When we became a country the very first building our Nation built was the U.S. Mint in Philadelphia. It wasn't the White House, not a Congressional building but the Mint. There was a drastic need to consolidate usable currency in our fledgling Country. As our Nation grew so did the need for more currency. In 1835 a Congressional bill passed for the expansion of three more mints. Philadelphia was, and still is today, the main mint and all other mints would be considered branch mints. All three of these branch mints would open their doors in 1838. They were to be located in New Orleans, Louisiana; Charlotte, North Carolina; and the "D" before there was a Denver Mint... Dahlonega, Georgia.
The very first question most people ask is "why there?" but there is good reason. Needless to say there was no Fed-Ex or UPS in 1835 and moving coins from Philadelphia to the rest of the country could be daunting to say the least! This also means moving gold into Philadelphia from other parts of the country was likewise a difficult venture. So, if you can't move the gold to the Mint, move the Mint to the gold! That is exactly what happened for Dahlonega as well as Charlotte. When we think Gold Rush most of us think of the California gold rush but most of us do not realize there was a gold rush in Georgia in the late 1820's. With gold readily available, Dahlonega was a prime location for the new branch mint. This is also why only gold coins were struck in Dahlonega, no silver at all. Since these coins were specifically made for circulation, only the smaller denominated coins were made here. They only produced the $1, $2 1/2, $3, and $5 gold coins and bore the single D mintmark of the Dahlonega mint.
In 1861, everything changed. With the Civil War raging, eventually the Dahlonega Mint fell under Confederate control and some believe coins were produced by the Confederates in 1861. The mint building would be mostly unused until it became the North Georgia College in 1873. Unfortunately, the original mint building would catch fire and burn down in 1878.
Today, Dahlonega struck coins are very rare and very pricey! Dahlonega coins are prized by collectors and no serious collection is without one. So the "D" we see today on modern coins is Denver but the "D" mintmark on older gold adds some serious credentials to those small gold coins of the Dahlonega mint.