Rock Creek

Geology of Rock Creek – Sapphires in Debris Flows

Map of Potentate Mining’s Rock Creek Property

In 1993 Dr. Barron completed an informal geological survey on part of the Gem Mountain property known as the Dann Placer, which at the time was incorrectly regarded by local miners as a perched alluvial bench above the Rock Creek valley. Dann Placer is known for phenomenal mining grades, often over 1,000 carats per cubic meter, but mining exposures showed no evidence of sorting and concentrating of the gravels and sapphires by rivers or streams.

The host material was a random mixture of rounded and angular rock fragments in a muddy and clay-rich matrix with the sapphires distributed throughout this matrix. There was only rudimentary bedding, unlike the typically well-bedded and sorted alluvial gravel deposits. Thus Dr. Barron had determined that the sapphire rich deposits are actually colluvium, or debris flow deposits, produced by a geological process known as mass wasting.

Mass wasting is effectively the formation of overlapping deposits from mudslides and landslides during humid events. Anything in the landslide’s path is caught up in the mudflow and so when it comes to rest it is a hodgepodge of everything that slid off the mountain.

The gullies and gulches throughout Gem Mountain have been subsequently reworked through the geological time frame and this colluvium was washed and sapphires naturally concentrated in the drainages as alluvial placer, producing the phenomenal grades that the old-timers worked.

In 1993, except for Dann Placer, the district was incorrectly considered as mined out with the resource effectively depleted in the gulches. As a result of testing due to the improved road access, it has become apparent that the areas between the gulches contain very significant concentrations of sapphires that were, and remain, virtually untouched. These areas between the old drainages are completely virgin ground which the old-timers had ignored for various reasons.

Firstly, because it was thought worthless; secondly, for the practical reason that a supply of water with sufficient hydraulic pressure was unavailable on the hill tops to process the colluvium for testing and sluicing; and thirdly, and most importantly, this was formerly US Forest Service land and hence out of the purview and ownership of the miners.

Almost all of the historic mining, even up to the 1990s, was only in the drainages and gulches of Gem Mountain. It was not until the consolidation of the ground by RY Timber that these hilltops were released from US Forest Service control and thus became private property.

In the summer of 2014, Potentate completed encouraging bulk sample testing on the south side of Gem Mountain on the high ground between Mink Gulch and Wild Cat Gulch and returned favorable values of in excess of 50 carats per cubic meter of gravels with some spectacular sapphires being recovered. Test mining, sampling and geological mapping will continue in 2015, and future seasons, especially in pursuit of the higher grade zones of sapphire occurrence.

The entire area of Gem Mountain is fortunate that it has never been subjected to alpine or continental glaciation, thus the mantle of sapphire-bearing residuum has not been removed. No convincing examples of sapphire in the matrix have been found to date, however we may speculate as to their likely origins.

Geological Origin of the Sapphires

Rock Creek Sapphires

There has been much said over the years about the possible origins of these considerable quantities of sapphires. Berg (2014) mentions that the source rocks, that are found to outcrop at Gem Mountain, may be volcanic rocks but there has been no bona fide occurrence of the sapphires having being found in-situ. Currently, the sapphires are found solely in colluvium and alluvial secondary concentrations in unsorted mud flows. It is clear from the sapphire distribution geographically that these debris flows of colluvium, and the secondary alluvial deposits in the gullies, have transported the sapphires only a short distance.

Unpublished microprobe studies have shown the presence of niobian rutile inclusions within the sapphires. In addition, niobian rutile megacrysts as large as 0.5 to 1 cm are often found in the jig concentrates along with the sapphires. The sapphires recovered in the jigs are typically rounded and resorbed-looking, much like the look of a candy that has been sucked, in a similar way to diamonds and pyrope garnets from kimberlite.

This implies that the sapphires are xenocrysts from a deeply-derived igneous host rock that on ascent and emplacement has plucked the sapphires from the lower crust or potentially even the upper mantle. About five kilometers from Gem Mountain there is a prominent outcrop of alkali basalt. Though this occurrence is not sapphire-bearing, alkali basalt is a known source rock for sapphires in Shandong, China, Queensland, Australia, and in Nigeria.

We can speculate that a basalt flow or sheet containing sapphire was entirely weathered away, possibly during a tropical regime, to leave only the resistant minerals behind. A ground magnetometer survey performed last year has isolated several anomalies which may be feeder dikes or blows of an alkali basalt or lamprophyre intrusive. These magnetometer surveys are clearly not caused by the rhyolite nor the quartzite country rock. The geophysical anomalies will be further tested in the summer of 2015. Once the elusive source rocks are defined using geophysics and trenching then it is anticipated that these areas may generate higher concentrations of sapphires to increase the sapphire resource on Gem Mountain.

Size Distribution of Rough Sapphires Recovered

Very comprehensive sampling and size distribution analysis completed on the sapphires recovered at Rock Creek was performed in the past by American Gem Corporation (1994-1996) for all stones down to less than 2 mm and it was determined that the vast majority would be less than 3.5 mm in dimension. Since the market for large quantities of sapphires of such small dimensions is limited, Potentate anticipates recovering only stones in excess of 3.5 mm. This would be accomplished by setting the screens in the jigs to 1/8 inch or approximately 3.5 mm.

Thus, sapphires under 3.5 mm would not be recovered and the size distributions would be skewed to a much larger size fraction. Potentate has found from its own testing program that, of the recovered sapphires from the heavy mineral concentrate, approximately 27% range in dimensions from 2.5 mm to 4.4 mm, and 65% from 4.4 to 8.5 mm. Large sapphires greater than 8.5 mm represented an astonishing 6.4% of the sampled areas, thus Potentate expects to produce a highly market acceptable product of larger blocky (high cutting yield) clean sapphire rough greater than 0.50 carats and up to 25 carats in size.