Abstract

The Mesquite mining district in southeastern California is presently the second largest gold producer in California. The district comprises two subparallel orebodies of Oligocene age that before mining were unconformably buried by a thin veneer of late Tertiary sedimentary rocks and minor basalt and Quaternary alluvium on the south piedmont slope of the Chocolate Mountains. This range forms part of the eastern margin of the Salton trough and is within an area cut by numerous, but now-inactive, strands of the Neogene San Andreas fault system. Extensive postmineral faulting, inferred to be related to the evolution of the San Andreas fault system since the middle Miocene, has disrupted, offset, and rotated parts of the orebodies. This dismemberment, occurring primarily along northeasterly striking, left-lateral, oblique-slip faults, has resulted in a stepped pattern of economic mineralization in map view on the piedmont. Mineralized faults were reactivated during this event; the reactivated faults have normal, thrust, and strike-slip displacements that accommodated relative rotations of blocks between the northeast-striking oblique-slip faults. The postmineral faulting converted gold-bearing quartz and carbonate veins to breccia or to clayey gouge without any accompanying hydrothermal mineralization. A palinspastic reconstruction of the orebodies sharpens their original geometric form.The two gold orebodies, thus reconstructed, consist of the large, Big Chief and Vista open pits and the small, lower economic-grade Cherokee and Rainbow open pits. Each orebody consists of upward-diverging, moderately to steeply dipping veins, breccias, and mineralized faults that cut amphibolite facies metamorphic rocks and granitoids of Jurassic and Cretaceous age. Published geochronologic data indicate the orebodies formed in the Oligocene. Northwesterly striking dextral strike-slip faults and subparallel veins and northerly striking extensional faults and veins are the dominant mineralized structures in the restored orebodies. These two ore-related fault sets mutually crosscut each other and they formed contemporaneously. Fault movement, vein formation, and mineralization were episodic and record progressive heterogeneous simple shear strains. Throughout the orebodies, the geometry of mineralized structures in their present map and cross-sectional views requires that they formed synchronously with episodic dextral strike-slip faulting under conditions of approximately north-south shortening and east-west extensional strains. Structural criteria include mineralized faults with subhorizontal to gently plunging slickensides; identifiable Riedel, conjugate Riedel, and P shears that define a dextral strike-slip fault system of mutually cross-cutting and braided faults; the character and relative orientation of extension veins with respect to the strike-slip faults; composite dilational jogs defined by the distribution of economic grades; flower structures; and strike-slip duplexes. The structural evidence is independent of scale and is present at the scale of (1) the bench wall, (2) the individual open pits, and (3) the entire mining district. The two orebodies are interpreted to have formed in areas of relative extension between subparallel dextral strike-slip faults in a combination of fault jogs and strike-slip duplexes. This structural setting for the Mesquite mining district cannot be reconciled with models that purport to link the formation of the orebodies to extensional deformation and detachment faulting during the Oligocene.

First Page Preview

First page PDF preview
You do not currently have access to this article.