A-type granite plutons and tin skarns in southeast Yukon: Mindy prospect and surrounding granites of 105C/9
Tim Liverton Watson Lake, Yukon
Liverton, T., 2016. A-type granite plutons and tin skarns in southeast Yukon: Mindy prospect and surrounding granite of 105C/9. In: Yukon Exploration and Geology 2015, K.E. MacFarlane and M.G. Nordling (eds.), Yukon Geological Survey, p. 151-164.
ABSTRACT In the southeast Yukon, immediately southwest of the mid-Cretaceous Cassiar suite plutons,is a northwest-trending suite of anorogenic one-mica granites called the Seagull suite. This suite is comprised of the Seagull and Hake batholiths, Ork and Thirtymile stocks and an un-named intrusion to the northwest. These B and F enriched granites are associated with various forms of tin mineralization, including skarns. The Mindy prospect in the Thirtymile Range contains a variety of metasomatic silicate and borate and fluoride minerals. Tin (Sn) mineralization is found as cassiterite and borate mineral phases. Mapping has shown that faulting active during metamorphism- metsomatism controlled the distribution of the skarn mineralization. Both mineral chemistry and structural control of mineralization have a significant effect on the economic potential of the Mindy prospect.
YUKON EXPLORATION AND GEOLOGY 2015 151 Yukon Geological Research
INTRODUCTION Skarn mineralization may have a variety of tin minerals: cassiterite, vonsenite [3(Fe,Mg)O·2Fe2O3·SnO2·3B2O3], A northwest-trending line of mid-Cretaceous granite 2+ 3+ hulsite [(Fe ,Mg)2·(Fe ,Sn)BO5], nordenskiöldine plutons in southeast Yukon (Seagull and Hake batholiths, [CaSn(BO ) ] and malayaite [CaSn(SiO )] associated with Ork stock, Thirtymile stock and one un-named stock) have 3 2 5 fluoborite [Mg3(F,OH)3BO3]. The borates and silicates been recognized as being distinct from the larger Cassiar are metallurgically refractory and only the cassiterite is suite of intrusions that are immediately to the east of the of economic interest; as a result, a total Sn analysis may Seagull suite trend. They are one-mica monzo to alkali give a misleading result for evaluation of a prospect. feldspar granites which have similar to A-type plutons The proportion of oxide tin (as cassiterite) to tin borates in chemistry, texture (occasional rapakivi texture) and and fluorides in the Mindy prospect is estimated to be fluorine-boron-tin (F-B-Sn) mineralization. Tin-rich skarns, somewhat greater than 50%. greisens and tin±niobium (Nb) ± tantalum (Ta) sheeted vein stockworks are associated with the Seagull batholith This paper discusses some of the 1992 work plus a but the Thirtymile and Ork stocks have peripheral tin or re-evaluation of Sn mineralogy through examination tungsten (W) skarns. of original thin sections and 23 new thin and polished sections cut from drill core stored in the H.S. Bostock Core The Mindy prospect (Fig. 1), tested with shallow diamond Library. drilling by Newmont Mining Corp. in 1981, is a greisen- altered skarn characterized by primary, high temperature calc-silicate assemblages that have been formed in, and GRANITES peripheral to, extensional faults above a buried stock and The Thirtymile Range contains mid-Cretaceous (101 Ma; have been replaced by iron-rich greisenized B-F-Sn skarn Liverton, 1992; Liverton et al., 2001) stocks of mineral assemblages. monzogranite to alkali feldspar granite according to the Streckeisen classification (LeMaitre, 2004). Two lithofacies of biotite-only granite make up the bulk of the outcropping Thirtymile stock to the northwest of the Mindy prospect (Fig. 2). A volumetrically minor hornblende-bearing porphyry forms a disaggregated syn-plutonic dike within the Thirtymile stock and is the only amphibole- bearing lithofacies seen in the pluton and the Seagull suite. A lithium-mica topaz leucogranite crops out at the southeast corner of the Thirtymile stock and forms ladder ALASKA dikes in the biotite-bearing ‘even-grained’ lithofacies. The leucogranite also forms 1-5 m dikes and three sills 65 o N that contain spectacular concentrations of topaz. The Dawson leucogranite may be classified as an alkali feldspar granite sensu stricto since the plagioclase is at least 95% albite. A similar lithofacies forms the just-exposed Ork stock and adjacent dikes that are 2.5 km southeast of the Mindy N.W.T. YUKON prospect. The Thirtymile and Ork leucogranites consist of a quartz-albite-orthoclase-zinnwaldite-topaz-fluorite mineral assemblage contianing accessory fergusonite. MINDY Whitehorse The Thirtymile and Ork stocks, together with the Seagull Watson L. and Hake batholiths, were considered as a sub-suite of the Teslin 60 o N Cassiar intrusions by Liverton (1992), called the Seagull-
o o Thirtymile suite and identified as having A-type affinity (Liverton and Alderton, 1994; Liverton and Botelho, 2001; 135 W 130W Liverton et al., 2005). Subsequent work on the Cretaceous Figure 1. Location of the Mindy prospect. granites of the Northern Cordillera added a further pluton to the NW as being part of this suite, renamed
152 YUKON EXPLORATION AND GEOLOGY 2015 Liverton - A-type granite and tin skarns in southeast Yukon: Mindy project
3 ? 1200 13 ? 8 5 ? 1600 12 ? ? 15
640,000E 1300 5 10
82 24 4 THIRTYMILE ? Moraine 20 8 8 5 ? STOCK 10 ? 3 ? ? 5 ? ? 32 ? 6 6 7 8 7 ? ? 20 ? ? 1400 30 8 7 6 25 6 ? 8 ? ? 6 6 ? 10 3 18 18 Min. & alt. 7 NOT 7 8 7 82 1300 6730,000N 88 8 ? 6 MAPPED NAD 27 5 ? 15 8 7 ? 10 10 5 10 8 8 9 4 9 6 18 La of Thrust 4 8 7 4,5 1600 8 Thirtymile 70 4 4 15 8 7 5 e 10 c ra altered t 5 7 9 e 10 bl MC si 7 os 4 P 16 9 10 9 projected NOT MAPPED 4 4 1926 Marble, HF 5 5 24 NOT MAPPED 5 4 5 ? 3 1 9 2 18 4 5 3 15 5 19 3 5 6 4 4 5 5 ? 5 3 20 5 1 1500 3 3 38 1 5 3 3 2 18 2 58 1200 4 3 3 Sheared volcs., 2 10 NOT qtzte, slaty mylonite 3 3 1100 1100 MAPPED 1300 18 2 2 10 3 9 3 5 14 2 10 Moraine ? 15 80 12 4 30 3 87 ? 9 1000 6 22 N 1997? ? 3 ? ? ? ? 20 5 ORK ? ? 19 Skarn
? ? ? 16 21 8
? 3 2 STOCK ? 10 3 float 3 3 6 ? 9 5 10 13 10
3 3
9 ? 3 16 2 ? ? 2 5 4 Moraine 3,4
? 9 26 5 1 0 1 2 3 4 “Ork” 3 ? 3 4 2 “Mindy” 10 2 Tarn ? ? Kilometres Sheared 10 2 15 3 5 ? volcanics ? at summit 9 4 Figure 2. Geology of the Thirtymile Range. Mapping 1987-1988, by T. Liverton. them as simply the Seagull suite, which is recognized remain as a melt to <650°C, producing a compositional as distinct from the Cassiar intrusions, and confirmed trend different to the ‘normal’ quartz enrichment process their anorogenic nature (Rasmussen, 2013). They are (Fig. 4a). Halogen-enriched granites fractionate to albite- metaluminous to weakly peraluminous and conform to enriched compositions, and due to protracted melt/ type A1 of Eby (1992), i.e., they are anorogenic (Fig. 3; aqueous fluid and fluid/vapour fractionation, as well as Liverton and Botelho, 2001). their reduced nature, are able to concentrate high field strength elements (HFSE) into the hydrothermal phase: The Thirtymile and Ork leucogranites are the most notably Sn, Nb, Ta (Lehman 1990). Shallow depth of chemically evolved (i.e., fractionated) granites in the emplacement of the pluton, and being located in the Northern Cordillera (Rasmussen, 2013), having Rb/Sr apical part of a batholith, is essential for the development contents exceeding 3000. They might be considered the of the F-B mineralization. An ‘inheritance’ of those metals plutonic analogue to the topaz rhyolites, such as found (i.e., that the source rocks were anomalous in HFSE) might from Colorado to New Mexico (Christianson et al., 1986). be possible (Lehmann, 1990) but fractionation processes They form the apical portion of batholiths which were in low-temperature reduced melts are most important for emplaced at shallow depth (no more than 2 km), mineralization. In the Seagull suite, halogen enrichment is as indicated by common miarolitic cavities and pod reflected in the unusual mineralogy at the Mindy and Ork pegmatites. These magmas were highly enriched in alkalis, prospects, these are greisenized skarn in the classification Rb, Li, with B and anions (F-, Cl-). This B and halogen of Kwak (1987). enrichment would have allowed the granitic magma to
YUKON EXPLORATION AND GEOLOGY 2015 153 Yukon Geological Research
Analyses of Fe-Li micas ranging from siderophyllite to granite carrying very little total iron is capable of exsolving zinnwaldite from the Seagull suite granites indicate Fe-rich hydrothermal fluid, but this is evident in the a sequence of Li-enrichment in the micas displaying retrograde/greisen stage skarn mineralization at the Mindy progressive fractionation from the least evolved lithofacies, prospect. the Thirtymile porphyry, through the Hake granite, Q Seagull granite, and the Thirtymile biotite granite, to the zinnwaldite of the leucogranite (leucogranite and a biotite-bearing lithofacies of the Thirtymile plutons are shown in Figure 4b,c). Biotite compositions indicate that these granites were distinctly reduced (Fig. 4c): biotite 2+ 2+ 3+ {Fe / (Fe +Fe )} = 0.811-0.965, magma ƒO2 ≤ NNO Quartz buffer (Liverton and Botelho, 2001). The progression to enrichment extremely Li-rich micas (trioctahedral 1M zinnwaldite) may 1
be a particular trait of the anorogenic granite. 2 3 The Li-mica leucogranite of the Mindy stock exhibits Increasing fluorine 4 three forms of hydrothermal mineralization at its southern trend contact: rare cm-scale beryl pegmatite, cm-scale lepidolite- Ab Or topaz greisen veins and, presumably the last event, Li - mica Even - grained Megacrystic Porphyry mm-scale joints that carry hematite. It is remarkable that a 1-4 show minimum - melting compositions for qtz-albite- orthoclase system at 1 kbar with 1 - 4 wt.% fluorine: (data from Manning,1982) Nb Si Lepidolite 80 b 10
Al Fe Zinnwaldite 1 A Si Fe
Biotite
50 40 A2
50 Al 20
1 Y Ce c Li - mica leucogranites Nb 0.9 3 2
0.8 Thirtymile biotite - bearing lithofacies 1 0.7 A FeO (FeO + Fe 0 )
0.6 0 0.1 0.2 Mg Fe* 2 Figure 4. (a) Proportions of quartz-albite-orthoclase for A the various lithofacies of the Thirtymile and Ork stocks compared to experimental data for a fluorine-bearing system. (b) Chemical data for Thirtymile and Ork micas. Y 3Ga Cation proportions of Al-Si-Fe for Fe-Li micas from Figure 3. Eby discriminant plots for anorogenic Thirtymile granites. (c) Chemical data for Thirtymile and granites showing compositions of the Thirtymile and Ork micas. Ferrous iron proportion to Mg/Fe ratio of Ork stocks (Eby, 1992). micas (FeO/(FeO + Fe2O3) vs. Mg/Fe* plot).
154 YUKON EXPLORATION AND GEOLOGY 2015
Liverton - A-type granite and tin skarns in southeast Yukon: Mindy project
1989 survey oriented on oriented survey 1989 Magnetic North Magnetic
T N Long.
Moraine calc - sil. dolomite from drill sections 9
8 1550 39 mylonite 1533.2 Quartzite o 1531.4 87 N dip 4
1533.2 Quartzite Silty cataclasite 1582.3 from sections Newmont 1 00N (1989 survey) 8+00 1+40W Talus 8 1623.6 1600 75 25 2 83 Magnetite H 20 1550 skarn 1566.0 G (float) F Cliff E Talus 1639.0 1571.3 D iff slope cl Moraine 5 of 3
se Ba 72 C 20 Quartzite, arkose 88 and silty cataclasite 1654.3 SURFACE of 40 DDH 81-9 60 o Bottom of upper 1654.7 SPECIMENS skarn at R.L. 1612.5 DDH 81-2 A= 125N, 110W 81-1 Fault interpreted 1660.4 B= 125N, 119W 81-3 (90 o ) 1650 C= 660N, 180W Top D= 670N, 285W E= 685N, 240W F= 690N, 270W 50o DDH 81-4 1581.2 G= 690N, 285W 61 H= 695N, 280W 1657.6 60o Approximate location of Newmont baseline 00E 81-5 Section Moraine Plateau Newmont 5+00N Fault interpreted
Fault interpreted DDH 81-8
1649.7 Faces
1577.5 LEGEND 60 Geological contact, dashed = approximate DDH 81-7 1634.5 from Newmont 1654.4 Pressure-solution cleavage, jointing coordinates 46 52 Small-scale folding: hinges
Skarn and marble float (Nebocat, 1982)
Diamond drill hole: horizontal projection TARN 1640.3 1650 shown for inclined holes (BQWL - 36.5mm 1627.0 51 core) 54 Fault Claim Post
Qtzte Instrument Station, showing R.L. Silty cataclasite R.L. 1575 Cataclasite Stratigraphy: 88 1588.2 83 DDH 81-6 87 Quartzite & arkose (phacoids) in 80 Diops. - act. from Newmont 24 BA skarn coordinates pelitic (silty) matrix. Minor skarns. R.L. Skarn 1617.0 Skarn: diopside/actinolite/pyrrhotite/fluorite
or magnetite/vonsenite/hulsite/fluoborite SE Marble: pure calcite to calc-silicate dolomite
Pelites, chert breccia, phyllonite MINDY PROSPECT 0 100 GEOLOGY AND LOCATION METRES Possible Scale = 1:2000 Metric OF NEWMONT’S 1981 1600 DIAMOND DRILL HOLES NTS 105 C-9, o o Surveyed using theodolite tachymetry. Vertical datum: Mindy Tarn = 1575 metres. 60 37’25” N Lat. 132 19’40” W Long. 00E (1989 survey) Figure 5. Detailed map of the Mindy prospect showing trend of the lower skarn horizon and location of diamond drill holes.
YUKON EXPLORATION AND GEOLOGY 2015 155 Yukon Geological Research
MINDY PROSPECT fluids into the carbonates. These faults have little obvious displacement and likely were associated with granite The Mindy prospect (Fig. 5) consists of sparse exposure intrusion. This is best shown to the west of Mindy where and frequent float of two marble-skarn horizons within a scheelite-bearing diopside skarn is developed alongside siliciclastic rocks that are similar to Proterozoic to lower one such fault (Figs. 2 and 5), which projects through Paleozoic continental assemblages, probably the rifted the mineralized portion of the Mindy prospect. An continental basement for the Yukon-Tanana terrane. interpretation of diamond drill sections (Liverton, 1990) Imbrication of the siliciclastic sequence is likely, and a indicates that the lower carbonate-skarn unit, the only phacoidal fabric predominates throughout the Range. potentially economic horizon, is boudinaged (Fig. 6). Mapping (Liverton, 1992) indicated the presence of at Away from the extensional faults it consists of pure calcite least three east-striking extensional faults that transect marble. On the Mindy property the carbonates locally dip the property and which have been interpreted to be the from 9° to 25° to the east. Two skarn horizons crop out at principal ‘plumbing’ for the introduction of hydrothermal the Mindy prospect. Sections are vertical
Section 00 N faces SE 81-1 81-2 81-3 81-4 81-5 81-8 81-7 81-6 collar in section 1650 F
17m into 105m section into 46m section 16 Sk Sk Sk 25m out 1600 2m out out Sk or marble No skarn 22m Po M M Sk M Sk into ? Sk M M Po Sk M ? 8 27 out Sk Sk M ? ? M App ? Sk 41m Sk roximate crop 1550 M projection of skarn out 96m Sk out in section out plane
Circles indicate where inclined holes cut the section plane. Distance that top and bottom of skarn / marble is projected is indicated. Contacts are interpolated allowing for dip in and out of section plane.
LONGITUDINAL SECTION LITHOLOGIES
Sk = skarn (scale does not permit mineralogy to be shown)
Po = pyrrhotite rich sections
81-1 81-3 81-4 M = marble
140o F 135 o 1650 F
1600 Sk Sk Sk M M Sk M 0 100 Sk Sk M METRES 1550 M Sk V /H = 1 Sk Long. section plane Long. Long. section plane Long.
SECTION THROUGH DRILLHOLES SECTION THROUGH DRILLHOLE 4 1 AND 3 FACING NE FACING NE Figure 6. Longitudinal and cross sections through drillholes 81-1 to 8, of the 1981 Newmont diamond drilling at Mindy prospect.
156 YUKON EXPLORATION AND GEOLOGY 2015 Jo
Jo 20% 47.67 m 6
Di Hed Jo 20% 48.33 m 10 Liverton - A-type granite and tin skarns in southeast Yukon: Mindy project Di Hed Jo 20% 50.94 m The upper skarn consists of vesuvianite-grossular-calcite The process of skarn formation has had an obvious 9 skarn with acicular vesuvianite crystals up to 40 mm long. structural control. Development of successive retrograde No tin mineralization has been noted from this horizon. andDi greisenized skarn assemblages has each accompaniedHed Jo 20% The margins of the coarse skarn grade into aphanitic fracturing51.55 of earlier m skarn. Cataclasis of pyroxene, that has Di ‘calc-silicate hornfels’ (diopside-hedenbergite) in contact a range of compositions from salite to ferrosalite,6 indicates with siliciclastic metsediments, that corresponds to the continuedDi early brecciation. Disequilibrium in compositionHed Jo 20% bimetasomatic skarn type of Kwak (1987). The garnet of over the scale52.05 m of a thin section is common in the central this horizon (approximately 21% andradite) is significantly mineralized part of the skarns (Fig. 7). 2 more aluminous than that of the lower skarn, presumably The most striking texture of later greisenized skarn is reflecting separate protoliths of different chemistry. Di Hed Jo 20% the intricately54.14 m banded ‘wrigglite’ (c.f., Kwak and Askins, The skarn of the lower horizon developed in the major 1981; Kwak, 1987). Wrigglite texture displays contorted16 marble unit adjacent to the extensional faults. Textural layers of magnetite and sulphides with phengite or talc, Di Hed evidence indicates that the original protolith controlled lesser amounts of fluorite, fluoborite and corrodedJo 20% relicts 59.64 m much of the skarn mineralogy, and that migration of many of chondrodite and vesuvianite. The wrigglite zones8 are chemical components during contact metamorphism and discordant to host-rock bedding, hence representing metasomatism was limited. The fine-grained hornfels S-C fractureDi systems and the wrigglite layers themselvesHed may Jo 20% fabric that developed during imbrication is preserved in be cut by60.00 several m generations of microfractures, calc-silicate mineralogy. <1 mm to 3 mm wide that carry phengite, talc and6 scarcer
ferrophengiteDi or serpentine. Hed Jo
Jo 20% Jo 20% 47.67 m 61.00 m 6 21
Di Hed Di Hed Jo 20% Jo 20% 48.33 m 62.12 m 10 41
Di Hed Di Hed Jo 20% Jo 20% 50.94 m 62.60 m 9 23
Di Hed Di Hed Jo 20% Jo 20% 51.55 m 63.86 m Di 6 Di 16
Di Hed Di Hed Jo 20% Jo 20% 52.05 m 67.35 m 2 7
Di Hed Di Hed Jo 20% Jo 20% 54.14 m 79.65 m 16 25
Di Hed Di Hed Jo 20% Jo 20% 59.64 m 80.55 m 8 18
Di Hed Di Hed Jo 20% Jo 20% 60.00 m 81.45 m 6 22
Di Hed Di Hed Figure 7. Proportions of diopside-johannsenite-hedenbergite in skarn pyroxenes from drill hole 81-3, from electron microprobe analyses. Each diagram showsJo 20% the range of compositions across one thin section. Numbers 61.00 m of analyses are shown in boxes to the right. 21
YUKON EXPLORATIONDi AND GEOLOGY 2015 Hed 157 Jo 20% 62.12 m 41
Di Hed Jo 20% 62.60 m 23
Di Hed Jo 20% 63.86 m Di 16
Di Hed Jo 20% 67.35 m 7
Di Hed Jo 20% 79.65 m 25
Di Hed Jo 20% 80.55 m 18
Di Hed Jo 20% 81.45 m 22
Di Hed Yukon Geological Research
Figure 8 gives a graphical log of DDH 2 to illustrate skarn • Early Fe-F-Sn mineralization: replacement of marble by distribution through the marble horizon and Figures 9-13 magnetite-fluorite-cassiterite. illustrate mineralogy, structure and texture of the skarn • Retrograde alteration: tremolite to actinolite along units. fractures altering garnet and pyroxene. The various stages of skarn mineralization are as follows: • Greisen alteration and mineralization: fluoborite • Primary skarn: garnet-vesuvianite, diopside ± larnite, alteration of pyroxene skarns, later (?) alteration wollastonite in calcite marble. Increasing Fe content to massive magnetite-vonsenite-hulsite-fluoborite from pyroxene to hedenbergite. Vein skarns of (often containing relict chondrodite); pyroxene skarn garnet ± arsenopyrite in pure marble. First fluorine replaced by fluoborite-vonsenite; voluminous wrigglite mineralization as chondrodite. skarn. • Late retrograde alteration: serpentine growth along fractures and grain
Qtz Diopside - forsterite - chondrodite boundaries. % Sn LOG (Newmont) Serpentine - magnetite 0.1 0.2 Grey to purple pelitic metasediment - Veins with fluoborite shows quartz layers of irregular selvedges Fluoborite thickness, often in S-folds. Has Pressure- occasional 0.5 - 1.5mm thick pyrrhotite - solution filled fractures at 0 - 30 degrees. Much 78.7 89.82 89.87 pyrrhotite veining from 80.2 - 80.3 Aspy Hulsite Qtz Pyrrhotite
80.3 MINERALOGY Banded pyrrhotite - diopside skarn. Pyrrhotite 81.25 - 81.31 (with minor chalcopyrite) up to 50%. Disharmonic folding (wrigglite) 82.17 - 82.23 Chondrodite in fluorite, mostly banded pyrrhotite in fluorite. Pale green diopside skarn 82.8 82.87 - 82.93 Fractured pyroxene skarn: actinolite - fluorite - phlogopite with pyrrhotite blebs. 83.01 - chlorite replacement
84.03 - 84.11 Relict chondrodite in magnetite and serpentine with fluoborite. Bands of fluoborite and pyrrhotite
Dark Grey, pyrrhotite - rich skarn. Wrigglite: relict diopside, rimmed by actinolite. Pyrrhotite, fluorite, silicate matrix varies from white to green 86.66 - 86.74 ferrophengite, arsenopyrite (very fine grained). Foliation is lenticular 86.78 - 86.82 Brecciated wrigglite. Brucite - phlogopite - magnetite. Zoned cassiterite 86.82 - 86.89 Pyrrhotite-magnetite skarn: garnet replaced by ferrophengite: has talc, or disharmonically folded (wrigglite). fluoborite, phengite. 0.65% Common 0.5mm wide pyrrhotite - filled Mag. One sinuous fluorite-fluoborite-phengite/bandylite vein. Arsenopyrite fractures ar at 45 deg. to core axis. Fine with the pyrrhotite magnetite from 86.8 - 87.7. Fractures // core axis contain fine-grained magnetite-hulsite- 0.61% arsenopyrite from 89.7 - 90.3. 89.82 - 89.83 Relict chondrodite and diopside. Veins have fluoborite selvedges that replace the silicates and contain some pyrrhotite. Centres are fluorite and magnetite with phengite rim and serpentine core. Borates in fractures 89.87 Chondrodite-vonsenite-magnetite-pyrite-fluoborite-talc-serpentine
Calcite with veins of ludwigite. Contact with hulsite - phlogopite / 91.9 White, fine-grained marble 92.00 - 92.06 magnetite-fluoborite wrigglite Ludwigite - mineralized fractures 0 - 10 deg. 92.07 Massive pyrrhotite with minor arsenopyrite and chalcopyrite replacement. 92.45 - 92.49 Chondrodite in pyrrhotite, relict wollastonite and talc Banded pyrrhotite skarn with irregular 92.84 - 92.92 V. little remnant ? chondrodite, skeletal pyrrhotite, in 50% wrigglite magnetite masses. Borates from 93.24 - 93.30 Wrigglite: chondrodite - magnetite - vonsenite - hulsite - fluoborite 93.24 - 93.45 93.60 - 93.66 93.66 - 93.71 Pyrrhotite, arsenopyrite, chalcopyrite replacement. Chondrodite in pyrrhotite, relict wollastonite 94.7 94.70 - 95.17 Banded magnetite - diopside skarn with fluoborite replacement Grey magnetite - fluoborite: ptygmatic pyrrhotite bands 94.89 - 94.96 Coarse chondrodite relicts with magnetite and diopside. Heavy 95.17 at centre of interval fluoborite replacement 95.63 - 95.72 Diopside replaced by fluoborite. a few scheelite crystals. Veins of fluorite and phlogopite cut the fluoborite. Late brucite veins. Pale green diopside skarn. 3mm fluorite 96.10 - 96.18 Wrigglite: opaques (probably largely pyrrhotite) with phengite and fluoborite. masses and pyrrhotite blebs. Some deep green amphibole masses to 15 mm long. 97.44 - 97.51 Coarse (to 5mm) pyroxene with actinolite and ? wollastonite. Hydrochlorborite (?) Veins at 98.04 98.04 - 98.12 Pyroxene skarn: 2mm wide fluorite - hydrochlorborite vein 98.44 at 32 deg. to core axis
Pyrrhotite Dark grey, slightly brecciated chert. P.s. Diopside 99.53 - 99.70 cleavage developed from 99.9 - 100.45 Magnetite 36 deg. 1mm quartz veins at 13 deg. and 0 deg. skarn Figure 8. Log of
101.3 Hulsite skarn horizon from Very brecciated chert. Fractures at 0 - 15 deg, Mindy DDH 81-2: some bearing pyrite. Chloritic sections. Fault zone 93.24 Aspy 93.29 98.04 98.11 102.5 Aspy Diopside - chondrodite - magnetite dip -60°; Az 006°; White quartz upper contact with chert at 75 deg. Tiny fractures at all angles from q Collar R.L. 0 - 60 deg. 103.68 104.0 1660.4 m.
158 YUKON EXPLORATION AND GEOLOGY 2015 Liverton - A-type granite and tin skarns in southeast Yukon: Mindy project
a
b
c
1 mm
Figure 10. Drillhole 81-4, 149.72 m. Plane polarized transmitted light on: ‘stitched’ mosaic of 60 photomicrographs. Sheared and retrograde altered skarn. Garnet in the bottom-right corner and centre, showing some zoning. Diopside is in the bottom-left corner and upper part of the image. Actinolite alteration has followed horizontal fractures which carry fluorite and some serpentine. Three en- echelon tension gashes in the top-left corner are filled with serpentine and quartz. These show evidence of at least six d crack-seal events. Scale bar 1 mm.
Figure 9. (a) Surface sample (670N, 285W): diopside skarn replaced by fluoborite (SW half of the image). Transmitted light, crossed polarizers, 1 mm scale bar. (b) Surface sample (125N, 110W): diopside skarn with remnant carbonate containing actinolite and crystals of (?) larnite. Serpentine alteration has followed cleavages and gain boundaries in the pyroxene. Transmitted light, crossed polarizers, scale bar 1 mm. (c) Surface sample (670N, 285W): detail of the fluoborite field in (9a). Acicular inclusions are vonsenite and the irregular SE-NW vein is serpentine. Plane polarized transmitted light, 0.5 mm scale bar. (d) DDH 81-2 sample (92.45 m): chondrodite in sulphide. Transmitted light, crossed polarizers, scale bar 0.5 mm.
YUKON EXPLORATION AND GEOLOGY 2015 159 Yukon Geological Research
a b
c d
Figure 11. (a) DDH 81-5B (109.06 m): cassiterite in fluorite with magnetite. The section has cut the cassiterite through a geniculate twin. Plane polarized transmitted light, 1 mm scale bar. (b) DDH 81-8 (49.0 m): wrigglite with cassiterite. Pyrite forms the wrigglite texture and the cassiterite is in a matrix of talc and possibly some phengite. Transmitted light, crossed polarizers, scale bar 1 mm. (c) DDH 81-5B (106.75 m): wrigglite texture shown by cassiterite (brown). Fluoborite and fluorite fill the atoll-like structure. The opaques are sulphides. Plane polarized transmitted light, 1 mm scale bar. (d) DDH 81-8 (48.92 m). Borates: vonsenite (acicular) and hulsite (curved). Incident (reflected) light with polarizers at 85°. Scale bar 1 mm.
SKARN CHEMISTRY the main skarn horizon is andradite-grossular, composed of <4% spessartine and <6% almandine components Compositions of pyroxenes, amphiboles, garnets and (the method of Droop (1987) was used to estimate Fe3+). vesuvianite were determined using energy dispersive X-ray The main skarn varies from 58-99% andradite, with the analysis with a Jeol electron microprobe on polished thin exception of the garnet found with vesuvianite of DDH sections and block specimens taken from diamond drill 81-3, 81.45 m (andradite ≈ 18%). core. The most complete section of pyroxene analyses through the lower (main) skarn was obtained from drill Chemical evolution of the skarn metsomatism is core of DDH 81-3. Two populations of pyroxene are interpreted to have begun as a high temperature alteration evident: a tight group of 0-12% hedenbergite (interpreted of the carbonate host rock by mostly silica introduction as the ‘primary’ pyroxene) and a group defined by a wide to produce the larnite high temperature (800°C) calcic variation of compositions from the same stratigraphic skarn. Subsequent garnet-vesuvianite skarns, followed by intervals, interpreted as being formed by successively diopside-rich primary skarns, and then successive iron-rich metasomatic fluid events (Fig. 7). The garnet of Fe-enrichment of the hydrothermal fluid resulted in a
160 YUKON EXPLORATION AND GEOLOGY 2015 Liverton - A-type granite and tin skarns in southeast Yukon: Mindy project range of pyroxene, from salite to hedenbergite; retrograde TIN MINERALIZATION alteration produced amphibole assemblages. Late introduction of HF produced the fluorite/chondrodite Although no quantitative analysis for tin was performed which was followed by the greisenized magnetite- on the silicate phases, the spectra from electron phengite-talc wrigglite skarns that contain the bulk of the microprobe energy-dispersive analyses were frequently tin. Cassiterite was probably first to crystallize in the F-rich examined qualitatively for tin peaks. None were found, massive magnetite stage, and the crystallization of borate- and tin levels in the garnet, pyroxene, amphiboles and fluoride minerals accompanied the later greisen alteration. vesuvianite are expected to be significantly less than The latest retrograde stage is represented by serpentine 0.5%. Tin mineralization does not obviously occur until formation and occasional bandylite [CuB(OH)4Cl] the early greisenized skarn stage, appearing first as (Fig. 12d). Prominent bleached zones as selvages to cassiterite in magnetite-phlogopite altered marble, and fractures in the siliciclastic unit above the lower skarn, later as fluoride-borate minerals accompanying magnetite- fluorite-fluoborite in the wrigglite skarn. The visible tin which have been attributed elsewhere to CaCl2 ‘exhaust solutions’ from metasomatism, indicate that abundant mineralization, as cassiterite, vonsenite or hulsite phases chloride was present with the boron to form this unusual (Fig. 12), accompanies a massive introduction of iron as mineral, bandylite. magnetite and often lesser amounts of pyrrhotite and a b
c d
Figure 12. (a) DDH 81-4 (150.0 m): birefringent, sector-twinned andradite. Transmitted light, crossed polarizers, scale bar 1 mm. (b) DDH 81-5B (105.08 m): microfaulting in skarn. Garnet at the bottom-left, vesuvianite at the top-right and actinolite retrograde alteration along the shear zone. Transmitted light, crossed polarizers, scale bar 1 mm. (c) DDH 81-2 (86.66 m): wrigglite comprised of magnetite and pyrrhotite (pale yellow) in fluorite and fluoborite. Incident (reflected) light with crossed polarizers. Scale bar 1 mm. (d) DDH 81-4 (148.4 m): bandylite (blue, hexagonal) in fluorite and serpentine. Plane polarized transmitted light, 0.1 mm scale bar.
YUKON EXPLORATION AND GEOLOGY 2015 161 Yukon Geological Research
pyrite into the skarn. In one specimen, pyrite alone has as cassiterite was introduced into the granite aureole with been observed to form wrigglite texture with the fluorides the first massive amounts of iron and fluorine. Subsequent and phengite (Fig. 12c). Nordenskiöldine was indicated by greisenization produced exotic minerals such as vonsenite, SEM scan from the sample collected at 685N, 290W hulsite and rare nordenskiöldine. At the Mindy prospect, (Fig. 5). A summary of skarn mineralogies (tin minerals in the mineralization has a structural control: skarns are bold) can be found in Tables 1 and 2. found within and adjacent to extensional faults that presumably formed the ‘plumbing’ for hydrothermal fluids released by a shallowly emplaced Li-mica leucogranite DISCUSSION stock. The approximately 100 Ma Seagull suite has peripheral Only cassiterite is viable as a tin ore, and at the Mindy Sn ± Ta ± Nb mineralization that is particular to southeast prospect only slightly more than half the tin minerals Yukon, and is consistent with the anorogenic nature of present are cassiterite which reduces any potential the intrusions. At the Mindy prospect, B-F rich magma grade. The structural control by faulting active during with a low solidus temperature and shallow depth mineralization has also reduced tonnage potential of the of emplacement allowed protracted melt-aqueous prospect by limiting the areal extent of the skarns. It is fluid interaction and development of a tin-enriched possible that the greater tin target at Mindy was never hydrothermal fluid. This fluorine and boron-rich nature of tested by drilling during the 1981 Newmont program: the A-type granite magma is reflected in greisenized skarn greisens would have been developed in the apical part mineralization at the Mindy prospect. Tin mineralization of the underlying granite, and these might be the larger tin target which is likely to be dominated by cassiterite and possibly contain wolframite. The Seagull batholith (105B/3) has nine tin skarns recorded in the Yukon MINFILE database. Of these, the JC and Val prospects have recorded the presence of borate or silicate tin minerals. Similar skarn prospects are likely to contain borates, but if only total tin is determined during assay, they would have an exaggerated estimate made of economic potential. An accompanying assay of oxide tin is needed to distinguish economically viable minerals (cassiterite) from total tin. Recognition of the structural control on the skarns (which was not emphasized during the 1981 work at Mindy) is also vital.
1 mm
Figure 13. Transmitted light with crossed polarizers: ‘stitched’ mosaic of approximately 45 photomicrographs, Massive magnetite-vonsenite mineralization, containing some chondrodite relicts, that has been cut by various generations of veins. The somewhat ptygmatic horizontal vein has fluoborite along its outer margins and euhedral magnetite in serpentine in the centre. A broad intermediate margin of talc contains corroded chondrodite. Magnetite, talc and fluoborite form the diagonal veins. Scale bar 1 mm.
162 YUKON EXPLORATION AND GEOLOGY 2015 Liverton - A-type granite and tin skarns in southeast Yukon: Mindy project
Table 1. Mineralogy of the skarn samples from 1981 Newmont diamond drilling. Drill hole Interval/Depth (m) Mineralogy 81-1 65.2-65.3 Rock is a quartzite, contains diopside, actinolite 81-1 67.0-67.08 diopside, carbonate, serpentine 81-1 69.55-69.62 diopside, actinolite, serpentine 81-1 73.4-73.49 cassiterite, magnetite, talc 81-1 75.42-75.49 wollastonite, fluoborite,vonsenite 81-1 76.52-76.56 pts diopside, hulsite, pyrrhotite. magnetite 81-1 76.60-76.67 diopside, carbonate, scapolite, magnetite, cassiterite, sulphide, phengite. 81-1 77.88-77.92 pts magnetite, wollastonite, fluoborite,cassiterite. 81-1 88.0-88.90 vesuvianite, diopside, serpentine 81-1 83.9-84.02 vesuvianite, diopside 81-1 88.4-88.61 Rock is a quartzite, contains biotite, cordierite 81-2 89.87 lts chondrodite, vonsenite, magnetite, pyrite, fluoborite, talc, serpentine 81-2 86.66-86.74 pts diopside, actinolite, pyrrhotite, fluorite, ferrophengite, pyrite, chalcopyrite 81-2 86.82-86.89 lts pyrrhotite, ferrophengite, talc, fluoborite, bandylite, arsenopyrite 81-2 89.87 lts chondrodite, vonsenite, magnetite, pyrite, fluoborite, talc, serpentine 81-2 92.45-92.49 pts pyrrhotite, arsenopyrite, chalcopyrite, chondrodite, wollastonite, diopside, talc 81-2 93.66-93.71 pts fluoborite, fluorite, pyrrhotite, arsenopyrite 81-2 96.10-96.18 pyrrhotite, phengite, fluoborite, serpentine. 81-4 70.17-70.25 pts diopside, magnetite, pyrrhotite, arsenopyrite, garnet 81-4 142.50 magnetite, sphalerite, talc, fluoborite, bandylite, phengite 81-4 148.30 pts magnetite, pyrrhotite, fluoborite, fluorite, arsenopyrite ferrophengite, magnetite, pyrrhotite, arsenopyrite, chalcopyrite, bandylite, 81-4 148.40-148.5 pts serpentine 81-4 149.72-149.82 lpts garnet, salite, ferroactinolite, pyrrhotite, fluorite 81-4 150.00 pts garnet, hedenbergite, actinolite, pyrrhotite 81-4 151.15-151.23 pts diopside, pyrrhotite 81-5B 66.72 pts garnet, salite, actinolite, pyrite 81-5B 103.47-103.51 andradite, diopside 81-5B 104.85-104.92 pts actinolite, fluorite,cassiterite, ferrophengite 81-5B 105.08 garnet, vesuvianite, actinolite 81-5B 106.75 pts magnetite, pyrrhotite, chalcopyrite, ferrophengite, cassiterite 81-5B 108.87 pts magnetite, pyrrhotite, arsenopyrite, fluorite, talc,cassiterite 81-5B 109.06-109.11 cassiterite, phengite, quartz, pyrrhotite 81-5B 111.25-111.34 pts fluorite, phengite, fluoborite, quartz, amphibole, pyrrhotite,cassiterite 81-8 48.92-49.0 lpts magnetite, hulsite, vonsenite 81-8 49.00 pts pyrrhotite, vonsenite, hulsite, fluoborite, serpentine 81-8 66.8 pts garnet, diopside Note: pts indicates a polished thin section of 25 x 50 mm, lts a 50 x 75 mm covered thin section, lpts a large polished thin section on a 50 x 75 mm slide. Tin minerals are shown in bold.
Grid location* Mineralogy 125N 110W – 1.5 m diopside, larnite, actinolite, fluorite 125N 110W – 4 m diopside, serpentine, fluorite, actinolite, carbonate 125N 119W – 1.5 m diopside, larnite, calcite, actinolite 660N 180W rock is a quartzite with phacoidal garnet and diopside 660N 180W (2nd slide) diopside, vesuvianite, garnet. 670N 285W diopside, magnetite, garnet, vesuvianite, epidote, borates 685N 240W Rock is a quartzite with a little plagioclase, carbonate layers 690N 270W chlorite, carbonate in siliciclastics, diopside, epidote 690N 285W garnet, vesuvianite, carbonate, pyroxene, sphene, epidote 695N 280W vonsenite Mineralogy of the *Coordinates refer to the original Newmont grid (Fig.5). Distances (e.g., 1.5 m) give depth Table 2. from top of a near vertical face. Tin minerals in bold. skarn hand samples.
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