Département de Minéralogie Rue des Maraîchers 13 CH-1211 Genève 4 Suisse - Switzerland

UNIVERSITE DE GENEVE

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Cours 3e cycle romand en Sciences de la Terre (September 6-23 2001)

   Magmatism and mineralization on a grande scale:
Mafic to silicic Tertiary volcanism of the northern
Rio Grande region, Colorado & New Mexico, USA

Professors Michael Dungan & Lluis Fontboté (University of Geneva)

Topic: The region of northern New Mexico and south-central Colorado offers an exceptional opportunity to study large-scale Tertiary ( 35 Ma to <1 Ma) volcanism ranging from basaltic lava fields to silicic calderas of several ages, some of which are associated with hydrothermal ore deposits and active geothermal systems. This excursion will focus on the three contiguous volcano-tectonic regions described below, wherein the volcanic evolution is well studied and for which there are more than 200 publications since 1980 on topics ranging from ore deposits, geophysics, tectonics and structure, volcanology and petrology, geochronology, sedimentology, and paleontology. Volcanic structures viewed on the outcrop scale will be integrated with features ranging from large calderas to regional-scale tectonics. The northern Rio Grande region is a microcosm of the Tertiary volcano-tectonic evolution of western North America, where intermediate-silicic Oligo-Miocene volcanism was succeeded by regional crustal extension and dominantly basaltic volcanism.

Valles caldera & Jemez volcanic field: The classic studies of the Bandelier Tuff by Smith and Bailey led to establishment of the genetic relationship between large ignimbrites and their associated calderas, the recognition of post-collapse caldera resurgence, and the concept of welding zonation in ignimbrites. Geochronologic and paleomagnetic studies of young volcanic rocks of the Valles caldera contributed to recognition of reversals of the Earth's magnetic field which led to first estimates of the GPTS. The caldera's internal structure has been revealed by recent geothermal exploration and scientific drilling. Finally, the deeply incised ignimbrite plateau on the eastern margin of the Jemez Mountains will allow us to examine simultaneously magnificent exposures of the variably welded rhyolitic Bandelier Tuff units and prehistoric cliff dwellings in the Bandelier National Monument.

Rio Grande Rift & Taos Plateau: The Tertiary Rio Grande rift comprises a series of structural basins flanked by large uplifts of Precambrian basement. These currently deforming and seismically active basins are filled with a combination of sedimentary and volcanic rocks that permit one to examine the interplay between extensional tectonics, volcanism, and basin sedimentation. Emphases of this part of the excursion will include: (1) examination of basin-marginal structures, accommodation zones, and syn-rift sedimentation, (2) the large Plio-Pleistocene Taos Plateau volcanic field (dominantly basaltic), and (3) the Questa caldera and associated intrusive rocks (27-22 Ma) plus associated molybdenum deposits.

Oligocene San Juan volcanic field and their ore deposits: This voluminous assemblage of calc-alkaline lavas, lahars, and tuffs (> 40,000 km³) includes the dominantly dacitic products of 17 ignimbrite calderas (among them the largest known ignimbrite, the Fish Canyon Tuff; 5,000 km³). The emphasis of this portion of the excursion will be: (1) large-scale caldera-related volcanism of both the central and southwest caldera clusters, particularly the densely welded ignimbrites and their resurgent and non-resurgent sources, and (2) the Creede and Summitville mining districts (guided by Dr. Phillip Bethke). We will stress the comparison between the relatively morphologically pristine Valles caldera versus the relationships that are accessible in the more deeply eroded San Juan volcanic field where primary volcanic morphology is rarely preserved.

Schedule: Participants should plan to arrive in Denver, Colorado on or before the evening of September 6. The excursion will return to Denver on the evening of September 21 (arrival in Switzerland on the 23rd).

Preparation: A geologic overview of the region will be presented in Geneva during the summer (date to be announced later). Participants are encouraged to acquaint themselves beforehand with key publications (list will be provided).

Costs: For participants from one of the CUSO universities we estimate that approximately two-thirds of the total costs of transportation (including airfare), housing, and meals will be borne by the 3e cycle. This course is also open to students at the diploma level. Space permitting, participants from non-CUSO universities may be allowed to take part in the excursion, but they will be responsible for their costs.

Organization: Prof. Michael Dungan (Michael.Dungan@terre.unige.ch) can provide additional information. Further details, including the final revised itinerary, will be posted on http://www.unige.ch/terre/admin/terre_sem.html. See also: Updates and Additional informations.


Please register for this course by contacting Mdm. Jacqueline.Berthoud (Jacqueline.Berthoud@terre.unige.ch). Deadline for registration is April 2, 2001. Maximum number of participants: 20

Note importante! Originellement, nous avions fixé le délai d'inscription au 1er mai 2001. Comme nous avons déjà trop d'inscriptions et que nous sommes dans l'obligation de faire des réservations fermes le plus tôt possible pour obtenir des vols bon marché, LE DERNIER DELAI POUR VOUS INSCRIRE A ETE FIXE AU 2 AVRIL 2001.

Itinerary for 3e Cycle Field Excursion: September, 2001
(version 24 August 2001)

Part 1: Española Basin-Jemez Volcanic Field (Late Tertiary-Pleistocene)

Day 1 (7/9): Denver to Florissant National Monument (fossil flora in lacustrine beds)

Day 2 (8/9): Santa Fe, NM: Stops include K-T boundary locality near Raton Pass and discussion of Laramide front-range structures from Raton to Las Vegas, NM (night in Santa Fe, NM)

Day 3 (9/9): Española Basin (Rio Grande Rift), Jemez volcanic field and Bandelier Tuff outflow (Pajarito Plateau), ending in Frijoles Canyon (Bandelier National Monument) (Days 3-5: motel in either Española or Los Alamos, NM)

Day 4 (10/9): Grand Tour of Valles caldera: resurgent dome and post-caldera volcanism.

Day 5 (11/9): New developments in Valles caldera volcanism and rift tectonics.

Part 2: San Luis Valley - Taos Plateau (Pliocene) - Questa Caldera (Miocene)

Day 6 (12/9): Embudo Fault Zone (accommodation zone) and Taos Plateau volcanic field; rift tectonics, sedimentation, and volcanism. (Day 6-7: Taos, NM)

Day 7 (13/9): Taos Plateau volcanism and rift tectonics.

Day 8 (14/9): Questa caldera and molybdenum deposit. (Questa, NM)

Part 3: San Juan Volcanic Field (Oligocene)

Day 9 (15/9): San Luis Hills. Introduction to San Juan volcanic field. (Antonito, CO)

Day 10 (16/9): Tour of the Platoro caldera complex (upper Conejos River-Platoro caldera-Alamosa River).

Day 11 (17/9): The Rio Grande Transect - From the eastern margin of the San Juan volcanic field to the core of the central San Juan caldera cluster.

Day 12 (18/9):
- Option 1 Epithermal high sulfidation Summitville mining district.
- Option 2 Mingled basalt-rhyolite complex of Handkerchief Mesa.

Day 13 (19/9): Creede caldera and Creed mining district (Philip Bethke)

Day 14 (20/9):
- Option 1: Lake City caldera and associated mineralization (Philip Bethke).
- Option II: Special topics on volcanology.

Day 15 (21/9):
- Group 1) Ore deposits at Cripple Creek, Colorado (Cresson Mine, Anglogold); late return to Denver.
- Group 2) Northern La Garita caldera; late return to Denver.

Departure from Denver (22/9) and arrival in Switzerland (23/9)