Cenozoic Tectonic Extension in the Sonoran Desert Region and Reconstruction of the Initial Distribution of Porphyry Copper Deposits

Arizona Geological Society

2024 Speaker Series

Tuesday, 9 January 2024 | 5:30 - 8:00 PM

Location:  Hexagon Mining Division Office

40 East Congress Street, Suite 150, Tucson, Arizona 85701

Parking: On the street or parking garage (Old Pueblo Parking)

Social Hour with Sandwiches from Beyond Bread (5:30-6:30 PM) Presentation (6:35 PM)

For those planning to attend the event, please register by 6:00 PM on Sunday, January 7, 2024

For those unable to attend, here is a

MS Teams URL for Remote Viewing

http://tinyurl.com/AGS-Jan9-JSpencer

for generously providing the venue and drinks

Cenozoic Tectonic Extension in the

Sonoran Desert Region and Reconstruction of the Initial Distribution of Porphyry Copper Deposits

Jon Spencer

Adjunct Researcher, Department of Geosciences

at the University of Arizona

(Arizona Geological Survey, retired 2015)

Abstract:  In the past ~30 Ma, severe tectonic extension in the Sonoran Desert region roughly doubled the land area and halved the crustal thickness. Before this time a high mountain range in this area was the source of clastic debris shed southwestward toward the Pacific continental margin and northeastward onto what is now the Colorado Plateau. Extensional demolition and lowering of this mountain range occurred during two episodes. The first episode, at~30-18 Ma, was characterized by low-angle normal faulting, metamorphic-core-complex genesis, and voluminous felsic magmatism. The second was characterized by high-angle normal faulting, subsidence of deep sedimentary basins, and basaltic magmatism. Evaluation of normal-fault offsets along transects through the Pineleño and Rincon Mountains, and in Tucson basin and the Sierrita Mountains, allow determination of approximate total extension in southeastern Arizona and conceptual restoration of this extension to reveal the late Laramide distribution of porphyry copper deposits. The current distribution of porphyry copper deposits is dominated by two belts, one extending approximately from La Caridad and Cananea (Sonora) through the Sierrita Mountains to Sacaton near Casa Grande. The other extends from Resolution and Miami-Inspiration, through Morenci to Chino in New Mexico. Restoration of extension yields a different geometry, as represented by three belts: (1) La Caridad – Cananea – Sierrita – San Manuel – Ray – Resolution – Miami-Inspiration, (2) Ajo – Silver Bell – San Manuel – Safford – Morenci, and (3) Sierrita – Rosemont – Johnson Camp – Chino – Hillsboro. Some deposits are located at the intersections of Belt 1 with belts 2 and 3. Belt 1 is approximately parallel to the Laramide continental margin. The distribution of porphyry copper deposits in this belt is almost certainly related directly to Laramide subduction geometry and tectonics. The strong alignments in belts 2 and 3, each oriented ~N80°E, are interpreted to represent lithologic and geochemical variations in the deep crust that were inherited from Paleoproterozoic crustal genesis. These belts are also aligned with the strike of generally steeply dipping foliations in Pinal Schist and related units and in lithologic layering in some Paleoproterozoic rock units. The interpretation that Paleoproterozoic lithologic variations in deep crust influenced the formation and distribution of porphyry copper deposits is consistent with previously proposed Paleoproterozoic deep crustal signatures in gold-silver ratios of produced Au-Ag ores (Titley, 2001, ECONOMIC GEOLOGY), Pb isotopic ratios in igneous rocks and sulfide minerals (Bouse et al., 1999, ECONOMIC GEOLOGY), and isotopic and REE indicators of primarily Proterozoic crustal origins of igneous rocks associated with porphyry copper deposits (Anthony and Titley, 1988, GEOCHIMICA…; Lang and Titley, 1998, ECONOMIC GEOLOGY).  

Bio: Jon Spencer received his Ph.D. in Geology from M.I.T. in 1981. After a year as a USGS post-doc he worked for 33 years for the Arizona Geological Survey (AZGS) as a Research Geologist and then Senior Geologist.  He was responsible for the AZGS bedrock geologic mapping program and, largely with joint Federal-State STATEMAP funding, supervised and participated in fieldwork that led to ~70 AZGS geologic maps. He also researched and published articles on metallic mineral deposits, radon and uranium, and the Pliocene Bouse Formation that was deposited when Colorado River water first reached the Mojave-Sonoran Desert region. He retired in 2015 but has continued to write and publish articles on the geology of southwestern North America. He lives in Tucson with his wife Margaret.

Hexagon Mining Division Office - 40 East Congress Street,

Suite 150, Tucson, Arizona 85701