A mining rebirth? - Historic US mines

Materials World magazine
,
1 Nov 2009

Canadian company Lincoln Mining Corporation is keen to start gold production at the Wilson and Wheeler mines in the USA. Historically yielding high levels of gold, the mines may yet offer more metal.

The California gold rush at Sutters Mill on the America River in the USA triggered a prospecting boom over Sierra Nevada, looking for the Mother Lode that was the source of the alluvial gold.

One of the subsequent finds was at Pine Grove near the California-Nevada border, some 20 miles south of Yerington. Gold was first discovered here in 1866 and, within a year, a town of 300 people was established. Mines were developed on both sides of the Pine Grove Creek valley where gold and silver was found in the bedrock. Twenty years later the town had grown to over 1,000 inhabitants and was producing over US$10,000 in gold every week from its three stamp mills.

Two main underground mines were developed, Wilson and Wheeler, and grades were reported at 1.40oz/t and 1.30oz/t respectively. Examination of the old mine pillars revealed a cut-off grade of 0.35-0.50oz. The mining boom ended in 1887 but some working continued into the 20th century when Pine Grove became a ghost town.

Over the following decades limited exploration took place, the most significant being the drilling undertaken by Canadian company Teck Resources in 1988-92. Despite 53,000ft of drilling in 160 holes, low gold prices in the early 1990s resulted in non-renewal of Teck’s leases and mining claims. Lincoln Mining Corporation, based in Vancouver, Canada, began consolidating a land position in the Pine Grove district in 2007 and had doubled its land position to approximately six square miles by early 2009. It has since expanded to seven square miles.

Geology rocks

The regional geology reflects the subduction of the Pacific plate under the North American plate. To the east of Pine Grove lies the Carlin-trend, one of the world’s most prolific gold fields. Here, mineralised fluids from deep batholiths were channelled into old carbonate rocks during an extensional structural stage at 30-42 million years. At Pine Grove, gold mineralisation is associated with a magmatic arc that some 100 million years earlier intruded sedimentary and volcanic rocks. Later, Tertiary volcanics and sedimentary were deposited over the Pine Grove mineralised system.

The most significant local geological feature is the Pine Grove fault, a northwest-striking, northeast-dipping normal fault, defining a 200m-wide extensional shear zone that forms part of the eastern boundary of the Pine Grove Hills structural block. The basement is the Lobell granodiorite and is host to the gold mineralisation. Within Lincoln’s claim block area, gold mineralisation is present in two historic mines. At Wheeler, gold-bearing quartz veins, emplaced in an early metamorphic event, show gold grains from 0.1mm to several millimetres in size. Pyrite and minor chalcopyrite are also present, the latter often coated with gold. The veins can assay over 100g/t gold.

On the opposite side of the valley at Pine Grove Creek is the Wilson mine. Here, mineralisation is confined to discrete tabular zones that dip gently to the north within the granodiorite. There are up to six mineralised zones separated by thicker unmineralised ryholite porphyry and dacite dykes. The mineralisation is traceable down-dip for 150m from the old workings, and for a further 350m the mineralised zones extend virtually flat down-dip to the north where gold-bearing veins have been encountered in drill holes.

The mineralisation at Wilson is much less disrupted than at Wheeler due to a lack of significant shearing events. Alteration at Wilson is similar to that found at Wheeler, although the intensity is weaker. The style of mineralisation is similar with gold bearing veins up to one-metre thick containing pods of sulphide mineralisation, and gold values up to 50g/t over 1.5m and copper values of up to 0.35%.

Detailed information?

‘Details of the 19th century underground workings are lost,’ says Paul Saxton, CEO of Lincoln Mining Corporation. ‘Only the historic reports of gold sold and the amount of tailings indicates the scale of the workings’.

The only verified information on the resources at Pine Grove is from Teck Resources’ drilling programme and some state mining reports. In total, 47,000m was completed by Canadian drilling contractor Major Drilling. At Wheeler, angled holes traced the mineralisation for 300m in the hanging and footwalls, whereas at Wilson the flat lying mineralisation could be traced by vertical holes, with the exception of six angled holes at 70m intervals that were drilled to test for steeper mineralisation controls.

Samples from the Teck drilling were assayed at Chemex Laboratories in Sparks, USA. The results show that gold mineralisation is present in most holes at Wheeler and Wilson with values ranging from three metres at 0.5g/t to 18.2m at 2.9g/t, using a minimum thickness of three metres and 0.5g/t as the cut-off grade. Maximum gold grades at Wheeler and Wilson are 17.1g/t and 16.6g/t respectively.

‘Analysis of the Teck samples has enabled Lincoln to produce an inferred [according to CIM resource classification] resource of 56,056oz at Wilson and 241,000oz at Wheeler using a cut off of 0.5g/t,’ says Saxton.

‘Reverse circulation drilling conducted by Teck provided drill cuttings which are of marginal use for metallurgical test work,’ he adds.

To overcome this, Lincoln has drilled two large diameter core holes (PQ 85mm and HQ 63.5mm) on each gold deposit. Drilling was aggravated by faulted rocks and clays, and, in the case of the Wilson deposit, two metres of underground mining voids. These large core holes have been twinned as far as possible with the Teck bores whose collars are still evident. Overall, some 245m of core have been recovered and are undergoing metallurgical testing at McClelland Laboratories in Reno, USA.

According to Saxton, Lincoln is rapidly advancing the project towards production. The company has recently taken aerial photographs of the entire property and is producing a new topographic base map suitable for engineering. Also, the Wilson and Wheeler mine leases are being surveyed to ensure property lines are exact and that state and federal jurisdictions are clearly identified for permitting purposes.

JBR Environmental Consultants, Reno, has been hired to expedite Government permits for a new mine. Lincoln is planning a 48-hole drilling programme at the Wilson and Wheeler sites to reaffirm and upgrade the identified gold resources. Application for this drilling has been submitted.

Further drilling within the Pine Grove mining district is also planned to potentially extend the deposits and/or discover satellite mineralisation. Bids are being received for a scoping study to be followed by a feasibility study, Saxton says.

Further information: Lincoln Mining Corporation