Paper No. 9
Presentation Time: 10:30 AM


GIRARDI, James D. and BARTON, Mark D., Dept. of Geosciences, University of Arizona, Tucson, AZ 85721,

New field, lab, and compilation studies document numerous iron-oxide-rich hydrothermal systems associated with Jurassic igneous centers and exposed over ~7 km of the uppermost Jurassic crust of the Mojave Desert and nearby regions. The investigated iron-oxide occurrences are part of a larger belt in the southwestern United States, and closely resemble the Jurassic-Early Cretaceous IOCG deposits of the coastal central Andes (Barton, 2009, SGA 10, p. 5-7). Composite exposures in the Mojave Region illustrate diverse mineralization characterized by >50% Fe, sparse sulfides, and variable Cu-Au-V-REE-P enrichments. They have acid altered tops, metal-poor Na±Ca altered bottoms, and skarns may form where carbonate rocks are present. This region allows comparison of igneous-hosted Kiruna-type magnetite-apatite-actinolite occurrences with carbonate-hosted magnetite-dominated iron-skarns, and speculation on their connections with economically important iron-oxide(-copper-gold) (= IOCG) mineralization. Detailed mapping and geochronology in the southern Palen Montains (SPM), CA, reveals a relatively intact Middle Jurassic hydrothermal system with deep Na±Ca alteration and magnetite-apatite mineralization and shallow (near surface) level pyrophyllite(-hematite) advanced argillic alteration. Alteration and mineralization features of the SPM are similar to region-wide patterns involving crustal level, host rocks, and structures as observed in dismembered Jurassic iron-oxide-rich hydrothermal systems.

Synthesis of data from 50 iron-oxide districts and occurrences, many with minor Cu(-Au), reveals co-location and alteration/mineralization similarities, suggesting similar origins for iron-skarn and Kiruna-type occurrences. In the Cordilleran context, arid paleoclimate including and evaporitic surficial fluid sources, mildly extensional tectonics, and diverse magmatic compositions are common to the setting of Mesozoic iron-oxide systems in North and South America. Differences in Cu(-Au) contents may reflect the scarcity of S necessary for sulfide formation in the largely terrestrial setting in southwestern North America whereas the relatively Cu-rich South American IOCG systems formed where marginal marine organic-bearing rocks were present (Barton, 2009, SGA 10, p. 5-7).