Canadian Mineralogist Vol. 18, pp. 59-70 (1980) CONTACT-METAMORPHICLAPIS LAZULI: THE ITALIAN MOUNTATNDEPOS|T8, COTORADO D. D. HOGARTH Department ol Geology, University of Ottawa, Ottawa, Ontario KIN 6Ns w. L. cnmrrN Mineralogisk4eologisk Museum, Sa:s' Gate 1, Oslo 5, Norway ABsrRAcr Mots-cl6s:lapis lazuli, Italian Mountain, Colorado, formation Belden, m6tamorphisme de contact, la- In the Italian Mountain area (Colorado), lazurite zurite, m6tasomatisme. formed early, apparently in equilibrium with diop- side, forsterite and perhaps barian phlogopite, whereas grossular formed later. Thomsonite re- INTRoDUcTToN presents a late-stage hydrothermal alteration of lazurite. Thomsonite, chlorite and hisingerite, just Lapis lazuli comprises a group of rare meta- beyond the lapis-lazuli zones, are also late minerals. morphic rocks having lazurite as an essential The deposits resulted from contact metamorphism constituent. The most common type, composed of subgreywacke and dolostone of the Pennsylvanian of coarse-grainedlazurite and diopside and in- Belden Formation by quartz monzonite and quartz diorite of the Tertiary Italian Mountain stocks. Na, terbedded with marble, is remote from any S, Ba, Sr, Cl and F were introduced in aqueous igneousrock that can be related to its formation. magmatic fluids. Early barite may have localized Recently, Kulke (1976a, b) and Hogarth & later barian phlogopite and lazurite. Late introduc- Griffin (1978) proposed that rosks of this type tion of Fe produced the Fe-rich silicates. unusual in Afghanistan and on Baffin Island constitute in lapis lazuli. Peak metamorpbis conditions were meta-evaporitesrecrystallized during high-grade P - 250 bars. T ry 600oC. regional metamorphism. The deposits in Gun- nison County, central Colorado, differ in many Keywords: lapis lazuli, Italian Mountain, Colorado, respects from these. They are comparatively Belden Formation, contact metamorphism, lazurite, metasomatism. fine grained, other minerals are significant, and the deposits lie within the contact-metamorphic Sovtvrlns aureole around an intrusive rock. Several occurrences are known, clustered in Dans la r6gion dite Italian Mountain (Colorado), two localities known as North Italian and Italian la lazurite s'est form6e t6t. vraisemblablement en Mountains. The lapis is restricted to impure Equilibre avec le diopside, la forst6rite et peut-etre marble layers of the Belden Formation @ennsyl- le phlogopite barifdre; le grossulaire a cristallis6 vanian) near their contact with quartz monzonite plus tard. La thomsonite repr6sente un stade tardif and quartz diorite of the Italian Mountain stocks d'alt6ration hydrothermale de la lazurite. Thom- (Tertiary). Locations are given in Figure 1. sonite, que chlorite et hising6rite, min6raux I'on Both localities are within the contact-meta- trouve au-deld des zones i lapis lazuli, sont aussi aureole of the Northern Intrusive Cen- de formation tardive. Ces gisements r6sultent morphic du comprising m6tamorphisme de subgrauwackes et de dolomies tre, the youngest of the three stocks du Belden (Pennsylvanien) au contact de monzo- the Italian Mountain Complex (Cunningham nite et diorite quartzifdres de plutons tertiaires. N4 1976). This northern stock is a composite in- S, Ba, Sr, Cl et F proviennent de fluides magma- trusive body with an outer zone of melanograno- tiques aqueux. Une premiEre g6n6ration de bary- diorite and quartz monzonite. The inner part, tine aurait amorc6 la cristallisation ult6rieure de made up of porphyritic quartz monzonite, is phlogopite barifdre et de lazurite. L'introduction tar- pierced by a plug of finer grained quartz mon- dive du fer a produit des silicates de Fe, rares dans interpreted as a vent facies by Cunning' le lapis lazuli. Les zonite, conditions m6tamorphiques ont (Fig. 1) atteint environ 250 bars et 600oC. ham. The Italian Mountain occurrences probably were also affected by contact metamor- (Traduit par la R6daction) phism during the intrusion of the quartz diorite 59 60 THE CANADIAN MINERALOGIST Frc. 1. Geology and lapis lazuli occurrences in the Italian Mountain area. The oorth-trending faults are part of the bastle Cieek Fault system. Geology aftei Cunningham (1973, 1976), with minor altera- iions after Truebe (1977) and peisonal observaiions by D.D. Hogarth. Drawing by E.W. Hearn (Uni- versity of Ottawa). from the Central Intrusive Centre, the oldest of fault zone (Frg. 1) predates the intrusive rocks' the three. and probably provided channels for movement The north-northwest-trending Castle Creek of fluiAs during and after the metamorphism. CONTACT.METAMORPHIC LAPIS LAZULI 61 The faults were reactivated in late Cenozoic time tain about llz km south of the Anderson oc- (Cunningham 1976). currence. Lapis lazuli is found in place at the upper occurrence(U in Fig. 1), prob- North ltalian Mountain and in float ably derived from nearby bedrock at the middle Locality A, the Anderson occurrence, has (M) and lower (L) @currences; adjacent blue produced a small amount of gem-quality lapis marble, in situ, contains scattered lazurite grains. lazuli over a number of years (Rosencrans1941, The upper occurrence is the largest and best Truebe 1977), The productive horizon has been exposed. Here lazurite occurs rn marble over stripped in an east-northeastdirection for 30 m. a width of 1.5 m but lapis lazuli of good color A pit, said to expose some lapis lazuli but now does not appear to exceed 2.5 bm in width. filled with debris, lies on strike 17 m to the west. The lazurite-bearinglayer can be followed about The rock dips 30 to 40o to the north and into 30 m down the slope of the mountain. It is cut the slope. Lapis lazuli of varying color is re- off by a fault to the east, but lazurite reappears stricted to a layer of dark tactite one m thick. in blue marble (specimen U20, Table 1) 15 m This layer is itself divided into centimetric to still further to the east. millimetric lazurite-rich sublayers alternating with lazurite-free horizons. MINERALoGY Lapis lazuli also occurs at two nearby local- ities. To the south, blocks of lapis are found The Italian Mountain peals have long been in a talus slope within an area of 0.4 hectares.At known as prolific mineral localities. Cross & one point (ocality B) an exposure of pale lapis Shannon (1927) described 28 minerals from several metres across appears to be in place. tastite developed in the Belden limestone. Al- The closest outcrop of the Italian Mountain though lazurite was not noted, the general as- stock is 40 m away. semblage is closely similar to that described The other locality, the Christopher occurrence, below. Grossular, diopside, scapolite,thomsonite, is on a spur of the steep northeast-facing slope sphene, calcite, pyrite and barite, described by of the mountain (ocality C). The rock here is Cross & Shannon,were also noted in our sfudies. intensely brecciated and composed of contorted The lapis lazuli occurrences comprise an in- fragments of limestone and mudstone cemented teresting mineral suite, but the size of grains and by coarse carbonate. It was mapped as part of their morphological developmentare no rival for the Castle Creek fault zone by Cunningham the minerals described by Cross & Shannon. (1976). Somewhat separated blocks of white, Rather, the grains are roundish to irregular and lazurite-bearing silicated limestone up to 25 cm microcrystalline, with individuals down to and long are found in the breccia over an area of beyond the resolution of the microscope. Speci- 8 m2. Some of the pieces contain sky-blue mens from the Christopher occurrence are ex- patches, but normally the color is very pale. tremely fine grained and quite unsuitable for Yellow and grey bloc*s are lazurite-free. point counting. The size of grains was a prime consideration in selecting specimensfor misro- Italian Mountain probe analysis. The Truebe occurrencesare on Italian Moun- From 54 thin-sectioned specimens of the lapis TAALE 1. UODAI ANALYSES OF HAIBIA. LAPIS IAZULI AND TAqIITE FR.M IrAIIEN Uq'NIAIN AiEA North ltali.u ltoutaLn Itali,e liloqtaLn A2 A4 AI0 A25 826 C2 C3 920 u2 3 '924 Diopaide Fo!sterlts 0.0 ]re. r Jzz.s]rz.o ]u.s )u.u )r.t jza.7 Iizrlte 0.0 Iho@onlts 12.3 ),., )rs.a \z.e ho.s ):.r ]zr.s Irr.2 Phlogoptt€ 11.3 0.0 0.0 0,4 0.0 0.0 9 Geaaularite 0.0 0.0 0.9 0.0 0.0 0.0 4.o 0.0 SaIDiLt9 0.0 h& 0.0 26-7 0.0 C:hlorit€ 0.0 gL6LDgerite 0.0 0.0 0.0 0.0 0.0 0.0 {.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Pyrlte 6 PyEhotlt€ LT.2 0.1 6.0 1.8 3.9 6 glEcl@a-pr€flxos A, B, C sdl U @lroslEnd to l@atlotra i.n Flgue l. l? Nodulo-ln 1l9ht yeII@ tactLte, 0.3 n abovE tapis lazult iayer. CoUect€d.. A4 Pale blue lapis lazull. WeaC €nil E6nch. Colleat€d. Al0 Brtgbt blu€ lapls lazulL. 10096 @t'orial oa slae of, t!e!ch. @ltscteal. 425 PaL6 blE-gteen 1ap16 lazuu.. at. Ada@, dlonor. 826 Pals blu6 lapLa Lazull. Contle of s@ll outcep. Collsctedt. q? g"iy tactLte. t@dlately 6a6t of tapL6-tazutL- itetpsLe. Clttecled. C3 Palo ble lapLs lazull f,&D pqt o! outcrop rtcbsst t! lazrlts, Cou.gct€d. U20 Bl@ @!ble. I0 E €ast of lapls lazqll. !. frueb€, do@. u23 Palo blE lapls lazul.l. ptt 25. collsctgd. O24 Brtgbt bl@ lapl,s Lazutl. plt 24. q. trEbo, doror. 62 THE CANADIAN MINERALOGIST All 10 specimens, as well as 2 specimens of relatively unaltered Belden Formation, were analyzed for 19 constituents by wet or instru- mental methods under the direction of E.J. Brooker, X-ray Assay Laboratories, Toronto. Lazurite crystals are optically isotropic and normally anhedral or ovoid, although tiny dode- cahedra were noted in thin section U24 (Italian Mountain) and in a thin section from locality B (North Italian Mountain).