Porphyry Copper Deposits

Porphyry Copper deposits are the world’s largest source of copper [Ref 1] and are distributed globally (Figure 1).

World wide, production by the ten largest producers amounted to 15.4 million tons in 1917 [Ref 2]; of these, the United States ranked fourth in production at 1.27 million tons, of which 68% was produced by mines in Arizona [Ref 3].

Numerous deposits are located in the geological Basin and Range Province of the Southwest, as shown in Figure 2. Among the currently active mines in Arizona are those operated by Freeport-McMoRan Inc, the Morenci, Bagdad, Safford, Sierrita and Miami mines [Ref 4] and those operated by ASARCO LLC, which are the Silver Bell, Mission complex mines, and the Ray complex mines [Ref 5].

Formation of Porphyry Copper Deposits and Ore Minerals

The ore bodies of Porphyry Copper deposits are formed by the intrusion of hydrothermal fluids emanating from a magma chamber several kilometers below the earth’s surface and the deposition of ore minerals as veins in pressure-induced fractures within a granitic porphyry (See figures and text in Ref 6). Chalcopyrite is the major copper mineral deposited [Ref 7, Page 4]. This initial mineralization results in grades of 0.3 to 0.9% copper and almost always less than 1% [Ref 6]. It is by Supergene Enrichment (Slides 6, 7, 8 in Ref 8], a secondary enrichment process, that the deposition and the accumulation of copper ore minerals above and below the water table increases the ore grade. As shown in Slide 8 of [Ref 9], oxidizing conditions in the ore body above the water table result in deposition of copper minerals such as azurite, malachite, and chrysocolla, and the sulfide minerals, chalcocite and bornite, form under a lesser concentration of oxygen below the water table.

The Ore Minerals

Peacock Ore

Many young mineral collectors, drawn by the brilliant spectrum of colors on their surfaces, have collected specimens of either oxidized Chalcopyrite or Bornite (Figures 10 and 12); these specimens typically are labeled as “Peacock Ore” or ‘Peacock Copper”. The color stems from a thin film formed by the oxidation of the mineral surface. The colors are caused by an optical effect due to light waves reflected by both the underlying surface of the mineral and the surface of the film, which reinforce each other. Reinforcement of the waves depends on the thickness of the film and the wavelength of the light [Ref 8, Equation 4(41)]. In thinner film the reflected light tends to the blue end of the spectrum and in thicker film, to the red end of the visible light spectrum [Ref 8, Equation 4(41)]. The copper sulfides Chalcocite and Covellite can also demonstrate blue to red reflections as seen in Figures 5 and 6.

Reference 1.  https://www.geologyforinvestors.com/porphyry-largest-source-copper/

Reference 2. https://investingnews.com/daily/resource-investing/base-metals-investing/copper-investing/copper-production-country/

Reference 3. http://azgs.arizona.edu/minerals/king-copper

Reference 4. https://www.fcx.com/operations/north-america

Reference 5. http://www.asarco.com/about-us/

Reference 6. https://www.911metallurgist.com/blog/geology-of-porphyry-copper-deposits

Reference 7.  https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0ahUKEwiGrK2ouK7aAhVK7IMKHc9lDTsQFghAMAY&url=https%3A%2F%2Fofmpub.epa.gov%2Feims%2Feimscomm.getfile%3Fp_download_id%3D517008&usg=AOvVaw2hyme1RJiqvtz5uQ0dreVM

Reference 8. https://www.slideshare.net/hzharraz/topic-9-supergene-enrichment

Reference 9. http://pages.physics.cornell.edu/p510/w/images/p510/1/14/Ss14_heavens.pdf

Thanksgiving Day 2017, my son Greg and I traveled to the old copper mining town of Bisbee for a two day exploration of the Bisbee Mining and Historical Museum, observing the, now non-operational, Lavender Open Pit Copper Mine, and soaking up the ambiance of this charming town perched on the low hills of the Mule Mountains. [Ref 1]

Mines 

We found that mining activity in Bisbee began with the staking of mining claims in 1877. It evolved from underground mining at the Copper Queen Mine to operation of the Lavender Pit and its cessation of operation.

The Copper Queen Mine – The Copper Queen was mined over an approximately 100 year period. Beginning with the staking of its claim in 1877 and ending in 1974. [Ref 2] During its operation, the Copper Queen Mine produced over eight billion tons of copper, gold production of almost three million ounces and over seven and a half million ounces of silver. [Ref 3] It has also yielded spectacular specimens of copper minerals.

The Lavender Pit – The Lavender Pit was named in honor of Harrison M. Lavender, Vice President and General Manager of the Phelps Dodge Corporation. [Ref 5] He conceived and carried out the open pit plan for continuing the mining activity at the site of the, former high-grade, Sacramento Hill Mine. The open pit mine opened in 1950 and continued until 1974. During this period the mine yielded about 600,000 tons of copper with ancillary production of gold and silver from ore averaging 0.7% copper. During operation about 250 million tons of waste were striped. Mining advanced by dynamiting 50-foot high ledges. Each blast resulted in the removal of about 75,000 tons of rock. Use of the1.2 tons of blasting material was strongly leveraged. 

The gem mineral turquoise formed as a secondary mineral through the chemical reaction from the primary copper sulfide and oxide ores. It occurred as stringers up to a few inches wide and small nuggets, were dispersed randomly throughout the ore body, and was recovered as a product of the mining activity by company personnel. [Ref 6] The typically deep blue in color, with red-brown veins or a chocolate brown matrix, is called Bisbee Blue in the trade. This is in recognition of its often deep blue color. [Ref 6]

The Bisbee Historical and Mining Museum – The Museum offers exhibits that trace the initial settlement of Bisbee upon the finding of copper and staking of mining claims in 1988, through the closing of mining activity in 1970. [Ref 7] The Museum also houses a world-class collection of copper minerals gathered early in the mining history of Bisbee and features, what must be called awesome, specimens. [Ref 8]

 References:

Ref 1 http://skywalker.cochise.edu/wellerr/geology-SEAZ/mules/Mules-list.htm

Ref 2 https://arizonadailyindependent.com/2015/06/04/history-of-the-warren-bisbee-mining-district/

Ref 3 http://skywalker.cochise.edu/wellerr/students/copper-mine/bisbee.htm

Ref 4 http://geology.byu.edu/Displays/minerals/malachite-after-azurite-37

Ref 5 http://clui.org/ludb/site/lavender-pit

Ref 6 http://www.turquoisemines.com/bisbee-turquoise-mine/

Ref 7 https://bisbeemuseum.org/bm-museum.aspx

Ref 8 http://www.my heraldreview.com/news/bisbee-mining-historical-museum-receives-mineral-donation/article_61b65698-6e44-11e5-b916-0fcc3f025977.html

Porphyry Copper Deposits

Porphyry Copper deposits are the world’s largest source of copper [Ref 1] and are distributed globally (Figure 1).

World wide, production by the ten largest producers amounted to 15.4 million tons in 1917 [Ref 2]; of these, the United States ranked fourth in production at 1.27 million tons, of which 68% was produced by mines in Arizona [Ref 3].

Numerous deposits are located in the geological Basin and Range Province of the Southwest, as shown in Figure 2. Among the currently active mines in Arizona are those operated by Freeport-McMoRan Inc, the Morenci, Bagdad, Safford, Sierrita and Miami mines [Ref 4] and those operated by ASARCO LLC, which are the Silver Bell, Mission complex mines, and the Ray complex mines [Ref 5].

Formation of Porphyry Copper Deposits and Ore Minerals

The ore bodies of Porphyry Copper deposits are formed by the intrusion of hydrothermal fluids emanating from a magma chamber several kilometers below the earth’s surface and the deposition of ore minerals as veins in pressure-induced fractures within a granitic porphyry (See figures and text in Ref 6). Chalcopyrite is the major copper mineral deposited [Ref 7, Page 4]. This initial mineralization results in grades of 0.3 to 0.9% copper and almost always less than 1% [Ref 6]. It is by Supergene Enrichment (Slides 6, 7, 8 in Ref 8], a secondary enrichment process, that the deposition and the accumulation of copper ore minerals above and below the water table increases the ore grade. As shown in Slide 8 of [Ref 9], oxidizing conditions in the ore body above the water table result in deposition of copper minerals such as azurite, malachite, and chrysocolla, and the sulfide minerals, chalcocite and bornite, form under a lesser concentration of oxygen below the water table.

The Ore Minerals

Peacock Ore

Many young mineral collectors, drawn by the brilliant spectrum of colors on their surfaces, have collected specimens of either oxidized Chalcopyrite or Bornite (Figures 10 and 12); these specimens typically are labeled as “Peacock Ore” or ‘Peacock Copper”. The color stems from a thin film formed by the oxidation of the mineral surface. The colors are caused by an optical effect due to light waves reflected by both the underlying surface of the mineral and the surface of the film, which reinforce each other. Reinforcement of the waves depends on the thickness of the film and the wavelength of the light [Ref 8, Equation 4(41)]. In thinner film the reflected light tends to the blue end of the spectrum and in thicker film, to the red end of the visible light spectrum [Ref 8, Equation 4(41)]. The copper sulfides Chalcocite and Covellite can also demonstrate blue to red reflections as seen in Figures 5 and 6.

Reference 1.  https://www.geologyforinvestors.com/porphyry-largest-source-copper/

Reference 2. https://investingnews.com/daily/resource-investing/base-metals-investing/copper-investing/copper-production-country/

Reference 3. http://azgs.arizona.edu/minerals/king-copper

Reference 4. https://www.fcx.com/operations/north-america

Reference 5. http://www.asarco.com/about-us/

Reference 6. https://www.911metallurgist.com/blog/geology-of-porphyry-copper-deposits

Reference 7.  https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0ahUKEwiGrK2ouK7aAhVK7IMKHc9lDTsQFghAMAY&url=https%3A%2F%2Fofmpub.epa.gov%2Feims%2Feimscomm.getfile%3Fp_download_id%3D517008&usg=AOvVaw2hyme1RJiqvtz5uQ0dreVM

Reference 8. https://www.slideshare.net/hzharraz/topic-9-supergene-enrichment

Reference 9. http://pages.physics.cornell.edu/p510/w/images/p510/1/14/Ss14_heavens.pdf

Thanksgiving Day 2017, my son Greg and I traveled to the old copper mining town of Bisbee for a two day exploration of the Bisbee Mining and Historical Museum, observing the, now non-operational, Lavender Open Pit Copper Mine, and soaking up the ambiance of this charming town perched on the low hills of the Mule Mountains. [Ref 1]

Mines 

We found that mining activity in Bisbee began with the staking of mining claims in 1877. It evolved from underground mining at the Copper Queen Mine to operation of the Lavender Pit and its cessation of operation.

The Copper Queen Mine – The Copper Queen was mined over an approximately 100 year period. Beginning with the staking of its claim in 1877 and ending in 1974. [Ref 2] During its operation, the Copper Queen Mine produced over eight billion tons of copper, gold production of almost three million ounces and over seven and a half million ounces of silver. [Ref 3] It has also yielded spectacular specimens of copper minerals.

The Lavender Pit – The Lavender Pit was named in honor of Harrison M. Lavender, Vice President and General Manager of the Phelps Dodge Corporation. [Ref 5] He conceived and carried out the open pit plan for continuing the mining activity at the site of the, former high-grade, Sacramento Hill Mine. The open pit mine opened in 1950 and continued until 1974. During this period the mine yielded about 600,000 tons of copper with ancillary production of gold and silver from ore averaging 0.7% copper. During operation about 250 million tons of waste were striped. Mining advanced by dynamiting 50-foot high ledges. Each blast resulted in the removal of about 75,000 tons of rock. Use of the1.2 tons of blasting material was strongly leveraged. 

The gem mineral turquoise formed as a secondary mineral through the chemical reaction from the primary copper sulfide and oxide ores. It occurred as stringers up to a few inches wide and small nuggets, were dispersed randomly throughout the ore body, and was recovered as a product of the mining activity by company personnel. [Ref 6] The typically deep blue in color, with red-brown veins or a chocolate brown matrix, is called Bisbee Blue in the trade. This is in recognition of its often deep blue color. [Ref 6]

The Bisbee Historical and Mining Museum – The Museum offers exhibits that trace the initial settlement of Bisbee upon the finding of copper and staking of mining claims in 1988, through the closing of mining activity in 1970. [Ref 7] The Museum also houses a world-class collection of copper minerals gathered early in the mining history of Bisbee and features, what must be called awesome, specimens. [Ref 8]

 References:

Ref 1 http://skywalker.cochise.edu/wellerr/geology-SEAZ/mules/Mules-list.htm

Ref 2 https://arizonadailyindependent.com/2015/06/04/history-of-the-warren-bisbee-mining-district/

Ref 3 http://skywalker.cochise.edu/wellerr/students/copper-mine/bisbee.htm

Ref 4 http://geology.byu.edu/Displays/minerals/malachite-after-azurite-37

Ref 5 http://clui.org/ludb/site/lavender-pit

Ref 6 http://www.turquoisemines.com/bisbee-turquoise-mine/

Ref 7 https://bisbeemuseum.org/bm-museum.aspx

Ref 8 http://www.my heraldreview.com/news/bisbee-mining-historical-museum-receives-mineral-donation/article_61b65698-6e44-11e5-b916-0fcc3f025977.html