By Ralph E. Pray, DSc, Metallurgical Engineering

Drill holes in a mineralized vein often penetrate hidden layers of poor ore which, after blasting, end up diluting the production quality. Disseminated gold ores carrying two ounces per ton may be identical in appearance to adjacent vein material assaying less than a tenth of an ounce. The operating miner needs some early control over what he sends to the mill. One way to do this may be to pan the cuttings prior to shooting. Before panning, the miner can grind the cuttings in the cap of an oxygen bottle using a steel bolt or old drill steel. Another way to get control is to assay the drill cuttings. An ounce or two of cuttings taken at different depths will identify grade zones behind the face. Following the assay, if the face area only is poor it can be shot as muck while the back of the hole is blocked off. If the back of the hole is poor it can be blocked off with fines or dowling while the face is shot as ore.

One of the disadvantages to this scheme is waiting for the assay. A bit of patience is required. Where a round of six or eight holes has penetrated five feet of vein the miner is used to loading and shooting. It's no fun toting sacks of wet muck to the Greyhound bus stop and waiting by the telephone to hear from some geezer who goes underground only at Disneyland. But it's done, and an example will show how it works.

The Test The Carbonate King zinc mine on Kokoweef Mountain in San Bernardino County, California, produced zinc mineral under government subsidy during war-time. More recently I sampled the old workings with the hope there were still values present. The owners were more interested in an "underground river of gold" tale than in talking about zinc, but we signed a mine agreement. No sulfides were present. The zinc carbonate ore appeared just like the limestone it occurred in. High-grade and low-grade looked identical.

The initial sample assays across vein remnants showed Zn and silver values of sufficient interest to seek an ore buyer.

Sample Site & Width  Zn(%)  Ag(opt)   
Bottom of shaft-left 8 ft  34.6  3.3   
Bottom of shaft-east 30 ft  10.4  6.0   
Bottom of shaft-right 6 ft  31.8  22.4   
50 ft-level 6 ft  34.9  3.0   
Shaft behind ladder 6 ft  33.6  6.4   
Track level near portal 2 ft  31.4  10.3   

Samples sent out to ore buyers resulted in a request from Sherwin Williams Chemical Company for additional rock samples. Their assay on the first sample was 32.3% Zn. They commented that this was an unusually high content for rough carbonate ore, and had trouble believing the sample was representative.

Unfortunately, Sherwin-Williams did not have a silver recovery circuit in their white paint plant in Coffeeville, Kansas. So Ag values were not considered. But they liked the second sample so much we received a purchase order for two carloads (120 tons) of as-mined ore.

The Operation Mining equipment was moved to the old zinc works. Mine Superintendent Frank Harris left the lab with a house trailer to live on the property and begin drilling with a small crew. There was no blasting. Drill cuttings were bagged, tagged, and taken to the Mobile Station at Bailey Road on the I-15 freeway west of Las Vegas for pick-up by Greyhound Bus Lines. I met the bus at the Pasadena Station and immediately assayed the samples for zinc. The first batch of ten drill-cuttings assayed as follows:

Sample No.  Zn (%) 
1 34.1 
10  2.0 

I called Frank at the Mobile Station with these numbers three hours after the samples arrived at the lab. He shot five of the holes as ore and five as waste. The ore was trucked to a gondola car at the nearby Nipton railroad siding. Waste was trucked out for road fill.

The process was patiently repeated until the high-grade was exhausted. We couldn't hold grade at 30% Zn, but managed to stay above the mid-twenties. The last ore bench in our main stope assayed 25.1% Zn from drill cuttings. The last car loads leaving the mine assayed 23.0% Zn.

The Shipment
Sherwin-Williams, calling this a "test lot," assayed the blended ore at 25.5% Zn and paid the rail freight. They processed the ore chemically to zinc oxide for the manufacture of white paint. This highly profitable shipment, which marketed 25,000 pounds of zinc metal as the carbonate, was, according to the California Division of Mines and Geology, the last zinc ore commercially mined in California. The results testify to the advantage of selective blasting in a small mine vein following the assay of drill cuttings.

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