Au and Ag Analysis of the Ann Mason Area, Yerington NevadaNancy De Witt, Ceara Purcell, and Hannah Aird

Department of Geological and Environmental Sciences, California State University, Chico

Alteration: First alteration

Types

● Secondary biotite and alkali feldspar

● Secondary biotite.

Identifiers

● Secondary biotite is identified by its degradation, the cleavage

appears in book-like stacks whereas in secondary biotite it is less

pronounced with a shreddy texture.

Second alteration

Types

● Secondary chlorite, epidote, and albite

● Secondary chlorite and epidote alteration

Identifiers

● Secondary chlorite can be identified by its alteration from biotite,

forming in fibrous crystals with pale green pleochroism in plain

polarized light (PPL) and anomalous birefringence in crossed

polarized light (XPL).

● Secondary epidote can be identified by its alteration from

plagioclase, forming in blocky crystals with pale yellow to

colorless pleochroism in PPL and second order blue birefringence.

● Secondary albite can be identified by its alteration from

plagioclase and/or alkali feldspar.

References1. Arif, J., Baker, T., 10 September 2004, Gold paragenesis and chemistry at Batu Hijau, Indonesia: implications for gold-rich porphyry copper deposits,

Mineralium Deposita, pg. 534.

2. Dilles, John H, Einaudi, Marco T., 1992, Wall-Rock Alteration and Hydrothermal Flow paths about the Ann-Mason Porphyry Copper Deposit, Nevada--A 6-km

Vertical Reconstruction, Economic geology and the bulletin of the Society of Economic Geologists, Vol. 87, iss. 8, pgs 1964.

3. Dilles, John H. et al, 2000, Overview of the Yerington Porphyry Copper District: Magmatic to Nonmagmatic Sources of Hydrothermal Fluids, Their Flow Paths,

Alteration Affects on Rocks, and Cu-Mo-Fe-Au Ores, pgs. 55 & 58.

4. Eliopoulos, Demetrios G. et al, 12 March 2014, Critical Factors Controlling Pd and Pt Potential in Porphyry Cu-Au Deposits: Evidence from the Balkan Peninsula,

Geosciences, pg. 35.

5. Kesler, Stephen E. et al, 1 Jun. 2002, Gold in porphyry copper deposits: its abundance and fate, Ore Geology Reviews, pg. 107 & 120.

6. Kulla, Greg et al. 2017 Updated Preliminary Economic Assessment on the Ann Mason Project Nevada, USA. Entrée Gold Inc. and Mason Resources Corp. 03

March 2017.

7. Liu, H., Chang, L. L. Y., 28 Feb. 1994, Phase relations in the system PbS-PbSe-PbTe, Mineralogical Society, vol. 58, pg. 568.

8. Sammelin (Kontturi), Monika, Wanhainen, Christina, an Martinsson, Olof, 18 Jul 2011, Gold mineralogy at the Aikik Cu-Au-Ag deposit, Gallivare area, northern

Sweden, vol. 133, is. 1-2, pg. 19.

9. Voudouris, Panagiotis et al, 2 May 2013, Extremely Re-Rich Molybdenite from Porphyry Cu-Mo-Au Prospects in Northeastern Greece: Mode of Occurrence,

Cause of Enrichment, and implications for Gold Exploration, Minerals, vol. 3, pg. 180.

10. Zhai, Degao and Liu, Jiajun, 18 Jun. 2014, Gold-telluride-sulfide association in the Sandaowanzi epithermal Au-Ag-Te deposit, NE China: implications for phase

equilibrium and physicochemical conditions, Mineralogy and Petrology, vol. 108, is. 6, pg. 853.

Geologic Background: The Ann Mason Area

● Located in Yerington District, Nevada.

● One of the four areas that make up the Yerington Batholith.

● Located at the southern region of the Yerington Batholith.

● A Cu porphyry system, associated with dike swarms and a series of

mines (Dilles and Einaudi, 1992).

What is a Cu porphyry system?

● A Cu porphyry system is where Cu orebodies are formed by

hydrothermal fluids, originating from an igneous body (magma

chamber) a few kilometers below the deposit itself.

In this Cu porphyry system

● The contact zone surrounding the igneous body created hornfels (a

metamorphic rock formed by the contact between mudstone /

shale, or other clay-rich rock) and endoskarn (lime-bearing siliceous

rock produced by the metamorphic alteration of limestone or

dolomite, forming inside the igneous body) with, primarily,

sedimentary brine dominated fluids.

● Then the intrusion of the Luhr Hill granite underwent crystallization

which led to aqueous saturated melt, creating a coexisting melt and

fluid.

● The fluids increased the volume of the intrusion, causing the

surrounding rock to fracture.

● As these fluids went through the fractures, they brought up sulfides

and precipitated them within those fractures (Dilles, 2000).

Project Goals: Yerington District, Nevada has an extensive history of

copper (Cu) mining and has provided significant geologic data of the

location, alteration, and histories of the Cu porphyry systems. There is

evidence of gold (Au) in the Yerington District from previous surveys on

Cu systems. While the presence of Au is known, its host mineral

assemblage have not been determined. Therefore, detailed

descriptions of ore mineral assemblages will allow for more efficient

mining.

Methods: This study looked at ten samples from the bornite-chalcopyrite zone as well as

data from twenty additional samples from the pyrite>chalcopyrite, chalcopyrite-pyrite, and

chalcopyrite-bornite mineralization zones. Using hand sample, thin section, geochemistry, and

SEM analysis, we were able to determine Au and Ag assemblages within their host material

and show how those assemblages compare to alteration types within the chalcopyrite-bornite

mineralization zone. During SEM analysis, the Element Energy Dispersive Spectroscopy (EDS)

system was used to calculate elemental spectrums of specific points or areas in thin section.

Figure 1: Note the three mineralization zones in the Ann Mason Area; py > cp, cp-py, and

cp-bn. This study focuses on the largest mineralization zone, cp-bn, and compares

mineralization trends of Ag, Au, and Pb (Kulla, 2017).

Sample Alteration Zone Host Rock AgAu AgAuTe AgPbSe AgTe PbSe

EG-AM-01a 2nd bio/chl+ab Qmp x x x

EG-AM-08 2nd bio/chl+ab Qmp x x x x

EG-AM-73 2nd bio+kfs/chl+ab Jpqm x x x x

EG-AM-18 2nd bio+kfs/chl+ep Qmp x x x

EG-AM-01b 2nd bio+kfs/chl+ep+ab Jpqm x x x

EG-AM-67 2nd bio+kfs/chl+ep+ab Qmp x x x x

EG-AM-45 2nd bio+kfs/chl+ep+ab Qmp x x x

EG-AM-57 2nd bio+kfs/chl+ep+ab Jpqm x x x x x

Table 1: The table above correlates alteration zones and host rock with different Ag, Au, and Pb assemblages. AgAu = electrum, PbSe = clausthalite, AgAuTe = gold silver telluride, AgPbSe = litochlebite, AgTe = hessite, Qmp = quartz monzonite, and Jpqm = porphyritic quartz monzonite.

Table 2: In the table above, Bn was shown to have the largest potential of hosting Ag, Au, and Pb minerals. Pb minerals were the most commonly found and occurred in a wider variety of textures, size, and position in the grain. Au minerals were only found in bornite, appeared as a smaller size range (particularly AgAu), and had only blocky textures. Te that was present with Ag and Au appears to be associated with larger grain sizes. Electrum (AgAu) was found only within bornite, but has been also been found in chalcopyrite in the Balkan Peninsula (Eliopoulos et al, 2014) and northern Sweden (Eliopoulos et al, 2014; Sammelin, 2011).Bornite = Bn, Ccp = Chalcopyrite, Enr = Enargite, and Pl = Plagioclase.

Figure 3: The table above compares concentrations of Cu, Ag, and Au in each sample on a logarithmic scale.

Conclusions: ● The presence of Au found exclusively in bornite rather than chalcopyrite is likely

due to the fact chalcopyrite can not structurally accommodate as much Au as bornite, making bornite the preferred host at high temperatures (Arif and Baker, 2004; Kesler et al 2002). At low temperatures, Au can be hosted in both bornite and chalcopyrite and will more likely form significant grains within both host minerals. Since porphyry deposits are high temperature systems and the order of Au is one magnitude higher in bornite as opposed to chalcopyrite, it provides further evidence of this Au mineralization trend. However, it should be noted that traces of Au are likely present in chalcopyrite and bornite, even at high temperatures, but are likely too small to be detected by SEM. Furthermore, the lack of Au in chalcopyrite could also be due to Au being lost to either high-temperature vapors or low temperature altering fluids (Kesler et al 2002).

● Te and Se are associated with Ag, Au, and Pb due to the magmatic-hydrothermal conditions, specifically alkaline to calc-alkaline magmatism (Zhai and Liu, 2014) and are commonly associated with extensional magmatism preceded by much older mantle metasomatism (Voudouris et al, 2013).

● PbSe has the highest diversity of textures and can be associated with Bn, Ccp, Enr, and Plg due to its wide solid state temperature range of 200°C to 1050°C (Liu and Chang, 1994).

Figure 5: EDAX map of PbSe enclosed in Ccp and Bn; Bn = bornite and Ccp = Chalcopyrite.

Figure 6: EDAX map of AgPbSe on the side of Ccp; Ccp = Chalcopyrite and Plg = plagioclase.

Figure 7: EDAX map of AgTe enclosed in Bn and Ccp; Bn = Bornite, Ccp = Chalcopyrite, and Qtz = Quartz.

Figure 8: EDAX map of AgAuTe enclosed in Bn and Ccp; Bn= Bornite, Ccp = Chalcopyrite, and Qtz = Quartz.

Figure 4: EDAX map of AgAu on the side of Bn; Bn = bornite and Plg = plagioclase.

Figure 2: EDAX map of various concentrations of elements that are present. Note Au is found in the center of the grain while Ag is found on the side of the grain. Al = aluminum, Si = silicon, S = sulfur, Ag = silver, K = potassium, Te = tellurium, Fe = iron, Cu = copper, Au = gold, Pb = lead, and Se = selenium.

Mineralization Zones: This study looks at the chalcopyrite-bornite,

mineralization zone in the Ann Mason Area and determines

accumulation of Ag, Au, and Mo within this mineralization zone.

Results: