Part 2 – History of Helvetia-Rosemont mining district, Pima County, Arizona

David-BriggsThis article presents the history of the Helvetia-Rosemont mining district from 1875 until the present time. Covering the pre-World War II history of the area, the first part of this two-part series deals with the settlers, the early mining camps where they lived, and the mining ventures that developed and worked many of its early mines. The final part of this series covers the post-World War II mining activities, which led to the discovery and development of the Rosemont copper deposit.

Helvetia-Rosemont (1945-1973)

During the period from 1945 to 1973, enormous strides were made in the understanding of the hydrothermal processes involved in ore formation. A conceptual model for porphyry copper deposits was also developed during this period. These advances proved to be very useful in evaluating the exploration potential of these large mineralized systems and resulted in the discovery of many large copper deposits in southeastern Arizona, including Mission (1951), Sierrita (1955), Twin Buttes (1956), Sacaton (1961), Florence (1970) and Red Mountain (1970).

Although the widespread distribution of copper in the Helvetia-Rosemont mining district interested many geologists during the late 1940s and 1950s, exploration efforts were challenged by the area’s highly complex structural setting. Mining companies eventually took up the challenge of evaluating the Helvetia-Rosemont mining district’s mineral potential. One of the first efforts was a churn drilling program conducted by the Lewisohn Copper Company in 1955-1956. It identified a large area of low-grade disseminated copper mineralization known as the Peach-Elgin deposit, which is located immediately northwest of the town site of Helvetia. The American Exploration and Mining Company explored the Ingersoll breccia pipe, located southeast Broad Top Butte during 1956, but failed to find economic mineralization.

 rosemont-site-drill-holesDrill Holes at the Peach-Elgin Copper Deposit (Photo by David Briggs, December 2013)

Having intermittently visited the Helvetia-Rosemont mining district over a period of more than a decade, the management of the Banner Mining Company decided it was time to explore the area. In April 1961, they acquired a large land package through four basic agreements, which included the extensive holdings of the Lewisohn estate. Exploration drilling commenced soon afterward, resulting in the discovery of the Rosemont deposit in drill hole G-33, which contained a 1,000-foot intercept, assaying more than 0.90% copper. Banner Mining Company acquired the Narragansett property in July 1963. By the end of 1963, this exploration effort had completed 16,541 feet of diamond and rotary drilling in the Helvetia-Rosemont mining district, including three diamond drill holes, totaling 4,300 feet at their Rosemont discovery.

panorama-of-RosemontPanorama of the Future Site of the Rosemont Copper Mine (Photo by David Briggs, August 2008)

The Anaconda Company acquired an option to lease Banner’s Arizona holdings in March 1963, which included the Helvetia-Rosemont property. Anaconda exercised its option in April 1964, initiating a 60-year lease. Over the next ten years, Anaconda geologists prepared detailed geological maps of the area and carried out extensive diamond drilling programs on the Peach-Elgin and Rosemont deposits. These studies included 113 diamond drill holes, totaling 136,838 feet in the Rosemont deposit.

Anamax Mining Company (1973-1986)

In January 1973, a 50/50 partnership, known as the Anamax Mining Company was formed by the Anaconda Company and Amax, Inc. to manage their holdings in southern Arizona. Amax, Inc. subsequently acquired the underlying ownership interest in these properties through its merger with the Banner Mining Company in June 1973.

Anamax developed plans to place Rosemont in production during the mid- to late 1970s. Their efforts included Anamax’s purchase of the 47,000-acre Empire Ranch in December 1974 and the Cienega Ranch in 1977 for the water rights. State-owned lands along North Canyon were also leased for use as a conventional tailings pond. Anamax also initiated the process to acquire title to 5,544 acres of public lands through a land exchange with the Coronado National Forest. Geological evaluation of its Rosemont-Helvetia properties included the completion of 52 diamond drill holes in the Rosemont deposit, totaling 54,350 feet by 1986.

Anamax’s mine plan of operation called for a 33,000 short ton/day conventional sulfide concentrator to treat the sulfide ores and a 4,100 short ton/day oxide treatment plant, which was similar to their existing facility at Twin Buttes. Total estimated water usage by this operation was 19,000 acre feet per year. The copper concentrate product was planned to be hauled by truck to a rail loading facility near Vail and shipped to the Hayden smelter.

In November 1985, Anamax ceased operations and permanently closed its Twin Buttes operation at Green Valley. This resulted in the liquidation of its assets, including its Helvetia-Rosemont property. Its Empire and Cienega ranch properties were acquired by real estate interests, who subsequently entered into a series of land exchanges, which transferred these properties to the Bureau of Land Management in June 1988. They ultimately became part of the Las Cienegas National Conservation Area in December 2000.

ASARCO, Inc. (1988-2004)

ASARCO, Inc. purchased the patented and unpatented mining claims in the Helvetia-Rosemont mining district from real estate interests in August 1988.

Over the next several years, ASARCO evaluated their mineral holdings in the Helvetia-Rosemont mining district and began drawing up plans for future mining operations at Rosemont. By 1995, they initiated a process to acquire title of the Forest Service lands required to facilitate the potential development of this mineral resource. This effort involved exchanging 2,222 acres of privately owned parcels within national forests of Arizona with an appraised value of $5.3 million for 13,272 acres of Forest Service lands in the Rosemont area, which had an appraised value of $5.1 million. This land exchange was opposed by local groups, including Save the Scenic Santa Ritas which was formed in 1996. In support of these groups, but with no legal authority or standing in the matter, the Pima County Board of Supervisors voted 4-1 to oppose ASARCO’s land exchange in May 1997. In February 1998, ASARCO suspended its efforts to get a land exchange in response to the falling price of copper, which had declined from $1.395/lb. in July 1995 to $0.755/lb. in February 1998.

Grupo Mexico acquired the Helvetia-Rosemont property in November 1999 through its merger with ASARCO, Inc. They continued to hold this property until June 2004, when it was sold to Triangle Ventures LLC. Over the 16-year period, the Helvetia-Rosemont property was held by ASARCO and its successor, Grupo Mexico, eleven diamond drill holes, totaling 14,695 feet were completed at Rosemont. Although ASARCO failed to complete their proposed land exchange, they did succeed in acquiring patents on 21 mining claims in the Rosemont area in December 1995.

Augusta Resource (2005-Present)

Soon after Triangle Ventures LLC purchased the Helvetia-Rosemont property, they offered to sell it Pima County, so it could be preserved as open space. After Pima County declined this offer, Triangle Ventures sold it to Augusta Resource Corporation in April 2005. Augusta Resource (Arizona) Corporation (renamed Rosemont Copper Company in July 2007) was incorporated under Arizona law to manage these assets.

After purchasing the Rosemont-Helvetia property, Augusta Resource commenced an in-fill drilling program to bring the resource estimate at Rosemont into compliance with National Instrument 43-101 standards set forth by Canadian securities regulations. Between mid-2005 and January 2007, Augusta Resource completed 55 diamond drill holes, totaling 96,129 feet. This program was designed to better define the geology and distribution of copper mineralization at Rosemont, as well as gather geotechnical data required to design a proposed pit. It also included a re-examination of all the drill core from previous studies at the site.

Diamond-DrillDiamond Drill Rig at Rosemont (Photo by David Briggs, January 2007)

In June 2006, Washington Group International completed a preliminary assessment and economic evaluation of the Rosemont copper project. Augusta Resource submitted a plan of operation to the U. S. Forest Service in July 2006. However, this plan was subsequently deemed incomplete and returned for further work. Augusta Resource resubmitted a revised plan of operation to the Forest Service in July 2007. M3 Engineering completed a positive feasibility study on the Rosemont copper project in August 2007. The Forest Service accepted Augusta Resource’s revised plan in March 2008 and commenced the process of evaluating this proposal under guidelines set forth in the National Environmental Protection Act of 1969 (NEPA).

The basic concept of Rosemont Copper’s proposed plan of operation was to minimize the proposed operation’s impacts through the use of proven technologies and innovative mining and reclamation practices. Estimated water consumption was drastically cut through the use of a dry stack tailings disposal system, which enabled much of the water that normally remains in a conventional tailings impoundment to be recycled by the ore treatment facility. A side benefit of using this system eliminated the need for a large tailings impoundment, which significantly reduced the size of the proposed operation’s areal footprint. Innovative mining practices allow reclamation to begin at startup and continue throughout the life of the project. Laboratory and on-site studies were also conducted to determine the best methods to reclaim the waste dumps and tailings disposal site.

part-2-hidden-valley-ranchReclamation Test Plot at Hidden Valley Ranch (Photo by Jan Rasmussen, September 2012)

Over the next several years, Rosemont Copper continued to evaluate the mineral potential at Rosemont and refine the economics of developing this resource. A 20-hole diamond drilling program (17,522 feet) was conducted from December 2007 through July 2008. Data from these studies were incorporated into an updated feasibility study completed by M3 Engineering in March 2009. This was followed by a twelve-hole diamond drilling program (18,874 feet), which was completed in February 2012.

In September 2010, Augusta Resource entered into a joint venture agreement with a Korean consortium, composed of the Korea Resources Corporation and LG International Corporation. This agreement allowed these companies to earn a 20% interest in the Rosemont property through partial funding of the project expenses.

After three and half years of study, the Coronado National Forest released a Draft Environmental Impact Statement on the Rosemont Copper project in October 2011 and commenced public hearings in November 2011. The public comment period ended in January 2012. Rosemont Copper received its Aquifer Protection permit from the Arizona Department of Environmental Quality in April 2012.

The feasibility study was updated again in August 2012, to incorporate updated resources and engineering information, as well as a mine plan in line with the Forest Service’s preferred alternative that resulted from the Draft Environmental Impact Statement. One of the modifications in this study was the elimination of the heap leach facility, which had been designed to treat the oxide resources that had been identified at Rosemont. Rosemont Copper received its Air Quality permit from the Arizona Department of Environmental Quality in January 2013.

After nearly two years of additional study, the Coronado National Forest published the Final Environmental Impact Statement on the Rosemont copper project on November 29, 2013. The Draft Record of Decision was officially released on December 13, 2013. With only a few more stages of the permitting process remaining to be completed, Rosemont Copper is expected to commence development of its 21st century copper project during the summer of 2014.


The short-lived nature of the early mining ventures in the Helvetia-Rosemont mining district is not unique. Many of Arizona’s early mining camps experienced similar boom-and-bust histories. Reasons for this include the heavy dependence on a single industry as were many of Arizona’s early mining camps. The major challenges faced in exploring, developing, mining and processing of the ores and providing the basic infrastructure required to support these remote communities significantly increased the risks of these early mining ventures. Access to regional transportation networks provided by the railroads was crucial for the transport of supplies to these remote mining camps and the shipment of their mineral products to market. All of these activities were labor intensive and costly, making the viability of many of Arizona’s early mining operations vulnerable to mild fluctuations in the commodity markets.

Finally, the nature of the low-grade disseminated copper mineralization that characterizes some porphyry copper deposits, like the one found in the Helvetia-Rosemont mining district offer few opportunities to develop the large, high-grade ore bodies that were required to sustain these early mining operations. In short, the technology and mining practices employed by these early mining ventures were poorly suited for the mineralization they encountered at these deposits.

However, despite all of the early challenges experienced by these early mining operations, investors have continued to support efforts to explore and develop mineral deposits throughout Arizona. Over the years, the knowledge gained from these early efforts, favorable commodity markets, improvements in infrastructure and advances in science, engineering and technology have made it possible to successfully develop and mine low-grade disseminated copper ores, such as those that are present at Mission, Sierrita and Rosemont.

David F. Briggs is a resident of Pima County and a geologist, who has intermittently worked as a consultant on the Rosemont Copper project since 2006. The opinions expressed in this document are those of the author and do not necessarily reflect those of Rosemont Copper.

Copyright (2014) by David F. Briggs. Reprint is permitted only if the credit of authorship is provided and linked back to the source.

About David F. Briggs 47 Articles
David F. Briggs is a retired geologist, whose work is feature by the Arizona Geological Survey. Briggs intermittently worked on the Rosemont project between 2006 and 2014. He has authored articles on Arizona’s mining history.