Introduction

This article pertains to the Morning Star mine, Monitor-Mogul mining district, Alpine County, California. It is a follow-up to the article I wrote on the district (Mindat article 2283). The Morning Star is a high-sulfidation epithermal deposit that produced copper-gold-silver ore. Based on the literature and the minerals observed in mine dump samples, the ore was apparently composed of copper sulfosalts (enargite, luzonite, and famatinite) mixed to varying degrees with pyrite, clay minerals, and altered rock. After analysis of selectively-sampled material, I was confident that significant silver was intimately associated with the copper sulfosalts. I was, however, unsure of the deportment of the silver. Did it occur as inclusions or intergrowths of distinct silver minerals, a trace component of the copper sulfosalts, or both? I was also interested in the paragenetic position of the silver mineralization. Was the silver deposited before, during, or after the copper? This has practical significance. If the silver was deposited at the same time as the copper sulfosalts (as a component of the copper sulfosalts, possibly exsolved later as distinct silver minerals), exploration for, and extraction of silver could be accomplished through visual recognition of the copper sulfosalts (with the caveat that certain copper sulfosalt bodies are richer in silver than others, as shown in my previous article). If the silver was deposited during a distinct hydrothermal event, earlier or later than the copper sulfosalts, there might be mineralization rich in silver but very different in appearance from the copper sulfosalt ore (for example, veinlets or grains of silver minerals in pyrite or altered rock); such mineralization could be more subtle or difficult to recognize than the copper sulfosalts. Lastly, if significant silver mineralization was in the form of supergene mineralization, this could mean that the bonanza ore mined from this deposit in the early days was superficial and would not be expected to continue at depth.

I chose three samples for further study. They represent different types of mineralization (in terms of texture and dominant copper sulfosalts) found at the Morning Star, but all of them represent material that was assayed and determined to have high silver content. The samples were sawn, and then analyzed with microprobe by Pavel Kartashov, who is thanked greatly for shedding light on what was previously a mystery.

I present the data in the order of the apparent paragenesis, when it seems possible to speculate on this. It should be kept in mind that the mineral identifications and interpretation are speculative. I take full responsibility for any errors in this article.

Sample 1

This sample is nearly pure enargite (based on XRD). It has an unusual, fibrous appearance that I have not observed in other enargite at the Morning Star mine.


Figure 1. Sample 1 (enargite)


A fire assay of a fragment gave the following result: 107.5 ounces silver/ton, and 1.02 ounce gold/ton. Microprobe analyses of the enargite (five analyses) indicate an antimony content ranging from 1.1 to 4%.

The backscattered SEM image in Figure 2, below, shows the gray enargite with bright, white inclusions of what appear to be at least two silver-copper-bismuth-sulfur minerals. These inclusions are elongate and more or less aligned.

Figure 2. Inclusions (white) in enargite (gray).


Figure 3, below, is a closer image.

Figure 3. Inclusions (white) in enargite (gray).


One of these inclusions has an apparent composition of 21% copper, 3% arsenic, 2.8% selenium, 51% silver, 7.5% bismuth, and 14% sulfur. Other inclusions have a higher bismuth:silver ratio. A few inclusions appear to be a lead-bismuth-silver-sulfur mineral. An unknown fraction of the copper and arsenic in the inclusion analyses could be spillover from the surrounding enargite. These inclusions were not conclusively identified and will be referred to as "bismuth-silver minerals."

The enargite appears to have suffered some leaching, either as a result of weathering or hydrothermal action. Figure 4, below, shows enargite with white bismuth-silver mineral inclusions. Note the "eaten away" areas on the right side of the grain.

Figure 4. Enargite (gray) with inclusions (white) and leached (?) area on right.


The somewhat even distribution and aligned nature of these bismuth-silver inclusions and lack of an apparent relationship to any fractures in the enargite suggest that they may be the result of exsolution from the enargite as it cooled. The apparent leaching appears to have occurred after the bismuth-silver inclusions were formed because the inclusions do not appear to be related in distribution to the leached pockets; presumably, if the leached areas were present when the bismuth-silver minerals were being deposited, this open space would have been exploited and filled in. Figure 5, below, shows an oxidized inclusion of bismuth-silver mineral (white) with leaching of enargite on the right margin of the inclusion. Presumably, the leached area would have been filled in by the bismuth-silver mineral if this open space was present and available at that time.

Figure 5. Oxidized bismuth-silver mineral (white) in enargite (gray); note open space to the right of the bismuth-silver mineral, apparently representing leached enargite.


The inclusion in Figure 5 is large and appears to be filling a space in between enargite crystals. This could possibly have resulted from migration of the exsolved bismuth-silver mineral into these available spaces. Figure 6, below, shows another apparent open-space filling. On the left, appearing to similarly fill space between enargite crystals, is an iron-arsenic-oxygen mineral, possibly scorodite.

Figure 6. Bismuth-silver mineral (white) and possible scorodite (dark gray, to left) as open-space fillings of enargite (light gray).


Figure 7, below, shows another apparent open-space filling. The light gray is a bismuth-silver mineral. On the left is an elongate, darker-gray inclusion of what is probably tetrahedrite with an apparent composition of 47% copper, 2.4% iron, 1.3% zinc, 4.9% arsenic, 13% antimony, 0.8% tellurium, 7% bismuth, and 24% sulfur.

Figure 7. Bismuth-silver mineral (light gray, to right) and tetrahedrite (elongate, medium gray, to left) in enargite (dark gray).


It was not possible to determine the possible paragenetic position of the tetrahedrite relative to the bismuth-silver minerals.

There also appears to be some late, supergene silver mineralization in this specimen associated with possible scorodite. In Figure 8, below, a grain of what is likely acanthite appears as a bright white spot against the darker possible scorodite. This possible acanthite is suspected to be supergene, as it is associated with oxidized areas in the enargite.

Figure 8. Possible acanthite (white) in possible scorodite dark gray).


A silver-selenium mineral that may be naumannite occurs in the same setting (associated with iron arsenate in open space). In Figure 9, below, the silver selenide appears as a white bleb and is being "targeted" by the microprobe.

Figure 9. Possible silver selenide (white).

Sample 2

This sample appears to be altered rock with abundant pyrite and masses of copper sulfosalt. One copper sulfosalt mass was analyzed with XRD and was found to be luzonite. Although enargite appears to be present, the appearance of the bulk of the copper sulfosalt (slightly bronzy color and an irregular fracture surface) suggests that luzonite is more abundant than enargite. A selective sampling of the copper sulfosalt (note: the sample was not pure copper sulfosalt; it only contained 34.8% copper) was analyzed and found to contain 61 ounces silver/ton.

Figure 10. Sample 2. Yellow "X" indicates analyzed area.


At least some of the pyrite appears to be earlier than the copper sulfosalt minerals because it forms rounded, remnant grains in it. Some pyrite is also present as bands or fracture-filling in the copper sulfosalt and apparently was deposited later or in between stages of copper sulfosalt deposition.

There is an unusual, concentric pattern of cracks in some of the sulfosalt masses that is visible in the photograph. These fractures appear to be leached or weathered. This fracturing does not appear to extend outside of the luzonite masses, hence, it appears not to be the result of teconic activity. One possible explanation is that the luzonite suffered shrinkage as it cooled. Regardless of the origin of the cracks, there appears to have been some leaching along them, either as a result of weathering or hydrothermal action.

For the reasons stated above, the bulk of the copper sulfosalt in the sample is assumed to be luzonite. Enargite (squarish grains with a black color and typical enargite cleavage) appears to be present in many of the sulfosalt masses as a rim on the outside of the luzonite. One luzonite mass has a vuggy center with euhedral sulfosalt crystals. This suggests that the copper sulfosalt mineralization proceeded from the outside in, with enargite being deposited earlier than the luzonite. Microprobe analysis cannot distinguish between enargite and luzonite, but the percent weight of antimony was used as a rough guide. Five spot analyses of what is apparently pure enargite (sample 1, above) from this deposit showed antimony ranging from 1.14 to 4%. Copper sulfosalts that otherwise appear to be (based on microprobe analysis) enargite or luzonite that have antimony values higher than this were considered likely to be luzonite. Enargite has been shown to have up to 6 wt.% antimony, so antimony contents higher than this suggest luzonite/famatinite (Springer, 1969).

In the copper sulfosalt (considered to be luzonite, as antimony content was above 6%), two bismuth-containing mineral inclusions were found. Figure 11, below, shows what might be bismuthinite (white), itself containing gray inclusions of a copper-bismuth-sulfur mineral, possibly emplectite:


Figure 11. Possible bismuthinite (white) with inclusions of a copper-bismuth-sulfur mineral (light gray) in luzonite (medium gray).


Figure 12, below, shows a white inclusion of a bismuth-silver-sulfur mineral with an apparent composition of 18% sulfur, 14% silver, and 64% bismuth. It is intergrown with another silver-bismuth mineral, which appears as gray.

Figure 12. Bismuth-silver-sulfur minerals (white and light gray) in luzonite (medium gray).


The texture of these bismuth minerals seems significant, especially in light of the next suspected mineralization event, described below. In Figure 12, a large, apparently leached fracture cuts across the bismuth minerals. If this open space was available when the bismuth minerals formed, it seems logical that it would have been filled in by them. Thus, it is suspected that these bismuth minerals were formed earlier than the leaching event, possibly as exsolutions as the aligned inclusions in the first sample are suspected to have formed.

It appears there may have been a later stage of mineralization that involved leaching and deposition of tellurium (and silver)-containing minerals. In Figure 13, below, what appears to be a leached fracture, visible in the top of the image, contains light grains of tetrahedrite-group minerals. This tetrahedrite contains silver (0-4.9%) and tellurium (3.8-13.1%), and some of it appears to be goldfieldite. One grain has a composition of 43% copper, 1.2% zinc, 4.9% silver, 3.2% arsenic, 12% antimony, 10.9% tellurium, and 24% sulfur. The copper sulfosalt surrounding this crack is assumed to be luzonite because two microprobe analyses in the vicinity yielded 4.3 and 4.6% antimony, which is higher than the concentration of that element in the apparently pure enargite sample (1.14 to 4%).

Figure 13. Open space (leached fracture?) in luzonite (light gray) with tetrahedrite (white, near top).



Figure 14, below, is a closer image, with one of the tetrahedrite grains "targeted."

Figure 14. Close up of tetrahedrite (white) from Figure 13.


Figure 15, below, shows tetrahedrite/goldfieldite (with a galena inclusion) apparently filling a leached pocket. Note the irregular margins of the tetrahedrite, which appear similar to the margins of the open, "leached" space. The presence of the galena inclusion suggests that it is also associated with the later tellurium-rich event.

Figure 15. Tetrahedrite (light gray) with inclusion of galena (small white spot) in luzonite (medium gray).


There is also a silver-rich mineral associated with these "leached" areas. One grain has a composition of 44% silver, 8.9% copper, 3.4% arsenic, 16% antimony, 5.5% tellurium, and 22% sulfur. It could possibly be a tellurium-rich freibergite, or a fine mixture of minerals. This silver-rich mineral is seen in Figure 16, below, as the lighter grains to the right.

Figure 16. Silver-rich mineral (light gray) near copper sulfosalts (darker grays).


In Figure 17, below, the silver-rich mineral ("targeted") occurs as lighter patches on what might be native sulfur.

Figure 17. Silver-rich mineral (light gray) on possible native sulfur (dark gray).


It appears clear that this silver-rich mineral is later than the enargite/luzonite. Like the tellurium-rich tetrahedrite, it may have been deposited in previously-leached areas. Its texture in Figure 16 suggests that it could possibly be the result of supergene or hydrothermal replacement of copper sulfosalt, perhaps tetrahedrite/goldfieldite.

No gold minerals were identified or imaged, but a separate grain of the copper sulfosalt (probably luzonite) in this sample was analyzed by Kerry Day and an inclusion (approximately 100 microns across) of a gold phase was found. Besides gold, the inclusion apparently contained tellurium, copper, iron, antimony, arsenic, and sulfur. The presence of tellurium suggests that the gold (whether as gold/electrum or as a telluride) is associated with the later stage tellurium-bearing tetrahedrite stage.

A possible alternative explanation for the textures observed in this sample could be that the tetrahedrite-bearing areas were more susceptible to weathering or hydrothermal leaching, and that the leaching event was spatially associated with, but later than the tetrahedrite. If this is the case, then the tetrahedrite mineralization may not have been later than the luzonite.

Sample 3

This sample represents the highest-grade material studied in this project. An analysis (acid digestion and AAS finish) of a small fragment indicated 314.5 ounces silver/ton.

Figure 18. Sample 3. Famatinite, luzonite, and pyrite. Yellow "X" indicates analyzed area.


Figure 19. Copper sulfosalt (probably famatinite and luzonite) in sample 3.


This sample appears to consist primarily of luzonite, famatinite, and pyrite. An XRD analysis of a sample from this rock indicated luzonite and famatinite. Microprobe analyses agree with these identifications. Spot analyses of the luzonite and famatinite also show high silver values (up to ~2% silver), but this may be due to fine, disseminated inclusions of a silver mineral (see below).

Figure 20, below, shows pyrite, luzonite, and famatinite in sample 3. Much of the pyrite is hosted by the copper sulfosalt and may be earlier, but in places the pyrite appears to have cut through the copper sulfosalts, suggesting that pyrite and copper sulfosalt deposition may have been contemporaneous or overlapping. Famatinite and luzonite are also intergrown so that it was not apparent that one was earlier than the other. "Islands" of either mineral were observed in the other.

Figure 20. Pyrite (dark gray) and luzonite and famatinite (both lighter gray).

As mentioned above, spot analyses of famatinite and luzonite gave high silver readings. Upon closer inspection, it is discovered that there are abundant inclusions (ranging in size up to ~10 microns across) of galena and a silver mineral in the copper sulfosalts and also in the pyrite. In Figure 20, the galena and silver mineral inclusions are the very light gray to white specks disseminated in the pyrite and copper sulfosalt.

Figure 21, below, shows galena (white) inclusions in pyrite.


Figure 21. Inclusions of galena (white) in pyrite.


Figure 22, below, shows inclusions (up to ~10 microns across) of a silver mineral in luzonite. Here, these inclusions appear to be mostly restricted to the copper sulfosalts, "avoiding" the pyrite islands.

Figure 22. Inclusions of a silver mineral (light gray) in luzonite (darker gray).


Because these inclusions are so small, determining their exact composition may not be possible due to spillover from the surrounding matrix, but it seems likely that they are acanthite. One inclusion yielded the following composition, according to microprobe: 72% silver, 5.6% copper, 5.2% arsenic, 1.2% antimony, and 16% sulfur. A smaller inclusion yielded less silver (59%) and more copper (13%), arsenic (7%), and antimony (3.6%). It seems likely that the copper, arsenic, and antimony in these analyses are spillover from the surrounding luzonite. These silver-bearing inclusions appear to be densely disseminated in the copper sulfosalts and it is possible that they account for the high silver (up to several percent) detected in spot analyses of luzonite/famatinite, rather than silver present as a trace component of the copper sulfosalts.

The disseminated nature of these silver-bearing mineral and galena inclusions and lack of an apparent relationship to fractures or other structures suggests that they may be the result of exsolution from the famatinite/luzonite. In one of the backscattered images, the inclusions appear to be restricted to the copper sulfosalts (see Figure 22), but in other places, these mineral inclusions are present in the pyrite. If the inclusions are the result of exsolution from copper sulfosalts, their presence in pyrite would require that these exsolved minerals migrated into the pyrite.

Discussion

The microprobe work has revealed a lot about this deposit. The mineralization is diverse, although many of the minerals remain unidentified, and the three samples studied were surprisingly different from each other.

A variety of silver-containing minerals occur as inclusions and fracture fillings in the copper sulfosalts at the Morning Star mine. It is not known what proportion of the silver in the deposit is accounted for by these minerals. In other deposits of this type, enargite and luzonite often contain silver as a trace element and thus contribute some of the silver. It is possible that the enargite, luzonite, and famatinite at the Morning Star mine contain silver, but the detection limit of the microprobe analyses (often approximately 0.5%) means that a significant amount (equivalent to over 100 ounces of silver/ton of pure sulfosalt) of silver could have gone undetected in the famatinite, luzonite, and enargite.

Besides the silver, significant gold may also be present in the copper sulfosalts and/or pyrite as inclusions or a trace component. Fire assays of copper sulfosalt-rich samples from the Morning Star mine consistently assay high in gold, although significant gold may be associated with the pyrite (see original article). The one gold-bearing inclusion found here (see under sample 2, above) suggests that gold is associated with tellurium. Based on the minerals identified in this work, it seems likely that this gold is associated with the tellurium-rich tetrahedrite mineralization event.

Once it is accepted that the only solid evidence regarding silver deportment in the Morning Star deposit consists of the silver-bearing mineral inclusions and fracture-fillings, the next question is the paragenetic position of these silver-bearing minerals, for the reasons stated in the introduction. Most of the silver-bearing minerals, and the bulk of the apparent silver mineralization, appear to be hypogene (deposited by hot, ascending fluids). The bismuth-silver minerals and the tetrahedrite/goldfieldite (which also apparently contains some silver) are assumed to be hypogene based on their textures and also because these minerals do not apparently form in supergene environments. The silver mineral (acanthite?) that occurs as inclusions in the famatinite/luzonite of sample 3 is also suspected to be hypogene. Although acanthite is a common supergene mineral, the texture (finely disseminated within the copper sulfosalt, and not forming a rim with typical supergene minerals around the copper sulfosalt) suggests hypogene deposition. The silver-rich mineral associated with the tetrahedrite in sample 2 remains unidentified. There does appear to be some supergene silver mineralization present in enargite in sample 1. The association of this suspect acanthite with apparent iron arsenate and the apparent lack of this mineral in other parts of the specimen that do not show evidence of oxidation both suggest this is supergene mineralization. It appears that the relative amount of silver contributed by supergene action was minor, at least in the samples studied. It is, of course, possible that the material studied here represents deeper, less weathered material, and that shallow, supergene-enriched ore was mined out in the early years of mining, but this is only speculation and there is no information available to support that possibility.

Moving on past the tentative conclusion that the silver mineralization is primarily hypogene, there remains the need to determine if the silver came in before, during, or after the copper. I have attempted to interpret the paragenetic relationships between the minerals observed, focusing on silver-containing phases. The textures observed suggest that some or most of the pyrite was deposited first, followed by the copper sulfosalts. Some pyrite appears to be contemporaneous with or later than copper sulfosalt. In the sample where both minerals occur abundantly, luzonite appears to be later than enargite, but this was not proven. The bismuth-silver minerals may have exsolved from the enargite and luzonite. The suspect acanthite in sample 3 may also have exsolved from copper sulfosalts. The tellurium-rich tetrahedrite and goldfieldite (and the unidentified silver-rich phase observed in sample 2) appear to be later, filling what appears to be open space related to a leaching event.

The apparent dissolution textures seen in sample 2 are reminiscent of textures described in the literature. Leroux (2010) found similar textures in enargite at a Turkish deposit; the leached spaces were filled with later galena. At the Lepanto, Philippines deposit, enargite appears to have been leached, and in these spaces, tennantite, goldfieldite, and tellurides were later deposited (Berger et al., 2014). These examples agree with the general pattern seen in high-sulfidation epithermal deposits: Enargite is deposited earlier, and later stages include tetrahedrite, chalcopyrite, and other low and intermediate-sulfidation minerals. Gold and silver often appear to be associated with these later-stage minerals (Heberlein, 2008; Henley et al., 2012; Kouzmanov and Ramboz, 2004; Leroux, 2010). The tetrahedrite/goldfieldite (and minor galena) filling fractures in the luzonite in sample 2 agree with this pattern.

Conclusions

It was confirmed in this work that silver occurs at the Morning Star deposit as inclusions and open space fillings of a variety of silver-containing minerals hosted in enargite, famatinite, and luzonite. The presence of silver as a trace component of enargite, famatinite, or luzonite could neither be confirmed nor denied, because of the methods used in the analysis.

Evidence was presented suggesting that hypogene silver mineralization (in the form of silver-containing mineral inclusions and open space fillings) was introduced with, and later than the enargite, famatinite, and luzonite. Evidence was also found of supergene silver mineralization, but no evidence was found that any enrichment occurred.

I welcome any opinions on the paragenetic relationships of the imaged textures; feel free to submit them as comments below.

Acknowledgements

Thanks to Pavel Kartashov and Kerry Day for their analytical work and Quintin Sahratian for cutting samples. Thanks also to various geologists, including Richard Henley, Haroldo Lledo, and Richard Sillitoe, for engaging in discussions regarding the possible paragenetic relationships of the minerals.

References

Berger, B.R., Henley, R.W., Lowers, H.A., and M.J. Pribil. 2014. The Lepanto Cu-Au deposit, Philippines: A fossil hyperacidic volcanic lake complex. Journal of Volcanology and Geothermal Research. 271: 70-82.

Chouinard, A. 2003. Alteration, Mineralization and Geochemistry of the High-Sulphidation Au-Ag-Cu Pascua Deposit, Chile-Argentina. Ph.D. thesis. McGill University, Montreal.

Heberlein, D. 2008. Spatial and temporal zonation at the El Indio Cu-Au-Ag deposit, Chile: Evidence for an evolving high sulphidation epithermal system. Terry Leach Symposium. Sydney, October 17, 2008.

Kouzmanov, K. and C. Ramboz. 2004. Genesis of high-sulfidation vinciennite-bearing Cu-As-Sn(Au) assemblage from the Radka epithermal copper deposit, Bulgaria: Evidence from mineralogy and infrared microthermometry of enargite. The Canadian Mineralogist. 42(5): 1501-1521.

Leroux. G.M. 2010. Stratigraphic and petrographic characterization of HS epithermal Au-Ag mineralization at the TV tower district, Biga Peninsula, NW Turkey. M.S. Thesis. University of British Columbia.

Springer, G. 1969. Compositional variations in enargite and luzonite. Mineralium Deposita. 4(1): 72-74.