Squyres said in the latest news conference the origin of this rock remains unexplained, so lets start off with some hypotheses.
This rock and rocks nearby certainly give the impression of being subjected to some type of chemical weathering either by liquid water alone or in combination with acids. This impression is made especially strong in the microscopic images.

I found these collected images in a post by Bruce Moomaw to the ISSDG forum:

taken from:

ISSDG · Inner Solar System Discussion Group (ISSDG)
From: "Bruce Moomaw" <rmoomaw@s...>
Date: Tue Jun 22, 2004 5:31 am
Subject: Latest interesting Mars photos
http://groups.yahoo.com/group/ISSDG/message/6594

The thinking now is that sulfuric acid was involved with the weathering seen at Meridiani and Gusev:

The Geology Of Mars Mid-'04
by Bruce Moomaw
Sacramento (SPX) Jun 08, 2004
"Now, what could have made this stuff? The MER science team is still not certain. But Nicholas Tosca of the State University of New York had run an extremely suggestive lab experiment just a few months earlier, in which he exposed basalt sand to various concentrations of sulfuric acid solution - and, lo and behold, ended up with a mixture of amorphous silicates, jarosite, magnesium and calcium sulfates, and the iron oxyhydroxide "goethite", which when dehydrated for just a few hundred thousand years turns into coarse gray hematite.
"And - as pointed out by other scientists such as Benton Clark and Jeffrey Kargel - it seems increasingly likely that sulfur in general, and sulfuric acid in particular, has played an important role in Mars' surface chemistry."
http://www.spacedaily.com/news/mars-mers-04zzzzx.html

The idea that sulfuric acid weathering may be involved brings to mind the possibility of acid rain and the dissolution of carbonate and sulfate that can cause:

Weathering Research Group.
http://www.qub.ac.uk/geog/documents/research/weathering/usd.html

Acid rain can be caused not only by pollution from human sources but also by volcanic activity.
The acid weathering may also involve carbonic acid which arises from interaction of water and carbon dioxide in the atmosphere on Earth.

The microscopic images of the stalks brought to mind the appearance of stalactites or stalagmites on Earth, especially in combination with their possible formation from acid weathering.
This page discusses stalactites with a biological influence:

Cave Slime
http://www.astrobio.net/news/article497.html

The stalks in Pot-of-Gold were not vertical however. But if you look at the Pancam image you can see there are several layers and interestingly the stalks in the same layer seem to be pointing in the same direction. Then perhaps they were formed in some manner similar to stalactites where instead of dripping acidic water from the ceiling, wind or flowing water induced their precipitated formation from acidic water, according to the direction of the wind or water flow.

Bob Clark