Porphyry Copper Systems of the Boulder batholith, Montana

Science Issue and Relevance

The composite Boulder batholith, a large mass of intrusive igneous rock, in southwest Montana hosts a variety of mineral deposit types including:  

1. important Cretaceous silver-polymetallic vein districts in the north, 

2. Cretaceous gold-silver-polymetallic veins overprinted by Paleocene porphyry copper-molybdenum stockwork deposits (complex system of structurally controlled or randomly oriented veins) and copper lodes (ore containing primarily metal that fills or is embedded in a fracture in a rock formation) in the south, 

3. unmined porphyry copper-molybdenum deposits of unknown age, origin, and formation, and  

4. historic skarn deposits in the Elkhorn Mountains. 

In 2022 and 2023, the USGS Earth Mapping Resources Initiative (EarthMRI) conducted high-resolution airborne magnetic and radiometric surveys flown over most of the exposed Boulder batholith and surrounding area in partnership with the Montana Bureau of Mines and Geology and Rio Tinto. The survey data provide geologic mapping support to understand the lithologies and hydrothermal alteration associated with the mineral systems. The Earth MRI effort has noted the potential for critical minerals, but their distribution, timing, and genetic relationship to the batholith and associated mineral deposits are not well understood. Understanding where critical minerals occur in high concentrations is deemed important for the Nation’s security. 

Methods to Address Issue

This project provides geologic support to better understand the mineral systems of the Boulder batholith with the overarching objectives being: 

• Development of new geophysical, geological, and metallogenic interpretations that are informed by new airborne geophysical data, 

• Development of geologic characterization to provide broader scale geologic, geochronologic, and petrogenetic (rock formation) interpretations in support of detailed Montana Bureau of Mines and Geology mapping work, 

• Development of new interpretations of the relations among volcanic and plutonic rocks and accompanying mineral systems. 

Porphyry copper systems, like that found near Butte, supply nearly three-quarters of the world's copper, half of the molybdenum, around one-fifth of the gold, most of the rhenium, and minor amounts of other metals such as silver, palladium, tellurium, selenium, bismuth, zinc, and lead. Such an abundance of metals makes these deposits an important geologic phenomena to understand. Our stakeholders in other government agencies that manage lands rely on our understanding of these deposits when making land management decisions. In addition, understanding the critical minerals in these deposits is important for national security as many of these minerals are needed for technology applications.