Abstract

The Central Mojave Silver-Barite District produced over $20,000,000 of silver, from 1880 to 1940. Epithermal silver-barite mineralization generally occurs as veins and disseminations in Miocene volcanics and superjacent volcan­iclastic sedimentary units. Smaller pockets of mineralization have also been prospected within the underlying Proterozoic, Waterman Gneiss.  The structural setting of the central Mojave is complex involving Holocene strike slip as well as Miocene detachment.  A model proposed by Jessey (1996) relates the mineralization to extension and detachment faulting. He suggests that meteoric water circulated to depth along the master detachment fault surface.  It was subsequently heated by, and perhaps commingled with, magmatic fluid, then rose upward along high angle faults. Ore deposition occurred at favorable structural loci within upper plate rocks.

This study seeks to examine both minor and trace elements within the Central Mojave District and relate their distribution to the proposed detachment model.  Thirty-six samples were collected from trines and prospect pits in upper and lower plate rock units of tour sub-districts (Calico, Mt. General, Waterman Hills and Mitchell Mange).  Each sample was crushed, screened and pelletized. Pellets were x-rayed with a Phillips PW 2400 X-Ray Fluorescence Spectro­photometer and trace/minor element concentrations determined with SEMI-Q software.

Two trends can be noted from the x-ray data. Volatile elements (chlorine and fluorine) are concentrated in upper plate rocks, particularly within those sub-districts where boiling of ore fluids has occurred.  This is consistent with deposition of ore mineralization from chloride-complexed solutions.  During boiling the complex breaks down releasing both the volatiles and complexed metals. Since boiling appears to be adiabatic, those sub-districts where mineralization was emplaced at or near the surface should have greater concentrations of volatiles.  Furthermore, as the metal-chloride complexes break clown, there should also be a coincident increase in base metal concentrations (i.e., copper, lead, silver). Available trace element data support this hypothesis. Those sub-districts with no demonstrable boiling should be paragenetically "clean" with only the most insoluble elements deposited (i.e., barium, iron and manganese.  As fluids circulated deeply along the detachment fault surface tectonic over-pressure prevented deposition of all but the most insoluble minerals (barite, and Fe-Mn oxides) resulting in the paragenetically simple ores of the lower plate (e.g., Mitchell Range).  As fluids moved upward along upper plate listric faults, pressure dropped, boiling ensued and metals were released from chloride complexes generating the base metals sulfides common in upper plate rocks (e.g., Waterman Hills). This study fails to explain the paragenetically more evolved ores of the Calico District where boiling has not been reported (Rosso, 1992).  Perhaps the depth of emplacement of Calico mineralization (>1 km) complicates the model.