Evidence that barelycontained nearlyexplosive venting of large volumes
Evidence that barely-contained, nearly-explosive, venting of large volumes of magmatic volatiles initiated porphyry Cu formation A Presentation to the MDRU February 16, 2012 Vancouver, British Columbia by Lawrence M. Cathles Cornell University Cathles and Shannon, EPSL 262 2007
Butte Montana: The richest hole on earth Big Butte Silicified Edge 4 km Diam ignimbr vent Headframes to Main Stage Veins Continental Pit Pre-main stage: in interval 66. 4 -64. 8 Ma veinlets vein halos Main-stage in interval 65 -62. 5 Ma ~3 m wide veins Host: 76 Ma Butte quartz monzonite
K alteration at Pre-main stage Butte Titley (1982) Most extensive alteration in porphyry systems Hb + K 2 SO 4 + H 2 SO 4 → bio + anhy + atz (@ 600˚C) Brimhall, 1977) 1800 ft rperv= 1220 m Hb Dr = 381 m sh io e m Fr o B 30 2 n r= m EDM Hb+K bio at 600°C Rock still plastic Aplite dikes →EDM Veinlets every ~5 cm i +1 o, n w o t% H K b GS acid attack 1800 ft H=7 k B 4000 ft elevation 4000 ft Valt = 1. 4 x 1010 m 3 D transition zone Sea level s 1 ille rts D e hn Rob o J er aft 5 97
Pervasive biotitization of hornblend “At Butte, Steve Roberts defined and mapped a zone of pervasive biotitization of hornblend, wherein BQM hornblend is converted to biotite, qtz, and anhydrite. Within this zone other BQM minerals are essentially unaffected, e. g. , plagioclase and K-feldspar are the same as in the fresh Butte Quartz Monzonite (BQM)…” (M. Reed, pc, 2006) hb Ca-poor, Fe-rich ferroactinolite Bio + qtz + anhydrite annite anhydrite quartz Hypothesis: biotitization of hb driven by diffusion of magmatic K, SO 2 from fractures to sites of hb within BQM at earliest stages of mineralization ?
Butte K Alteration was produced in ~900 years within a much larger volume of rock heated to ~600°C Dr m transition zone b H no K , Bio wt% +1 rperv= 1220 m 30 =2 1800 ft Hb sh io B no 4000 ft elevation m 3 e Fr Valt = 1. 4 x 1010 1. Alteration volume measures total fluid vented 2. Taper(S/Dr) measures flow rate through vein 3. Division gives duration of venting
Leached Rims on Ore Fragments Cathles, 1979
Theory = two mass balances…. . DE[m 2/yr] = effective diffsn const [-] = m 3 fluid to alter m 3 rock = v[m 3/m/yr] = volume flux in vein assumes taper is linear Mass Balance #2 Mass Balance #1 moles c in pores moles c to alter rock ½ z f r f Cc + rr G z moles in = moles consumed = average flux Cc from 0 to t Note- don’t need to know aperature of vein
Some further relations…. m 3 fluid per m of fracture
Numeric verification ar e lin 1 T=600 C rf=400 kg/m 3 rr=2712 kg/m 3 t=5 f=0. 01 S=0. 02 m D=1. 19 m 2/yr 2 3 10 2 50 (125 m 3/m/yr) 1. 48 yr 3 318 m/yr 1 2. 1 -5 0 1 x 1 taper is linear t=0. 45 yr t=4. 5 yr 100 t/m/yr
3 D relationships…. . fluid Vol needed m 3 fluid/s rock volume Vol fluid/vol rk m 3 fluid/mvein-s Assumes rpervasive>>Dr mvein/m 2 bounding surface
Time to produce potassic alteration at Butte Hb h es Bio Fr Dr 30 =2 Hb 1800 ft , no K Bio wt% +1 rperv= 1220 m m Source intrusion 8. 3 km , , 5% magmatic venting porphyry no 4000 ft elevation transition zone Dr=1250 ft = 380 m rpervasive = 4000 ft = 1220 m DKrock = 1 wt% (Dilles, pc, 2007) Kfluid = 0. 3 wt % (Rusk et al. , 2004) DE= 5. 23 x 10 -5 m 2/yr S = 2. 35 cm factive = 0. 3
~Volume Heated to 600°C • Very large rock volume heated to 600°C in 1000 yrs • Potassic alteration was an isothermal reaction front • Porphyries and acid alteration retrograde • Exploration target?
Flow shut off before vent areas cooled NO VEINS EVER FORMED IN CORE OF SYSTEM Ac id As T Al te ra tio n • Very large rock volume heated to 600°C in 1000 yrs • Potassic alteration was an isothermal reaction front • Porphyries and acid alteration retrograde • Exploration target?
Mineralization and acid alteration retrograde Hot , flui dn ot a no halos ever formed cid crkl Coo ler, EDM As T Q-mo Ac id fluid very acid BQ Al te ra tio n EDM PGS Flow shut off before vent areas cooled NO VEINSEVER FORMED IN CORE OF SYSTEM GS
Pre-main stage: barely-contained explosion Main stage ruptured • Main stage Butte ruptured (chalcocite veins 3 m wide) • Pinotubo porphyries exploaded • Pebble dikes Headframes to Main Stage Veins Continental Pit (pre-main stage)
2 S 1. 6 5. 6 10 cm t 1000 3190 yrs 80 factive 1/10 1/3 1/2 t 3333 1000 667 2 S Uncertainties Geiger et al (2002) f 0. 1 0. 5 1% t 100 30 19 t 16, 700 1000 317 yrs [K]800°C Deformation 0. 7 [K]600°C f = 0. 3 Giggenbach (1984), const. Na Venting at max already waining 1000 years remains a reasonable estimate
Conclusions • • • Initial volatile release very rapid (1000 yrs Butte) Intensively fractures host (dilation, uplift) Potassically enriches very large volume Heats to ~600°C an even larger volume Mineralization and acid alteration occurs as venting wanes and system cools Explains deposit facies (pegmatitic crackel veinlets>EDM->QS, large vein and veinlet) and tendency to explode (Main stage Butte, Pinotubo) Implications (magma dynamics, exploration (thml halo, …) Issues- how vent so rapidly (latent heats, etc. ) Utility of back-of-envelope calculations
References Cathles, L. M. , and Shannon, R. , 2007, How potassium silicate alteration suggests the formation of porphyry ore deposits begins with the nearly explosive but barren expulsion of large volumes of magmatic water, Earth Planet. Sci. Lett. , 262 p 92 -108. Geiger, S. , Haggerty, R. , Dilles, J. H. , Reed, M. H. , and Matthai, S. K. , 2002, New insights from reactive transport modeling: the formation of the sericite vein envelopes during early hydrothermal alteration at Butte, Montana, Geofluids, 2, p. 185 -201 Rusk, B. G. , Reed, M. H. , Dilles, J. H. , Klemm, L. M. , and Heinrich, C. A. , 2004, Compositions of magmatic hydrothermal fluids determined by LA-ICP-MS of fluid inclusions from the porphyry copper-molybdenum deposit at Butte, MT, Chemical Geology, 210, p. 173 -199. Rusk, B, Reed, M. , 2004, Butte Excursion Guidebook prepared for Penrose Conference held at Yellowstone National Park. Rusk, B, 2004, Fluid evolution in the hydrothermal system that formed the porphyry copper-molybdenum deposit at Butte, Montana, powerpoint presentation prepared for the Yellowstone Penrose Conference subsequently distributed on the web.
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