Even though ARPA does not specifically mention "metal detecting", you need to understand what ARPA says about digging artifacts.
Below is a brief definition of the Montana law according to the ARPA law:
Code Book: Montana Code Citation: §22-3- 432 Section Title: Antiquities: permits Summary: (1) Prohibits a person from excavating, removing or restoring any heritage property or paleontological remains on lands owned by the state without first obtaining an antiquities permit from the historic preservation officer. (2) Requires strict compliance with the following guidelines: (a) permits may be granted only to reputable museums, universities, colleges or other historical, scientific or educational institutions, societies or persons with a view toward dissemination of knowledge about cultural properties, provided that the historic preservation officer must be satisfied that the applicant possesses the necessary qualifications to guarantee the property excavation of those sites and objects; and (b) a permit must specify that a summary report of such investigation, containing relevant maps, documents, drawings and photographs, must be submitted to the historic preservation officer, within a specific time period to be determined by the historic preservation officer. (3) Directs that all heritage property and paleontological remains collected under an antiquities permit are the permanent property of the state and must be deposited in museums or other institutions within the state or loaned to qualified institutions outside the state, unless otherwise provided for in the antiquities permit. (4) Declares that an antiquities permit is not a substitution for any other type of permit that a state agency may require for other purposes. Primary Topic: Archeological Activities
In addition to the above mentioned ARPA law, if you are doing any treasure hunting in Montana, you must be aware of another law, the National Historic Preservation Act. (NHPA)
As confusing as the above laws may seem, if you are metal detecting in Montana, do not dig anything that you believe is an artifact, or any item that is older than 100 years, especially if you are on any State Lands.
If you want to detect on private property, then be sure you get written permission from the landowner.
For detecting in Montana, town, village and city parks, you'll need to check with those local officials.
I cannot be responsible for any outdated laws from the time of this posting.