Geology and mineralization in the vicinity of the Morning Star precious-metal deposit of the Ivanpah Mountains, San Bernardino County, California

Abstract

Disseminated precious-metal mineralization occurs at the Morning Star deposit, in the upper-plate of the Morning Star thrust which represents intermediate age (105 and 90 Ma) faulting along the Mesozoic foreland fold and thrust belt (MFFTB). Deformation along the Morning Star thrust consists of both ductile and brittle events, and is different than deformation on other structures in the MFFTB in the northeastern Mojave Desert. The different style and episodic nature of deformation, together with the spatial relationship to the Teutonia batholith, accounts for mineralization on the Morning Star thrust and the absence of mineralization on other thrusts in the belt.

Two stages of mineralization have been identified in the Morning Star deposit. Six distinct types of electrum have been identified. Electrum occurs as free grains, fracture fillings, or as inclusions in minerals during primary mineralization, and as rims around early electrum or as intergrowths with covellite and acanthite during secondary mineralization. Textures and compositions of the electrum and Ag+Au-sulfides indicate Au and Ag remobilization was isochemical in the lower portions of the deposit, while remobilization at upper levels of the deposit resulted in gold enrichment.

Hydrothermal fluids (H₂O-CO₂-6 wt.% NaCl), that were driven by Late Cretaceous igneous activity, scavenged metals from the host rocks to form the Morning Star mineralization. Early mineralization is syn-tectonic, but the bulk of the mineralization is post-tectonics. The bulk of the main stage mineralization was precipitated into open spaces at temperatures between 280° and 330°C by a combination of reduction and increased acidity of the fluid due to wall rock reactions. Ore grades have subsequently been increased by Au and Ag remobilization.