The Eyebrow is one of the archaeological sites covered by Patricia Flint in her excellent UM master's thesis on local archaeology. This quarry is no doubt the reason the creek just to the east is called Flint Creek, and that the Hellgate River was called the Arrowstone by early geographers.
The term "flint" is a slight misnomer, as "flint" is a variety of chert, a sedimentary form of silica, while the silica at the Eyebrow is hydrothermal in origin, and would better be described as chalcedony. Nonetheless, since "Chalcedony Creek" doesn't have the same ring as "Flint Creek", we will refer to the stone at the site as "flint" for historical reasons.
Just to the south of the Eyebrow is Henderson Creek. Apparently a party traveling near the landmark of the Eyebrow ("Arrow Point") tried a pan in the nearby creek, got some gold, and reported it back to Angus McDonald. An account by McDonald (see P. 201) confirms that he assayed gold from Flint Creek prior to 1850, when he reported it to John Owen and some friends - apparently including William Graham - and that the first discovery of gold in what would become Montana was made at Flint Creek. Graham provides the additional information that McDonald then sent out a prospecting party, but they got little done because hostile Blackfeet were in the area. Still, by Graham's account, confirmed by McDonald, it is apparent that some gold was recovered very early in Henderson Gulch, in fact before the gold discoveries in the Bitterroot and Gold Creek.
Below are two maps - a Google Earth aerial view, and a snippet of Lonn et al's 2010 geologic map of the Missoula East quad. In the air photo from Google Earth, you can see why the "Eyebrow" got its name - a patch of timber on the hill indeed looks much like an eyebrow. You can also get an idea of how close the flint quarry at the Eyebrow is to the major placer gold mines along Henderson Creek. The portion of Henderson shown has been extensively dredged. On the geologic map, the Eyebrow is in about the center of the map, in Cambrian sedimentary rocks (Hasmark Formation) with Tertiary andesite and basalt a short distance to the west... the likely source of the hydrothermal fluids that formed the flint.