AN INVESTIGATION OF QUARTZ-SULFIDE VEIN FORMATION AND DEPTH CONDITIONS BY QUARTZ AND FLUID INCLUSION PETROGRAPHIC ANALYSES FROM THE WINSTON AND DIAMOND HILL MINING DISTRICTS, ELKHORN MOUNTAINS, MONTANA

Porphyry Cu-(Mo-Au) style mineralization is widely distributed throughout the polyphase Boulder Batholith, but few studies have been conducted on the nature of quartz-sulfide veining from early, 71-77 Ma, satellite intrusions compared to the well-studied Butte district. Porphyry deposits form at different depths ranging from ≲ 1.5-10 km. Quartz veins in these deposits have distinct textures and fluid inclusion characteristics that reflect the depth of emplacement of the causative intrusion. A reconnaissance petrographic study was conducted on quartz-sulfide veins from the Winston and Diamond Hill mining districts to interpret the depth and conditions of their formation.

At the Winston district, quartz-sulfide vein samples were collected from the Edna and Vosburg stocks. Petrographic analyses of vein samples at both locations reveal an early generation of anhedral quartz with secondaries of abundant CO₂-rich fluid inclusions. The aqueous-carbonic inclusions indicate entrapment at very high pressure from a deeply emplaced intrusion (≳ 7-8 km). South of the Winston district, vein samples were studied from the Diamond Hill quartz monzonite intrusion. Quartz ± magnetite ± sulfide veins contain early anhedral quartz with abundant decrepitated fluid inclusions. The early quartz is overgrown by a later quartz generation that exhibits euhedral terminations. Both types of quartz lack CO₂ inclusions and are instead cross-cut by hypersaline liquids. The hypersaline inclusions have small vapor bubbles and homogenize by halite dissolution instead of vapor disappearance. Vapor-rich fluid inclusions are largely absent, suggesting the hypersaline inclusions did not form by fluid immiscibility and indicate depths of ≳ 5-6 km. This can occur by chloride-saturated magmatic fluids that are the first to exsolve from plutons under deep and high confining pressures. Veins from the Winston and Diamond Hill districts were formed by two types of deep intrusions that are characterized by CO₂-rich and high salinity fluid inclusions, respectively. Therefore, these veins express notable differences compared to classic shallow- to intermediate-depth porphyry Cu deposits that show ubiquitous evidence for immiscibility. The veins from these satellite intrusions appear to resemble a deep, reduced intrusion-related system.