ZIRCON AGE AND GEOCHEMISTRY OF GRANITES RELATED TO PORPHYRY AND POLYMETALLIC MINERAL DEPOSITS OF THE EASTERN ELKHORN MOUNTAINS AND BOULDER BATHOLITH, MONTANA

New U-Pb and Ar-Ar data for the Elkhorn Mts, which lie east of the Boulder batholith and west of the east-vergent Lombard thrust, constrain the age of polymetallic Au-Ag veins, Pb-Zn-Ag carbonate replacements, and small Cu skarns. Porphyry intrusions with Cu-Au-bearing sulfides, quartz veins, and hydrothermal biotite were intersected by drill core at Radersburg by Exxon Minerals in the 1970’s, and contain sulfides in the Diamond Hill and Vosburg stocks.

The lower and middle members of the Elkhorn Mountains Volcanics (~86-83 Ma) are gently folded. An Ar-Ar age of 85.3 ± 0.3 Ma (hbl) from an andesite dike may be part of this magmatic event. Younger pyroxene-biotite monzodiorite is undated, but a series of granite stocks and dikes range from 78 to 70.5 Ma and are likely more shallowly emplaced than those at Butte (~7-8 km). The oldest in the Radersburg area are shallowly emplaced chlorite±epidote-altered andesite porphyry sills and dikes of the Rattlesnake intrusion (Ar-Ar hbl 78.2 ± 0.1 Ma). The Rattlesnake intrusion is cut by the Congress Mine porphyry (U-Pb zr 75.9 Ma), associated with productive Au-Cu veins and deeper porphyry Cu-Au mineralization at Radersburg. Post-ore lamprophyre dikes (Ar-Ar bi, hbl, 78.0 ± 0.1 Ma), suggest a short duration of magmatism at Radersburg. Four of the granites dated by U-Pb zircon are 76-77 Ma, similar to the 76.3 Ma Butte granite age, and therefore likely part of the Boulder Batholith. The Vosburg porphyry is 70.5 ± 0.4 Ma and is the only Boulder Batholith granite recognized so far that is intermediate in age between the Butte granite and the 66 Ma porphyries of the Butte district.

Zircon Ti-compositions imply a range of temperatures (~1000-700°C) and are lowest (≥700°C) in the mineralized Congress Mine and Vosburg porphyries (Table 1). Alike other ore-related granitic suites, both Yb/Gd and Eu/Eu*(cn) increase as Ti-in-zircon temperature decreases. Anomalous values (Eu/Eu*>0.5) characterize the mineralized granites as at Butte, but not the Lone Mt and South Fork stocks.

Table 1: LA-ICP-MS zircon data				Lo & Hi are minima & maxima
Intrusion	Comp’n	Age (±2s )	Low Ti-zr T		Hi Eu/Eu*cn	Hi Yb/Gd
Vosburg Ppy	Gd-Gr	70.5 ± 0.4	700°C		0.90	9
Diamond Hill	Gd	77.1 ± 1.6	800°C		0.75	6
South Fork	QMD	76.1 ± 1.3	800°C		0.40	4
Congress Ppy	Gd	75.9 ± 0.7	700°C		0.85	11
Lone Mt	Gd-QM	76.1 ± 0.8	850°C		0.50	5
Butte Qz Ppy	Gr	~66 Ma	750°C		0.85	25