Clay Mineralogy of Phyllic and Intermediate Argillic Alteration at Bingham, Utah

Abstract

The earliest alteration mineral zones in the porphyry Cu-Mo-Au ore deposit at Bingham, Utah, include the outermost propylitic and inner potassic alteration of igneous rocks. Innermost phyllic and intermediate argillic alteration is superimposed on propylitic and potassic alteration. Petrographic, X-ray diffraction, and electron microprobe analyses are used to characterize the clay mineralogy of phyllic and intermediate argillic alteration. Phyllic alteration affects monzonite near the contact with the quartz monzonite porphyry in the southwest and northeast portions of the deposit and consists primarily of veins and vein selvages of illite interstratified with an average of 7 percent smectite (R >3 ordering). The veins and selvages also contain small amounts of smectite and kaolinite. The illite/smectite interstratification is a mixture of 1M and 2M polytypes. Structural formulas for the illite show less K and Al and more Si than muscovite, octahedral Mg, and both a tetrahhedral and an octahedral layer charge. The Kubler crystallinity index and smectite content increase systematically as particle size decreases, consistent with an increase in smectite in the interstratification in the smaller size fractions. Intermediate argillic alteration consists of smectite, illite, and kaolinite that occur primarily as a replacement of plagioclase phenocrysts. A few veins of intermediate argillic minerals are present in the quartz monzonite porphyry. Illite is interstratified with an average of 6 percent smectite (R >3 ordering), but a few samples contain a regular interstratification of illite with 40 to 50 percent smectite (R1 ordering). The smectite replacing plagioclase is montmorillonite, and smectite that occurs in one vein is beidellite. Statistical trends in intermediate argillic alteration mineralogy within the quartz monzonite porphyry show that illite and kaolinite abundances increase generally upward and toward the northeast. Smectite abundance increases generally downward and toward the southwest. In a cross section near the center of the deposit, abundances of kaolinite and illite decrease as abundance of smectite increases with increasing depth. The occurrence of interstratified illite/smectite averaging 6 to 7 percent smectite together with discreet smectite suggest formation near 200°C. A temperature difference between phyllic and intermediate argillic alteration could not be resolved from the mineralogy. Temperature estimates from fluid inclusions for phyllic alteration at other porphyry copper deposits generally range from 200° to 350°C. The activity ratio of Na + /K + for the fluid calculated from Na + /K + in illite averages 5.0 for a phyllic vein that crosscuts propylitic alteration, 1.4 for a phyllic vein that crosscuts potassic alteration, and 2.8 for intermediate argillic alteration superimposed on potassic alteration. Log f H 2 O / f HF of the hydrothermal fluid computed from the fluorine content of illite averages 6.1 for a phyllic vein that crosscuts propylitic alteration, 5.8 for a phyllic vein that crosscuts potassic alteration, and 5.7 for intermediate argillic alteration. Log f H 2 O / f HF of fluids forming late-stage biotite is about 5.0. Intermediate argillic alteration could not be correlated with sulfides in the quartz monzonite porphyry. An increase in illite upward and toward the northeast may correlate with massive sulfides in sediments recently discovered in that region.