MINERALOGY-101: ENVIRONMENTS & ASSOCIATIONS – Massive Sulfides

MASSIVE SULFIDES

Deposits of massive sulfides are found in several forms. Probably the two most prevalent types are *stratabound* deposits and *vein deposits.* The later are most often found along fault zones of one type or another. Both are usually associated with metamorphic rocks, though deposits in other settings are also known. I’m most familiar with the metamorphic stuff, so I’ll be concentrating on those here.

The stratabound deposits occur as layers sandwiched in between other layers of rock, and originated as sulfide-rich beds of volcanic materials – cooled lava flows, ash layers, and layers of sediments eroded off of volcanic island ranges or mountain chains. Volcanism often brings materials rich in sulfur and metals to the surface – laying them down in thick beds on the ground or sea floor. The ones I’m familiar with started out as rhyolitic tuffs and magnesium, iron and aluminum rich materials that metamorphism converts to *amphibolites* - schists and granulites composed largely of amphibole minerals such as hornblende and actinolite, usually with a fair amount of chlorite mixed in. The metamorphism of these rocks takes place after deep burial when tectonic forces exert high pressures and bring the rocks up to geologically moderate to high temperatures. Here in the Northeastern US such beds are found in Taconian age rocks created by volcanic island arcs that formed off the early coast of New England, and were then caught up in the “vise” of these islands and the seafloor colliding with the edge of the continent and being driven up onto the continental margin as the arcs and seafloor were driven land-ward by seafloor spreading further out, in the middle of the ancient Iapetus Ocean. During the height of this process there was a fair amount of hydrothermal activity – ocean water trapped in the rocks and brought up to high temperatures by the collision. These hydrothermal fluids served to enrich the stratabound massive sulfide beds by geochemically gathering in the sulfur and metal and depositing them along certain horizons or zones in the rocks.

There is evidence in New England which suggests that the stratabound massive sulfide “beds” actually occur in thrust fault zones that are regional in scale (see the post below on ultramafic tectonic slivers): The shearing forces of the island-continent collision created highly fractured and “mobile” zones at certain horizons in the rocks as they were metamorphosed, and these zones of weakness became the areas where the hydrothermal fluids gathered in the metals and sulfur, depositing them. So technically – for the New England occurrences at least – the term “stratabound” is something of a misnomer. These deposits are closely related to the more typical fault zone deposits we see in the region as well.

Fault zone deposits of massive sulfides (whether along thrust faults or typical high angle faults, normal or reverse) are pretty much self-explanatory: Hydrothermal fluids rich in metals and sulfur coursing through the sheared rocks of fault zones deposit sulfide minerals as ore bodies along those faults. Here in New England quite a few of them occur due to magmatic doming during the Mesozoic era: Large bodies of magma – deep-seated liquid rock – slowly work their way towards the surface from the depths at which they are created. If they approach close enough to the surface their buoyancy actually lifts the overlying “veneer” of crustal rock; and since that rock is solid and somewhat brittle, it fractures from the stress and great blocks shift in relation to one another – faulting. [The magmas here, by the way, eventually cooled down and crystallized into plutons and a small batholith, called the “White Mountain Batholith.” These rocks contain pegmatites, and dikes and sills of pegmatite were forced out into the country rock – where we find a number of pegmatite minerals. See the post below on pegmatites for more info on these.]

Around where I live, we find quite a few fault zone deposits of massive sulfides, many of which have seen mining activity – people searching for ores of iron, sulfur, lead, zinc, copper, silver and gold. These deposits and the mines developed in them pretty much form a “halo” around the perimeter of the White Mountain Batholith. And it is my belief that the source of the metals and sulfur in these deposits are the older, Taconian age, *volcanogenic* stratabound massive sulfides discussed above. Just a little theory that I have – and that I am presently working on trying to prove or disprove. (Hey – the materials had to come from somewhere, and around here the stratabound massive sulfide “beds” are almost the only likely source…) Most of these deposits are rich in minerals such as pyrite (lots and lots of pyrite!), galena, sphalerite, and hematite - with some chalcopyrite and traces of arsenopyrite and other “primary” sulfide minerals. And massive sulfides usually carry a tiny percentage of silver and gold – though rarely (if ever…) in sufficient concentrations to be profitable on their own; as miners learned (to their dismay) when they tried to develop gold and silver mines on these deposits. If they didn’t capitalize on the base metal ores they were finding – turning the mines into lead, iron and zinc mines – they went bust. Those that read the writing on the wall (or in the rock, as the case may be) faired better, making a living for a space of time from base metals – with the occasional shipment of gold and silver as a bonus. Even most of the supposed “copper” mines around here really weren’t: Chalcopyrite – the principal copper ore – is also rather scarce. Some of the mines capitalized on the pyrite’s sulfur content, shipping ground pyrite to Boston for processing to us the sulfur in the production of industrial sulfuric acid. In fact, the most profitable mines in the region’s massive sulfide deposits were those that went that route – considering all other ore production as subsidiary to sulfur production. The old Davis Pyrite Mine in Rowe, Mass., and the Ely Mine in VT are a couple of examples of such mines.

Whether we’re talking stratabound or faultbound, the mineral assemblages found in these deposits are largely a combination of sulfides (such as those mentioned above) and quartz. In places the sulfides are overshadowed by metallic oxides – particularly hematite, magnetite, and a bewildering array of manganese oxides that are damn near impossible to identify without high-tech lab equipment. These generally end up being called either “psilomelane” or “wad.” In one stratabound deposit down in the Berkshire Hills of western Massachusetts, the manganese was so rich it produced a deposit of rhodonite, rhodochrosite, and “wad” that was rich enough to be mined for the manganese – the old Betts Manganese Mine in Plainfield, Mass. A couple of places where the materials were particularly iron-rich are the Forge Hill Iron Mine in West Hawley, Mass. (specular hematite and magnetite), and at the Franconia Iron Mine on Ore Hill in the town of Sugar Hill, NH (magnetite.)

Often these deposits produce suites of secondary minerals – minerals that form after the deposit has basically been created. Percolating ground water – either geothermally heated to become hydrothermal fluid or simply cool water perking down through fractures and interstices as “epigene” or “meteoric” water – dissolve the primary minerals and redeposit them as crystals of the same minerals or chemically combine the constituents as new minerals. At one site where I am currently doing some study, this sort of activity has produced a major suite of unusual secondary minerals – unusual, at least, for New England: malachite, brochantite, linarite, native silver, acanthite, pyrite and chalcopyrite, cuprite sphalerite, cerussite and anglesite, posnjakite, schulenbergite, metazeunerite – and a few things we haven’t even identified yet. (We are up to “Unknown #37” – but some of the earlier ones have been identified since being listed as mystery minerals; so there are currently 15 to 20 which remain to be determined.) Anyway, good old Ma Nature seems to have had a field day with the fault zone sulfide deposit at the Mascot Lead Mine here in Gorham, NH - where I just happen to live! (Talk about the luck of the draw…) [PS: June 2001 update: We have determined that some of the secondaries found in one of the mine dumps formed in the dumps, not in the ore lode. Schulenbergite and posnjakite are two of them. "Modern mineralogy" :~} ]

More typical deposits tend to have fewer – if any – secondaries. The nearby Nay-Fogg Mine in West Milan, NH, hasn’t produced any good secondary minerals that I know of. The Silver Lake Lead Mine in Madison has produced a few – though nothing like the plethora found at the Mascot Mine. The only other “rich” mine in New England that comes to mind is the old Manhan Lead Mine in Easthampton, Mass. – “Loudville” (the mining village near the mine that got its name because the miners were - well – a rather *loud* bunch…) That mine has produced secondaries such as wulfenite, pyromorphite, cerussite, and anglesite – the latter two in just about every conceivable crystal habit! There are even, I believe, still a few “mystery minerals” from there that await identification. (I have a shoe-box full of them that I inherited from an old friend who got them from another old friend of ours that studied the Manhan Mine minerals and wrote a paper on them for the “Mineralogical Record” back in the late ‘70s. Some day – if I ever find the time – I’m going to dig up that box and have some fun playing with Fred Lincks’ “Manhan Mystery Minerals.” Good hearted Fred – rest his soul – introduced me to the mineralogy of that mine back in the early ‘80s. Until I got to know him, I’d had no idea what a wealth of minerals the site produced. I thought it was just a place to go for quartz crystals, galena, and an occasional find of wulfenites. Little did I know…)

Finally, localities such as these often also produce “accessory” minerals – minerals that formed in the rocks around the deposit. Such as finds of almandine and epidote crystals at Ore Hill in Sugar Hill, NH, and finds of ferrohornblende crystals in the amphibolite containing the hematite and magnetite at the Forge Hill Mine in West Hawley, Mass. But here we are drifting afield from stratabound massive sulfides and getting into the metamorphic mineral environment – which I’ll discuss in another post to the Mineralogy-101 series.

Keep on rockin’!

Al Plante

From Alan Plante - June 01, 2001 at 23:09:46
Email: sheral[ ]ncia.net