Homestake Mining Company -- McLaughlin Mine
Geology of the McLaughlin Deposit
By Dean Enderlin, 2002

Geology of the McLaughlin Deposit continued . . . Page 5 of 5

Pyrite (Fool's Gold)

Pyrite was widespread in the McLaughlin deposit, but generally not coarse-grained. The crystal cluster shown at right (with a vial of Mother Lode placer gold for comparison), came from an altered zone in sheared Knoxville mudstone. Such shear zones occasionally yielded pockets rich in beautiful large pyritohedrons such as this one. D. Enderlin photo.

   
Barite

Barite was relatively common along the western margin of the McLaughlin deposit. It formed as barium-enriched acid-sulfate waters from the hot springs percolated back down into portions of the deposit. Such sulfate-rich zones often also hosted stalactitic marcasite. D. Enderlin photo.

   
Barite with Quartz

The photo at right shows a cavity lined with barite crystals that have been encrusted with drusy quartz . The sample was collected from Quartz Adit Hill at the north end of the McLaughlin deposit. D. Enderlin photo.

   
Aragonite

Carbonates were widespread throughout the McLaughlin deposit. Magnesite and dolomite crystals were especially common. Calcite and aragonite were less common. The sample at left shows prismatic aragonite crystals collected from vugs in metabasalt on the west margin of the South Pit. D. Enderlin photo.

   
Chalcedony

Chalcedony, the fibrous form of quartz, was widespread through all but the deepest areas that were mined in the pit. The photo at right shows colloform banding (irregular swirls) in chalcedony from an orebody known as "40-R." The sample also contains gold which isn't visible at this magnification. D. Enderlin scan.

   
Chalcedony

Chalcedony can grow in countless shapes and patterns. At left is a spectacular sample collected from the North Pit. The chalcedony bands grew as crusts on freestanding scepters of calcite or dolomite. The carbonate scepters dissolved as the chalcedony formed, leaving the eye-shaped openings. Collectors often refer to banded chalcedony as "agate." D. Enderlin photo.

   
Marcasite

The photo at right shows botryoidal and stalactitic marcasite. Marcasite is an iron sulfide, like pyrite. In this case, it formed in an acid-sulfate water recharge zone, where iron-rich acidic water was reduced by ascending hydrogen sulfide. Such recharge zones are known as "zones of mixing," and their mineralogy can be quite complex.  In some areas of the South Pit, the marcasite stalactites grew to over 20 cm in length. D. Enderlin photo.

   
Framboidal Pyrite

Under high magnification, even a common mineral like pyrite can reveal peculiar textures! The photomicrograph image at left shows framboidal pyrite from the McLaughlin deposit. Debate continues over the origin of the growth forms called "framboids." Do bacteria create them?  Maybe. The number of doctoral dissertations on the subject would probably fill this CD-ROM! Homestake Mining photo.

   
Barite

Because of its tendency to grow well-formed crystals, barite from the McLaughlin mine is one of the more collectible minerals. This sample came from the South Pit. D. Enderlin photo.

   
Chalcedony

Chalcedony is found in a variety of forms at the McLaughlin mine. One of the more peculiar growth patterns is shown at left. These are stalactites formed in a vein cavity. We can assume that the cavity in which these formed was not a pathway for upwelling fluids. Instead, it was a gas-filled void in which fluids descended from above. This sample exemplifies the complex fluid pathway system of the hot springs. D. Enderlin photo.

   
Resiny Opal

"Root beer brown" was indicative of higher grade ore in the South Pit of the McLaughlin mine. The coloration is due to finely disseminated hydrocarbon residue in the vein matter. In the sample at right, the dark areas are resin-brown pigmented opal. The opal initially formed as an amorphous silica gel deep in the hot springs. Such gels typically dehydrate and recrystallize through time, to form chalcedony or quartz. Hydrocarbon residues are known to inhibit such recrystallization, which is why the opal was preserved in this case. It is thought that the close association of gold with resin-colored zones is a coincidence, where both were preserved by favorable reducing conditions. The gold affinity for organic matter (which is often seen in other world gold deposits) has never been clearly demonstrated at McLaughlin. D. Enderlin photo.

   
Calcite Pseudomorphs

Calcite and quartz have opposing stabilities when it comes to favorable precipitating conditions. Where one forms, the other often dissolves either completely or partially. This is one of the most obvious of the many complex reactions that go on in a hot springs. Hot springs geochemistry differs from laboratory chemistry because in a hot springs the "beaker" is reacting with the solution that boils inside of it! So, at any given point in the roots of the hot springs, the water chemistry is different.

In the example above left, bladed calcite was encrusted with chalcedony. As the chalcedony layers grew over the calcite, the calcite dissolved, leaving only a mold of the original eye-shaped crystals. These legacy crystal forms or "pseudomorphs," are very common in the boiling zone of a hot springs where pressure/temperature conditions are constantly shifting. These beautiful patterns exemplify the violent conditions in the heart of the ancient hot springs that formed the McLaughlin deposit. D. Enderlin scan.

Animated globe courtesy NOAA NESDIS National Geophysical Data Center 
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