Skarns
Notes based on Evans Ch 13
• general location and rock types
• calc silicate minerals
• Ts 400-650oC. and lower? (Pb-Zn skarns)
• barran skarns (met pet) metasomatic rocks (from lmst)
calc-silicate hornfels: met. of impure lmst and dol (no metasomatism)
if already mineralized: look like skarns (reaction skarns)
• exo skarns vs endoskarns (c.f. greisens)
generally exo contains more min. where both occur but both may be mineralized.
• Types of exoskarns: calcic (e.g. andradite, diopside) or magnesian (e.g. forsterite)
• Majority of deposits are in calcic exoskarns
• Named on basis of major metal. - Table 13.1.
Cu, W, Fe, Pb-Zn, Mo, Sn
U-REE? Mary Kathleen deposit
• may be associated with endogranitic porphyry-style mineralization,
e.g. vein and greisen deposits. - Bingham
• Zoning - e.g. Fig. 13.1 - in exoskam
also in endoskarn
bt ® amph ® px ® gnt /contact
• Distribution of endoskarns
absent from cupolas with Porphyry-style mineralization.
mainly on peripheries where fluid flow was in or along contact.
original contact may be indistinguishable.
No mention of hydrous and anhydrous skarns.
Examples
• Cu skarns - Copper Canyon, Nevada (Fig. 13.2)
Mem้, Haiti (Fig. 13.3)
• Fe skarns - Sarbai, USSR (Russia) - Fig. 13.4 Siberia
Marmosa, Canada*
• W Skarns - most of world's W production.
King Island, Australia; Salau, France (Pyrenees)
Not that much W worldwide.
King Island Scheelite, Fig. 13.5
mass balance calculations: addition of Si, Fe, Al (Edwards et al.)
loss of Ca and CO2
Kwak. et al: suggest these estimates are in error.
best site for ore = where hornfels in marble (doesn’t explain why hornfels is imp.)
hornfels = pre-skarn contact metamorphism
• Talc and Graphite
graphite: minor from skarns (preexisting C) - reaction skarn?
~ 70% of world talc (Mg6[Si8O20](OH)4)
may be v. high grade: almost 100% talc; ~ 400oC.
Genesis of skarn deposits
(1) isochemical metamorphism
(2) multiple stages of metasomatism
(3) retrograde alteration
(Fig. 13.9)
proximal vs distal skarns
stage 1
marble, hornfels, quartzite
reaction skarns
talc, wollastonite
stage 2
magmatic fluids 400 - 800oC
minerals mainly anhydrous
ore stage
Stage 3
hydrous alteration
epidote, actinolite
sulphides
structurally controlled
may extend beyond skarn into marble
Distal skarns
stages 1 and 2 may not form
210 - 350oC
? "the metam. (1 & 2) development at deep levels is likely to be more extensive than at shallower levels . . . . . "
stage 3 better devel. than 1 and 2 at shallow levels.
(permeability ? )
Sources of "introduced materials"
most = magmatic
easy to supply Si, what about Fe?
Ausable - no intrusion: Fe remobilized from metam. rx
Isotopes: lots of magmatic input
FI : correlation between chemistry and type of pluton
lower Ts for Pb-Zn.
ZONING - Fig. 13.10