MARYSVILLE -- A step through the portal of the Drumlummon Mine is a step back in time.

About a billion years ago, limestone was deposited here, when Montana was at the bottom of a shallow sea. About 70 million years ago, molten granite intruded into fractures in the limestone. About a century ago, Tommy Cruse discovered gold and silver flakes among those rocks. About five decades ago, the whitewashed plywood doors at the entryway closed when the last miner left.

Until earlier this summer, only the occasional adventurers explored the 29 miles of shafts that were drilled, chipped and blasted into a honeycomb maze. Water flooded the lower levels. Few paid attention to the mine that made Marysville and millionaires.

Yet a Toronto-based Canadian company believes there's still pay dirt left, missed by the previous miners who removed 586,000 ounces of gold and almost 5 million ounces of silver from 1 million tons of ore. RX Exploration has been drilling deep into the granite and limestone inside the Drumlummon Mine for the past six months, pulling out core samples to assay.

They like what they're seeing.

They want to go for the gold.

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"This was the largest underground gold producer in the state," Joe Bardswich, chief mining engineer for RX, says as he dons his hard hat before entering the mine last week. "The main way they explored in the old days was to find a vein and (bore) into it, hopefully finding increasing grades. But they were limited how far they could go with their drills."

Legal problems and nature's flooding of the lower mine shafts also put a crimp on exploration and production, which ended around 1953. RX wants to pump out and treat an estimated 100 million gallons of arsenic-tainted water, then use the latest available technology -- which basically involves more powerful drills and equipment than that of the late 1800s-- to search for precious metals.

Bardswich is a large man who speaks with a hint of a Canadian accent. He's also a legally "qualified person" who can attest to results from core sampling. He says it looks promising.

In the contractor's work trailer outside the mine portal, Bardswich pulls a 2 3/4-inch diameter, foot-long stone cylinder from a cardboard box full of similar samples (including one core sample that cut through a wooden retaining wall; it's marked as "low-grade pine.") The granite sample has been sawn in half, and in the sunlight filtering through a window, Bardswich twists it back and forth.

Flecks of gold shimmer on a black background of solid rock known as tetrahedrite. The flecks are difficult to see with the naked eye, but they speak volumes to Bardswich, who adds that it's not uncommon to find silver next to the gold.

"Generally, you'll find fine gold and fine silver tied up in minerals. There are no big 60-ounce chunks here," Bardswich said. "It's not like in the movies."

But RX says its "assay" -- a type of analysis -- show that these flakes can add up quickly

"We've been getting about an ounce of gold per ton of rock, and about 22.9 ounces of silver per ton," Bardswich says. "We think there's two main veins with three or four minor ones."

News releases from RX put it another way. They figure the mine is good for around 70,700 ounces of gold, plus 1.9 million ounces of silver, in about 155,000 tons of rock.

With gold hovering around $900 per ounce, that's about $63 million.

Silver is only $12 per ounce, but 1.9 million ounces is worth almost $23 million.

A lot of overhead costs are involved, but Bardswich can't help but smile at the math.

Pete Strazdas, a consultant for RX, pulls open one of the portal doors so a group of people, including scientists from the Montana Department of Environmental Quality, can enter the Drumlummon. The mine is named after the parish in Ireland where Cruse was born.

The musky scent of moisture and mud hangs heavy in the air. The chocolate-colored muck is ankle deep on the floor and Seibert "Smitty" Smith, the mine safety officer, warns to walk either between the ore carts' two rusty metal rails or on the right side. Deep holes on the left side are easily disguised under the pools of turbid water.

At times, both sides of the mine's walls can be touched simultaneously with outstretched arms. At first the granite walls are exposed, then the tunnel segues into an area where the stone is covered by heavy wooden timbers.

Overhead, what looks like a chain-link fence is bolted into the mine walls, holding back tons of fallen rock.

Below the fence, hanging about 5 feet in the air, are cords and pipes carrying water, air to breathe and to power pneumatic tools, and electricity.

Deeper into the mine, the whoosh of a fan steadily increases until it's too loud for conversation. A few hundred feet inside, Bardswich pauses, noting that when they began looking at reopening the Drumlummon, cave-ins prevented people from getting beyond here.

They've cleared away the fallen rock and installed a second set of plywood doors to help control air movement. Slowly, the darkness gives way to a fairly well-lit cavern, which is about 30-by-60 feet wide, with a 50-foot ceiling full of "stopes" or holes where miners blasted upward so the ore would fall down and they could cart it out to the mine portal in rail cars. Some stopes are as wide as 40 feet.

It's also in this room, known as the No. 1 Shaft Station, where the group pauses to marvel at the historic miners' creativity. Someone used red bricks to create a retaining wall, which is perfectly curved to follow the natural rock lines. It's a testament to a bricklayer's skill, as well as to the money that was thrown around a century ago when the Drumlummon was producing millions of dollars in gold and silver.

"That's incredible. I've been in a bunch of mines, and I've never seen anything like it," says Catherine Dreesbach, a DEQ mining engineer.

Smitty proudly notes that the only other underground mine with a brick wall like this that he knows of is in South Dakota.

Along with the upward stopes, shafts here drop down 1,200 feet to the 1,600-foot mine level. The Drumlummon Vein intersects the No. 1 Shaft Room, north to south, horizontally at the 400-foot level. The Castletown Vein branches out toward the northeast at the 600-foot level. We're 1,100 feet inside the mine.

"We're not looking for gold here," Bardswich notes. "We're going farther in."

Instead, it's in this room, where exploration equipment is stockpiled, that RX wants to install a water treatment system as part of the effort to remove the water that has flooded the lower level shafts since the early 1900s.

Arsenic levels range from 10 to 20 parts per billion in water in the Drumlummon, sometimes exceeding the federal human health standard of 10 ppb. The average arsenic concentration in nearby Silver Creek is 3 ppb.

Generally, the treatment system under consideration at this point calls for the water to be pumped from below through pipes into cylinders standing in this room, which would be filled with iron filings encased in tall tubes. Arsenic in the mine's water would bond with the iron. That treatment system is called "adsorption," Strazdas said.

Ion exchange is another option, where resins strip out the arsenic.

RX wants to start discharging treated water at a rate of 40 gallons per minute, eventually increasing to 400 gpm, via a pipe that would be perforated and buried two feet underground in an old rail bed outside the mine. Water entering the infiltration line would need to meet state water quality standards; tests indicate it would, according to DEQ documents.

In some mines, water moving across exposed rocks creates acid runoff. Warren McCullough, DEQ environmental management bureau chief and Wayne Jepson, a DEQ hard rock hydrologist, say that doesn't seem to be the case here.

"Basically, the water in the mine is of better quality than the water in Silver Creek, with the exception of arsenic," Jepson said, adding that the mine water's pH is neutral.

Jepson adds that they're sampling water from a variety of pools, since it can be "highly variable," with water clean in one zone and contaminated in another.

At a fork in the tunnel, a low-profile "mucker" machine sits, waiting to scoop blasted or fallen rocks and take them outside. It's articulated, or jointed, in the center, allowing the large machine to curve around the tunnel walls. The mucker is a huge step up from wheelbarrows and buckets.

Water occasionally drips from the ceiling, and the group stops to look into a cavern where it sounds like a waterfall is contributing to the side room's pool. It's hard to tell the pool's depth or the water's quality. One of two miners currently "mucking" around in here laughs, joking that this is where they swim in the summer.

An "area of refuge" is on the right a bit farther down the tunnel. It's equipped with bottled air and water, lights and food, plus a telephone to call the surface. If there's ever a problem, like the recent cave-ins that killed miners in Utah and West Virginia, those who can't get out try to get in here. It's a sobering thought.

Bardswich notes that they have at least a year of exploration ahead of them -- and some permits to apply for -- before any mining gets under way that puts them in the black. So far, RX has spent about $2 million on sampling, plus cleaning out fallen rocks and reinforcing walls and ceilings.

"Part of the exploration would be drafting on a vein that's producing ore," Bardswich said. "Once you establish that and have an area built out ... you can start making money on it. While you do that, you also keep exploring so you always have ore ahead of you."

Their underground exploration permit allows them to take out up to 10,000 tons of ore for testing and possibly recovering of gold and silver. If they don't disturb more than five acres above ground -- which is part of the current plan -- they can operate with little government oversight under a small miners' exclusion.

If their theory pans out, Bardswich said he would be happy if mining lasted here for 15 years, employing up to 300 people.

"But you can have a small, very profitable mine that runs only three to five years," Bardswich noted.

The flip side, McCullough adds, is that some mines operate for a century. It all depends on the geology.

RX said a new mill to extract the gold from the ore might be built about three miles down valley. But initially, the rock might be trucked to Montana Tunnels, or mills near Radersburg or Phillipsburg.

Mining is a patient person's profession. RX is moving inward here at a rate of about 10 feet per day, with technology not that far removed from the past. Miner Aaron Edmondson grips a jack-leg drill, showing how he bores 30 or so holes, then uses dynamite to blast the rock free.

About 2,500 feet in, the tunnel ends near a cave-in and cache of explosives. Bardswich says they're thinking about hooking around and tunneling backward to connect with a new portal they hope to build.

It's at this tunnel's end that the old miners signed sooty signatures on the walls, probably using carbon flames from their headlamps. Fatso Haley was here in 1925. So was Jack Smigaj and Jim Obernford. B. O'Conell drew a face in 1912. Smitty -- not the one who's here today -- left his mark on March 11, 1917. Apparently he wasn't the most popular guy, because someone else wrote that "Smitty eats ..." -- let's just call it manure.

"You don't see this stuff every day even if you work in a mine," Dreesbach says, once again surprised by the Drumlummon.

Today, though, this is where the tour ends. A few photographs and 20 minutes later, the group returns to the stinging bright sunshine and turns off the headlamps.

McCullough blinks as his eyes adjust, and looks at the portal as Edmondson closes the plywood doors. McCullough has to sign off on the permits to move forward, and he's pleased with what he's seen so far.

"It seems like a solid, well-thought-out and well-financed project," said McCullough, who's been working in and around mines for about 30 years, including 13 years with DEQ. "It doesn't look like they'll have nearly the challenges I've seen with other operations around the state."

Reporter Eve Byron: 447-4076 or eve.byron@helenair.com