From the Wikipedia: The Black Diamond Coal Mining Company was formed in 1861, consolidating the Cumberland and Black Diamond coal mines in the region of Mount Diablo, in Contra Costa County, California. During its years of operation as a mining company, it established three towns: Nortonville , California, Southport , Oregon, Black Diamond and Franklin Washington. The company's mines in California and its settlement of Nortonville later became part of the Black Diamond Mines Regional Park and a California Historical Landmark. Several railroad lines were built in California and Washington to support the company's mines, and the company operated numerous ships to transport its coal. As the mines played out and petroleum became the more common source of energy, the company closed its mines and transitioned into real estate as the Southport Land and Commercial Company.
We know that miners from the Nortonville/Somersville area were transferred north to Black Diamond WA in 1885 after the closure of the Black Diamond mines in California. Whilst there is no shortage of historical resources and information on the Black Diamond/Franklin communities in Washington the Southport settlement seems to have disappeared without trace.
Around 1876 a man named B.B Jones opened the Southport mine and the town grew up to provide accommodation for the workforce. By 1882 Southport boasted a population of 100 (approx), a hotel/saloon and several stores. It was a company town of the Black Diamond Coal Mining Company. It never had its own post office instead relying on the office at nearby Coos City for postal services. The town site is situated on Southport Lane, off of US Highway 101 (see map in the right hand column). Many miners from both Southport and Nortonville moved to the new mine at Black Diamond WA after it opened in 1885. After this date the town went into decline and eventually disappeared although it is still marked on an 1896 topographic map . The mine was leased (but not worked) by a rival coal company for some years after this date.
The Black Diamond Coal Company's history with the Southport Mine is detailed in this brief excerpt from the excellent Southport Land & Commercial Company History
" In the late 1870s the Company acquired two coal bearing properties near present‑day Coos Bay, in southeast Oregon. Exploratory tunnels were dug at the 956‑acre Black Diamond Mine on the peninsula formed by Haynes and North Sloughs, but the mine was never fully developed. In contrast, the 2,636‑acre Southport Mine on Isthmus Slough, across from Coos City, became very prosperous, with the small town of Southport, Oregon growing up in the canyon below the mine. A few years after its initial development, the Company leased the Southport Mine, the mine’s railroad and a nearby wharf to a competing coal mining company. "
There are other ghost towns in the area around Southport, notably Coos City (mentioned above) and Beaver Hill. The latter is a small former mining settlement, the site of which can be viewed in Google Maps . There are no remains above ground but the layout of the former town and pit workings can be seen quite clearly particularly in 3d view.