English: The town of Butte, Montana is known as the “Richest Hill on Earth” and "The Mining City". The Butte Mining District has produced gold, silver, copper, molybdenum, manganese, and other metals.

The area's bedrock consists of the Butte Quartz Monzonite (a.k.a. Butte Pluton), which is part of the Boulder Batholith. The Butte Quartz Monzonite ("BQM") formed 76.3 million years ago, during the mid-Campanian Stage in the Late Cretaceous. BQM rocks have been intruded and altered by hydrothermal veins containing valuable metallic minerals - principally copper sulfides. The copper mineralization has been dated to 62-66 million years ago, during the latest Maastrichtian Stage (latest Cretaceous) and Danian Stage (Early Paleocene). In the supergene enrichment zone of the area, the original sulfide mineralogy has been altered.

In this outcrop at the Orphan Boy Mine, the rusty brown areas at left are iron oxide-stained hydrothermal vein rocks. The whitish-colored areas at right represent an argillic alteration zone formed next to the vein - the original vein fluids have altered some of the host rock minerals to clay. Kaolinite can be expected in the inner, higher-temperature argillic zone and montmorillonite in the outer, lower-temperature argillic zone.

Hydrothermal vein systems alter their surrounding country rocks to various degrees. The suite of minerals formed by hydrothermal alteration changes with distance from the veins and with depth - this results in an alteration mineral zonation, which can be used to explore for valuable ore deposits. The degree of alteration is also due to the residence time of the hydrothermal fluids and how intensely fractured the wall rocks are.

Wall rock alteration can be simple recrystallization, resulting in a crystal size change. However, alteration usually results in the formation of new minerals. The most readily exchanged materials in alteration environments are Na, K, Ca, Mg, Fe, Al, silica, and carbonate. The effects of alteration are seen by discoloration (often yellowing and red staining), a change in density/specific gravity (the rocks become heavier or lighterweight), and softening of the rocks.

Important types of alteration assemblages include: 1) Silicification - this involves adding silica to the system. 2) Silication - this refers to adding silica to carbonate rocks and moving carbonate out of the system. This results in the formation of skarn deposits and contact metamorphic deposits with reactive country rocks. 3) Carbonatization - this involves adding CO2 to a system to produce carbonate minerals such as dolomite, siderite, calcite, etc.

Wall rock alteration styles vary with temperature. High-temperature alteration involves greisenization, tourmalinization, garnetization, and the formation of potassium silicates. Moderate-temperature alteration involves sericitization, argillization (dickite, kaolinite), serpentinization, epidotization, and zeolitization. Low-temperature alteration includes opalization, chalcedonization, argillization (kaolinite, montmorillonite), propylitization, alunitization, adularitization, serpentinization, epidotization, and zeolitization. Other wall rock alteration styles include the formation of quartz (at high to low temperatures), jasperoid (at moderate to low temperatures), and pyrite (at high to low temperatures).

Potassic zone alteration assemblages will have potassium feldspar and biotite mica - may also expect sericite, quartz, and anhydrite. Phyllic zone alteration assemblages will have secondary quartz, sericite, and pyrite. Argillic zone alteration assemblages will have clays, with kaolinite and dickite in the inner, higher-temperature zone and montmorillonite in the outer, lower-temperature zone. Propylitic zone alteration assemblages will have epidote, chlorite, carbonate, and pyrite.

Locality: grounds of the Orphan Boy Mine, western side of the town of Butte, southwestern Montana, USA (vicinity of 46° 00' 42.36" North latitude, 112° 34' 05.50" West longitude)