Santa Catalina Mountains

The Pirate Fault of Canada del Oro

pirate-fault

The Pirate fault forms the western boundary of the Santa Catalina Mountains near Tucson and separates the mountains from the Cañada del Oro basin to the west. The fault occurs just east of the communities of Saddlebrooke, Catalina, and Oro Valley. Remnants of this fault, exposed for about 15 miles along the mountain front, are described in a paper from the Arizona Geological Survey (see reference below). The paper describes geological features of 10 sites along the fault trace.

 

The AZGS says that this fault represents an expression of Basin & Range faulting which was active between 12 million and 6 million years ago. Vertical displacement on the fault is estimated to be about 2.5 miles with the west side down relative to the Santa Catalina Mountains uplift on the east. The fault dips from 50° to 55° west along its entire trace. The Basin & Range era was a time of crustal extension which formed much of the topography in Southern Arizona.

According to AZGS: “ Following cessation of active uplift, the fault was buried under detritus eroded from the uplifted Santa Catalina block and, currently, is being exhumed by the down-cutting Cañada del Oro and its tributaries. This field examination reveals the fault to have left a sparse but diverse collection of remains implying a varied history of fault development and evolution.”

“Deposition of basin-fill material in the Cañada del Oro basin culminated in Pleistocene time (1-2 Ma) following cessation of active uplift on the Pirate fault. Alluvium deposited during this latter time forms the high-stand surface of coalescent alluvial fans composed mostly of detritus eroded from the Santa Catalina Mountains.” That material contains placer gold deposits. The gold was derived from gold-bearing quartz veins in the Santa Catalina Mountains.

The Pirate fault disappears beneath alluvium to both the south and north. If one projects the northern trace, the Pirate fault could intersect the southeast-to-northwest trending Mogul fault. Indeed, near the projected intersection is a decorative stone quarry whose source rock is highly fractured, deformed, and altered bedrock that may be evidence of the projected fault intersection.

Parts of the exposed Pirate fault are stained red by hematite, an iron oxide, suggesting that mineralizing hydrothermal solutions were present during the development of the fault. The exact nature of this mineralization is enigmatic and according to the AZGS, “would seem to defy ready explanation.” “The picture that emerges is that of the Pirate fault as a geologic entity whose tenure as an active participant in the extensional Basin-Range tectonic event has left behind a somewhat sparse and locally enigmatic set of remains from which to infer, caveat emptor, its past.”
Reference:

Hoxie, D.T., Exhuming the Remains of the Inactive Mountain-Front Pirate Fault, Santa Catalina Mountains, Southeastern Arizona. Arizona Geological Survey, Contributed Report CR-12-F, 18p.

Free download: http://repository.azgs.az.gov/sites/default/files/dlio/files/nid1483/cr-12-f_pirate_fault_report_v.1.pdf

See also: The Gold of Cañada del Oro

The Basin & Range Province of North America

The Pirate Fault of the Santa Catalina Mountains

The Pirate fault on the west side of the Santa Catalina mountains defines the western escarpment of the range and parallels Canada del Oro wash.

The Arizona Geological Survey has just published a new paper on the fault:

Citation: Hoxie, D.T., 2012, Exhuming the Remains of the Inactive Mountain-Front Pirate Fault, Santa Catalina Mountains, Southeastern Arizona. Arizona Geological Survey Contributed Report CR-12-F, 18 p. You can download the 24Mb report here.

Pirate-fault-zone

The Arizona Geological Survey describes the Pirate fault system as follows:

Not so long ago, mountain building was alive and well in southern Arizona. [The Pirate] fault demarcates uplift of the Santa Catalina structural block to the east, from the down-dropped, alluvial Cañada del Oro basin to the west. Vertical displacement on the nearly 16 mile long fault is about ~ 2.5 miles or 13,200 feet, which occurred over a six million year span from ~ 12 to 6 million years before the present.

Once buried under detritus eroded from the uplifted Santa Catalina Mountains, the Pirate fault is currently being exhumed by the downcutting Cañada del Oro and its tributaries. “This field examination reveals the fault to have left a sparse but diverse collection of remains implying a varied history of fault development and evolution”, says Hoxie. In detailed mapping of the trace of the Pirate fault zone, he identified a number of sites of exposed fault rocks and fault-related features, ten of which are described in detail. Key observations include identifying local, hematite-rich zones and noting the presence of small-volume, mafic dikes that intruded fault breccia near the end of active uplift on the Pirate fault.

The report includes an annotated satellite image of the Santa Catalina Mountains – Cañada del Oro Basin and a suite of maps showing the distribution of geologic units – alluvium, granites (emplaced between about 70 to 26 million years before the present), and the Pinal Schist, a 1.6 billion-year-old metamorphic rock – that crop out along the fault.

Pirate-fault-outcrop-300x257The photo on the left shows an outcrop of the steeply dipping Pirate fault. The photo above shows several faults in the Santa Catalina Mountains including the Catalina detachment fault which forms the scarp on the north side of Tucson.

 

Oracle Ridge mine in the Santa Catalina Mountains may re-open next year

The Oracle Ridge mine is a small, underground copper mine in the Santa Catalina Mountains about 3 miles northeast of, and downhill from, Summerhaven. This mine is on private property within Coronado National Forest. The mine was operated intermitently, most recently from 1991-1996. See location map below. The mine is being developed by a junior Canadian mining company, Oracle Ridge Copper (project website).

Oracle-Ridge-locaton-300x240

 The ore is predominantly bornite ( Cu5FeS4) hosted by limestone. Chalcopyrite ( CuFeS2) occurs in the lower reaches. Ore grade is considerably higher than that at the open pit mines in the area. The ore also contains silver and gold.

Exploration in the area may have begun as early as 1870 and the first major operation dates from 1881 according to the Department of the Interior. The Oracle Ridge mine is principally a contact metamorphic skarn similar to the Rosemont deposit.

The company anticipates employing about 200 people, running the mine 24 hours per day, 365 days per year. The anticipated mine life is 11 years.

The ore will be mined and crushed underground, then conveyed to a plant immediately outside the mine. The mine will produce 140 tons of concentrate (about 30% copper) a day which will be trucked off the mountain and transported to a smelter. The company has not announced which smelter, but ASARCO’s Hayden smelter is the closest.

Tailings will be contained by a double-lined, dry stack method with water reclaimed and recycled. The old tailings will be incorporated in the new tailings piles on private land. The entire operation would be contained within 45 acres. The water supply will come from underground workings and wells.

The photo below shows the mine area. The gray areas are tailings from past operations. The company claims that new operations would not increase the existing footprint.

Oracle-Ridge-view

 See also:

Old mines of the Tucson Mountains

Rosemont’s dry-stacked tailings will be greener than those near Green Valley

Saginaw Hill, another old mine in a Tucson area neighborhood

Sierrita Mine is only U.S. source of Rhenium

Surprising Structure of the Copper Deposits near Green Valley, Arizona

The I-10 copper deposit

The Pontatoc mine in a north Tucson neighborhood