Occurrence and Alteration of Phosphate Minerals at the Stewart

American Mineralogist, Volume 70, pages 395408, 1985 Occurrenceand alteration of phosphateminerals at the Stewart Pegmatite, Pala District,lSan Diego County, California J,c,MssE. Stncr,svr exo GonooN E. BnowN, Jn. Department of Geology St anford U niuer sity, St anford, Califurnia 94 3 0 5 Abstract Parageiibticrelationships among lithiophilite Li(Mn, Fe)2+ PO4 and its alteration products are describedfor one of the complex granitic pegmatitesin the Pala pegmatitedistrict, San Diego County, California. At the Stewart pegmatite, lithiophilite occurs in the upper- intermediate,microcline-quartz zone while spodumeneand amblygonite are found in the quartz core. The sequenceof primary mineral formation within the pegmatite reflectsan increasein the activities of both lithium speciesand volatile components(phosphorus and fluorine).Extensive alteration oflithiophilite involved oxidation, hydration, and cation leaching, but suprisingly little metasomatism.Secondary minerals presentinclude sicklerite,hureaulite, purpurite, stewartite,phosphosiderite, several incompletely identified phases,and various manganeseoxides. Secondaryphosphates formed during the initial stagesof alteration pseudomorphouslyreplaced lithiophilite as a result of its limited hydrothermalinterac- tion with late-stagepegmatitic fluids. Later membersof the alteration sequencerepresent supergene weathering products. The Stewart pegmatite crystallized from a highly- diflerentiatedgranitic magmaat shallowcrustal depths(about 3-5 km). The lack of extensive metasomaticreplacement, which is so evident among the phosphatemineral assemblagesof other granitic pegmatites,is thought to primarily be a result of the relatively rapid cooling and volatile fluid loss at the shallow depths of formation of the Stewart pegmatite.Under theseconditions, there was little opportunity for metasomaticreaction to take place between lithiophilite and residualpegmatitic fluids. Introduction generalsusceptibility to alteration, the potential usefulness of the phosphateminerals as indicators of changingcon- For almost a century the granitic pegmatitesof southern ditions during the post-crystallizationhistory of a pegma- California have been a well-known source of gemstones tite has long been recognized(Mason, 1941; Fisher, 1958; and other minerals(Kunz, 1905;Merrill, 1914;Donnelly, eerni, 1970;van Wambeke,l97L; Moore, 1982).More- 1936;Jahns and Wright, 1951; Sinkankas,1957; Foord, over, the widespreadoccurrence and diversity of the peg- 1976').ln addition to gem material, some of thesepegma- matitic phosphatesalso provide a rationale for using these tites contain interesting accessoryminerals, The present mineralsto better understandthe detailsof pegmatitecrys- study of the primary phosphatemineral lithiophilite and its tallization. However, the complex paragenesesof these secondary alteration products from the famous Stewart phaseshave only recentlybegun to be elucidated(Moore, pegmatitenear Pala was undertakento better understand 197Q,1971, 1972a, 1981, 1982; Moore and Molin-Case, the occurrenceand paragenesisof phosphateminerals in 1974i Fontan et al., 1976i Fransolet, 1976; Shigley and granitic pegmatites. Brown, 1980;Miicke, 1981;Segeler et al., 1981;London Phosphateminerals such as apatite, amblygonite,lithi- and Burt, l982al. While the broad outlines of phosphate ophilite and others are common minor constituentsof nu- crystal chemistryand the crystal structuresof a number of merousgranitic pegmatites(Moore, 1973,1982\. These peg- phosphateminerals have been establishedas a result of matitic phosphatesare generally found to be altered by theserecent studies, details of the thermodynamicrelation- oxidation, hydration, cation leaching,and metasomaticre- phosphateminerals have yet to be addressed placementreactions. Such alteration is responsiblefor most ships among manner. of the nearly 150 secondaryphosphate species now recog- in a comprehensive (1912) first describedthe phosphate minerals nized from pegmatites.The extent of this phosphatealter- Schaller from Pala, but unfortunatelyhis completestudy of pegma- ation appearsto be somewhatgreater than is the casefor tite mineralogy of this area was never published.Beyond many of the associatedpegmatitic silicates. Because of this some additional information reported by Murdoch (1943) and Jahns and Wright (1951),there has been little subse- I Presentaddress: Research Department, Gemological Institute quent investigation of the Pala phosphates.This is in of America,1660 Stewart Street, Santa Monica, California 904O4. nrarked contrast with the extensiverecent studies of phos- 000H04x/85/0304-{395$02.00 395 3% SHIGLEY AND BROWN: PHOSPHATE MINERALS, STEWAR.TPEGMATI.IE Table 1. Primary and secondaryphosphate minerals from the consistingof hundredsof individual plutons ranging up to Stewartpegmatite. severalkilometers or larger in diameter.These plutons in- trude into prebatholithicmetasedimentary and metavolca- Mineral* Chmlcal fomula** nic rocks. They are composedof older gabbro grading to tonalite, granodiorite, quartz monzonite, younger LlEbaophlIlte Li (Mn, Fe) 2+Po4 and granite; with the first two rock types being the most vol- Slcklerite r,rr_*tunf]-'rel+)roo uminousconstituents. Hureaulite r"y (po3oH) {u", l+turo;oGo4) 2 2 The pegmatitedikes at Pala are generallyconfined to the Purpurlte (m, re) 3+?oo gabbroic plutons of the batholith, and are believedto rep- resent products of the final stagesof its magmatic differ- 3+po4 Heterosite (Fe,Mn) entiation and crystallization (Jahns, 1947; Jahns and Phosphosiderlte r.3+(nzo)zpor, Wright, 1951).The observeduniformity in the attitude and S lewar t ite u"2+1nro)o 2H20 tabular form of these pegmatiteshas {r.]+{on)2e12o) 2(po I ). been taken as evi- Anblygonlre+ 5 (Li,Na)A1(Po4) (F,oH) dencefor their formation by the crystallizationof late-stage residual magmasas vein fillings along sheetlike fractures Trlpllte+q (Mn,re)2+r(Po4) in the older and more competentgabbro. The pegmatites q Trlphylite+ Li(Ie,Mn)- PO4 range from centimetersto metersin thicknessand up to a kilometer in exposedlength. They are composedprimarily List does not include several incompleiely idef,tlfled of feldspars,qvartz, and micas,and most can be considered phosphate phases nored during lhls study. Taken fron Moore (1982) and I'leischer (l983). "simple" pegmatiteson the basis of their homogeneous t Primry phosphate ninerals. Reported to occur by Jahns and Wrtghr (1951), but not internal structure.However, several ofthe larger dikessuch exelned during this study, as the Stewarthave a more complexinternal structureand diverse mineralogy. Lithium minerals and gem materials, as well as the more unusualaccessory minerals such as the phosphates,are virtually restrictedto these"complex" peg- phate minerals from other granitic pegmatites(see Miicke, matites. 1981;Segeler et al., 1981;London and Burt, 1982a).Thus, the purposesof the presentinvestigation were (1) to care- The Stewart pegmatite fully reexamine phosphate the mineralogy of one of the The Stewartpegmatite occurs as an elongateoutcrop of Pala pegmatites to extendSchaller's earlier work in light of light-coloredrock on the south-facingslope of Tourmaline more recent ideas; (2) to use this information to evaluate Queen mountain two kilometers north of Pala. It extends the geologic pegmatite; history of the and (3) to compare over a distanceof a kilometer with a northerly strike and a these minerals with the phosphatesreported from other moderatewesterly dip, and attains a maximum thicknessof granitic pegmatites.The Stewartpegmatite was chosenfor 25 meters.The StewartLithia mine, near the southernend study becauseit contains abundant lithiophilite crystals, of the diko, is a major gem producer(Jahns et al., 1974\. and its interior portions are partly accessiblethrough the While an accurate modal composition is unavailable, existingunderground workings of the StewartLithia mine. Jahns(1953) published the following bulk compositionfor Table 1 lists the phosphateminerals reported from the pegmatite. Data gatheredon the phosphatemineralogy and alteration sequenceare used to interpret aspectsof the overall geologic history of the Stewart pegmatite in the context of the Jahns-Burnhammodel of granitic pegmatite formation (Jahnsand Burnham, 1969;Jahns, 1982). Geologic setting Pala is located in the northwest corner of San Diego County (Fig. 1). Several hundred granitic pegmatites, chiefly dikes, are exposedon the hillsidesimmediately sur- rounding the town. The geology of the area has been de- scribed by Larsen (1948, 1951)and Jahns (1954a 1954b, r979\. Much of the Pala pegmatitedistrict is underlain by ig- neous intrusive rocks of the Cretaceous-agePeninsular Ranges (or Southern California) batholith. U-Pb and K-Ar radiometric age dates for batholithic rocks fall within the 130-90m.y. time span (Banks and Silver, 1969; 20 XIrl Krummenacher et al,, 1975; Dalrymple, 1976). This Fig. 1. Map of San Diego County showing the location of the orogenic-typebatholith is a large,composite intrusive body StewartLithia mine near Pala. SHIGLEY AND BROWN: PHOSPHATE MINERALS, S.TEWARTPEGMATI,IE 397 the southern portion of the pegmatite (in wt.% oxides): SiOz 74.9,AlzO3 14.9, CaO 0.1,NarO 3.6,K2O 5.2,LizO 0.7,F 0.4, H2O 0.4, total 100.2.Major constituentsinclude microcline-perthite, albite, quartz, muscovite, lepidolite, spodumene,amblygonite, and tourmaline,while additional acc€ssoryminerals are rich in Li, P, Be, B, and Mn (for details,see Jahns and Wright, 1951).Within the pegmatite, mineral

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