ECONOMIC GEOLOGY WITH WHICH IS INCORPORATED THE AMERICAN GEOLOGIST VOL. XIV DECEMBER, I9I 9 No. 8 PRIMARY (HYPOGENE) SULPHATE MINERALS IN ORE DEPOSITS2 B. S. BUTLER. CONTENTS. PAOE. Introduction ......................................................... Sulphate Minerals .................................................... Character of Ore Solutions as Indicated by Volcanic Emanations ....... 590 Conditions of Format'ion of Sulphur Trioxide ......................... 593 Formation of Sulphuric Acid in Nature ............................... 594 Formation of Primary Sulphate Minerals ............................. 596 Summary ............................................................ 6o8 Acknowledgment .................................. -................... 609 INTRODUCTION. In studyingore depositsthe writer has severaltimes been con.- fronted with the problemof accountingfor the formation of sulphateminerals that are apparentlyprimary. These minerals occur at placeswhere no evidencecan be found of their forma- tion by surface oxidation. Either they were formed directly 'by igneousemanations or, if they were formed by surfaceagencies, those agenciesmust have effectedthe depositionof other asso- ciatedminerals that would ordinarily •beregarded as of hypogene origin. An attemptwill be madeto analyzethis problemby start- ing with the belief, 'basedon deductionsfrom field observations, that the sulphatesin certaindeposits have •een formed directlyby x Published by permission of the Director, United State Geological Survey. 58x 5.82 B. S. BUTLER. igneousemanations. To avoidcomplications the occurrenceof the mineralsin depositsthat are not generallyregarded as associated with igneousrock is discussedonly incidentally. It is recognized that nearly all the sulphatesthat occur as apparently primary mineralsin ore depositsassociated with igneousrock have also been formed where they are not associatedwith igneous rocks. It is likewise recognizedthat in the formations of at least some of the primary sulphatesin ore depositsthat are associatedwith igneousrocks surface agenciesmay have played a part. The general conclusionreached is that some of the metals which at high temperatureare combinedwith oxygen,as the temperature is reduced,give up oxygen,and 'boththe metalsand the oxygen combinewith sulphur, producingsulphides of the metals and oxides of sulphur. At suitabletemperatures the sulphitesand sulphatesare doubtlessformed and the lesssoluble sulphates are deposited. Moreover, it is believedthat certain conditionslead to the formation of free sulphuric acid which, on reaction with potassium-aluminumrocks, forms alunite. SULPHATE MINERALS. Brief summaries.of the occurrenceof the principal primary sulphateminerals in igneousrocks or of the sulphateminerals that are apparentlyprimary in veins which are believedto be associatedwith igneousrocks are given below. No attempt is made to includein this summaryoccurrence of theseminerals in other relations,or to give a completelist of localities. Hauynite and Noselite.--tlauynite, Na2Ca(NaSO4' A1) A12Sia- O• and noselite,Na4(NaSO4'A1)'Al•SiaO1• are primary min- erals in certain rather uncommontypes of eruptive rocks such as thoseof the CrippleCreek volcano. It is noteworthythat these minerals are characteristiccomponents of volcanic rocks,whereas the closelyrelated chlorideminerals are componentsof plutonic or deep-seatedrocks. Barite.--Barite, BaSO4_,is perhapsthe most commonprimary sulphatevein mineral. It occursusually if not invariably in PRIM.4RI / SULPH.4TE MINER.4LS IN ORE DEPOSITS. 583 'depositsformed at moderateand low temperature2 or amongthe laterminerals of depositsthat containminerals deposited at high temperature. Barite also occurswhere it is not known to be relatedto igneousrocks. It is difficultto determineeven approximately the temperature at which any mineral depositwas formed, but Lindgrena has grouped the minerals in such deposits.in three classes--those formedat low temperature(200 ø C. or less,perhaps much less), at moderatetemperatures (t75 ø to 3ooøC.), and at high tem- peratures(300 ø to 575ø C.). Contact depositsmay have been formed at much higher temperaturesnear that of magmas,which range from 8ooø to •4ooø C. ,'Inhydrite.--Anhydrite,CaSO4, has only recentlybeen recog- nized as a primary vein mineral and its associationsindicate depo- sition at moderate temperature. Anhydrite has the unusual propertyof decreasingin solubilitywith increaseof temperature, beingbut slightlysoluble at 2ooø C.4 It might be supposedthat anhydritewould form most abund- antly in depositsthat replacedlimestone, but the recordedoccur- rencesdo not justify this supposition. Probably the ,abundant ca•'bondioxide that must necessarilybe presentin solutionsthat are replacing limestone inhibits the precipitation of calcium sulphate. Lindgren5 first observedanhydrite as a vein-formingmineral in the Cactus mine, Utah, where it occurs in a vein in monzonite associatedwith tourmaline,hematite, pyrite, chalcopyrite,barite and siderite. Anhydrite, barite and siderite were among the latestminerals to form. Lindgrengave the followingsuggestion' as to the origin of anhydrite: •-Emmons, W. I-I., "A Genetic Classification of Minerals," Ecoa. Gvox.., Vol. 3, p. 6•8, x9o8. a Lindgren, Waldemar, "Mineral Deposits," McGraw Hill Publishing Co., New York, x9x3. 4 Melcher, A. C.,/lm. Chem. Soc. Jour., Vol. 32, pp. 50-66, x9•o. 5Lindgren, Wald.emar, "New Occurrence of Willemite and Anhydrite," Science,.new ser., Vol. 28, p. 933, x9o8. 584 B. S. BUTLER. It is suggestedas a possibilitythat during the later part of mineraliza- tion the anhydrite was precipitated by a reaction between ascending solutionsof sodiumsulphate and descendingsolutions containing calcium carbonate. The writerø acceptedthis interpretation,but with reservations and doubtsthat are amongthe motiveswhich led to the prepara- tion of this paper. Anhydriteoccurs in the Bully Hill district,Calif., in lodesin alaskite porphyry and is associatedwith pyrite, chalcopyrite, sphaleriteand barite. Graton7 apparentlyregards it as having beendeposited directly from ascendingsolutions. Boyle8 alsoregards the gypsum and anhydriteof the Bully Hill district as of deep-seatedorigin, supposingthat the calciumwas derived from the limestonethrough which the solutionspassed. Anhydriteis reportedfrom the Cobredistrict, Santiago, Cuba. The depositsare in andesitetuff cut by dikesof andesite. The associatedminerals are pyrite, chalcop.yriteand quartz? Geijer1ø describes anhydrite in depositsin Swedenassociated with tremolite, galena,chalcopyrite and pyrite. ß Hewett,TM and Miller and Singewaldx2 have describedthe re- markabledeposits of the Minasragra vanadiummine of Peru. Hewett is quotedby Miller and Singewaldas stating that the Veta Madre, which is a mixture of earthy material, disseminated sulphideof vanadium,and anhydrite,the last largely alteredto gypsumfor I2o feet below the surface,represents shale that 'has 6 Butler, B. S., "Geologyand Ore Depositsof the San Franciscoand Adja- cent Districts, Utah," U.S. Geol. Survey Prof. Paper 80, p. 124, I913. 7 Graton, L. C., "The Occurrenceof Copper in Shasta County, Calif.," U.S. Geol. Survey Bull. 43o, p. IOO, I9m. s Boyle, A. C., Jr., "Geology and Ore Depositsof the Bully Hill Mining District, Calif.," Am. Inst. Min. Eng. Trans., Vol. 48, p. III, I915. • Emerson, E. H., "Geologia de las minas," Bol. de Minas, Cuba, No. 4, PP. 47-52, I918. •o Geyer, Per, Falutraktens berggrund ack malmfyndighet'er, Geol. Survey of Sweden. Aorsbok, I916, p. I56. • Am. Inst. Min. Eng. Trans., Vol. 40, pp. 274-299, 19o9. • Miller, B. L., and Singewald, J. T., "The Mineral Deposits of South America," pp. 487-49x, McGraw-Hill Book Co., Inc., New York, I919. PRIM•IRY SULPH•ITE MINERSILS IN ORE DEPOSITS. 585 'beenmore or lesssaturated by sulphideof vanadiumand replaced by anhydrite. Bastin•a has describedprimary anhydriteand gypsumin the Bradencopper deposits of Chile. The rocksand ores of the dis- . trict recorda complicatedseries of igneousevents, including ex- trusions,intrusions and three distinctperiods of mineralization. The first mineralization producedextensive tourmalinization with the depositionof relativelysmall amountso.f pyrite and chalcopyrite.The mineralsof the secondperiod were mainly quartz, pyrite and chalcopyritewith small amountsof black tourmaline and a little biotite. ß The metallic minera!sof the third period includepyrite, chal- copyrite, bornire, galena, sphalerite, molybdenite,tennantite, enargiteand hfibnerite,and the gangueminerals. siderite, rhodo- chrosite,calcite, anhydrite, gypsum and barite. The mineraliza- tion of the third periodis thoughtby Bastin to have taken place at lower temperaturethan the earlier periodsand was character- ized by solutionof tourmalineas contrastedwith depositionof that mineral in the first two periods. Anhydrite is also presentin the copperdeposits of Cuka- Dulkan at Bor, Serbia24 It is regardedby Lazarevic,however, as secondary. Anhydrite occursmost abundantlywhere it is not associated with either igneousrocks or veins. Gypsum.--Gypsumis a commonmineral in ore depositsbut has doubtlessusually been formed'by the alteration of anhydrite or by reaction'between solutions of sulphuricacid (proc[ucedby the oxidationof sulphides)and calcium-bearingminerals. As already noted,Bastin considers gypsum in the Bradenmine as primary. Adolph Knopf has kindly furnishedthe following note on the occurrenceof gypsumat the Utica mine,Calif. The ore on the 2,•00-foot level of the Utica mine, on the Mother