n , t h i s i s not e n t i r e l y true of the East Showing. An examination of miner-a l i z e d sections from d r i l l holes J 3 3 , J3h and J35 i n d i c a t e s also that sulphide m i n e r a l i z a t i o n i s not n e c e s s a r i l y accompanied by c a l -c i t e , although t h i s mineral i s conspicuous i n most mineralized sec-t i o n s . Thin sections of specimens J33-55U' and J35-12U 1 show s u l -phide m i n e r a l i z a t i o n to be c l o s e l y associated with zones where s i l -i c a t e minerals have strongly but not massively developed i n the carbonate rock. - • The stained polished surface of specimen J33—55U* shows sulphide m i n e r a l i z a t i o n to occur i n a c a l c i t i c zone. As i n the specimens from the Main Showing, small p a r t i c l e s of dolomite, which are probably uhreplaced r e s i d u a l s , occur i n the c a l c i t e , i n some cases c l o s e l y associated with s p h a l e r i t e and p y r i t e . Since t h i n section J3U-55U* was not stained, a precise d i s t i n c t i o n between c a l c i t e and dolomite could not be made. In the t h i n section, how-ever, the sulphides are seen to be very c l o s e l y associated with small aggregates of s i l i c a t e minerals (Plate VII B). Serpentine, which appears to be an a l t e r a t i o n product of f o r s t e r i t e , i s the most abundant s i l i c a t e mineral present. Irregular aggregates of sulphide minerals, most commonly s p h a l e r i t e , embay or occur as i s o l a t e d grains i n serpentine masses. In some cases remnants of f o r s t e r i t e grains occur as cores i n the serpentine masses. A few unaltered grains of f o r s t e r i t e occur i n portions of the section where sulphides are absent or weakly developed. This suggests that the s e r p e n t i n i z a t i o n has been accomplished by the solutions which deposited the sulphides. Grains of a micaceous mineral which i s bel i e v e d to be phlogopite occur i n a l l parts of the s e c t i o n but are most abundant adjacent to sulphide grains. In some cases laths of phlogopite occur w i t h i n serpentine masses. The sulphides have shown a d i s t i n c t preference for areas adjacent to s i l i c a t e grains but there i s l i t t l e evidence to suggest that the s i l i c a t e s have been replaced i n preference to the carbonate. In some cases islands of f o r s t e r i t e which have op-t i c a l c o n t i n u i t y are embayed by c a l c i t e . The abundance of c a l c i t e i n the section, and the f a c t that i t replaces f o r s t e r i t e suggests that t h i s mineral has at l e a s t i n part been introduced, probably at a l a t e stage. The t h i n s ection of specimen J35-12k' shows p y r r h o t i t e and to a l e s s e r extent p y r i t e to occur i n a zone where s i l i c a t e minerals have been developed. This t h i n s ection was not stained but the stained polished surface shows c a l c i t e to be present i n abundance. Irregular masses and small blebs of dolomite occur with-i n the c a l c i t e . S i l i c a t e minerals form approximately 25 percent of the section. The s i l i c a t e minerals present are diopside, f o r -s t e r i t e and phlogopite, diopside being the most abundant. V»hile several large, almost complete, grains of diopside occur i n the section, this mineral and f o r s t e r i t e occur f o r the most part as c l u s t e r s of small i d e n t i c a l l y oriented grains which are replaced by carbonate (probably c a l c i t e ) . The sulphides appear to replace the carbonate-silicate mixture. Unreplaced grains and small masses of s i l i c a t e minerals and carbonate are i n some cases completely embayed by the l a r g e r sulphide masses. Veinlets of p y r r h o t i t e f i n g -er out into the silicate-carbonate aggregate from these l a r g e r s u l -phide masses. A study of the stained polished specimen suggests that the carbonate replacing the s i l i c a t e s i s c a l c i t e . I t seems quite probable that the c a l c i t e has been introduced at a l a t e stage, replacing the dolomite and to some extent the s i l i c a t e s formed by the thermal metamorphism of the dolomite. Serpentine has not been extensively developed i n t h i s section although a few grains of diop-side have been p a r t i a l l y a l t e r e d to t h i s mineral. Laths of phlogo-p i t e are scattered throughout the s e c t i o n . Discussion From the d e t a i l e d study of a l i m i t e d number of specimens, no r e a l evidence has been found to ind i c a t e that the processes of dolomitization and m i n e r a l i z a t i o n are r e l a t e d . A study of diamond d r i l l cross sections of the mineralized zones, however, does indi c a t e that these mineralized zones are i n general enveloped by dolomite or rock which i s dolomitic. Calcareous dolomitic sections, which i n f i e l d observation would probably be considered to be p a r t i a l l y dolomitized limestone, when studied i n stained t h i n section appear to be p a r t i a l l y " c a l c i f i e d " dolomite. From the l i m i t e d number of specimens studied, however, i t cannot be s a i d that a l l s o - c a l l e d dolomitic limestones i n the area are " c a l c i f i e d " dolomites. Evid-ence of " c a l c i f i c a t i o n " was observed In both mineralized zones within dolomite and unmineralized dolomites some distance from mineralized zones. Quite extensive thicknesses of pure limestone marble do occur i n the d r i l l hole cross sections. These limestone sections probably represent o r i g i n a l limestone. Most of the better sulphide m i n e r a l i z a t i o n occurs i n c a l -c i t i c zones within broad envelopes which are predominantly dolomitic. Some sulphides, u s u a l l y disseminated, however, do occur i n dolomite with no associated c a l c i t e . The paragenetic r e l a t i o n s h i p of c a l -c i t e and dolomite w i t h i n the mineralized zones i s not as c l e a r as that i n the unmineralized zones. The general impression gained by the w r i t e r i s that dolomite has been replaced by the sulphides to be l a t e r p a r t i a l l y replaced by c a l c i t e i n the mineralized zones. Zones i n which s i l i c a t e minerals are strongly developed such as those c o n s i s t i n g of massive wol l a s t o n i t e , diopside, f o r s t e r i t e and i t s a l t e r a t i o n product serpentine, appear to be generally unfavor-able to sulphide m i n e r a l i z a t i o n . Zones, however, i n which small aggregates of f o r s t e r i t e and serpentine, and to a l e s s e r extent diopside, occur i n intimate mixture with dolomite (now l a r g e l y replaced by c a l c i t e ) appear to be quite favorable to sulphide min-e r a l i z a t i o n . There i s no evidence to i n d i c a t e that a l l the c a l c i t e observed i n the mineralized zones and " c a l c i f i e d " dolomite zones has been formed by dedolomitization. One i n c i d e n t i n the metamor-phic process, however, has been dedolomitization* The presence of c a l c i t e envelopes surrounding masses of f o r s t e r i t e (now l a r g e l y serpentine) suggests that a form of dedolomitization has occurred. The thermal metamorphism of a s i l i c e o u s or s i l i c a t e d dolomite w i l l produce f o r s t e r i t e and c a l c i t e according to the following r e a c t i o n : 2CaMg(00^)2 - 1 - S i 0 2 »2MgO.Si0 2 -f- aCaCO^ + 2C0 2 Since no f r e e quartz was observed i n any of the t h i n sections, i t i s believed that the s i l i c a necessary f o r t h i s r e a c t i o n was i n t r o -duced, although i t i s quite p o s s i b l e that t h i s mineral may have been a primary constituent l o c a l i z e d i n some horizons of the o r i -g i n a l limestone. I t i s generally believed that the fundamental cause of dedolomitization i s found i n the f a c t that magnesium car-bonate has a much lower d i s s o c i a t i o n temperature than calcium car-bonate. I t i s f o r t h i s reason that magnesia takes precedence over lime i n the reactions which accompany the thermal metamorphism of a dolomite. According to Harker (1°!?0, p. 8*U) the f i r s t mineral to form, and with a l i m i t e d supply of s i l i c a , the only mineral, i s f o r s t e r i t e . I f the r e a c t i o n which produced f o r s t e r i t e proceded on a molecular basis , the r e a c t i o n would be accompanied by a volume decrease of about 22 percent. The volume of c a l c i t e formed with respect to f o r s t e r i t e would have a r a t i o of almost 2 : 1. The proportion of c a l c i t e to f o r s t e r i t e observed i n some of the c a l c i t e -f o r s t e r i t e associations appears to be approximately t h i s r a t i o , but i n most cases the r a t i o of c a l c i t e to f o r s t e r i t e i s much greater. This excess of c a l c i t e and the f a c t that c a l c i t e appears to replace f o r s t e r i t e s trongly suggests that some c a l c i t e at l e a s t was i n t r o -duced. The product of the thermal metamorphism of limestone i n the presence of s u f f i c i e n t s i l i c a i s a wollastonite rock. The presence of a l i t t l e magnesia w i l l bring about the formation of some diop-s i d e . I f s i l i c a i s present i n a dolomitic rock or i s introduced i n greater quantities than would s u f f i c e to convert a l l the magnes-i a to f o r s t e r i t e , diopside w i l l be formed. Harker (1950, p. 85) states that the formation of diopside does not i n i t s e l f import dedolomitlzation but, i f the mineral i s subsequently converted to serpentine and c a l c i t e , the same r e s u l t i s reached i n d i r e c t l y . I t i s quite p o s s i b l e that the serpentine and c a l c i t e i n some of the specimens examined was formed by the a l t e r a t i o n of diopside. The diopside, f o r s t e r i t e and c a l c i t e assemblage, accord-ing to Bqwen (I9I4.O), i s one formed under conditions of medium temp-erature metamorphism. Turner (19k8, p. 73) notes that the der i v -a t i v e s of dolomitic limestones with s i l i c a as the sole impurity f a l l w i t h i n the l e f t hand h a l f of F i g . 1. Si0 2 F i g u r e C a l c i t e P e r i c l a s e In order of decreasing s i l i c a , and increasing magnesia content, the s i l i c a - d e f i c i e n t assemblages are: c a l c i t e - w o l l a s t o n i t e - d i o p s i d e , c a l c i t e - d i o p s i d e - f o r s t e r i t e and c a l c i t e - f o r s t e r i t e - p e r i c l a s e . Since no p e r i c l a s e (or brucite) was found i n any of the sections examined, i t might be concluded that s i l i c a was present or was intrduced i n s u f f i c i e n t amounts to form the s i l i c a t e s , or that the metamorphism took place under conditions of high pressure which are s a i d to be unfavorable to the formation of p e r i c l a s e . The formation of c a l c i t e by the type of dedolomitlzation j u s t described cannot be c a l l e d upon i n most cases to explain the close a s s o c i a t i o n of c a l c i t e and sulphide m i n e r a l i z a t i o n . The close ass o c i a t i o n of sulphide minerals with small masses of f o r s t e r i t e (now l a r g e l y a l t e r e d to serpentine) as observed i n specimens from the Sast Showing may, however, i n d i c a t e that c a l c i t e formed by the ther-mal metamorphism of a s i l i c e o u s or s i l i c a t e d dolomite i s amenable to sulphide replacement. The greater part of the sulphide m i n e r a l i z -a t i o n , however, occurs i n zones i n which magnesian s i l i c a t e s have not been extensively developed. Several tentative possible explan-ations f o r the a s s o c i a t i o n of c a l c i t e and sulphide minerals within dolomitized zones i n which s i l i c a t e minerals have not been exten-s i v e l y developed follow: (1) The c a l c i t i c zones represent portions of the o r i g i n a l limestone which have been incompletely dolomitized. The islands of dolomite occurring i n the c a l c i t e , i n many cases rimming sulphide minerals, may represent metasomes of replacing dolomite. The f a c t that the c a l c i t e i n the mineralized zones lias a very low degree of transparency might be taken as an i n d i c a t i o n that i t i s primary. I f such i s the case, zones of undolomitized or p a r t i a l l y dolomitized. limestone were more amenable to sulphide replacement than the enclosing dolomite. (2) Perhaps the process of dolomitization was followed by one of p a r t i a l dedolomitization i n zones where sulphide minerals were deposited. The magnesia formed by the p a r t i a l thermal d i s s o c i a t i o n of the dolomite was removed by solutions r i c h i n C0 2, leaving c a l c i t e . The process of dedolomitization as. o u t l i n e d by Faust might be pro-gressive, that i s , dolomitization accomplished by the e a r l y emplacement of a magma i s followed by dedolomiti-zation at a l a t e r stage i n the hydrothermal a c t i v i t y . This process of l a t e r dedolomitization might be more l o c a l i z e d than the preceding process of dolo m i t i z a t i o n , following c h i e f l y the channelways and zones of weakness where sulphides were deposited. I f such i s the case, the isl a n d s of dolomite w i t h i n the c a l c i t i c zones are r e s i d -uals which have not been dedolomitized. While no concrete evidence was observed i n a study of specimens to support t h i s explanation, some suggestive evidence was observed. The c a l c i t e i n some of the s u l -phide zones i s rather vuggy— a l o s s of volume may have been brought about by dedolomitization. Pyrrhotite ap-pears to be more common i n the mineralized zones of the Main Showing than i n those of the East Showing. S i m i l a r -l y c a l c i t e appears to be more abundant i n the mineralized zones of the Main Showing than i n those of the East Show-i n g . I f p y r r h o t i t e can be considered a high temperature mineral, one might conclude from these observations that higher temperatures p r e v a i l e d where c a l c i t e i s more abun-dant. Since the process of dedolomitization as proposed by Faust requires f a i r l y high temperatures, the c a l c i t e may have been formed as a r e s u l t of t h i s process. (3) The c a l c i t e associated with the sulphide m i n e r a l i z a t i o n may have been transported by hydrothermal solutions at a l a t e stage i n the hydrothermal process. The c a l c i t e has p a r t i a l l y replaced the dolomite i n the zones where s u l -phides were deposited. Residual masses of dolomite have 6; been l e f t w i t h i n the c a l c i t e and as p a r t i a l rims around sulphide masses. The c a l c i t e has been l o c a l i z e d by the same s t r u c t u r a l controls as the mi n e r a l i z i n g s o l u t i o n s . The paragenesis of the dolomite, sulphides and c a l c i t e , i f i t can be r e l i e d upon, would i n d i c a t e that c a l c i t e i s the l a t e r mineral. According to Faust, the MgO derived from the incom-p l e t e thermal d i s s o c i a t i o n of a dolomite i s more soluble than CaCO^ i n waters r i c h i n GO^. At a l a t e stage i n the hydrothermal a c t i v i t y , i t seems quite conceivable that the hydrothermal solutions w i l l become poor i n magnesia due to the f a c t that t h i s material has been l a r g e l y removed from the zone of thermal d i s s o c i a t i o n . c a l c i n a t i o n . The hydrothermal solutions w i l l then become r e l a t i v e l y r i c h i n lime. These l a t e r s o l u t i o n s , r i c h i n lime as compared to magnesia, may account f o r the replacement of dolomite by c a l c i t e i n zones where sulphide minerals have been de-p o s i t e d . The magnesia taken into s o l u t i o n by the replace-ment of dolomite i n the mineralized zones w i l l be t r a n s -ported elsewhere. These so l u t i o n s , enriched i n magnesia at a l a t e stage i n the hydrothermal a c t i v i t y may be c a l l -ed upon to account f o r the dolomite which commonly f i l l s post-ore f r a c t u r e s . Of the three explanations proposed f o r the a s s o c i a t i o n of c a l c i t e and sulphide minerals w i t h i n dolomitized zones, the w r i t e r i s of the opinion that the l a t t e r i s the most applicable to the phenomenan as observed i n specimens from the Jackpot property. Conclusions While dolomite appears to be very favorable host rock f o r lead-zinc ore, t h i s f a v o r a b i l i t y i s probably not e n t i r e l y respon-s i b l e f o r the l o c a l i z a t i o n of the ore w i t h i n dolomitized envelopes i n limestones. The f a c t that the dolomitic a l t e r a t i o n g e n e r a l l y precedes ore deposition and i s favorable to ore deposition may be merely a very p r o v i d e n t i a l circumstance. Evidence suggests that the occurrence of dolomitic aureoles enveloping lead-zinc deposits i n limestone i s an inherent c h a r a c t e r i s t i c i n a great many deposits of t h i s type. The zones of dolomitic a l t e r a t i o n are undoubtedly l o c a l i z e d by s t r u c t u r a l c o n t r o l s . Lead-zinc ores are probably to a lar g e extent l o c a l i z e d by the same general s t r u c t u r a l controls and may therefore occur f o r the most part w i t h i n dolomitized zones. The general s t r u c t u r a l l o c a l i z a t i o n of ore within dolomitized zones i s probably f u r t h e r modified by the favorable c h a r a c t e r i s t i c s which are a t t r i b u t e d to dolomite as a host rock. From both the poin t of v±evr of t h e i r magnesium content and t h e i r amenability to the release of magnesium, dolomites or dolomitic limestones provide a most adequate source of magnesium f o r the dolomitization of limestone. The mechanism proposed by Faust f o r the release of magnesium from dolomites and i t s trans-p o r t a t i o n to outlying areas where the dolomitization of limestone i s accomplished f u l f i l s the requirement that dolomitization precedes sulphide mineralization and dolomitizing solutions are directly but not genetically related to magmatic solutions. Field evidence indicates that the mineralized zones of the Jackpot property occur within broad dolomitic envelopes i n lime-stone. The writer's study of specimens from this property indicates that dolomitization preceded the deposition of sulphide minerals. No evidence was found to indicate the manner i n which dolomitization was accomplished. Practically a l l significant sulphide mineraliz-ation i s associated with calcite either within pure dolomite or with-in dolomite i n which s i l i c a t e minerals have been strongly, although not massively, developed. In either case the calcite i s a late min-eral being for the most part at least, post-sulphide i n age. The dedolomitization accompanying the thermal metamorphism of the dolo-mite i s undoubtedly responsible for the presence of some calcite i n the s i l i c a t e zones but i t i s believed that additional calcite has been subsequently introduced. Sections of 'carbonate rock, which from a f i e l d study might be considered dolomitic limestone, when studied i n thin section appear to be pa r t i a l l y "calcified" dolomite rather than par t i a l l y dolomit-ized limestone. The areas surrounding mineralized zones probably were originally completely dolomitized but have been subsequently par t i a l l y replaced by calcite. A dedolomitized dolomite i n the v i c i n i t y of the parent magma of the ore-bearing solutions might be c a l l e d upon as a source f o r the l a t e r c a l c i t e i n both the mineralized and unmineralized zones w i t h i n the dolomite. At a l a t e stage i n the hydrothermal a c t i v i t y , when the magnesia has been l a r g e l y removed from the thermally d i s -sociated dolomite, the hydrothermal solutions might have become r e l a t i v e l y r i c h i n the l e s s soluble calcium carbonate. The calcium carbonate c a r r i e d by these solutions may have replaced dolomite i n s t r u c t u r a l l y favorable zones of the previously dolomitized lime-stone, mineralized zones bein^ e s p e c i a l l y favorable. Suggestions for Future Study From a review of the l i t e r a t u r e pertaining to dolomiti-zation and the relationship of dolomite to lead-zinc deposits, and also from a study of specimens from a lead-sinc deposit occurring i n dolomitized limestone, the writer concludes that the problem of the association of dolomite and ore warrants much further i n v e s t i -gation. Some suggestions w i l l be made with regard to the phases of the problem which most require study and several possible means of acquiring a more complete knowledge of the problem w i l l be pro-posed. One basic requirement i s a knowledge of the process or processes by which limestone i s altered to dolomite. Laboratory experiments have been only moderately successful i n duplicating the apparently simple reaction by which calcium i n limestone i s p a r t i a l l y replaced by magnesium to form dolomite. Additional ex-perimentation i n t h i s phase of the problem i s e s s e n t i a l . Con-t r o l l e d laboratory experiments on the synthesis of dolomite under conditions which might be applicable to i t s formation i n nature could be best accomplished by the cooperative ef f o r t s of chemists and geologists. The f i e l d relationships of dolomite and ore must be stud-ied i n greater d e t a i l . Staining methods are applicable, to some extent, to f i e l d use. General relationships between c a l c i t e , dolo-mite and ore i n diamond d r i l l core and i n reasonably smooth-surfaced rock specimens can be determined i n the f i e l d by the copper n i t r a t e s t a i n i n g method. The determination of general f i e l d r e l a t i o n s h i p s w i l l f a c i l i t a t e the s e l e c t i o n of specimens from representative zones which require d e t a i l e d study. Many d e t a i l e d r e l a t i o n s h i p s can be determined from a binocular microscope study of stained polished surfaces. A more d e t a i l e d study may be accomplished by a petro-graphic examination of stained t h i n s e c t i o n s . Staining i s e s s e n t i a l i f the o v e r a l l p i c t u r e of the a s s o c i a t i o n of c a l c i t e and dolomite i n a t h i n section i s to be gained from a petrographic study. A c a r e f u l study should be made of the r e l a t i o n s h i p of dolomitized zones to possible s t r u c t u r a l c o n t r o l s . For optimum conditions of study, deposits should be selected i n which the s t r u c -t u r a l p i c t u r e i s simple and which e x h i b i t a minimum amount of con-ta c t metamorphic e f f e c t s . I f the s t r u c t u r a l p i c t u r e i s complex or i f the dolomitized zones have been thermally metamorphosed, the r e l a t i o n s h i p of the dolomitized zones to possible s t r u c t u r a l con-t r o l s may be obscured. Appendix Staining Technique Principles Underlying Staining Methods which Distinguish  Calcite from Dolomite The success of stains i n distinguishing c a l c i t e from dolo-mite i s dependent on the following two facts: (1) Calcite has different chemical properties than dolomite. A reagent must be selected which reacts with one mineral only, or reacts with the two minerals at different rates. (2) No complete mineralogical gradation exists between c a l -c i t e and dolomite. I f the two minerals formed a complete isomorphous seri e s , staining methods would probably not provide a sharp index of conroosition. Staining methods i n general employ the s a l t of a strong acid and a weak base, the solution of which acts l i k e a weak acid. The s a l t chosen i s such that upon attacking the c a l c i t e , i t leaves a coating of a compound that i s colored or that can be made to ac-quire a color. According to Rodgers (1°U0, p. 788), a l l rhombohedral carbonates except c a l c i t e react to weak acid i n the same way as dolomite, and the orthorhombic carbonates (except cerussite) react i n the same way as c a l c i t e . Separation can therefore be effected between these two groups. In the calcite-rhodochrosite isomorphous series, d i s t i n c t i o n can be made between pure c a l c i t e and material high i n manganese. The Staining Methods Several s t a i n i n g methods have been devised f o r d i s t i n g -uishing c a l c i t e from dolomite. Some of these stains are more ex-acting than others. Leroy (19U9, p. 162) l i s t s the following f i v e s t a i n i n g methods i n the order of h i s preference: (1) Fairbank's Method (2) Copper N i t r a t e Method (3) S i l v e r Nitrate Method (II.) Lemberg Method (5) Potassium Ferricyanide Method Most writers who have used s t a i n i n g methods to d i s t i n g -uish c a l c i t e from dolomite pr e f e r the Fairbank's method, although i t i s s a i d to have some disadvantages. The color contrast provided by thi s method i s not p a r t i c u l a r l y good i n dark calcareous rocks, es-p e c i a l l y f o r photographic purposes. The f i l m of s t a i n i s s a i d to check and peel i n some cases. Leroy's second choice, the Copper Nit r a t e method, as des-cribed by him, does not make use of the most recent modifications made i n the procedure. The w r i t e r found the modified Copper N i t r a t e method to give excellent s t a i n i n g r e s u l t s . While no other method was used by the writer, some s t a i n i n g r e s u l t s of other methods used by Jory (1950) were observed and considered i n f e r i o r to those obtain-ed from the modified Copper Nitrate method. 70 History of the Copper Ni t r a t e Staining Method Staining by copper n i t r a t e s o l u t i o n appears to have been introduced i n t o the E n g l i s h l i t e r a t u r e by Holmes (1921, p. 267). He, however, found i t necessary to powder the mineral to be tested and b o i l the powdered mineral f o r a few minutes i n a concentrated copper n i t r a t e s o l u t i o n . By t h i s method, the c a l c i t e p a r t i c l e s acquired a strong green c o l o r a t i o n while the dolomite remained un-a f f e c t e d . No reagent was used to f i x the s t a i n . Ross (1935, p. 8) appears to have been the f i r s t to make extensive studies of pol i s h e d surfaces stained by immersion f o r several hours i n a c o l d d i l u t e s o l u t i o n of copper n i t r a t e . A f t e r several hours of immersion i n the s o l u t i o n , the more r e a d i l y attack-ed c a l c i t e acquires a f a i n t blue-green s t a i n of copper carbonate. The f a i n t blue-green s t a i n of copper carbonate so obtained i s i n no way f i x e d to the c a l c i t e but rubs o f f r e a d i l y . The blue-green s t a i n acquired i n the n i t r a t e s o l u t i o n i s washed i n ammonium hyd-roxide, which produces a deep blue s t a i n of C U X N H ^ J J ^ O H ^ on the c a l c i t e . Ross found the s t a i n to be coherent and f i r m . The blue s t a i n provided a d i s t i n c t i v e contrast with other minerals and photo-graphed extremely w e l l . The method provided, i n Ross 1 case, a means of d i s t i n g u i s h i n g pure c a l c i t e from s i d e r i t e , ankerite, rhodochrosite, and manganiferous c a l c i t e , a l l of which are unaffected by copper n i t r a t e . Rodgers (19k0, p. 788), using the same general procedure as Ross, r e f i n e d the s t a i n i n g method and made an e f f o r t to f u r t h e r increase the c o l o r contrast between c a l c i t e ( s t a i n e d blue) and dolo-mite (white). By a s e r i e s of laboratory t e s t s on calcite-dolomite specimens, conducted under v a r i a b l e concentrations of n i t r a t e s o l -u t i o n and v a r i a b l e immersion times, Rodgers a r r i v e d at what he be-l i e v e d to be the optimum conditions f o r s t a i n i n g according to the requirements of the study. Keith (19U6, p. 971), used the copper n i t r a t e s t a i n i n g method as described by Rodgers to d i s t i n g u i s h between c a l c i t e and dolomite. He concluded that t h i s method produced l e s s uniform s t a i n -ing r e s u l t s than the Lemberg method. Staining Procedure as Outlined by Rodgers Rodgers concluded that a molar s o l u t i o n of copper n i t r a t e produced the most s a t i s f a c t o r y r e s u l t s . A molar s o l u t i o n of copper n i t r a t e i s prepared by adding the given amounts of e i t h e r of the following compounds to 1000 gm of water: 188 gm Cu(N0 3) 2 255 gm Gu(N0 3) 2.3H 2 0 332 gm Cu(M03)2.6H20 The specimen to be stained must have a f a i r l y w e l l p o l i s h -ed surface. The p o l i s h e d surface i s immersed i n the n i t r a t e s o l u -t i o n i n such a way that i t i s elevated from the f l o o r of the cont-aining v e s s e l . The p o l i s h e d surface must be f r e e from contamination which might prevent the surface from wetting. I t i s advisable to wet the polished surface p r i o r to immersion so a i r bubbles do not adhere to i t . Rodgers recommends immersion f o r a period of f i v e to s i x hours a t room temperature unless there i s an excess of c a l c i t e . For specimens high i n c a l c i t e , two and one-half to four hours i s s u f f i c i e n t , depending on the purpose of the study. On removal from the n i t r a t e s o l u t i o n , the polished surface i s immersed, without washing and befor drying, i n ammonium hydroxide. A few seconds of immersion i n the ammonium hydroxide i s s u f f i c i e n t to f i x the s t a i n , a longer immersion has no apparent undesirable e f f e c t s . A f t e r the s t a i n i s f i x e d , the specimen i s washed and may be s l i g h t l y buffed to remove any excess p r e c i p i t a t e . A f t e r the ammonia treatment the s t a i n i s permanent and w i l l stand up to considerable b u f f i n g . Dolomite, e s p e c i a l l y a f t e r long immersions i n the n i t r a t e s o l u t i o n , may develop a f a i n t green haze. For optimum conditions of d i s t i n c t i o n between c a l c i t e and dolomite, the c a l c i t e s t a i n should be deep and uniform while the dolomite has not begun to s t a i n on the borders to b l u r r the contacts. Some lack of uniformity i n the coat-ing on the c a l c i t e may be preferable to the l a c k of d i s t i n c t i v e contacts. In specimens cons i s t i n g l a r g e l y of c a l c i t e , long immer-sions i n the n i t r a t e s o l u t i o n r e s u l t s i n a heavy s t a i n and s t r u c -t u r a l d e t a i l s may be obscured. S t r u c t u r a l d e t a i l s are best observed with t h i n but uniform s t a i n s . This s t a i n i n g method can not be r e l i e d upon f o r c a l c i t e -dolomite d i s t i n c t i o n s i n rocks having porous or weathered surfaces. These surfaces soak up the s o l u t i o n and react with the ammonium hydroxide regardless of the mineralogical composition of the rock. On reasonably w e l l polished surfaces of compact rocks the s t a i n i s r e l i a b l e . The s t a i n appears to be very s e l e c t i v e even i n rocks containing very f i n e aggregates of c a l c i t e . The copper n i t r a t e s o l u t i o n i s very stable and i s not af f e c t e d by standing unused. The s o l u t i o n can be used to s t a i n a great many specimens before becoming exhausted, but should be f i l t -ered o c c a s i o n a l l y . The ammonia s o l u t i o n q u i c k l y turns a deep blue c o l o r but t h i s does not cause anomalous s t a i n i n g . The ammonia sol u -t i o n can be used as long as i t smells strongly of ammonia. Observations and Modifications of the Writer The surface of diamond d r i l l core of compact rocks i s s u f f i c i e n t l y smooth to provide diagnostic d i s t i n c t i o n s between c a l -c i t e and dolomite, although smoother surfaces are desirable f o r de-t a i l e d study. Very good s t a i n i n g r e s u l t s were obtained on surface ground v/ith #2l|0 Carborundum and #95 A l o x i t e on a cast i r o n l a p . Specimens ground successively with the two aforementioned abrasives and then h i g h l y polished on a f e l t lap d i d not appear to s t a i n as simply w e l l as those Aground with the abrasives. With equal immersion times i n the n i t r a t e s o l u t i o n , the s t a i n acquired by the more h i g h l y p o l -ished surfaces was thinner and p a l e r i n color than those acquired by the les s h i g h l y polished surfaces. The more h i g h l y p o l i s h e d surfaces probably r e s i s t the wetting a c t i o n of the n i t r a t e s o l u -t i o n to a greater extent than do the rougher surfaces, thus the chemical r e a c t i o n i s retarded. With respect to the strength of the ammonium hydroxide s o l u t i o n that i s used to f i x the s t a i n , Rodgers merely states that a strong s o l u t i o n should be used. The w r i t e r found that very strong solutions tend to dissolv e the s t a i n rather than f i x i t . A rather d i l u t e s o l u t i o n of ammonium hydroxide i n which the odor of ammonia can be d i s t i n c t l y detected was found to be of s u f f i c i e n t strength to f i x the s t a i n . A 2N. s o l u t i o n of NH^OH produced very s a t i s f a c t -ory r e s u l t s . Even greater calcite-dolomite contrast can be attained by etching the polished surfaces i n d i l u t e hydrochloric a c i d p r i o r to s t a i n i n g . Polished surfaces thus treated were immersed f o r about one minute i n f i v e percent hydrochloric a c i d and then stained i n the usual manner. The dolomite, unetched by the aci d , stands out d i s t i n c t l y against the blue stained c a l c i t e of lower r e l i e f . Most sulphide and s i l i c a t e minerals are likewise unaffected by the d i l u t e hydrochloric a c i d and stand out from the c a l c i t e a f t e r etching. The etching process f a c i l i t a t e s the observation of a p a r t i a l three dimen-s i o n a l p i c t u r e of the mineral a s s o c i a t i o n s . The d i s t i n c t i o n between c a l c i t e and unetched minerals can be very e f f e c t i v e l y shown i n photo-graphs of the etched stained surfaces taken under oblique l i g h t i n g . The shadows cast by the unetched minerals e f f e c t i v e l y accentuate the r e l i e f (Plate V B). 75 The Preparation of Stained Thin Sections The copper n i t r a t e s t a i n i n g method provides an equally-diagnostic means of d i s t i n g u i s h i n g c a l c i t e from dolomite i n t h i n section (Plate I I B). The technique used i n preparing stained t h i n sections -will be described here. One side of the rock s l i c e from which the t h i n section i s to be made i s ground and polished. The polished surface of the rock s l i c e i s stained and mounted with Canada balsam on a glass s l i d e with the stained surface f a c i n g the g l a s s . The mounted rock s l i c e i s then ground to the desired thickness from the unstained side and sealed with a cover g l a s s . I f the t h i n s e c t i o n i s stained on the side adjacent to the cover glass a f t e r the se c t i o n has been mounted and ground to the desired thickness, some anomalous s t a i n i n g w i l l probably r e s u l t . Porous areas of the s l i d e or areas from which mineral grains have been plucked w i l l trap the n i t r a t e s o l u t i o n and s t a i n , regardless of the mineral composition. Also, the immersion i n the n i t r a t e s o l u t i o n of the mounted s l i d e may di s i n t e g r a t e the Canada balsam. The stained c a l c i t e has a b l u i s h green c o l o r when viewed i n t h i n s e c t i o n . Presumably the yellow t i n t of Canada balsam super-imposed on the blue s t a i n produces t h i s greenish c o l o r . The t e x t -u r a l r e l a t i o n s of c a l c i t e aggregations are l a r g e l y obscured by the almost opaque s t a i n (Plate I I B). 76 Merits of the Copper N i t r a t e Staining Method (1) I t appears to be as d e l i c a t e as other methods (2) The s t a i n does not shrink or crack (3) The s t a i n i s u s u a l l y very uniform (U) The s t a i n does not rub o f f e a s i l y (5) The s t a i n i n g can be accomplished a t room temperatures (6) The solutions are e a s i l y prepared and are stable (7) The s t a i n shows up equally w e l l on l i g h t or dark-colored specimens (8) The s t a i n contrasts w e l l with other minerals i n photo-graphs (9) The s t a i n i s appl i c a b l e to both p o l i s h e d rock surfaces and t h i n sections The one obvious disadvantage of t h i s s t a i n i n g method with respect to some of the other methods i s the time required f o r s t a i n -i n g . Staining by some of the other methods requires only a few min-utes. 78 P l a t e I I x 60 A. Specimen J 1 5 - 1 0 5 1 - s t a i n e d t h i n s e c t i o n . C a l c i t e ( b l u e ) cuts and replaces an o p t i c a l l y continuous dolomite g r a i n ( w h i t e ) . Pyrite(yel3ow) i s c l o s e l y a s s ociated with a g r a i n of dolomite. B. Specimen J 1 5 - 1 0 5 ' - s t a i n e d t h i n s e c t i o n . Photograph shows the a s s o c i a t i o n of c a l c i t e ( b l a c k ) and dolomite(white) i n " c a l c i f i e d " dolomite. A. J a c k p o t s u r f a c e s p e c i m e n - s t a i n e d p o l -i s h e d s u r f a c e . R e s i d u a l b a n d s o f d o l o m i t e ( w h i t e ) i n c a l c i t e ( d a r k g r e y ) . S p h a l e r i t e ( b l a c k ) i s c o n f i n e d t o t h e c a l c i t e a r e a s . A v e i n o f l a t e d o l o m i t e ( u p p e r r i g h t ) c u t s o l d e r d o l o m i t e , c a l c i t e a n d s p h a l e r i t e . x 30 B. J a c k p o t s u r f a c e s p e c i m e n - s t a i n e d p o l -i s h e d s u r f a c e . A n e n l a r g e d s e c t i o n o f t h e d o l o m i t i c p o r t i o n o f t h e s p e c i m e n s h o w n i n P l a t e I I I A. C a l c i t e ( b l a c k ) i s b e l i e v e d t o r e p l a c e d o l o m i t e ( g r e y ) . F i n g e r s o f c a l c i t e p e n e t r a t e d o l o m i t e g r a i n b o u n d a r i e s . P l a t e IV x 15 J a c k p o t s u r f a c e specimen- etched and s t a i n -ed s u r f a c e . E n l a r g e d s e c t i o n o f upper r i g h t hand p o r t i o n o f specimen shown i n P l a t e I I I A. A v e i n o f d o l o m i t e ( w h i t e ) c u t s e a r l i e r d o l o m i t e ( l i g h t g r e y ) , c a l c i t e ( d a r k g r e y ) and s p h a l e r i t e ( b l a c k ) . P l a t e V A. Specimen J 1 5 - 1 5 6 1 - s t a i n e d p o l i s h e d s u r f a c e . M i n e r a l i z e d bands of c a l c i t e (dense l i g h t e r g r e y ) o c c u r w i t h i n d o l o m i t e ( m o t t l e d d a r k e r g r e y ) . The s u l p h i d e s a r e shown t o o c c u r a l m o s t e x c l u s i v e l y w i t h i n the c a l c i t e bands. x 30 B. Specimen J 1 5 - 1 5 6 ' - e t o t e e d and s t a i n e d s u r f a c e . E n l a r g e d p o r t i o n o f m i n e r a l i z e d band shown i n P l a t e V A. The s u l p h i d e s s p h a l e r i t e ( b l a c k ) and p y r r h o t i t e ( g r e y s hagreen s u r f a c e ) o c c u r i n most cases c l o s e l y a s s o c i a t e d w i t h b l e b s o f d o l o m i t e ( g r e y smooth s u r f a c e ) i n a groundmass o f c a l c i t e ( w h i t e ) . B l e b s o f d o l o m i t e and s u l p h i d e m i n e r a l s a r e l e s s commonly i n d i v i d u a l l y embayed by c a l c i t e . 82 Plate VI A. Specimen J 1 5 - 3 2 ? ' - s t a i n e d pol ished surface. Serpentine lenses(black) are shown to be enveloped by halos of c a l c i t e ( w h i t e ) w i t h i n dolomite (grey) . x 15 B. Specimen J 1 5 - 3 2 7 1 - s t a i n e d pol ished surface. An enlarged sect ion of the lower r i g h t hand port ion of the spec-imen shown i n Plate VI A. C a l c i t e v e i n l e t s are seen to transect the serpentine lenses . 83 Plate V I I A. S p e c i m e n J 1 5 - 1 7 1 1 - s t a i n e d t h i n s e c t i o n . The s u l p h i d e s s p h & l e r i t e ( b r c w n ) and p y r i t e ( y e l l o w ) a r e shown i n most c a s e s t o be p a r t i & ] ] y rimmed by d c l o m i t e ( w h i t e ) w i t h i n C E l c i t e ( b l u e ) . A mass o f p y r i t e ( r i ^ h t c e n t e r ) i s s e e n t o r e p l a c e o p t i c a l l y c o n t i n u o u s i s l e n d s o f d o l o m i t e . B. S p e c i m e n J 3 3 - 5 5 4 1 - u n s t a i n e d t h i n s e c t i o n . S p h a ] e r i t e ( b r o w n ) i s shown t o be c l o s e l y a s s o c i a t e d w i t h a mass o f s e r p e n t i n e ( g r e y ) w h i c h c o n t a i n s r e s i d u a l c o r e s o f f o r s t e r i t e . The g r e a t e r p a r t o f t h e c a r b c n a t e ( w h i t e ) i s . b e l i e v e d t c be c a l c i t e . BIBLIOGRAPHY !•• Ambrose,-J.W. (19h3)'- B r u c i t i c limestones and h a s t i n g s i t e syenite near Wakefield Quebec; Trans. Royal Soc. Can., ser. 3 , v o l . 37, sec. U, pp. 9 - 2 2 . 2 . Andrews, K.W. ( 1950): An X-ray examination of a sample of pure c a l c i t e and of s o l i d s o l u t i o n e f f e c t s i n some nat-u r a l c a l c i t e s ; Min. Mag., v o l . 2 9 , no. 208, pp. 8 5 - 9 8 . 3. Bain, G.W. (1936): Mechanics of metasomatism; Ec. Geol. 3 1 ( 5 ) , pp. 505-526. h» Bain, H.F., Van Hise, C.R., (1901): Preliminary report on the lead and Adams, G.I. and zinc deposits of the Ozark u p l i f t ; U.S. Geol. Surv. 22nd Ann, Rept. Pt. 2 , pp. 2 0 9 , 211. 5 . Balconi, M. and F e r r a r i o , L. ( 1939): A c t i o n on dolomite of water r i c h i n carbon dioxide; P e r i d i c o mineral. (Rome) 10 , pp. 397-iiOii 6 . Bar, Otto (1921*): Versuch einer Losung des dolomit problems auf phasentheoretischen Grundlage; Senckkenberg. naturf G e s e l l i , Bd. VI, pp. 116-118. 7. Bastin, E.S. et a l (1939): Contributions to a knowledge of the - lead and zinc deposits of the M i s s i s s i p p i V a l l e y region; Geol. Soc. Amer. Special Paper No. -21+. 8 . Bateman, A.M. (19U6): Economic Mineral Deposits; Wiley and Sons, New York. 9. Behre, C.H. (1937): The Wisconsin lead d i s t r i c t ; Preliminary paper, '.Ec. Geol. 3 2 ( 6 ) , pp. 783-809. 10. Behre, C.H. (I9I4O): Some observations i n ore -•—search; Min. Tech. T.P. 1209, p. 21 . 11 . Berg, L.G. (19U3): Influence of s a l t admixtures on the d i s s o c i a -t i o n of dolomite; Compt. Rend. (Doklady) Acad. S c i . U.R.S.S., v o l . 3 8 , pp. 2l|-27. 1 2 . 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