sczn2_g3-b4 alter engl short lima2012r (1)

L. Fontboté. Alteration, p. 1. 9/17/12

201209


© L. Fontboté (2003)

kaol-alun

residual silica

kaol-ser

Refugio HS Au Deposit, Maricunga Belt, northern Chile.!(Photo LF 1994.10-4/06)

kaol-alun

Typical zonation formed by very acid fluids


Orebody!

Alteration I!

Alteration II!

Orebody!

Alteration I!

Alteration II!

Hydrothermal alteration, which, for the study of primary mineral assemblages is a nuisance, can be very helpful:

(I) as a guide for exploration, (Ii) as an aid to determine the chemical nature of hydrothermal solutions and their sources, and (Iii) to understand the evolution of the hydrothermal system.

The study of active hydrothermal systems (e.g. well studied in New Zealand, Iceland, Yellowstone ...) is an important source of information for studying the formation of hydrothermal deposits.


Hydratation - "base exchange"!


Equilibrium curves experimentally determined in chloride solutions by Meyer and Hemley (1967) for a K2O-Al2O3-SiO2-H2O system; and b Na2O-Al2O3-SiO2-H2O system; as a function of temperature. Quartz present and pressure at 1000 bar. c K2O-Na2O-Al2O3-SiO2-H2O system at 400°C and 1000 bar total pressure; quartz present (After Meyer and Hemley 1967)!

K-Feldspar !KAlSi3O8 Pyrophyllite!Al2Si4O10(OH)2 Musc., Ser. !KAl3Si3O10(OH)2 Kaolinite !Al2Si2O5(OH)4 Albite !NaAlSi3O8 Paragonite !NaAl3Si3O10(OH)2 Pyrophyllite!Al2Si4O10(OH)2 Kaolinite !Al2Si2O5(OH)4!

Evolution of a cooling fluid without SO2

K-feldspar + H+

musc +qtz+ K+

musc + H+

kaol + K+


Equilibrium curves experimentally determined in chloride solutions by Meyer and Hemley (1967) for a K2O-Al2O3-SiO2-H2O system; and b Na2O-Al2O3-SiO2-H2O system; as a function of temperature. Quartz present and pressure at 1000 bar. c K2O-Na2O-Al2O3-SiO2-H2O system at 400°C and 1000 bar total pressure; quartz present (After Meyer and Hemley 1967)!

K-Feldspar !KAlSi3O8 Musk., Ser. !KAl3Si3O10(OH)2 Kaolinite !Al2Si2O5(OH)4 Albite !NaAlSi3O8 Paragonite !NaAl3Si3O10(OH)2 Pyrophyllite!Al2Si4O10(OH)2


K-Feldspat !KAlSi3O8!muscovite !KAl3Si3O10(OH)2!kaolinite !Al2Si2O5(OH)4!alunite ! !KAl3(SO4)2(OH)6!

Stability relations of alunite, kaolinite, muscovite and K-feldspar as a function of the activities of K2SO4 and H2SO4 (After Hemley et al. 1969)!


Diagramatic representation of hydrothermal alteration in rocks containing A dominant alkali-feldspar and B dominant calcic plagioclase (after Hemley and Jones, 1964 in Pirajno, 1992)!

Albite !NaAlSi3O8!

Montmorrillonite !

(Na,Ca)0,3(Al,Mg)2Si4O10(OH)2·n(H2O)!

Paragonite !NaAl3Si3O10(OH)2!

Kaolinite !Al2Si2O5(OH)4!

Alunite !KAl3(SO4)2(OH)6!

Seedorf, 2008


Seedorf, 2008

NOTE: all alteration types refer to envelopes on veins or volumes of rocks in which envelopes have coalesced or in which alteration is pervasive. Reference is not to large volumes of rocks which may reflect numerous different alteration events. !

Dominant mineral are in bold letters!FELSIC IGNEOUSROCKSalteration type hydrothermal minerals key reactions, or key

assemblagecomments

potassic (PT ou K) bio(phl)-ksp, anh, alb,qtz, (ser, chl, epi, adl,mgt,rut)

plg -> ksp; hbl ->bio/phl, (qtz, rut, mgt,anh); ign bio -> phl,rut;plg -> alb-olg;

PT is rarely pervasive, mostly limited tovnlt envlps. Rarely forms in andesiticwall rocks. Absent in mafic-umfc rocks

biotitic (BI ou EDM, = earlydark mica, Butte))

bio seams, vnlts, bio after hbl(no sec ksp)

hbl -> bio (phl); ign bio ->bio/phl, rut;

a wk form of PT; commonly pervasive in appearance(related to microfrcts); zones out to AC or PR as BIenvlps on fracts)

sodic (NA) olg-act, alb-chl, spn, (sca,cpx, epi)

ksp -> olg/alb; mfcs -> act orpyx & spn or rut

NA alt’n is similar to weak forms of endoskarn, i.e.,it grades into calcic metasomatism, but plg in NA ismore sodic than in endoskarn; NA may grade intoPR. Includes Na-Ca of Dilles and Einaudi (1992).

propylitic (PR ou P) chl, alb, cal, epi, (act,mgt, py, cly, mtm)

felds -> alb & epi, mfcs-> chl & epi

minor to no addition of non-volatilecomponents. Much of literature on B.C.alkalic porphyries refers to “intense PR”,but this probably is NA

includes Na-Ca


intermed argillic (IA) mtm, kao, chl, cly, (ser,epi, py)

plg -> mtm (kao); mfcs -> chl; ksp is stable ormetastable;

A weak form of hydrolysis. Commonsupergene assemblage; much of the IAalt’n thought to be hyopogene in porphdeposits (e..g., Lowell & Guilbert)probably is supergene. Note the generalabsence of SE/IA envelopes on porphyry-style “D-vns”.

sericitic (SE ou S) ser-qtz-py, chl, ill, hem mtm (kao) -> ser; ksp ->ser (qtz); chl -> ser-py

whether SE or IA depends on alt’n state ofksp; if ksp gone, then SE, but don’t forgetthe scale factor--I’m referring to alt’nalong individual fract’s or vnlt’s. Inepithermal systems, SE is dominated byill.

adv’d argillic (AA) kao, pyo, qtz, sil, chd,py, hem, alu, (dik, zuy,adl, toz, tou, dia, cor)

ser -> kaol/pyroph; ser -> alunite; commonlyabundant py

The most intense form of hydrolysis (H-metasomatism). Mention of kaol alonewon’t do it, for it could be supergene orpart of an IA assemblage. Alunite is acommon campanion of AA, but again donot rely on this phase for classification forit could be supergene. In peraluminousgranitoids, intense hydrolysis starts athigher T (greisens).




Magmatic &

hydrothermal biotite -  Magmatic (“book”) biotite

-  Hydrothermal (“shreddy”) biotite: disordered appareance, alteration

product on another mineral, commonly amphibol or another

ferromagnesian mineral


Potassic alteration: Intimately associated with initial

stockwork veining at T>400°C

Cation exchange between cooling magmatic fluid and wall rocks

NaAlSi3O8 + K+ = KAlSi3O8 + Na+ rock fluid rock fluid


FUS-93-8

albite-oligoclase rims

actinolite


Quartz-sericite veins (D-veins), Yerington, Nevada, USA


Quartz-sericite veins (D-veins), El Salvador, Chile

HS for instance some polymetallic Cordilleran

or Butte-type veins

Einaudi, M, (1996)

A-B qtz veinlets

D veins


Veinlets in porpyhry systems

(according to Einaudi, 1996; based son the work by Gustafson and Hunt (1975) on El Salvador, Chile)

Note the preserved fabric

Sierra Gorda, North Chile



Typical vuggy (or residual) silica in the central part of the Marcapunta Au-Ag prospect, Colquijirca District, Central Peru.!

(Photo 1998.3/11) !© L. Fontboté (2003)



K2Al6(SO4)4(OH)12




Yerington skarn, Nevada,

sczn2_g3-b4 alter engl short lima2012r (1)

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