Hydrothermal alteration associated with the Iron Hat iron skarn deposit, eastern Mojave Desert, San Bernardino County, California

Abstract

The Iron Hat deposit, which occurs in the Marble Mountains, San Bernardino County, California, is a typical example of Mojave Desert iron skarn deposits. Three episodes of skarn formation in calcareous wall rocks adjacent to granitic intrusions have been recognized:1. Early contact skarn formation generated iron-poor garnet, pyroxene, and minor vesuvianite, with retrograde epidote, actinolite, calcite, chlorite, and late-stage vein quartz + or - specular hematite at temperatures < or = 600 degrees C; X (sub CO 2 ) varied between <0.001 and 0.1. Prograde fluids were NaCl-H 2 O-rich, with maximum salinities near 50 equiv wt percent NaCl. Some prograde contact skarn growth was associated with boiling or condensing fluids. Isotopic analyses of vein and residual calcite in contact skarn and adjacent marble can be explained by decarbonation of the marble and interaction with an infiltrating fluid in equilibrium with the pluton during skarn formation and by superimposed low-temperature exchange with evolved meteoric fluids.2. Intermediate-age skarn formed around felsic dikes and produced epidotized felsic dikes with alteration envelopes of iron-rich garnet and pyroxene, minor wollastonite, and retrograde actinolite, chlorite, calcite, quartz, and specular hematite.3. Magnesian-magnetite skarn was the last skarn formed and occurs as fault-controlled clinohumite-magnetite-serpentine bodies at contacts between dolomitic marble and granite. Fluid involved in the precipitation of magnetite skarn was enriched in CO 2 and KCl relative to that which formed contact skarn. Relatively CO 2 -rich conditions (X (sub CO 2 ) approximately 0.1) during this skarn-forming event are consistent with delta 18 O analyses of magnetite and calcite in ore zones, which suggest that these minerals precipitated from 18 O-enriched fluids. This is explicable through mixing of H 2 O-rich fluids in equilibrium with the pluton with CO 2 -rich fluids produced during devolatilization of the adjacent marble. Minimum temperatures of 375 degrees C are suggested by mineral assemblages in ore zones and by isotopic fractionation between coexisting magnetite and calcite.Two distinct alteration events are recorded in the associated pluton:1. Pervasive albitization of granite and felsic dikes occurred penecontemporaneously with contact skarn and dike skarn development but prior to iron ore deposition. Fluid inclusion and isotopic data suggest that the fluid responsible for albitization had a large magmatic component or that it isotopically equilibrated with the pluton at temperatures as high as 600 degrees C.2. Late-stage quartz + or - specular hematite alteration is found in all three types of skarn and in all igneous rocks. Temperatures of 200 degrees to 250 degrees C and NaCl-CaCl 2 -H 2 O + or - KCl fluids with salinities of 0 to 22 equiv wt percent NaCl are indicated by fluid inclusion studies. Drastic whole-rock 18 O depletions associated with this event suggest that the fluid responsible for alteration was largely meteoric in origin.