Elemental dispersion associated with alteration and mineralization at the hall (nevada moly) quartz monzonite-type porphyry molybdenum deposit, with a section on comparison of dispersion patterns with those from climax-type deposits

Abstract

Each of two molybdenum-bearing, quartz monzonitic stocks were emplaced at 66–70 m.y.B.P. and underwent continuous inward crystallization which was interrupted three times by the successively deeper release of hydrothermal fluids. The stocks are concentrically zoned texturally and each contains three stacked molybdenum orebodies and overlapping zones of alteration and mineralization. Mapping and petrographic/fluid inclusion studies indicate that in each alteration/ mineralization pulse, early quartz-molybdenite ± chalcopyrite ± pyrite veinlets with K-feldspar alteration envelopes ( T = 470–750°C; wt.% NaCl eq. = 8–19%) were followed by quartz-muscovite ± pyrite ± flourite (greisen) veinlets with envelopes of coarse-grained (1–23 mm) muscovite. Later fluids in some pulses formed quartz-magnetite veinlets or chlorite-enveloped (quartz-pyrite-chalcopyrite ± magnetite) veins, but all pulses were ultimately closed by quartz-galena-sphalerite-chalcopyrite ± tetrahedrite ± pyrite veins having no alteration envelopes ( T = 370–450°C; wt.% NaCl eq. = 30–41%). Regional structure and paleomagnetic data indicate that during Tertiary time, the stocks were tilted eastward about 70°. Alteration/mineralization zoning is consistent with such a tilt and trace-elemental dispersion patterns are interpreted in this context. Molybdenum orebodies are related to quartz-molybdenite veinlets, with successively deeper orebodies related to veinlets produced by successively deeper phases of each stock. Copper highs (200–1000 ppm) generally occur with molybdenum highs (400–2000 ppm) as a result of the nearly ubiquitous occurrence of chalcopyrite in the quartz-molybdenite veinlets. Fluorine highs (1000–21000 ppm) are related to greisen veinlets and occur 30–100 + m outward of molybdenum highs. Tin anomalies are generally weak (5–25 ppm) and either coincide with fluorine highs (in one stock) or are sporadically distributed (in the other stock). Tungsten anomalies are also weak (5–18 ppm) and although locally present with tin and fluorine highs, are more typically sporadic in distribution. Anomalies of lead (300–5000 ppm), zinc (200–4000 ppm), manganese (150–3200 ppm), and silver (3–30 ppm) are related to base-metal veins and occur 40–150 m outward of molybdenum highs. Arsenic is sporadically distributed in general, but correlates well with lead-zinc-manganese-silver highs where copper highs are also present. Dispersion patterns for lithium, bismuth, thallium, antimony, niobium, and gold are also briefly discussed.