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Scientific Investigations Report 2010–5070–D

Arc-Related Porphyry Molybdenum Deposit Model

By Ryan D. Taylor, Jane M. Hammarstrom, Nadine M. Piatak, and Robert R. Seal, II

Chapter D of Mineral Deposit Models for Resource Assessment

Thumbnail of and link to report PDF (17.9 MB)Abstract

This report provides a descriptive model for arc-related porphyry molybdenum deposits. Presented within are geological, geochemical, and mineralogical characteristics that differentiate this deposit type from porphyry copper and alkali-feldspar rhyolite-granite porphyry molybdenum deposits. The U.S. Geological Survey’s effort to update existing mineral deposit models spurred this research, which is intended to supplement previously published models for this deposit type that help guide mineral-resource and mineral-environmental assessments.

Arc-related porphyry molybdenum deposits are a substantial resource for molybdenum metal and may have anomalous concentrations of tungsten. The deposits contain low-grade ore (0.03–0.22 percent molybdenum) as molybdenite, but are large-tonnage, making them amenable to bulk mining open-pit techniques. The mineralizing system usually has fluorine contents of less than 0.1 percent. The cogenetic intrusion is a differentiated calc-alkaline granitoid, typically granodiorite to quartz monzonite in composition, with low rubidium and niobium, and moderate to high strontium concentrations. Metals and hydrothermal fluids are sourced from these intrusions, with an additional meteoric fluid component contributing to peripheral alteration but not adding more metal. The lithology of the surrounding country rocks is not important to the formation of these deposits, but a surrounding carbonate unit may be altered to skarn that contains economic mineralization. The creation of contact-metamorphosed hornfels adjacent to the intrusion is common.

Formation of arc-related porphyry molybdenum deposits typically occurs within a continental arc environment related to arc-continent or continent-continent collision and subduction. Few deposits are found in an island arc setting. Most classified arc-related porphyry molybdenum deposits are located in the western cordillera of North America, notably in British Columbia and Alaska.

Hydrothermal alteration provides a key component to the identification of a deposit. Alteration usually is zoned from a core of potassic plus/minus silicic alteration outwards through phyllic to propylitic alteration. Argillic alteration may be irregular in shape and will overprint earlier hydrothermal alteration.

Exploration should be limited to magmatic arc belts that have been unroofed and eroded to levels of a few kilometers depth. Important geological vectors toward areas of higher grade mineralization include intensity of hydrothermal alteration, veining, and faulting. Anomalous levels of molybdenum, tungsten, copper, lead, or zinc in soils, tills, stream sediments, and drainage waters may indicate the presence of an arc-related porphyry molybdenum deposit. Geophysical exploration techniques have been met with minimal success because of the overall low concentration of associated sulfide and oxide minerals.

Geoenvironmental concerns are generally low because of low volumes of sulfide minerals. Most deposits are marginally acid-generating to non-acid-generating with drainage waters being near-neutral pH because of the acid generating potential of pyrite being partially buffered by late-stage calcite-bearing veins. The low ore content results in a waste:ore ratio of nearly 1:1 and large tailings piles from the open-pit method of mining.

First posted June 15, 2012

For additional information contact:
USGS Central Mineral and Environmental Resources Science Center
Box 25046, MS-973
Denver Federal Center
Denver, CO 80225

http://minerals.cr.usgs.gov/

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Suggested citation:

Taylor, R.D., Hammarstrom, J.M., Piatak, N.M., and Seal, R.R. II, 2012, Arc-related porphyry molybdenum deposit model, chap. D of Mineral deposit models for resource assessment: U.S. Geological Survey Scientific Investigations Report 2010–5070–D, 64 p.



Contents

Abstract

Introduction

Deposit Type and Associated Commodities

Historical Evolution of Descriptive and Genetic Knowledge and Concepts

Regional Environment

Physical Description of Deposit

Hypogene Ore Characteristics

Hypogene Gangue Characteristics

Hydrothermal Alteration

Supergene Ore Characteristics

Supergene Gangue Characteristics

Weathering/Supergene Processes

Geophysical Characteristics

Geochemical Characteristics

Petrology of Associated Igneous Rocks

Petrology of Associated Sedimentary Rocks

Petrology of Associated Metamorphic Rocks

Theory of Deposit Formation

Exploration/Resource Assessment Guides

Geoenvironmental Features and Anthropogenic Mining Effects

Mine Waste Characteristics

Knowledge Gaps and Future Research Directions

Acknowledgments

References Cited

Appendix 1

Appendix 2


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