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As governments promise to become more open and accessible, so too must their archive buildings. In fact, the conflicting demands on a contemporary archive building epitomise those on public buildings in general. An archive must provide a secure stronghold for irreplaceable documents, a stable internal environment that preserves these documents and a welcoming reading room.

Due to open later this month, the £3.8m Jersey Archive satisfies all these warring requirements thanks to the inspired design of architect MacCormac Jamieson Prichard, famous for the Ruskin Library in Lancaster, in association with local firm BDK Architects. The building was developed on behalf of the self-governing States of Jersey, and the environmental controls were designed by services engineer Ove Arup & Partners.

The Jersey government decided to set up an archive service within a purpose-designed building in 1993, three years after thieves had made off with plastic bin-liners containing priceless records of the German occupation. It appointed Denise Williams to run the service, draw up the building brief and manage the project. Williams had undergone a literal trial by fire in her previous post at Norwich when the archive building went up in flames, but this has not dampened her enthusiasm for public accessibility. "People get a thrill from seeing the real objects," she says.

At the Jersey Archive, the conflicting requirements of the brief have been dealt with in a manner that MacCormac Jamieson Prichard revels in. Although a modest 1885 m² in area and housing just 12 staff, the building has been articulated into four main elements containing its four prime functions. These functions are distinguished in contrasting form, materials and colour.

Facing the street is a crimson-painted block behind a clear-glazed window wall. It is self-evidently the public foyer, fronted by a "shop window" that reveals the building's interior to passers-by. Stretching backwards behind the foyer block is a two-storey linear block behind solid granite walls with punched-through windows. It contains the offices and workshops for cataloguing and conserving the documents. Jutting out of the upper storey of the linear block is a timber-clad carriage-like structure. This encloses the main reading room, where the public can view the precious original documents. David Prichard, partner in MacCormac Jamieson Prichard, likens its design to an oak treasure chest. Finally, at the far end of the linear block rises a four-storey windowless bastion in white render. This contains the strongrooms in which the records are stored.

The sequence of the four building elements is significant, Prichard explains. "There is a progression from the glazed public foyer at the front of the building, through increasingly controlled spaces, to the most secure space at the back of the site. We have tried to make the building legible, with one end transparent and the other end blank."

Another severe design constraint came with the site, which is an abandoned granite quarry about 10 minutes' walk from the centre of St Helier. It is a horseshoe-shaped bowl encircled by 18 m high granite cliffs, with a narrow mouth facing the street. Its one saving grace is that the granite is a beautiful warm russet colour with bold striations.

The sequence of four building elements has been so carefully tailored to fit the granite bowl that it has generated its mirror image of four distinct external spaces. The foyer block fills the mouth of the quarry facing the street, including a canopy, or porte-cochère, over the vehicular entrance. It has been set back from the street to create a paved public square, although its effect is marred by an asbestos-cement shed opposite.

The rest of the building stretches into the quarry in an L-shaped formation, with the largest external space opening out to the south. This is the main visitor courtyard, from which all the elements of the building are visible. It takes the satisfyingly crisp shape of a quarter-circle, as it is completely encircled by the timber "treasure chest" of the reading room, the white-rendered bastion block and the curving natural rock face. The courtyard is attractively landscaped by a lawn, a wide parapet wall containing bronze castings of historic documents, and an arc of parking spaces below a timber pergola.

The buildings enclosing the courtyard have been designed with the rich layering and texturing that MacCormac loves. The ground-floor walls are faced in coursed rough granite rubble, as if they were extensions of the natural quarry walls. The reading room is clad in horizontal tongue-and-groove boarding in American oak, with narrow banding in black-painted steel. It is supported on cantilevered beams in precast concrete with exposed aggregate. The bastion block is faced in smooth white render, and preceded by a stair tower in ribbed concrete blockwork. And, overhead, the roofs are zinc monopitches with wide overhangs, separated from the walls below by clerestory strips – glazed windows for the reading room and louvred steel vents for the bastion block.

This abundance of materials just manages to escape visual overload, as the granite walls, American oak cladding – once it has weathered – ribbed concrete blockwork and projecting granite beams are all variations of beige in colour. Two projecting pairs of steel rainwater pipes add an extra sculptural layer, if a spindly one, to the architectural composition.

Inside the building, the progression from open public areas to the secure strongroom starts at the foyer facing the street. This is a light and airy double-height space with a York stone floor and enclosed by window walls. After registering at the reception desk and depositing coats and bags, visitors proceed up an open-tread staircase and into the white-plastered corridor of the linear block.

On the right side of the corridor, green-lacquered MDF panels enclose the reading room. The room is further separated from the linear block by what Prichard calls "a layer of light": continuous strips of glass set into the floor and below the ceiling.

The reading room's design as a treasure chest celebrates the interface between public and secure areas. Its interior has the formal, ceremonial feel of an arcane gentlemen's club meeting room. It is dominated by an elongated oak table of reading desks, which are separated down the centreline by an acid-etched glass screen and equipped with fibre-optic reading lights resembling laboratory faucets. At one end stands a separate desk, where an invigilator keeps an eye on the precious documents.

The room is lined in more green-lacquered MDF panels, chosen, Prichard says, for the contemplative mood that the colour induces and its connotations of traditional leather-topped desks. A high ceiling of precast concrete panels is raised above clerestory windows, which fill the room with an even daylight. Two low-level horizontal strips of windows frame views of the courtyard garden.

Computerised reading rooms, along with staff offices and conservation workshops, are all ranged on the north side of the linear block, where they benefit from soft north light. Small square windows frame views of the surrounding cliff-faces.

Many of the architectural features contribute to passive environmental control in this naturally ventilated building. Windows have been restricted in number and size to reduce heat gain and loss, and, where they face south, are shaded by oversailing eaves or metal brises-soleil. Walls finished in hardwall plaster and precast concrete ceilings absorb fluctuating temperatures.

The strongrooms are four windowless floors in the bastion block, behind four-hour fire-compartment walls and doors. The archives are stored in a manually operated racking system on rollers. Stable temperature and humidity conditions are maintained to the latest British Standard without air-conditioning.

The Jersey Archive is basically a high-security repository, in which the island's precious documents must be doubly secured against theft and the damaging fluctuations of the external atmosphere. Its location inside a disused quarry and the client's requirement for a low-energy building could have made it even more forbidding. The architectural achievement is that the resulting building is such a welcoming and uplifting piece of civic architecture.