Minerals of the Masonic Mining District, Mono County, CaliforniaLast Updated: 1st Oct 2018
By Kyle Beucke
The Masonic mining district is a lesser-known gold district in Mono County, California. It was the scene of mining excitement in the early 1900s, and its mines produced an estimated 55,800 ounces of gold and 39,000 ounces of silver. The district appears to have received little attention regarding geology and mineralogy, with the notable exceptions of Vikre et al. (2015) and Johnson (1951). The original focus of my interest in the district was to understand the nature of the ore that was mined and how the gold occurred, including its paragenetic position in the mineralization. It soon became apparent that there is a wide variety of secondary minerals present at Masonic, including some that are rare. These secondary minerals were studied by friends and will be the focus of another article. The goal of this article is to provide a brief introduction to the district and describe the hypogene mineralogy, focusing on the occurrence of gold.
The Masonic mining district is located northeast of Bridgeport, California and it is reached via CA-182 and Masonic Road. Masonic Road is unpaved (dirt), but it is wide for much of its length and is usually in excellent condition. Most of the Masonic district is public land within the Humboldt-Toiyabe National Forest, but it includes some patented mining claims, which are private property.
The initial discovery of gold in the Masonic area was made by placer miners in the 1860s (Vikre et al., 2015). Apparently, the name Masonic was chosen by the miners because most of them were Freemasons. There was little development for the next several decades. This may have been for several reasons, including (1) interest being focused on nearby mining districts, (2) the placer gold possibly being sparse or very fine, and (3) the fact that the hard-rock gold deposits at Masonic do not occur as large, well-defined, continuous quartz veins and may not have been recognized by the early miners. Finally, around 1900-1902, the Jump Up Joe and Pittsburg-Liberty claims were located and underground mining began.
Figure 3. Pittsburg-Liberty mine. c. 1907? Reno Gazette-Journal, July 6, 1907. Obtained from Ancestry.com
The first recorded production at Masonic was in 1907, when seventeen tons of rich ore valued at $1,040 per ton (containing approximately 50 ounces of gold per ton based on period gold prices) were shipped from the Pittsburg-Liberty mine to the Selby smelter in the San Francisco Bay Area of California (Eakle and McLaughlin, 1917).
Later shipments of rich ore, valued at over $700 per ton, were soon made from the Pittsburg-Liberty, which was the most important mine in the district and produced an estimated $600,000 in gold (or about 30,000 ounces) (Johnson, 1951). Veins at the Pittsburg-Liberty were mined to a depth of 150 feet (Whitney and Whitney, Inc., 1984). The mineralization at the nearby Serita mine was discovered later; its production takes second place at approximately 20,000 ounces of gold. This mine has been spelled in other ways, including Sarita and Cerrita, but Serita appears to be the original spelling. After the Pittsburg-Liberty and Serita, the Chemung mine was also a significant producer. Newspaper articles from the early 1900s suggest small amounts of high-grade ore were removed from many other mines in the district.
While Masonic was largely a gold camp, silver is also present in the ore, although it usually accounted for a small fraction of the total value. Some of the mines, however, were noted for high-grade silver. One of these is the Perini mine, in the northeastern portion of the district. A newspaper article reported ore worth $177 per ton, mostly in silver. Based on the silver price of that time, this means the ore contained at least 100 ounces of silver per ton ("Perinfield camp is scene of important strike," 1927).
Most of the ore mined in the district was apparently extracted from underground workings, but some was taken from surface cuts. The largest underground stope (i.e., cavity resulting from the removal of ore) at the Serita mine was at least 80 feet long, 60 feet high, and 12-30 feet wide, and the ore removed from this stope during the first year it was mined averaged $33.00 per ton, or approximately 1.5 ounces of gold per ton (Bray and Fraser, 1922). Most mining in Masonic took place from 1902 to 1920, and the last significant mining at Masonic appears to have been the open pit worked at the Serita mine in the 1950s.
Recovery methods and ore grades
As mentioned above, the first recorded district production consisted of the shipment of rich ore to smelters, including the Selby smelter. The process of smelting achieves a high recovery of gold, but transportation and treatment costs would have made this an option viable only for very rich ore, and records suggest that the ore shipped probably contained at least five ounces of gold per ton. The building of mills in the Masonic district made local treatment of moderate-grade ore economical. As far as I know, the first mill operated in the district was the Pittsburg-Liberty, a 10-stamp, steam-powered amalgamation mill used from 1907 onward (Eakle and McLaughlin, 1917). This mill processed ore from underground operations at the Pittsburg-Liberty and Serita mines. By 1915, the Pittsburg-Liberty mill was modified to include fine grinding and cyanide vat leaching and was used to treat ore from the Serita mine for at least two years. The mill at this point had a capacity of 75 tons per day (Bray and Fraser, 1922; Whitney and Whitney, Inc., 1984). Ore was transported from the Serita mine to the mill by means of an aerial tramway, the remains of which are still present as of 2017. At least 18,000 tons of ore from the Serita mine with an average grade of approximately one ounce of gold per ton were processed. Recovery was reportedly good, although there appears to have been some loss of gold through leakage of the cyanide solution (Bray and Fraser, 1922; Whitney and Whitney, Inc., 1984). In what was apparently the last mining in the district, cyanidation was employed at the Chemung mill to treat ore from the Serita open pit and other district mines (Johnson, 1951; Steinpress, 1984). The remains of the Chemung mill are seen in the photograph at the beginning of this article. Approximately 11,500 tons of ore with an average grade of 0.17 ounce of gold per ton were treated at the Chemung mill during this activity; this was probably the lowest-grade ore mined at Masonic (Whitney and Whitney, Inc., 1984).
The silver ore at the Perini mine was apparently (based on limited observation of mine dump material and one fire assay; see below) enargite, and this almost certainly would have required smelting. A newspaper article indicates that it was the intention of the operators of the mine to ship the rich ore to a smelter ("Perinfield camp is scene of important strike," 1927).
During the 1970s and 1980s, Masonic was again explored for gold. Sampling, mapping, and drilling were done with attention being focused on the Serita mine area, where there was estimated to be 180,000 tons of ore with an average grade of 0.15 ounce of gold per ton of ore (Vikre et al., 2015; Whitney and Whitney, Inc., 1984). Mine dumps were included in the resource estimate, not surprising when considering the cut-off grade for ore at the Serita mine during the early, pre-1950s period of mining was reportedly $15.00 per ton, or approximately 0.75 ounce of gold per ton. Metallurgical tests in the 1970s and 1980s indicated that heap leaching would give poor gold recoveries and that fine grinding was necessary for good extraction (Steinpress, 1984). This is likely because of the fine-grained gold contained in a fine-grained silica gangue, although occurrence of gold in refractory minerals may have also played a role.
Du Bray et al. (2016) and Vikre et al. (2015) provide an excellent introduction to the geology of the Masonic mining district. The Masonic mining district, as well as other districts in the area such as Bodie, are located within the Bodie Hills volcanic field, and these gold deposits are associated with the magmatic activity that occurred there.
Briefly, the major rock types present in the Bodie Hills volcanic field include older, basement rocks, primarily Mesozoic granitic rock of the Sierra batholith and meta-volcanics and meta-sediments of Paleozoic to Jurassic age, and younger volcanics (Vikre et al., 2015).
The Bodie Hills volcanic field developed as part of the subduction-fed Cenozoic volcanic activity that swept through Nevada and into California before being starved of magma as a result of the passage of the Mendocino Triple Junction and termination of subduction. The extinct volcanoes associated with this activity have been referred to as the ancestral Cascade arc to distinguish them from the modern Cascade volcanoes, which are still active. Volcanic activity took place in the Bodie Hills from approximately 15 to 6 Ma, and approximately 25 distinct eruptive centers have been identified in the area. The first activity involved several large stratovolcanoes of trachyandesite composition. One of these was the Masonic stratovolcano, which covered the area of the present-day Masonic mining district. This stratovolcano is thought to have had a diameter of 16-18 kilometers and it was composed of layers of lava flows and debris-flow beds. Masonic Mountain (composed of Mesozoic granitic rock) is located at the approximate center of the volcano. Later volcanic activity in the field included lava domes and more silica-rich magma (du Bray et al., 2016).
The Masonic stratovolcano was cut by north and northeast-trending faults that have been interpreted as strike-slip faults (du Bray et al., 2016). These faults allowed fluids and gasses to rise, alter the wall rock, and deposit ore minerals.
Masonic, like the better-known Goldfield, Nevada district, has been classified as a high-sulfidation epithermal gold deposit. All epithermal gold deposits, considered to include a spectrum from low to high-sulfidation, are thought to form at relatively shallow depths and they are often hosted in volcanic rock. Research suggests that high-sulfidation deposits involve a greater input of magmatic volatiles, and perhaps magmatic fluid, compared to low-sulfidation epithermal deposits (Arribas, 1995). Magmatic gases and fluids are thought to have supplied most of the acidic, rock-altering fluids and metals in these systems. Some characteristics of high-sulfidation epithermal deposits present in the Masonic district include: (1) the presence of hydrothermal alunite as an alteration and gangue mineral - high-sulfidation deposits have also been termed “quartz-alunite,” (2) the presence of “high-sulfidation” sulfides and sulfosalts, such as enargite and the famatinite-luzonite series, and (3) enrichment in certain elements, including arsenic, bismuth, and tellurium, that are also typically enriched in modern volcanic fumarole settings. High-sulfidation deposits are hypothesized to be the underground plumbing systems of fumaroles (Henley et al., 2012). For a review of the characteristics of high-sulfidation deposits, see Hedenquist et al. (2000) and Arribas (1995). It should be noted that some of the small mines or prospects in the Masonic area, such as the Sunday mine east of the Pittsburg-Liberty, apparently have a different origin than the high-sulfidation gold deposits. These deposits are not discussed in this paper.
The “ledges” of Masonic, described by Johnson (1951), are a hydrothermal alteration-related feature typical of high-sulfidation epithermal deposits. They are also present, and closely associated with ore, in the Goldfield, Nevada district. These ledges represent faults through which acidic fluids rose, working their way into the surrounding volcanic rock and altering it in a characteristically-zoned sequence. The central and most altered portion immediately adjacent to the fault is typically rock that has been leached of its constituent minerals except for silica. It often has a vuggy texture derived from the leaching of the feldspar phenocrysts from the original porphyritic rock, and it is termed “vuggy silica,” which, in the context of this deposit type, specifically refers to porphyritic rock that has been leached of feldspars, possibly including later silicification, rather than a generic silica gangue with a vuggy texture. Figure 5, below, shows a sample from the Serita mine that is presumed to be vuggy silica.
Figure 5. A sample from the Serita mine that appears to be volcanic or volcaniclastic rock that has been leached of feldspar, leaving "vuggy silica." Field of view: ~1.5 cm.
The vuggy silica is brittle and it often has been fractured by later movement, presumably fault-related. Ore minerals and additional silica are often deposited in these fractures and in the vugs in the altered rock left by the previous leaching event. It is thought that the brittleness of the “vuggy silica” makes it a more likely host for ore compared to other types of rock alteration, for example those characterized by kaolinite. As we leave the central “core” of the vuggy silica, there is often a zone of alunite and quartz alteration. The alunite can sometimes be seen in place of the original feldspar phenocrysts in such rocks. As mentioned above, alunite is an alteration mineral typical of high-sulfidation deposits. Moving further out from the core, other alteration minerals, including kaolinite, typically become dominant.
The silica “ledges” often protrude above the surrounding rock, probably because the vuggy silica and quartz-alunite rocks are more resistant than surrounding alteration types (Hedenquist et al., 2000). These ledges are often light-colored and stained red with iron oxides possibly derived from weathered pyrite. Once associated with gold deposits in a new district, these prominent features certainly would have become the focus of prospecting in a district like Masonic, in the same way that quartz vein outcrops attracted attention in many mining districts. These ledges, however, are not guaranteed to have economic gold mineralization, as the earlier alteration stage is sometimes not followed by later fracturing and precious-metal mineralization.
Precious-metal mineralization at Masonic occurs in two forms: (1) Near vertical veins and breccias and (2) hydrothermal sedimentary beds. The veins, present at the Pittsburg-Liberty and other local mines, are hosted in trachyandesite and pre-Tertiary rock that has been replaced by quartz, alunite, and pyrite. The matrix of the veins is quartz with variable amounts of alunite, pyrite, kaolinite, barite, sulfides, and sulfosalts (Vikre et al., 2015).
The second deposit type at Masonic is represented by the ore at the Serita and Lakeview mines (Vikre et al., 2015). In this case, the hydrothermal alteration and mineralization apparently do not follow structure (at least within the mined area), but instead are restricted to volcaniclastic beds. The mineralizing fluids appear to have moved laterally through the relatively permeable volcaniclastics. The volcaniclastic rock has been leached and altered to a mixture of quartz, alunite, and kaolinite and fine-grained quartz, pyrite, and alunite was deposited in the open space. A portion of the mineralization, referred to as “pool sinter,” is distinctive in appearance, being composed of layered, light-brown, very fine-grained quartz with sedimentary textures. These pool sinters may have formed at the paleosurface. Similar textures have been observed at other high-sulfidation gold deposits. For example, at the Lepanto (Philippines) district, there are finely-laminated siliceous sediments composed of silica, titanium dioxide (probably anatase or rutile), pyrite, tennantite, and chalcopyrite. Berger et al. (2014) hypothesized that this laminated silica was deposited in a hyperacidic lake environment as amorphous silica and later crystallized to quartz.
A late stage of gray, cryptocrystalline silica (see top of specimen in Figure 6, above) forms a band on top of, or occurs as veinlets cutting the pool sinter, indicating that it was deposited relatively late in the hydrothermal sequence. Based on assays and microprobe work, these gray bands contain Bi, Te, and Ag minerals. Based on assays, the pool sinters also contain gold, although the deportment of gold in this material is mostly a mystery. Only one grain with (possibly supergene) gold was found in similar material in this work (see Figures 11 and 26).
Occurrence of gold at Masonic
The available literature provides little detail regarding the deportment of gold at Masonic. Most accounts suggest that the bulk of the gold at Masonic is in the form of native gold, that this gold is very fine-grained (Johnson (1951) reports that it appeared as “purplish clouds”), therefore likely not easy to recognize, and that it is contained within a fine-grained silica gangue, often referred to as chalcedony. Johnson suggests that the gold is relatively late-stage and occurs in the last "chalcedony" band deposited. He also reported finding flakes of native gold in a copper sulfosalt that he thought might be enargite or famatinite, suggesting that some gold may have been deposited with the copper sulfosalts. Tellurides have been reported from Masonic, but this could have been an assumption based on the recognition of tellurium in the ore rather than the identification of a telluride mineral. The first published mention of tellurium in the Masonic ore may have been in 1908, when the ore of the Pittsburg-Liberty mine was described as "quartz carrying some sulphides and tellurium" (Staff correspondents, 1908).
If indeed most of the gold at Masonic occurs as very fine-grained gold in fine-grained silica, this was likely a decisive factor in the mining history of the district. First, fine-grained gold would presumably result in very fine-grained placer gold (if any), which would be more difficult to prospect for and process. It could explain the apparent lack of significant placer gold at Masonic (as noted by Vikre et al., 2015). Second, the processing of the hard-rock gold ore would be more challenging, as fine-grained silica gangue would mean that very fine grinding would be necessary to liberate (expose) the gold particles and make them accessible to mercury or cyanide.
Vikre et al. (2015) reported the following gold minerals as inclusions in copper sulfosalts at the Pittsburg-Liberty mine: Electrum, Au-Cu alloy, Au-Cu-Ag alloy, a Au-Ag-S mineral, a Au-Ag-Se mineral, and a Au-Ag-Cu-Se mineral. The inclusions that were illustrated are approximately 5-20 microns in size, and the inclusions appear to be locked within, on the outer surfaces of, or in fractures in grains of copper sulfosalts.
Few minerals other than gold and quartz are mentioned in the descriptions of Masonic gold ore. As mentioned above, tellurium was recognized to be present in the ore, at least at the Pittsburg-Liberty mine. Copper minerals were reported, but little attention appears to have been given to them. Weed (1918) mentioned that the ore at the Pittsburg-Liberty mine contained copper along with gold and silver, and later exploration reports indicate copper minerals (in one report, they are suspected to include tetrahedrite) being present in minor amounts in some areas in the Serita mine. This evidence suggests that typical ores were not notably rich in copper, or at least not rich enough to either warrant consideration of the value of the copper in the ore or be concerned about its impact on gold recovery.
Samples were obtained from the Masonic district and studied in an attempt to determine how gold occurs at Masonic and how the gold is related paragenetically to the other minerals present. Most of these samples were collected personally, but of necessity some were obtained from other investigators. The following ten figures are shown to illustrate the different macroscopic styles of gold mineralization found in this work.
Figure 7, below, shows a sample of quartz vein from the Pittsburg-Liberty mine. The dark mineral grains are mostly famatinite-luzonite, goldfieldite, and Cu-Bi minerals, in that order of abundance. Native gold and sparse Au-Ag telluride occur with the goldfieldite. This sample assayed 1.8 ounces of gold and 4.4 ounces of silver per ton.
A sample from the Serita mine is shown in Figure 8, below. In this sample, altered rock has been coated (on the right and top), cut (light veinlets), and apparently impregnated (note the change in color and texture from the lower-left portion of the rock to the areas closer to the silica coatings and veinlets) with bluish-white, fine-grained silica. Tiny, dark particles of unidentified minerals (some apparently oxidized) occur in this fine-grained silica (possibly the "purplish clouds" described by Johnson (1951)). Tiny particles of gold were seen in the fine-grained silica, especially the coating on the right side that is richer in mineral grains. The gold is often in close association with the tiny, dark grains of unidentified minerals. This sample assayed 0.51 ounce of gold per ton, and it appears to be representative of the Serita mineralization based on the descriptions found in the literature (porous "chert" or "chalcedony," with the gold occurring as fine particles in bluish-gray "chalcedony"). This and other samples (see Figure 10, below) from the Serita mine illustrate the disseminated nature of the mineralization at this mine and suggest that mineralization at this mine was more challenging to recognize compared to the veins found in other mines (for example, the Pittsburg-Liberty). The style of mineralization at the Serita mine is also more conducive to larger-scale, open-pit mining, which only took place at that mine.
A close-up of a veinlet of the fine-grained silica cutting the altered rock is shown in Figure 9, below.
The sample shown in Figure 10 (below) is from the Serita mine and appears to be volcanic or volcaniclastic rock that was altered and silicified. It was apparently impregnated throughout with fine-grained silica that lines open space as well. Tiny, dark grains are disseminated through the rock. Microprobe work determined that these dark grains are goldfieldite and an unidentified Bi-Ag-Se-S mineral (described below); the two minerals often appear to be intergrown. Very fine-grained gold is associated with these two minerals. This sample assayed 2.3 ounces of gold and 3.2 ounces of silver per ton.
Figure 11, below, shows a sample from the Serita mine. The light brown/gray material appears to be altered and silicified volcanic rock. Microcrystalline silica has completely impregnated the rock and lines open spaces. The dark gray band is fine-grained silica with yellow "needles" that appear to be oxidized Bi-Te minerals. Material with this appearance (dark, fine-grained silica containing yellow "needles" coating or cutting through altered rock or "pool sinter") is common at the Serita mine. Silver and gold are present in this material, and silver to gold ratios appear to be higher compared to other samples. Silver minerals occur as tiny, black grains in the dark, fine-grained silica rims. Gold, though present in some form (based on fire assays), was only identified in one grain, apparently a mixture with a Ag-Hg halide mineral (described below). This sample assayed 0.8 ounce of gold and 40 ounces of silver per ton.
The mines in the northeastern portion of the Masonic district were apparently less productive than those in the southwestern area. However, gold (and silver) mineralization is present there and ore was produced. Figure 12, below, shows breccia found near a shaft in the area labelled as the "Alton Jack" mine by Vikre et al. (2015). This mine may have been part of the True Friend mine, based on descriptions of the location and workings found in old reports, and the Jump Up Joe is another mine that is believed to be in the vicinity. This shaft and the workings in the immediate vicinity will be referred to as the Alton Jack mine, with the caveat that there is significant uncertainty regarding the mine names and locations in this portion of the district. The Alton Jack mine breccia is well-oxidized. Fragments of altered rock contain what appear to be casts of weathered-out pyrite crystals. The rock fragments are cemented by relatively coarse-grained quartz that is mainly comb-textured. No fine-grained silica was observed, which is different from what is seen in material from the mines further to the southwest. The quartz appears to have at one time contained abundant sulfides and/or sulfosalts, as brown, orange, and yellow crumbling material is abundant in the quartz and often is present within what appear to be casts resulting from the weathering and dissolution of metallic minerals. A layer of quartz contains fine-grained black inclusions, giving the quartz a black appearance. These black inclusions appear to be suspended in the quartz crystals. Black bands/rings of this quartz can be seen in Figure 13. A portion of this sample was analyzed and found to be well-mineralized, with 0.78 ounce of gold per ton, 2.1 ounces of silver per ton, 2360 ppm As, 571 ppm Bi, 451 ppm Cu, 2650 ppm Sb, etc. Tiny grains of gold were observed in this sample and in every case the gold was in or adjacent to the black quartz. One especially large (~0.3 mm) grain of gold was found in this sample (see Figure 27). Based on the elements found to be enriched in this sample and the minerals present in other Masonic samples, it seems likely that copper sulfosalts were present prior to being weathered out. The visible gold may be hypogene gold that was deposited in the quartz and resisted weathering, or it could be a supergene product of the weathering of sulfides or sulfosalts. Another similar-appearing sample from the same location was assayed and yielded 0.53 ounce of gold per ton.
In the hope of learning what minerals were originally present (pre-weathering) in the material depicted in Figure 12, an adit dump a short distance downhill was visited. Material was found that consists of altered rock fragments cemented by quartz (some of it comb-textured) (see Figure 14, below). Pyrite and copper sulfosalts (relatively abundant goldfieldite, identified with EDS, and also famatinite-luzonite, based on appearance) occur in the quartz. The copper sulfosalt makes up perhaps 1-2% of the sample (visual estimate). An assay of this material yielded 0.21 ounce of gold per ton.
Unoxidized breccia was also found near a mine dump that may be on the same "ledge" in the other direction, uphill. This may be the Jump Up Joe mine. The breccia (see Figure 15, below) consists of quartz and altered rock fragments with pyrite and copper sulfosalts (perhaps 2%, based on visual estimate). Most of the copper sulfosalt appears to be famatinite-luzonite, based on appearance and EDS analyses, but some goldfieldite is present (EDS analysis of one grain) and enargite as well. A fire assay of this material gave 0.56 ounce of gold per ton. ICP-MS analysis on the same sample indicated 1.4 ounces of silver per ton.
Figure 16, below, shows a sample found on a dump near the Perini mine, in the far northeastern portion of the Masonic district. It consists of relatively coarse-grained enargite and pyrite in what appears to be altered volcanic rock. According to newspaper articles, the Perini mine ore contained gold and silver but was especially rich in silver. Besides the enargite and pyrite, no other metallic minerals were recognized in this material. There appears to be some crystalline quartz and barite, but no fine-grained silica was observed. An assay of a sample of this material with approximately 20% enargite (visual estimate) was assayed and yielded 116.2 ounces of silver and 0.15 ounce of gold per ton; this is the highest silver assay and the greatest silver:gold ratio of the samples analyzed in this work. I suspect that this material is representative of the high-grade silver ore mined at the Perini mine. As no other metallic minerals were identified in the material, the silver may occur in the enargite as a minor component of the enargite, as microscopic inclusions of one or more silver minerals, or both. Such ore would likely have required smelting to recover the silver. Although high-sulfidation epithermal deposits appear to be more commonly valued more for their gold content, silver-rich examples occur, including some with silver-bearing enargite. An example of such a deposit is the Morning Star mine in the Monitor-Mogul district, Alpine County, California, where the enargite and luzonite contain inclusions of silver minerals.
Results of the study of occurrence of gold at Masonic
Gold was found to occur in the following forms in the Masonic district in this work:
Telluride (petzite?): One ~5 micron inclusion of an apparent gold-silver telluride was found in goldfieldite from the Pittsburg-Liberty mine. The composition (Au: 23%; Ag: 40%; Te: 26%; Se: 5%) suggests that it is petzite.
Gold and gold alloys: The most common form of gold recognized in this work is native gold containing small amounts of silver or copper. In the southwestern portion of the district (including the Serita and Pittsburg-Liberty mine), this gold appears to occur as little clusters of tiny grains in very fine-grained silica, and if goldfieldite is visible in the sample, the gold is often closely associated with that sulfosalt. In samples with little or no visible goldfieldite, the gold appears to frequently be associated with a white, fine-grained substance suspected to be silica, possibly opal (see Figure 18, below). Observed textures suggest that the deposition of the white silica followed that of the gold.
Samples from the Pittsburg-Liberty mine contain more abundant sulfosalts and they have more apparent stages of hydrothermal activity (compared to those from the Serita mine). In these samples, the gold, white silica, and goldfieldite (when present in this stage) appear to follow (paragenetically) famatinite-luzonite associated with “needles” of bismuth minerals. The gold (and goldfieldite, if present) is then followed by famatinite-luzonite without noticeable bismuth minerals. This, in turn, is followed by a second generation of goldfieldite. This will be discussed below, under Paragenesis.
In two Serita mine samples, including the one shown in Figure 8 (above), clusters of tiny grains of visible gold occur in fine-grained silica in close association with tiny, more or less weathered grains of minerals that were not identifiable (in one specimen, EDS analyses suggest they include a Bi-Se(S?) mineral and possibly goldfieldite) (see Figures 19 and 20).
As mentioned earlier, in samples where goldfieldite and gold are visible these two minerals appear to be strongly associated with each other. The gold occurs as clusters or bands of tiny grains or flakes that are concentrated in silica near the goldfieldite. The gold often appears to merge into the goldfieldite, suggesting that the gold was deposited with the goldfieldite or at least that the deposition of the two minerals was not separated by another hydrothermal event. Backscattered electron images reveal more textural detail. In Figure 25 (below), some of the gold particles appear to be locked within the goldfieldite, suggesting that the gold was deposited before or with the goldfieldite. In multiple analyzed goldfieldite grains, including that shown in Figure 25, gold particles appear to be concentrated near the outer portions of the goldfieldite. In the sample in Figure 25, the gold appears to cut across bands representing compositional zoning in the goldfieldite. Such textures suggest that the gold may have been deposited after the goldfieldite. It is also possible that the gold was exsolved from the goldfieldite and was remobilized and concentrated in open spaces.
Based on microprobe analyses, gold grains occurring near or inside (as inclusions) goldfieldite contain minor silver (up to 3%) and copper (up to 6%). The gold shown in the images to this point is assumed to be hypogene. Gold that is possibly of supergene origin was found (with microprobe) in a single grain of a Ag-Hg-S-halide mineral in a Serita mine sample. Within this halide grain, there are tiny spots of gold (see Figure 26, below). This gold may be a product of the weathering of a previous gold-containing mineral.
Native gold was observed in the oxidized material from the Alton Jack mine shown in Figures 12 and 13. Because this material is thoroughly oxidized, it is possible that the gold is supergene. The gold was seen to occur in or near bands of crystalline quartz with a dark appearance resulting from fine-grained black inclusions. Fine-grained silica was not observed in this material. The largest grain of gold seen in Masonic in this work, approximately 0.3 mm wide, was found in this material (see Figure 27, below).
As a possible minor component of a Bi-Ag-Se-S mineral: This mineral, described below, was found to be associated with goldfieldite in the sample from the Serita mine shown in Figure 10 (see Figure 28, below). Microprobe analyses indicate that gold is present (~1-3% by weight, although the lower values are below or just reach the detection limit). This gold could occur as a minor component of the mineral, or it could occur as very fine-grained gold or gold minerals in close association with the Bi-Ag-Se-S mineral.
Previous reports suggested that native gold is the most significant form of gold at Masonic. The samples studied in this work support that idea because native gold was the most frequently-identified form of gold. Gold was found to occur in silica as grains that range up to approximately 50 microns (0.05 millimeters) and in goldfieldite as inclusions that range up to approximately 10 microns (0.01 millimeters). The exception to this generalization is the oxidized breccia at the Alton Jack mine, where visible gold grains measuring up to ~0.3 mm across were found. This coarse gold at the Alton Jack may be supergene. Very pure (high-fineness) gold is an important hypogene ore mineral at other high-sulfidation epithermal gold deposits, for example, the El Indio deposit in Chile (Jannas and Araneda, 1985). However, an unknown proportion of the gold at Masonic is present as a telluride (apparently petzite) and possibly as a minor component of the Bi-Ag-Se-S mineral observed at the Serita mine. In addition, it is possible that gold is present as a trace component of sulfosalts and pyrite, although even economically-significant concentrations of gold in this form would likely not be detectable with the microprobe analyses used in this work.
The presence of gold as telluride, as a possible minor component of the Bi-Ag-Se-S mineral, and (possibly) as a trace component of sulfosalts and sulfides has metallurgical implications. Reports indicate that recoveries of gold were poor during experiments involving cyanide leaching of coarse-crushed ore from Masonic. This suggests that conventional heap-leaching of open pit gold ore at Masonic would not be successful. It was apparently assumed that this poor gold recovery was due to very fine-grained gold being locked inside very fine-grained silica, making the gold difficult to liberate and expose to the cyanide solution. In other words, in ore broken to fragments of a certain size, a smaller particle of gold in finer-grained silica would be more likely to remain encapsulated within silica compared to a larger particle of gold in coarse-grained quartz. The native gold observed in the southwestern portion of the Masonic district does appear to be very fine-grained (smaller than 50 microns across) and it does appear to have been deposited in very fine-grained silica. The mineralization style observed at the Alton Jack mine is very different. There, coarser gold (possibly supergene) occurs in what appears to be coarser-grained quartz that apparently contained unidentified sulfides/sulfosalts that were later weathered out.
The portion of the gold in the Masonic district that is accounted for by tellurides and other complex minerals, however much that may be (and it could be very little), appears to have received little attention. If this portion is significant, it would likely further complicate recovery of gold via heap leaching. It is also possible that ore mined during early operations was more oxidized and therefore possibly more amenable to cyanide leaching compared to the remaining (deeper?) gold mineralization, because gold contained as tellurides or as a component of sulfides and sulfosalts could have weathered out as free gold.
The following list includes minerals previously reported from Masonic as well as those identified in this work. Only minerals presumed to be hypogene (deposited by ascending hydrothermal solutions) are dealt with here. Supergene (resulting from post-hydrothermal reworking involving meteoric water) minerals will be the focus of another article. Minerals reported from the Chemung mine could have come from other mines, because the Chemung mill processed ore from several district mines, including the Serita.
Acanthite: Argentite was reported from the Lakeview and Red Rock mines by Vikre et al. (2015). Acanthite appears to occur abundantly in one Serita mine sample collected in this study; this sample is shown in Figure 11. This sample includes fine-grained bluish-gray silica that contains abundant pale yellow to orange Bi-Te-O minerals. It also contains black grains up to 100 microns across that appear to be acanthite (but with tellurium up to 11% and selenium up to 4.4 %). It is not known if this acanthite is hypogene or supergene (both are common in nature), but backscattered electron images suggest the grains are pure and they do not appear to be replacing other minerals, which suggests a hypogene origin.
Alunite: Alunite is a common mineral in the Masonic district, both as a component of the rock alteration as well as a gangue mineral (Vikre et al., 2015). In this work, well-crystallized alunite was found in material from many district mines, including the Pittsburg-Liberty, Mellor, and Serita. Some of the alunite from the Serita mine occurs as clear blades (R. Housley, pers. comm.).
Anatase: Based on Raman spectroscopy, anatase occurs in the "pool sinter" at the Serita mine, but distinct crystals were not observed. Anatase was also seen as white, gray, or pale yellow/green powder in vugs at this mine (R. Housley, pers. comm.). Anatase may be a common product of hydrothermal alteration of certain rock-forming minerals in high-sulfidation epithermal deposits. It was found in the laminated silica sediments at Lepanto, which are thought to have formed in a hyperacidic lake environment (Berger et al., 2014). The Masonic "pool sinter" may have formed in a similar setting.
Arsenopyrite: A mineral from the Pittsburg-Liberty mine may be arsenopyrite, based on a couple analyses (EDS and Raman spectroscopy) (A. Kasatkin and R. Housley, pers. comm.). These identifications are somewhat in doubt, partly because of the difficulty of identifying this mineral with these methods and partly because arsenopyrite is characteristic of low-sulfidation epithermal deposits and is not commonly reported from high-sulfidation deposits.
Barite: Barite was reported to occur in breccia at Masonic (Vikre et al., 2015). Barite (confirmed with Raman spectroscopy) was found in material from the Pittsburg-Liberty and Serita mines (R. Housley, per. comm.). In material from the Pittsburg-Liberty mine, barite often forms euhedral crystals in quartz vugs (see Figure 29).
Bismuthinite: Bismuthinite was listed by Johnson (1951). In the current study, bismuthinite (with minor selenium) was only found as inclusions in goldfieldite in quartz vein material from the Pittsburg-Liberty mine. So far, all the free (i.e., hosted in silica as opposed to inclusions in other metallic minerals) dark, acicular crystals from the Pittsburg-Liberty mine that have been analyzed are Bi-Cu-S minerals (see below, under cuprobismutite).
Cassiterite: A microscopic grain approximately 10 microns in diameter in quartz near goldfieldite from the Pittsburg-Liberty mine was tentatively identified as cassiterite based on microprobe analysis.
Chalcopyrite: Chalcopyrite was reported to occur in breccia from the Chemung mine by Vikre et al. (2015).
Cinnabar: Cinnabar was listed from the Masonic district by Johnson (1951). In the current work, cinnabar was not identified. Veins of fine-grained silica were found near the Serita and Lakeview mines that contain a fine-grained, bright red mineral. This was suspected to possibly be cinnabar, but it appears to be hematite, based on XRF and wet chemistry analyses (R. Housley, pers. comm.).
Cuprobismutite: Unidentified Bi-Cu-S minerals were reported from the Pittsburg-Liberty mine as inclusions in copper sulfosalts by Vikre et al. (2015). Black, metallic needles are common in quartz vein material the Pittsburg-Liberty mine. They are associated with goldfieldite and famatinite-luzonite. EDS analyses of multiple specimens indicates one or more Bi-Cu-S minerals. One XRD analysis of black needles associated with goldfieldite indicated cuprobismutite (A. Kasatkin, pers. comm.). This represents the only definitive identification of a needle-like Bi-Cu mineral occurring free (not as an inclusion) in the Masonic district.
Diaspore: Diaspore is reported to occur as an alteration mineral in the Masonic district (Vikre et al., 2015).
Dickite: Dickite is reported to occur as an alteration mineral in the Masonic district (Vikre et al., 2015). Pale yellow powdery material lining cavities in samples from the Serita mine produced a Raman spectrum consistent with a mixture of dickite and a sulfate (R. Housley, per. comm.).
Enargite: Enargite was reported to occur in veins and breccias of many Masonic mines and in the layered "pool sinter" at the Serita mine by Vikre et al. (2015). In this work, enargite appeared to be much less abundant than famatinite-luzonite and goldfieldite in the more productive portion of the Masonic district; it is more abundant to the northeast. Grains with the appearance of enargite (black, with distinct cleavage) were seen in a couple specimens from the Pittsburg-Liberty mine; one such grain was analyzed with EDS and the composition was consistent with enargite. The enargite at that mine appears to be associated with, and possibly intergrown with famatinite-luzonite. A single crystal of what appears to be enargite was observed in a sample found on an adit dump north of the Serita mine (possibly the Waldo Consolidated), and a small amount may be present in the hypogene (unoxidized) material found at the Alton Jack mine shown in Figures 14 and 15. Enargite appears to be the most common copper sulfosalt at a mine shaft dump southeast of the Masonic town site. Enargite is the only copper sulfosalt observed in material from near the Perini mine, where samples were found with abundant (comprising up to approximately 20% of one sample) enargite and pyrite in what appears to be altered rock. A sample of enargite-rich material from the Perini mine assayed 116 ounces of silver per ton, suggesting that the enargite may contain silver as inclusions of a silver mineral or as a trace component (see Figure 16).
Famatinite-luzonite series: Johnson (1951) reported a copper sulfosalt, gray in color with a reddish tint, to be common at the Serita and Pittsburg-Liberty mines. He suspected this mineral might be enargite or famatinite. Vikre et al. (2015) reported famatinite to be present at the Pittsburg-Liberty mine. In the current work, a dark mineral with a bronzy tint was found to be common in Masonic. Grains from the Pittsburg-Liberty mine were identified as famatinite-luzonite with Raman and microprobe analyses (R. Housley, per. comm.). These grains are distinguished from goldfieldite by the bronzy tint and the lack of triangular crystal form; grains of goldfieldite are black, often have a triangular shape, and weather to form bright green Te-O minerals. Microprobe analyses indicate that famatinite-luzonite grains vary in As-Sb ratio and are often zoned. Famatinite-luzonite appears to be the most abundant and widespread copper mineral in the Masonic district, based on the samples studied.
Some of the famatinite-luzonite appears to be intergrown with Cu-Bi-Se-S minerals. Figure 34 shows such a specimen. This texture may be the result of exsolution.
Figure 34. One or more unidentified Bi-Cu-Se-S minerals (white) in famatinite-luzonite (gray). Pittsburg-Liberty mine.
Gold: Native gold is common at Masonic and may account for a majority of the gold in the ores of the district. The occurrence of native gold at Masonic is described in the previous section.
Goldfieldite: Goldfieldite appears to be a common copper sulfosalt in the Masonic district and it may be the most abundant tellurium-bearing hypogene mineral there. In addition, goldfieldite (or its green oxidation products) appears to be the most commonly-observed and readily-recognizable metallic mineral associated with visible gold. Goldfieldite is common in veins and breccias at the Pittsburg-Liberty mine (Vikre et al., 2015). It also occurs at the Serita mine, where it was found (associated with gold) disseminated in altered rock. It is distinguished from famatinite-luzonite by its black color and often triangular cross-section, and from enargite by the irregular fracture lacking distinct cleavage. The goldfieldite often contains selenium and some of it is zoned (see Figure 26). Goldfieldite was less common in material from mines northeast of the Pittsburg-Liberty (True Friend and Jump Up Joe) and was not observed at the Perini mine).
Gypsum: Gypsum was identified in material from the Pittsburg-Liberty mine (R. Housley, pers. comm.).
Hübnerite: Hübnerite was identified in one specimen from the Serita mine as approximately 1 mm wide clusters of black, blade-like crystals in a vug lined with quartz crystals (M. Chorazewicz, per. comm.).
Illite: Illite was reported as an alteration mineral by Vikre et al. (2015).
Kaolinite: Kaolinite was reported as an alteration and gangue mineral by Vikre et al. (2015).
Montmorillonite: Montmorillonite was reported as an alteration mineral by Vikre et al. (2015).
Naumannite: Naumannite was reported to occur in breccia from the Chemung mine (as noted elsewhere, material from the Chemung mine may be from other district mines) and as inclusions in copper sulfosalts at the Pittsburg-Liberty mine (Vikre et al., 2015). Unidentified selenides and a fine-grained black mineral high in silver were reported from Masonic by Johnson (1951). Silver selenides were not identified in this work, but acanthite (some apparently containing minor selenium) and a Bi-Ag-Se-S mineral (see below) were found.
Petzite (?): Unidentified tellurides have been reported previously from the Masonic district. The only telluride found in this work consisted of a single inclusion of a Au-Ag telluride with the composition of petzite that was found in goldfieldite from the Pittsburg-Liberty mine (see Figure 17).
Pyrite: Pyrite is widely distributed in the Masonic district, but it is a minor mineral in most of the material studied. It is disseminated in altered rock and it also occurs in veinlets and breccias (Johnson, 1951; Vikre et al., 2015). Crystal forms observed include cubes and octahedrons; octahedral pyrite, some of it fairly coarse-grained, is common on the dumps of the Gold Fund mine. Pyrite does not appear to be closely associated with gold in the material studied from the most productive (southwestern) portion of the district. It appears to be more abundant in mineralized samples from the northeastern area (Alton Jack and Perini mines). There is no evidence that pyrite at Masonic is a significant host of gold. One sample of altered rock with ~20% pyrite (visual estimate) from the Gold Fund mine, in the northeastern part of the district, was found to contain only 0.02 ounce of gold per ton, suggesting that there may be low levels of gold associated with this pyrite but not to an extent that would have been economically significant during historical mining activities.
Pyrophyllite: Pyrophyllite was reported as an alteration mineral at Masonic (Vikre et al., 2015).
Pyrrhotite: Pyrrhotite was reported to occur in breccia at the Chemung mine (Vikre et al., 2015).
Quartz: Quartz is common in the Masonic district as a rock alteration mineral and as a vein/veinlet filling and breccia matrix (Vikre et al., 2015). Large, well-defined veins are not characteristic of the gold deposits in the district.
Sphalerite: Sphalerite was reported to occur in breccia at the Chemung mine (Vikre et al., 2015).
Stibnite: Stibnite was listed by Johnson (1951), but it was not reported by Vikre et al. (2015) and it was not found in this work. It seems likely that one of the Bi-Cu-S minerals or the Hg-Sb mineral described below, all of which occur at Masonic as needles, were misidentified as stibnite.
Sulfur: Native sulfur was reported to fill cavities at the Success mine (Johnson, 1951). In this work, one specimen was found near the Chemung mill with sulfur filling cavities and forming a few sharp crystals. This rock may have come from the Success mine as sulfur was reported from that mine and ore from the Success and other district mines was worked at the Chemung mill (P. Vikre, pers. comm.). This rock also contained a Hg-Sb mineral that is possibly livingstonite (see below, under unidentified hypogene minerals).
Tennantite: Dark grains from the Pittsburg-Liberty mine may be tennantite, based on an EDS analysis (A. Kasatkin, pers. comm.).
Unidentified hypogene minerals
Minerals that could not be identified were found during microprobe work. Some of the most interesting (primarily those that contain precious metals) will be mentioned here.
Bi-Ag-Se-S mineral: A mineral was found in two Serita mine samples that may account for a majority of the silver in these samples. In one sample, this mineral also appears to be closely associated with gold. Figure 38 shows a grain consisting of this mineral apparently intergrown with goldfieldite.
The Bi-Ag-Se-S mineral is often weathered to some degree, as judged by the oxygen detected with the microprobe. One of the fresher (least oxidized) grains has a composition of 56% Bi, 25% Se, 8% Ag, and 7% S. Based on analyses of other grains, this mineral appears to contain some copper (up to 7%) and lead (up to 3%) as well. The composition is a close match for the AgBi3Se5 phase found by Kovalenker and Plotinskaya (2005) in a deposit on the Kamchatka Peninsula. Interestingly, microprobe analyses suggest that the mineral at Masonic also contains small amounts of gold, ranging from none or just below the limit of detection up to 2.8% gold. Microprobe analyses of at least two grains that consisted of intergrowths of the Bi-Ag-Se-S mineral with goldfieldite revealed neither silver nor gold in goldfieldite or its weathering products, whereas the Bi-Ag-Se-S mineral has silver as a major component and usually has some gold as well. Higher gold values in the Bi-Ag-Se-S mineral appear to be associated with oxidation. The gold could occur either as a minor component of the mineral or as a fine-grained intergrowth of gold or a gold mineral with the Bi-Ag-Se-S mineral. Either way, it appears that deposition of gold coincided with that of the Bi-Ag-Se-S mineral.
Hg-Sb mineral: A vuggy, silicified rock with abundant native sulfur was found near the Chemung mill (see Figures 27-29). As noted earlier, this rock may have come from the nearby Success mine. In addition to sulfur, sparse metallic gray needles were found in this rock. One needle was analyzed by EDS and it was found to be a Hg-Sb-S mineral, possibly livingstonite.
Ag-Hg mineral: One grain from a Serita mine sample appears to consist of an intergrowth of several minerals, one of which might be either a Bi-Hg-Ag-Se-S-O or a Ag-Hg mineral with a Ag:Hg ratio of 4.5 (it is uncertain if this grain is a pure phase or an intergrowth of multiple phases). Mercury minerals were rarely found in this work. The only other mercury minerals recognized are the possible livingstonite (see above) and a grain found at the Serita mine that might be an intergrowth of a Ag-Hg halide and fine-grained gold (see Figure 26), possibly weathering products of the unidentified Ag-Hg mineral.
Other unidentified phases were reported by Vikre et al. (2015), including a copper sulfide with the composition of digenite. They also reported Cu-As-O minerals in apparent hypogene intergrowths with copper sulfosalts.
Brief note regarding supergene Bi-Te minerals at the Serita mine
A wide variety of supergene minerals containing Cu, Bi, Te, As, Se, etc. are present at the Masonic district. Some of them are well-crystallized, although sparse. One distinctive type of material common at the Serita mine is described above and shown in Figure 11. In this material, the dark gray fine-grained silica contains yellow-orange minerals that often appear to fill casts after acicular crystals of some unknown mineral that presumably weathered out. Based on microprobe analyses, these pale minerals are largely one or more Bi-Te-O minerals. Antimony and arsenic are often significant (up to ~30%), and minor amounts of silver are present as well (up to ~3%). It appears likely that multiple minerals are involved. Significant gold (based on fire assays) is also associated with these samples, and it is possible that gold may occur in some form associated with the pale minerals. Gold is often closely associated with bismuth minerals in other mineral deposits, and it is possible gold was associated with whatever hypogene bismuth minerals were present in this material before being oxidized.
Figure 40 (below) is a backscattered electron image of an especially complex grain from the Serita mine sample shown in Figure 11. It may have originally been an intergrowth of barite and one or more Bi-Te minerals that was then oxidized. Certain areas are rich in silver, probably in the form of acanthite. Besides being oxidized (assuming the minerals were not originally deposited as oxides), this grain has apparently also been leached (presumably pre-oxidation) by hydrothermal fluids. It is tempting to speculate that the silver was originally contained in a bismuth-containing mineral and was liberated during oxidation to form the acanthite. This is an example of the messy, oxidized material from the Sarita mine that was very difficult to study.
Although little work has been done by me in the northeastern portion of the Masonic district, the mineralization there is different from that seen to the southwest (e.g., Pittsburg-Liberty and Serita mines). The most obvious differences are the dominant metallic minerals and silica texture. Enargite appears to be sparse in samples from the southwestern mines; it was not observed at the Serita mine (although one crystal was observed in a sample from a dump of an adit to the north of the Serita mine that was evidently driven in an attempt to access the Serita mineralization at depth), and it appears to be sparse and subordinate to goldfieldite and famatinite-luzonite at the Pittsburg-Liberty. To the northeast, samples with abundant enargite and no other recognizable copper sulfosalt were found at the Perini mine. The Perini mine samples are sulfide-rich (including abundant pyrite) and fine-grained silica was not observed. Goldfieldite appears to be less common in material from the northeastern mines, although it is present in samples from the vein(s) exploited at the Jump Up Joe and True Friend mines. Macroscopically-visible bismuth minerals were not seen in material from mines to the northeast, whereas needle-like crystals of various bismuth minerals were seen at the Pittsburg-Liberty and Serita mines. The abundant fine-grained silica (including pool sinter) found in the southwestern area versus the mascroscopically-crystaline quartz and more abundant sulfides and sulfosalts found in the northeastern area suggest that a deeper level of exposure may be represented by the material to the northeast (see Sillitoe, 1999). Visible gold that was apparently hypogene was also only seen at the Pittsburg-Liberty and Serita mines. Differences in depth of exposure across the district may be responsible for differences in gold grade (see Conclusion, below).
Textures of silica and metallic minerals seen in hand samples from Masonic show that there were multiple stages of mineralization. Banding (at least of silica) is common, but cross-cutting relationships are difficult to find, making interpretation of paragenesis challenging. Recognition of which stages of mineralization involved deposition of gold could have practical implications. In many high-sulfidation epithermal gold deposits (including the famous El Indio deposit in Chile), there is an early stage of enargite, luzonite, and pyrite followed by a stage including tetrahedrite-tennantite, gold, and more abundant silica. The later, gold-rich stage is considered to represent a lower sulfidation event relative to the earlier stage that contains enargite and luzonite (Claveria, 2001; Heberlein, 2008; Jannas and Araneda, 1985). Little information has been published regarding the paragenesis of the metallic mineralization at Masonic other than the recognition that there were multiple stages of silica and mineral deposition and Johnson's (1951) suggestion that the bulk of the gold was deposited after the copper sulfosalts.
Vikre et al. (2015) analyzed samples from the Masonic district for elements including gold, silver, copper, arsenic, antimony, bismuth, and tellurium. These samples included altered rock with low precious metal contents and samples very rich in gold. After examining this data, it is apparent that copper, bismuth, and tellurium all appear to be associated with gold, although some of the gold-rich (i.e., ore grade) samples have copper contents that would probably not correspond to visible copper minerals. However, this does not necessarily mean that all of these elements (Au, Cu, Bi, and Te) were deposited as minerals simultaneously; they could have been deposited separately in sequence or they could have partially overlapped.
In the hope of better understanding the paragenetic position of the gold mineralization at Masonic, several samples containing gold (based on assay results) were studied. All were examined under the microscope and some were sawn and microprobed. Metallic minerals were identified with a combination of EDS and visible characteristics. The primary goals were to (1) find gold or gold-bearing minerals, and (2) determine the paragenetic relationships between the metallic minerals (including the gold). This attempt to determine paragenesis is tentative and admitted to be imperfect. This work was focused on material found in the southwestern portion of the district, including the two most productive mines (the Pittsburg-Liberty and Serita mines). The mines to the northeast (i.e., Alton Jack and Perini mines) received less attention in this work and appear to represent different styles of mineralization.
Cross-cutting relationships were rarely apparent. The following criteria were used to determine paragenetic relationships. Some textures show crustification of one phase upon another; the (apparently) crustifying phase was considered to be the later one. Material from the Pittsburg-Liberty mine shows roughly banded veins where it is apparent that there is a sequence of deposition, but it is sometimes unclear in what direction the sequence progressed. The following textures were found and were used to “orient” the layers. If one side of the banding terminates in a layer of terminated quartz crystals and then open space, it was assumed that the quartz crystals represent the latest stage of mineralization and each band of mineralization, progressively further away from the quartz crystals, was considered to be earlier. The white fine-grained silica described elsewhere in this article sometimes fills open quartz vugs and is laminated. Within a specimen, these “geopetal” textures often appear to be aligned with each other and with the layering of metallic minerals and quartz, suggesting that these veins were more-or-less horizontal when forming. The geopetals often did not completely fill the open space, and the flat upper surfaces were used to align the specimen “right side up.” In these cases, banding of mineralization in the specimen was assumed to have progressed from the bottom upwards.
One interesting texture that appears to be common at Masonic but is of uncertain significance may represent leaching. The leaching apparently occurred prior to weathering because the enclosing silica gangue appears to fill in all of the available space and, in many cases, there do not appear to be secondary weathering products associated with the texture. Figure 40 (above) shows a complex grain from the Serita mine with apparent leaching of unidentified hypogene minerals prior to weathering. Figure 41 (below) shows a triangular grain of goldfieldite (with a tiny, anvil-shaped inclusion of gold in the center) partially coated with famatinite-luzonite (slightly darker gray, on right side). Both the goldfieldite and the luzonite have pitted outer surfaces and appear to have been leached. The texture suggests that the goldfieldite was coated with famatinite-luzonite prior to both minerals being leached. This leaching appears to be associated with fine-grained silica.
One sample that shows an unusually large number of distinct mineralization events will be discussed here to illustrate the apparent complexity of the Masonic mineralization. This sample, from the Pittsburg-Liberty mine, assayed 1.8 ounces of gold per ton but had no visible gold. Microprobe work revealed gold and telluride inclusions in goldfieldite (see Figure 25).
Figure 42 (below) shows one fragment of this studied sample with the location of each of the following, described stages of mineralization marked with yellow numbers. Besides the metallic minerals, quartz appears to have been deposited more or less throughout the history recorded in this specimen.
1. Bronzy grains, apparently famatinite-luzonite (based on analysis of similar-appearing grains), appear to have been deposited relatively early (see Figure 43).
2. Forming what appears to be a band deposited over the previous mineral is a fine-grained mix of famatinite-luzonite grains and Bi-Cu minerals that occur as needles (both analyzed with microprobe) (see Figure 44).
3. Apparently deposited next is goldfieldite, which forms large crystals with a triangular cross-section (see Figure 45). Tiny grains of gold and an inclusion of gold-silver telluride were observed in this goldfieldite.
4. The next apparent event was the deposition of a very fine-grained white substance, possibly silica (see Figure 46). It fills cavities in the quartz and is apparently deposited over (later than) the goldfieldite. This white silica forms geopetal textures in which the laminations are assumed to represent the original orientation of the vein. The banding in this vein material appears to represent sequential deposition of metallic minerals on a more-or-less horizontal plane.
5. The next event appears to be another generation of famatinite-luzonite (without apparent “needles” of bismuth minerals) deposited over (later than) the goldfieldite (see Figure 47).
6. There appears to be a second generation of goldfieldite occurring as smaller crystals (but still with the triangular cross-section) (see Figure 48). Larger “needles,” probably of some bismuth mineral, were also seen associated with this goldfieldite.
This paragenetic sequence appears to agree with other samples from the Pittsburg-Liberty mine, although interpreting paragenesis in many specimens is complicated by the fine-grained nature of many of the metallic minerals (making identification difficult) and the cryptic textures.
The following observations related to paragenesis at Masonic seem significant:
1. Apparently, there were two repetitions of a sequence consisting of famatinite-luzonite followed by goldfieldite.
2. Gold appears to be closely associated with the first generation of goldfieldite. It may have also been deposited at other times. For example, samples of quartz from the Alton Jack and a prospect/mine north of the Serita with grains of famatinite-luzonite (identification based on appearance and EDS analyses), pyrite, sparse enargite, and no other recognizable metallic minerals assayed 0.02 and 0.06 ounce of gold per ton. A sample containing abundant enargite from near the Perini mine assayed 0.15 ounce of gold per ton. A sample from the Gold Fund mine with abundant pyrite but no visible sulfosalts or other sulfides assayed 0.02 ounce of gold per ton. This suggests that gold mineralization, although lower in grade, may have accompanied the deposition of enargite, famatinite-luzonite, and pyrite.
Early deposition of luzonite followed by a later stage involving goldfieldite, tellurides, and gold agrees with the sequence observed in some other high-sulfidation epithermal gold deposits (Arribas, 1995).
Similarity to Goldfield
The similarity between the gold deposits at Masonic, California and Goldfield, Nevada is not surprising, as they are both high-sulfidation epithermal deposits. The similarity of the rock alteration, including the "ledges," has already been mentioned. Regarding the ore itself, similarities between the two districts include: (1) elevated bismuth and tellurium, with bismuth and tellurium minerals, including goldfieldite and bismuthinite and (2) very fine-grained, high-fineness gold (Sung et al., 2005). The apparent deposition of goldfieldite after famatinite-luzonite at Masonic agrees with the paragenesis reported by Sung et al. (2005) for Goldfield. They found that gold in Goldfield was deposited in multiple stages, with enargite, goldfieldite, and bismuthinite.
This work supports the idea that a significant portion or majority of the gold in the most productive (southwestern) area of Masonic occurs as very fine-grained native gold in fine-grained silica. In addition, an unknown portion of the gold occurs in tellurides, apparently including petzite. Some gold may occur as part of the unidentified Bi-Ag-Se-S mineral found at the Serita mine. Gold probably occurs as a trace component of various sulfides and sulfosalts, although it is not known how much of it occurs in this form. Studies have shown that significant concentrations of gold can occur in copper sulfosalts (for example, enargite) and bismuth minerals (for example, tellurobismuthinite) (Chouinard, 2003; Ciobanu et al., 2009). The presence of gold as a component of tellurides or the Bi-Ag-Se-S mineral or as a trace element in sulfides or sulfosalts may be of significance to any future gold exploration and mining activities at Masonic, as gold in these forms would likely be more difficult and expensive to extract by cyanide than native gold.
Silver, which was reportedly of minor economic significant in most Masonic mines, is present in acanthite, in an unidentified Bi-Ag-Se-S mineral, in at least two Hg-Ag minerals (one being a halide), in some unidentified form associated with enargite (at the Perini mine), and almost certainly as a minor component of the copper sulfosalts.
The sequence of mineralization in the southwestern portion of the district appears to have been complex, with at least two stages of famatinite-luzonite and goldfieldite deposition, although these are preliminary observations hampered by the scarcity of clear cross-cutting relationships. Goldfieldite appears to follow luzonite, in agreement with the pattern seen in other high-sulfidation epithermal deposits. In samples from the Pittsburg-Liberty mine, the visible gold appears to have precipitated well within (i.e., bracketed by) copper sulfosalt mineralization. This observation differs from that made by Johnson (1951), who suggested that the bulk of the gold was deposited after copper sulfosalts, although his observations may have been made on specimens where later (post-gold) stages of mineralization were not represented. The visible gold in this portion of the district always appears to be associated with fine-grained silica and often with goldfieldite.
The highest-grade gold mineralization (>2 ounces of gold per ton) studied in this work, and that with grades sufficient to have been of economic interest in the early days of mining, involved particles of native gold in fine-grained silica (often associated with goldfieldite). This apparently hypogene gold in fine-grained silica was only seen in material from the Pittsburg-Liberty and Serita mines, which are also the most productive mines in the district. This appears to have been a distinct, gold-rich hydrothermal event. In these samples, the fine-grained silica appears to be associated with extensive dissolution of copper sulfosalts and other metallic minerals. The significance of the apparent leaching to gold mineralization is unknown, but they could be related, as both leaching and gold are associated with the fine-grained silica. The oxidation that appears to be associated with the gold at the Serita mine is also suspicious. It seems possible that the gold-depositing event may have involved oxidized fluids that perhaps also partially dissolved the pre-existing metallic minerals, resulting in the textures seen in backscatter SEM images of Serita mine material. Vikre et al. (2015) suspected that some of the oxidized minerals at the Serita mine could be hypogene.
There are at least two explanations for the apparent lack of the visible gold+fine-grained silica mineralization outside of the highly productive "core" of the Masonic district. First, it is possible that the mineralization at the Pittsburg-Liberty and Serita represents a shallower portion of the hydrothermal system, and that the gold and fine-grained silica was only deposited at the shallower depths. At least in the case of the Serita mine, this seems reasonable. Second, it is possible that the gold and fine-grained silica followed fractures that were simply not open at the time of deposition in the veins of the other mines.
In gold-mineralized samples from the northeastern portion of the district (e.g., Alton Jack and Perini mines), fine-grained silica is either absent or less abundant. Samples with pyrite, enargite, famatinite-luzonite, and (sometimes) goldfieldite and no other visible metallic minerals contain gold (based on assays), although often somewhat lower in grade compared to samples from the more productive mines. This suggests that gold may have been deposited with these minerals as well, either as inclusions or as a trace component. The most gold-rich (>0.5 ounce of gold per ton) material studied from the northeastern area is the breccia from the Jump Up Joe (unoxidized) and Alton Jack (oxidized); the Jump Up Joe breccia may be representative of the pre-oxidation Alton Jack breccia. Gold content (based on assays) of samples with goldfieldite are higher than those with only enargite and/or famatinite-luzonite, lending further support to the idea that gold was preferentially deposited in association with goldfieldite.
If the bulk of the hypogene gold in the northeastern mines was in the form of inclusions of gold or trace amounts of gold in the structure of sulfides/sulfosalts, then this was likely a decisive factor in the history of these mines, as upon exploration below the zone of oxidation, the veins would have shifted from possibly profitable ore with gold that could be cheaply extracted with simple amalgamation to sulfide/sulfosalt containing gold that was not extractable with this method. If this mineralization was not sufficient to ship to a smelter (and samples assayed from the northeastern area in this work, with the exception of the rich silver ore from the Perini mine, do not appear to be rich enough to have been shipable in early days), hitting the water table may have doomed the mine.
Dr. Peter Vikre is thanked for his work on the district and for generously sharing his knowledge of the mineral deposits of the area. Dr. Robert Housley, Marek Chorazewicz, Joe Marty, Dr. Anthony Kampf, Dr. Pavel Kartashov, Kerry Day, and Anatoly Kasatkin are thanked for their work on this project. Dellilah Sabba is thanked for her photograph of the Chemung mill, which is at the top of this article. Dellilah Sabba and Dulce Bustamante reviewed the manuscript. Dan Evanich contributed mineral photographs. David Lowe and others accompanied me in the field.
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