CONTACT METAMORPHISM, MINERALIZATION, AND DETRIAL ZIRCON INTERPRETATION FROM SILVER HILL SPOKANE WASHINGTON

Silver Hill, in Spokane County, Washington, hosts an abandoned tin-tungsten mine from the early 1900’s. Detrital zircon analysis yielded unexpectedly young detrital zircon (DZ) ages in the host rock. Twenty-seven of 113 zircon grains were about 48 million-years-old from a rock mapped as Mezoproterozoic metasedimentary rocks, or approximately 1.45 billion years old Belt Super group. Why and how did these young zircon crystals form? There are a few possible reasons why very young zircons are present in the quartzite, but one key factor is that the proximal and sometimes cross-cutting (intruding) granite is also about 48 million years old (Stephens et al., 2017). Possible hypotheses for these young DZ ages:

1) Zircon grains were deposited with ash fallout, buried, and then tectonically ex-hummed within the error (2%) of age determination (about a million years).

2) Young zircon grains were from contamination during sample preparation.

3) Zircon grains crystallized during contact metamorphism from the granite.

Younger zircons in the quartzite were small, 20 micron and equant, suggesting that they were not eroded or transported. Burial, lithification, uplift and high temperature (600 to 900 degrees Celsius) would suggest recrystallization by contact metamorphism. Obliterated small and equant zircons crystal associated in an ash deposit. Contamination was discluded because small grains of zircon were observed in micro-probe analyses which was done at WSU to analyze the samples of granite and quartzite. This data yielded the different types of feldspar's within the rock and examined the grains that were in contact with one another to analyze using two-feldspar temperature and pressure ratios. This was to determine where in the thermal gradient the young zircon crystals grew and to show it was not deep enough for complete melt of the rock. Silver Hill is the roof pendant of this intrusion. The similarities of age of the granite and the younger zircon grains suggest contact metamorphism. The presence ore: sheelite, wolframite (Tungsten ore) and Cassidertie (tin ore) also suggest shallow hydro-thermal enrichment associated with intrusions and contact metamorphism. Therefore, this study concludes that the host rock is Neoproterozoic, the granite is Eocene, and that the ore enrichment was likely coeval with the intrusion.