Seg Mapping Course

7/28/2019 Seg Mapping Course

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SEG MAPPING COURSEDECEMBER 2012


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DAY 1:Goldfield Au Deposit is a high sulfidation system and sits on the edge of a epithermal system (Qtz,Alunite, Kaolinite) and a porphyry system (Pyrophylite, Diaspore).

Fig. 1: Geologic Map of Goldfield District. Note the pink coloured rock in the middle which is part of a caldeira. It consistsmaily of Rhyolite.

The system is centered by a caldeira consisting of rhyolite (21Ma). This is surrounded by Dacite andOrdovician/Silurian rocks. Fault zones are surrounding the system (see red pins in fig.1), which aresteeply dipping in the North and shallow dipping in the West. Most of the deposits are situated in oraround these faults. Au mineralization sits in silicified areas. These areas are normally surrounded byargilitic alteration and then prophylithic alteration.Main Au is associated with:

1. Quartz2. Sulfide

Sulfides that can be found in this system:- Enargite: Cu 3AsS4 - Luzonite: Cu 3AsS4 (Low T version of Enargite)- Famatinite: Cu 3SbS4 (As from Enargite replaced by Sb)

If you get a As or Sb sulfide, depends on your temperature/geochem.

- Chalcopyrite: CuFeS 2


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- Bismuthinite: Bi 2S3 - Tennatite: Cu 12As4S13 - Tetrahydrite: Cu 12Sb4S13 (Sb replacing As in Tennantite)- Goldfieldite: Cu 12Te 4S13

Cu can be replaced by Ag, Zn, Fe. Zn decreases going to the South. To the north you have moreTe(Goldfieldite), to the SW you have more As(Tennantite) and Sb is everywhere.

- Argentite: Ag2S (very early weathered to Chlorargyrite)- Chlorargyrite: AgCl (Cerargyrite)

If you have a system with As/Sb and very little Fe, it tends to make Energite/Luzonite/Famatinite. No orvery little chalcopyrite will be made. If any Covellite is in the system, this will be very late.

SILLICATESAlunite : KAl3(SO4)2(OH)6 (Alunite is a family of minerals with Al in it)

Figure : Alunite cristals in carbonaceous rock

If Al is replaced by Fe you get Jarosite: KFe 3(SO4)2(OH)6 K can be substituted by Na, Ca, Ba depending on T in the system.Na/K can also be replaced by Argento Jarosite or by H to form Hydronium Jarosite: H 3OFe3(SO4)2(OH)6


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Alunite is a sulfate mineral (lots of Sulfur) and need a lot of water, oxidating and highly acidicenvironment to be formed. (Acid sulfidation systems). You need a lot of sulfur (S 6+) to produce alunite.This environment can only be found in the subsurface.Alunite in hydrothermal systems (replacement for mainly felspars) are white/yellow.In supergene environments its really fine grained, harder, milkier, translucent, glassy and has a greatvariety of colors (fig. 2)

Chemical reaction of feldspar replaced by Aluninte:3KALSi3O8 + 2H

+ + 2SO42- + 2H2S + O2 + 2H2O KAl3(SO4)2(OH)6 + 9SiO2 + 2K

+(aq)

If you add K + on the right you have to balace with H + on the left.

Kaolinite : Al2Si2O5(OH)4 (Also called Dickite which is the high T variety of Kaolinite)K-Mica: KAl3Si3O10(OH)2 (Sericite, Illite)K-Mica with some Fe/Mg will produce Phengite. Biotite can have upto 3% Ti

Figure : Dickite filling fractures in an Andesite unit

Pyrophylite : Al2Si4O10(OH)2Kaolinite + (Si)2 Pyrophylite + H 2O


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Figure : Pyrophylite(yellowish/pearly colored) in carbonaceous rock (looks rectangular in field specimen)

Diaspore : AlOOH (No Silica)


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Figure : Diaspore cristals on a fracture surface. These cristals are rhomboedral and transparent.

The goldsystem at Goldfield is formed at T=250; this is the reason why there is no Arsenopyrite (formedat T=450)

Which system is more sulfidized:a. A system with Enargite: Cu 3AsS4 b. A system with Tenantite: Cu 12As4S13

4Cu3AsS4 Cu12As4S13 + 3S0(g) (Cu = Constant)

To form Enargite, you need Tenantite and 3 mols S, so a system with enargite is more sulfidized. This iswhy enargite is found in the upper layers and Tenantite in the lower layers.

Vuggy Silica: has cavities of Feldspar, Biotite mainly. Even Al gets out of system pH < 2

Staining in rock fractures that are dark (black) with rainbow colored shine (green/purple/blue): thesecolors are produced by Fe-ox/hydrox. Typical of goethite surfaces.

Ferrolysis: Fe 2+(from Py) + H 2O Fe(OH)3(Ferryhydrite) + 3H+


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Dickite vs Kaolinite in the field: Dickite is more pearlish, soapish, yellowish than Kaolinite.Which system is more sulfidized:

a. A system with Bornite: Cu 5FeS4 b. A system with Chalcopyrite: CuFeS 2

5 CuFeS2 + 2S0(g) 4 FeS2 + Cu5FeS4 (Cu = Constant)

You need more Sulfur to form Bornite, so this system is more sulfidized.

2 reaction predominant in magmatic hydrothermal systems:1. Fe2+ Fe3+ 2. S2- SO42- (Alunite, Jarosite, Barite, Anhydrite, Gypsum)

System with anhydrite is sulfate rich. Py-Alunite, Py-Gypsum, Py-Anhydrite are examples of S 2- and SO 42-

in equilibrium.

Oxides:

Prominent source of Py- Goethite: has an enormous pH range in which it is stable, it has a brown color and the streak

color is orange-brown(more brownish)- Jarosite: low T/pH stable mineral, is more yellowish- Hematite: weakly acidic to alkaline, is dark colored, streak is red

Goethite and Hematite form from Ferrihydrite by two different and competitive mechanisms:Goethite crystals form in solution from dissolved Fe 3+ ions produced by the dissolution of Ferrihydrite,whereas Hematite forms through an internal dehydration and rearrangement within the Ferrihydriteaggregates (Schwertmann, 1959; Schwertmann and Fischer, 1966; Fischer and Schwertmann, 1975).Therefore, Goethite should be favored as the concentration of Fe 3+ ions in equilibrium with Ferrihydrite

increases, and hematite should be favored as the concentration decreases. The concentration and formof Fe 3+ ions in equilibrium with Ferrihydrite depend strongly on pH. Goethite is strongly favored wherethe concentration of monovalent Fe 3+ ions, either Fe(OH) 2

+ or Fe(OH) 4-, is at a maximum. The maximum

for Fe(OH) 2+ activity is at pH 4, and that for Fe(OH) 4

-, within the pH range tested at pH 12. On the otherhand, Hematite shows maximum formation where these concentrations are at their minimum, i.e.,around pH 8, which is also the point of zero charge of Ferrihydrite. Below pH 4, although (Fe(OH) 2

+)increases further, it is overridden by the concentration of the divalent Fe(OH) 2+ ions which appear to beless favorable for Goethite crystal growth than the monovalent form. This situation may retard theformation of Goethite but not that of Hematite, so that relatively more Hematite is formed. The lowersuitability of Fe(OH) 2+ compared to Fe(OH) 2

+ can be explained as follows: Fe 3+ ions feeding the growingGoethite crystal must be discharged at the crystal surface before being built into the crystal; thisdischarge is probably easier for monovalent than it is for divalent ions. More Hematite forms as acidityincreases below pH 2. At very high pHs (> I M OH-), where crystallization is very rapid and only goethiteformes, the crystals consist of very thin (-100 A) and long (up to 5/xm) needles, indicating an increasinglystrong retardation of crystal growth in the a- and b-directions and a rapid growth in the c-direction. Inanalogy to the strongly acid range this could indicate the formation of divalent Fe(OH) 5

2- ions under theinfluence of an extremely high (OH -) instead of monovalent Fe(OH) 4

- ions at lower (OH)-, the formerbeing less suitable for crystal growth than the latter. This could explain why the crystallinity ofGoethiteformed at pH 12 was lower than that formed at pH 10 and below, although Goethite was the only phaseto form at pH 12.


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Weathered rock with phenocrists:KaoliniteKfsp Kmica Kaolinite/Dickite: what happened with the K

3KAlSi3O8 + 2H+ KAl3Si3O10(OH)2 + 2K

+(aq) + 6SiO 2(aq) (K-spar to K-mica)KAl3Si3O10(OH)2 + 2H

+ + 3H2O 3Al2Si2O5(OH)4 + 2K+(aq) (K-mica to Kaolinite) +

6KAlSi3O8 + 6H+ + 3H2O 3Al2Si2O5(OH)4 + 2K

+ (K-spar to Kaolinite)

Oxidation of Py:Py Ferryhydrite Goethite/JarositeFe(OH)3 FeOOH + H2OMerian: yellow: more hydrated/red: less hydrated

No Mo in epithermal systems. Mo is transported as an oxi anion as (OH) - in hydrothermal systems.

Field Mapping

Field Mapping was done in Cristal Spring Formation (Carbonates) and Rattle Snake Formation.

Cristal Spring formation is of Neoproterozoic age and consist of Dolomite altered to package of Tremolite, minor Talk, Chlorite and as residual of the Dolomite, Calcite. Fe-ox from the Dolomite hereand there. In faults you can see nice formed bx with Hematite and Goethite. This formation wasintruded by a Pyroxene Diabase (dark green color). In field you will see Tremolite, Talc, Dolomite.Diabase doesnt have silica. Where did the silica come from?

Exercise:1. Dolomite to Tremolite

5CaMg(CO 3)2 + 8SiO2 + H2O Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2

2. Dolomite to Talc3CaMg(CO 3)2 + 4SiO2 + H2O Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2

3. Tremolite to Talc3Ca2Mg5Si8O22(OH)2 + 6CO2 + 2H2O 5Mg 3Si4O10(OH)2 + 4SiO2 + 6CaCO3

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